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SURGICAL ANATOMY OF THE HAND AND UPPER EXTREMITY

SURGICAL ANATOMY OF THE HAND AND UPPER EXTREMITY

JAMES R. DOYLE, M.D. Emeritus Professor of Surgery (Orthopaedics) John A. Burns School of Medicine University of Hawaii Honolulu, Hawaii Editor-in-Chief The Journal of Techniques in Hand and Upper Extremity Surgery

MICHAEL J. BOTTE, M.D. Co-Director Hand and Microsurgery Service Division of Orthopaedic Surgery Scripps Clinic La Jolla, California Orthopaedic Surgery Service San Diego VA Health Care System Clinical Professor Department of Orthopaedic Surgery University of California, San Diego School of Medicine San Diego, California

Illustrated by Elizabeth Roselius with contributions by Christy Krames

Acquisitions Editor: Robert Hurley Developmental Editor: Keith Donnellan Production Editor: Thomas J. Foley Manufacturing Manager: Benjamin Rivera Cover Designer: Christine Jenny Compositor: Lippincott Williams & Wilkins Desktop Division © 2003 by LIPPINCOTT WILLIAMS & WILKINS 530 Walnut Street Philadelphia, PA 19106 USA LWW.com All rights reserved. This book is protected by copyright. No part of this book may be reproduced in any form or by any means, including photocopying, or utilized by any information storage and retrieval system without written permission from the copyright owner, except for brief quotations embodied in critical articles and reviews. Materials appearing in this book prepared by individuals as part of their official duties as U.S. government employees are not covered by the above-mentioned copyright. Printed in China Library of Congress Cataloging-in-Publication Data Doyle, James R. Surgical anatomy of the hand and upper extremity / James R. Doyle and Michael J. Botte. p. ; cm. Includes bibliographical references and index. ISBN 0-397-51725-4 1. Hand—Anatomy. 2. Arm—Anatomy. I. Botte, Michael J. II. Title. [DNLM: 1. Arm—anatomy & histology. 2. Hand—anatomy & histology. WE 805 D754s 2003] QM 548 .D69 2003 611′.97—dc21 2002030007 Care has been taken to confirm the accuracy of the information presented and to describe generally accepted practices. However, the authors and publisher are not responsible for errors or omissions or for any consequences from application of the information in this book and make no warranty, expressed or implied, with respect to the currency, completeness, or accuracy of the contents of the publication. Application of this information in a particular situation remains the professional responsibility of the practitioner. The authors and publisher have exerted every effort to ensure that drug selection and dosage set forth in this text are in accordance with current recommendations and practice at the time of publication. However, in view of ongoing research, changes in government regulations, and the constant flow of information relating to drug therapy and drug reactions, the reader is urged to check the package insert for each drug for any change in indications and dosage and for added warnings and precautions. This is particularly important when the recommended agent is a new or infrequently employed drug. Some drugs and medical devices presented in this publication have Food and Drug Administration (FDA) clearance for limited use in restricted research settings. It is the responsibility of the health care provider to ascertain the FDA status of each drug or device planned for use in their clinical practice. 10 9 8 7 6 5 4 3 2 1

To Julie Kaye Frances and Robert E. Carroll, M.D., friends and mentors. J.R.D. To my mother, Verona Louise Minning-Botte, M.D., and my father, Joseph Michael Botte, M.D. For their love, encouragement, and support and for being the best teachers that I ever had. M.J.B.

CONTENTS

Contributing Authors ix Foreword by David P. Green xi Preface xiii SECTION I: SYSTEMS ANATOMY 1 1

Skeletal Anatomy 3

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Muscle Anatomy 92 Appendix 2.1. Muscles of the Hand and Forearm and Arm: Origin, Insertion, Action, and Innervation 180 Appendix 2.2. Muscle Compartments and Fascial Spaces of the Upper Extremity 183 Appendix 2.3. Human Forearm Muscle Difference Index Values: A Comparison of Architectural Features of Selected Skeletal Muscles of the Upper Extremity 184

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Brachial Plexus 297 Vincent R. Hentz and Y. Mark Hong

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Arm 315

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Elbow 365

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Forearm Part 1: Flexor Forearm 407 Part 2: Extensor Forearm 461

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Wrist 486 Richard A. Berger, James R. Doyle, and Michael J. Botte Hand Part 1: Palmar Hand 532 Part 2: Dorsal Hand 642

Nerve Anatomy 185 Appendix 3.1. Dermatomes of the Upper Extremity 226

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SECTION II: REGIONAL ANATOMY 295

Vascular Systems 237

Appendix: Anatomic Signs, Syndromes, Tests, and Eponyms 667 Subject Index 693

CONTRIBUTING AUTHORS

Richard A. Berger, M.D., Ph.D. Professor, Departments of Anatomy and Orthopaedic Surgery, Mayo Clinic, Rochester, Minnesota Vincent R. Hentz, M.D. Chief, Hand Division, and Professor of Functional Restoration (Hand), Department of Surgery, Stanford University School of Medicine, Palo Alto, California Y. Mark Hong, B.S. Department of Surgery, Stanford University School of Medicine, Palo Alto, California

FOREWORD

The best surgeons are those well versed in anatomy. A surgeon can never learn too much anatomy, but up until now, he or she had to go to many sources to glean a broad base of anatomical knowledge. My own career illustrates this point. As a medical student, I began with Gray’s massive and dry tome, learning anatomy for the sake of anatomy, with no clinical relevance. Then, as a resident, I discovered Hollinshead’s three-volume text that added functional implications. I also found, at that time, Henry’s classic book with its quaint Irish-English prose and manual mnemonics. As a young surgeon, I sought out books that would give me quick, snapshot glimpses of anatomy that I could memorize and carry in my head at least until the next day in the operating room. Grant’s Atlas was the first of these, which was later replaced on my shelf by McMinn and Hutchings’ magnificent atlas with its lifelike-quality color plates. Specialized texts such as Sunderland and Spinner have described wonderfully detailed and precise anatomy, but with a limited focus. Now the hand and upper extremity surgeon has what all of the above resources offered and more, packed into a single volume. The thoroughness of Gray, the practical applications of Hollinshead, and the clarity of McMinn and Hutchings have been blended into one unified source. More than sixty crisp photographic prints depict detailed cadaver anatomy with a precision and clarity that rivals McMinn. Most of the drawings were created by Elizabeth Roselius, a master among contemporary medical illustrators. The exceptionally high quality of these illustrations is complemented by a text that is not only thorough, but also replete with clinical applications.

Another pleasant surprise in this text is the appendix of anatomic signs, syndromes, tests, and eponyms, where even the surgeon who has studied the history of surgery will find new or more accurate information. Practical lessons in the Greek and Latin derivations of words explain why similarsounding words that evolve from separate sources have different meanings. One of the authors, James R. Doyle, was the first to describe in detail the flexor pulley system in the fingers (1975) and later in the thumb (1977). Jim Doyle has studied the anatomy of the hand throughout his entire professional career with the eye of an artist who can perceive details better than most of us, with an inherent tenacity fired even harder during a fellowship year with Robert E. Carroll, and with an exquisite and careful attention to detail. This book is the culmination of his life-long dedication. Michael Botte, his co-author, brings to this project the thoroughness and precision of a true scientist, and his input is significant. The collaboration of Doyle and Botte has produced a remarkable piece of work that will benefit not only the entire surgical community, but our patients as well. Every serious hand surgeon will find a readily accessible spot on his or her bookshelf for this text. David P. Green, M.D. Clinical Professor Department of Orthopaedics University of Texas Health Science Center at San Antonio San Antonio, Texas

PREFACE

Our goal has been to assemble between two covers a comprehensive collection of anatomical material designed to aid the hand and upper extremity surgeon in the evaluation and treatment of patients. A comprehensive knowledge of anatomy is a major prerequisite for safe and effective surgery. Although written by hand surgeons for hand surgeons, the authors believe that this text will also be useful to hand therapists, anatomists, neurologists, neurosurgeons, sports medicine surgeons and physicians, physiatrists, and bioengineers because it is a compendium of anatomic knowledge. The science of anatomy and the art of surgical technique are intertwined topics that are not easily separated. Although this book is not designed as a text on operative surgery, overlap with surgical technique is inevitable, appropriate, and complementary to the goal of the book. Although another anatomy textbook may seem redundant, we hope the reader will agree that this text represents a unique and current collection of material, which may not be conveniently found elsewhere. Much of the information can be found in other resources such as texts and journals but we hope the reader, who like us has had difficulty recalling where we found a particular bit of information that we now need to review or utilize in a timely fashion, will come to value this comprehensive resource. Our primary goal in this text is to provide a readily available source for this information that is user friendly, easily portable, and clinically relevant. We hope that the arrangement, clarity, and brief yet comprehensive presentations of these topics will be of sufficient uniqueness to earn the designation of original, but we readily admit that paraphrasing and adoption of others’ original concepts have been used (although we have done our best to give credit where it was due). The words of Anatole France (passed on to us by Adrian Flatt) bear repetition here, “When a thing has been said and said well, have no scruples, take it and copy it.” The text is divided into sections on Systems Anatomy and Regional Anatomy, followed by an Appendix on Anatomic Eponyms, Signs, Syndromes and Tests. The section on Regional Anatomy represents the practical component of this text because it provides the reader with anatomic landmarks, relationships, surgical approaches, clinical correlations, and the anatomy of selected anatomic variations found in that region. The student of anatomy

will also recognize the immense value of the systems approach, found in the section on Systems Anatomy, in providing a comprehensive and overall view of a given anatomic structure or system. The authors take great pride in the color photographs of fresh cadavers used in this text. A quote by Emanuel Kaplan, about color photographs, from his foreword to Milford’s 1968 classic monograph on Retaining Ligaments of the Digits of the Hand seems appropriate here as well, “The natural color illustrations add precision and eliminate the imaginary interpretive creativity leading to error.” We hope that the quality of our color photography can approach that of Milford and warrant the affirmation of Kaplan on the value of natural color photographs. We hope these color photographs, along with the excellent illustrations, will serve to make the anatomy in this text as realistic as possible. We also believe that the combination of these two art forms along with the descriptive text will provide the reader with appropriate information, which will permit accurate preoperative evaluation, diagnosis, and effective surgical treatment. Anatomy, as a surgeon must deal with it, is three dimensional, but only two dimensions can be portrayed in a text. This fact should immediately indicate to the reader that there is no substitute for personal experience in the dissection room. In a two-dimension text, structures are often portrayed as lying side by side when in reality they may be vertically arrayed. A good example is the usual depiction of the radial and ulnar arteries in the proximal forearm. The ulnar artery is depicted as lying to the ulnar side of the radial artery in the same anatomic plane whereas, in reality, the ulnar artery is deep and ulnar to the radial artery and is often difficult to find. The reader should also note that the anatomic variations included are those that the authors perceive to have some practical clinical relevance to the region and that the list of variations is not encyclopedic. Reported differences in anatomy may be due to anatomic variations as well as inter-observer variability and subsequent interpretation of the observation. It would seem reasonable to assume that all observers of a particular region or segment of anatomy would see or observe the same things and interpret what they saw in a similar uniform fashion. Such is not the case, and although many points of anatomy are agreed upon there are many that are not. Two

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Preface

illustrative examples come readily to mind: (1) the arcade of Struthers’ in the arm and (2) the location of the sesamoid bones about the MP joint of the thumb. Some authors describe in detail the arcade of Struthers’ 8 cm above the medial humeral epicondyle and attach clinical significance to it as a potential site of ulnar nerve compression in the arm. Others claim that it does not exist or at least that they have never seen it and thus it has no clinical relevance. The location of the ulnar and radial sesamoid bones about the MP joint of the thumb have been reported to be in the adductor pollicis and flexor pollicis brevis tendons respectively or in the palmar plate where they possess articular cartilage and articulate with the thumb metacarpal; two entirely different pictures of the same structures. Interobserver variability may be illustrated by the imperfect, yet humorous, analogy of six blind persons examining a camel. Each of their descriptions are based upon their particular location about the camel. Their significant inter-observer variation results in a series of descriptions that would confound even a camel veterinarian. The authors include themselves in those observers who may be subject to imperfect observation as well as faulty interpretation. Thus, there may be a lively correspondence and commentary generated by this text. We believe that studies that have large numbers of specimens in their data base have a greater potential for reflecting what might be considered more common and thus likely to be encountered in the day to day practice of surgery. Studies with small numbers of specimens in which several patterns or categories of anatomical arrangement are noted tell us that significant variation exists. It may not tell us the true incidence of a given pattern or arrangement in spite of the authors’ conscientious reporting of one, two, or three cases in their series which demonstrated a particular pattern or arrangement. Such studies though, are still

important and tell us that significant variation exists in that particular structure or region and that the surgeon must be prepared to encounter such an arrangement or even a new and unreported pattern or arrangement. By now, the reader has begun to appreciate the fact that anatomy is not a “fixed” science, but rather an evolving or developing endeavor with many remaining challenges and opportunities. All authors have their own methods for placing thoughts on paper. This quote from Wallace Stegner1, although directed at the writer of autobiography or fiction, seems appropriate, “You take something that is important to you, something you have brooded about. You try to see it as clearly as you can, and to fix it in a transferable equivalent. All you want in the finished print is the clean statement of the lens, which is yourself, on the subject that has been absorbing your attention.” The authors wish to recognize their debt to those surgeons and anatomists who have studied and described their anatomic findings in the upper extremity and to our many mentors and colleagues who have taught, encouraged, and inspired us. Finally, the authors wish to acknowledge their debt to Robert Hurley and Keith Donnellan of the editorial staff at Lippincott Williams & Wilkins who have patiently guided and encouraged us throughout this process in such a competent and professional manner. We also owe a great debt to Elizabeth Roselius, medical artist, for her understanding of complex anatomic concepts and her ability to convert those concepts into clear and concise drawings. James R. Doyle, M.D. Michael J. Botte, M.D. 1

Stegner WE, Where the bluebird sings to the lemonade springs. New York: Random House, 1992.

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I SYSTEMS ANATOMY

1 SKELETAL ANATOMY MICHAEL J. BOTTE

The skeletal anatomy of the upper limb is divided into the shoulder girdle, the arm, elbow, forearm, carpus, and hand. The scapula, clavicle, and sternum comprise the skeletal shoulder girdle. The mid-portion of the humerus comprises the skeletal arm. The distal humerus and proximal ulna and radius form the skeletal elbow. The radius and ulna and associated soft tissues comprise the skeletal forearm. The carpus consists of the distal radius and ulna along with the eight carpal bones: the scaphoid, lunate, capitate, trapezium, trapezoid, triquetrum, hamate, and pisiform. The hand contains 19 bones: 5 metacarpals, 5 proximal phalanges, 4 middle phalanges, and 5 distal phalanges. The skeleton of the upper limb is attached relatively loosely to the trunk. The clavicle provides the only direct skeletal connection of the upper limb to the axial skeleton, articulating through the sternoclavicular joint. The upper limb is substantially stabilized to the thorax by muscles of the soft tissue scapulothoracic articulation. This relatively loose attachment maximizes upper limb mobility and flexibility, allowing rotation and translation of the scapula on the thorax. The loose connection of the upper limb to the trunk is in contrast to the lower extremity, where the majority of the stabilization is through the skeletal connection of the hip joint. In the following sections, each bone and associated joint of the upper limb is discussed. The ossification centers, descriptive osteology, articulations, muscle attachments, and clinical implications are discussed. Osseous anomalies or variations, when significant, are described as well. CLAVICLE Derivation and Terminology The clavicle derives its name from the Latin clavis, meaning “key” (1–3). The plural of clavicle is claviculae (1,3). The clavicle has been referred to alternatively as the clavicula. Clavicular indicates “relating to the clavicle” (1,3).

Ossification Centers The clavicle begins to ossify earlier than any other part of the skeleton (4,5). It has three ossification centers, two primary centers for the shaft and one secondary center for the medial end (Fig. 1.1). The primary centers for the shaft consist of a medial and a lateral center, both of which appear during the fifth or sixth week of fetal life. The centers fuse to each other approximately 1 week later. The secondary ossification center is located at the sternal end of the clavicle and first appears approximately the eighteenth or twentieth year, usually about 2 years earlier in women. The secondary center unites with the remaining portion of the clavicle at approximately the twenty-fifth year. An acromial secondary center sometimes develops at 18 to 20 years of age, but it usually is small and fuses rapidly with the shaft (2,6). The clavicle does not ossify in quite the normal manner of endochondral ossification, as occurs in most of the skeleton. Although the medial and lateral ends of the clavicle do undergo endochondral ossification, the mid-portion is formed by a process that shares features of both endochondral and intramembranous ossification. The clavicle is preformed of cartilage in embryonic life, but does not proceed with endochondral ossification in the conventional manner. Instead, the cartilage model simply serves as a surface for the deposition of bone by connective tissues. Eventually, the cartilage is resorbed and the clavicle becomes fully ossified (7–10). [The process is shared by the mandible. The remaining long bones of the upper extremity are formed by conventional endochondral ossification (7).] Osteology of the Clavicle The clavicle is a curved, roughly “S”-shaped long bone that lies subcutaneously along the anterolateral base of the neck. When viewed from its superior side, the clavicle shape resembles the letter “F,” with the concavity of the medial curve being directed posteriorly, and the concavity of the lateral portion directed anteriorly (Figs. 1.2 and 1.3). It

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Systems Anatomy

FIGURE 1.1. Illustration of right clavicle showing the three centers of ossification. There are two primary centers (medial and lateral) for the shaft and one secondary center for the medial end.

forms the most anterior portion of the shoulder girdle, and is subcutaneous along its entire course. It is directed nearly horizontally toward the acromion of the scapula, located immediately superior to the first rib. The clavicle consists of cancellous bone surrounded by cortical bone (see Figs. 1.2 and 1.3). The cortical bone is thicker in the intermediate or shaft portion, and relatively thin at the acromial and sternal ends. The clavicle is unique in that, unlike most other long bones, it usually has no medullary cavity (5). This is related to its unique form of ossification, which consists of both endochondral and intramembranous ossification. The clavicle has specific differences in men and women and can be used to determine sex of a skeleton or specimen. The clavicle in general is shorter, thinner, less curved, and smoother in women than in men. Midshaft circumference of the clavicle is a reliable single indicator of sex, especially combined with the bone weight and length (11,12). In persons who perform heavy manual labor, the clavicle becomes thicker and more curved, and its ridges become more distinct for muscular attachment. For its descriptive osteology, the clavicle is discussed here from lateral to medial, beginning with the acromial portion and moving to the lateral one-third, medial two-thirds, and the sternal portion. Acromial Portion of the Clavicle The most laterally positioned part of the clavicle the acromial portion, which contains the articulation for the

acromion of the scapula and the associated attachments of the acromial clavicular ligaments. The acromial portion of the clavicle is somewhat flattened and is wider compared with the mid-portions. The superior surface is flat, with a rough ridge along the posterosuperior portion. The anterior surface of the acromial portion is concave and smooth, the posterior surface convex and smooth, and the inferior somewhat convex and rough. On the inferior surface, there are multiple small foramina for nutrient vessels. The articular surface is oval and directed obliquely and inferiorly. The rim of the articular margin is rough, especially superiorly, for attachment of the thick acromioclavicular ligaments. The acromial portion of the clavicle projects slightly superiorly to the acromion of the scapula. The acromioclavicular joint is palpable approximately 3 cm medial to the lateral border of the acromion. Lateral Third of the Clavicle The lateral third of the clavicle is wider and flatter than the more medial portion. This portion has distinct superior, inferior, anterior, and posterior surfaces. The superior and inferior surfaces are flat. The posterior surface is rounded, convex, and slightly thickened. The anterior surface is mildly concave, and becomes wider and rough in the most lateral portion as it approaches the acromion. The posterior and anterior portions have roughened areas for the attachment of the trapezius and deltoid muscles, respectively. On its inferior surface in the lateral third, there is the conoid

FIGURE 1.2. Right clavicle, superior surface, showing muscle origins (red) and insertions (blue).

1 Skeletal Anatomy

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FIGURE 1.3. Right clavicle, inferior surface, showing muscle origins (red) and insertions (blue).

tubercle for attachment of the conoid ligament (the medial portion of the coracoclavicular ligament). Lateral to the conoid tubercle is the trapezoid line, an oblique line on the undersurface for attachment of the trapezoid ligament (which is the lateral portion of the coracoclavicular ligament). Medial Two-Thirds of the Clavicle The medial two-thirds of the clavicle is more rounded than the sternal end or the lateral thirds, and becomes slightly wider from lateral to medial. Anteriorly, the surface is straight or curved with a mild convexity. Along this anterior surface is the large origin of the clavicular head of the pectoralis major. The posterior border of the clavicle in the medial twothirds is smooth and concave, and oriented toward the base of the neck. The posterior border widens as it approaches the sternum. Posteriorly and inferiorly, there is the small attachment area for the origin of the sternohyoid muscle, which extends into the sternal region. Also along the posterior border, on the superior margin, is the area of origin of the sternocleidomastoid muscle. On the posterior border of the inferior surface of the lateral two-thirds is a rough tubercle, the conoid tubercle, for attachment of the conoid ligament. From the conoid tubercle to the costal tuberosity (see later), there is a large attachment area for the insertion of the subclavius muscle. This surface also gives attachment to a layer of cervical fascia, which envelops the omohyoid muscle. In the medial portion of the medial two-thirds, the clavicle becomes slightly wider and thicker, especially when viewed from above or below. In this medial portion, the clavicle is rougher both anteriorly and posteriorly. On the inferior surface of the medial clavicle extending into the sternal portion is a delineated long roughened area, the costal tuberosity, which is approximately 2 cm in length. The costoclavicular ligament attaches in this area. The rest of the area is occupied by a groove, which gives attachment to the subclavius muscle. The clavipectoral fascia, which

splits to enclose the subclavius muscle, is attached to the margins of the groove. The brachial plexus is located deep to the mid-portion of the clavicle. The mid-portion of the clavicle is formed by the intersection of two curves of the bone, anteriorly convex on the lateral portion, and anteriorly concave in the medial portion. At the junction of these two curves, the clavicle overlies the divisions of the brachial plexus and the subclavian vessels. Sternal Portion of the Clavicle At the sternal end, the clavicle becomes wider at the midportion, but not in general as wide as the acromial end. The relative widths of the bone can be used for easy determination between the sternal and acromial ends. As the sternal end flares out, it becomes rough and more irregular. The sternal end usually is easily palpable. The sternal portion contains a sternal articular surface for the manubrium of the sternum. The sternoclavicular joint contains the articular disc. There is a triangular surface for articulation with the cartilage of the first rib in this area on the inferior surface of the clavicle. Surrounding the articular surfaces is a rim that is roughened for the attachments of the sternoclavicular and costoclavicular ligaments. The sternal end of the clavicle lies slightly above the level of the manubrium and hence usually is palpable. This area is covered by the sternal end of the sternocleidomastoid muscle. On the inferior surface of the sternal portion there is a rough, raised ridge, the costal tuberosity, which extends into the medial third of the clavicle (see earlier). The costoclavicular ligament attaches to the costal tuberosity. Associated Joints The clavicle articulates with the acromion of the scapula laterally (acromioclavicular joint), and with the manubrium of the sternum and cartilage of the first rib medially (sternoclavicular joint; Fig. 1.4).

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Systems Anatomy

FIGURE 1.4. Superior portion of anterior manubrium showing medial clavicles and sternoclavicular joints.

The acromioclavicular joint between the lateral end of the clavicle and the acromion of the scapula is stabilized by several structures: the acromioclavicular ligaments, coracoclavicular ligament, and joint capsule. The acromioclavicular ligament crosses the acromioclavicular joint, most developed on the superior portion of the joint. The ligament is oriented along the axis of the clavicle. It attaches to the roughened areas on the adjacent ends of the clavicle and acromion. The coracoclavicular ligament stabilizes the acromioclavicular joint by anchoring the clavicle to the coracoid of the scapula. It is more efficient in stabilizing the acromioclavicular joint than the acromioclavicular ligaments, even though it does not cross the joint. It consists of two parts: the trapezoid ligament (located laterally) and the conoid ligament (located medially). The trapezoid ligament, as its name implies, is quadrangular or trapezoid in shape. It is broad and thin, and crosses from the upper coracoid surface to the trapezoid line on the inferior surface of the clavicle. It follows an oblique or almost horizontal direction, ascending laterally as it crosses from the coracoid process to the clavicle above. The conoid ligament, located medial and slightly posterior to the trapezoid ligament, attaches from the root of the coracoid process in front of the scapular notch, and ascends superiorly to attach to the conoid tubercle of the undersurface of the lateral clavicle. It is a dense ligament, roughly triangular in shape. At the sternal articulation, the sternoclavicular joint is located at the superior portion of the manubrium. The first costal cartilage is located inferior to the sternoclavicular joint. The inferior surface of the medial end of the clavicle articulates with a small portion of the first costal cartilage.

The sternoclavicular articulation involves the medial end of the clavicle, which articulates with both the sternum (at the sternoclavicular or clavicular notch) as well at the adjacent superior surface of the first costal cartilage. An articular disc composed of fibrocartilage lies between the end of the clavicle and the sternum. The medial end of the clavicle is convex vertically but slightly concave anteroposteriorly, and therefore the shape often is described as “sellar” (pertaining to a saddle, saddle-shaped) (1,3). The articular disc of the sternoclavicular joint is flat and generally circular, attached superiorly to the superoposterior border of the clavicular articular surface (see Fig. 1.4.). The disc is centrally interposed between the articulating surfaces of the clavicle and sternum, and divides the joint into two cavities, each of which is lined with synovial membrane. The articular disc is thicker peripherally and in the superoposterior portion. The disc is attached inferiorly to the first costal cartilage near its sternal junction. In the remaining portion of the disc’s circumference, it is attached to the joint capsule of the sternoclavicular joint. Most of the motion at the sternoclavicular joint occurs between the articular disc and the clavicle, with less movement occurring between the articular disc and the sternum (5). The ligaments and soft tissues that stabilize the sternoclavicular joint include the joint capsule, the anterior sternoclavicular ligament, the posterior sternoclavicular ligament, the interclavicular ligament, and the costoclavicular ligament (4,5) (see Fig. 1.4). The joint capsule lies deep to the ligaments, and completely surrounds the articulation. The stability of the joint is shared by the joint capsule and the associated ligaments. The joint capsule varies in thickness and strength. The anterior and posterior portions usually are thicker and stronger, reinforced by the anterior and posterior sternoclavicular lig-

1 Skeletal Anatomy

aments. The joint capsule is reinforced by the interclavicular ligament superiorly. The inferior portion of the sternoclavicular joint capsule is thin, and resembles areolar tissue (4). The anterior sternoclavicular ligament is broad and covers the anterior portion of the sternoclavicular joint (see Fig. 1.4). It is attached superiorly to the upper and anterior portion of the medial end of the clavicle. The ligament passes obliquely downward and medial from the clavicle to the sternum. The ligament attaches to the superior part of the manubrium. The sternocleidomastoid muscle passes over the anterior sternoclavicular ligament. The joint capsule and articular disc lie posterior to the anterior sternoclavicular ligament. The posterior sternoclavicular ligament also is broad, similar to the anterior sternoclavicular ligament. The ligament spans the posterior portion of the sternoclavicular joint, attached to the superior portion of the medial end of the clavicle. It passes obliquely inferiorly and medially (similar to the anterior sternoclavicular ligament), to attach inferiorly to the dorsal portion of the superior manubrium. The articular disc and synovial membranes of the sternoclavicular joint lie anteriorly. The sternohyoid and the sternothyroid muscles lie posteriorly. The interclavicular ligament connects the medial ends of the two clavicles and is attached to the superior border of the manubrium. The ligament spans from one clavicle to the other, stretching along the superior border of the manubrium. It is of variable size between individuals and forms the floor of the jugular notch (see Fig. 1.4). Anterior to the interclavicular ligament is the sternocleidomastoid muscle. Dorsal to the ligament are the sternohyoids. The interclavicular ligament adds considerable strength to the superior portion of the sternoclavicular joint capsule. The costoclavicular ligament is located at the inferior border of the medial end of the clavicle, outside of and just lateral to the joint capsule (see Fig. 1.4). It helps stabilize the medial end of the clavicle to the superior portion of the medial part of the cartilage of the first rib. The ligament has an oblique orientation, extending medially and inferiorly from the inferomedial clavicle to reach the superior portion of the costal cartilage. The clavicle has a slight ridge on its inferomedial end, the costal tuberosity, to which the costoclavicular ligament attaches. Anterior to the costoclavicular ligament lies the tendon of the origin of the subclavius muscle. Posterior to the costoclavicular ligament is the subclavian vein. Muscle Origins and Insertions Muscle attachments to the clavicle include the trapezius, pectoralis major, deltoid, sternocleidomastoid, subclavius, and sternohyoid (see Figs. 1.2 and 1.3). The trapezius inserts onto the superolateral shaft. The clavicular head of the pectoralis major originates from the anteromedial por-

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tion of the shaft. The deltoid originates from the anterolateral portion of the shaft. The sternocleidomastoid muscle originates from the superomedial portion of the shaft. The subclavius inserts onto the inferior surface of the middle third of the shaft. The sternohyoid originates from the inferomedial surface (2,4,5). Clinical Correlations: Clavicle Relationship to the Brachial Plexus The mid-portion of the clavicle lies approximately over the divisions of the brachial plexus. The clavicle is an important bony landmark in planning incisions for supraclavicular or infraclavicular brachial plexus exploration. It is a useful landmark in the orientation and identification of structures in brachial plexus. Although rare, neurovascular compression of the brachial plexus can occur with clavicular fractures (13). Clavicle Shaft Fractures The clavicle is one of the most commonly fractured bones (14). Fractures most often occur at the junction of the lateral one-third and medial two-thirds, its weakest portion (5,15,15a). The distal portion usually is displaced inferiorly, in part because of the weight of the shoulder. The proximal portion is displaced little. Nonunion is rare, but usually occurs in the middle third (16). The clavicle commonly is injured because of its subcutaneous location. Neer Classification of Distal Clavicle Fractures n Type 1: A nondisplaced, nonarticular fracture of the distal clavicle, with the acromioclavicular joint and ligaments intact. n Type 2: A displaced fracture of the distal clavicle that is interligamentous (fracture extends between the conoid ligament medially and trapezoid ligament laterally). The conoid ligament is torn, the trapezoid ligament remains attached to the distal segment, and the medial segment is displaced superiorly (due to loss of the conoid ligament). The distal fragment remains aligned to the acromioclavicular joint (due to stabilization of intact trapezoid ligament). n Type 3: An intraarticular fracture of the distal clavicle that is lateral to the coracoclavicular ligaments. There is no displacement because the ligaments are intact (17–19). Acromioclavicular Separation Injury at the acromioclavicular joint (AC separation) has been classified by several descriptions. One of the most

8

Systems Anatomy

widely used classifications divides the injury into three types. Type I is a partial tear of the ligaments, involves no joint subluxation, and usually is treated symptomatically. There is minimal widening (if any) of the acromioclavicular joint space, which normally measures 0.3 to 0.8 cm. Type II involves a more extensive but incomplete tear, with partial subluxation seen radiographically. Widening of the acromioclavicular joint or bone surfaces can be 1 to 1.5 cm. There usually is an associated increase in the coracoclavicular distance by 25% to 50%. Treatment also is symptomatic, often with shoulder support with an immobilizing device. Type III is a complete disruption of the ligaments with dislocation of the clavicle from the acromion. There is marked widening of the acromioclavicular joint, usually greater than 1.5 cm. It often is treated surgically with internal fixation and repair or reconstruction of the ligaments (17). Recently, these injuries have been classified into six types (20–22). Types I, II, and III are similar to the traditional classification system. A type IV injury is rare, and involves posterior dislocation of the distal end of the clavicle. The clavicle is displaced into or through the trapezius muscle. Shoulder motion therefore usually is more painful than with the type III injury. The type V injury is an exaggeration of type III in which the distal end of the clavicle appears to be grossly displaced superiorly toward the base of the neck. The apparent upward displacement is the result of the downward displacement of the upper extremity. There is more extensive stripping of soft tissues of the clavicle and the patient usually has more pain than in the type III injury. The type VI injury involves a subcoracoid dislocation of the distal clavicle. There is an inferior dislocation of the distal clavicle (inferior to the coracoid process) and posterior to the biceps and coracobrachialis tendons. Because of the amount of trauma required to produce a subcoracoid dislocation of the clavicle, there may be associated fractures of the clavicle and upper ribs or injury to the upper roots of the brachial plexus. Management of types IV, V, and VI usually involves operative repair/reconstruction (20–22). Type III injuries have been further divided into additional variants, including those in children and adolescents involving a Salter type I or II fracture through the physis of the distal clavicle, or a complete separation of the acromioclavicular articular surfaces combined with a fracture of the coracoid process (22). Sternoclavicular Separation Sternoclavicular separation is rare compared with AC separation. Posterior dislocations may cause pressure on the great vessels or airway located posterior to the joint. Computed axial tomography is helpful in determining the direction of subluxation/dislocation. Reduction of the posterior dislocation is safest in the operating room in the presence of

a general or thoracic surgeon if damage has occurred or is discovered involving the vessels or airway. Posttraumatic Osteolysis of the Distal Clavicle After injury to the shoulder, such as a type I injury to the acromioclavicular joint, resorption of the distal end of the clavicle occasionally may occur. The osteolytic process, which is associated with mild to moderate pain, usually begins within 2 months after the injury. Initial radiographs show soft tissue swelling and periarticular osteoporosis. In its late stage, resorption of the distal end of the clavicle results in marked widening of the acromioclavicular joint (17). Cleidocranial Dysostosis Cleidocranial dysostosis is a partial or complete absence of the clavicle. It is associated with abnormal ossification of the skull bones (23). Patients with congenital absence of the clavicle have shown little or no limb dysfunction; however, after clavicular excision (for trauma or tumor), noted findings have included weakness, drooping of the shoulder, and loss of motion (15,19,24). Clavicular Dysostosis Clavicular dysostosis is a result of incomplete union of the two ossification centers of the clavicle (23). SCAPULA Derivation and Terminology The scapula derives its name from the Greek for “spade” (1,3). The plural of scapula is scapulae (1). Graves’ scapula indicates a scapula in which the vertebral border is concave. Scaphoid scapula indicates a scapula in which the vertebral border is concave (same as Graves’ scapula). Winged scapula indicates a scapula that is positioned with the vertebral border prominent (1). Ossification Centers and Accessory Bones The scapula has seven to eight ossification centers: one for the body, two for the coracoid process, two for the acromion, one for the medial (vertebral) border, and one for the inferior angle (Fig. 1.5). Additional centers may be present to help form the inferior and superior portions of the glenoid cavity (4,5). The body begins to ossify at approximately the second month of fetal life, forming an irregular quadrilateral plate of bone near the scapular neck, adjacent to the glenoid cavity. The plate extends to form the major part of the scapula.

1 Skeletal Anatomy

9

FIGURE 1.5. Illustration of right scapula showing several centers of ossification. The scapula may have seven to eight (or more) ossification centers: one for the body, two for the coracoid process, two for the acromion, one for the medial (vertebral) border, and one for the inferior angle. Additional centers may be present to help form the inferior and superior portions of the glenoid cavity.

The spine extends up from the dorsal surface of this plate approximately the third month of fetal life. At birth, the major part of the scapula is osseous. The glenoid cavity, coracoid process, the acromion, and the vertebral border and inferior angle remain cartilaginous at birth. An ossification center appears in the middle of the coracoid process during the first year after birth. This ossification center joins the rest of the scapula at approximately the fifteenth year. Between the fourteenth and twentieth years, ossification of the remaining parts of the scapula takes place in quick succession. Ossification of these parts occurs in the following order: the base of the coracoid process, the base of the acromion, the ossification centers in the inferior angle and adjacent part of the medial border, the tip or lateral portion of the acromion, and the remainder of the medial border (2,4,5). The base of the acromion is formed from three or four ossification centers. It is partially formed by an extension from the spine of the scapula (from the ossification center of the body), and partially from the two centers of the acromion (which previously have united to each other). The tip of the coracoid process may develop a separate ossifica-

tion center. These various centers join the body by the twenty-fifth year. Persistence of an ossification center of the acromion that does not fuse with the others or with the scapula can present as an accessory bone, the os acromiale. An os acromiale usually is located at the lateral margin of the acromion, is of variable size and shape, and usually is bilateral (25). It also is possible for the os acromiale to exist as a small accessory ossicle directly above the greater tuberosity of the humerus. This ossicle is separated from the acromion by approximately 1 cm, and usually is somewhat circular in shape. The superior third of the glenoid cavity may be ossified from a separate center, or may ossify from an extension of the center at the base of the coracoid. When ossification is from a separate center, the center usually ossifies between the tenth and eleventh years. This superior portion of the glenoid then joins the rest of the scapula between the sixteenth and eighteenth years. An epiphyseal plate or crescentic epiphysis also may appear for the lower part of the glenoid cavity, which is thicker peripherally. This rim converts the flat cavity into the gently concave fossa that is present in the adult glenoid (2,4,5).

10

Systems Anatomy

Osteology of the Scapula The scapula is a large, flat, triangular bone that spans the dorsal aspect of the second through seventh ribs (Figs. 1.6 to 1.8). Its synovial articulations include those with the humerus and the clavicle. In addition, the scapula is stabilized to the dorsal surface of the thorax by muscle, forming the scapulothoracic articulation. The main processes (acromion, coracoid, and subchondral portions of the glenoid) as well as the thicker portions of the body contain trabecular bone (see Figs. 1.6 to 1.8). The remaining portions generally consist of thin cortical bone. The central portions of the supraspinous fossa and most of the infraspinous fossa consist of thin cortical bone. Occasionally the bone is so thin that it may appear translu-

cent or may have areas that are incompletely ossified, being filled with connective tissue. Osteology measurements are given in Figure 1.9 and Table 1.1. The mean length of the scapulae from the superior angle to the inferior angle is 15.5 cm. The width of the scapula from the medial border to either the superior or inferior rim of the glenoid is approximately 10.6 cm. The scapula is significantly larger in men than women (26) (Table 1.1). For descriptive osteology, the scapula has two surfaces, the costal (anterior) and the dorsal (posterior). It contains the process of the acromion, the coracoid, and the spine. It has three borders: superior, medial (or vertebral), and lateral (or axillary). It has three angles: inferior, superior, and lateral (26,27).

FIGURE 1.6. Right scapula, anterior surface, showing muscle origins (red) and insertions (blue).

1 Skeletal Anatomy

11

FIGURE 1.7. Right scapula, posterior surface, showing muscle origins (red) and insertions (blue).

Surfaces of the Scapula The costal surface forms the large subscapular fossa, a slightly concave surface for the origin of the subscapularis (see Fig. 1.6). The medial two-thirds of the subscapular fossa is roughened, with ridges that course laterally and superiorly. These ridges give origin to tendinous attachments of the subscapularis. Along the medial border of the costal surface is a long, thin rim that provides the insertion of the serratus anterior. The dorsal surface is slightly convex from superior to inferior. It contains the two fossae for the supraspinatus and infraspinatus, separated by the prominent spine of the scapula. The supraspinatus fossa, which is much smaller than the infraspinatus, is smooth, concave, and broader at its medial aspect than its lateral border. It is bordered by the spine inferiorly, the coracoid process laterally, and the superior and medial rim of the scapula superiorly and medially, respectively. The supraspinatus muscle originates from the medial two-thirds of the fossa (see Fig. 1.7). The infraspinatus fossa is approximately three times larger than the supraspinatus fossa. It has a slight concavity

superiorly to inferiorly, especially along the medial border. There is a slight convexity throughout its central portion, and a deep groove near the axillary border. The attachments of the infraspinatus are located on the lateral third of the fossa (see Fig. 1.7). There is a slight bony ridge that runs along the lateral border of the dorsal surface of the scapula. The ridge runs from the lower part of the glenoid cavity, downward and backward to the medial border, to an area approximately 2 to 3 cm superior to the tip of the inferior angle. This ridge serves for the attachment of a fibrous septum that separates the infraspinatus from the teres major and teres minor. The surface between the ridge and the lateral border is narrow in the superior two-thirds. In this area, the ridge is crossed near its center by a groove that contains the circumflex scapular vessels. This ridge provides attachment for the teres minor superiorly and for the teres major inferiorly. The area of origin of the teres major is broader and somewhat triangular. The latissimus dorsi muscle glides over the lower region, and frequently a few muscle fibers arise at the inferior angle of the scapula. The teres muscles are separated from each other by a fibrous septum that extends along an

12

Systems Anatomy

FIGURE 1.8. Right scapula, lateral view, showing glenoid cavity and profile of coracoid process, acromion, and body.

oblique line from the lateral border of the scapula to an elevated ridge (2,4,5). Processes of the Scapula The scapula has three main processes: the acromion, the coracoid process, and the spine of the scapula (see Figs. 1.6 to 1.9). The acromion is a lateral extension of the spine. The process becomes flattened as it extends laterally, overhanging the glenoid, and forms the most superior portion or “summit” of the scapula (see Fig. 1.9A–E). The shape is variable, with a flat configuration in 23%, curved in 63%, and hook-shaped in 14% (26). The mean length of the acromion in the anteroposterior plane is 4.8 cm. The mean width of the acromion in the mediolateral plane is 2.19 cm, and the mean thickness is 9.4 mm. The narrowest portion forms a neck, the diameter of which is 1.35 cm (26) (Table 1.1). The acromion is located an average distance of 16 mm from the glenoid (26). The superior surface is rough and

convex and provides attachment for the thick acromioclavicular ligaments and a portion of the deltoid muscle. The remaining portions are subcutaneous and smooth. The inferior surface of the acromion is smooth and concave. The lateral border is thick and irregular and usually has three or four tubercles for the tendinous origins of the deltoid muscle. The medial border is shorter than the lateral and concave. In this area, the acromion provides a portion of the attachment of the trapezius muscle. On this medial border, there is a small oval area of articular cartilage for articulation with the acromial end of the clavicle. The apex of the acromion is a small area where the medial and lateral borders intersect. In this area, the coracoacromial ligaments form their attachment. Inferiorly, where the lateral border of the acromion becomes continuous with the lower border of the crest of the spine, the acromial angle is located. The acromial angle can be palpated subcutaneously and used as a landmark. The coracoid process is a thick, curved projection of bone that projects anteriorly, superiorly, and medially from

B

A

C

D FIGURE 1.9. A: Anterior view of the right scapula showing the standard terminology of the anatomic regions. B: Posterior view of the right scapula showing terminology and general measurements. The measurements include [1] the maximum length of the scapula; [2] the width of the scapula measured to the posterior rim of the glenoid; [3] the width of the scapula measured to the anterior rim of the glenoid (also shown in Fig. 1-9C); [4] the inferior scapular angle; [5] the anteroposterior thickness of the medial border of the scapula measured halfway along the medial edge of the scapula and 1 cm from the edge; and [6] the distance from the superior rim of the glenoid to the base of the suprascapular notch. The measurement values are shown in Table 1.1. C: The right scapula (superior view as shown in the inset) showing the measurement of the spine. The measurements include [7] the length of the scapular spine measured from the medial edge of the scapula where it meets with the scapular spine to the lateral edge of the acromion; [8] the distance from the medial edge of the scapula where it meets with the scapular spine to the edge of the spinoglenoid notch; [9] the anteroposterior width of the spine measured 1 cm from the medial edge of the scapula; [10] the anteroposterior width of the spine measured 4 cm from the medial edge of the scapula; [11] the anteroposterior width of the spine at the lateral edge (spinoglenoid notch); and [12] the anteroposterior thickness of the acromial neck at its thinnest diameter. Also shown is measurement [3], which is the width of the scapula measured on the anterior surface. The measurement values are shown in Table 1.1. D: Scapular measurements of the length [13], width [14], and thickness [15] of the acromion, and the coracoacromial distance [16], as seen from the superior view of the right scapula. The measurement values are shown in Table 1.1. (continued on next page)

14

Systems Anatomy

F

E

G

FIGURE 1.9. (continued) E: Lateral view of the right scapula, showing the coracoacromial distance [16], the minimal distance between coracoid and acromion [17], and the dimensions of the glenoid fossa [18–20]. The measurement values are shown in Table 1.1. F: Measurements of the thickness of the scapular head [21,22] and glenoid tilt angle [23] as seen from the inferior view of the right scapula. The measurement values are shown in Table 1.1. G: Dimensions of the coracoid process of the right scapula as seen from the anterior view. Measurements include the length of the coracoid from the tip of the coracoid to the point at which the coracoid angulates inferiorly [24]; the coracoid thickness measured in the superoinferior direction 1 cm from the tip of the coracoid [25]; and the distance from the tip of the coracoid to the base of the suprascapular notch [26]. The measurement values are shown in Table 1.1. (From Von Schroeder HP, Kuiper SD, Botte MJ. Osseous anatomy of the scapula. Clin Orthop 383:131–139, 2001.)

155.0 106.0 106.9 36.1 3.8 31.8 133.6 85.5 7.3 17.9 46.1 13.5 48.0 21.9 9.4 27.1 15.5 28.6 26.0 36.4 22.0 12.9 7.9 45.3 10.6 50.7

1-9C 1-9C 1-9C 1-9C 1-9C 1-9C 1-9D 1-9D 1-9D 1-9D,E 1-9E 1-9E 1-9E 1-9E 1-9F 1-9F 1-9F 1-9G 1-9G 1-9G

Average

1-9B 1-9B 1-9B,C 1-9B 1-9B 1-9B

Figure

4.7 1.2 4.8

1.8 3.3 2.9 3.6 3.5 3.0 3.7

2.2 5.1 3.7 1.1 4.5

11.8 8.7 1.2 3.2 6.3

16.0 8.5 9.7 2.5 0.7 2.9

SD

All

35 8 40

13 25 22 30 17 8 0

10 38 15 8 22

113 71 6 11 38

127 92 89 30 3 28

Min

54 12 58

19 34 32 43 30 18 17

18 57 27 12 39

153 101 10 26 59

179 122 126 42 5 39

Max

42.3 9.8 47.7

14.9 25.8 23.6 33.6 19.4 11.0 8.1

12.1 43.6 20.4 8.7 24.6

124.8 78.5 7.3 17.0 41.2

140.8 99.0 98.9 34.8 3.6 30.6

Average

3.0 1.3 3.0

1.8 0.9 0.9 1.7 2.2 1.8 3.6

1.0 3.6 2.2 0.8 2.5

5.9 5.4 1.4 3.1 2.2

11.9 3.4 4.5 2.1 0.7 2.3

SD

36 8 40

13 25 22 30 17 8 0

10 38 17 8 22

115 71 6 11 38

127 92 89 30 3 28

Min

Female

47 12 52

19 27 25 36 24 14 14

14 51 23 10 29

136 88 10 21 44

160 103 106 38 5 34

Max

48.3 11.4 54.0

16.1 30.9 27.8 38.0 24.7 14.5 8.0

14.2 50.9 22.6 9.8 28.7

140.9 91.1 7.2 18.3 50.9

166.4 112.3 113.4 36.4 3.9 32.6

Average

3.4 0.8 3.6

1.5 3.1 3.0 3.3 2.8 3.2 3.0

1.7 3.5 4.6 0.9 5.2

10.0 6.6 1.3 3.8 4.9

11.4 5.7 7.8 1.6 0.7 2.4

SD

Male

aNumbers correspond to those used in figures; all measurements are in millimeters except 4 and 23, which are in degrees. AP, anteroposterior; Max, maximum; Min, minimum; med, medial; ML, mediolateral; NS, not significant; Post, posterior; SI, superoinferior; SD, standard deviation. bp < .05. From von Schroeder HP, Kuiper SD, Botte MJ. Osseous anatomy of the scapula. Clin Orthop 383:131–139, 2001.

General measurements 1 Length of scapula 2 Post. glenoid–med. scapula distance 3 Ant. glenoid–med. scapula distance 4 Inferior angle (degrees) 5 Thickness of medial edge 6 Superior glenoid to notch Scapular spine 7 Length of spine 8 Length of base of spine 9 Spine thickness at 1 cm 10 Spine thickness at 4 cm 11 Spine thickness laterally Acromion 12 Acromial neck diameter 13 AP length of acromion 14 ML width of acromion 15 Thickness of acromion 16 Coracoacromial distance Glenoid and head of scapula 17 Superior glenoid-acromial distance 18 AP diameter of glenoid 19 Diameter of glenoid to notch 20 SI length of glenoid 21 Thickness of head at 1 cm 22 Thickness of head at 2 cm 23 Glenoid tilt angle (degrees) Coracoid process 24 Length of coracoid 25 Thickness of coracoid 26 Distance from coracoid to notch

Measurementa

TABLE 1.1. MEASUREMENTS OF THE SCAPULA

41 10 48

14 25 23 32 21 8 3

10 44 15 8 24

123 78 6 14 41

143 101 99 34 3 29

Min

53 12 58

19 34 32 42 30 18 13

17 57 27 11 39

153 101 10 26 59

179 122 126 39 5 38

Max

b

b

b

NS

b

b

b

b

b

NS

b

b

NS

b

b

b

NS NS

b

b

b

NS

b

b

b

b

Sex Difference

16

Systems Anatomy

the upper portion of the neck of the scapula (see Figs. 1.6–1.8, and 1.9A, D, E, G; Table 1.1). It is located approximately 5.07 cm from the notch of the scapula (26). The coracoid measures approximately 4.53 cm long and 1.06 cm thick. The base is broad and the anterior portion projects anteriorly. The coracoid process has a concave surface that faces laterally. It is smooth to accommodate the gliding of the subscapularis, which passes just inferior to it. The distal portion curves upward to angle more horizontally, and its outer surface is rough and irregular for attachment of the pectoralis minor. The pectoralis minor insertion is along the anterior rim; the coracobrachialis and short head of the biceps originate more laterally toward the tip. The clavipectoral fascia also attaches to the apex. The attachments of the trapezoid and conoid ligaments are located just medial to the pectoralis minor insertion. The coracoid is roughened along this rim for the ligament and muscle attachments. The coracoid process usually is palpable through the anterior deltoid, and can be used as a valuable bony landmark The spine of the scapula spans from the medial border (at the junction of the upper and middle thirds of the medial border) of the scapula to the acromion (see Figs. 1.6, 1.7, and 1.9A–C). The length of the spine from the medial edge to the lateral edge of the acromion is approximately 13.3 cm, with the length of the base 8.5 cm. The anteroposterior width of the spine at 1 and 4 cm from the medial edge is 7 mm and 18 mm, respectively (26) (Table 1.1). The upper and lower borders are rough to accommodate muscular attachments. The dorsal border forms the crest of the spine. The crest of the spine is subcutaneous and easily palpable. Borders of the Scapula The scapula has three borders: superior, medial, and lateral (see Figs. 1.6, 1.7, and 1.9A). The superior border is the shortest and the bone here is the thinnest. The edge can be somewhat sharp. The shape of the border is concave, extending from the medial angle to the base of the coracoid process. The scapular notch is a semicircular groove in the rim of the superior border. It is located at the lateral part of the superior border, with its base approximately 3.2 cm from the superior rim of the glenoid (26). It is formed partly by the base of the coracoid process. The superior rim of the suprascapular notch is crossed by the superior transverse ligament. The ligament may be ossified. The suprascapular notch has been shown to exist as an osseous foramen in approximately 13% of specimens (26). The suprascapular nerve passes through the suprascapular notch, which is transformed into a foramen by the ligament. This is a potential area of suprascapular nerve entrapment. The suprascapular artery passes dorsal to the ligament, and does not enter the notch (28). The portion of the superior border adjacent to the notch also provides attachment for the omohyoid muscle.

The lateral border begins at or above the inferior margin of the glenoid cavity (see Figs. 1.6, 1.7, and 1.9A). It inclines obliquely downward and medially to the inferior angle. Below the glenoid cavity, there is a roughed area, the infraglenoid tubercle, which is approximately 2.5 cm long. This area gives origin to the long head of the triceps brachii muscle. The inferior third of the lateral border is thin and sharp, and provides attachment of a portion of the teres major posteriorly. The subscapularis originates anteriorly on a portion of its anterior surface. The medial (vertebral) border is the longest of the three borders of the scapula (see Figs. 1.6, 1.7, and 1.9A). It extends from the superior angle to the inferior angle. The border is slightly arched with a posterior convexity. This border is intermediate in thickness between the superior and lateral borders, measuring approximately 4 mm thick at 1 cm from the edge (26). The portion superior to the spine forms an obtuse angle of approximately 145 degrees with the portion inferior to the spine. The border has an anterior and posterior lip, with an intermediate narrow area. The anterior lip provides attachment for the serratus anterior muscle. The posterior lip provides attachment for the supraspinatus muscle above the spine and the infraspinatus below the spine. The narrow area between the two lips provides insertion for the levator scapulae muscle above the triangular area, which marks the beginning of the spine. The insertion of the levator scapulae may extend along the major portion of the dorsal rim of the medial border superior to the spine (5,28). The rhomboid minor muscle inserts on this edge inferior to the levator scapulae at the level of the spine. The rhomboid major inserts on the rim just inferior to the attachment of the rhomboid minor (and inferior to the spine). The insertion of the rhomboid major may extend along the major portion of the dorsal rim of the medial border inferior to the spine (5,28). At the level of the spine, the rhomboid minor also inserts into a fibrous arch that attaches to the base of the spine. Angles of the Scapula The scapula has three angles: the superior, inferior, and lateral angles (see Figs. 1.9A, 1.16, 1.17, 1.19A). The superior angle is formed by the junction of the superior and medial (vertebral) borders. This region is thin, smooth, and rounded, and gives attachment for a portion of the levator scapulae muscle. It measures approximately 80 degrees. The inferior angle is formed by the junction of the medial (vertebral) border and the lateral (axillary) borders. It measures approximately 25 degrees. The inferior angle, in contrast to the superior angle, is thick and rough. The dorsal surface provides attachment for the teres major and, in some individuals, a few fibers of the latissimus dorsi (see Fig. 1.17). The lateral angle is the thickest part of the bone, and the adjacent broadened portion of the bone sometimes is

1 Skeletal Anatomy

referred to as the head of the scapula. It measures approximately 90 degrees. The broadened area is connected to the rest of the scapula by a slightly constricted neck. This area of the scapula forms part of the shoulder joint. The most lateral portion becomes the glenoid, an oval, slightly concave surface. The surface of the glenoid is relatively shallow. The mean size of the glenoid is 2.9 cm in anteroposterior width by 3.6 cm in superoinferior length (26) (Table 1.1). It faces posteriorly by approximately 8 degrees (26). Its laterally facing articular surface is deepened and broadened by the glenoid labrum, which is a circumferential rim of fibrocartilage. The glenoid labrum plays an important role in stabilizing the shoulder. Superior to the glenoid, near the base of the coracoid process, there is a slight elevation, the supraglenoid tubercle, which provides the origin of the long head of the biceps brachii. Associated Joints The scapula articulates with the acromial end of the clavicle at the acromioclavicular articulation (see earlier, under Clavicle), and articulates with the proximal humerus at the glenoid articulation. The scapula slides and rotates on the thorax, stabilized by muscular attachments, and forms the soft tissue scapulothoracic articulation. Muscle Origins and Insertions Muscle attachments include the trapezius, deltoid (deltoideus), supraspinatus, infraspinatus, levator scapulae, minor and major rhomboids (rhomboideus), serratus anterior, teres major, teres minor, subscapularis, triceps, long and short heads of the biceps, coracobrachialis, pectoralis minor, and the omohyoid (see Figs. 1.6 and 1.7). The costal (anterior) surface provides the origin for the subscapularis. The dorsal (posterior) surface provides the origins for the supraspinatus (from the supraspinatus fossa) and infraspinatus (from the infraspinatus fossa). The spine contains part of the insertion of the trapezius as well as a portion of the origin of the deltoid. The dorsal portion of the acromion contains additional areas for the origin of the deltoid. The coracoid process contains the origins of the coracobrachialis and the short head of the biceps as well as the insertion of the pectoralis minor. The dorsal rim of the medial (vertebral) border receives the insertions of the levator scapulae and the minor and major rhomboid muscles. The levator scapulae insertion is located superior to the level of the spine, the rhomboideus minor insertion is located at the level of the spine, and the superior rhomboideus major insertion is located inferior to the level of the spine. The serratus anterior inserts along the anterior (costal) surface of the medial border. The teres minor and the teres major originate along the dorsal rim of the lateral border. The teres minor origin lies superior to the teres major. The origin of the long head of the triceps is located

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inferior to the glenoid. The long head of the biceps originates superior to the glenoid. The omohyoid inserts on the upper rim of body, superior to the supraspinatus fossa. Clinical Correlations: Scapula Failure of Bony Union Congenital failure of bony union between the acromion and spine may occur. The junction may be stabilized by fibrous tissue or may exist as a defect in the scapula. This may be mistaken for a fracture of the acromion, when in reality it represents a chronic fibrous union. Os Acromiale The base of the acromion is formed from three or four ossification centers. Persistence of one of the individual ossification centers of the acromion that does not fuse with the others or with the scapula can present as an accessory bone, the os acromiale. The os acromiale can be mistaken for a fracture of the acromion or humerus, or can resemble calcific tendinitis of the supraspinatus tendon. The os acromiale usually can be detected because it usually is located at the lateral margin of the acromion; it is of variable size and shape but usually is rounded and bilateral (25). It may exist as a small accessory ossicle directly above the greater tuberosity of the humerus, separated from the acromion by approximately 1 cm, and usually is somewhat circular (25). The Acromion as a Bony Landmark The lateral border of the acromion usually is palpable. It allows orientation for operative procedures in the vicinity of the subdeltoid bursa or rotator cuff. The Acromion’s Role in Impingement Syndrome Impingement of the rotator cuff usually involves thickening of the acromion. The portion that usually is most thickened or responsible for impingement is the anterior portion, which often develops an exostosis or large osteophyte. The Coracoid Process as a Bony Landmark With the arm by the side, the tip of the coracoid process is oriented anteriorly. It can be palpated by applying deep pressure through the anterior portion of the deltoid muscle approximately 2.5 cm below the lateral part of the clavicle on the lateral side of the infraclavicular fossa. Because muscles (pectoralis minor, short head biceps, coracobrachialis) and ligaments (coracoclavicular and coracoacromial ligaments) attach to the coracoid process, and because of the close vicinity of the musculocutaneous

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nerve, the coracoid is a valuable palpable landmark for orientation in terms of these structures. The coracoid process also serves as a valuable landmark for operative approaches to the glenohumeral joint and the brachial plexus. In addition, the base of the coracoid process forms a portion of the suprascapular notch. It can be a potential aid in the localization of the suprascapular nerve and suprascapular notch. Suprascapular Nerve Entrapment The suprascapular notch is converted to a foramen by the attachment of the superior transverse ligament, which crosses across the upper open end of the notch (29,30). The ligament may be ossified. [The suprascapular notch has been shown to exist as an osseous foramen in approximately 13% of specimens (26).] The suprascapular nerve passes through the notch, and is susceptible to nerve compression in this area. This condition occasionally is seen in patients with inflammatory conditions or in young, active athletes and is characterized by localized pain or atrophy of the supraspinatus and infraspinatus. Treatment includes conservative management (antiinflammatory medications, possible cortisone injections, and activity modification). If it is refractory to medical treatment or if localized atrophy is present, operative nerve decompression usually is warranted. Winging of the Scapula Winging of the scapula is a deformity in which the scapula angles up from the thorax (scapula alta), usually due to muscular imbalance. It often is caused by neuropathy of the long thoracic nerve and weakness of the serratus anterior, or by neuropathy of the spinal accessory nerve with weakness of the trapezius muscle (30,31). HUMERUS

In the proximal end of the humerus, ossification begins in the head of the bone during the first year (or earlier in some individuals). The center for the greater tuberosity begins to ossify during the third year, and the center for the lesser tuberosity begins to ossify during the fifth year. The centers for the head and tuberosities usually join by the sixth year, forming a single large epiphysis that fuses with the body in approximately the twentieth year (see Fig. 1.10B). In the distal end of the humerus, ossification begins in the capitulum near the end of the second year and extends medially to form the major part of the articular end of the bone. The center for the medial part of the trochlea appears at approximately 10 years of age. The medial epicondyle begins to ossify at approximately the fifth year, and the lateral epicondyle at approximately the twelfth or thirteenth year. The lateral epicondyle and both portions of the articulating surface (having already joined together) unite with the body. At approximately the eighteenth year, the medial epicondyle is joined to the body of the humerus. Osteology of the Humerus The humerus is the largest bone in the upper extremity. Each end of the humerus is composed of cancellous bone covered by thin cortical bone. The diaphysis consists of thick cortical bone throughout its length, with a well defined medullary canal. The medullary canal extends the entire length of the humerus. At the proximal and distal metaphyses, the medullary canal changes to cancellous bone, and the outer cortex becomes thinner (Figs 1.11 to 1.13). For descriptive osteology, the humerus can be described in terms of the proximal end, the shaft (diaphysis or body), and the distal end (Fig. 1.14; see Figs 1.11 to 1.13). The proximal end consists of the head, anatomic neck, surgical neck, and the greater and lesser tuberosities. The distal end includes the capitulum, trochlea, and medial and lateral condyles and epicondyles.

Derivation and Terminology Humerus is derived from the Latin humer, meaning “shoulder” (3). The plural of humerus is humeri. Ossification Centers The humerus has eight ossification centers: one each for the body, the head, the greater tuberosity, the lesser tuberosity, the capitulum, and the trochlea, and one for each epicondyle (Fig. 1.10). The ossification center for the body appears near the central portion of the bone at approximately the eighth week of fetal life. Ossification soon extends to either end of the bone, so that at birth the humerus is nearly completely ossified, with only the ends remaining cartilaginous.

Proximal End of the Humerus The head of the humerus forms nearly half of a sphere (see Figs. 1.11 to 1.13). With the arm at the side of the body, the humeral head is directed medially, superiorly, and slightly posteriorly, thus facing the glenohumeral joint. The entire smooth area, covered by hyaline cartilage, articulates with the glenoid of the scapula. The anatomic neck of the humerus denotes an obliquely oriented margin or circumference line that extends along and inferior to the articular portion of the head (see Figs. 1.11 and 1.12). In this area there is a groove that encircles the articular portion. The groove is well delineated along the inferior half. In the superior portion, the groove narrows to separate the head from the greater and lesser

1 Skeletal Anatomy

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FIGURE 1.10. A: Illustration of humerus showing centers of ossification. There are eight ossification centers: one each for the shaft, the head, the greater tuberosity, the lesser tuberosity, the capitulum, and the trochlea, and one for each epicondyle. B: Schematic illustration of proximal and distal humerus in a young adult showing epiphyseal lines. The dark lines indicate the attachment of the articular capsule.

A

B

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FIGURE 1.11. Right humerus, posterior aspect.

tuberosities. The circumference of the anatomic neck provides attachment for the articular capsule of the shoulder joint. In this area, there are numerous foramina for nutrient vessels (4). The surgical neck is located distal to the anatomic neck (see Figs. 1.11 and 1.12). It is the area of the junction of the shaft with the proximal end of the humerus, just distal to the head and tuberosities. As opposed to the anatomic neck, there is no groove that delineates the surgical neck. Its name derives from the common occurrence of fractures in this area, many of which are managed by operative methods. The greater tuberosity is located lateral to the head and lateral to the lesser tuberosity (see Figs. 1.11 to 1.13). The greater tuberosity often is referred to as the greater tubercle in anatomy textbooks (4,5). The upper surface is rounded and contains three flat impressions for muscle insertion. The superiormost portion of the greater tuberosity provides insertion for the supraspinatus. The middle impression is for the infraspinatus, and the inferiormost impression for the teres minor. The insertion site for the teres minor lies approximately 2.5 cm distal to the insertion of the supraspinatus, and a portion of the teres minor inserts onto the shaft. The lateral surface of the greater tuberosity is rough and convex. It merges distally into the lateral surface of the shaft of the humerus. The lesser tuberosity is smaller but more prominent than the greater tuberosity (see Figs. 1.12 to 1.13). The lesser tuberosity often is referred to as the lesser tubercle in anatomy textbooks (4,5). It is located anteriorly, adjacent to the anatomic neck. The anterior surfaced of the lesser tuberosity provides the major points of insertion of the subscapularis. The greater and lesser tuberosities are separated from each other by a deep groove, the bicipital groove (intertubercular groove, intertubercular sulcus; see Figs. 1.12 and 1.13A). The tendon of the long head of the biceps brachii muscle coursers along and within this groove, along with a branch of the anterior humeral circumflex artery, which travels superiorly to supply a portion of the shoulder joint. The bicipital groove courses obliquely downward and ends in the proximal third of the humeral shaft. The upper portion of the bicipital groove is lined by a thin layer of cartilage and covered by an extension of the synovial membrane of the shoulder. The lower portion of the groove becomes progressively shallow and provides the insertion of the latissimus dorsi. On either side of the bicipital groove there is a crest of bone. These are the crests of the greater and lesser tuberosities, also known as the bicipital ridges. Distal to the greater and lesser tuberosities, the circumference of the bone narrows to where the shaft joins the proximal portion. This is the surgical neck of the humerus. In the distal portion of the bicipital groove, the latissimus dorsi inserts just medial to the groove. The pectoralis major tendon inserts just lateral to the groove, slightly distal to the insertion of the latissimus dorsi.

1 Skeletal Anatomy

FIGURE 1.12. Right humerus, anterior aspect.

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22

A

B FIGURE 1.13. A: Right humerus, anterior aspect, showing muscle origins and insertions. B: Right humerus, posterior aspect, showing muscle origins (red) and insertions (blue).

1 Skeletal Anatomy

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FIGURE 1.14. Distal humerus, inferior surface, showing articular surface and contours of trochlea and capitulum.

Shaft of the Humerus The shaft of the humerus, also anatomically referred to as the body, spans the portion of the humerus from the surgical neck proximally and to the area just proximal to the portion referred to as the distal extremity (see Figs. 1.11 to 1.13). (The distal extremity includes the condyles, capitulum, and trochlea, as discussed later.) The shaft of the humerus is cylindrical in the proximal portion, but becomes progressively flatter and somewhat triangular distally. In the distal portion of the shaft, the bone actually has three surfaces, but two borders (the medial and lateral borders). The surfaces of the shaft of the humerus consist of an anterolateral surface, an anteromedial surface, and a posterior (or dorsal) surface. The anterior surface is divided into the anterolateral and anteromedial surfaces by an oblique ridge that starts proximally and laterally at the greater tuberosity and extends distally to end near the medial epicondyle. The anterolateral surface of the proximal humeral shaft provides the elongated insertion area of the pectoralis major muscle, which attaches along the distal part of the crest of the greater tuberosity (see earlier, under The Proximal End of the Humerus). Lateral and distal to the insertion of the pectoralis major is an oblong area that provides the insertion point of the deltoid muscle. This area, known as the deltoid tuberosity, is located on the anterolateral surface of the humerus and consists of a raised, slightly triangular elevation. Distal and anterior to the deltoid tuberosity, extending along the anterolateral surface of the humeral shaft, there is a relatively large, broad, slightly concave area that provides the origin area for the brachialis. Also distal to the deltoid tuberosity is the radial sulcus (radial groove), which extends obliquely distally, spiraling along the lateral shaft, and provides the path for the radial nerve and profunda brachii artery (see Figs. 1.11 and 1.13B). The radial sulcus is bordered on one side by the origin of the lateral head of the triceps, the deltoid tuberosity, and the origin of the brachialis (all located lateral and proximal to the groove). On the other side of the radial sulcus is the origin of the

medial head of the triceps, located medial and distal to the sulcus. The anteromedial surface of the humeral shaft contains a portion of the bicipital groove proximally. The tendon of the latissimus dorsi inserts into or along the medial crest of the intertubercular groove in the area just distal to that traversed by the bicipital tendon. Distal and medial to this area near the medial border, is the insertion area of the teres major. In the midportion of the anteromedial shaft, near the medial border of the humerus is the insertion area of the coracobrachialis. In the distal portion of the anteromedial surface of the humerus, the bone is flat and smooth, and provides for the large origin of the brachialis muscle. The dorsal surface of the humerus slightly rotates from proximal to distal, so that the proximal portion is directed slightly medially, and the distal portion is directed posteriorly and slightly laterally. The surface of the posterior surface of the humerus is nearly completely covered by the lateral and medial heads of the triceps brachii. The lateral head arises from the proximal portion, on the lateral half of the bone, just lateral to the radial sulcus. The origin of the medial head of the triceps begins on the proximal third of the posterior surface of the humerus, along the medial border of the bone and the medial distal border of the radial sulcus. This large origin area extends the length of the posterior humerus, covering the major portion of the posterior half of the humerus. The triceps origin extends distally to end as far as distal as the posterior portion of the lateral epicondyle, just proximal to the origin of the anconeus muscle. The medial and lateral borders run the entire length of the humerus. The medial border of the humerus extends from the lesser tuberosity to the medial epicondyle. The proximal third of the medial border consists of a prominent crest, the crest of the lesser tuberosity. The crest of the lesser tuberosity provides the insertion area of the tendon of the teres major. More distally, in the mid-portion of the shaft and located on the medial border is a rough impression for the insertion of the coracobrachialis. Distal to this area is the entrance of the nutrient canal into the humerus. A second nutrient canal may exist at the starting point of the

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radial sulcus. The anterior portion of the distal third provides the origin area for the brachialis muscle (see above under shaft of the humerus). The posterior portion of the distal third and medial border of the medial and distal thirds of the shaft provide the wide origin area of the medial head of the triceps. The distal third of the medial border is raised into a ridge, the medial supracondylar ridge. This ridge becomes more prominent distally. The medial supracondylar ridge provides an anterior lip for a portion of the origin of the brachialis muscle. The ridge also provides a posterior lip for a portion of the medial head of the triceps brachii. The medial intermuscular septum attaches in an intermediate portion of the medial supracondylar ridge. The lateral border of the humerus extends from the dorsal part of the greater tuberosity to the lateral epicondyle. The lateral border separates the anterolateral surface of the humerus from the posterior surface. The proximal half of the lateral border is rounded and indistinctly marked, serving for the attachment of part of the insertion of the teres minor, and the origin of the lateral head of the triceps brachii. The sulcus or groove for the radial nerve (see above) crosses the central portion of the lateral border of the humerus. The distal part of the lateral border forms a rough, prominent margin, the lateral supracondylar ridge. The lateral supracondylar ridge provides the attachment area for several structures. Superiorly, there is an anterior lip for the origin of the brachioradialis muscle. Distal to this area, the lateral supracondylar ridge provides an area for the origin for the extensor carpi radialis longus. Distally, there is a posterior lip for a portion of the origin of the medial head of the triceps brachii. The intermediate portion of the lateral supracondylar ridge provides the attachment site for the lateral intermuscular septum. Distal Portion of the Humerus The distal portion of the humerus is often referred anatomically as the distal extremity of the humerus (see Figs. 1.11 to 1.14). The distal portion is flat, widened, and ends distally in a broad, articular surface. The distal portion contains the two condyles, medial and lateral (see Fig. 1.14). The lateral portion of the distal articular part consists of a smooth, somewhat semi-spherical shaped capitulum of the humerus. The capitulum is covered with articular cartilage on its anterior surface and articulates with the fovea of the head of the radius. Proximal to the capitulum, there is a slight depression in the humerus, the radial fossa. The radial fossa provides a space for the anterior border of the head of the radius when the elbow is fully flexed. Just medial to the capitulum is a slight shallow groove, in which the medial margin of the head of the radius articulates. Just proximal to the capitulum on the anterior surface of the humerus are several small foramina for nutrient vessels. The medial side of the articular surface of the distal humerus is comprised of the spool-shaped trochlea (see

Fig. 1.14). The trochlea occupies the anterior, inferior, and posterior surfaces of the condyle. The trochlea has a deep groove between two well demarcated borders. The lateral border is separated from the capitulum by the shallow groove. The medial border of the trochlea is thicker, wider, and more prominent, and projects more distally than the lateral border. The grooved portion of the articular surface of the trochlea is shaped well to fit the articular surface of the trochlear notch of the ulna. The trochlea is wider and deeper on the dorsal surface than on the anterior surface. Proximal to the anterior portion of the trochlea is a small depression, the coronoid fossa. The coronoid fossa provides a space for the coronoid process of the ulna during flexion of the elbow. Proximal to the posterior part of the trochlea is a deep, triangular depression, the olecranon fossa. The olecranon fossa provides a space to accept the most proximal portion of the olecranon when the elbow is extended. The olecranon fossa and the coronoid fossa are separated from each other by a thin, sometimes translucent partition of bone. The partition may be perforated to produce a supratrochlear foramen. The fossae are lined by a synovial membrane that extends from the elbow joint. The margins of the fossae provide attachment for the anterior and posterior ligaments and joint capsule of the elbow. Above the medial and lateral condyles are the epicondyles. These projections provide the attachment for several muscles. The medial epicondyle is larger and more prominent than the lateral epicondyle. The medial epicondyle contains the origin of the extrinsic flexor pronator muscles of the forearm and flexor muscles of the hand and wrist. These include the pronator teres, flexor carpi radialis, palmaris longus, flexor digitorum superficialis, flexor digitorum profundus, and flexor carpi ulnaris. The ulnar collateral ligaments of the elbow joint also originate from the medial epicondyle. On the posterior surface of the medial epicondyle is a shallow groove in which the ulnar nerve traverses. The lateral epicondyle is smaller and less prominent than the medial epicondyle. The lateral epicondyle contains the origin of several muscles, including the wrist and digit extrinsic extensor muscles and the supinator. Muscle attachments to the lateral epicondyle include the supinator, extensor carpi radialis longus, extensor carpi radialis brevis, extensor digitorum communis, extensor carpi ulnaris, extensor digiti minimi, and anconeus. The lateral epicondyle also provides attachment for the radial collateral ligament of the elbow joint Associated Joints The humerus articulates with the scapula at the glenohumeral joint, with the ulna at the ulnohumeral joint (trochleoulnar joint), and with the radius at the radiocapitellar joint.

1 Skeletal Anatomy

Muscle Origins and Insertions Muscle attachments include 24 muscles (see Figs. 1.13A–B). The greater tuberosity provides the insertion of the supraspinatus, the infraspinatus, and the teres minor. The lesser tuberosity affords the insertion of the subscapularis. The pectoralis major inserts to the anterior bicipital groove, the teres major inserts to the posterior bicipital groove, and the latissimus dorsi inserts to the central portion or crest of the bicipital groove. The shaft of the humerus provides the insertion of the deltoid and coracobrachialis, and the origins of the brachialis and the triceps (medial and lateral heads). The lateral shaft and epicondyle is the area of origin of the brachioradialis; the medial epicondyle provides the origin of the pronator teres, the flexor carpi radialis, the palmaris longus, the flexor digitorum superficialis, the flexor digitorum profundus, the flexor carpi ulnaris, and the anconeus. The lateral epicondyle provides origin for the extensor carpi radialis longus and brevis, the extensor digitorum communis, extensor digiti minimi, extensor carpi ulnaris, and anconeus.

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of the humeral tuberosities at the insertion of the rotator cuff, or a humeral head that is slightly superiorly located or mildly superiorly subluxated. Magnetic resonance imaging (MRI) can demonstrate soft tissue changes such as bursal inflammation, thickening and effusion, and inflammatory changes or partial tearing of the rotator cuff before osseous changes seen by standard radiographs (17). Neer Classification of Impingement Syndrome (32) n Stage I: Local edema or hemorrhage; reversible condition. Usual age group: young, active individuals involved in sports requiring excessive overhead use of arm. n Stage II: Fibrosis, thickening of subacromial soft tissue, rotator cuff tendinitis, and possible partial tear of rotator cuff; manifested by recurrent pain. Usual age group: 25 to 40 years. n Stage III: Complete rupture of rotator cuff, progressive disability. Usual age group: over 40 years.

Clinical Correlations: Humerus

Fractures of the Proximal Humerus

The Surgical Neck

Neer has classified fractures of the proximal humerus as to the number of segments (18):

The surgical neck, located at the junction of the head (and tuberosities) with the shaft, is an area of frequent fracture, hence its name. Fractures of the surgical neck are much more common than in the anatomic neck, and usually are the result of a direct impact or a fall onto the elbow with the arm abducted. Deformity of fractures of the surgical neck usually include adduction or medial displacement of the shaft due to the pull of the pectoralis major, teres major, and latissimus dorsi. The proximal fragment may be abducted by the pull of the supraspinatus muscle. The Anatomic Neck Fractures rarely occur along the anatomic neck. When fractures do occur in this location, it usually is in an older patient and often is the result of a fall onto the shoulder. Because the shoulder capsule attaches to the bone distal to the anatomic neck, fractures of the anatomic neck usually are intracapsular. Impingement Syndrome Impingement syndrome of the shoulder refers to a condition in which the supraspinatus tendon and subacromial bursa are chronically or repetitively entrapped between the humeral head inferiorly and either the anterior acromion itself, spurs of the anterior acromion or acromioclavicular joint, or the coracoacromial ligament superiorly (17). Osseous findings seen radiographically can include thickening or proliferation of the acromion, spurring at the anteroinferior aspect of the acromion, degenerative changes

n One-part fractures of the proximal humerus are fractures with minimal or no displacement or angulation. n Two-part fractures consist of two major displaced fragments. This can include a displaced fracture of either the greater or lesser tuberosity, fracture of the surgical neck, or fracture of the anatomic neck. n Three-part fractures consist of three major displaced fragments. This can include fractures of both the greater and lesser tuberosities, or a combination of fracture of one of the tuberosities and fracture of the surgical neck. n Four-part fractures consist of four displaced fragments, such as those involving both tuberosities as well as the surgical neck. Anterior Dislocation of the Shoulder In this injury, the humeral head dislocates anterior to the glenoid; it accounts for 97% of shoulder dislocations. It usually is diagnosed on anteroposterior radiographs. Definitive radiographic diagnosis is by the transscapular (“Y” view) or axillary view. Hill-Sacks Lesion This is a defect in the posterolateral aspect of the humeral head resulting from anterior dislocation (often associated with recurrent injuries). The lesion occurs when the dislocated humeral head strikes the inferior margin of the glenoid, producing a “hatchet” compression fracture defect of the humeral head. It usually is demonstrated on the antero-

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posterior view radiograph of the shoulder with the humerus internally rotated. The presence of this lesion is virtually diagnostic of previous anterior dislocation (17). Bankart Lesion Injury to the anterior-inferior cartilaginous labrum, which is usually associated with an avulsion of the inferior glenohumeral ligament from the anterior-inferior glenoid rim. Associated from anterior dislocation of the glenohumeral joint. It may affect only fibrocartilaginous portion of the glenoid, but is commonly noted in association with a fracture of the anterior aspect of the inferior osseous rim of the glenoid. The Bankart lesion is less commonly seen than the Hill-Sacks lesion. The presence of this lesion is virtually diagnostic of previous anterior dislocation (17). Posterior Dislocation of the Shoulder This accounts for 2% to 3% of shoulder dislocations. It can occur from direct force or a blow to the anterior shoulder, from indirect force applied to the arm combining adduction, flexion, and internal rotation, or it can be associated with severe muscle contraction from electric shock or convulsive seizures. The humeral head is located posterior to the glenoid fossa, and usually impacts on the posterior rim of the glenoid. The shoulder usually is positioned or locked in adduction and internal rotation. Standard radiographs may not demonstrate the lesion (because the humeral head lies directly posteriorly, and radiographs may appear unremarkable on standard anteroposterior views). Injury can be demonstrated by either an axillary view (often difficult to obtain because of the arm locked in adduction) or by a special anteroposterior view with the patient rotated 40 degrees toward the affected side. With this view, the normal clear space of the glenohumeral joint is obliterated by the overlap of the humeral head located posterior and slightly medial to the surface of the glenoid. Fractures of the Shaft Proximal to the Insertion of the Deltoid Muscle If a fracture of the humeral shaft occurs just proximal to the insertion of the deltoid, the proximal fragment of the humerus usually is adducted or pulled medially by the pectoralis major, latissimus dorsi, and teres major. The distal fragment usually is displaced or angulated laterally (apex medially, or fracture in valgus) because of the deltoid. Fractures of the Humeral Shaft Distal to the Insertion of the Deltoid Muscle If a fracture of the humeral shaft occurs just distal to the insertion of the deltoid, the proximal fragment usually is displaced laterally by the deltoid and supraspinatus muscle. The distal fragment usually is pulled medially and upward by the triceps, biceps, and the coracobrachialis muscles.

Fractures of the Humeral Shaft Associated with Radial Nerve Palsy Up to 18% of humeral shaft fractures have an associated radial nerve injury (33–36). Most nerve injuries represent a neurapraxia or axonotmesis, and 90% resolve in 3 to 4 months (37–39). This injury often is referred to as the Holstein-Lewis fracture, which describes an oblique fracture of the distal third of the humerus. However, radial nerve palsy is associated most commonly with fractures of the middle third of the humerus (34,38). Supracondylar Fractures The area of bone at the supracondylar level is relatively thin, and fractures through this area are common, especially in children. Structures at risk for injury in supracondylar fractures include the brachial artery and median nerve anteriorly and the radial nerve laterally. Brachial artery injury subsequently is associated with compartment syndrome of the forearm. Supracondylar Process In approximately 1% of upper extremities, there is a downward-curved, hook-shaped process of bone that emanates from the medial cortex approximately 5 cm proximal to the medial epicondyle. It can be associated with a connecting fibrous band (ligament of Struthers), which can be a proximal extension of the pronator teres. The median nerve may pass deep to the supracondylar process and ligament, and may be subject to compression, resulting in median neuropathy. The brachial artery also may pass deep to the ligament (28,40–43). Lateral Epicondylitis Lateral epicondylitis commonly is referred to as tennis elbow. It is thought to consist of either chronic inflammation, partial tear, or “overuse injury” of the common extensor origin. Chronic or repetitive wrist or digital extension often is associated with the onset of symptoms. The extensor carpi radialis brevis often is implicated as the main muscle involved. Although management usually is conservative (activity modification, antiinflammatory medications, splinting, cortisone injections), severe and refractory cases can be managed with operative exploration and release, debridement, or repair of the extensor carpi radialis brevis origin or other involved muscle. Medial Epicondylitis Medial epicondylitis commonly is referred to as golfer’s elbow. Similar to lateral epicondylitis, it is though to consist of either chronic inflammation, partial tear, or overuse injury of the common flexor pronator muscle origin.

1 Skeletal Anatomy

Chronic or repetitive wrist or digital flexion often is associated with symptoms. Osteochondrosis Osteochondrosis (osteochondritis dissecans, osteonecrosis) of the capitellum of the humerus is referred to as Panner’s disease. ULNA Derivation and Terminology The ulna derives its name from the Latin word meaning “the arm” or “the elbow” (1,3). The plural of ulna is ulnae (1).

FIGURE 1.15. Illustration of ulna, showing the three centers of ossification. There is one center in the shaft (body), one in the proximal portion (proximal extremity), and one in the distal end (distal extremity).

27

Ossification Centers and Accessory Bones The ulna has three ossification centers: one in the shaft (body), one in the proximal portion (proximal extremity), and one in the distal end (distal extremity). The mid-portion of the shaft is the first ossification center to appear, becoming visible at approximately the eighth week of fetal life (Figs. 1.15 and 1.16). The ossification centers then extend through the major part of the shaft. At birth, the distal portions and the major part of the olecranon remain cartilaginous. Between the fifth and sixth years, a center in the central portion of the ulnar head appears and soon extends into the styloid process. At approximately the tenth year, a center appears in the olecranon near its outer portion. Most of the ossification of the olecranon actually develops from proximal extension from the center of the shaft (2,4,5). Several accessory bones can be associated with the distal ulna. These accessory bones, if present, usually are the result of secondary or additional ossification centers that do not fuse with the distal ulnar or associated carpal bones. Accessory bones associated with the distal ulna include the os triangulare (os intermedium antebrachii, os triquetrum secundarium), the os ulnostyloideum, and the os pisiforme secundarium (ulnare antebrachii, metapisoid) (see Fig. 1.27B) (44–46). The os triangulare is located distal to the head of the ulna, between the ulnar head, lunate, and triquetrum. The os ulnostyloideum is located in the vicinity of the ulnar styloid. The os pisiforme secundarium is located between the distal ulna and pisiform, close to the proximal edge of the pisiform.

FIGURE 1.16. Illustration of proximal and distal ulna in a young adult, showing epiphyseal lines.

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Accessory bones also can occur from other causes such as trauma (46) or heterotopic ossification of synovial tags (47). Therefore, anomalous, irregular ossicles or small, rounded bones of abnormal size or shape may be encountered that do not fit a specific described accessory bone or location. Osteology of the Ulna The ulna is located in the medial aspect of the forearm lying parallel to the radius when the forearm is supinated. It is a true long bone with a triangular cross-section proximally that becomes rounded distally. The ulna consists of a shaft with thick cortical bone and a long, narrow medullary canal (Figs. 1.17 to 1.20). The cortex of the ulna is thickest along the interosseous border and dorsal surface. In the proximal and distal ends of the ulna, the cortical bone becomes thinner, and the medullary canal is replaced with cancellous bone. The cortical bone remains relatively thick along the posterior portion of the olecranon. The ulna is anatomically divided into three main portions: the proximal end (proximal portion, proximal extremity), the shaft (body), and the distal end (distal portion, distal extremity) (Fig. 1.21; see Figs. 1.19 and 1.20). The proximal end contains the hook-shaped olecranon and the coronoid process to form the medial hinge-like portion of the elbow. The shaft consists of the major portion of the body between the proximal and distal portions. The distal end consists of the head and styloid process. In general, the ulna becomes progressively smaller and thinner from proximal to distal. Proximal Ulna The proximal end of the ulna consists of the olecranon, the coronoid process, the trochlear notch, and the radial notch (see Fig. 1.21A–F). The olecranon is the large, thick curved portion of the proximal ulna. The most proximal portion of the olecranon is angled slightly forward or distally to form a prominent lip that passes into the olecranon fossa of the humerus when the elbow is extended. The base of the olecranon is slightly constricted where it joins the shaft of the ulna, forming the narrowest part of the proximal ulna. The posterior surface of the olecranon is triangular and smooth. This prominent area, easily palpable through the skin, is covered by the olecranon bursa. The superior (or most proximal) surface of the olecranon is somewhat quadrilateral in shape and has a rough surface for the insertion of the triceps tendon. The anterior surface of the olecranon is concave and smooth, and is lined with articular cartilage to form the proximal portion of the trochlear notch. There usually is a nonarticular zone in the mid-portion of the articular surface (see later discussion of trochlear notch). The elbow joint capsule attaches to the anterior aspect of the superior surface of the olecranon. The medial portion of the olecranon provides

FIGURE 1.17. Right ulna and radius, anterior aspect, showing muscle origins (red) and insertions (blue).

attachment for the oblique and posterior parts of the ulnar collateral ligament. The medial aspect of the olecranon also provides an area for the origin of a portion of the flexor carpi ulnaris muscle. The posteromedial portion also provides a part of the origin of the flexor digitorum superficialis. The lateral portion of the olecranon provides the insertion of the anconeus muscle (see Fig. 1.18).

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29

FIGURE 1.18. Right ulna and radius, posterior aspect, showing muscle origins (red) and insertions (blue). FIGURE 1.19. Right ulna and radius, anterior aspect.

The coronoid process is a triangular eminence that projects from the anterior surface of the ulna, roughly at the junction of the shaft with the proximal portion (see Fig. 1.19). Its base arises from the proximal and anterior part of the shaft. The superior surface of the coronoid process is smooth and concave, and forms the inferior portion of the trochlear notch. Its inferior surface is concave and

rough. At the junction of the coronoid with the shaft of the ulna is a thickened, rough eminence, the tuberosity of the ulna. This tuberosity provides the attachment area for the brachialis as well as the oblique cord of the radius. The lateral surface of the coronoid contains the radial notch,

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Systems Anatomy

FIGURE 1.20. Right ulna and radius, posterior aspect.

which is a narrow, rounded, oblong depression lined with articular cartilage. The radial notch articulates with the rim of the radial head during forearm supination and pronation. The medial surface of the coronoid process provides the area of attachment of the anterior and oblique portions of the ulnar collateral ligament. At the anterior portion of the medial surface of the coronoid is a small, rounded eminence for the origin of three humer-

oulnar heads of the flexor digitorum superficialis. Posterior to this eminence, a slight ridge extends from the medial aspect of the coronoid distally. Along this ridge arise the proximal portions of the insertions of the flexor digitorum profundus, along with the ulnar head of the pronator teres. In addition, a small ulnar head of the flexor pollicis longus may arise from the distal part of the coronoid process (see Fig. 1.17).

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31

A

B

C

D FIGURE 1.21. A: Proximal right ulna, lateral aspect. B: Right elbow, medial aspect, showing capsular attachment and medial ligaments. C: Right elbow, lateral aspect, showing capsular attachment and lateral ligaments. D: Right elbow, sagittal section. E: Proximal radioulnar joint, with radial head removed, showing annular ligament. (continued on next page)

The trochlear notch of the ulna is a large concave depression that is semilunar in shape and formed by the coronoid process and the olecranon (see Figs. 1.19 and 1.21A,E, and F). The trochlear notch, covered anteriorly by articular cartilage, provides the articular surface for the trochlea of the humerus. The articular surface of the trochlear notch has an area near its mid-portion that contains a central transverse area that usually is deficient in articular cartilage. This area

subdivides the articular surface into a proximal portion (on the anterior surface of the olecranon) and a distal portion (on the anterosuperior surface of the coronoid). At this mid-portion of the trochlear notch, the borders are slightly indented near its middle, creating a narrow portion in the proximal ulna. The radial notch of the ulna is the articular depression on the lateral aspect of the coronoid process (see Figs. 1.19,

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F

E

G

H FIGURE 1.21. (continued) E: Proximal radioulnar joint, with radial head removed, showing annular ligament. F: Proximal ulna, with proximal radius removed to show annular ligament and radial notch. G: Right elbow, anterior aspect, showing synovial membrane. The capsule has been removed and the articular cavity distended. H: Right elbow, posterior aspect, showing synovial membrane. The capsule has been removed and the articular cavity distended.

1 Skeletal Anatomy

and 1.21A,E, and F). The notch is narrow, oblong, and lined with articular cartilage. The notch articulates with the circumferential rim of the radial head. The anterior and posterior margins of the radial notch provide the attachment areas for the annular ligament. Shaft (Body) of the Ulna The shaft (or body) of the ulna is triangular in cross-section in the proximal two-thirds, but becomes round in the distal third. Longitudinally, the proximal half of the shaft is slightly convex dorsally and concave anteriorly. The distal half (and sometimes central portion) becomes longitudinally straight. The distal half of the shaft may be slightly concave laterally and convex medially. In cross-section, the triangular shape presents an anterior, posterior, and medial surface, as well as an anterior border, posterior border, and interosseous border (each of which is located at the apex of the triangular cross-sectional shape). The interosseous ligament is attached along the interosseous border apex of the triangle, and there is no true lateral surface in this region of the bone. More distally, the bone becomes progressively circular in cross-section. The shaft flares slightly distally as it enlarges into the ulnar head. The three borders of the ulnar shaft are the anterior, posterior, and interosseous borders. The anterior border of the ulna begins proximal at the prominent medial angle of the coronoid process and extends distally along the anteromedial aspect of the shaft to terminate anterior and medial to the styloid process of the head of the ulna. The anterior border is best defined in its proximal portion, and becomes rounder, smoother, and less clearly defined in the central distal portion as the shaft becomes progressively circular in circumference distally. In this central portion of the anterior border, the ulna provides a large surface origin for the flexor digitorum profundus muscle (see Fig. 1.17). The distal one-fourth of the anterior border is referred to as the pronator ridge and provides origin for the pronator quadratus (4). The posterior border of the ulna begins proximally at the apex of the triangular subcutaneous surface of the olecranon (see Fig. 1.18). The posterior border extends distally along the mid-posterior portion of the shaft, to terminate posterior to the styloid process. The posterior border is well defined along its proximal one-third to three-fourths; however, as the ulna becomes more circular in cross-section distally, the distal portion of the posterior border is more rounded, smooth, and poorly defined. In the well defined proximal portion, the posterior border of the ulna gives rise to the attachments of an aponeurosis, which provides a common origin for the flexor carpi ulnaris, the extensor carpi ulnaris, and the flexor digitorum profundus (see Fig. 1.18). The posterior border separates the medial and posterior surfaces of the ulna.

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The interosseous border of the ulna is well defined and can be somewhat sharp in its central portion (see Figs 1.17 to 1.20). The interosseous border actually extends along the lateral margin of the ulna, beginning at the radial notch and curving slightly anteriorly as it extends distally. A proximal portion of the interosseous border is referred to as the supinator crest, providing a ridge for the attachment of a portion of the supinator muscle. In the distal one-fourth of the shaft, the interosseous border is less well defined. The interosseous ligament attaches along the interosseous border and is thickest at its attachment in the central portion of the interosseous border. The interosseous ligament provides a partition that separates the anterior and posterior surfaces of the ulna. There are three surfaces of the shaft of the ulna: the anterior, posterior, and medial surfaces. The anterior surface of the ulna lies between the interosseous border (located laterally) and the anterior border (located medially). The anterior surface is wide in its proximal portion and slightly concave along the proximal one-half or three-fourths of the shaft. In this broad proximal portion, the surface is slightly roughened and provides the large origin of the flexor digitorum profundus (see Fig. 1.17). The origin of the flexor digitorum profundus extends to cover most of the anterior surface, from the proximal third to the distal end of the middle third. The distal fourth of the anterior surface is covered by the pronator quadratus, which takes origin from an oblique oval area (see Fig. 1.17). The nutrient canal enters the ulna at the anterior surface at the junction of the proximal and middle thirds. A branch of the anterior interosseous artery enters at this site. The posterior surface of the shaft of the ulna is the area between the posterior border and the interosseous border (see Figs. 1.18 and 1.20). This surface is somewhat laterally located along the shaft and is broad proximally, where the posterior edge is well defined. The middle portion of the posterior surface is narrower, straight, and begins to loose the definition of the posterior edge as the shaft becomes progressively rounder in cross-section. In the distal third, the posterior surface is round and flares slightly as the ulnar head is formed. In the proximal portion, there is an oblique line or ridge, which begins proximally at the dorsal end of the radial notch and continues distally (see Fig. 1.18). There is a triangular surface proximal to this ridge that provides the insertion area for the anconeus muscle. The proximal part of the ridge also provides a portion of the origin area for the supinator. Along the mid-portion of the posterior surface of the ulnar shaft, there is a central, longitudinal ridge that is referred to as the perpendicular line (4). This perpendicular line provides an attachment for the extensor carpi ulnaris. The medial part is smooth, and covered by the extensor carpi ulnaris. The lateral part is wider and rougher, and provides the origin for the supinator, the abductor pollicis longus, the extensor pollicis

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longus, and the extensor indicis proprius. Also attaching in the vicinity of the perpendicular line is an aponeurosis that provides a common attachment for the extensor carpi ulnaris, flexor carpi ulnaris, and flexor digitorum profundus (Fig. 1.18). The medial surface of the shaft of the ulna is the area between the posterior border and the medial border. The medial surface is broad proximally and slightly concave in its proximal two-thirds. As the shaft extends distally, the medial surface becomes more narrow and round, and slightly convex. The medial surface flares at the distal end to form the head of the ulna. The proximal three-fourths of the medial surface of the ulna provides a portion of the origin of the flexor digitorum profundus (Fig. 1.18).

Associated Joints The ulna articulates by synovial joints with the humerus and radius. The distal ulna also articulates with the carpus through the ulnocarpal joint, a nonsynovial joint that is capable of load transfer. Proximally, the ulna articulates with the humerus through the hinge-like ulnohumeral joint (see Fig. 1.21A–F). A proximal articulation also exists with the radial head, the proximal radioulnar joint. The outer margin of the radial head articulates with the radial notch of the ulna (see Figs. 1.17 to 1.21). Distally, the head of the ulna articulates with the radius to form the distal radioulnar joint. This synovial joint normally does not communicate with the radial carpal joint.

Distal Ulna The distal portion of the ulna consists of the head and styloid process (Fig. 1.22; see Figs. 1.17 to 1.20). The head of the ulna is a rounded, partially spherical eminence that forms from the flare of the distal shaft. The head is covered in its distal and lateral surfaces with articular cartilage. Distally, it articulates with the proximal surface of the triangular fibrocartilage complex and ulnocarpal ligaments. The lateral, anterior, and medial surfaces of the ulnar head articulate with the ulnar notch of the distal radius to form the distal radioulnar joint. The styloid process is a narrow, nonarticular prominence based posterior and slightly medial to the ulnar head. The styloid process extends distally to become the most distal portion of the ulna. It provides attachment for the triangular fibrocartilage complex and ulnocarpal ligaments. The tendon of the extensor carpi ulnaris passes through a shallow groove located between the head and styloid process on the posterior surface of the distal ulna.

Muscle Origins and Insertions A variable number of muscles attach to the ulna, usually at least 12 (see Figs. 1.17 and 1.18). The olecranon provides attachment for the triceps insertion, anconeus insertion, and origin of the ulnar head of the flexor carpi ulnaris (medial aspect). The base of the coronoid process provides the insertion area for the brachialis. The proximomedial ulna also provides the attachment for a portion of the origin of the flexor digitorum superficialis and flexor digitorum profundus (whose origin extends into the shaft). Medial to the insertion of the brachialis, the proximal shaft or base of the coronoid process provides part of the origin for the pronator teres. The proximolateral anterior ulna provides the origin for the supinator. Occasionally, a small portion of the origin of the flexor pollicis longus arises from the proximal ulna (see Fig. 1.17). The dorsal shaft of the ulna provides attachment for the common aponeurosis to the extensor carpi ulnaris, flexor carpi ulnaris, and flexor

FIGURE 1.22. Axial view of right distal radius and ulna, showing configuration of distal radioulnar joint, the carpal articular surface, and distal end of ulnar head and styloid process.

1 Skeletal Anatomy

digitorum profundus, and the origin of the abductor pollicis longus, extensor pollicis longus, and extensor indices (Fig. 1.18). On the anterior aspect of the shaft of the ulna, the flexor digitorum profundus occupies a vast origin area, covering the major portion of the anterior shaft. Distally, the medial aspect of the anterior shaft provides the origin for the pronator quadratus (Fig. 1.17). Clinical Correlations: Ulna Olecranon Osteotomy (Nonarticular Portion) On the central portion of the articular surface of the proximal ulna, in the trochlear notch, there is an area in the joint that is devoid of articular cartilage. In this area, the olecranon is slightly narrower. An olecranon osteotomy placed in this area can avoid injury to the articular surface. Fractures of the Olecranon Several classification systems have been described for fractures of the olecranon (17,47a). A modification of the Colson classification recently has been popularized (48): n Type I: fracture of the olecranon that is nondisplaced n Type II: fracture of the olecranon that is displaced but without elbow instability n Type III: fracture of the olecranon that is comminuted, but without elbow instability n Type IV: fracture of the olecranon that is comminuted, unstable, and with elbow instability Fractures of the Coronoid Fractures of the coronoid has been classified into three types (49): n Type I: fracture of the coronoid involving only the tip n Type II: fracture of the coronoid involving one-half or less of the coronoid n Type III: fracture of the coronoid involving more than one-half (50) Nightstick Fracture This is a single-bone forearm fracture involving the shaft of the ulna, often nondisplaced or minimally displaced (51). It was originally described from nightstick injury, when the forearm is placed above the shoulder to protect the face or body from blow from nightstick. Monteggia Fracture This fracture of the proximal third of the ulna and a concomitant anterior dislocation of the radial epiphysis was

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described by Monteggia in 1814 (52). The classification has been modified by Bado to include four subtypes (53): n Type I: Anterior dislocation of the radial head with associated anteriorly angulated fracture of the ulna shaft n Type II: Fracture of the ulnar diaphysis with posterior angulation at the fracture site and a posterolateral dislocation of the radial head n Type III: Fracture of the ulnar metaphysis with a lateral or anterolateral dislocation of the radial head n Type IV: Fracture of the proximal third of the radius and ulna at the same level with an anterior dislocation of the radial head Fracture of the Ulnar Styloid and Implications for Attached Ligaments Because of the attachments of the triangular fibrocartilage complex, fracture of the ulnar styloid may represent avulsion fracture or concomitant injury to the triangular cartilage complex. Accessory Bones Several accessory bones can be associated with the distal ulna and may be mistaken for fractures. An accessory bone usually represents the residual of a secondary ossification center that does not fuse with the associated bone, but it also may arise from trauma or from heterotopic ossification of synovial tags (46,47). The accessory bones associated with the distal ulna include the os triangulare (located distal to the distal end of the ulna, between the ulna, lunate and triquetrum), the os ulnostyloideum (located in the vicinity of the ulnar styloid), and the os pisiforme secundarium (located between the pisiform and distal ulna; see Fig. 1.27B) (46) (see descriptions earlier, under Ossification Centers and Accessory Bones). Disagreement exists as to the origin of the os triangulare (25,46). It has been classified as soft tissue calcification, an old avulsion fracture, or as arising from a secondary ossification center (from the ulna styloid). It has been reported to be present bilaterally without preexisting history of trauma, which supports its existence as a true independent ossicle (25). Schultz (25) has emphasized that differentiation of an accessory bone from a recent or nonunited fracture of the ulna styloid may be difficult. Differentiation from a fracture of the ulnar styloid may be assisted by noting the length and completeness of the ulna styloid. If the styloid process is of normal contour and no defects are present indicating the location of an avulsed fragment, the area of ossification probably represents an accessory bone. At times, the ulna styloid may arise from a separate center of ossification, and failure of fusion of this center leads to disruption of the normal contour of the styloid. In a recent fracture, the fracture line is found dividing

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the ulna styloid without the presence of dense opposing surfaces. Comparative radiographs can assist in the diagnosis if the condition is found to be bilateral. Arthritis of the Distal Radioulnar Joint Loss of congruity of the distal radioulnar joint can occur from angulation or joint disruption in distal radius fractures (Colles’ fracture), or from dislocation or subluxation from Galeazzi-type fractures or fractures of the radial head with concomitant injury to the interosseous ligament resulting in proximal translation of the radius (Essex-Lopresti fracture). Positive Ulnar Variance Positive ulnar variance can be associated with shortening of the radius either from congenital or traumatic causes. Positive variance can lead to increased force transmission through the ulnocarpal joint or to impingement of the ulnar head on the lunate or triquetrum. Operative management can consist of shortening of the ulna, distal ulna resection, or lengthening of the distal radius. Negative Ulnar Variance Negative ulnar variance is associated with Kienböck’s disease. In the absence of arthritic or degenerative changes, management may consist of lengthening the ulna or shortening the radius to produce a neutral ulnar variance. RADIUS Derivation and Terminology The radius derives its name from the Latin for spoke (i.e., of a wheel) (1). The plural of radius is radii (1). Ossification Centers The radius contains three ossification centers: one for the proximal portion, one for the shaft (body), and one for the distal portion (Figs. 1.23 and 1.24). The ossification center for the shaft first becomes visible in the mid-portion of the bone at approximately the eighth week of fetal life. Ossification begins in the distal end during the second year of life. Ossification of the proximal end becomes visible during the fifth year. The proximal epiphysis fuses with the ossification center of the shaft at 15 to 18 years of age. The distal epiphysis fuses to the shaft between the seventeenth and twentieth year. Occasionally, an additional center is visible in the radial tuberosity, which appears at approximately the fourteenth or fifteenth year. Accessory bones can be associated with the distal radius. These include the os radiostyloideum and the os

FIGURE 1.23. Schematic illustration of the radius, showing ossification centers. There are three centers: one for the proximal portion, one for the body (shaft), and one for the distal portion.

radiale externum (parascaphoid) (see Fig. 1.27B) (25,46). The os radiostyloideum usually is located at the lateral aspect of the distal radius, in the vicinity of the radial styloid. The os radiale externum is located slightly distal to the site of the os radiostyloideum, between the radial styloid and the scaphoid. An accessory bone, if present, usually is the result of a secondary or additional ossification center that does not fuse with the associated bone. That associated with the distal radius usually is from a secondary or additional ossification center of the radial styloid (46) (see Fig. 1.27B). Accessory bones also can occur from other causes, such as trauma (46) or heterotopic ossification of synovial tags (47). Therefore, anomalous, irregular ossicles or ossicles of abnormal size or shape may be encountered that do not fit a specific described accessory bone or location.

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FIGURE 1.24. Proximal and distal radius in a young adult, showing epiphyseal lines.

Osteology of the Radius The radius lies laterally in the forearm, has a long, narrow shaft, and is widened proximally and distally to form the head and styloid process, respectively. The radius consists of three major parts: the proximal portion (proximal extremity), the shaft (body), and the distal portion (distal extremity). The radius lies parallel to and is slightly shorter than the ulna (see Figs. 1.17 to 1.20). The proximal end is much smaller than the distal portion. At the elbow, the radial head articulates with the capitulum of the humerus and with the radial notch of the proximal ulna. At the wrist, the distal radius articulates with the scaphoid and lunate at the radiocarpal joint, and with the head of the ulna at the distal radioulnar joint. The proximal and distal articulations with the ulna provide forearm pronation and supination. The distal end articulation at the radiocarpal joint provides wrist extension, flexion, and radial and ulnar deviation. The radiocarpal joint usually transfers most of the force from the wrist to the radius, and subsequently to the elbow. The internal structure of the radius is that of a long bone with a long, narrow medullary cavity (see Figs. 1.17 to 1.20). The medullary canal is enclosed by thick cortical bone, which is strongest and thickest along the interosseous border. The cortex becomes thinner at the proximal and distal ends of the radius. At the proximal end, the shaft flares out to form the head, with a central, cup-shaped area of the head containing relatively thick subchondral bone. The trabeculae of the proximal and distal radius are arranged into a somewhat arched pattern. Proximally, the trabeculae pass proximally from the cortical layer of the shaft to the fovea of the head of the radius. These trabeculae are crossed by transverse trabeculae that are oriented parallel to the surface of the fovea. In a similar manner, the

trabeculae of the distal radius are arranged so that they extend longitudinally from the cortical bone and course to the articular surface. Additional trabeculae cross parallel to the surface of the joint. Proximal Radius The proximal end of the radius consists of the head, neck, and the tuberosity (see Figs. 1.17 to 1.20). The head is shaped somewhat like a thick disc or short cylinder. The proximal surface forms a shallow cup, the central portion of which is the fovea. The fovea of the radial head articulates with the capitulum of the distal humerus. The articular margin or periphery of the head is smooth and approximately 5 to 10 mm high. The radial head is thickest in the medial portion where it articulates with the radial notch of the ulna. The radial head is slightly shorter in the lateral portions, where it is surrounded by the annular ligament. The head is connected to the smooth, narrower radial neck. The neck is cylindrical and has a thick cortex. The head overhangs the neck, giving a slight mushroom-like appearance. On the posterior aspect of the neck there is a slight ridge or roughened surface for the insertion of a portion of the proximal supinator. The anterior surface of the neck is smooth. Along the anterior undersurface of the rim formed by the junction of the radial head and radial neck there are several small nutrient foramina. The tuberosity of the radius lies on the anteromedial aspect of the proximal radius, distal to the neck. The tuberosity is rough on its most medial and posterior aspects for the insertion of the biceps tendon. On its most anterior aspect, the tuberosity is smooth, in which a bursa is interposed between the tendon and the radius.

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Shaft of the Radius The shaft of the radius, often referred to in anatomic textbooks as the body, consists of the major portion of the bone between the head and the distal end (2,4,5). In the proximal portion, the shaft is round or cylindrical where it joins the radial neck. More distally, the shaft becomes triangular in cross-section, with an apex directed toward the ulna where the interosseous ligament attaches. The triangular cross-sectional area of the shaft results in three surfaces (anterior, posterior, and lateral) separated by three borders (anterior, posterior, and interosseous). The interosseous border along the medial aspect is sharp along its margin, except proximally near the tuberosity. The shaft gradually increases in size from proximal to distal. The shaft of the radius is gently curved, convex dorsally and laterally. The anterior (palmar, volar) surface is correspondingly gently curved concave volarly. The interosseous border, on the medial aspect, is gently curved concave ulnarly. The anterior border is located on the anterolateral surface of the shaft. It separates the anterior and lateral surfaces. It is well defined in its proximal and distal portions, but poorly defined in its central or middle portion, where the border is more rounded and less distinct. The anterior border starts proximally from the distal portion of the tuberosity and extends longitudinally to reach the anterior part of the base of the styloid process. The proximal third of the anterior border of the radius is elevated to form a slight ridge known as the anterior oblique line of the radius. The anterior oblique line is sharper and more defined in its distal portion, forming a palpable crest along the lateral margin of the anterior surface. The anterior oblique line provides the area of origin of the flexor digitorum superficialis and flexor pollicis longus muscles. Proximal and lateral to the anterior oblique line, the area on the radius provides a portion of the insertion of the supinator muscle. In the distal part of the shaft of the radius, along the distal one-fourth, the anterior border is more clearly defined than the central portion. This part of the anterior border provides the insertion area of the pronator quadratus and attachment of the dorsal carpal ligaments. The distal portion of the anterior border continues distally and slightly laterally, and terminates in a small tubercle on the anterolateral surface. This tubercle, located at the base of the styloid process, provides the insertion attachment for the brachioradialis muscle (Fig. 1.17). The posterior border begins proximally at the posterior aspect of the neck of the radius and extends distally to the posterior aspect of the base of the styloid process. The posterior border separates the posterior surface of the radius from the lateral surface. The border actually is rounded and not clearly defined, especially in the most proximal and distal aspects. It is best defined in its middle third, where it is slightly roughened.

The interosseous border extends along the medial aspect of the radius in proximity to the ulna. Proximally, the interosseous border is poorly defined. Distal to the radial tuberosity, the interosseous border changes from a rounded contour to a sharp, somewhat rough, prominent edge. The edge is the most prominent and thickest at the junction of the proximal third and distal two-thirds. A the distal portion of the interosseous border, approximately 5 cm from the distal end of the radius, the interosseous border divides into two ridges that continue to form the anterior and posterior margins of the ulnar notch. This creates a triangular surface between the ridges, known as the medial surface of the distal radius (5). This triangular area serves as an insertional area for a portion of the pronator quadratus. In this distal area, the divided interosseous border separates the anterior surface of the radius from the posterior surface. Along its sharp distal three-fourths, the interosseous border provides the attachment for the interosseous ligament, connecting the radius to the ulna. The anterior surface of the shaft of the radius lies between the anterior and interosseous borders. The surface is concave in its proximal three-fourths, but becomes slightly broader and flatter in its distal fourth. The large concave proximal surface provides the origin for the flexor pollicis longus. The muscle covers the major surface area of the anterior surface. The flatter, broader distal portion of the anterior surface is covered by the pronator quadratus. Distal and radial to the attachment of the pronator quadratus, in the palmar aspect of the radial styloid, there is a triangular area separated from the shaft by a slight ridge. This triangular area is roughened and provides attachment for the radiocarpal ligaments. Several nutrient foramina are present on the distal anterior surface of the radial metaphysis. Near the midpoint or in the vicinity of the junction of the proximal and middle thirds of the anterior surface, there usually is a nutrient foramen and canal. The foramen receives a branch from the anterior interosseous artery. The nutrient vessel enters the radius with a somewhat proximally directed course. The posterior surface of the radius lies between the posterior and interosseous borders. It is flat, slightly convex, or slightly rounded along most of its course. In the proximal third, it is smooth and may be slightly concave, providing for the attachment of the supinator, which covers the posterolateral surface of the proximal radius. Just distal to the attachment of the supinator is the oblique insertion area of the pronator teres, which extends to the lateral surface. In the middle third of the posterior surface, the surface is broad and may become slightly concave, providing origin for the abductor pollicis longus and extensor pollicis brevis. In the distal third of the posterior surface of the radius, the surface is broad, convex, irregular, and grooved, providing the passage and routing of the dorsal extensor tendon compartments (see later, under Distal Radius) (Fig. 1.18).

1 Skeletal Anatomy

The lateral surface of the radius is a gently convex surface lying between the anterior and posterior borders. It generally is smooth, rounded, and remains convex along its entire surface. In the proximal portion, it provides a portion of the attachment of the supinator muscle. In the central portion there is a slightly roughened oval area for the insertion of the tendon of the pronator teres. In the distal portion of the lateral surface, the surface is smooth where the tendons of the abductor pollicis longus and extensor pollicis brevis muscles cross. Distal Radius The distal portion of the radius includes the metaphyseal and epiphyseal regions. This portion of the radius is quadrilateral in cross-section and encompasses the widest portion of the radius. Anatomic features include the anterior, posterior, medial, and lateral surfaces; the styloid process; the dorsal (Lister’s) tubercle; the ulnar notch; and the radiocarpal and distal radioulnar joint articular surfaces. The lateral surface flares out gradually from the shaft, extending further along the lateral margin to form the styloid process. The styloid process is conical. A rough area at the base of the styloid provides the attachment for the brachioradialis. This lateral surface is slightly rough, and projects distally to terminate in the tip of the styloid. The distal area of the styloid provides attachment for the articular capsule and the capsular thickening of the collateral ligament. On the lateral surface of the radial styloid, there is a flat groove for the passage the abductor pollicis longus and extensor pollicis brevis tendons. The process is easily palpable and serves as a useful anatomic landmark to mark the lateral margin of the radiocarpal joint. The anterior surface of the distal radius is concave, palmarly directed, and widened or flared out from the contour of the shaft. The surface is rough for the attachment of the palmar radiocarpal ligaments, and multiple small foramina are present to provide vascularity to this metaphyseal area of the radius. The anterior surface has a thick, prominent ridge, which is palpable approximately 2 cm proximal to the thenar eminence. A portion of the anterior surface is covered by the pronator quadratus, of which there are attachments that extend distally to the area adjacent to the area of the attachment of the wrist capsule and radiocarpal ligaments. The medial surface of the distal radius consists of the ulnar notch and the articular surface for the ulnar head, comprising the radial component of the distal radioulnar joint. The ulnar notch is narrow, smooth, concave in the anteroposterior plane, and roughly triangular, with the widest portion distally. The margins of the articular surface are bordered by a slight ridge, further defining the ulnar notch. Small nutrient foramina are present just proximal to the articular margin of the distal radioulnar joint.

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The posterior (dorsal) surface of the distal radius flares out gradually from the shaft. It is irregular, rough, convex, and contains multiple small vascular foramina for the distal radial metaphysis. In the mid-portion of the posterior distal radius is the prominent dorsal (Lister’s) tubercle. It lies from 5 to 10 mm from the distal joint surface. A portion of the extensor retinaculum attaches to Lister’s tubercle. On the medial aspect of the dorsal tubercle is a deep, smooth groove for passage of the extensor pollicis longus tendon. On the most lateral aspect of the posterior distal radius, there are less defined grooves, from lateral to medial, for passage of the abductor pollicis longus, extensor pollicis brevis, extensor carpi radialis longus, and extensor carpi radialis brevis, respectively. The groove that contains the extensor carpi radialis longus and brevis is broad and shallow, and subdivided into two parts by a slight ridge to allow passage of each of the two tendons, with the longus located lateral to the brevis. On the ulnar aspect of the posterior distal radius, ulnar to Lister’s tubercle, are faint grooves for passage of the extensor indicis and extensor digitorum communis. The extensor indicis tends to pass slightly deeper than the extensor digitorum communis. In this vicinity, along the dorsal margin of the distal radius and adjacent to the cortex, the posterior interosseous nerve courses. The distal margin of the posterior surface of the distal radius is rough to provide for the attachment of the dorsal radiocarpal ligaments. The carpal articular surface of the distal radius is roughly triangular (apex lateral), smooth, concave, and curving and extending distally along the lateral margin. The base of the triangle intersects the articular surface of the distal radioulnar joint. On the carpal articular surface, there is a slight division by a mild anteroposterior ridge. This divides the articular surface into lateral and medial parts. The lateral part is triangular and contains the scaphoid fossa. The medial portion is more quadrangular and contains the lunate fossa. The distal radiocarpal articular surface is concave and slightly oval, elongated from anterior to posterior. Between the distal radioulnar joint and the radiocarpal joint there is a slight separation of the articular surfaces by a prominent ridge. This ridge, located in the ulnar notch, provides the radial attachment for the triangular fibrocartilage. Associated Joints At the proximal end, the head of the radius articulates with the capitulum of the humerus and with the radial notch of the ulna (see Fig. 1.21B–F). At the distal end, the radius articulates, through its ulnar notch, with the head of the ulna to form the distal radioulnar articulation. Also at the distal end, the radius articulates with the scaphoid and the lunate at the radiocarpal joint. The scaphoid articulates

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with the scaphoid fossa of the distal radius. The specific articulation with the scaphoid is referred to as the radioscaphoid joint or, depending on the specific location in the radioscaphoid joint, the articulation can be referred to as the styloscaphoid joint [descriptive because of its significance for arthritis and the scapholunate advanced collapse (SLAC) wrist]. The specific articulation with the lunate is referred to as the radiolunate joint. The lunate articulates with the lunate fossa of the distal radius. The interosseous ligament between the radius and the ulna can be considered a nonsynovial articulation.

interosseous ligament between the radius and ulna may allow proximal migration of the radius. This injury was described by Essex-Lopresti in 1951 (61,62). At the wrist, the proximal migration of the radius results in relative shortening of the radius, producing a relative positive ulnar variance. Management in the acute setting includes reconstruction or metallic prosthetic replacement of the radial head (to regain proximal support), and, as needed, pinning of the distal radius and ulna to hold the reduced distal radioulnar joint accurately. Galeazzi’s Fracture

Muscle Origins and Insertions There usually are nine muscles that attach to the radius (see Figs. 1.17 and 1.18). The biceps insertion attaches to the radial tuberosity. The supinator originates from the oblique ridge of the proximal medial aspect. The flexor digitorum superficialis originates along an oblique line on the anterior proximal and central diaphysis. The flexor pollicis longus origin covers the anterior shaft of the radius. The insertion of the pronator quadratus attaches to the distal anterior diaphysis and metaphysis. The midshaft on the radial aspect provides the insertion of the pronator teres. The origins of the abductor pollicis longus and extensor pollicis longus attach to the posterior midshaft. The brachioradialis inserts into the lateral aspect of the distal radius, just distal to the styloid. Clinical Correlations: Radius The Oblong Shape of the Scaphoid Fossa The oblong shape of the scaphoid fossa of the distal radius influences radioscaphoid arthritis, as can be demonstrated with the SLAC wrist from scapholunate instability. The scaphoid fossa of the radius is somewhat oblong in shape, and accepts the oblong articular surface of scaphoid. The lunate fossa of the radius is more nearly spherical, and accepts the more hemispherical articular surface of the lunate. With scapholunate instability, mobility of the scaphoid in the oblong fossa is not as well tolerated because areas of stress concentration result if the scaphoid rotates abnormally. The more spherical shape of the radiolunate articulation can tolerate motion of the lunate more easily, without stress concentration. Therefore, in long-standing scapholunate instability, arthritic changes usually develop first in the radioscaphoid joint (styloscaphoid joint), whereas the radiolunate joint may be relatively well preserved until the latest stages (54–60). Essex-Lopresti Lesion Fracture of the radial head (which results in the loss of proximal support of the radius) along with injury to the

Fracture of the distal radial shaft with an associated dislocation or subluxation of the distal radioulnar joint was described by Galeazzi in 1934 (63–65). The fracture usually occurs at the junction of the middle and distal thirds of the radius, and usually has a transverse or short oblique configuration. Open reduction with internal fixation (ORIF) usually is the preferred method of treatment (65). Fracture Classification of the Radial Head Fractures of the radial head have been described by Mason in 1954 (65a), and recently modified by Hotchkiss. The Hotchkiss classification is as follows (62): n Type I: Nondisplaced or minimally displaced fracture of the radial head or neck. Forearm rotation is limited only by acute pain and swelling. Intraarticular displacement of the fracture is less than 2 mm. Treatment usually is sling immobilization and active motion as early as tolerated. n Type II: Displaced (>2 mm) fracture of the head or neck, motion may be mechanically limited or incongruous, without severe comminution (repairable by ORIF), and fracture involves more than a marginal lip of the radial head. Treatment is variable, and includes either ORIF (recently more popular), early motion without excision, or excision. n Type III: Severely comminuted fracture of the radial head and neck, not reconstructible, and requires excision for movement. Treatment usually is excision, with possible prosthetic replacement to improve valgus stability and prevent proximal translation of the radius. Colles’ Fracture Colles described this fracture of the distal radius in 1814 (66). The fracture involves the distal metaphysis, which is dorsally displaced and angulated, and usually occurs within 2 cm of the articular surface. The fracture may extend into the distal radiocarpal joint. Classic features include dorsal angulation (silver fork deformity), dorsal displacement, radial angulation (loss of radial inclination), and radial

1 Skeletal Anatomy

shortening. There often is accompanying fracture of the ulnar styloid, which may signify avulsion of the triangular fibrocartilage complex (67). Barton’s Fracture Barton described this fracture of the distal radius in 1838 (68). The fracture is a fracture–dislocation in which the rim of the distal radius, dorsally or palmarly, is displaced with the hand and carpus (68,69). The fracture differs from the Colles’ or Smith’s fracture in that the dislocation is the most clinically and radiographically obvious abnormality, with the radial fracture noted secondarily. The volar Barton’s fracture is similar to the Smith’s type III fracture, where both involve palmar dislocation of the carpus associated with an intraarticular distal radius component. Smith’s Fracture

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CARPUS General Aspects The carpus consists of eight carpal bones, arranged in a proximal and a distal row, each row containing four bones. The proximal row includes (from lateral to medial) the scaphoid, lunate, triquetrum, and pisiform. The pisiform is located palmar to the plane of the remaining three carpal bones of the proximal row, and the pisotriquetral joint is separated from the joining articulations. The distal row includes (from lateral to medial) the trapezium, trapezoid, capitate, and hamate (Figs. 1.25 and 1.26). The proximal row is convex proximally and concave distally. The proximal row articulates proximally with the distal radius and with the triangular fibrocartilage complex, forming the radiocarpal and ulnocarpal joint. The proximal row articulates distally with the distal carpal row, forming the midcarpal joint.

Smith described an additional fracture pattern of the distal radius in 1854. In this fracture, often called reverse Colles’ fractures, the distal radial fragment is palmarly angulated or displaced, producing a “garden spade” deformity (69,70). The hand and wrist are displaced forward or palmarly with respect to the forearm. The fracture may be extraarticular, intraarticular, or part of a fracture–dislocation (67,70,71). The classification modified by Thomas includes type I, which is extraarticular; type II, which crosses into the dorsal articular surface; and type III, which is intraarticular and similar to the volar Barton’s fracture–dislocation. Chauffeur’s Fracture This fracture of the radial styloid was described originally because of the mechanism of injury, whereby the hand crank of early automobiles would backspin to strike the wrist. The fracture, if displaced, is treated with ORIF. If the fracture is displaced more than 3 mm, there may be an associated scapholunate dissociation, which may benefit from repair of the ligament as well as ORIF of the styloid (67,72). Accessory Bones Accessory bones, the os radiostyloideum and the os radiale externum, are located in the vicinity of the radial styloid (25,46) (see Fig. 1.27B). The os radiale externum is located slightly distal to the site of the os radiostyloideum. If present, these accessory bones can be mistaken for a fracture. An accessory bone usually represents the residual of a secondary ossification center that does not fuse with the associated bone, but it also may arise from trauma or from heterotopic ossification of synovial tags (46,47) (see description earlier, under Ossification Centers and Accessory Bones).

FIGURE 1.25. Skeletal hand and wrist, palmar aspect.

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FIGURE 1.26. Skeletal hand and wrist, dorsal aspect.

The four bones of the distal row articulate distally with the five metacarpal bones and with each other. The bones of the distal carpal row are straighter in alignment across the wrist than the proximal row, especially at their distal articulations with the metacarpal bones. On the dorsal surface of the carpus, a gentle convex arch is formed by the arrangement of the proximal and distal rows. On the palmar surface, however, a deep concavity if formed, designated the carpal groove. The carpal groove is accentuated by the palmar projection of the pisiform and hook of the hamate medially, and by the projection of the scaphoid tuberosity and trapezial ridge laterally. The midcarpal joint and the radiocarpal joint usually do not communicate with each other; if communication does occur, as seen through flow of dye from an arthrogram,

there is either a tear or incompetence of the scapholunate or lunotriquetral ligaments. The vascular supply to the carpus is through two main systems, the dorsal carpal vascular system and the palmar carpal vascular system (73) (see Fig. 1.29). The dorsal and palmar systems consist of a series of dorsal and palmar transverse arches that are connected by anastomoses formed by the radial, ulnar, and anterior interosseous arteries. The specific vascular patterns in each carpal bone (intraosseous vascularity) are described in the section on osseous anatomy (73). The ossification of the carpus may be quite variable (5) (see Fig. 1.26). The carpal bones usually are cartilaginous at birth, with the exception of the capitate and the hamate, where ossification already may be present. Each carpal bone ossifies from one center; the capitate usually is first and the pisiform usually last, but variability may exist in the order of ossification of the other carpal bones (74–76) (Fig. 1.27). The specifics of ossification of each carpal bone are discussed separately later. The carpus can be associated with several accessory ossicles (46) (see Fig. 1.27B; Table 1.2). In general, the development of these accessory bones is from an additional or anomalous secondary ossification center, and therefor the accessory bones are described later under sections on ossification. Accessory bones however, also can occur from other causes such as trauma (46) or heterotopic ossification of synovial tags (47). Anomalous, irregular ossicles or ossicles of abnormal size or shape thus may be encountered that do not fit a specific described accessory bone or location. In addition to accessory bones, congenital fusions (or coalitions) have been noted to occur in most of the carpal articulations (see Fig. 1.27C). Congenital coalitions are thought to occur either by the fusion of two ossification centers or by the nonseparation of two cartilage elements, resulting in one bone (46,77). SCAPHOID Derivation and Terminology The scaphoid (os scaphoideum, os naviculare manus, carpal navicular) derives its name from the Greek skaphe, which means “skiff ” or “light boat.” Scaphoid therefore denotes “boat-shaped” (1). The word navicular is derived from the Latin navicula, also indicating a boat. Ossification Centers and Accessory Bones The scaphoid usually has one ossification center (see Fig. 1.27A). It begins to ossify in the fourth year in girls, and the fifth year in boys (74). Occasionally, an additional ossification center fails to unite, forming an accessory ossicle, the os centrale (centrale dorsale, episcaphoid). The os centrale

A

B

FIGURE 1.27. A: Schematic illustrations showing times of ossification of the carpus and hand. B: Accessory ossicles of the wrist: schematic illustration of the carpus showing the various accessory bones and approximate locations. C: Possible sites for carpal coalitions. (B and C after O’Rahilly R. Developmental deviations in the carpus and the tarsus. Clin Orthop 10:9–18, 1957.)

C

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TABLE 1.2. ACCESSORY BONES OF THE WRIST Os capitatum secundarium (carpometacarpale V) Os centrale (centrale dorsale, episcaphoid) Os epilunatum (centrale II) Os epitrapezium Os epitrapezoideum (trapezoideum dorsale) Os epitriquetrum (epipyramis, centrale IV) Os gruberi (carpometacarpale VI) Os hamulare basale (carpometacarpale VII) Os hamuli proprium Os hypolunatum (centrale III) Os hypotriquetrum Os metastyloideum Os parastyloideum (carpometacarpale III) Os paratrapezium Os pisiforme secundarium (ulnare antebrachii, metapisoid) Os praetrapezium (carpometacarpale I) Os radiale externum (parascaphoid) Os radiostyloideum Os styloideum (carpometacarpale IV) Os subcapitatum Os trapezium secundarium (multangulum majus secundarium, carpometacarpale II) Os trapezoideum secundarium (multangulum minus secundarium) Os triangulare (intermedium antebrachii, triquetrum secundarium) Os ulnare externum Os ulnostyloideum Os vesalianum manus (vesalii, carpometacarpale VIII) From O’Rahilly (44–46).

occurs between the scaphoid, trapezoid, and capitate bones (see Fig. 1.27B). During the second prenatal month, it is a cartilaginous nodule usually fusing with the scaphoid. Besides the os centrale, an additional ossification center may give rise to two large portions of the scaphoid. If these fail to fuse, the result is a bipartite scaphoid (25). Bipartite scaphoids are rare, usually bilateral, and can be distinguished from a fracture by the smooth cortical edges, lack of history of trauma, and absence of displacement or degenerative changes (25). Several other accessory bones can be associated with the scaphoid. These accessory bones, if present, usually

are the result of secondary or additional ossification centers that do not fuse with the scaphoid. These include the os centrale, the os radiale externum (os parascaphoid), the os epitrapezium, os epilunatum (os centrale II), and the os radiostyloideum (see Fig. 1.27B) (25,46). The os centrale is located between the scaphoid, capitate, and trapezoid. The os radiale externum is located at the distal lateral margin of the scaphoid tubercle, adjacent to the trapezium. The os epitrapezium is located just distal to the site of the os radiale externum at the distal lateral aspect of the scaphoid in close proximity to the trapezium. The os epilunatum is located in the region between the scaphoid and lunate, at the more distal aspect of the scapholunate articulation. The os radiostyloideum is located in the vicinity of the radial styloid, slightly proximal to the lateral mid-portion of the scaphoid (46) (see Fig. 1.27B). Osteology of the Scaphoid The scaphoid is the largest bone of the proximal carpal row, located proximally and radially (Fig. 1.28; see Figs. 1.25, 1.26, 1.37, and 1.38). It consists internally of cancellous bone, surrounded by a cortical shell (see Fig. 1.28). The cortex of the distal pole (tuberosity) is relatively thick. The axis of the scaphoid is directed distally, laterally, and palmarly. It rests in a plane at approximately 45 degrees to the longitudinal axis of the wrist (67). Articular cartilage covers 80% of the surface (67). The major portions include the tuberosity (located palmarly and distally), the body, and the proximal pole. The central narrow portion of the body is the waist. The palpable scaphoid tuberosity is located at the base of the thenar eminence and usually is in line with the radial border of the long finger. The tuberosity extends palmarly, and is more readily palpable with the dorsiflexed wrist in radial deviation (which increases the palmar flexion of the scaphoid and thus directs the tuberosity into the palm, where is becomes easily palpable). When the wrist is ulnarly deviated, the palmar flexion of the scaphoid decreases, and thus the tuberosity is more difficult to palpate. The dorsal surface is rough, grooved, and narrower than the palmar

A

B FIGURE 1.28. Right scaphoid. A: Dorsal aspect. B: Palmar aspect.

1 Skeletal Anatomy

surface. A dorsal groove courses the entire length of the scaphoid, and provides for the attachment of ligaments and vessels. The rough dorsal area in the region of the waist contains small vascular foramina, more of which usually are located slightly distally (78). These foramina allow entrance of the vital dorsal ridge vessels, a leash of vessels that supply vascularity to the body and, through retrograde flow, to the proximal pole (73,79). The lateral surface, directed proximally and radially, is convex and covered with articular cartilage. The most medial surface, which articulates with the lunate (lunate surface), is located ulnarly, has a flat, semilunar shape, and contains a relatively small surface area for lunate articulation. It is covered with articular cartilage. The portion articulating with the capitate is large, concave, and faces distomedially, and is covered with articular cartilage. The most distal portion articulates with the trapezium and trapezoid. This distal portion is a continuous, slightly convex surface. This distal articulation usually has two parts or “facets,” separated by a small ridge. The presence and morphology of the articular facets is variable; in approximately 25% of specimens, there may be a palpable but not readily visually identifiable separation of the facets, and the two facets may not be distinguishable at all in approximately 19% (see below, Anomalies and Variations). Two distinct facets are present in at least 82% of specimens. The medial facet articulates with the trapezoid, and the lateral facet articulates with the trapezium. Each facet is covered with articular cartilage. The articular surfaces of the proximal portion of the scaphoid (including those articulating with the capitate, lunate, and distal radius) are all covered with articular cartilage, and thus do not provide any soft tissue attachments for vascularity. Hence, the vascular supply to the proximal pole is from retrograde flow from the dorsal ridge vessels located at the level of the waist.

Anomalies and Variations in Morphology of the Scaphoid There is anatomic variability in the morphology of the distal articular surface of the scaphoid that articulates with the trapezium and trapezoid. The joint may or may not contain two distinct facets. Viegas and coworkers have shown that in 81.2% of scaphoids studied, there was a distinctly separate facet for the trapezoid articulation and another distinct facet for the trapezium, with an interfacet ridge separating the two. The interfacet ridge was both visible and palpable in 56.4% of wrists. In 24.8% of wrists, the scaphoid was found to have a palpable, but not readily visually identifiable interfacet ridge. In the remaining 18.8% of wrists, the scaphoid had a smooth distal articular surface without a visually or palpably identifiable ridge between the area of trapezial or trapezoidal articulation on the scaphoid (80,81).

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Vascularity of the Scaphoid The scaphoid receives its vascular supply mainly from the radial artery. Vessels enter in the limited areas dorsally and palmarly that are nonarticular areas of ligamentous attachment (79,82–84) (Fig. 1.29). The dorsal vascular supply to the scaphoid accounts for 70% to 80% of the internal vascularity of the bone, all in the proximal region (79) (see Fig. 1.29A). On the dorsum of the scaphoid, there is an oblique ridge that lies between the articular surfaces of the radius and of the trapezium and trapezoid. The major dorsal vessels to the scaphoid enter the bone through small foramina located on this dorsal ridge (79,82,84,85). The dorsal ridge is in the region of the scaphoid waist. At the level of the intercarpal joint, the radial artery gives off the intercarpal artery, which immediately divides into two branches. One branch runs transverse to the dorsum of the wrist. The other branch runs vertically and distally over the index metacarpal. Approximately 5 mm proximal to the origin of the intercarpal vessel at the level of the styloid process of the radius, another vessel is given off that runs over the radiocarpal ligament to enter the scaphoid through its waist along the dorsal ridge. In 70% of specimens, the dorsal vessel arises directly from the radial artery. In 23%, the dorsal branch has its origin from the common stem of the intercarpal artery. In 7%, the scaphoid receives its dorsal blood supply directly from the branches of both the intercarpal artery and the radial artery. There are consistent major communications between the dorsal scaphoid branch of the radial artery and the dorsal branch of the anterior interosseous artery. No vessels enter the proximal dorsal region of the scaphoid through the dorsal scapholunate ligament, and no vessels enter through dorsal cartilaginous areas. The dorsal vessels usually enter the scaphoid through foramina located on the dorsal ridge at the level of the scaphoid waist. However, in a few of the studied specimens, the vessels enter just proximal or distal to the waist. The dorsal vessels usually divide into two or three branches soon after entering the scaphoid. These branches run palmarly and proximally, dividing into smaller branches to supply the proximal pole as far as the subchondral region. The palmar vascular supply accounts for 20% to 30% of the internal vascularity, all in the region of the distal pole (79,85) (see Fig. 1.29B). At the level of the radioscaphoid joint, the radial artery gives off the superficial palmar branch. Just distal to the origin of the superficial palmar branch, several smaller branches course obliquely and distally over the palmar aspect of the scaphoid to enter through the region of the tubercle (79,83). These branches, the palmar scaphoid branches, divide into several smaller branches just before penetrating the bone. In 75% of specimens, these arteries arise directly from the radial artery (79). In the remainder, they arise from the superficial palmar branch of the radial artery. Consistent anastomoses

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A FIGURE 1.29. A: Classic depiction of dorsal pericarpal arterial network.

exist between the palmar division of the anterior interosseous artery and the palmar scaphoid branch of the radial artery, when the latter arises from the superficial palmar branch of the radial artery. There are no direct communicating branches between the ulnar artery and the palmar branches of the radial artery that supply the scaphoid. Vessels in the palmar scapholunate ligament do not penetrate the scaphoid. The palmar vessels enter the tubercle and divide into several smaller branches to supply the distal 20% to 30% of the scaphoid. There are no apparent anastomoses between the palmar and dorsal vessels (79). Associated Joints The scaphoid articulates with five bones: the radius proximally, the lunate medially, the capitate medially and distally,

and the trapezoid and trapezium distally (see Figs. 1.25, 1.26, 1.28, 1.37, and 1.38). The proximal lateral portion of the scaphoid sits in the scaphoid fossa of the radius, forming the radioscaphoid joint. In the distal portion of the radioscaphoid joint, where the mid-lateral portion of scaphoid articulates with the radial styloid, the specific portion of the joint can be referred to as the styloscaphoid joint (descriptive because of its significance for arthritis and SLAC wrist). The articulation with the lunate, forming the scapholunate joint, has a relatively small surface area, in part because of the narrow crescent shape of the lunate, which may contribute to the difficulty in performing arthrodesis of this joint. The scaphocapitate articulation has a relatively large surface area, usually allowing successful arthrodesis of this joint. The distal articulation of the scaphoid with the trapezoid and trapezium is referred to as the triscaphe joint.

1 Skeletal Anatomy

B FIGURE 1.29. (continued) B: Classic depiction of palmar pericarpal arterial network. (A and B after Taleisnik J. The vascular anatomy of the wrist. In: Taleisnik J, ed. The wrist. New York: Churchill Livingstone, 1985:51–78.) AIA, anterior interosseous artery; DMCA, dorsal metacarpal artery; PF, perforating branches; PMA, palmar metacarpal artery; CPDA, common palmar digital artery; PDA, proper palmar digital artery. (continued on next page)

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Systems Anatomy

C

D FIGURE 1.29. (continued) C: Drawing of the arterial supply of the lateral aspect of the wrist. D: Schematic drawing of the dorsum of the wrist, showing vascular contributions to the carpal bones.

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FIGURE 1.29. (continued) E: Schematic drawing of the palmar aspect of the wrist, showing the vascular contributions to the carpal bones. (C–E after Gelberman RH, Panagis JS, Taleisnik J, et al. The arterial anatomy of the human carpus: part I. the extraosseous vascularity. J Hand Surg [Am] 8: 367, 1983.)

E

Muscle Origins and Insertions A small portion of the abductor pollicis brevis may originate from the palmar surface of the scaphoid tuberosity. (The major portion of origin of the abductor pollicis brevis usually is from the proximal part of the palmar surface of the trapezium.) A portion of the transverse carpal ligament also attaches to the medial portion of the scaphoid tuberosity (see Fig. 1.37). Clinical Correlations: Scaphoid The scaphoid is the most commonly fractured bone of the carpus (86). It is susceptible to fractures at any level [approximately 65% occur at the waist, 15% through the proximal pole, 10% through the distal body, 8% through the tuberosity, and 2% in the distal articular surface (67)]. Scaphoid fractures have a relatively high incidence of nonunion (8% to 10%), frequent malunion, and late sequelae of carpal instability and posttraumatic arthritis (67). The relatively small surface area of the scapholunate joint (due in part to the narrow crescent shape of the lunate) probably contributes to the difficulty in achieving operative arthrodesis of this joint. While, the relatively large surface area of the scaphocapitate joint facilitates successful operative arthrodesis.

Arthrodesis of the triscaphe joint stabilizes or “anchors” the distal portion of the scaphoid, and thus prevents collapse into palmar flexion, as is seen when there is disruption of the scapholunate ligaments. Therefore, triscaphe arthrodesis has been described for treatment of scapholunate instability. The retrograde vascularity of the scaphoid enters the dorsal waist through the dorsal ridge vessels (73,79), and these vessels should be protected during dorsal exposure of the scaphoid. Avascular necrosis of the proximal pole of the scaphoid is due to disruption of the retrograde vessels that supply the proximal pole. Preiser’s disease describes avascular necrosis of the scaphoid, usually occurring in the proximal pole (87,88). Accessory Bones Several accessory bones can be associated with the scaphoid and may be mistaken for fractures. An accessory bone usually represents the residual of a secondary ossification center that does not fuse with the associated bone, but it also may arise from trauma or from heterotopic ossification of synovial tags (46,47). The accessory bones associated with the scaphoid include the os centrale (located between the scaphoid, capitate, and trapezoid), the os radiale externum (located at the distal radial border of the scaphoid tuberosity), the os epitrapezium (located between the scaphoid and

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trapezium), os epilunatum (located between the scaphoid and lunate), and the os radiostyloideum (located near the radial styloid at the lateral border of the waist of the scaphoid; see Fig. 1.27B) (25,46) (see descriptions earlier, under Ossification Centers and Accessory Bones). The os centrale exists as a free bone in lower primates (25). The Bipartite Scaphoid A bipartite scaphoid may be mistaken for a fracture. A bipartite scaphoid arises from the failure of fusion of two significant ossification centers. It often is bilateral. The bipartite scaphoid may be distinguished from a fracture from the lack of trauma history, bilaterality, and absence of displacement or degenerative changes. It is possible to injure the bipartite scaphoid, resulting in pain and a radiographic appearance resembling a fracture. Symptoms from injury to a bipartite scaphoid usually resolve with a course of protection or immobilization. LUNATE (OS LUNATUM, SEMILUNAR)

epitriquetrum is located between the lunate, hamate, and triquetrum, just ulnar to the site of the os hypotriquetrum. The os triangulare is located between the lunate, triquetrum, and the distal ulna (46) (see Fig. 1.27B). Osteology of the Lunate The lunate is crescentic, concave distally and convex proximally (Fig. 1.30; see Figs. 1.25, 1.26, 1.37, and 1.38). It consists internally of cancellous bone, surrounded by a cortical shell (see Fig. 1.30A,B). The dorsal and palmar surfaces are rough for the attachment of carpal ligaments. The palmar surface is roughly triangular and is larger and wider than the dorsal portion. The smooth, convex proximal articular surface articulates with the lunate fossa of the distal radius and with a portion of the triangular fibrocartilage on its proximoulnar aspect. The lateral surface is crescent shaped, flat, and narrow, with a relatively narrow surface area with which it contacts the scaphoid. The medial surface is square or rectangular, fairly flat, and articulates with the triquetrum. The distal surface is deeply concave and articulates with the proximal portion of the capitate.

Derivation and Terminology The lunate derives its name from the Latin luna, meaning “moon” (1), and is so named because of its crescent or moon shape (as visualized on the lateral projection). The British literature may refer to the lunate as the semilunar, derived from semi, meaning “half ” or “partly,” and lunar, meaning “moon” (2). Ossification Centers and Accessory Bones The lunate is cartilaginous at birth. It usually has one ossification center that begins to ossify during the fourth year (74) (see Fig. 1.27A). Variation in the ossification has been noted, with ossification taking place at from 1.5 to 7 years of age in boys, and between 1 and 6 years of age in girls (89). Double ossification centers in the lunate also have been noted (90,91). Several accessory bones can be associated with the lunate. Accessory bones, if present, usually are the result of a secondary or additional ossification center that does not fuse with the associated bone. Those associated with the lunate include the os epilunatum (os centrale II), the os hypolunatum (os centrale III), the os hypotriquetrum, the os epitriquetrum (epipyramis, os centrale IV), and the os triangulare (os intermedium antebrachii, os triquetrum secundarium) (see Fig. 1.27B) (46). The os epilunatum is located between the lunate, scaphoid, and capitate, along the distal border of the scaphoid and lunate. The os hypolunatum is located between the lunate and the capitate, just ulnar to the site of the os epilunatum. The os hypotriquetrum is located in the vicinity of the lunate, capitate, proximal pole of the hamate, and the triquetrum. The os

Anomalies and Variations in Morphology of the Lunate Differences in lunate morphology have been discussed by Taleisnik, Zapico, Viegas, Shepherd, and others (85,92a–95). The lunate has been divided into three types, based on whether its proximal aspect is curved or angulated. The lunate shape is evaluated by measurements of the angle between the lateral scaphoid side and the proximal radial side of the lunate. The type I lunate has an angle greater than 130 degrees and is present in approximately 30% of those studied. The type I lunate has been associated with an ulnar minus wrist. The type II lunate has an angle of approximately 100 degrees, and is present in approximately 50%. The type III lunate has two distinct facets on the proximal surface, one that articulates with the radius and another that articulates with the triangular fibrocartilage. The type III lunate is the least common, present in approximately 18% (85). The separate ulnar facet on the proximal lunate, when present, has been noted to vary in size between subjects (93). Two types of lunate osseous morphology, based on the presence or absence of a medial facet for hamate articulation, have been noted and described by Viegas and coworkers, Burgess, and Sagerman et al. (81,94–97). A type I lunate is one in which there is no medial facet. Its reported incidence is between 27% and 34.5% (81,94–96). A type II lunate has a medial facet that articulates with the hamate. The reported incidence is between 65.5% and 73%. The size of the medial facet in the type II lunate ranges from a shallow, 1-mm facet to a deep, 6-mm facet. In the type II lunate with a large medial facet, there occasionally has been

1 Skeletal Anatomy

A

51

B

C FIGURE 1.30. Right lunate. A: Proximolateral aspect. B: Distomedial aspect. C: Patterns of intraosseous blood supply to the lunate (see text). (C after Gelberman RH, Bauman TD, Menon J, et al. The vascularity of the lunate bone and Kienbock’s disease. J Hand Surg [Am] 5:272, 1980.)

associated ridging on the capitate and hamate (81,95). When the facet is large, it is easily identifiable radiographically and can be distinguished easily from the type I lunate. However, when the medial facet is small in the type II lunate, it may be difficult to distinguish it from a type I lunate (81,97). With the type II lunate, carpal kinetics and kinematics are different than in wrists with the type I lunate. The type II lunate has been shown to be associated with an increased incidence of cartilage erosion on the proximal pole of the adjacent, articulating hamate (see later, under Clinical Implications).

the articular surfaces for the triquetrum and the capitate, there usually is a narrow strip of articular surface for articulation with the proximal portion of the hamate. A curved ridge separates the articular surfaces for the hamate and capitate. Contact with the hamate is maximized when the carpus is ulnarly deviated. Proximally, on the ulnar aspect of the proximal articular surface of lunate, the lunate articulates with a portion of the triangular fibrocartilage complex. Muscle Origins and Insertions There are no muscle origins or insertions on the lunate.

Associated Joints The lunate articulates with five bones: the radius, scaphoid, capitate, hamate, and triquetrum (see Figs. 1.25, 1.26, 1.30, 1.37, and 1.38). The lunate articulates with the radius on its proximal surface; it lies in the lunate fossa of the radius, located on the ulnar aspect of the distal radius. The lunate articulates with the scaphoid along the lunate’s radial surface, with a relatively small, crescent-shaped articular surface area. The lunate articulates with the capitate distally, where the proximal pole of the capitate sits in the distal, crescent-shaped articular surface of the lunate. The lunate articulates with the triquetrum medially. In this area, the articular surface of the lunate is rounded or oval. Between

Vascularity of the Lunate The lunate receives its blood supply from both palmar and dorsal sources or from the palmar aspect alone (see Fig. 1.29A,B). In 80% of specimens, the lunate receives nutrient vessels from both the palmar and dorsal surfaces. In 20% of specimens, it receives nutrient vessels from the palmar surface alone. Except for these relatively small dorsal and palmar surfaces, the lunate is covered by articular cartilage, and thus no other vessels enter the bone. The vessels entering the dorsal surface are from branches of the dorsal radiocarpal arch, the dorsal intercarpal arch, and occasionally from smaller branches of the dorsal branch of the anterior

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interosseous artery (73,98,99) (Fig. 1.29A). On the palmar aspect, the lunate nutrient vessels are supplied by the palmar intercarpal arch, the palmar radiocarpal arch, and communicating branches from the anterior interosseous artery and the ulnar recurrent artery (Fig.1.29B). The vessels that enter dorsally are slightly smaller than those entering palmarly. Major vessels branch proximally and distally after entering the bone and terminate in the subchondral bone. The dorsal and palmar vessels anastomose intraosseously just distal to the mid-portion of the lunate. The proximal pole has relatively less vascularity. There are three major intraosseous patterns. These patterns take the shape of the letters “Y,” “I,” or “X” (see Fig. 1.30C). The Y pattern is the most common, occurring in 59% of studied specimens. The stem of the “Y” is oriented dorsally or palmarly with equal frequency. The I pattern occurs in approximately 30% of specimens, and consists of a single dorsal and a single palmar vessel. The single dorsal and single palmar vessels anastomose in a straight line, thus forming the “I”-shaped pattern. The X pattern occurs in 10% of specimens and consists of two dorsal and two palmar vessels that anastomose in the center of the lunate, thus forming an “X” (73,98,99). In 20% of studied specimens, a single palmar supply was noted. This pattern consists of a single large vessel that enters on the palmar surface and branches in the lunate to provide the sole blood supply. Clinical Correlations: Lunate The lunate and triquetrum usually begin to ossify in the fourth and third years, respectively. Rarely, fusion of these two ossification centers occurs, resulting in lunotriquetral coalition. Of all of the carpal coalitions, lunotriquetral is one of the most common (44,46,100–104). Lunate ossification may be delayed in a variety of syndromes, including epiphyseal dysplasias and possible homocystinuria (81,105). Complete absence of the lunate also has been reported (106). The lunate has been divided into three types, based on whether its proximal aspect is curved or angulated (see earlier, under Anomalies and Variations). The lunate shape is evaluated by measurements of the angle between the lateral scaphoid side and the proximal radial side of the lunate. The type I lunate has an angle greater than 130 degrees and is present in approximately 30% of those studied. The type I lunate has been associated with an ulnar minus wrist. Two types of lunate morphology based on the presence or absence of a medial facet have been described by Viegas and coworkers (see earlier, under Osteology) (81,94,95, 107,108). The carpal kinetics and kinematics have been shown to be different in wrists with the two types of lunate (109). The type II lunate contains a medial facet for articulation with the hamate. This has been associated with an increased incidence of cartilage erosion with exposed bone

on the proximal pole of the hamate. These erosions usually are not identifiable by radiography. The incidence of hamate proximal pole erosions has been noted to be as high as 44% with the type II lunate (containing the medial facet articulating with the hamate). This is in contrast to the type I lunate (which contains no medial facet), in which hamate erosions or lesions were noted only in 0% to 2% (81, 95,107). A triangular shape of the lunate on radiographs may indicate a lunate dislocation, or tilting of the lunate in either direction (dorsiflexion or palmar flexion). Dislocation of the lunate (or perilunate dislocation) is the most common type of carpal dislocation. The relatively small contact surface area between the lunate and scaphoid (due, in part, to the narrow crescent shape of the lunate) probably contributes to the difficulty in achieving operative arthrodesis of the scapholunate joint. Although fractures through the central portion of the lunate are rare, loss of vascularity (Kienböck’s disease) is associated initially with increased radiodensity, followed by flattening or osseous collapse with fragmentation/fracture in the later stages (110–112). The Stages of Kienböck’s Disease (110,111) n Stage I: Normal appearance on radiographs, possible linear or compression fracture on tomogram. Avascular changes visualized on MRI. Bone scan shows abnormal uptake. n Stage II: Bone density changes (sclerosis), slight collapse of radial border. n Stage III: Fragmentation, collapse, cystic degeneration, loss of carpal height, capitate proximal migration, scaphoid rotation (scapholunate dissociation). n Stage IV: Advance collapse, scaphoid rotation, sclerosis, osteophytes of the radiocarpal joint. Accessory Bones Several accessory bones may be associated with the lunate and can be mistaken for fractures. An accessory bone usually represents the residual of a secondary ossification center that does not fuse with the associated bone, but it also may arise from trauma or from heterotopic ossification of synovial tags (46,47). The accessory bones associated with the lunate include the os epilunatum (located between the lunate, scaphoid, and capitate), the os hypolunatum (located between the lunate and capitate), the os hypotriquetrum (located between the lunate, capitate, proximal pole of the hamate, and the triquetrum), os epitriquetrum (located between the lunate, triquetrum, and proximal pole of the hamate), and the os triangulare (located between the lunate, triquetrum, and distal ulna; see Fig. 1.27B) (46) (see descriptions earlier, under Ossification Centers and Accessory Bones).

1 Skeletal Anatomy

TRIQUETRUM (OS TRIQUETRUM, TRIQUETRAL BONE, CUNEIFORM) Derivation and Terminology The name triquetrum is derived from the Latin for “threecornered” (1). The older British literature refers to the triquetrum as the cuneiform, derived from the Latin cuneus, meaning “wedge,” and forma, meaning “likeness” or “form” (2). Ossification Centers and Accessory Bones The triquetrum is cartilaginous at birth. It has one ossification center that begins to ossify during the third year (74) (see Fig. 1.27A). Several accessory bones can be associated with the triquetrum. Accessory bones, if present, are usually the result of an additional or secondary ossification center that does not fuse with the associated bone. Those associated with the triquetrum include the os hypotriquetrum, the os epitriquetrum (os epipyramis, os centrale IV), the os triangulare (os intermedium antebrachii, os triquetrum secundarium), and the os ulnare externum (46) (see Fig. 1.27B). The os hypotriquetrum is located in the vicinity of the triquetrum, lunate, capitate, and the proximal pole of the hamate. The os epitriquetrum is located between the triquetrum, lunate, and proximal hamate, just ulnar to the site of the os hypotriquetrum. The os triangulare is located between the triquetrum, lunate, and the distal ulna (46) (see Fig. 1.27B).

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medial surface of the hamate. The dorsal surface is rough for the attachments of carpal ligaments. The palmar surface contains two regions: medial and lateral. On the medial region of the palmar surface is the articular surface for the pisiform. This relatively small articular surface is round or oval. The lateral portion of the palmar surface is rough and nonarticular and provides attachments for carpal ligaments. The lateral surface of the triquetrum forms the base of the pyramid, which is flat and quadrilateral, for articulation with the lunate. The medial and dorsal surfaces may be somewhat confluent. The medial surface is the pointed summit of the pyramid, and provides attachment for the ulnar collateral ligament of the wrist. Associated Joints The triquetrum articulates with three bones: the lunate, the pisiform, and the hamate (see Figs. 1.25, 1.26, 1.31, 1.37, and 1.38). The articulation with the lunate on the radial surface of the triquetrum is roughly square or rectangular, or oval. The triquetrum articulates with the pisiform palmarly. The articular surface for the pisiform is round or oval in shape. The articulation with the hamate is based distally and slightly radially. The articular surface for the hamate is smooth, curved, and slightly oval or triangular, extending along the distoradial surface of the triquetrum. Muscle Origins and Insertions There are no muscle origins or insertions on the triquetrum.

Osteology of the Triquetrum The triquetrum is pyramid-shaped and located on the proximoulnar aspect of the carpus (Fig. 1.31; see Figs. 1.25, 1.26, 1.37, and 1.38). Internally, the triquetrum consists of cancellous bone, surrounded by a cortical shell (see Fig. 1.31). The triquetrum has several surfaces, including the proximal, distal, lateral, dorsal, and palmar. The proximal surface faces slightly medially, and contains both a rough, nonarticular portion, and a lateral, slightly convex articular portion that may “articulate” with the triangular fibrocartilage complex. The distal surface is directed laterally and contains both concave and convex surface portions. The distal surface is curved and smooth for articulation with the

FIGURE 1.31. Right triquetrum. Distoradial aspect.

Vascularity of the Triquetrum The triquetrum receives its blood supply from branches from the ulnar artery, the dorsal intercarpal arch, and the palmar intercarpal arch (see Fig. 1.29A,B). Nutrient vessels enter from the intercarpal arches and pass through its two nonarticular surfaces, on the dorsal and palmar aspects. The dorsal surface of the triquetrum is rough for attachments of associated carpal ligaments. This dorsal surface contains a ridge that runs from the medial to the lateral aspect. Two to four vessels enter this dorsal ridge and radiate in multiple directions to supply the dorsal 60% of the bone. This network is the predominant blood supply to the triquetrum in 60% of specimens (73,99). The palmar surface contains an oval facet that articulates with the pisiform. One or two vessels enter proximal and distal to the facet. The vessels have multiple anastomoses with each other and supply the palmar 40% of the bone. This palmar vascular network is predominant in 20% of specimens (99). Significant anastomoses between the dorsal and the palmar vascular networks have been found in 86% of specimens studied (99).

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Clinical Correlations: Triquetrum The triquetrum and the lunate usually begin to ossify in the third and fourth years, respectively. Rarely, fusion of these two ossification centers occurs, resulting in lunotriquetral coalition. Of all of the carpal coalitions, lunotriquetral is one of the most common (44,46,100–104). Fractures of the triquetrum result from a direct blow or from an avulsion injury that may include ligament damage. The most common fracture is probably the impingement shear fracture of the ulnar styloid against the dorsal triquetrum, occurring with the wrist in extension and ulnar deviation, particularly when a long ulnar styloid is present (97,113,114). An avulsion component also may be present. A small bone fragment located dorsal to the triquetrum is seen best on the lateral radiograph. Accessory Bones Several accessory bones may be associated with the triquetrum and can be mistaken for fractures. An accessory bone usually represents the residual of a secondary ossification center that does not fuse with the associated bone, but it also may arise from trauma or from heterotopic ossification of synovial tags (46,47). The accessory bones associated with the triquetrum include the os hypotriquetrum (located between the triquetrum, lunate, capitate and the proximal pole of the hamate), the os epitriquetrum (located between the triquetrum, lunate, and proximal pole of the hamate, just ulnar to the site of the os hypotriquetrum), the os triangulare (located between the proximal triquetrum, lunate, and the distal ulna), and the os ulnare externum (located at the distal end of the triquetrum and adjacent to the ulnar border of the distal hamate; see Fig. 1.27B) (46) (see descriptions earlier, under Ossification Centers and Accessory Bones). PISIFORM (OS PISIFORME) Derivation and Terminology The name pisiform is derived from the Latin pisum, meaning “pea,” and forma, meaning “likeness,” “shape,” or “form” (1). Pisiform thus denotes “pea shaped.” Ossification Centers and Accessory Bones The pisiform is cartilaginous at birth. It has one ossification center that begins to ossify in the ninth or tenth year in girls, and in the twelfth year in boys (74) (see Fig. 1.27A). It usually is the last carpal bone to ossify (5). There is an accessory bone that can be associated with the pisiform. The os pisiforme secundarium, also known as the os ulnare antebrachii or the os metapisoid, is located at the proximal pole of the pisiform (46) (see Fig. 1.27B). The os pisiforme secundarium, if present, usually is the result of

an additional, secondary ossification center that does not fuse with the pisiform. Osteology of the Pisiform The pisiform is the smallest carpal bone. It is situated at the base of the hypothenar eminence on the medial side of the wrist (Fig. 1.32; see Figs. 1.25 and 1.37). It lies palmar to the triquetrum, in a plane palmar to the other carpal bones. The pisiform actually is a sesamoid bone in the tendon of the flexor carpi ulnaris. It consists internally of cancellous bone, surrounded by a cortical shell (see Fig. 1.32). It is generally spherical, although there is a slight long axis in the distolateral direction (4,5). The pisiform is flat on its dorsal surface, where the only articular surface is located. It articulates only with the triquetrum. The pisotriquetral joint is not a portion of the radiocarpal joint, and there usually is not a communication between these joints. The palmar surface of the pisiform is round and rough, and provides attachments for the flexor carpi ulnaris (proximally) and the abductor digiti minimi (distally). The lateral and medial surfaces are rough. The lateral surface usually contains a shallow groove that lies adjacent to the ulnar artery. Associated Joints The pisiform articulates with the triquetrum dorsally (see Figs. 1.25, 1.32, and 1.37). This articular facet is flat and oval, and is located slightly proximal on the dorsal surface. Muscle Origins and Insertions The flexor carpi ulnaris inserts onto the proximal palmar edge of the pisiform, forming a crescent-shaped insertion that is convex proximally and concave distally. The abductor digiti minimi (quinti) originates on the distal portion of the pisiform, forming an oval origin area. The pisiform is enclosed in these myotendinous structures (see Fig. 1.37). There are no muscle origins or insertions on the dorsal surface of the pisiform. Vascularity of the Pisiform The pisiform receives its blood supply through the proximal and distal poles from branches of the ulnar artery (see Fig. 1.29A,B). The pisiform is a sesamoid bone in the tendon of the flexor carpi ulnaris. The tendon attaches to the pisiform proximally, and the proximal blood supply enters in this area. One to three vessels penetrate inferior to the triquetral

FIGURE 1.32. Right pisiform. Dorsal aspect.

1 Skeletal Anatomy

facet. These proximally entering vessels divide into multiple branches. Two superior branches run parallel beneath the articular surface of the facet, and one or two inferior branches run along the palmar cortex and anastomose with the superior branches (99). The distal vascular supply includes one to three vessels that enter inferior to the articular facets, divide into superior and inferior branches, and run parallel to the palmar cortex. These distally entering vessels anastomose with the proximal vessels. The superior vessels run deep to the articular facet and communicate with the proximal superior vessels, forming an arterial ring deep to the facet. There are multiple anastomoses between the proximal and the distal vascular networks. Clinical Correlations: Pisiform Fracture of the pisiform can occur with a fall on the dorsiflexed, outstretched hand. Avulsion of its distal portion with a vertical fracture can occur from a direct blow while the pisiform is held firmly against the triquetrum under tension from the flexor carpi ulnaris (67,113,115). Accessory Bones There is an accessory bone that can be associated with the pisiform, the os pisiforme secundarium (Fig. 1.27B). It is located at the proximal pole of the pisiform, and, if not appreciated, it may be mistaken for a fracture. An accessory bone usually represents the residual of a secondary ossification center that does not fuse with the associated bone, but it also may arise from trauma or heterotopic ossification of synovial tags (46,47). HAMATE (OS HAMATUM, UNCIFORM) Derivation and Terminology Hamate is derived from the Latin hamulus, meaning “hook,” and hamatum, meaning “hooked” (1). The hamate also may be referred to as the unciform bone, derived from the Latin uncus, also meaning “hook,” and forma, meaning “likeness,” “shape,” or “form” (2).

55

Ossification Centers and Accessory Bones The hamate is cartilaginous at birth. It has one ossification center that begins to ossify at the end of the third month. Of all the carpal bones, the hamate usually is the second to ossify (after the capitate) and, on occasion, ossification already may have started at birth (5,74–76) (see Fig. 1.27A). Several accessory bones can be associated with the hamate. Accessory bones, if present, usually are the result of a secondary or additional ossification center that does not fuse with the associated bone. Those associated with the hamate include the os hamuli proprium, os hamulare basale (carpometacarpale VII), os hypotriquetrum, os epitriquetrum (os epipyramis, os centrale IV), os ulnare externum, os vesalianum manus (os vesalii, os carpometacarpale VIII), os gruberi (os carpometacarpale VI), and os capitatum secundarium (carpometacarpale V) (see Fig. 1.27B) (46). The os hamuli proprium is a secondary ossification center in the hook of the hamate that does not fuse with the body. It is located in the palmar aspect of the mid-body, where the hook usually is located. The os hamulare basale is located between the distal body of the hamate and the base of the ring finger metacarpal. The os hypotriquetrum is located proximal to the proximal pole of the hamate, adjacent to the lunate, capitate, and triquetrum. The os epitriquetrum is located proximal to the proximal pole of the hamate, adjacent to the triquetrum and lunate, just ulnar to the site of the os hypotriquetrum. The os ulnare externum is located ulnar to the distal body of the hamate, distal to the triquetrum. The os vesalianum manus is located proximal to the small finger metacarpal, near the styloid. The os gruberi is located at the radiodistal margin of the body of the hamate, between the hamate, capitate, and the base of the ring and base of the long finger metacarpals. The os capitatum secundarium is located just radial to the site of the os gruberi, at the radiodistal margin of the hamate body and between the capitate and bases of the ring and long finger metacarpals (46) (see Fig. 1.27B). Osteology of the Hamate The hamate consists of a body, a proximal pole, and a hook (hamulus; Fig. 1.33; see Figs 1.25, 1.26, 1.37, and 1.38). It

B

A FIGURE 1.33. Right hamate. A: Medial aspect. B: Inferolateral aspect.

56

Systems Anatomy

consists internally of cancellous bone, surrounded by a cortical shell (see Fig. 1.33). The hamate is an irregularly shaped bone with an unciform hamulus (hook). The hook is located on the distal portion of the palmar surface, slightly closer to the medial aspect. The hook projects palmarly from the rough palmar surface. The hook is slightly curved, with its convexity medial and concavity lateral. The tip of the hook has a slight lateral inclination and serves as a point of attachment for a portion of the transverse carpal ligament. The hook of the hamate and the pisiform contribute to the medial wall of the carpal tunnel. The convex (medial) side of the hook is rough. The concave (lateral) side is smooth where the adjacent flexor tendons to the small finger pass. At the base of the hook, on the medial side, there may be a slight transverse groove in which the terminal deep branch of the ulnar nerve may contact as it passes distally. The body of the hamate is somewhat triangular or cuneiform (wedge shaped), with a wide distal portion and a narrowing into an apex proximolaterally. The dorsal and palmar surfaces of the body are largely nonarticular, and are rough for attachments of the carpal ligaments. The distal, wide surface of the hamate consists of the articular surfaces for the base of the small and ring finger metacarpals. The articular surface thus has two facets, one for each metacarpal, separated by a slight intraarticular ridge. The facet for the ring finger metacarpal is smaller than that for the small finger metacarpal. The proximal surface narrows into a thin margin of the wedge-shaped body. At the tip of the proximal surface there usually is a small, narrow facet for articulation of the lunate. The hamate may be in contact with the lunate only during ulnar deviation of the wrist. The medial surface of the body of the hamate is broad and somewhat rectangular. In contains the relatively large articular surface for articulation with the triquetrum. The surface is curved, with a convexity proximally that becomes concave distally. At the distal aspect of the medial side of the body, there is a narrow medial strip that is nonarticular. On the lateral surface of the body of the hamate, the relatively large surface is nearly completely articular, with the exception of a small area on the distal palmar angle. The proximal portion or the lateral aspect is convex, and the distal portion is slightly concave. The lateral aspect articulates with the capitate. Associated Joints The hamate articulates with five bones: the triquetrum, the capitate, the base of the ring and small finger metacarpals, and the small articulation with the lunate (see Figs. 1.25, 1.26, 1.33, 1.37, and 1.38). The hamate articulation with the triquetrum is along the proximal and medial aspects, through a relatively large, oval-shaped articular surface area. The hamate articulates with the capitate along its lateral surface, also involving a relatively large, oval articular surface area. The hamate articulates with the base of the

metacarpals through two facets, one to the small and one to the ring finger. The articulation with the small finger metacarpal usually involves a much larger articular facet. In addition, the very most proximal portion articulates with the lunate, especially when the wrist is ulnarly deviated. Muscle Origins and Insertions The opponens digiti minimi and flexor digiti minimi originate from the palmar ulnar surface of the hook of the hamate (see Figs. 1.37 and 1.38). In addition, a small portion of the flexor carpi ulnaris may insert into the palmar aspect of the hamate (the major insertion of the flexor carpi ulnaris is into the proximal portion of the palmar surface of the pisiform) (2). There are no muscle origins or insertions on the dorsal surface of the hamate. Vascularity of the Hamate The vascularity of the hamate is supplied from three main sources: the dorsal intercarpal arch, the ulnar recurrent artery, and the ulnar artery (see Fig. 1.29A,B). The vessels enter through the three nonarticular surfaces of the hamate, which include the dorsal surface, the palmar surface, and the medial surface through the hook of the hamate. These nonarticular surfaces of the hamate are somewhat rough for attachment of carpal ligaments. The dorsal surface is triangular in shape and receives three to five vessels. These branch in several directions to supply the dorsal 30% to 40% of the bone (73,99). Small foramina usually are easily visible on the dorsal surface. The palmar surface also is triangular and usually receives one large vessel that enters through the radial base of the hook. It then branches and anastomoses with the dorsal vessels in 50% of studied specimens (73,99). The hook of the hamate receives one or two small vessels that enter through the medial base and tip of the hook. These vessels anastomose with each other but usually not with the vessels to the body of the hamate. Clinical Correlations: Hamate Fracture of the hook of the hamate often occurs in sportsrelated use of clubs, bats, or racquets (116). Direct force exerted by these objects against the hypothenar eminence or transverse carpal ligament has been implicated (67,116). Fracture of the hook of the hamate often is not visible on standard radiographs. It may be visualized with the carpal tunnel view. Alternatively, trispiral, computed tomography or MRI may show difficult-to-visualize fractures. Untreated displaced fractures of the hook of the hamate may lead to attrition rupture of the flexor tendons to the small finger because these tendons pass against the hook and can be subject to wear from contact and friction against

1 Skeletal Anatomy

a jagged fracture surface. A patient with a hook of the hamate fracture may perceive pain on the dorsum of the hamate and palpation over the hook on the palmar side usually elicits tenderness. The incidence and location of arthrosis and chondromalacia (with cartilage erosions and exposed subchondral bone) is among the highest at the proximal pole of the hamate. Chondromalacia was found in 16.8%; arthrosis with exposed subchondral bone was found in 28.2% (94). Arthrosis at the proximal pole of the hamate also is associated with the presence of a mid-carpal plica. A mid-carpal plica was identified in 1% of 393 wrists. All wrists that had a mid-carpal plica also were found to have arthrosis at the proximal pole of the hamate (94). Accessory Bones Several accessory bones may be associated with the hamate and can be mistaken for fractures (Fig. 1.27B). An accessory bone usually represents the residual of a secondary ossification center that does not fuse with the associated bone, but it also may arise from trauma or heterotopic ossification of synovial tags (46,47). The accessory bones associated with the hamate include the os hamuli proprium (located in the area of the hook), the os hamulare basale (located at the distal margin of the hamate, in the vicinity of the bases of the long and ring finger metacarpals), the os hypotriquetrum (located proximal to the proximal pole of the hamate, adjacent of the lunate, capitate, and triquetrum), the os epitriquetrum (located proximal to the proximal pole of the hamate, in the vicinity of the lunate, capitate, and triquetrum, just ulnar to the site of the os hypotriquetrum), the os ulnare externum (located ulnar to the body of the hamate, just distal to the triquetrum), the os vesalianum manus (locate ulnar and slightly distal to the hamate, near the styloid process of the base of the small finger metacarpal), the os gruberi (located at the distoradial corner of the hamate, adjacent to the capitate and bases of the long and ring finger metacarpals), and os capitatum secundarium (located at the distoradial corner of the hamate, adjacent to the capitate and bases of the long and ring finger metacarpals, just radial to the site of the os gruberi; see Fig. 1.27B) (46) (see descriptions earlier, under Ossification Centers and Accessory Bones). CAPITATE (OS CAPITATUM, OS MAGNUM)

57

Ossification Centers and Accessory Bones The capitate usually is cartilaginous at birth. It has one ossification center that begins to ossify in the second month. Of all the carpal bones, the capitate (or hamate) usually is the first to ossify, and occasionally ossification already may have started at birth (5,74–76) (see Fig. 1.27A). Several accessory bones can be associated with the capitate. Accessory bones, if present, usually are the result of a secondary or additional ossification center that does not fuse with the associated bone. Those associated with the capitate include the os subcapitatum, os capitatum secundarium (carpometacarpale V), os gruberi (os carpometacarpale VI), os hypotriquetrum, os epitriquetrum (epipyramis, os centrale IV), os hypolunatum (os centrale III), os epilunatum (os centrale II), os centrale (os centrale dorsale, os episcaphoid), os metastyloideum, os parastyloideum (os carpometacarpale III), and os styloideum (carpometacarpale IV) (see Fig. 1.27B) (25,46). The os subcapitatum is located adjacent to the central portion of the body of the capitate. The os capitatum secundarium is located at the distoulnar corner of the capitate, adjacent to the distal hamate, and the bases of the longer and ring finger metacarpals. The os gruberi is located just ulnar to the site of the os capitatum secundarium, at the distoulnar corner of the capitate and adjacent to the bases of the ring and long metacarpals. The os hypotriquetrum is located ulnar to the base of the capitate, proximal to the proximal pole of the hamate, and adjacent to the triquetrum and lunate. The os epitriquetrum is located just ulnar to the site of the os hypotriquetrum, proximal to the proximal pole of the hamate, and adjacent to the triquetrum and lunate. The os hypolunatum is located just proximal to the proximal margin of the capitate, between the lunate and adjacent to the proximal pole of the scaphoid. The os epilunatum is located between the capitate, lunate, and scaphoid, just radial to the site of the os hypolunatum. The os centrale is located between the capitate, scaphoid, and trapezoid. The os metastyloideum is located at the distoradial aspect of the capitate, between the trapezoid and base of the index finger metacarpal. The os parastyloideum is located at the distoradial aspect of the capitate, slightly distal to the site for the os metastyloideum, between the capitate and base of the index and long finger metacarpals. The os styloideum is located at the distal aspect of the capitate, just ulnar to the site for the os parastyloideum, between the capitate and the base of the index and long finger metacarpals (46) (see Fig. 1.27B).

Derivation and Terminology The name capitate is derived from the Latin caput, meaning “head.” Capitate denotes “head-shaped” (1). It also has been suggested that the word capitate indicates the “head” of the wrist because it is the largest bone of the carpus. The older British literature may refer to the capitate as the os magnum, derived from magnum, indicating “large” (2).

Osteology of the Capitate The capitate is the largest and centrally located carpal bone, containing articulations with the lunate, scaphoid, trapezoid, the long, index, and ring finger metacarpals, the hamate, and the triquetrum (Fig. 1.34; see Figs. 1.25, 1.26,

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Systems Anatomy

B

A FIGURE 1.34. Right capitate. A: Medial aspect. B: Lateral aspect.

1.37, and 1.38). It consists internally of cancellous bone, surrounded by a cortical shell (see Fig. 1.34). It is elongated in the proximo distal direction, and thus contains a longitudinal axis. There is a slight concavity to the dorsal, radial, and ulnar surfaces, thereby producing a “waist” that is narrowed and located slightly proximal to the transverse midline. The dorsal surface is larger than the palmar surface. Both are rough for attachment of carpal ligaments. The palmar surface is flat or slightly convex. The proximal pole is rounded. The distal end is flattened with slightly squared corners on the medial and lateral aspects. The distal surface, which is transverse to its axis, is triangular (apex located palmarly), with both a concave and a convex component. The distal articulation is mainly with the base of the long finger metacarpal. There are slight variations as to the specific articulations distally (see later, under Anomalies and Variations). The medial and lateral borders are somewhat concave. The lateral border usually has a narrow concave strip for the medial side of the base of the index metacarpal. The dorsal medial angle of the distal aspect usually (approximately 86% of wrists) has a facet for the articulation with the base of the ring finger metacarpal. This small facet may be absent in 14% (81,94,117). The relatively large head of the capitate, consisting of the proximal rounded pole, projects into the concavity formed by the lunate and scaphoid. The proximal surface articulates with the lunate and the proximal portion of the lateral surface articulates with the scaphoid. Along the distolateral surface, there is a separate facet for the trapezoid. This facet may be separated from the facet for the scaphoid by a rough interval. The medial surface of the capitate has a relatively large, concave facet for the hamate. Anomalies and Variations in Morphology of the Capitate The distal aspect of the capitate articulates mainly with the base of the long finger metacarpal. In 84% to 86% of wrists, the capitate also has a small, narrow facet for articu-

lation with the base of the ring finger metacarpal (94,117,118). The capitate–ring finger metacarpal articulation, when present, usually is easily identifiable on standard radiographs (118). A separate facet for articulation with the ring finger metacarpal was found to be absent on the capitate in 14% of wrists (81,94,117). Associated Joints The capitate articulates with seven bones, largely with the lunate, scaphoid, trapezoid, the base of the long finger metacarpal, and the hamate (see Figs. 1.25, 1.26, 1.34, 1.37, and 1.38). There are smaller articulations with the base of the index and ring finger metacarpals, and, with the wrist in certain positions (radial deviation), with the triquetrum. The capitate articulates with the lunate proximally, where the capitate’s proximal pole sits deep in the crescent-shaped fossa of the lunate, forming a major portion of the mid-carpal joint. The capitate also articulates with the scaphoid proximally and radially; the articular surface of the capitate is irregular and somewhat oval, and encompasses the proximal portion of the lateral border of the capitate. The capitate articulates with the trapezoid on the distal portion of its lateral border through a relatively small articular surface area. Distally, the capitate articulates largely with the base of the long finger metacarpal. On the distal radial corner of the capitate, there is a smaller articulation with the ulnar proximal corner of the base of the index metacarpal. Along a small strip of the distal ulnar corner of the capitate, there also is a narrow articulation with the radial proximal corner of the base of the ring finger metacarpal. (Thus, the capitate articulates with three metacarpals: the index, long, and ring fingers.) Along the entire concave ulnar border of the capitate, there is a long, somewhat ovoid articulation with the body and proximal pole of the hamate. At the proximal ulnar border of the capitate there is a potential small articulation with the triquetrum when the wrist is radially deviated.

1 Skeletal Anatomy

59

Muscle Origins and Insertions

Accessory Bones

Approximately half of the oblique head of the adductor pollicis (adductor pollicis obliquus) originates from the distal radial part of the palmar surface of the capitate (see Figs. 1.37 and 1.38). The base of the long finger metacarpal serves for the other, distal half of the origin of the oblique head; the trapezoid also may contain a small portion of the origin of the oblique head of the adductor pollicis. There are no muscle origins or insertions on the dorsal surface of the capitate.

Several accessory bones may be associated with the capitate and can be mistaken for fractures. An accessory bone usually represents the residual of a secondary ossification center that does not fuse with the associated bone, but it also may arise from trauma or from heterotopic ossification of synovial tags (46,47). The accessory bones associated with the capitate include the os subcapitatum (located adjacent to the distal body), the os capitatum secundarium (located between the capitate and bases of the long and ring finger metacarpals), the os gruberi (located between the capitate and bases of the ring and long finger metacarpals, just ulnar to the site for the os capitatum secundarium), the os hypotriquetrum (located between the capitate, proximal pole of the hamate, triquetrum, and lunate), the os epitriquetrum (located between the capitate, proximal pole of the hamate, triquetrum, and lunate, just ulnar to the site for the os hypotriquetrum), the os hypolunatum (located between the capitate, lunate, and scaphoid, just ulnar to the site of the os epilunatum), the os epilunatum (located between the capitate, lunate, and scaphoid), the os centrale (located between the capitate, scaphoid, and trapezoid), the os metastyloideum (located between the capitate, trapezoid, and base of the index finger metacarpal), the os parastyloideum (located between the capitate and bases of the index and long finger metacarpals), and the os styloideum (located between the capitate and bases of the index and long finger metacarpals, just ulnar to the site for the os parastyloideum; see Fig. 1.27B) (46) (see descriptions earlier, under Ossification Centers and Accessory Bones).

Vascularity of the Capitate The capitate receives its vascularity from both dorsal and palmar sources. The main vascularity originates from vessels from the dorsal intercarpal and dorsal basal metacarpal arches, as well as from significant anastomoses between the ulnar recurrent and palmar intercarpal arches (see Fig. 1.29A,B). The vessels that enter the capitate penetrate through the two nonarticular surfaces on the dorsal and palmar aspects of the bone. The dorsal surface of the capitate is rough for attachments of the dorsal carpal ligaments. The dorsal surface is broad, relatively wide, and contains a deeply concave portion. Two to four nutrient vessels enter the distal two-thirds of the dorsal concavity. Smaller vessels occasionally enter more proximally, near the neck. Multiple small foramina usually are visible in this dorsal portion of the capitate. The entering dorsal vessels course palmarly, proximally, and ulnarly within the capitate in a retrograde fashion to supply the body and head. This dorsal supply continues palmarly and proximally, eventually reaching the vicinity of the convex rough palmar surface. Terminal vessels reach the proximal palmar head and terminate just deep to the articular surface (73,99). The palmar vascular contribution is through one to three vessels. These vessels enter the palmar surface on the distal half of the capitate and course proximally in a retrograde fashion. Small foramina may be visible in this palmar area of the capitate. In 33% of studied specimens, the vascularity to the capitate head originated entirely from the palmar surface. There are notable anastomoses between the dorsal and the palmar blood supplies in 30% of specimens studied (73,99).

TRAPEZOID (OS TRAPEZOIDEUM, OS MULTANGULUM MINUS, LESSER MULTANGULAR) Derivation and Terminology The name is derived from the Latin trapezoides and the Greek trapezoeides, both indicating “table-shaped.” This has been extrapolated to denote a four-sided plane, with two sides parallel and two diverging (1). The word multangular pertains to “many-sided.” Ossification Centers and Accessory Bones

Clinical Correlations: Capitate The capitate is rarely fractured because of its protected position in the carpus. The “naviculocapitate syndrome” consists of fracture of the capitate and the scaphoid, with the proximal capitate fragment rotated 90 to 180 degrees. The articular surface thus is displaced anteriorly or faces the fracture surface of the capitate neck (119). (Also known as scaphocapitate syndrome.)

The trapezoid is cartilaginous at birth. It has one ossification center that begins to ossify during the fourth year in girls and in the fifth year in boys (74) (see Fig. 1.27A). Several accessory bones can be associated with the trapezoid. Accessory bones, if present, usually are the result of a secondary or additional ossification center that does not fuse with the associated bone. Those associated with the trapezoid include the os trapezoideum secundarium (mul-

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tangulum minus secundarium), the os metastyloideum, the os centrale (centrale dorsale, episcaphoid), and the os trapezium secundarium (multangulum majus secundarium, carpometacarpale II) (see Fig. 1.27B) (46). The os trapezoideum secundarium is located at the distal radial corner of the trapezoid, between the trapezoid and the radial base of the index finger metacarpal. The os metastyloideum is located at the distal ulnar corner of the trapezoid, between the trapezoid and the ulnar base of the index finger metacarpal. The os centrale is located between the trapezoid, scaphoid, and capitate. The os trapezium secundarium is located at the radial margin of the trapezoid, between the trapezoid, trapezium, and base of the thumb and index metacarpals (46) (see Fig. 1.27B). Osteology of the Trapezoid The trapezoid is a small, irregular carpal bone, with somewhat of a mushroom, wedge, or T-shape, larger dorsally than palmarly (Fig. 1.35; see Figs. 1.25, 1.26, 1.37, and 1.38). It consists internally of cancellous bone, surrounded by a cortical shell (see Fig. 1.35). The trapezoid is the smallest bone in the distal carpal row. When viewed dorsally, the dorsal surface is oval, elongated in the radioulnar direction. Its dorsal surface is rough. The smaller palmar portion is a projection from the wide dorsal portion, connecting to the dorsal portion slightly laterally. When viewed palmarly, the palmar portion is round or slightly squared. The distal surface articulates with a groove in the base of the index metacarpal. The distal surface is triangular, with the apex palmar. This distal articular surface is convex, containing two smaller concave facet-like surfaces located radially and ulnarly. The medial surface articulates with the distal, radial part of the capitate. The medial articular surface on the trapezoid is narrow and concave from dorsal to palmar. The narrow lateral surface of the trapezoid is convex and smooth and articulates with the trapezium. The proximal portion articulates with the scaphoid tuberosity articular surface, forming the ulnar facet of the triscaphe joint. Associated Joints The trapezoid articulates with four bones: the base of the index finger metacarpal, the capitate, the scaphoid, and the trapezium (see Figs. 1.25, 1.26, 1.35, 1.37, and 1.38).

Along its distal surface, the trapezoid articulates with base of the index metacarpal, where the trapezoid sits in a groove of the metacarpal. The trapezoid articulates along its ulnar border with the capitate, where the trapezoid contains a small rectangular facet on the ulnar aspect near the palmar surface. The trapezoid articulates proximally with the scaphoid, forming the ulnar component of the triscaphe joint. The trapezoid also articulates radially with the trapezium, where a convex surface of the lateral border of the trapezoid sits in a concave articular surface of the trapezium. The four articular surfaces of the trapezoid all connect with each other, each separated by a relatively sharp edge. Muscle Origins and Insertions The trapezoid gives origin to one, and possibly two muscles: the deep head of the flexor pollicis brevis, and, variably, to a small portion of the origin of the adductor pollicis (oblique head; see Figs. 1.37 and 1.38). The deep head of the flexor pollicis brevis originates from the palmar aspect of the trapezoid. (The superficial head originates from the transverse carpal ligament and from the palmar aspect of the trapezium.) The flexor pollicis brevis inserts into the radial sesamoid and into the radial aspect of the base of the proximal thumb metacarpal. A small portion of the origin of the adductor pollicis oblique head (adductor pollicis obliquus) may originate from the distal ulnar corner of the palmar surface of the trapezoid. (The major origins of the adductor pollicis obliquus are from the base of the long metacarpal and distal portion of the palmar surface of the capitate.) There are no muscle origins or insertions on the dorsal surface of the trapezoid. Vascularity of the Trapezoid The trapezoid is supplied by branches from the dorsal intercarpal and basal metacarpal arches and the radial recurrent artery (see Fig. 1.29A,B). The nutrient vessels enter the trapezoid through its two nonarticular surfaces on the dorsal and palmar surfaces. The main blood supply of the trapezoid is from the dorsal supply. The dorsal surface is broad and flat, where the nonarticular surface serves for attachment of carpal ligaments. Three or four small vessels enter the dorsal surface in

B

A FIGURE 1.35. Right trapezoid. A: Medial aspect. B: Inferolateral aspect.

1 Skeletal Anatomy

the central aspect of the rough surface. Multiple small foramina usually are visible in this dorsal area. After penetrating the subchondral bone, the vessels branch to supply the dorsal 70% of the bone. These dorsal vessels provide the primary vascularity of the trapezoid (99). The palmar blood supply provides vascularity to approximately 30% of the trapezoid. The palmar surface is narrow, flat, and relatively small, and contains a small nonarticular portion where ligaments attach. In this area, one or two small vessels penetrate the central palmar portion. After entering the palmar surface of the trapezoid, the vessels branch several times to supply the palmar 30% of the bone. The palmar vessels do not anastomose with the dorsal vessels (99). Clinical Correlations: Trapezoid Fractures of the trapezoid are rare because of its protected position and its shape. Axial loading of the second metacarpal can cause dorsal (or, more rarely, palmar) dislocation, with associated rupture of the capsular ligaments (120). Because of the wedge or mushroom shape of the trapezoid (with the wide portion dorsally), dislocations are much more apt to occur dorsally than palmarly. Oblique radiographs and tomography may be helpful to visualize trapezoid fractures because the trapezoid is difficult to visualize on routine posteroanterior, anteroposterior, or lateral views of the wrist.

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Ossification Centers and Accessory Bones The trapezium is cartilaginous at birth. It has one ossification center that begins to ossify during the fourth year in girls and the fifth year in boys (5,74) (see Fig. 1.27A). Several accessory bones can be associated with the trapezium. Accessory bones, if present, usually are the result of a secondary or additional ossification center that does not fuse with the associated bone. Those associated with the trapezium include the os trapezium secundarium (multangulum majus secundarium, carpometacarpale II), the os praetrapezium (carpometacarpale I), the os paratrapezium, the os epitrapezium, the os radiale externum (parascaphoid), and the os trapezoideum secundarium (multangulum minus secundarium) (see Fig. 1.27B) (46). The os trapezium secundarium is located between the trapezium and the ulnar base of the thumb metacarpal. The os praetrapezium is located between the distal aspect of the trapezium and the thumb metacarpal. The os paratrapezium is located between the distoradial aspect of the trapezium and the radial base of the thumb metacarpal. The os epitrapezium is located at the proximal aspect of the trapezium, between the trapezium and distoradial aspect of the scaphoid. The radiale externum is located between the trapezium and the distal scaphoid, proximal to the site of the os epitrapezium (46) (see Fig. 1.27B).

Accessory Bones

Osteology of the Trapezium

Several accessory bones may be associated with the trapezoid and can be mistaken for fractures. An accessory bone usually represents the residual of a secondary ossification center that does not fuse with the associated bone, but it also may arise from trauma or heterotopic ossification of synovial tags (46,47). The accessory bones associated with the trapezoid include the os trapezoideum secundarium (located between the trapezoid, index finger metacarpal, and trapezium), the os metastyloideum (located between the trapezoid, base of the index finger metacarpal, and the capitate), the os centrale (located between the trapezoid, scaphoid, and capitate), and the os trapezium secundarium (located between the trapezoid, trapezium, and the vicinity of the bases of the index and thumb metacarpals; see Fig. 1.27B) (46) (see descriptions earlier, under Ossification Centers and Accessory Bones).

The trapezium is the most radially located carpal bone, assuming a functionally strategic position at the base of the thumb metacarpal and positioned just distal to the scaphoid (Fig. 1.36; see Figs. 1.25, 1.26, 1.37, 1.38, and 1.39). It consists internally of cancellous bone, surrounded by a cortical shell (see Fig. 1.36). The trapezium has an irregular shape. The dorsal and palmar surfaces are rough. The dorsal surface is wide and may contain a slight indentation or groove along which the radial artery passes. The palmar surface is narrow and contains a deep groove on the palmar ulnar surface. The groove forms the osseous portion of the fibroosseous tunnel containing the flexor carpi radialis tendon. Radial to the groove is a distinct longitudinal ridge (trapezial ridge) running in the proximodistal direction. The trapezial ridge provides attachment for a portion of the transverse carpal ligament (flexor retinaculum). The trapezial ridge and palmar surface of the trapezium also provide origins for the abductor pollicis brevis, opponens pollicis, and flexor pollicis brevis muscles. The lateral surface of the trapezium is broad and rough for attachment of carpal ligaments. The trapezium contains four articular surfaces for articulations with the scaphoid, trapezoid, index finger metacarpal, and the thumb metacarpal. The proximal articular surface is relatively small, and contains the facet for the scaphoid. The distal articular surface is relatively large and oval and saddle shaped. This distal

TRAPEZIUM (OS TRAPEZIUM, OS MULTANGULUM MAJUS, GREATER MULTANGULAR) Derivation and Terminology The name is derived from the Latin and Greek trapezion, indicating an irregular four-sided figure. The word multangular denotes “many-sided.”

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A

B FIGURE 1.36. Right trapezium. A: Palmar aspect. B: Medial aspect.

articular surface articulates with the thumb metacarpal. This large sellar (“saddle-shaped”) joint allows unique mobility. The surface shape has been found to be fundamentally different in men and women. The surface area also is significantly smaller in women (121). The ulnar aspect of the trapezium is concave, and contains the articular surface for the trapezoid. A small area on the distal ulnar aspect contains a narrow oval facet for articulation with the radial base of the index finger metacarpal. Associated Joints The trapezium articulates with four bones: the scaphoid, thumb metacarpal, trapezoid, and a small portion of the index metacarpal (see Figs. 1.25, 1.26, and 1.36 to 1.38). The trapezium articulates proximally with the scaphoid, forming an important component of the triscaphe joint. The articular surface on the trapezium for the scaphoid is somewhat square or rectangular. Distally and radially, the trapezium articulates with the thumb metacarpal through a saddle-shaped articulation. The trapezium articulates with the trapezoid along its medial border, where the articular surface on the trapezium is somewhat square. Distally and medially, there is a relatively small articulation of the trapezium with the index metacarpal. This joint surface on the trapezium is somewhat square or rectangular. Muscle Origins and Insertions The palmar surface of the trapezium contains origins of the three thenar muscles: abductor pollicis brevis, flexor pollicis brevis (superficial head), and opponens pollicis (see Figs. 1.37 and 1.38). These muscles attach to the palmar surface or just lateral to the trapezial ridge. Although the flexor carpi radialis does not actually insert into the trapezium, it traverses through a fibroosseous tunnel along the ulnar aspect of the trapezium. There are no muscle origins or insertions on the dorsal surface of the trapezium.

Vascularity of the Trapezium The vascularity of the trapezium is from vessels from the distal branches of the radial artery (see Fig. 1.29A,B). Nutrient vessels enter the trapezium through its three nonarticular surfaces. These surfaces are the dorsal and lateral aspects, which are rough and serve as sites for ligamentous attachment, and the prominent palmar tubercle from which the thenar muscles arise. Dorsally, one to three vessels enter and divide in the subchondral bone to supply the entire dorsal aspect of the bone. Palmarly, one to three vessels enter the mid-portion and divide and anastomose with the vessels entering through the dorsal surface. Laterally, three to six very fine vessels penetrate the lateral surface and anastomose freely with the dorsal and palmar vessels. The dorsal vascular supply usually supplies most of the vascularity. There are frequent anastomoses among all three systems. The associated dorsal, palmar, and lateral surfaces of the trapezium contain multiple foramina for the nutrient vessels (83,99). Clinical Correlations: Trapezium Fracture of the articular surface of the trapezium is produced by the base of the thumb metacarpal being driven into the articular surface of the trapezium by the adducted thumb (67,122). Avulsion fractures caused by capsular ligaments can occur during forceful deviation, traction, or rotation (115). Fracture of the trapezial ridge may occur from a direct blow to the palmar arch or forceful distraction of the proximal palmar arch to result in avulsion of the ridge of the trapezium by the transverse carpal ligament (123,124). The carpal tunnel view radiograph may be required to visualize this fracture. Accessory Bones Several accessory bones may be associated with the trapezoid and can be mistaken for fractures. An accessory bone

1 Skeletal Anatomy

usually represents the residual of a secondary ossification center that does not fuse with the associated bone, but it also may arise from trauma or heterotopic ossification of synovial tags (46,47). The accessory bones associated with the trapezium include the os trapezium secundarium (located between the trapezium and the base of the thumb metacarpal), the os praetrapezium (located between the distal trapezium and central portion of the base of the thumb metacarpal), the os paratrapezium (located between the trapezium and the radial aspect of the base of the thumb metacarpal), the os epitrapezium (located between the trapezium and scaphoid), the os radiale externum (located between the trapezium and scaphoid, just proximal to the site for the os epitrapezium), and the os trapezoideum secundarium (located between the trapezium, trapezoid, and basses of the index and thumb metacarpals; see Fig. 1.27B) (46) (see descriptions earlier, under Ossification Centers and Accessory Bones).

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METACARPALS (OSSA METACARPALIA) Derivation and Terminology The word metacarpal is derived from the Greek meta, which indicates “beyond,” “after,” or “accompanying,” and karpos, which means “wrist.” Therefore, metacarpal denotes “beyond or after the wrist.” General Features The five metacarpals are named for their associated digit, that is, thumb metacarpal, index finger metacarpal, long finger metacarpal, ring finger metacarpal, and small finger metacarpal. Although the metacarpals often are indicated by number (thumb as the first metacarpal, small finger as the fifth metacarpal), confusion has arisen as to which is the first and which is the fifth. Therefore, identifying each by associated digit is preferable. Despite their small size, the metacarpals are true long bones (4,5) (Figs. 1.37 to 1.39; see Figs. 1.25 to 1.27A).

FIGURE 1.37. Bones of right hand, palmar aspect, showing muscle origins (red) and insertions (blue).

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FIGURE 1.38. Bones of right hand, dorsal aspect, showing muscle origins (red) and insertions (blue).

A

B

FIGURE 1.39. Right thumb metacarpal. A: Lateral (radial) aspect. B: Medial (ulnar) aspect.

Each has an expanded proximal base, an elongated diaphysis (shaft or body), and a distal head. The head and bases consist internally of cancellous bone, similar to other long bones. The shaft has a thickened cortex that gradually thins at the diaphyseal–metaphyseal junction. A medullary canal lies in the shaft. Variation exists as to the relative lengths of the metacarpals (125,126). The long finger metacarpal usually appears as the longest, although the index finger metacarpal often is the longest or of equal length to the long finger metacarpal (125,126). The metacarpal of the ring finger usually is shorter than that of the index finger. The small finger metacarpal usually is the shortest. The metacarpal of the ring and little finger may be unproportionately shorter than those of the index and long fingers, resulting in an asymmetry to the hand (125,126). With a clenched fist, the metacarpal head of the long finger often appears to be the most prominent. This is due in part to its greater length,

1 Skeletal Anatomy

but also to the relatively “shorter” position of the index metacarpal, which is recessed into the carpus slightly more than the long finger metacarpal. This results in the long finger metacarpal appearing longer clinically. Posner and Kaplan have described the relative length relationships in terms of ratio of metacarpal size to the corresponding phalanges (125) (Table 1.3). The relative lengths of the proximal phalanges compared with the corresponding metacarpals are as follows: index, 1:1.6 to 2.4; long, 1:1.4; ring 1:1.3 to 1.5; little, 1:1.7 (125,126). The base of each metacarpal flares from the shaft into a wide proximal end. The flared base is cuboidal, wider dorsally than palmarly. The shafts of the metacarpals are curved longitudinally, with a slight convexity dorsally and concavity palmarly. The radial and ulnar aspects of the shafts also are curved in a slight concavity, presenting a surface for attachment of the interosseous muscles. On the palmar surface of the shaft is a prominent ridge that separates the attachments of adjacent palmar interosseous muscles. The dorsal surface is flattened and somewhat triangular, with the apex proximal. The flattened dorsal surface allows easy gliding of the overlying extrinsic extensor tendons. The triangular outline forms a ridge that runs along the dorsal aspect of the metacarpal, separating two sloping surfaces that provide attachments for the dorsal interosseous muscles. The head of each metacarpal is slightly thicker in the dorsopalmar direction. The articular surface of each head is smooth, oblong, convex, and flattened from side to side. On the radial and ulnar aspects of each head, at the level of the dorsal surface, there is a tubercle that provides purchase for a portion of the collateral ligaments. Between the tubercles on the palmar side, there is a hollow fossa for the attachment of a portion of the collateral ligament of the metacarpophalangeal joint and for the joint capsule. The dorsal surface of the head is broad and flat and accommodates the overlying extrinsic extensor tendon. The palmar aspect of the head contains a groove lying along the junction of the articular surface and the nonarticular portion of the head. The extrinsic flexor tendons pass through the groove, which helps form part of the fibroosseous tunnel of the flexor sheath. TABLE 1.3. RATIOS OF THE BONES OF THE FINGERS

Index Middle Ring Small

Distal Phalanx

Middle Phalanx

Proximal Phalanx

Metacarpal

1 1 1 1

1.1–1.4 1.3–1.8 1.3–1.7 1.0–1.2

1.8–2.8 2.2–2.7 2.0–2.8 1.6–2.2

3.2–4.3 3.0–3.9 3.0–3.6 2.7–3.9

From Posner MA, Kaplan EB. Osseous and ligamentous structures. In: Spinner M, ed. Kaplan’s functional and surgical anatomy of the hand, 3rd ed. Philadelphia: JB Lippincott, 1984:23–50.

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The articular surfaces are convex from dorsal to palmar and from radial to ulnar, although there is less convexity transversely. The metacarpal heads articulate with the proximal phalanges distally and the bases articulate with the distal carpal row. The bases of the metacarpals also articulate with each other (with the exception of the thumb metacarpal). The metacarpals to the index, long, ring, and small finger converge proximally. The thumb metacarpal, relative to the other metacarpals, is positioned more anteriorly and rotated medially on its axis through approximately 90 degrees, so that its morphologic dorsal surface faces laterally and its morphologic palmar surface faces medially. This rotation of the thumb allows it to flex medially across the palm so that it can be rotated into opposition with each finger. The motion of opposition consists of flexion and medial rotation (pronation) of the thumb across the palm, so that the pulp of the thumb faces the pulp of the lesser digits. The metacarpals can be associated with several sesamoid bones. In general, a sesamoid is a bone that develops in a tendon and occurs near a joint. By its location, the sesamoid serves to increase the functional efficiency of the joint by improving the angle of approach of the tendon into its insertion (25). Sesamoids are variably present. They are most common at the metacarpophalangeal joint of the thumb, in the intrinsic tendons that flex the metacarpophalangeal joint. Sesamoids also often are present at the metacarpophalangeal joint of the index and small finger, and at the interphalangeal joint of the thumb. Occasionally, one or two sesamoids may be present at any of the metacarpophalangeal joints of the hand (25). In addition to their variable presence, a sesamoid may exist as a bipartite sesamoid. They also may be fractured, resulting in two small fragments with an irregular margin between them. The metacarpals can be associated with several accessory ossicles. In general, the development of these accessory bones is from an additional or anomalous secondary ossification center, and therefore the accessory bones are described later under sections on ossification. Accessory bones, however, also can occur from other causes such as trauma (46) or heterotopic ossification of synovial tags (47). Therefore, anomalous, irregular ossicles or ossicles of abnormal size or shape may be encountered that do not fit a specific described accessory bone or location. The accessory bones located in the vicinity of the metacarpals, if present, usually are near the base, between the metacarpal and adjacent carpal bone. They usually form from a secondary ossification center of the carpal bone (46). THUMB METACARPAL (OSSA METACARPALIA I) Ossification Centers and Accessory Bones The thumb metacarpal has two ossification centers, one primary center in the midshaft and one secondary center in

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the base (see Fig. 1.27A). This is in contrast to the remaining metacarpals, which have one primary ossification center in the shaft and one secondary center in the head. Ossification in the midshaft begins in approximately the ninth week of prenatal life. Ossification in the base begins late in the second year in girls, and early in the third year in boys. The ossification centers unite before the fifteenth year in girls and before the seventeenth year in boys (127). Several accessory bones can be associated with the thumb metacarpal, usually located near or around the base and in close proximity to the trapezium. These accessory bones, if present, usually are the result of a secondary or additional ossification center that does not fuse with the associated bone. Those close to the thumb metacarpal usually are secondary ossification centers of the trapezium. These accessory bones include the os trapezium secundarium (multangulum majus secundarium, carpometacarpale II), the os praetrapezium (carpometacarpale I), and the os paratrapezium (46) (see Fig. 1.27B). The os trapezium secundarium is located between the ulnar base of the thumb metacarpal and the distal margin of the trapezium. The os praetrapezium is located between the thumb metacarpal (in the mid-portion of the base) and distal aspect of the trapezium. The os paratrapezium is located between the radial base of the thumb metacarpal and the distoradial aspect of the trapezium (46) (see Fig. 1.27B).

ening on the radial and ulnar borders. The articular surface at the base, which appears concave when viewed from the medial lateral direction and convex when viewed from the anteroposterior direction, is saddle shaped to accommodate the saddle shape of the trapezial articular surface. The base of the thumb metacarpal articulates only with the trapezium. This complex joint surface configuration plays an important role in the mechanism of opposition of the thumb. It represents half of the saddle joint that it forms with the corresponding surface of the trapezium (125). The articular surface is demarcated from the shaft by a thick, crestlike ridge that extends around the circumference, clearly separating the articular surface from the shaft. On the lateral (palmar) aspect of the base of the thumb metacarpal lies the insertion area for the abductor pollicis longus. There usually is a small tubercle at the lateral metacarpal base for the insertion of this tendon. On the ulnar aspect of the base lies the area of origin for the first palmar interosseous muscle. This muscle origin may extend distally to include a portion the ulnar aspect of the shaft. There are no articular facets present on the sides of the thumb metacarpal because this metacarpal does not articulate with any other metacarpal, in contrast to the remaining metacarpals, each of which articulates at the base with its adjacent metacarpal. Shaft of the Thumb Metacarpal

Osteology of the Thumb Metacarpal As emphasized by Williams [Gray’s Anatomy (5)], caution needs to be exercised when describing the thumb metacarpal because its position of rotation creates confusion in describing the various surfaces. Morphologic terms are used, but are supplemented in places by their topographic equivalents. For instance, the dorsal (lateral) surface of the thumb can be considered to face laterally; its long axis diverges in a distal lateral direction from the carpus. The thumb metacarpal is short and thick, and differs in shape and configuration from the metacarpals of the digits (see Figs. 1.25, 1.26, and 1.37 to 1.39). It is more stout, its shaft is thicker and broader, and it diverges to a greater degree from the carpus than the other metacarpals. The metacarpal contains the widened base, a narrow shaft, and a rounded head. The head and the base of the thumb metacarpal internally consist of cancellous bone surrounded by a relatively thin cortical shell (see Fig. 1.39). The shaft consists of thick cortical bone encircling the open medullary canal. At the head and at the base, the medullary canal rapidly changes to cancellous bone. Base of the Thumb Metacarpal The base of the thumb metacarpal differs greatly from all the other metacarpals. The base flares into a wider trumpetshaped expansion, with a prominent palmar lip and thick-

The shaft of the thumb metacarpal is thick and broad. The average thickness in the midshaft normally varies from 6 to 11 mm. The dorsal surface of the shaft is flat and wide, usually noticeably thicker and wider than the other metacarpals. Its anteroposterior thickness is relatively less pronounced, and in cross-section, the shaft is oval or somewhat triangular (apex palmar). It is mildly longitudinally convex along its dorsal surface. It also is mildly longitudinally concave palmarly, radially, and ulnarly. The palmar (medial) surface of the shaft is divided by a blunt ridge into a larger lateral (anterior) part, which gives rise to the opponens pollicis muscle, and a smaller medial (posterior) part, which gives origin to the lateral head of the first dorsal interosseous muscle (see Figs. 1.37 and 1.38). Head of the Thumb Metacarpal The head of the thumb metacarpal is rounded but less convex than the other metacarpals. The head also is much less spherical than the heads of the other metacarpals. It is thus more suited for hingelike motion than it is for more universal joint motion (which is possible to a greater degree with the other metacarpals). The articular surface is wide and flat and has a quadrilateral appearance. The articular surface extends much further palmarly than it does dorsally. The head of the thumb metacarpal is thicker and broader transversely. On the palmar aspect at the ulnar and radial

1 Skeletal Anatomy

angles, there are two articular eminences or tubercles which articulate the thumb sesamoid bones. The lateral articular eminence is larger than the medial. The associated sesamoid bones lie within the two heads of the flexor pollicis brevis. Associated Joints The head of the thumb metacarpal articulates with the base of the proximal thumb phalanx (Fig. 1.40; see Figs. 1.25, 1.26, and 1.37 to 1.39). The base of the thumb metacarpal articulates with the trapezium. Unlike the remaining metacarpals, the thumb metacarpal does not articulate with its adjacent (index) metacarpal. Muscle Origins and Insertions Four muscles usually attach to the thumb metacarpal: abductor pollicis longus, opponens pollicis, first dorsal interosseous and, inconsistently, a small portion of the origin of the flexor pollicis brevis (most of which originates from the palmar trapezium) (4,5) (see Figs. 1.37 and 1.38). In addition, the adductor pollicis and flexor pollicis brevis muscles insert into the closely associated thumb sesamoid bones, located palmar (medially) to the head of the thumb metacarpal.

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The abductor pollicis longus inserts into a tubercle located on the dorsal (lateral) aspect of the base of the thumb metacarpal. The opponens pollicis, which originates mainly from the transverse carpal ligament as well as from the palmar trapezium, inserts into a long, oval area along the radiopalmar aspect of the shaft of the thumb metacarpal. The first dorsal interosseous muscle is a bipennate muscle with two heads of origin, one on the thumb metacarpal and one on the index finger metacarpal. On the thumb metacarpal, the muscle has its origin along the dorsomedial aspect of the shaft of the thumb metacarpal. (On the index metacarpal, the second head originates along the radial aspect of the shaft.) The first dorsal interosseous inserts on the radial base of the proximal phalanx of the index finger and acts to abduct the index finger at the metacarpophalangeal joint. There is disagreement over the attachment of a first palmar interosseous muscle to the thumb. Although there are three distinct palmar (volar) interossei, some accounts describe four palmar interossei (128). When four are described, the first palmar interosseous consists of a small group of muscle fibers that takes origin from the ulnar side of the thumb metacarpal and blends with the oblique head of the adductor pollicis to insert with it on the ulnar side of the thumb. The continuity of this slip with the origin of the

FIGURE 1.40. Frontal section through articulations of the carpus.

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adductor pollicis from the bases of the index and long finger metacarpals, and its insertion with the adductor pollicis, seem to be sufficient reason for calling it a part of the adductor pollicis rather than a first palmar interosseous. Some authors have called this same slip the deep head of the flexor pollicis brevis. Functionally, the entire adductor pollicis is similar to a palmar interosseous (4,5,128). The origin of the flexor pollicis brevis usually is from the transverse carpal ligament, as well as from the trapezoid (deep head) and trapezium (superficial head). However, there may be a small slip of fibers that originates from the base of the thumb metacarpal on the palmar, medial aspect. These fibers join the superficial belly and continue to insert on the radial sesamoid (4). Clinical Correlations: Thumb Metacarpal The thumb metacarpal ossifies somewhat like a phalanx. For this reason, the thumb skeleton has been considered to consist of three phalanges. However, others have considered the distal phalanx of the thumb to represent fused middle and distal phalanges, a condition occasionally seen in the fifth toe (129). When the thumb has three phalanges, the metacarpal usually has a distal and proximal epiphysis. It occasionally bifurcates distally, the ulnar portion having no distal epiphysis and bearing two phalanges, and the radial bifurcation showing a distal epiphysis and three phalanges (130). The existence of only a distal metacarpal epiphysis may be associated with a greater range of movement at the metacarpophalangeal joint. In the thumb, it is the carpometacarpal joint that has the wider range, and a basal epiphysis in the first metacarpal may be attributable to this (4,5). However, a distal epiphysis has been noted rarely in the thumb metacarpal, and a proximal epiphysis has been noted rarely in the index metacarpal (4,5). In 1543, Vesalius originally suggested that the thumb had three phalanges, considering the thumb metacarpal as the proximal phalanx (4,5). Sesamoid Bones Sesamoid bones are common at the metacarpophalangeal joints of the thumb and index and small fingers, and the interphalangeal joint of the thumb. They may be mistaken for fractures, and can themselves be fractured or develop as bipartite sesamoids, further confusing the clinical impression. Schultz provides guidelines for distinguishing sesamoids from fractures (25). Multipartite sesamoids usually are larger than a normal or fractured sesamoid. Multipartite sesamoids have smooth, more regular opposing surfaces with cortical margins, and may be bilateral. In an acute fracture, the line of fracture is sharp, irregular, assumes any shape, and may be displaced. At times, it may be necessary to see fracture healing before the diagnosis can be made (25,131–139).

Accessory Bones Several accessory bones may be associated with the thumb metacarpal and can be mistaken for fractures. An accessory bone usually represents the residual of a secondary ossification center that does not fuse with the associated bone, but it also may arise from trauma or heterotopic ossification of synovial tags (46,47). The accessory bones associated with the thumb metacarpal are usually in the region of the base, representing secondary centers associated with the trapezium (see Fig. 1.27B). These accessory bones include the os trapezium secundarium (located between the thumb metacarpal and the distal ulnar corner of the trapezium), the os praetrapezium (located between the central portion of the base of the thumb metacarpal and the distal margin of the trapezium), and the os paratrapezium (located between the radial aspect of the base of the thumb metacarpal and the distal radial corner of the trapezium (46) (see Fig. 1.27B and descriptions earlier, under Ossification Centers and Accessory Bones).

INDEX FINGER METACARPAL (OSSA METACARPALIA II) Ossification Centers and Accessory Bones The index metacarpal (second metacarpal) has two ossification centers, one primary center in the shaft and one secondary center in the head (see Fig. 1.27A). Ossification in the midshaft begins in approximately the eighth or ninth week of prenatal life. Ossification in the secondary head center appears in the second year in girls, and between 1.5 to 2.5 years in boys. These secondary ossification centers usually first appear in the index metacarpal, and sequentially appear in the order of long finger, ring finger, and, last, the small finger. The secondary ossification in the head of the index metacarpal unites with the shafts at approximately the fifteenth or sixteenth year in women, and the eighteenth, nineteenth, or twentieth year in men (127). Several accessory bones can be associated with the index finger metacarpal, usually located at the base between the metacarpal and the trapezoid. These accessory bones, if present, usually are the result of a secondary or additional ossification center that does not fuse with the associated bone. Those associated with the index metacarpal usually are from a secondary ossification center of the trapezoid. These include the os trapezoideum secundarium (multangulum minus secundarium), the os metastyloideum, and the os parastyloideum (os carpometacarpale III) (see Fig. 1.27B) (46). The os trapezoideum secundarium is located at the radial base of the index metacarpal and the distal radial corner of the trapezoid. The os metastyloideum is located between the ulnar base of the index finger metacarpal, the distal ulnar corner of the trapezoid, and the distoradial corner of the capitate. The os parastyloideum is located

1 Skeletal Anatomy

between the ulnar base of the index metacarpal, the distoradial corner of the capitate, and the radial base of the long finger metacarpal. It is located just radial to the site for the os styloideum (which is associated with long finger metacarpal; see Fig. 1.27B) (46). Osteology of the Index Metacarpal The index metacarpal often is the longest metacarpal and usually has the largest base. It comprises a widened proximal base, a narrow curved shaft, and a rounded head (Fig. 1.41; see Figs. 1.25, 1.26, 1.37, and 1.38). The head and base consist internally of cancellous bone surrounded by a relatively thin cortical shell (see Fig. 1.41). The shaft consists of thicker cortical bone that encircles the open medullary canal. At the base and the neck, the medullary canal rapidly changes to cancellous bone. Base of the Index Finger Metacarpal The base of the index metacarpal has a unique groove or fork in the dorsopalmar direction. The fork is widened proximally, slightly larger medially than laterally, and open toward the carpus for articulation with the trapezoid. The trapezoid thus is nestled securely by the base of the index metacarpal. Medial to the groove in the base of the metacarpal there is an extension of bone forming a ridge that articulates with the capitate. On the lateral aspect of the base, near the dorsal surface, is a quadrilateral facet for articulation with the trapezium. Dorsal to the trapezial facet A

B

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is a roughened area for the insertion of the extensor carpi radialis longus. On the palmar surface of the base is a small tubercle or ridge that provides attachment for the insertion of the flexor carpi radialis. The medial side of the base of the index metacarpal is thickened, forming the larger half of the metacarpal base. This portion articulates with the base of long finger metacarpal through a prominent thickening, the styloid process of the base of the long metacarpal (125, 126). This articulation includes a long facet, narrow in its central area. The base of the index metacarpal thus includes a total of four articular facets. The ulnar side of the base of the index metacarpal, which articulates with the styloid process of the long metacarpal, has a small, roughened area just distal to the articular facet for insertion of strong interosseous ligaments. These ligaments hold the base of the index and long finger metacarpals together. There is a slight depression between the two halves of the base of the metacarpal that usually contains several small foramina for nutrient arteries that arise from the dorsal carpal arch. Similar to the dorsal surface, the palmar surface of the metacarpal has a roughened area with multiple foramina for the palmar nutrient arteries entering the base (125). Shaft of the Index Finger Metacarpal The shaft of the index metacarpal is curved, convex dorsally and concave palmarly. It has a flat, triangular dorsal surface immediately proximal to the head. The shaft is oval or slightly triangular in cross-section, flattened dorsally. The dorsal surface is broad more distally, but proximally the dorsal surface narrows to a ridge. The dorsal surface is lined by lateral ridges that converge toward the dorsum, approximately at the junction of the distal two-thirds with the proximal third, to form a single ridge running proximally and ending at the apex of the forked base. The palmar surface of the shaft is smooth in the central area, but becomes more irregular at the proximal and distal ends. The metacarpal has converging borders that begin at the tubercles, one on each side of the head for the attachment of collateral ligaments. Along the shaft of the index metacarpal three interosseous muscles originate, two dorsal interosseous and one palmar interosseous. Proximally, the lateral surface inclines dorsally for the ulnar head of the first dorsal interosseous muscle. The medial surface inclines similarly, and is divided by a faint ridge into two areas: a palmar strip for origin of the first palmar interosseous and a dorsal strip for the origin of the radial head of the second dorsal interosseous muscle (2,4,5). At the junction of the shaft and head, several small foramina usually are present for the entrance of nutrient vessels. Head of the Index Finger Metacarpal

FIGURE 1.41. Right index finger metacarpal. A: Dorsolateral aspect. B: Medial aspect.

The head of the index metacarpal is rounded and slightly elongated in the dorsopalmar axis. Although the head may

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be irregular, it has a smooth convex area that extends further in the palmar–distal direction than in the mediolateral direction. The extraarticular areas of the head are roughened and contain medial and lateral tubercles at the articular margins for attachment of the collateral ligaments and joint capsule. The tubercles are located on the dorsal half of the side of the metacarpal head. Along with the tubercles, there is a slight elevated ridge that surrounds the articular smooth area. The articular surface extends further over the palmar aspect than over the dorsal aspect. There is a small depression just proximal to the articular surface over the mid-dorsal aspect of the head for the attachment of the capsule of the metacarpophalangeal joint. On the medial and lateral surface of the metacarpal head are longitudinal furrows just proximal to the articular margin to assist the passage of the tendons of the interosseous muscles. At the margin of the articular surface, there are multiple small vascular foramina in which vessels from the attaching soft tissues enter the head. Associated Joints The base of the index metacarpal articulates largely with the trapezoid, which lies in the groove at the metacarpal base (see Figs. 1.25, 1.26, 1.37, 1.38, 1.40, and 1.41). In addition, the ulnar aspect of the base of the metacarpal contains a small articular surface for articulation with the capitate, and a more distal and ulnar articulation with the neighboring long finger metacarpal. On the radial aspect of the base of the index metacarpal, there also is a small articular surface for articulation with the trapezium. The index metacarpal usually does not articulate with the thumb metacarpal. The head of the index metacarpal articulates with the base of the proximal phalanx of the index finger. Muscle Origins and Insertions Six muscles attach to the index metacarpal: the flexor carpi radialis, the extensor carpi radialis longus, the first and second dorsal interosseous muscles, the first palmar interosseous muscle, and, often, a relatively small portion of the origin of the adductor pollicis oblique head (see Figs. 1.37 and 1.38). The flexor carpi radialis inserts into the palmar aspect of the base of the index metacarpal. The insertion point usually is wide, encompassing most of the width of the base of the index metacarpal. The extensor carpi radialis longus inserts into the dorsal aspect of the base of the index metacarpal. The insertion point usually is slightly radial to the longitudinal midline of the metacarpal (4). The first dorsal interosseous muscle (ulnar head) originates from the radial aspect of the shaft of the index metacarpal. This muscle belly joins the belly originating

from the ulnar aspect of the thumb metacarpal (radial head), thus forming a bipennate muscle with a common insertion. The first dorsal interosseous muscle inserts into the radial aspect of the base of the proximal phalanx of the index finger. Considerable variations exist as to the bone versus soft tissue insertion of the interosseous muscles (into either the proximal phalanx or the extensor aponeurosis). In the index metacarpal, most, if not all fibers insert into bone (140), whereas the remaining dorsal and palmar interosseous muscles show variation as to bone versus extensor insertion. See discussions of individual muscles in Chapter 2. Most of the bony insertion of the first dorsal interosseous probably is functionally advantageous, whereas the bony insertion of a strong first dorsal interosseous muscle helps stabilize the index finger during pinch and grasp, resisting the force exerted by the thumb by producing reciprocal abduction of the proximal phalanx of the index finger. The second dorsal interosseous muscle (radial head) originates from the ulnar aspect of the shaft of the index metacarpal. This muscle belly joins the belly originating from the radial aspect of the shaft of the long finger metacarpal (ulnar head), thus forming a bipennate muscle with a common insertion. The second dorsal interosseous then inserts into either the lateral base of the proximal phalanx of the long finger, or the extensor aponeurosis (approximately 60% bone, 40% extensor hood) (140). The first palmar interosseous muscle originates from the palmar aspect of the ulnar side of the index metacarpal shaft. The first palmar interosseous muscle inserts into the extensor aponeurosis or, to a variable degree, into the base of the ulnar aspect of the proximal phalanx of the index finger. The palmar interosseous muscles function largely to adduct and flex the proximal phalanx. Throughout the extensor aponeurosis, the interosseous muscles also assist with extension of the middle and distal phalanges. A small portion of the adductor pollicis oblique head may originate from the base of the index metacarpal. This usually is in the proximal, ulnar corner of the metacarpal on the palmar side. Most of the origin of the oblique head of the adductor pollicis attaches to the capitate and to the base of the long finger metacarpal. Clinical Correlations: Index Finger Metacarpal The base of each metacarpal, including the index metacarpal, is somewhat cuboid, wider dorsally than palmarly. This results in a slightly wedge-shaped bone, with the apex palmar. With this configuration, subluxation or dislocation of the base of the index metacarpal on the trapezoid usually occurs in a dorsal direction. Palmar dislocation of the base of the index metacarpal is understandably rare, usually prevented by the wide dorsal portion of the base.

1 Skeletal Anatomy

Sesamoid Bones Sesamoid bones are common at the metacarpophalangeal joints of the thumb and the index and small fingers, and the interphalangeal joint of the thumb. They may be mistaken for fractures, and can themselves be fractured or develop as bipartite sesamoids, further confusing the clinical impression. Schultz provides guidelines for distinguishing sesamoids from fractures. Multipartite sesamoids usually are larger than a normal or fractured sesamoid. Multipartite sesamoids have smooth, more regular opposing surfaces with cortical margins, and may be bilateral. In an acute fracture, the line of fracture is sharp, irregular, assumes any shape, and may be displaced. At times, it may be necessary to see fracture healing before the diagnosis can be made (25). Accessory Bones Several accessory bones may be associated with the index finger metacarpal and can be mistaken for fractures. An accessory bone usually represents the residual of a secondary ossification center that does not fuse with the associated bone, but it also may arise from trauma or heterotopic ossification of synovial tags (46,47). The accessory bones associated with the index metacarpal usually are in the region of the base, representing secondary ossification centers associated with the trapezoid (see Fig. 1.27B). These accessory bones include the os trapezoideum secundarium (located at the radial base of the index metacarpal and the distal radial corner of the trapezoid), the os metastyloideum (located between the ulnar base of the index finger metacarpal, the distal ulnar corner of the trapezoid, and the distoradial corner of the capitate), and the os parastyloideum (located between the ulnar base of the index metacarpal, the distoradial corner of the capitate, and the radial base of the long finger metacarpal; see Fig. 1.27B) (46) (see descriptions earlier, under Ossification Centers and Accessory Bones).

LONG FINGER METACARPAL (OSSA METACARPALIA III) Ossification Centers and Accessory Bones The long finger metacarpal (third metacarpal) has two ossification centers, a primary ossification center in the shaft and a secondary center in the head (see Fig. 1.27A). Ossification in the midshaft begins in approximately the ninth week of prenatal life. Ossification in the secondary center in the head appears in the second year in girls, and from 1.5 to 2.5 years in boys. These secondary ossification centers usually appear first in the index metacarpal, and sequentially appear in the order of long finger, ring finger, and, last, the small finger. The secondary ossification in

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the head of the long finger metacarpal unites with the shaft at approximately the fifteenth or sixteenth year in women, and the eighteenth or nineteenth year in men (127). On the dorsal aspect of the long finger metacarpal there is a raised, thickened protuberance of bone often referred to as the styloid. This styloid process may have a separate ossification center, or form a separate ossicle (see later) (46, 127). Several accessory bones can be associated with the long finger metacarpal, usually located at the base between the metacarpal and the trapezoid. These accessory bones, if present, usually are the result of a secondary or additional ossification center that does not fuse with the associated bone. Those associated with the long finger metacarpal usually are from a secondary ossification center of the base of the metacarpal (from the ossification center of the styloid) or from a secondary center of the capitate. These include the os styloideum (os carpometacarpale IV), the os parastyloideum (os carpometacarpale III), the os subcapitatum, the os capitatum secundarium (os carpometacarpale V), and the os gruberi (os carpometacarpale VI) (see Fig. 1.27B) (46). The os styloideum is located at the radial corner of the base of the long metacarpal, between the bases of the long and index metacarpal and the distal radial corner of the capitate. The os parastyloideum is located just radial to the site of the os styloideum, at the radial corner of the base of the long metacarpal, and between the base of the index metacarpal and distal radial corner of the capitate. The os subcapitatum is located proximal to the mid-portion of the base of the long finger metacarpal, adjacent to the central portion of the body of the capitate. The os capitatum secundarium is located at the ulnar base of the long finger metacarpal, between the metacarpal and the distoulnar corner of the capitate, and close to the hamate and base of the ring finger metacarpal. The os gruberi is located just ulnar to the site of the os capitatum secundarium, at the ulnar corner of the base of the long finger metacarpal, between the long and ring finger metacarpals, the distoulnar corner of the capitate, and the distoradial corner of the body of the hamate (46) (see Fig. 1.27B). Osteology of the Long Finger Metacarpal The long finger metacarpal usually is the second longest metacarpal, second only to the index finger metacarpal (Fig. 1.42; see Figs. 1.25, 1.26, 1.37, and 1.38). Similar to the other metacarpals, the long finger metacarpal consists of a widened proximal base, a narrow curved shaft, and a rounded head. The head and base are composed internally of cancellous bone surrounded by a relatively thin cortical shell (see Fig. 1.42). The shaft consists of thicker cortical bone that encircles the open medullary canal. At the base and at the neck, the medullary canal rapidly changes to cancellous bone.

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B

FIGURE 1.42. Right long finger metacarpal. A: Lateral aspect. B: Medial aspect.

Base of the Long Finger Metacarpal The base of the long metacarpal is unique in that it contains the styloid process, a short, consistent projection that extends proximally from the radial side of the dorsal surface (125). The base of the long finger metacarpal articulates largely with the capitate by a facet that is convex anteriorly and dorsally concave, where it extends to the styloid process on the lateral aspect of its base (see Fig. 1.42). Through the styloid process, there also is a narrow articulation for the index metacarpal base comprising a narrow, striplike facet, constricted centrally and somewhat hourglass-shaped. There also may be a small articulation with the trapezoid on the radial base of the styloid. The articular and size relationships of the bases of the index and long metacarpals are variable. When the styloid process of the long finger metacarpal is short, the ulnar part of the base of the index metacarpal may articulate with a small portion of the capitate. On the radial side of the base of the long metacarpal, just distal to the articular surface for the index metacarpal base, there is a rough area for insertion of the intermetacarpal interosseous ligament (125). The long finger metacarpal base also has an articulation with the ring finger metacarpal. It consists of two oval articular facets. The palmar facet may be absent; however, less frequently the two facets may be connected proximally by a narrow bridge (4,5). This double facet articulates with a similar double facet on the radial side of the base of the ring finger metacarpal. There usually is a rough, raised area between the two facets, just distal or palmar to the articular surface. This rough area serves for the attachment of the associated interosseous intermetacarpal ligament. On the palmar sur-

face of the base of the metacarpal, there may be a roughened or raised area for a small portion of the insertion of the flexor carpi radialis. (The major insertion point for the flexor carpi radialis is at the palmar base of the index metacarpal.) The dorsal surface of the base of the long finger metacarpal contains a roughened or slightly raised area for insertion of the extensor carpi radialis brevis. The insertion point is slightly radial to the midline of the shaft of the metacarpal. On the widened, rough areas on the dorsal and palmar surfaces of the base of the index metacarpal, there usually are several small foramina for the nutrient arteries. On the palmar surface of the base, there also is a portion of a long longitudinal crest that extends to the shaft. This crest serves for the origin of the adductor pollicis, and joins a similar crest or roughened area on the capitate, which also provides attachment for the adductor pollicis. Shaft of the Long Finger Metacarpal The shaft of the long finger metacarpal is curved, convex dorsally and concave palmarly. To a large degree, the long metacarpal resembles the index metacarpal. In cross-section, the shaft of the long finger metacarpal is oval or triangular, with the apex palmar. The dorsal surface of the shaft is smooth to allow passage of the extrinsic extensor tendons. The dorsal surface is somewhat flat, and is triangular with the apex proximal. The dorsal surface widens slightly from proximal to dorsal. There are two faint longitudinal lateral ridges that form the edges of this dorsal triangle and converge toward the proximal third of the dorsal surface. A single ridge continues proximally toward the base. The exten-

1 Skeletal Anatomy

sor digitorum communis crosses close to the triangular portion of the dorsal surface. On its lateral surface, the ulnar head of the second dorsal interosseous muscle originates. This lateral surface is demarcated by the lateral ridges on the dorsal surface. On the medial surface, the radial head of the third dorsal interosseous muscle originates. At the junction of the shaft and head, several small foramina usually are present for the entrance of nutrient vessels. There is no consistent nutrient vessel in the shaft. The metacarpal receives most of its vascularity from the base and from the head and neck regions (125). Head of the Long Finger Metacarpal The head of the long finger metacarpal is similar to that of the index metacarpal. It is rounded, and slightly elongated in the dorsopalmar axis. In the anteroposterior plane, the head is round, smooth, and convex, flatter on the medial and lateral sides. The articular surface extends much more palmarly than dorsally, thus providing for more flexion of the proximal phalanx. The head is roughened medially and laterally, with medial and lateral tubercles at the articular margins for attachment of the collateral ligaments and joint capsule. Palmar to the tubercles, on the medial and lateral aspects of the head, there are grooves in which the tendons of the interosseous muscles pass. On the palmar surface of the head, just proximal to the articular margin, there are two tubercles for the insertion of the palmar joint soft tissues. Also in this region, at the margin of the articular surface, the bone is rough, and there are multiple small vascular foramina for nutrient vessels. Associated Joints The base of the long finger metacarpal articulates largely with the distal end of the capitate (see Figs. 1.25, 1.26, 1.37, 1.38, 1.40, and 1.42). In addition, on the lateral base of the long finger metacarpal, there is a narrow, hourglass-shaped articular surface for articulation with the base of the index metacarpal. The styloid process may articulate with the trapezoid. On the medial base of the long finger metacarpal, there is a similar strip or pair of circular articular areas for articulation with the base of the ring finger metacarpal. Distally, the long finger metacarpal articulates with the base of the proximal phalanx of the long finger.

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hand, it does not need a muscle to adduct it into this position.) The long finger metacarpal also may receive attachments from the insertion of the flexor carpi radialis (4,5). However, most of the insertion of the flexor carpi radialis is into the base of the index finger metacarpal. The extensor carpi radialis brevis tendon inserts into the dorsal base of the long finger metacarpal. The point of insertion usually is radial to the midline of the shaft of the metacarpal (4). The second dorsal interosseous muscle (ulnar head) originates along the shaft of the lateral border of the long finger metacarpal. These fibers are joined by fibers of the second interosseous that originate from the medial border of the adjacent index finger metacarpal (radial head), thus forming a bipennate muscle. The second dorsal interosseous then inserts into either the lateral base of the proximal phalanx of the long finger, or into the extensor aponeurosis (approximately 60% bone, 40% extensor hood) (140). The third dorsal interosseous muscle (radial head) originates along the shaft of the medial border of the long finger metacarpal. These fibers are joined by fibers of the third dorsal interosseous that originate from the lateral border of the ring finger metacarpal (ulnar head), thus forming a bipennate muscle. The third dorsal interosseous then inserts into either the medial base of the proximal phalanx of the long finger, or into the extensor aponeurosis (approximately 6% into bone, 94% into extensor aponeurosis) (140,141) The oblique head of the adductor pollicis originates largely from the palmar aspect of the base of the long finger metacarpal. The remaining fibers of the oblique head originate from the palmar capitate or trapezoid. The fibers of the oblique head of the adductor pollicis join the fibers from the transverse head, and collectively insert into the ulnar sesamoid. The transverse head of the adductor pollicis originates from the palmar shaft of the long finger metacarpal. These fibers join the fibers of the oblique head, and insert into the ulnar sesamoid of the thumb metacarpal. The flexor carpi radialis may insert partially into the radial aspect of the base of the long finger metacarpal. Most of the insertion of this muscle, however, is into the base of the index metacarpal.

Muscle Origins and Insertions

Clinical Correlations: Long Finger Metacarpal

There are five major muscle attachments to the long finger metacarpal. These include the extensor carpi radialis brevis, the second and third dorsal interosseous muscles, the oblique head of the adductor pollicis, and the transverse head of the adductor pollicis (see Figs. 1.37 and 1.38). The long finger metacarpal does not give origin to a palmar interosseous muscle. (Because it lies in the midline of the

The base of the long finger metacarpal is somewhat cuboid, wider dorsally than palmarly. This results in a slightly wedge-shaped bone, with the apex palmar. The styloid process at the base of the metacarpal adds to the width dorsally. With this configuration, subluxation or dislocation of the base of the long finger metacarpal on the capitate usually occurs in a dorsal direction. Palmar dislocation of the

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base of the long finger metacarpal is understandably rare, usually prevented by the wide dorsal portion of the base. Accessory Bones Several accessory bones may be associated with the long finger metacarpal and can be mistaken for fractures. An accessory bone usually represents the residual of a secondary ossification center that does not fuse with the associated bone, but it also may arise from trauma or heterotopic ossification of synovial tags (46,47). The accessory bones associated with the long finger metacarpal usually are in the region of the base, representing secondary ossification centers of the styloid of the metacarpal, or arise from a secondary center of the capitate (see Fig. 1.27B). These accessory bones include the os styloideum (located at the radial corner of the base of the long metacarpal, between the bases of the long and index metacarpal and the distal radial corner of the capitate), the os parastyloideum (located just radial to the site of the os styloideum, at the radial corner of the base of the long metacarpal, and between the base of the index metacarpal and distal radial corner of the capitate), the os subcapitatum (located proximal to the mid-portion of the base of the long finger metacarpal, adjacent to the central portion of the body of the capitate), the os capitatum secundarium (located at the ulnar base of the long finger metacarpal, between the metacarpal and the distoulnar corner of the capitate, and close to the hamate and base of the ring finger metacarpal), and the os gruberi (located just ulnar to the site of the os capitatum secundarium, at the ulnar corner of the base of the long finger metacarpal, between the long and ring finger metacarpals, the distoulnar corner of the capitate, and the distoradial corner of the body of the hamate; see Fig. 1.27B) (46) (see descriptions earlier, under Ossification Centers and Accessory Bones). RING FINGER METACARPAL (OSSA METACARPALIA IV)

Several accessory bones can be associated with the ring finger metacarpal, usually located at the base between the metacarpal and the hamate or capitate. These accessory bones, if present, usually are the result of a secondary or additional ossification center that does not fuse with the associated bone. Those associated with the ring finger metacarpal usually are from a secondary ossification center of the neighboring hamate or capitate, or from a secondary ossification center in the styloid of the base of the adjacent long finger metacarpal. These accessory bones include the os gruberi (carpometacarpale VI), the os capitatum secundarium (carpometacarpale V), and the os hamuli proprium. The os gruberi is located at the radial corner of the base of the ring finger metacarpal, between the base of the long metacarpal and distoulnar corner of the capitate. The os capitatum secundarium is located just radial to the site of the os gruberi, between the radial corner of the base of the ring finger metacarpal and the proximal ulnar corner of the long finger metacarpal (between the distal margins of the capitate and hamate). The os hamuli proprium is associated more closely with the hamate, proximal to the base of the ring finger metacarpal (46) (see Fig. 1.27B). Osteology of the Ring Finger Metacarpal The ring finger metacarpal is intermediate in size between the long finger and small finger metacarpals, and noticeably shorter and thinner than the index and long metacarpals (Fig. 1.43; see Figs. 1.25, 1.26, 1.37, and 1.38). It is similar in overall shape to the other metacarpals, containing a widened proximal base, a narrower curved shaft, and a rounded head. It most resembles the long finger metacarpal, especially in the head and shaft; however, the base shows distinct differences (see later). Internally, it also is similar to the remaining metacarpals. The head and base consist internally of cancellous bone surrounded by a relatively thin cortical shell (see Fig. 1.43). The shaft consists of thicker cortical bone that encircles the open medullary canal. At the base and at the neck, the medullary canal rapidly changes to cancellous bone (127).

Ossification Centers and Accessory Bones The ring finger metacarpal (fourth metacarpal) has two ossification centers, a primary ossification center in the shaft and a secondary center in the head (see Fig. 1.27A). Ossification in the midshaft begins in approximately the ninth week of prenatal life. Ossification in the secondary center of the head appears in the second year in girls, and from 1.5 to 2.5 years in boys. These secondary ossification centers usually first appear in the index metacarpal, and sequentially appear in the order of long finger, ring finger, and, last, the small finger. The secondary ossification in the head of the ring finger metacarpal unites with the shaft at approximately the fifteenth or sixteenth year in women, and the eighteenth or nineteenth year in men (127).

Base of the Ring Finger Metacarpal The base of the ring finger metacarpal is relatively small and quadrilateral, usually containing two proximal articular facets and articular surfaces on the radial and ulnar aspects of the base for the adjacent metacarpals. There is considerable variation in the shape of the base of the ring finger metacarpal, and it has been described in several ways (see Anomalies and Variations, later) (81,94, 108,117,118,142). The proximal articular surface is quadrangular, is directed somewhat medially, and is convex anteriorly and dorsally concave. There is a proximal elevation on the dorsal surface that divides the articular surface into radial and ulnar parts, or facets. The radial facet of the ring

1 Skeletal Anatomy A

75

B

FIGURE 1.43. Right ring finger metacarpal. A: Lateral aspect. B: Medial aspect.

finger metacarpal articulates with the ulnar third of the distal articular surface of the capitate. The ulnar facet of the metacarpal articulates with the radial facet of the hamate. The metacarpal’s articular portion for the capitate usually involves only a small oval or square facet. Also on the radial aspect of the base of the ring finger metacarpal, there is a set of two oval or round facets for articulation with the adjacent long finger metacarpal base. On the ulnar side of the base of the ring finger metacarpal, there is an oval, or narrow, oblong facet, usually with a concave surface, for articulation with the adjacent base of the small finger metacarpal. The roughened area between the two proximal articular facets provides an area of attachment for the interosseous intermetacarpal ligament. On the base of the metacarpal, on the dorsal and palmar surfaces just distal to the articular margin, there are multiple small foramina for nutrient vessels (5,125). Shaft of the Ring Finger Metacarpal The shaft of the ring finger metacarpal is slender and curved. It is convex dorsally and concave palmarly. To a large degree, the ring finger metacarpal shaft resembles that of the index and long finger metacarpals, although it is noticeably shorter and thinner. The metacarpal may taper proximally, so that the narrowest portion is at the junction of the base and the shaft. In cross-section, the metacarpal is round, oval, or slightly triangular (apex palmar). On the medial aspect of the shaft is a slight concavity for the origin of the radial head of the fourth dorsal interosseous muscles. On the lateral aspect, there is a slight concavity for the origin of the ulnar head of the third dorsal interosseous muscle. On the lateral surface of the shaft of the ring finger metacarpal there is a faint ridge that separates the attach-

ments of the second palmar interosseous muscle from the ulnar head of the third dorsal interosseous muscles. The dorsal surface of the shaft is smooth and somewhat flat to allow passage of the extrinsic extensor tendons. The triangular flattened area present on the index and long metacarpals also is present on the ring finger metacarpal. The palmar surface, which is concave, is slightly flatter in the proximal half. Along the distal half of the palmar surface, the surface tends to form a slight longitudinal ridge along the midline of the cortical surface. There is no consistent nutrient vasculature in the shaft of the metacarpals. The metacarpals receive most of their vascularity from the base and the head and neck regions (125). Head of the Ring Finger Metacarpal The head of the ring finger metacarpal is similar to that of the index and long metacarpals. It is round, but slightly elongated in the dorsopalmar axis. The head is roughened medially and laterally, with medial and lateral tubercles at the articular margins for attachment of the collateral ligaments and joint capsule. At the margin of the articular surface, there are multiple small vascular foramina through which vessels from the attaching soft tissues enter the head. Anomalies and Variations in Morphology of the Ring Metacarpal Recent studies on the carpometacarpal joints have shown that the base of the ring finger metacarpal has considerable variation in morphology (81,94,108,117,142–145). The general shape of the base, noted to be relatively flat or conical, usually is readily identifiable on standard radiographs (117). With regard to articular morphology, the base of the

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ring finger metacarpal articulates with the hamate and, to a variable degree, with the capitate (81,118,142,143, 146–148). The base of the ring finger metacarpal and the associated articulations appear to exhibit more variation than any of the other carpometacarpal joints (118). Five different types of ring finger metacarpal base have been described with regard to shape and articular configurations. n Type I contains a broad base that articulates with the hamate and has a single dorsal facet extension that articulates with the capitate. This type was present in approximately 39% of wrists. n Type II contains a broad base that articulates with the hamate, and two facet extensions (one dorsal and one palmar) that articulate with the capitate. Type II was present in approximately 8% of wrists. n Type III contains a relatively narrow base that articulates only with the hamate. Type III was present in 9% of specimens. n Type IV contains a broad base that articulates with the hamate and a separate, single dorsal facet that articulates with the capitate. Type IV was present in approximately 34% of wrists. n Type V contains a broad base that articulates with the hamate and the capitate. Type V was present in approximately 9% of wrists (81,94,108,117). Associated Joints The base of the ring finger metacarpal articulates largely with the distal end of the radial articular facet of the hamate (see Figs. 1.25, 1.26, 1.37, 1.38, 1.40, and 1.43) (Also see Anomalies above). In addition, on the medial base of the ring finger metacarpal, there is a narrow, oval or hourglassshaped articular surface for articulation with the base of the small finger metacarpal. On the lateral base of the long finger metacarpal, there is a similar strip or pair of circular articular areas for articulation with the base of the long finger metacarpal. On the lateral base of the ring finger metacarpal, between the articular facets for the hamate and the long finger metacarpal, there is a small oval articular area for the capitate. Distally, the ring finger metacarpal articulates with the base of the proximal phalanx of the ring finger. Muscle Origins and Insertions There are three major muscle attachments to the ring finger metacarpal. These include the origins of the third dorsal interosseous (ulnar head), the origin of the fourth dorsal interosseous (radial head), and the origin of the second palmar interosseous (see Figs. 1.37 and 1.38). The third dorsal interosseous muscle (ulnar head) originates along the shaft of the lateral border of the ring finger metacarpal. These fibers are joined by fibers of the third

dorsal interosseous that originate from the medial border of the adjacent long finger metacarpal (radial head), thus forming a bipennate muscle. The third dorsal interosseous then inserts into the either the medial base of the proximal phalanx of the long finger, or into the extensor aponeurosis (approximately 6% bone, 96% extensor hood) (140). The fourth dorsal interosseous muscle (radial head) originates along the shaft of the medial border of the ring finger metacarpal. These fibers are joined by fibers of the fourth dorsal interosseous that originate from the lateral border of the small finger metacarpal (ulnar head), thus forming a bipennate muscle. The fourth dorsal interosseous then inserts into either the medial base of the proximal phalanx of the ring finger, or into the extensor aponeurosis (approximately 40% bone, 60% extensor aponeurosis) (140,141). The second palmar interosseous muscle originates from the lateral palmar border of the shaft of the ring finger metacarpal. The muscle inserts into the extensor aponeurosis of the ring finger, or into the radial side of the base of the proximal phalanx of the ring finger. Clinical Correlations: Ring Finger Metacarpal The joint surfaces at the base of the ring and small finger metacarpals are saddle-shaped or flat, respectively, and are not confined by the borders of the adjacent metacarpal bases or carpal bones (as with the index and long metacarpals). This allows, in part, the greater motion at the carpometacarpal joints of the small and ring finger, compared with the relatively restricted motion of the carpometacarpal joints of the index and long fingers. The base of each metacarpal, including that of the ring finger, is somewhat cuboid, wider dorsally than palmarly. This results in a slightly wedge-shaped bone, with the apex palmar. With this configuration, subluxation or dislocation of the base of the ring finger metacarpal on the hamate usually occurs in a dorsal direction. Palmar dislocation of the base of the ring finger metacarpal is understandably rare, usually prevented by the wide dorsal portion of the base. Accessory Bones Several accessory bones may be associated with the ring finger metacarpal and can be mistaken for fractures. An accessory bone usually represents the residual of a secondary ossification center that does not fuse with the associated bone, but it also may arise from trauma or heterotopic ossification of synovial tags (46,47). The accessory bones associated with the ring finger metacarpal usually are in the region of the base, representing secondary ossification centers from the capitate, hamate or a secondary center of the base of the metacarpal (46) (see Fig. 1.27B). These accessory bones include the os gruberi (located at the radial corner of the base of the ring finger metacarpal, between the base of the

1 Skeletal Anatomy

long finger metacarpal and distoradial corner of the hamate), the os capitatum secundarium (located just radial to the site of the os gruberi, between the radial corner of the base of the ring finger metacarpal, the proximal ulnar corner of the long finger metacarpal, and the distal margins of the capitate and hamate), and the os hamuli proprium (which is more closely associated with the hamate, located proximal to the base of the ring finger metacarpal; see Fig. 1.27B) (46) (see descriptions earlier, under Ossification Centers and Accessory Bones).

A

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B

SMALL FINGER METACARPAL (OSSA METACARPALIA V) Ossification Centers and Accessory Bones The small finger metacarpal (fifth metacarpal) has two ossification centers, a primary ossification center in the shaft and a secondary center in the head (see Fig. 1.27A). Ossification in the midshaft begins in approximately the ninth week of prenatal life. Ossification in the secondary center of the head appears in the second year in girls, and from 1.5 to 2.5 years in boys. The secondary ossification centers usually appear last in the small finger metacarpal (usually appearing first in the index metacarpal, and sequentially in the long finger, ring finger, and, last, the small finger). The secondary ossification in the head of the small finger metacarpal unites with the shaft at approximately the fifteenth or sixteenth year in women, and the eighteenth or nineteenth year in men (127). An accessory bone can be associated with the small finger metacarpal, the os vesalianum manius (os vesalii, os carpometacarpale VIII). It usually is located at the ulnar base of the metacarpal, distal to the ulnar aspect of the hamate. An accessory bone, if present, usually is the result of a secondary or additional ossification center that does not fuse with the associated bone. That associated with the small finger metacarpal may be from a secondary ossification center of the base of the metacarpal or from a secondary center of the hamate (46) (see Fig. 1.27B). Osteology of the Small Finger Metacarpal The small finger metacarpal usually is the thinnest and smallest of the metacarpals, although the thumb metacarpal, which is much thicker, may be shorter. The overall shape of the small finger metacarpal is similar to that of the other metacarpals, containing a widened proximal base, a narrower curved shaft, and a rounded head (Fig. 1.44; see Figs. 1.25, 1.26, 1.37, and 1.38). It differs most in the shape and characteristics of the base. Internally, the small finger metacarpal is similar to the other metacarpals. The head and base consist of cancellous bone surrounded by a relatively thin cortical shell (see Fig. 1.44). The shaft consists of thicker cortical bone that encircles the open

FIGURE 1.44. Right small finger metacarpal. A: Lateral aspect. B: Medial aspect.

medullary canal. At the base and at the neck, the medullary canal rapidly changes to cancellous bone (127). Base of the Small Finger Metacarpal The base of the small finger metacarpal is larger than that of the ring finger, and slopes proximally and ulnarly. The medial portion of the base is nonarticular and contains a thickening of bone or a tubercle for insertion of the extensor carpi ulnaris. The lateral base of the small finger metacarpal articulates with the ulnar facet of the distal hamate. The articular surface on the metacarpal is transversely concave, and convex from palmar to dorsal. To some degree, this articular surface, which is saddle-shaped, is not unlike the articular surface of the base of the thumb metacarpal. This configuration contributes to the relatively greater motion at the hamate–small finger metacarpal joint compared with the carpometacarpal joints of the index and long finger rays. The overall area of the articular surface at the base is oval or quadrangular and directed somewhat laterally. On the lateral aspect of the base of the small finger metacarpal, there is an oval or narrow facet for articulation with the ring finger metacarpal base. Shaft of the Small Finger Metacarpal The shaft of the small finger metacarpal is slender and curved. It is convex dorsally and concave palmarly. To a large degree, the small finger metacarpal shaft resembles that of the other metacarpals, although it is noticeably shorter and thinner. On the dorsal portion of the lateral aspect, there is a slight concavity for the origin of the ulnar head of the fourth dorsal interosseous. On the palmar portion of the lateral aspect, there is a slight concavity for the

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origin of the third palmar interosseous muscle. On the medial surface of the shaft of the small metacarpal, there is a concavity for attachment of the opponens digiti minimi. The dorsal surface of the shaft of the small metacarpal is smooth to allow passage of the extrinsic extensor tendons. The dorsal surface of the shaft may appear somewhat triangular, similar to the other metacarpal shafts. The shaft of the small metacarpal may taper or become somewhat constricted at the junction of the proximal shaft with the base. This area may be the narrowest portion of the metacarpal. Head of the Small Metacarpal The head of the small finger metacarpal is similar in shape to that of the other metacarpals, although noticeably thinner and smaller. It is rounded, and slightly elongated in the dorsopalmar axis. The head is roughened medially and laterally, with medial and lateral tubercles at the articular margins for attachment of the collateral ligaments and joint capsule. At the margin of the articular surface, there are multiple small vascular foramina through which vessels from the attaching soft tissues enter the head. Associated Joints The base of the small finger metacarpal articulates largely with the distal end of the hamate, through the ulnar distal articular facet of the hamate. In addition, on the lateral base of the small finger metacarpal, there is a narrow, oval or hourglass-shaped articular surface for articulation with the base of the ring finger metacarpal (see Figs. 1.25, 1.26, 1.37, 1.38, 1.40, and 1.44). Distally, the small finger metacarpal articulates with the base of the proximal phalanx of the small finger. Muscle Origins and Insertions There are three major muscle attachments to the small finger metacarpal. These include the origins of the fourth dorsal interosseous (ulnar head), and the insertion of the opponens digiti minimi, and the origin of the third palmar interosseous (see Figs. 1.37 and 1.38). The fourth dorsal interosseous muscle (ulnar head) originates along the shaft of the lateral border of the small finger metacarpal. These fibers are joined by fibers of the fourth dorsal interosseous that originate from the medial border of the ring finger metacarpal (radial head), thus forming a bipennate muscle. The fourth dorsal interosseous then inserts into either the medial base of the proximal phalanx of the ring finger, or into the extensor aponeurosis (approximately 40% bone, 60% extensor aponeurosis) (140,141). The third palmar interosseous muscle originates from the lateral palmar border of the shaft of the small metacarpal. The muscle inserts into the extensor aponeuro-

sis of the small finger, or into the radial side of the base of the proximal phalanx of the small finger. The extensor carpi ulnaris tendon inserts into the dorsomedial base of the small finger metacarpal. There usually is a thickening of bone or a small tubercle for insertion of the tendon. The opponens digiti minimi inserts into the medial border of the shaft of the small finger metacarpal. Clinical Correlations: Small Finger Metacarpal The joint surfaces at the base of the small and ring finger metacarpals are saddle-shaped and flat, respectively, and are not confined by the borders of the adjacent metacarpal bases or carpal bones (as are the index and long finger metacarpals). This allows, in part, the greater motion at the carpometacarpal joints of the small and ring finger, compared with the relatively restricted motion of the carpometacarpal joints of the index and long fingers. The base of each metacarpal, including that of the small finger, is cuboid, wider dorsally than palmarly. This results in a somewhat wedge-shaped bone, with the apex palmar. With this configuration, subluxation or dislocation of the base of the small finger metacarpal on the hamate usually occurs in a dorsal direction. Palmar dislocation of the base of the small finger metacarpal is understandably rare, usually prevented by the wide dorsal portion of the base. Sesamoid Bones Sesamoid bones are common at the metacarpophalangeal joints of the thumb and index and small fingers, and the interphalangeal joint of the thumb. They may be mistaken for fractures, and can themselves be fractured or develop as bipartite sesamoids, further confusing the clinical impression. Schultz provides guidelines for distinguishing sesamoids from fractures. Multipartite sesamoids usually are larger than a normal or fractured sesamoid. Multipartite sesamoids have smooth, more regular opposing surfaces with cortical margins, and may be bilateral. In an acute fracture, the line of fracture is sharp, irregular, assumes any shape, and may be displaced. At times, it may be necessary to see fracture healing before the diagnosis can be made (25). Accessory Bones The os vesalianum manus (os vesalii, os carpometacarpale VIII) is an accessory bone that may be located at the ulnar base of the small finger metacarpal, distal and ulnar to the hamate. If present, it can be mistaken for a fracture. An accessory bone usually represents the residual of a secondary ossification center that does not fuse with the associated bone, but it also may arise from trauma or heterotopic ossi-

1 Skeletal Anatomy

fication of synovial tags (46,47) (see Fig. 1.27B and descriptions earlier, under Ossification Centers and Accessory Bones). PHALANGES Derivation and Terminology The word phalanx is derived from the Greek word for a line or array of soldiers (1). General Features Each digit has three phalanges: proximal, middle, and distal. The thumb has two phalanges: proximal and distal. The proximal and middle digital phalanges all share a similar internal structure, whereas the distal phalanges are markedly different (see later). The phalanges are true long bones with a well defined medullary canal (Fig. 1.45; see Figs. 1.25, 1.26, 1.27A, 1.37, and 1.38), and contain, from proximal to distal, a base, shaft (diaphysis), neck, and head. Each head consists of two condyles. The distal phalanx does not have a true head, but instead terminates in the distal tuft. At either end, the bone becomes wider to form the base and head, with the cortex becoming thinner and the

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internal portion being replaced with cancellous bone. In the diaphysis, similar to other long bones, the cortex is thick, and the medullary canal is open. The proximal phalanges are the longest and largest, the distal the shortest and smallest. Collectively, the three phalanges of the middle finger (long finger) are the longest, resulting in the middle finger usually having the greatest length. The ring finger usually is second in length, and the small finger usually is the shortest. The index finger usually is slightly shorter than the ring finger, but may be equal to or longer than the ring finger (125). Each of the phalanges has two ossification centers (see Fig. 1.27A). The primary center is located in the diaphysis and the secondary center is in the proximal portion, in the epiphysis. Ossification begins prenatally in the shafts at the following periods: distal phalanges, eight or ninth week; proximal phalanges, the tenth week; middle phalanges, the eleventh week or later. The epiphyseal centers appear in the proximal phalanges early in the second year in girls, and later in the second year in boys. In the middle and distal phalanges, the epiphyseal centers appear in the second year in girls, and in the third or fourth year in boys. All of the epiphyses unite approximately the fifteenth to sixteenth year in women, and the seventeenth to eighteenth year in men (5). Because of the differences of the phalanges of the digits and the thumb, their osteology is discussed separately from that of the digital phalanges. PROXIMAL PHALANX OF THE DIGITS Ossification Centers The proximal phalanx of each digit has two ossification centers, one in the shaft and one in epiphysis at the base (Table 1.4; see Fig. 1.27A). The primary ossification in the shaft begins prenatally in approximately the tenth week. The secondary ossification in the base appears early in the second year in girls and later in the second year in boys. The times of ossification of the secondary center of the proximal phalanx vary slightly among the different digits, as described in the following sections (149) (Table 1.4). Ossification of Index Finger Proximal Phalanx In the index finger proximal phalanx, the basal epiphysis first appears in boys at 15 to 18 months of age and in girls at the 9 to 13 months of age. The epiphysis fuses to the shaft in boys between 16 and 17 years of age and in girls between 14 and 15 years of age (149). Ossification of Long Finger Proximal Phalanx

FIGURE 1.45. Illustration of digit showing metacarpophalangeal and interphalangeal joints, palmar aspect.

In the long finger proximal phalanx, the basal epiphysis first appears in boys at 15 to 18 months of age and in girls at 9

TABLE 1.4. APPEARANCE OF OSSIFICATION CENTERS IN THEIR NORMAL SEQUENCE AND DATES OF COMPLETE OSSIFICATION AND FUSION ACCORDING TO W. GREULICH AND S. I. PYLE Sex Capitate Hamate Distal epiphysis of radius Basal epiphysis of proximal phalanx middle finger Basal epiphysis of proximal phalanx index finger Basal epiphysis of proximal phalanx ring finger Capital epiphysis metacarpal index finger Basal epiphysis of distal phalanx of thumb Capital epiphysis of middle metacarpal Capital epiphysis of ring metacarpal Basal epiphysis of proximal phalanx little finger Basal epiphysis of middle phalanx middle finger Basal epiphysis of middle phalanx ring finger Capital epiphysis of metacarpal little finger Basal epiphysis of middle phalanx index finger Triquetrum Basal epiphysis of distal phalanx middle finger Basal epiphysis of distal phalanx ring finger Basal epiphysis of first metacarpal Basal epiphysis of proximal phalanx of thumb Basal epiphysis of distal phalanx little finger Basal epiphysis of distal phalanx index finger Basal epiphysis of middle phalanx of little finger Lunate Lunate Trapezium Trapezoid Scaphoid Distal epiphysis of ulna Pisiform Sesamoid of abductor pollicis

Male Female Male Female Male Female Male Female Male Female Male Female Male Female Male Female Male Female Male Female Male Female Male Female Male Female Male Female Male Female Male Female Male Female Male Female Male Female Male Female Male Female Male Female Male Female Male Female Male Female Male Female Male Female Male Female Male Female Male Female Male Female

First Appearance (mos.)

Adult Status (yrs.)

Birth–3 Birth–3 3 3 12–15 9–15 15–18 9–12 15–18 9–13 15–18 9–12 15–20 9–13 15–18 12–15 15–20 9–13 15–20 9–12 18–24 15–18 18–24 15–18 18–24 15–18 24–30 15–17 24–32 15–18 24–36 18–25 18–24 18–24 18–24 18–24 24–32 18–22 24–32 18–22 36–42 18–24 36–42 24–30 42–48 24–32 32–42 30–36 32–42 30–36 31/2–5 yrs. 36–50 5–6 yrs. 31/2–4 yrs., 2 mo. 5–6 yrs., 4 mo. 31/2–4 yrs., 4 mo. 5 yrs., 3 mo.–6-10 yrs. 51/2–61/2 yrs.

17–18 15–16 14–15 12–13 18–19 17–18 15–17 14–16 16–17 14–15 16–17 14–15 16–17 15 15–151/2 13–131/2 16–17 15 16–17 14–15 16–17 14–15 16–17 14–15 17 15 16–17 14–15 16–17 14–15 15–16 15–16

From, Greulich WW, Pyle SI. Radiographic atlas of skeletal development of the hand and wrist, 2nd ed. Stanford, Stanford University Press, 1959, with permission.

17–18 16–17 12–13 11

1 Skeletal Anatomy

to 12 months of age. The epiphysis fuses to the shaft in boys between 15 and 17 years of age and in girls between 14 and 16 years of age. Ossification of Ring Finger Proximal Phalanx In the ring finger proximal phalanx, the basal epiphysis first appears in boys at 15 to 18 months of age and in girls at 9 to 12 months of age. The epiphysis fuses to the shaft in boys between 16 and 17 years and in girls between 14 and 15 years of age. Ossification of Small Finger Proximal Phalanx In the small finger proximal phalanx, the basal epiphysis first appears in boys at 18 to 24 months of age and in girls at 15 to 18 months of age. The epiphysis fuses to the shaft in boys between 16 and 17 years of age and in girls between 14 and 15 years of age. Osteology of the Proximal Phalanx The proximal phalanx consists of a base, shaft, and head. The proximal phalanx of each digit is similar. The proximal phalanx of the long finger usually is the longest, followed, in decreasing order of size, by the ring, index, and small finger proximal phalanges. The thumb proximal phalanx, described separately later, usually is approximately the length of the small finger proximal phalanx, although the thumb proximal phalanx is much thicker and wider. Base of the Proximal Phalanx The base of each phalanx flares out from the shaft. There is a slight convexity to the dorsal surface of the base. The palmar base is concave, terminating in a thickened ridge or lip that borders the palmar surface of the base at the joint. On the palmar surface of the base of the proximal phalanges there is a slight groove to accommodate passage of the flexor tendons.

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Shaft of the Proximal Phalanx The shaft of each phalanx is smooth, convex dorsally, and concave palmarly, and narrows slightly medially and laterally from proximal to distal, terminating in the narrow neck. This tapering is more pronounced in the middle phalanx compared to the proximal phalanx. The shaft of the proximal phalanx is oval in cross-section, with a slight squaring on the volar aspect as the medial and lateral surfaces meet the palmar surface. The shaft of the phalanges tapers from proximal to distal in both the frontal and sagittal sections, resulting in a narrow distal portion of the shaft. This narrow portion, located just proximal to the head, often is referred to as the neck. Head of the Proximal Phalanx The neck of each proximal phalanx widens abruptly to form the head of the phalanx. The head consists of two condyles. The articular surface has a slight depression seen in the anteroposterior plane, demarcating the two condyles. The articular surface extends further palmarly than dorsally to allow the greater amount of flexion (and relatively limited extension). The articular surface is rounded, as noted on the lateral projection. The head does not have the marked increase in thickness in the anteroposterior direction, as is present in the heads of the metacarpals. Associated Joints The proximal phalanx articulates with the head of the associated metacarpal at each metacarpophalangeal joint, and with the base of the associated middle phalanx at the proximal interphalangeal joint (Fig. 1.46; see Fig. 1.45). The metacarpophalangeal joint is a multiaxial joint that allows movement in the medial and lateral directions, as well as slight rotation, because of the more spherical shape of the metacarpal head and the concavity of the base of the proximal phalanx. The joint is stabilized by the collateral ligaments, accessory collateral ligaments, volar plate, joint capsule, and intrinsic and extrinsic overlying tendons.

FIGURE 1.46. Illustration of digit showing metacarpophalangeal and interphalangeal joints, lateral aspect.

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The proximal interphalangeal joints are stabilized by the collateral ligaments, accessory collateral ligaments, volar plate, joint capsule, and overlying intrinsic and extrinsic tendons. It is a hinge joint, unlike the multiaxial metacarpophalangeal joint. Thus, the proximal interphalangeal joint does not produce the medial and lateral motions or the slight rotation of which the metacarpophalangeal joint is capable. The condyles of the proximal phalanx are symmetric, adding to the stability (and lack of motion) in the medial and lateral planes.

to 18 months of age. The epiphysis fuses to the shaft in boys between 16 and 17 years of age and in girls between 14 and 15 years of age. Ossification of Ring Finger Middle Phalanx In the ring finger middle phalanx, the basal epiphysis first appears in boys at 18 to 24 months of age and in girls at 15 to 18 months of age. The epiphysis fuses to the shaft in boys at approximately 17 years of age and in girls at approximately 15 years of age.

Muscle Origins and Insertions Several muscles insert into the base of the proximal phalanges. The palmar interosseous muscles insert into the ulnar base of the index proximal phalanx, and the radial bases of the ring and small finger proximal phalanges. The flexor digiti minimi and abductor digiti minimi insert into the ulnar base of the small finger proximal phalanx. The first dorsal interosseous inserts, in part, to the radial base of the index finger proximal phalanx. The second and third dorsal interosseous muscles insert, in part, into the radial and ulnar bases of the long finger proximal phalanx, respectively. The fourth dorsal interosseous inserts, in part, into the ulnar base of the ring finger proximal phalanx. The amount of insertion into bone versus that into the extensor mechanism tends to decrease consecutively from the index, long, and ring fingers. This mechanism is complex, and is described in detail in Chapter 2 (26,141) (Figs. 1.37 and 1.38). MIDDLE PHALANX OF THE DIGITS Ossification Centers The middle phalanx of each digit has two ossification centers, one in the shaft and one in epiphysis at the base (see Fig. 1.27A and Table 1.4). The primary ossification in the shaft begins prenatally in approximately the eleventh week or later. The secondary ossification in the base appears early in the second year in girls and in the third or fourth year in boys. The times of ossification in the secondary center of the middle phalanx vary slightly among the different digits, and are described in the following sections (149) (Table 1.4). Ossification of Index Finger Middle Phalanx In the index finger middle phalanx, the basal epiphysis first appears in boys at 24 to 32 months of age and in girls at 15 to 18 months of age. The epiphysis fuses to the shaft in boys between 16 and 17 years of age and in girls between 14 and 15 years of age. Ossification of Long Finger Middle Phalanx In the long finger middle phalanx, the basal epiphysis first appears in boys at 18 to 24 months of age and in girls at 15

Ossification of Small Finger Middle Phalanx In the small finger middle phalanx, the basal epiphysis first appears in boys at 42 to 48 months of age and in girls at 24 to 32 months of age. The epiphysis fuses to the shaft in boys between 16 and 17 years of age and in girls between 14 and 15 years of age. Osteology of the Middle Phalanx The middle phalanges of the digits are similar to each other. Overall, the middle phalanges are shorter than their associated proximal phalanges. The length ratio between the proximal and middle phalanges varies, even in the same individual and in the same hand. In general, the ratio of length of the proximal phalanx to length of the middle phalanx is between 2:1 and 1.3:1, regardless of finger (125) (Table 1.3). The middle phalanx of the long finger usually is the longest, the ring and index middle phalanges are similar (although either may be the longer), and the small finger middle phalanx usually is the shortest. Each phalanx has a base, shaft, and head. Although the general appearance of the middle phalanges is similar to that of the proximal phalanges, distinct differences exist. The palmar aspect of the middle phalanx shaft is not as concave as is the palmar aspect of the proximal phalanx. The lateral crests are thicker in the middle phalanx, and tend to be wider and rougher, occupying the midpart of the phalanx. The nutrient foramina may be more visible or more numerous on the palmar aspect just proximal to the head. In the middle phalanx, the dorsal aspect of the shaft is more narrow proximal to the head and widens to a steeper degree toward the base. The dorsal aspect of the shaft is more convex, smooth, and more nearly round than is the dorsal aspect of the proximal phalanx. The heads of the middle and proximal phalanx are similar in configuration (125). Base of the Middle Phalanx The base of each phalanx flares out from the shaft on the dorsal, medial, lateral, and palmar surfaces. On the dorsal aspect of the base, there is a transverse ridge along the most proximal rim, separating the base from the articular surface. The ridge is more accentuated in its mid-portion, forming a dor-

1 Skeletal Anatomy

sal lip that extends proximally over the joint. This elevated mid-portion forms a tubercle that provides insertion for the central slip of the extensor mechanism. On the lateropalmar aspect of the base, there is a prominent tubercle that terminates in a ridge on the medial and lateral aspects of the base. This tubercle provides insertion of the collateral ligaments. Although the palmar base is concave or flat, it terminates in a thickened ridge or lip on the midpoint that borders the palmar surface of the base at the joint. This tubercle is just distal to the articular surface of the base. Just distal to this tubercle are multiple small foramina for nutrient vessels. The articular surface of the base is divided into facets, consisting of two concave depressions for the two condyles of the head of the proximal phalanx. The two articular facets are separated by a dorsopalmar articular crest that corresponds to the intercondylar depression of the head of the proximal phalanx. This crest extends toward the dorsal tubercle of the base dorsally, and toward the palmar tubercle on the base volarly (125). The palmar tubercle of the base of the middle phalanx forms a palmar prominence in relation to the shafts of the middle and proximal phalanges, and provides mechanical advantages for the function of the flexor digitorum superficialis (125). The presence of the nutrient foramina in the protected areas under the tendon insertion is functionally advantageous because this allows movement of the flexor tendons without interfering with the entering vessels (125). Shaft of the Middle Phalanx The shaft of the middle phalanx is shorter than that of the proximal phalanx (125) (Table 1.3). The middle phalanx can be as much as half the length of the corresponding proximal phalanx, with the ratio of length of the proximal phalanx to length of the middle phalanx between 2:1 and 1.3:1 (125). The shaft of the middle phalanx is less convex dorsally and less concave palmarly compared with the proximal phalanx. The proximal half of the middle phalanx is wider in proportion to the distal half, compared with the proximal phalanx. The radial and ulnar borders of the shaft are concave, and when viewed from dorsally, the shaft has a slight hourglass shape, with the narrowest portion located slightly distal to the mid-portion. There are prominent crests on the proximal half of the shaft on both the radial and ulnar aspects. On the palmar aspect of the middle phalanx, along the radial and ulnar portions of the proximal half, the cortex is rough for the insertion of the flexor digitorum superficialis. This rough area tends to blend with the roughened proximal shaft and base, for attachment of the volar plate and joint capsule. The narrow portion of the shaft just proximal to the head of the middle phalanx often is referred to as the neck. Head of the Middle Phalanx The head of the middle phalanx is similar to that of the proximal phalanx, although much smaller. The head of

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each phalanx widens abruptly from the neck of the shaft. The head consists of two condyles. The articular surface has a slight depression seen in the anteroposterior plane, demarcating the two condyles. The articular surface extends further palmarly than dorsally to allow the greater amount of flexion (and relatively limited extension). The articular surface is rounded, as noted on the lateral projection. The head does not increase in thickness in the anteroposterior direction, as do the heads of the metacarpals. DISTAL PHALANX OF THE DIGITS Ossification Centers The distal phalanx of each digit has two ossification centers, one in the shaft and one in the epiphysis at the base (see Fig. 1.27A and Table 1.4). The primary ossification in the shaft begins prenatally in the eight or ninth week. The secondary ossification in the base appears early in the second year in girls and in the third or fourth year in boys. The times of ossification of the secondary center of the distal phalanx vary slightly among the different digits, and are described in the following sections (149) (Table 1.4). Ossification of Index Finger Distal Phalanx In the index finger distal phalanx, the basal epiphysis first appears in boys at 36 to 42 months of age and in girls at 24 to 30 months of age. The epiphysis usually fuses to the shaft in boys between 17 and 18 years of age and in girls between 15 and 16 years of age. Ossification of Long Finger Distal Phalanx In the long finger distal phalanx, the basal epiphysis first appears in boys at 18 to 24 months of age and in girls at 18 to 24 months of age. The epiphysis usually fuses to the shaft in boys between 17 and 18 years of age and in girls between 15 and 16 years of age. Ossification of Ring Finger Distal Phalanx In the ring finger distal phalanx, the basal epiphysis first appears in both boys and girls at 18 to 24 months of age. The epiphysis usually fuses to the shaft in boys at approximately 17 to 18 years of age and in girls at approximately 15 to 16 years of age. Ossification of Small Finger Distal Phalanx In the small finger distal phalanx, the basal epiphysis first appears in boys at 36 to 42 months of age and in girls at 18 to 24 months of age. The epiphysis usually fuses to the shaft in boys between 17 and 18 years of age and in girls between 15 and 16 years of age.

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Osteology of the Distal Phalanx The distal phalanges differ in size, shape, and contour from the proximal and middle phalanges. Each has a base, a shaft, and distal tuft. Although the base and, to some degree, the shaft share similarities to the proximal and middle phalanges, the tuft is quite different in size and configuration. When compared with each other, the distal phalanges of the long and ring finger tend to be similar in length, followed by the slightly smaller index distal phalanx, followed in turn by the shortest small finger distal phalanx. In some individuals, the long finger distal phalanx may be up to 2 mm longer than the others (125). All of the distal phalanges are much shorter and thinner than the distal phalanx of the thumb. The widths of all of the distal phalanges are similar, with the exception of the small finger, which usually is thinner. In general, the shape and overall outline of the base of the distal phalanges are similar to those of the middle phalanges. The shaft of the distal phalanges differs slightly from those of the proximal and middle phalanges, with the distal phalanx containing a shaft that is shorter, narrower, and straighter and lacking the curved contours (convex dorsally) of the others. The distal phalanx terminates in the roughened distal tuft that is wider than the shaft. The average length ratios of the middle phalanx to the distal phalanx (with the middle phalanx used as a unit) are as follows: index, 1:0.6 to 1:0.9; long, 1:0.6 to 1:0.7; ring, 1.0.6 to 1:0.7; small 1:1 to 1:0.8 (125) (Table 1.3). Base of the Distal Phalanx The base of the distal phalanx usually has the same width (or is slightly wider) than the adjacent head of the middle phalanx. In general shape, it resembles the base of the middle phalanx, although much smaller. On the dorsal aspect, the base flares out dorsally and centrally, creating a ridge that separates the articular surface from the shaft. The dorsal base is roughened slightly and forms a raised area, the dorsal tubercle. The dorsal tubercle provides the insertion site of the extensor digitorum communis (and extensor indicis proprius on the index distal phalanx). On the radial and ulnar aspects of the base are bone prominences known as the lateral tubercles. The lateral tubercles are roughened and raised, and serve for the attachment of the collateral ligaments and joint capsule of the distal interphalangeal joint. The lateral tubercles are most pronounced on the volar half of the base. On the volar surface of the base, there is a palmar lip or ridge along the joint margin, known as the volar tubercle (125). The surface of the palmar aspect of the base is, however, somewhat flatter and rougher, and irregular. This area provides the insertion site for the flexor digitorum profundus. In this area, multiple small foramina are present for passage of the nutrient vessels.

Shaft of the Distal Phalanx The shafts of the distal phalanges are short and thin compared with the shafts of the middle and proximal phalanges. The shafts also are much shorter and thinner than that of distal phalanx of the thumb. The shaft is wide proximally and becomes progressively thinner as the tuft is approached. The narrowest portion of the shaft is just proximal to the formation of the tuft. The dorsal surface of the shaft is rounded and slightly convex, but much less so than that of the middle and proximal phalanges. On the palmar surface, the shaft is slightly concave, but to a lesser degree than in the middle and proximal phalanges. The medial and lateral surfaces are rounded, and the widest portion of the shaft is slightly volar. On cross-section, the shaft of the distal phalanx thus is oval or slightly triangular, with the base on the volar half of the shaft. Tuft of the Distal Phalanx The distal phalanges terminate in a roughened, wide portion known as the tuft. The tuft consists of a thicker ridge of bone that is crescent-shaped and lines the distal portion of the distal phalanx. The crest is symmetric when viewed from the palmar or dorsal aspect. When viewed dorsally, the tuft is a thicken margin along the distal aspect of the phalanx, usually a few millimeters thick. When viewed from the palmar surface, the margin of the tuft is thicker and extends more proximally. The medial and lateral portions of the tuft on the volar surface extend a few millimeters more proximal into the shaft than the central volar portion. This thickened area thus forms a horseshoe shape, opened proximally. On the medial and lateral surfaces of the tuft, the thickest portion extends obliquely, from proximal volar to distal dorsal. Several small foramina are visible on the distal tuft for entrance of nutrient vessels. These are most numerous on the palmar surface. The tuft provides for the attachment of the septa that help support, stabilize, and anchor the pulp of the digit to the distal phalanx. Associated Joints The base of the distal phalanx articulates with the head of the middle phalanx through the distal interphalangeal joint. The distal interphalangeal joint is a hinge joint. The joint surface of the base of the distal phalanx has two facets, medial and lateral, which articulate with the corresponding medial and lateral condyles of the head of the middle phalanx. The joint is stabilized by the collateral ligaments, accessory collateral ligaments, the volar plate, and extrinsic tendons of the flexor digitorum profundus and extensor digitorum communis (and extensor indicis proprius of the index finger). Muscle Origins and Insertions The flexor digitorum profundus inserts into the palmar surface of the base of the distal phalanx. The extensor digito-

1 Skeletal Anatomy

rum communis inserts into the dorsal surface of the base of the distal phalanx. The extensor indicis proprius also inserts into the dorsal surface of the base of the distal phalanx, slightly ulnar to the extensor digitorum communis.

THUMB PROXIMAL PHALANX Ossification Centers The thumb proximal phalanx has two ossification centers: a primary center in the shaft and a secondary center in the epiphysis (at the base; see Fig. 1.27A and Table 1.4). Ossification begins prenatally in the shafts, usually in the tenth week. Ossification in the epiphyseal center appears in the mid-portion in the second year in girls (18 to 22 months of age), and in the later months of the second year or in the early months of the third in boys (24 to 32 months of age). The epiphysis unites to the shaft at approximately the fifteenth to sixteenth year in girls and in the seventeenth to eighteenth year in boys (5,149). Osteology of the Thumb Proximal Phalanx The proximal phalanx of the thumb consists of a base, shaft, and a head. Overall, the proximal phalanx resembles the other proximal phalanges, but in general is shorter, with a length approximately that of the proximal phalanx of the small finger. It is thicker than that of the small finger.

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Shaft of the Thumb Proximal Phalanx The shaft of the proximal phalanx of the thumb is approximately the length of the proximal phalanx of the small finger (although it actually may be shorter). The shaft is relatively thick, especially in its proximal portion, compared with the other proximal phalanges. The shaft is rounded and smooth, and in cross-section it is round or oval, slightly flatted palmarly, and, to a lesser degree, flattened dorsally. The shaft does not have the lateral crests seen on the proximal phalanges of the digits. Very seldom can a small foramen for a nutrient vessel be identified on the shaft (125). Head of the Thumb Proximal Phalanx The head of the thumb proximal phalanx resembles the head of the other proximal phalanges. The head is slightly larger, with a wider articulating surface. The articular surface extends more palmarly, and when viewed in the lateral projection, the articular surface appears symmetrically rounded. (There is no increase in thickness in the anteroposterior direction, as is noted in the heads of the metacarpals). It has a well defined margin separating the articular surface from the palmar and dorsal surfaces of the shaft. The head has two condyles, easily visualized in the anteroposterior plane. The medial and lateral surfaces of the head are flat and roughened to provide attachment for the collateral ligaments and joint capsule. The flattened areas laterally give the squared appearance of the head as seen on the anteroposterior view. Several small foramina are located just proximal to the articular surface, especially on the palmar surface, providing access for nutrient vessels.

Base of the Thumb Proximal Phalanx The base of proximal phalanx of the thumb is similar to the base of the proximal phalanges of the digits. The base flares out from the shaft, more noticeably on the palmar surface than dorsally. The dorsal surface of the base is flatter than the palmar surface, and has a slight convexity. There also is a slight crest or roughened area that separates the articular surface from the shaft. This dorsal roughened area provides the insertion site for the extensor pollicis brevis. The palmar base is concave, terminating in a thickened ridge that borders the palmar surface of the base at the metacarpophalangeal joint. On the palmar surface of the base of the proximal phalanges there is a slight groove to accommodate the flexor tendons. This groove is better delineated in the proximal phalanges of the digits compared with that of the thumb. The articular surface at the base of the proximal thumb phalanx differs slightly from those of the digital proximal phalanges. In the thumb, the articular surface at the base is flatter and less concave to accommodate the articular surface of the head of the thumb metacarpal, which tends to be flatter and less spherical than in the other metacarpals.

Associated Joints The proximal phalanx of the thumb articulates proximally with the head of the thumb metacarpal at the thumb metacarpophalangeal joint. The proximal phalanx of the thumb articulates distally with the base of the thumb distal phalanx. The thumb metacarpophalangeal joint is similar to that of the other metacarpophalangeal joints; however, because of the shape of the adjoining joint surfaces, the joint is more hingelike instead of multiaxial, as in the others (125). The metacarpophalangeal joint of the thumb is associated with two sesamoid bones located in the volar plate or thenar tendons. The lateral sesamoid usually is slightly larger than the medial sesamoid. The joint is stabilized by collateral ligaments, accessory collateral ligaments, and joint capsule, along with the intrinsic muscles (flexor pollicis brevis, abductor pollicis brevis, extensor pollicis brevis, and adductor pollicis) and the overlying extrinsic tendons (flexor pollicis longus and extensor pollicis longus). The interphalangeal joint of the thumb is a hinge joint, larger than the interphalangeal joints of the digits. It is stabilized by the collateral ligaments, accessory collateral

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ligaments, volar plate, and overlying extrinsic tendons (flexor pollicis longus and extensor pollicis longus). Muscle Origins and Insertions Several muscles insert into the base of the thumb proximal phalanx. The extensor pollicis brevis inserts into the dorsal surface of the base. The abductor pollicis brevis inserts into the radial aspect of the base. The flexor pollicis brevis inserts into the palmar base. The adductor pollicis inserts into the ulnar aspect of the base. THUMB DISTAL PHALANX Ossification Centers The thumb distal phalanx has two ossification centers: a primary center in the shaft and a secondary center in the epiphysis (at the base; see Fig. 1.27A and Table 1.4). Ossification begins prenatally in the shaft, usually in the eighth or ninth week. Ossification in the epiphyseal center appears in the second year in girls (12 to 15 months of age), and in the later months of the second year in boys (15 to 18 months of age). The epiphysis unites to the shaft at approximately the thirteenth year in girls and in the fifteenth year in boys (5,149). Osteology of the Thumb Distal Phalanx The distal phalanx of the thumb is markedly larger; it is longer, thicker, and wider, than the distal phalanges of the other digits. However, other than the size, the overall characteristics and osteology are similar to those of the other distal phalanges. The distal phalanx of the thumb consists of a base, shaft, and a tuft. The tuft occasionally is incorrectly referred to as the head. Base of the Thumb Distal Phalanx The base of the thumb distal phalanx is wide and thick, with pronounced flaring medially and laterally. There is a ridge along the dorsal, medial, and lateral surfaces, outlining the articular surface. The dorsal base is roughened and has a thick crest just distal to the articular surface. The crest is more elevated in the central portion and provides the attachment site of the extensor pollicis longus. On the medial and lateral surfaces of the base, there is pronounced flaring. Each side has small, irregular tubercles for attachment of the collateral ligaments and joint capsule. These tubercles are more accentuated in the thumb than in the phalanges. The palmar surface of the base is flatter (or even slightly concave) compared with the flared dorsal base. With less flaring and a flatter surface, the palmar base joins the shaft in a more gradual manner. The palmar surface is rough for attachment of the flexor pollicis longus. The

articular surface at the base is divided by a slight midcrest into two concave surfaces. These surfaces articulate with the condyles of the head of the proximal phalanx. The base of the thumb proximal phalanx has multiple small foramina for nutrient vessels. These are most easily visualized on the palmar surface. Shaft of the Thumb Distal Phalanx The shaft (and overall length) of the thumb distal phalanx is longer and wider than those of the digits. The shaft is rounded on the dorsal and lateral surfaces, and somewhat flat or slightly convex on the palmar surface. On cross-section, the shaft is oval or hemispherical in shape, and appears much flatter than in the other digits. Specific foramina for nutrient vessels usually are not visualized on the shaft. Tuft of the Thumb Distal Phalanx The distal tuft is a thickened, widened distal tip that expands abruptly from the shaft. On the anteroposterior projection, the tuft is oval, triangular, or somewhat diamond-shaped. It contains a thickened rim along the distal, medial, and lateral margins. The palmar surface of the tuft is smoother and less pronounced, and blends with the shaft in a gradual manner. The tuft is roughened to provide for the attachments of the many septa that help support, stabilize, and anchor the pulp of the distal portion of the thumb. Associated Joints The distal phalanx of the thumb articulates with the head of the proximal phalanx through the interphalangeal joint of the thumb. The articular surface of the base of the distal phalanx is divided into two concave surfaces that articulate with the two corresponding condyles of the head of the proximal phalanx. The joint is a uniaxial hinge joint, stabilized by two collateral ligaments, two accessory collateral ligaments, a volar plate, and a joint capsule. The extrinsic tendons of the extensor pollicis longus and flexor pollicis longus move the joint, as well as adding stabilization. Muscle Origins and Insertions There are two muscle insertions on the thumb distal phalanx. The extensor pollicis longus inserts into the base of the phalanx on the dorsal surface. The flexor pollicis longus inserts into the base of the phalanx on the palmar surface. REFERENCES 1. Dorland’s illustrated medical dictionary, 28th ed. Philadelphia: WB Saunders, 1994. 2. Pick TP, Howden R. Gray’s anatomy: descriptive and surgical. Philadelphia: Running Press, 1974:134–171.

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75. Shakir A, Zaini S. Skeletal maturation of the hand and wrist of young children in Baghdad. Ann Hum Biol 1:189–199, 1974. 76. Wingerd J, Peritz E, Sproul A. Race and stature differences in the skeletal maturation of the hand and wrist. Ann Hum Biol 1: 201–209, 1974. 77. Smitham JH. Some observations on certain congenital abnormalities of the hand in African natives. Br J Radiol 21:513–518, 1948. 78. Obletz BE, Halbstein BM. Non-union of fractures of carpal navicular. J Bone Joint Surg 20:424-428, 1938. 79. Gelberman RH, Menon J. The vascularity of the scaphoid bone. J Hand Surg [Am] 5:508–513, 1980. 80. Moritomo H, Viegas SF, Nakamura K, et al. The scaphotrapezio-trapezoidal joint. Part 1: an anatomic and radiographic study. J Hand Surg [Am] 25:899–910, 2000. 81. Viegas SF. Variations in the skeletal morphology of the wrist. Clin Orthop 383:21–31, 2001. 82. Barber H. The intraosseous arterial anatomy of the adult human carpus. Orthopedics 5:1–19, 1972. 83. Grettve S. Arterial anatomy of the carpal bones. Acta Anat 25: 331–345, 1955. 84. Taleisnik J, Kelly PJ. Extraosseous and intraosseous blood supply of the scaphoid bone. J Bone Joint Surg Am 1966;48:1125–1137. 85. Taleisnik J. The vascular anatomy of the wrist. In: Taleisnik J, ed. The wrist. New York: Churchill Livingstone, 1985:51–78. 86. Amadio PC, Taleisnik J. Fractures of the carpus. In: Green DP, Hotchkiss RN, Pederson WC, eds. Green’s operative hand surgery, 4th ed. New York: Churchill Livingstone, 1999:809–864. 87. Preiser G. Zur Frage der typischen traumatischen Ernahrungsstorungen der kurzen Hand- und Fusswurzelknochen. Fortschr Geb Roentgenstr 17:360–362, 1911. 88. Vidal MA, Linscheid RL, Amadio PC, et al. Preiser’s disease. Ann Chir Main Memb Super 10:227–235, 1991. 89. Poznanski AK. Hand and radiologic diagnosis: with gambits and pattern profiles, 2nd ed. Philadelphia: WB Saunders, 1984:67–96. 90. Case JT. Borderlands of the normal and early pathologic and skeletal roentgenology, 10th ed (2nd American ed). New York: Gruhn and Stratton, 1961:87–93. 91. Edggimann P. Zur Bipartition des Lunatum. Radiol Clin Biol 20:65–70, 1951. 92. Patterson RM, Elder K, Brewer L, et al. Normative carpal bone anatomy measured by computer analysis of three-dimensional reconstructions of computed tomography images. J Hand Surg [Am] 20:923–929, 1995. 92a. Zapico JMA. Malacia del semilunatar. Doctoral thesis. Valladolid, Spain: Universidad de Valladolid, Secretariado de Publicaciones, 1966. 93. Shepherd FJ. A note on the radiocarpal articulation. J Anat 25: 349, 1980. 94. Viegas SF, Patterson RM, Hokanson JA, et al. Wrist anatomy: incidence, distribution and correlation of anatomy, tears and arthritis. J Hand Surg [Am] 18:463–475, 1993. 95. Viegas SF, Wagner K, Patterson RM, et al. The medial (hamate) facet of the lunate. J Hand Surg [Am] 15:564–571, 1990. 96. Burgess RC. Anatomic variations of the midcarpal joint. J Hand Surg [Am] 15:129–131, 1990. 97. Sagerman SD, Hauck RM, Palmer AK. Lunate morphology: can it be predicted with routine x-ray films? J Hand Surg [Am] 20:38–41, 1995. 98. Gelberman RH, Bauman TD, Menon J, et al. The vascularity of the lunate bone and Kienbock’s disease. J Hand Surg [Am] 5: 272–278, 1980. 99. Panagis JS, Gelberman RH, Taleisnik J, et al. The arterial anatomy of the human carpus: II. the intraosseous vascularity. J Hand Surg [Am] 8:375–382, 1983. 100. Metz VM, Schimmerl SM, Gilula LA, et al. Wide scapholunate

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joint space in lunotriquetral coalition: a normal variant? Radiology 188:557–559, 1993. Minnaar AB de V. Congenital fusion of the lunate and triquetral bones in the South African Bantu. J Bone Joint Surg Br 34: 45–48, 1952. Simmons BB, McKenzie WD. Symptomatic carpal coalition. J Hand Surg [Am] 10:1990–1992, 1985. Cope JR. Carpal coalition. Clin Radiol 25:261–266, 1974. Carlson DH. Coalition of the carpal bones. Skeletal Radiol 7: 125–127, 1981. Morreels CL Jr, Fletcher BD, Weilbaecher RG, et al. Roentgenographic features of homocystinuria. Radiology 90: 1150–1158, 1968. Postacchini F, Ippolito E. Isolated absence of human carpal bones. Teratology 11:267–272, 1975. Viegas SF. The lunohamate articulation of the midcarpal joint. Arthroscopy 6:5, 1990. Viegas SF, Hillman G, Elder K, et al. Measurement of carpal bone geometry by computer analysis of 3D CT images. J Hand Surg [Am] 18:341–349, 1993. Nakamura K, Beppu M, Matsushita K, et al. Biomechanical analysis of the stress force on midcarpal joint in Kienbock’s disease. Hand Surg 2:101–115, 1997. Stalh F. On lunatomalacia (Kienbock’s disease): a clinical and roentgenological study, especially on its pathogenesis and the late results of immobilization treatment. Acta Chir Scand 45[Suppl 126]:1–133, 1947. Lichtman DM, Alexander AH, Mack GR, et al. Kienbock’s disease: update on silicone replacement arthroplasty. J Hand Surg [Am] 7:343, 1982. Marak FM. Avascular necrosis of the carpal lunate. Clin Orthop 10:96–107, 1957. Botte MJ, Gelberman RH. Fractures of the carpus, excluding the scaphoid. Hand Clin 3:149–150, 1987. Levy M, Fischel RE, Stern GM, et al. Chip fractures of the os triquetrum: the mechanism of injury. J Bone Joint Surg Br 61: 355–357, 1979. Bryan RS, Dobyns JH. Fractures of the carpal bones other than lunate or navicular. Clin Orthop 149:107–111, 1980. Stark HH, Chao EK, Zemel NP, et al. Fracture of the hook of the hamate. J Bone Joint Surg Am 71:1202–1207, 1989. Viegas SF, Crossley M, Marzske M, et al. The fourth carpometacarpal joint. J Hand Surg [Am] 16:525–533, 1991. El Bacha AN, Tubiana R. The hand, vol II. Philadelphia: WB Saunders, 1985:158–168. Fenton RL. The naviculo-capitate fracture syndrome. J Bone Joint Surg Am 38:681–684, 1956. Kuhlmann JN, Fournol S, Mimoun M, et al. Fracture of the lesser multangular (trapezoid) bone. Ann Chir Main Memb Super 5:133–134, 1986. Ateshian GA, Rosenwasser MP, Mow VC. Curvature characteristics and congruence of the thumb carpometacarpal joint: differences between female and male joints. J Biomech 25: 591–607, 1992. Cordrey IJ, Ferrer-Torells M. Management of the fractures of the greater multangular: report of five cases. J Bone Joint Surg Am 42:1321–1322, 1963. Palmer AK. Trapezial ridge fractures. J Hand Surg [Am] 6:561, 1981. Botte MJ, von Schroeder HP, Gellman H, et al. Fracture of the trapezial ridge. Clin Orthop 276:202–205, 1992. Posner MA, Kaplan EB. Osseous and ligamentous structures. In: Spinner M, ed. Kaplan’s functional and surgical anatomy of the hand, 3rd ed. Philadelphia: JB Lippincott, 1984:23–50. Kaplan EB. Functional and surgical anatomy of the hand, 2nd ed. Philadelphia: JB Lippincott, 1965.

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127. Joseph J. Further studies of the metacarpophalangeal and interphalangeal joints of the thumb. J Anat 85:221–229, 1951. 128. Hollinshead WH. Anatomy for surgeons: the back and limbs.. Philadelphia: Harper and Row, 1982:259–562. 129. Broom R. The origin of the human skeleton: an introduction to human osteology. London: Witherby, 1930. 130. Nicholson GW. Studies on tumour formation: sacro-coccygeal teratoma with three metacarpal bones and digits. Guys Hosp Rep 87:46–106, 1937. 131. Bell MJ, McMurtry RY, Rubenstein J. Fracture of the ulnar sesamoid of the metacarpophalangeal joint of the thumb: an arthrographic study. J Hand Surg [Br] 10:379–381, 1985. 132. Clarke P, Braunstein EM, Ewissman BN, et al. Case reports: sesamoid fracture of the thumb. Br J Radiol 56:485, 1983. 133. Gibeault JD, Saba P, Hoenecke H, et al. The sesamoids of the metacarpophalangeal joint of the thumb: an anatomical and clinical study. J Hand Surg [Br] 14:244–247, 1989. 134. Hansen CA, Peterson TH. Fracture of the thumb sesamoid bones. J Hand Surg [Am] 12:269–270, 1987. 135. Ishizuki M, Nakagawa T, Ito S. Hyperextension injuries of the metacarpophalangeal joint of the thumb. J Hand Surg [Br] 19: 361–367, 1994. 136. Jones RP, Leach RE. Fracture of the ulnar sesamoid bone of the thumb. Am J Sports Med 8:446–447, 1980. 137. Stener B. Hyperextension injuries to the metacarpophalangeal joint of the thumb: rupture of ligaments, fracture of sesamoid bones, rupture of flexor pollicis brevis. An anatomical and clinical study. Acta Chir Scand 125:275–293, 1963. 138. Trumble TE, Watson HK. Posttraumatic sesamoid arthritis of the metacarpophalangeal joint of the thumb. J Hand Surg [Am] 10:94–100, 1985. 139. Wood VE. The sesamoid bones of the hand and their pathology. J Hand Surg [Br] 9:261–264, 1984. 140. Eyler DL, Markee JE. The anatomy and function of the intrinsic musculature of the fingers. J Bone Joint Surg Am 36:1381, 1954. 141. Smith RJ. Intrinsic muscles. Instr Course Lect 1975. 142. Lewis OJ. Joint remodeling and the evolution of the human hand. J Anat 123:157–201, 1977 143. Steele DJ, Bramblett CA. The anatomy and biology of the human skeleton. College Station, TX: Texas A & M University Press, 1988:176–177. 144. Bergman RA, Thompson SA, Afifi AK, et al. Compendium of human anatomic variation. Baltimore: Urban and Schwarzenberg, 1988:206–227. 145. Marzke M, Wullstein K, Viegas SF. Variability at the carpometacarpal and midcarpal joints involving the fourth metacarpal, hamate, and lunate in catarrhini. Am J Phys Anthropol 93:229–240, 1994. 146. Shipman P, Walker A, Bichell D. The human skeleton. Cambridge, MA: Harvard University Press, 1985:111–117. 147. Anderson JE. Grant’s atlas of anatomy, 8th ed. Baltimore: Williams & Wilkins, 1983. 148. Warwick R, Williams PL. Gray’s anatomy, 35th British ed. Philadelphia: WB Saunders, 1973:340–341, 436–440. 149. Greulich WW, Pyle SI. Radiographic atlas of skeletal development of the hand and wrist, 2nd ed. Stanford, CA: Stanford University Press, 1959.

SUGGESTED READINGS Bennett EH. Fractures of the metacarpal bones. Dublin J Med Sci 73: 72–75, 1882. Bennett EH. On fractures of the metacarpal bone of the thumb. BMJ 2:12–13, 1886.

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Berger RA, Beckenbaugh RD, Linscheid RL. Arthroplasty in the hand and wrist. In: Green DP, Hotchkiss RN, Pederson WC, eds. Green’s operative hand surgery, 4th ed. New York: Churchill Livingstone, 1999:147–191. Bigliani LU, Flatow EL, Pollock RG. Fractures of the proximal humerus. In: Rockwood CA Jr, Green DP, Bucholz RW, et al, eds. Rockwood and Green’s fractures in adults, 4th ed. Philadelphia: Lippincott–Raven, 1996:1055–1107. Bowers WH. The distal radioulnar joint. In: Green DP, Hotchkiss RN, Pederson WC, eds. Green’s operative hand surgery, 4th ed. New York: Churchill Livingstone, 1999:987–1032. Butters, KP. Fractures and Dislocation of the Scapula. In: Rockwood CA Jr, Green DP, Bucholz RW, et al, eds. Rockwood and Green’s fractures in adults, 4th ed. Philadelphia: Lippincott–Raven, 1996: 1163–1192. Cabanela ME, Morrey BF. Fractures of the proximal ulna and olecranon. In: Morrey BF, ed. The elbow and its disorders. Philadelphia: WB Saunders, 1993. Christie A. Prevalence and distribution of ossification centers in the newborn infant. Am J Dis Child 77:355–361, 1949. Colles A. On the fracture of the carpal extremity of the radius. Med Classics 4:1038–1042, 1940. Craig SM. Anatomy of the joints of the fingers. Hand Clin 8: 693–700, 1992. Eaton RG. Joint injuries of the hand. Springfield, IL: Charles C Thomas, 1971. Engebretsen L, Craig E. Disorders of the acromioclavicular joint. In: Peimer CA, ed. Surgery of the hand and upper extremity. New York: McGraw-Hill, 1996:225–231. Engebretsen L, Craig E. Disorders of the scapula. In: Peimer CA, ed. Surgery of the hand and upper extremity. New York: McGraw-Hill, 1996:233–239. Engebretsen L, Craig E. Disorders of the sternoclavicular joint. In: Peimer CA, ed. Surgery of the hand and upper extremity. New York: McGraw-Hill, 1996:219–224. Failla JM. Hook of hamate vascularity: vulnerability to osteonecrosis and nonunion. J Hand Surg [Am] 18:1075–1079, 1993. Fernandex DL, Palmer AK. Fractures of the distal radius. In: Green DP, Hotchkiss RN, Pederson WC, eds. Green’s operative hand surgery, 4th ed. New York: Churchill Livingstone, 1999:929–985. Flatt AE. The care of minor hand injuries. St. Louis: CV Mosby, 1959. Francis CC, Werle PP. The appearance of centers of ossification from birth to five years. Am J Phys Anthropol 24:272–299, 1939. Gad P. The anatomy of the volar part of the capsules of the finger joints. J Bone Joint Surg Br 49:362–367, 1967. Garcia-Ellias M. Carpal instabilities and dislocations. In: Green DP, Hotchkiss RN, Pederson WC, eds. Green’s operative hand surgery, 4th ed. New York: Churchill Livingstone, 1999:865–928. Gardner E, Gray O, O’Rahilly R. Anatomy, 3rd ed. Philadelphia: WB Saunders, 1969. Gelberman RH, Gross MS. The vascularity of the wrist: identifying arterial patterns at risk. Clin Orthop 202:40–49, 1986. Gelberman RH, Salamonh PB, Jurist JM, et al Ulnar variance in Kienbock’s disease. J Bone Joint Surg Am 57:674–676, 1975. Glickel SZ, Barron OA, Eaton RG. Dislocations and ligament injuries in the digits. In: Green DP, Hotchkiss RN, Pederson, WC, eds. Green’s operative hand surgery, 4th ed. New York: Churchill Livingstone, 1999:772–808. Goss CM. Gray’s anatomy, 26th ed. Philadelphia: Lea & Febiger, 1954. Green DP, Butler TE. Fractures and dislocations in the hand. In: Rockwood CA Jr, Green DP, Bucholz RW, et al, eds. Rockwood and Green’s fractures in adults, 4th ed. Philadelphia: Lippincott–Raven, 1996:607–744. Greenspan A. Upper limb II: distal forearm, wrist, and hand. In:

Greenspan A, ed. Orthopedic radiology: a practical approach, 3rd ed. Philadelphia: Lippincott Williams & Wilkins, 2000:151–195. Haines RW. The extensor apparatus of the finger. J Anat 65:251–259, 1951. Handley RC, Pooley J. The venous anatomy of the scaphoid. J Anat 178:115–118, 1991. Hardman TG, Wigoder SB. An unusual development of the carpal scaphoid. Br J Radiol 1:155–158, 1928. Howard LD Jr. Fractures of the small bones of the hand. Plast Reconstr Surg 29:334, 1962. James JIP. Fractures of the proximal and middle phalanges of the fingers. Acta Orthop Scand 32:401–412, 1962. Jit I, Singh S. Estimation of stature from clavicles. Indian J Med Res 44:137–155, 1956. Kjaer-Petersen K, Langhoff O, Andersen K. Bennett’s fracture. J Hand Surg [Br] 15:58–61, 1990. Kuczynski K. The proximal interphalangeal joint: anatomy and causes of stiffness in the fingers. J Bone Joint Surg Br 50:656–663, 1968. Kuhlmann JN, Guerin-Surville H, Boabighi A. Vascularization of the carpus: a systematic study. Surg Radiol Anat 10:21–28, 1988. Kuschner SH, Shepard L, Stephens S, et al. Fracture of the index metacarpal base with subluxation of the trapeziometacarpal joint: a case report. Clin Orthop 264:197–199, 1991. Landsmeer JMF. Anatomical and functional investigations of the human finger, and its functional significance. Acta Anat 24[Suppl]:1–69, 1955. Last RJ. Anatomy: regional and applied, 4th ed. London: J and A Churchill, 1966. Lee MCH. The intraosseous arterial pattern of the carpal lunate. Acta Orthop Scand 33:43–55, 1963. Leibovic SJ, Bowere WH. Anatomy of the proximal interphalangeal joint. Hand Clin 10:169–178, 1994. MacConaill MA. The mechanical anatomy of the carpus and its bearings on some surgical problems. J Anat 75:166–175, 1941. McMurrich JP. The nomenclature of the carpal bones. Anat Rec 8: 173–182, 1914. Mestdagh H, Bailleu JP, Chambor JP, et al. The dorsal arterial network of the wrist with reference to the blood supply of the carpal bones. Acta Morphol Neerl Scand 17:73–80, 1979. Meyer DB. The prenatal development of the skeleton. Thesis. Detroit: Wayne State University, 1957. Meyers MH, Wells R, Harvey J. Naviculocapitate fracture syndrome. J Bone Joint Surg Am 53:1383–1386, 1971. Milford LW. Retaining ligaments of the digits of the hand: gross and microscopic anatomic study. Philadelphia: WB Saunders, 1968. Minamikawa Y, Horii E, Amadio EPC, et al. Stability and constraint of the proximal interphalangeal joint. J Hand Surg [Am] 18: 198–204, 1993. Minne J, Depreux R, Mestagh H, et al. Les pedicules arteriels du massif carpien. Lille Med 18:1174–1185, 1973. Monahan PRW, Galasko CSB. The scapho-capitate fracture syndrome: a mechanism of injury. J Bone Joint Surg Br 54:122–124, 1972. Noback CR, Robertson GGT. Sequences of appearance of ossification centers in the human skeleton during the first five prenatal months. Am J Anat 89:1–28, 1951. Norris TR, Okamura G. Anterior and multidirectional shoulder instability. In: Peimer CA, ed. Surgery of the hand and upper extremity. New York: McGraw-Hill, 1996:273–298. Oberlin C, Salon A, Pigeau I, et al. Three-dimensional reconstruction of the carpus and its vasculature: an anatomic study. J Hand Surg [Am] 17:767–772, 1992. Patel MR, Pearlman HS, Bassini L, et al. Fractures of the sesamoid bones of the thumb. J Hand Surg [Am] 15:776–781, 1990.

1 Skeletal Anatomy Poehling GG, Ruch DS. Wrist arthroscopy: anatomy and diagnosis. In: Green DP, Hotchkiss RN, Pederson WC, eds. Green’s operative hand surgery, 4th ed. New York: Churchill Livingstone, 1999: 192–199. Pollock RG. Fractures of the proximal humerus: shoulder region. In: Peimer CA, ed. Surgery of the hand and upper extremity. New York: McGraw-Hill, 1996:241–258. Ramamurthy S, Hickey R. Anesthesia. In: Green DP, Hotchkiss RN, Pederson WC, eds. Green’s operative hand surgery, 4th ed. New York: Churchill Livingstone, 1999:22–48. Resnic D, Manolagas SC, Fallon MD. Histogenesis, anatomy, and physiology of bone. In: Resnick D, ed. Bone and joint imaging, 2nd ed. Philadelphia: WB Saunders, 1996:1–11. Rosenwasser MP. Fractures of the humerus: diaphysis. In: Peimer CA, ed. Surgery of the hand and upper extremity. New York: McGrawHill, 1996:259–271. Rother P, Druger G, Schramek G. Proportions of the femur and humerus in relation to bone length. Anat Anz 160:65–76, 1985. Seitz WH Jr. Fractures of the distal radius. In: Peimer CA, ed. Surgery of the hand and upper extremity. New York: McGraw-Hill, 1996: 637–666. Silverman FN. A note on the os lunatotriquetrum. Am J Phys Anthropol 13:143–146, 1955. Sinberg SE. Fracture of a sesamoid of the thumb. J Bone Joint Surg 22:444–445, 1940. Slattery PG. The dorsal plate of the proximal interphalangeal joint. J Hand Surg [Br] 15:68–73, 1990. Smith RJ. Intrinsic contracture. In: Green DP, Hotchkiss RN, Peder-

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son WC, eds. Green’s operative hand surgery, 4th ed. New York: Churchill Livingstone, 1999:604–618. Smith RJ. Non-ischemic contractures of the intrinsic muscles of the hand. J Bone Joint Surg Am 53:1313–1331, 1971. Stern PJ. Fractures of the metacarpals and phalanges. In: Green DP, Hotchkiss RN, Pederson WC, eds. Green’s operative hand surgery, 4th ed. New York: Churchill Livingstone, 1999:711–771. Steyn M, Iscan MY. Osteometric variation in the humerus: sexual dimorphism in South Africans. Forensic Sci Int 106:77–85, 1999. Streatfeild T, Griffiths HF. Fracture of a sesamoid bone. Lancet 1: 1117, 1934. Szabo RM, Sutherland TB. Acute carpal fractures and dislocations. In: Peimer CA, ed. Surgery of the hand and upper extremity. New York: McGraw-Hill, 1996:711–726. Tagare HD, Elder KW, Stoner DM, et al. Location and geometric description of carpal bones in CT images. Ann Biomed Eng 21: 715–726, 1993. Travaglini E. Arterial circulation of the carpal bones. Bull Hosp J Dis Orthop Inst 20:19–36, 1959. Tubiana R, Valentin P. The anatomy of the extensor apparatus of the fingers. Surg Clin North Am 44:897–918, 1964. Vance RM, Gelberman RH, Evans EF. Scaphocapitate fracture. J Bone Joint Surg Am 62:271–276, 1980. Weiland AJ. Small joint arthrodesis. In: Green DP, Hotchkiss RN, Pederson WC, eds. Green’s operative hand surgery, 4th ed. New York: Churchill Livingstone, 1999:95–107. Williams CS, Gleberman RH. Vascularity of the lunate: anatomic studies and implications for the development of osteonecrosis. Hand Clin 9:391–398, 1993.

2 MUSCLE ANATOMY MICHAEL J. BOTTE

The following sections describe the anatomic features of skeletal muscles of the upper extremity. Each is provided as a reference for a specific muscle, and is not intended for the purpose of planning operative approaches. A summary of muscle origin, insertion, innervation, vascular supply, and action is listed initially, followed by a general description of the gross anatomic features, actions and biomechanical aspects, variations and anomalies, and clinical implications of the anatomy (1–14). At the end of this chapter are several appendices for further reference. Appendix 2.1 summarizes the general features of each muscle for muscle comparison. Appendix 2.2 lists the skeletal muscles as to extremity compartments, from the standpoint of compartment syndrome. Appendix 2.3 lists muscle difference index values. These values are comparisons of the architectural features of several muscles of the forearm. The architectural difference index allows comparison of the relative differences (or similarities) of each skeletal muscle with regard to design and function, based on architectural properties (15). DELTOID MUSCLE (DELTOIDEUS) Derivation and Terminology. Deltoid is derived from the Latin deltoides, which means “triangular in shape or form” (1,2). Origin. Lateral third of clavicle, acromion, and inferior edge of spine of the scapula. Insertion. Deltoid tuberosity of the lateral humerus. Innervation. Axillary nerve (C5, C6). Occasionally, a contribution from C4 also may be present in the axillary nerve (3–8). Vascular Supply. Acromial and deltoid branches of the thoracoacromial artery; posterior and anterior circumflex humeral arteries; subscapular artery, and deltoid branch of the profunda brachii. The thoracoacromial, posterior and anterior circumflex humeral arteries, and the subscapular artery all arise from the axillary artery (3–11). Principal Action. Abduction, forward flexion, and extension of the humerus.

Gross Anatomic Description: Deltoid Muscle The deltoid is a relatively thick, curved muscle in the shape of an isosceles triangle, with the apex pointed inferiorly. It occupies and comprises the deltoid muscle compartment of the shoulder (Appendix 2.2). The deltoid surrounds the humeral head and glenohumeral joint on all aspects except medially and inferiorly, and, when viewed from above, the muscle appears somewhat U-shaped, with the open portion facing medially. The muscle has a broad origin, expanding anteriorly from the lateral third of the anterior clavicle, laterally from the superolateral aspect of the acromion, and posteriorly along the inferior edge of the spine of the scapula (Fig. 2.1). Based on the origin, the muscle has three subdivisions: a clavicular, acromial, and a (scapular) spinous part. The clavicular and spinous parts consist of long muscle fiber bundles that coalesce laterally and inferiorly at the insertion to help form a “V” or inverted triangle shape. At the insertion, the fibers converge into a short, thick tendon that attaches to the deltoid tuberosity of the lateral mid-diaphysis of the humerus (Fig. 2.2). The tendon of the deltoid also may give off an expansion into the brachial deep fascia that may reach the forearm. The anterior and posterior portions of the muscle converge directly into the insertion. The mid-portion, from the acromion, however, is multipennate. In this portion, four or five intramuscular septa or tendinous expansions descend superiorly from the lateral aspect of the acromion. Similarly, from the inferior insertional area, three septa or tendinous expansions ascend from the insertion site. The septa from the acromion above run obliquely and insert or interdigitate with the separate septa from the insertion site below (3,4,11–14). In addition, there is interdigitation of the tendons from the clavicular and spinous portions. The septa are interconnected with short muscle fibers that provide powerful traction. The muscle fasciculi are large, and produce a coarse longitudinal striation. The deltoid is responsible for creating the rounded profile of the shoulder (9–13). The deltoid muscle is innervated by the axillary nerve (C5, C6), which leaves the posterior cord of the brachial

2 Muscle Anatomy

plexus and courses posteriorly through the quadrangular space to reach the deep surface of the deltoid muscle. The nerve then crosses from posterior to lateral along the deep surface of the muscle approximately 5 cm distal to the acromion. The axillary nerve gives off motor branches along its course, and courses anteriorly as far as the anterior edge of the deltoid muscle. Although the axillary nerve comprises mostly fibers from C5 and C6, it may contain a contribution from C4. Actions and Biomechanics: Deltoid Muscle The deltoid is able to contract certain portions or parts independently of others. Thus, parts of the muscle can act

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separately as well as together. When the entire muscle contracts, the humerus can be abducted slightly beyond 90 degrees (3,4). (Additional humeral abduction actually is produced in conjunction with scapular rotation.) The anterior (clavicular) fibers contracting independently assist the pectoralis major in producing forward flexion and internal rotation of the humerus. The posterior fibers contracting independently can assist the latissimus dorsi and teres major in producing extension (to approximately 45 degrees) and external rotation of the humerus. The clavicular and spinous portions can contract simultaneously to assist with stabilization of the humerus. The central portion of the muscle is multipennate. This central (acromial) portion assists with strong abduction of the humerus. Aided by the supraspinatus, it can abduct the humerus until the joint

A FIGURE 2.1. Anterior (A) and posterior (B) views of the scapula, showing muscle origins (red) and insertions (blue). (continued on next page)

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B FIGURE 2.1. (continued)

capsule is tense inferiorly. The trapezius also assists the deltoid with humeral abduction. In general, the most effective abduction takes place with the humerus in external rotation. When the abduction takes place in the plane of the body of the scapula, scapular rotation can be fully effective in assisting with humeral abduction and raising the arm above the head. During humeral abduction, the central (acromial) fibers of the deltoid contract strongly, aided by the anterior (clavicular) and posterior fibers, both of which help prevent departure of the humerus from the plane of motion. In the early stages of abduction, there is an upward traction force on the humeral head produced by the deltoid. The humeral head is prevented from translating upward by the synergistic downward pull of the subscapularis, infraspinatus, and teres minor (3,4). Electromyography suggests that deltoid contributes little to internal or external rotation, but confirms that it does take part in most other shoulder movements. When a weight or load produces a downward drag on the upper extremity, the deltoid and the supraspinatus contract to help resist the downward force. Other common actions of the deltoid include assisting to produce arm swinging during ambulation, and helping to

forward flex the arm to position the hand at various heights during manual tasks (3,8,11,16). Anomalies and Variations: Deltoid Muscle Several variations of the muscle belly of the deltoid have been noted (11). Each of the three parts may appear as a separate muscle, so that there is a split in the muscle mass or of the distal insertion tendon. A separate clavicular part is the most common of these anomalies. The acromial and spinous parts also may appear as a separate muscle. The deep portion of the deltoid may be separated from the major mass portion of the muscle, and this deep portion may insert into the shoulder capsule or extend distally onto the humerus (11). Portions of the deltoid may be absent, especially those originating from the clavicle or acromion. Several accessory muscle or tendon slips may attach to the deltoid. These muscle slips can connect to the fascia covering the infraspinatus muscle or connect directly to the trapezius muscle. Muscle slips also attach to the vertebral or axillary borders of the scapula. An accessory tendon of

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A

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B FIGURE 2.2. Anterior (A) and posterior (B) views of the humerus, showing muscle origins (red) and insertions (blue).

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insertion has been noted to extend to the radial side of the forearm. The muscle belly of the deltoid may coalesce with adjacent muscles, and appear structurally joined to these muscles. Coalescing muscles include the pectoralis major, trapezius, infraspinatus, brachialis, and brachioradialis (11). Clinical Correlations: Deltoid Muscle Axillary palsy can produce severe deltoid atrophy. This, in turn, results in prominence of the acromion, which can simulate dislocation of the shoulder joint. The distance between the acromion and humeral head is increased to the extent that a fingertip may be inserted between them (3,4). Deltoid paralysis from axillary nerve injury is a well recognized complication of shoulder dislocation, especially anterior and inferior (luxatio erecta) dislocations (17). Central nervous disorders such as stroke can result in deltoid paralysis. The paralysis can result in inferior subluxation of the humeral head, which can secondarily result in traction on the brachial plexus with associated pain or limb paresthesias. Thickening of the distal edge of the deltopectoral fascia may produce compression of the median nerve (11,17,18). CORACOBRACHIALIS MUSCLE Derivation and Terminology. The coracobrachialis derives its name from its origin from the coracoid process and its insertion into the brachium. Coracoid is derived from the Greek korakoeides, which means “crowlike” or “like a crow’s beak” (korax = raven, and eidos = appearance), and pertains to the coracoid process, which resembles a bird’s beak. The word brachialis is derived from the Latin and Greek brachialis and brachion, respectively, which designate or pertain to the arm (1,2). Note that brachi and brachial pertain to arm, and should not be confused with brachy (from Greek brachys), which refers to “short” (i.e., brachydactyly for short digits). Origin. Apex of the coracoid process, along with the conjoined tendon of the short head of the biceps. Muscle fibers of the coracobrachialis also may originate from the tendon of the short head of the biceps along the proximal 10 cm of the tendon. Insertion. Medial humeral diaphysis, over a 3- to 5-cm insertional impression on the cortex. The area of insertion is roughly at the junction of the proximal and middle thirds of the humerus, between the attachment of the triceps and the brachialis. Innervation. Musculocutaneous nerve (C5, C6, C7). Vascular Supply. Muscular branches from the axillary artery, the brachial artery, and the anterior circumflex humeral artery (3,11).

Principal Action. Forward flexion and adduction of the humerus. Gross Anatomic Description: Coracobrachialis The coracobrachialis is a relatively long and slender muscle, somewhat cylindrical in shape. Along with the biceps brachii and brachialis, the coracobrachialis helps comprise the anterior muscle compartment of the arm (Appendix 2.2). The coracobrachialis helps form the inconspicuous rounded ridge on the upper medial side of the arm. The pulse of the brachial artery often can be seen or palpated in the depression posterior to the coracobrachialis. The muscle fibers extend obliquely and in a parallel fashion. The muscle usually contains an aponeurotic band that continues from the deep surface of the muscle to the insertion. The muscle originates from the apex of the coracoid process, along with the conjoined tendon of the short head of the biceps (see Fig. 2.1). Muscle fibers of the coracobrachialis also may originate from the tendon of the short head of the biceps along the proximal 10 cm of the tendon. The muscle may be separated into two heads or parts, separated by the musculocutaneous nerve, which passes in between. When the superficial and deep parts are clearly defined, the tendon of origin of the superficial part may be clearly separated from that of the deep part and may be closely associated with the tendon of the short head of the biceps brachii (3,4,8,11,19–25). The muscle extends from the coracoid process in a distal direction toward the medial diaphysis of the humerus. The muscle is cylindrical or fusiform in shape. The muscle inserts into the medial humeral diaphysis over a 3- to 5-cm insertional impression on the cortex (see Fig. 2.2A). The area of insertion is roughly at the junction of the proximal and middle thirds of the humerus, between the attachment of the triceps and the brachialis. At the insertion point, there also may be two separate tendons from the superficial and deep parts of the muscle (3,4,8,11). The musculocutaneous nerve, derived mostly from C5, C6, and C7, innervates the coracobrachialis. The nerve exits the brachial plexus from the lateral cord near the level of the acromion. The branch to the coracobrachialis usually is the first (most proximal) motor branch from the musculocutaneous nerve, followed by motor branches to the biceps and then the brachialis. After exiting from the lateral cord, the musculocutaneous branch to the coracobrachialis enters the proximal third of the medial aspect of the muscle and crosses through the muscle from medial to lateral near its midline. Flatow and colleagues (26) and Eglseder and Goldman (27) have quantified the anatomic aspects of coracobrachialis innervation in relation to the coracoid process. The distance from the coracoid process to the point where the musculocutaneous nerve enters the coracobrachialis muscle averages between 46 and 56 mm (range, 31 to

2 Muscle Anatomy

82 mm) (26,27). Small nerve twigs to the coracobrachialis (proximal to the main nerve trunk) enter the muscle as close as 17 mm distal to the coracoid process, with an average of 31 mm. The authors note that the frequently cited range of 5 to 8 cm below the coracoid for the level of penetration cannot be relied on to describe a “safe zone” because 29% of the nerves entered the muscle proximal to 50 mm below the coracoid (74% if the proximal twigs are considered) (26). The musculocutaneous nerve exits the coracobrachialis muscle at a mean of 75.5 mm distal to the coracoid process (27). Actions and Biomechanics: Coracobrachialis The coracobrachialis functions mainly to assist with flexion and adduction of the humerus. With the humerus in extension, the coracobrachialis assists in returning the humerus to a neutral position. In abduction, the coracobrachialis acts with the anterior fibers of the deltoid to stabilize the humerus in the plane of motion. The coracobrachialis also helps stabilize and maintain the head of the humerus in the glenoid fossa. Theoretically, the coracobrachialis can help rotate the scapula if the humerus is stabilized (3,8,11). Anomalies and Variations: Coracobrachialis There are several variations of the insertion of the coracobrachialis, including those more proximal and those more distal on the humerus. Those more proximal than the proximal diaphysis include insertions into the surgical neck of the humerus or capsule of the shoulder joint (28). The coracobrachialis brevis (or coracobrachialis superior, coracobrachialis rotator humeri) is an anomalous muscle that arises from the coracoid process and inserts proximally, into the bicipital ridge of the humerus in the proximal diaphysis, approximately 1 cm distal to the lesser tuberosity (3,11). This muscle may represent a remnant of a separate portion of the muscle formed embryologically. Those inserting more distally may include attachment sites along the medial margin of the humerus, or a separate insertion in the medial distal humerus or medial epicondyle. The distal insertion may consist of an elongated tendinous extension. The coracobrachialis inferior or coracobrachialis longus denotes an anomalous muscle that inserts much farther distally than usual (3,11). These often insert into either the distal medial aspect of the humerus, into the fibrous band of the medial intermuscular septum, or into the ligament of Struthers. The muscle also may extend distally into the medial supracondylar ridge, medial epicondyle, or an anomalous supracondylar process. The coracobrachialis inferior or coracobrachialis longus has been referred to as Wood’s muscle, based on Wood’s descriptions of several muscle variations in 1870 (8,11,18).

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Similar to the anomalous distal insertions, the coracobrachialis may have several accessory slips that attach to the muscle distally. These include extensions to the medial epicondyle, the medial intermuscular septum, or the distal medial aspect of the humerus (11,21). Muscle or tendon slips have been noted to extend to various structures in the shoulder area, including the tendons of the latissimus dorsi and teres major, or to the lesser tuberosity of the humerus (22,23). Among these is the coracobrachialis minor (le court coracobrachialis of Cruveilhier), an accessory muscle that arises from the coracoid process and crosses the radial nerve in the axilla and inserts into the tendinous part of the latissimus dorsi (11). Complete absence of the coracobrachialis can also occur. An anomalous muscle has been noted to arise from the medial aspect of the distal half of the humerus, between the coracobrachialis and brachialis, passing obliquely across the front of the brachial artery and median nerve and attaching with the common origin of the forearm flexor muscles (22,23). It did not appear to be an additional head of the coracobrachialis, biceps, or brachialis. The muscle appeared to place the median nerve and brachial artery at risk for compression; the authors suggest that the existence of this muscle be kept in mind in a patient presenting with a high median nerve palsy together with symptoms of brachial artery compression (22,23). Several variations in the musculocutaneous innervation of the coracobrachialis have been noted, with most of the differences involving the path of the nerve before muscle innervation (11,21,26,27,29–33). Although the motor branch from the musculocutaneous nerve usually pierces the muscle and travels in its substance, the nerve may not pierce the muscle in the proximal portion. Instead, the nerve may continue along with or in the substance of the median nerve, travel distally along the muscle, and then, as a single trunk or as several branches, pass between the biceps and brachialis, supplying these muscle as well as the coracobrachialis from the more distal aspect. This variation has been suggested to occur in approximately 20% of arms (11). Alternatively, the motor nerve to the coracobrachialis may split, with a branch entering and supplying the muscle, and then a portion may rejoin the main musculocutaneous nerve trunk. The nerve also may pass posterior to the coracobrachialis or between it and the short head of the biceps muscle before innervating the coracobrachialis. Rarely, the lateral cord may enter as a nerve into the coracobrachialis and then divide into the musculocutaneous nerve and the lateral head of the median nerve. Clinical Correlations: Coracobrachialis Muscle Several operative procedures involve mobilization or exposure of the coracoid process. The musculocutaneous nerve and motor branches to the coracobrachialis muscle are at

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risk for injury. The points of innervation of the musculocutaneous nerve to the coracobrachialis have wide variability, with muscular branches entering the coracobrachialis from 31 to 82 mm distal to the coracoid process (26). In the past, there has been a frequently cited “safe zone” of 5 to 8 cm distal to the coracoid for the level of penetration of these nerve branches. This safe zone cannot be relied on, however, because of the established variability of the nerve. This variability should be kept in mind when exposing or mobilizing the coracoid process, and the vicinity of the musculocutaneous nerve and its branches should be appreciated. The coracobrachialis, including the axillary vessels, can be used as a local muscle flap for coverage of exposed infraclavicular or postmastectomy defects (34). Isolated musculocutaneous nerve palsy has been noted to occur. Atraumatic palsy (35) as well as palsies associated with heavy exercise or violent extension of the elbow have been reported (36–39). It can occur bilaterally (40). The coracobrachialis, however, usually is spared weakness, and the area of compression is thought to be possibly within the muscle itself (39). The syndrome usually produces weakness of the biceps brachii and brachioradialis, with sensory abnormalities along the lateral forearm. It usually resolves with rest, but may take weeks or months (see later, under Clinical Correlations: Biceps Brachii). The wide variation in the course of the musculocutaneous nerve before and inside the coracobrachialis, and the high percentage of anomalies, emphasize the complexities and irregularities of this anatomic region with regard to surgical approaches (11,21,26,27,30–32). BICEPS BRACHII MUSCLE Derivation and Terminology. Biceps is derived from the Latin and Greek bi, meaning “two,” and the Latin caput, meaning “head.” Biceps thus refers to “two heads.” Brachialis is derived from the Latin and Greek brachialis and brachion, respectively, which designate or pertain to the arm (1,2). Note that brachi and brachial pertain to the arm, and should not be confused with brachy (from Greek brachys), which refers to “short” (i.e., brachydactyly for short digits). Origin. Short head: from the coracoid process, in the conjoined tendon of the coracobrachialis. Long head: from the supraglenoid tubercle of the scapula and from the posterior part of the glenoid labrum by a long tendon of the origin approximately 9 cm long. Insertion. The bicipital tuberosity of the radius and into the bicipital aponeurosis, which inserts into the deep fascia on the ulnar aspect of the forearm. Innervation. Musculocutaneous nerve (C5, C6). Vascular Supply. The brachial artery and the anterior circumflex humeral artery. The short head may receive a branch from the axillary artery (3,8,11,19). Principal Action. Flexion and supination of the forearm.

Gross Anatomic Description: Biceps Brachii The biceps brachii is a relatively large, thick, and roughly fusiform muscle comprising a major portion of the anterior muscle compartment of the arm (Appendix 2.2). The muscle has two heads, arising from two separate origins. The muscle heads then partially coalesce into a single large muscle belly, although it still grossly retains some features of two separate heads (41). The short head arises from the tip of the coracoid process, originating as a thick, flat tendon that is conjoined with the origin of the coracobrachialis muscle (see Fig. 2.1). The short head then separates, and the muscle belly becomes more defined. The muscle fibers of the short head descend from the dorsomedial surface of the tendon, in a vertical fashion, and join the fibers of the long head. The fibers increase in number from proximal to distal as the muscle approaches the insertion. The long head arises from a rough or raised point just superior to the rim of the glenoid fossa, known as the supraglenoid tubercle of the scapula. It is intracapsular at its origin. From the origin, there is a well defined, long, stout tendon that is approximately 9 cm long. The tendon runs from the apex of the glenoid cavity enclosed in a double tubular sheath that is an extension of the synovial membrane of the joint capsule. The tendon is intracapsular as it crosses and then arches over the head of the humerus. It emerges from the joint posterior to the transverse humeral ligament. The tendon then descends in the intertubercular sulcus of the humerus, where it is held in place by the transverse humeral ligament and a fibrous expansion from the tendon of the pectoralis major. At the myotendinous junction, the muscle belly of the long head joins the belly of the short head. The muscle fibers extend distally and obliquely. The two bellies appear joined together, and form a single elongated belly. The two heads, however, can be separated from each other to within approximately 7 cm of the elbow joint. The muscle fibers then form a terminal tendon in the distal fourth of the arm. The fibers coalesce and become tendinous, taking the shape of a flattened or oval tendon. As the tendon approaches its insertion point, it spirals from proximal to distal, so that the anterior surface turns to face laterally. The tendon passes between the brachioradialis and the pronator teres. It then inserts into a rough posterior attachment area of the radial tuberosity (Fig. 2.3A). There is a bursa in the vicinity of the tendon that separates the tendon from a smooth anterior area of the tuberosity. Proximal to the elbow joint, the tendon also has a broad medial fascial expansion, the bicipital aponeurosis. This aponeurosis actually forms in the proximal part of the terminal tendon and is first identifiable as a vertical septum between the two heads of the biceps. More distally, it becomes a broadened and flattened aponeurosis. Muscle fibers insert on the sides of the septum and surfaces of the aponeurosis, the long

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B FIGURE 2.3. Anterior (A) and posterior (B) views of the radius and ulna, showing muscle origins (red) and insertions (blue).

head chiefly on the deep surface, and the short head primarily on the superficial surface. This fascial attachment extends distally and medially superficial to the brachial artery to coalesce with the deep fascia of the distal upper arm and proximal forearm. This is in the vicinity of the origin of the flexor–pronator muscles of the forearm. The tendon often can be split as far distally as the radial tuberosity,

where the anterior and posterior layers can be traced back to the separate bellies of the short and long head, respectively (3,4,10–13,42). The biceps muscle is innervated by the musculocutaneous nerve (C5, C6). Although each head receives its own nerve branch, the two branches may extend together as a small common nerve trunk. Several separate smaller

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branches may enter the muscle on the deep surface in the proximal portion of the middle third. A distinct intramuscular fissure in each head has been noted where the nerve enters the muscle (11,43). The path and variations of the musculocutaneous nerve, including the branch patterns to the biceps and brachialis muscles, have been studied by Yang et al. (43) and Chiarapattanakom and colleagues (44). In microdissections of 24 fresh-frozen cadaver specimens, Yang et al. found that the motor branch to the biceps exited from the musculocutaneous nerve 119 mm distal to the coracoid process. Variations were seen in the innervation of the two heads of the biceps. A common primary motor branch that bifurcated to supply the two heads was seen in 20 specimens (type I). Two specimens had two separate primary branches originating from the main musculocutaneous nerve trunk to individually supply each head of the biceps (type II). The third variation (type III), also seen in two specimens, was similar to type I, but with an additional distal motor branch innervating the common belly of the biceps muscle. The motor branch to the brachialis muscle exited from the musculocutaneous nerve 170 mm distal to the coracoid process. The motor branches to the biceps and brachialis muscles may be dissected proximally from their points of exit from the main trunk of the musculocutaneous nerve for mean distances of 44 and 53 mm, respectively. These variations have clinical application in the operative exposure of the musculocutaneous nerve, especially in performing intercostal nerve to musculocutaneous motor branch transfer for elbow flexion in patients with brachial plexus injuries (43). In a subsequent study, Chiarapattanakom and colleagues studied 112 musculocutaneous nerves from 56 cadavers (44). There were three distinct types of branching patterns for the biceps innervation: in 62%, there was one branch only; in 33%, there were two branches; and in 5%, there were three branches. The origin of the first branch averaged 130 mm from the acromion, regardless of branch type. The maximum distance between the first and second branch was 53 mm. In 92%, there was only one branch to the brachialis muscle (44).

Actions and Biomechanics: Biceps Brachii The biceps is one of the primary flexors of the elbow. Flexion of the elbow is most effective with the forearm in supination. The biceps is also the strongest supinator of the forearm, especially with rapid or resisted movements. The short head, from its origin on the coracoid process, can assist with adduction and forward flexion of the humerus. The long head, from its origin just above the glenoid, assists in stabilizing the humeral head in the glenoid cavity. The long head can specifically help to prevent superior migration of the humeral head during contraction of the deltoid.

Based on cross-sectional analysis of the major elbow muscle flexors, the biceps brachii appears to contribute 34% of flexion torque, with the brachialis contributing 47% and the brachioradialis 19% (20).

Anomalies and Variations: Biceps Brachii Several variations of the biceps have been noted (11,22,23, 41,42). Most of these consist of accessory heads or interconnecting anomalous muscles bellies. Accessory heads are often associated with variations in the musculocutaneous innervation or with abnormal courses of the axillary and brachial arteries (41). There may be an absence of one or both heads of the biceps brachii or both heads may be separate along their complete course from origin to insertion. Both heads may also be coalesced along most of their course. Supernumerary heads are common, occurring in over 10% of specimens (11,45,46). An accessory head may arise from the coracoid process, capsule of the shoulder joint, tendon of the pectoralis major, or the region of the deltoid insertion (21,22,42). A third or fourth (humeral) head has been found in approximately 12% to 14% of arms (11,45,47,48). It usually arises from the proximal humerus in the region of the greater tuberosity. Less commonly, two accessory heads may arise together from the neck of the humerus or posterior to the tendon of the pectoralis. These two anomalous heads may be joined to the pectoralis tendon. The lateral of the two accessory slips usually joins the long head of the biceps and the medial head usually joins the short head. A third head often arises from the superomedial part of the brachialis and attaches to the bicipital aponeurosis and medial side of the tendon of insertion (3). This head often is located deep to the brachial artery. It also may consist of two slips that extend distally, one slip superficial and one deep to the brachial artery. Muscle or tendon slips may extend from the lateral aspect of the humerus or intertubercular sulcus to join the main muscle belly of the biceps. The most common anomalous slip arises from the humerus near the insertion of the coracobrachialis and extends distally between the coracobrachialis and brachialis. This anomalous slip usually joins the short head, but most of the fibers pass into the part of the tendon that forms the bicipital aponeurosis. This slip also may be completely separated and terminate entirely in the bicipital aponeurosis (11). An accessory slip may arise from the deltoid. Several variations have been noted at the distal end of the muscle, including various muscular or tendinous slips that extend from the biceps to the distal humerus, ulna, radius, forearm fascia, or neighboring muscles (11,49,50). Supernumerary heads may extend to or from the biceps to the brachialis, brachioradialis, pronator teres, flexor carpi radi-

2 Muscle Anatomy

alis (FCR), flexor digitorum profundus (FDP), intermuscular septum, or medial epicondyle (11,49,50). Muscle coalitions from the biceps have been noted where the muscle “fuses” with the belly of neighboring muscles, including the pectoralis major and minor, coracobrachialis, and brachialis (11). Attachments from a muscular or tendinous extension from the distal biceps to the palmaris longus have been noted (11). Attachments from a muscular or tendinous extension from the distal biceps to the extensor carpi radialis brevis (ECRB) have also been noted (51). An anomalous muscle has been noted to arise from the medial aspect of the distal half of the humerus, between the coracobrachialis and brachialis, passing obliquely across the front of the brachial artery and median nerve and attaching with the common origin of the forearm flexor muscles (22,23). The muscle did not appear to be an additional head of the coracobrachialis, biceps, or brachialis. The muscle, however, appeared to place the median nerve and brachial artery at risk for compression. The authors suggest that the existence of this muscle be kept in mind in a patient presenting with a high median nerve palsy together with symptoms of brachial artery compression (22,23). As noted previously in the descriptions of the coracobrachialis, several variations in the course and innervation of the musculocutaneous nerve to the coracobrachialis, biceps, and brachialis have been noted. Most variations involve the path of the nerve before muscle innervation (11,26,27, 29–33,41). Although the motor branch from the musculocutaneous nerve usually pierces the coracobrachialis and travels in its substance, the nerve may not pierce the muscle. Instead, the nerve may continue along with or in the substance of the median nerve, travel distally along the coracobrachialis, and then, as a single trunk or as several branches, pass between the biceps and brachialis, supplying these muscles as well as the coracobrachialis from the more distal aspect. This variation has been suggested to occur in approximately 20% of arms (11,21,26,27,29–33,41). The musculocutaneous nerve may be absent. The biceps (and brachialis) can receive its innervation directly from the median nerve (32). Clinical Correlations: Biceps Brachii Rupture of the biceps tendon is among the most common of closed tendon ruptures. These occur either proximally in the tendon of the long head (or short head) (52–56), or distally, at or near the insertion (57–68). Bicipital tendinitis occurs in the tendon of the long head, usually along the anterior shoulder in the intertubercular groove. Chronic tendinitis is associated with tendon rupture, as well as a high incidence of associated related shoulder problems, including impingement syndrome and frozen shoulder (69–76).

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The lacertus fibrosus is a structure known to cause or contribute to median nerve compression in the forearm (49,50). Proximal paralysis of the brachial plexus involving the C5 and C6 nerve roots (Erb-Duchenne palsy) results in paralysis of the biceps. If C7 remains intact, the innervation of the triceps remains intact. Functioning of the triceps in conjunction with paralysis of the biceps results in the elbow positioned in full extension. To restore elbow flexor power, several operative flexorplasty procedures have been described (77–80). These include proximal transfer of the forearm flexor–pronator or wrist extensor mass (which increases their moment arm across the elbow and enhances their ability to act as secondary elbow flexors) (81–85), transfer of part or all of the pectoralis major (with or without transfer of the pectoralis minor) (86–89), transfer of the latissimus dorsi (90–94), anterior transfer of the triceps tendon (95–97), and transfer of the sternocleidomastoid (98). Proximal transfer of the flexor pronator muscle origin is known as the Steindler flexorplasty (81–85), described in 1918 (81). Weakness of the biceps brachii and brachialis muscle due to isolated palsy of the musculocutaneous nerve has been reported. It can follow heavy exercise (36–39) or occur atraumatically (35). Bilateral palsy also has been noted (40). Violent extension of the forearm may be a factor. The syndrome features painless weakness of the biceps and brachialis, sensory loss in the distal lateral forearm, and a history of recent vigorous upper extremity resistive exercises. Loss of contour of the biceps has been noted (38). The syndrome usually resolves with rest, but may take weeks or months (37). The musculocutaneous nerve usually is injured distal to the innervation of the coracobrachialis. It has been postulated that nerve entrapment or stretching occurs where the nerve passes through the coracobrachialis (38). The condition should not be confused with C5 and C6 radiculopathy, brachial plexopathy, or rupture of the biceps brachii muscle belly or tendon. With 6 weeks of heavy isometric strength training, the strength of the elbow flexors can be increased by 14%, with a mean increase in cross-sectional area of 5.4% (99). Male and female percentage increases in strength and muscle size are similar (no significant differences) (99). The variations of the musculocutaneous innervation to the biceps (described earlier under Gross Anatomic Description: Biceps Brachii) should be appreciated when planning intercostal to musculocutaneous nerve transfer to restore elbow flexion in the patient with brachial plexus palsy (43). BRACHIALIS Derivation and Terminology. Brachialis is derived from the Latin and Greek brachialis and brachion, respectively,

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which designate or pertain to the arm (1,2). Note that brachi and brachial pertain to “arm,” and should not be confused with brachy (from Greek brachys), which refers to “short” (i.e., brachydactyly for short digits). Origin. Distal two-thirds of the anterior humerus, medial and lateral intermuscular septa. Insertion. Proximal ulna, base of the coronoid process, anterior capsule of the elbow. Innervation. Musculocutaneous nerve (C5, C6). Additional innervation is from small branches from the radial and median nerves. Vascular Supply. Muscular branches from the brachial artery, ulnar artery, superior and inferior ulnar collateral arteries, anterior ulnar recurrent artery, radial collateral branch of the profunda brachii, and radial recurrent artery (3,4,11). Principal Action. Flexion of the forearm. Gross Anatomic Description: Brachialis The brachialis is a relatively large, wide muscle, and along with the biceps brachii and coracobrachialis, the brachialis comprises the anterior muscle compartment of the arm (Appendix 2.2). The brachialis originates on the distal twothirds of the anterior humerus (see Fig. 2.2). The attachment area of the origin is long and wide, commencing proximally along the anterior and posterior margins of the insertional tendon of the deltoid and extending distally along the anterior humerus to end in an inverted “V” at the level just proximal to the elbow capsule. The origin may extend to within 2.5 cm of the articular surface of the elbow, ending proximal to the radial and coronoid fossae (3,4,11). At the level of the humerus below the midshaft, the muscle envelops the distal humerus on the anterior, lateral, and medial aspects to partially surround the shaft, covering approximately two-thirds of the bone circumference. The muscle also arises from the medial intermuscular septum and from the lateral intermuscular septa proximal to the origin of the brachioradialis and extensor carpi radialis longus (ECRL), with more attachments from the medial side. The muscle belly is somewhat flat, and is convex anteriorly and concave posteriorly as its extends distally. The muscle fiber bundles descend in a specific pattern. The middle bundles descend in a straight vertical direction. The medial bundles descend in an oblique course, from medial to lateral. The lateral bundles also descend in an oblique course, from lateral to medial. In the distal fourth of the muscle, the myotendinous junction begins. A portion of the dorsal side of the lateral edge initially becomes tendinous. This tendinous portion enlarges as the muscle extends distally, and an additional tendinous portion joins the myotendinous junction on the anterior surface of the muscle proximal to the elbow joint. The tendon thickens and converges as it extends distally. It passes along the anterior capsule of the elbow joint and inserts onto a roughened area on the anterior aspect of the base of the coronoid process

(see Fig. 2.3A). Cage and colleagues studied the anatomic aspects of the brachialis in reference to the coronoid process and associated fractures (100). The brachialis was found to have a musculoaponeurotic insertion that included the elbow capsule, coronoid, and proximal ulna. The bony insertion averaged 26.3 mm in length, with its proximal margin averaging 11 mm distal to the coronoid tip. The tip of the coronoid process usually was not covered by capsule or muscle attachments (in only 3 of 20 specimens did the capsule actually insert onto the tip) (100). In general, it was found that the brachialis insertion was more along the distal portion of the base of the coronoid, and only in Morrey type III fractures (those through the base of the coronoid) would the fracture fragment be large enough to include the brachialis bony insertion (100). The brachialis is innervated by the musculocutaneous nerve (C5, C6). The nerve passes from medial to lateral between the brachialis (located posterior to the nerve) and the biceps (located anterior to the nerve). A motor branch usually enters the brachialis on the anterior surface in the proximal and medial portions of the muscle. The radial nerve (C7) may supply a small branch to the distal lateral part of the muscle (101). The median nerve also may supply a small branch to the medial side of the brachioradialis (3,4,11). As noted earlier in the discussion of the corocobrachialis and biceps brachii, the path and variations of the musculocutaneous nerve, including the branch patterns to both the biceps and brachialis muscles, were studied in detail by Yang and colleagues (43). In 24 fresh-frozen cadaver specimens, the motor branch to the brachialis muscle exited from the musculocutaneous nerve a mean of 170 mm distal to the coracoid process. A single primary motor branch (type I) was seen in most specimens, and the rare specimen (type II) showed two separate primary motor branches innervating the muscle. The motor branches to the biceps and brachialis muscles may be dissected proximally from their points of exit from the main trunk of the musculocutaneous nerve for mean distances of 44 and 53 mm, respectively. These variations have clinical significance for the operative exposure of the musculocutaneous nerve, especially in performing intercostal nerve to musculocutaneous motor branch transfer for elbow flexion in patients with brachial plexus injuries (43). In a subsequent study, Chiarapattanakom and colleagues dissected 112 musculocutaneous nerves in 56 cadavers (44). In 92% of specimens, there was one motor branch to the brachialis muscle. It always emerged from the main trunk distal to the nerve to the biceps and averaged 170 mm from the acromion (44). Actions and Biomechanics: Brachialis The brachialis provides strong flexion to the forearm, in both pronation and supination. Based on cross-sectional analysis of the major elbow muscle flexors, the brachialis appears to contribute 47% of flexion torque, with the biceps brachii contributing 34% and the brachioradialis

2 Muscle Anatomy

19% (20). The brachialis also has a probable contribution as a secondary stabilizer of the elbow joint (100). Anomalies and Variations: Brachialis The muscle belly of the brachialis may be divided into two or more separate heads or bellies (11). When the brachialis exists as two separate heads, each head commences on either side of the deltoid tuberosity (one anterior and one posterior to the deltoid insertion). If two or more muscle bellies exist, the distal insertion becomes more variable or irregular, to include several additional anomalous insertional sites. These insertion sites include (besides portions of the coronoid process) the radius on or below the bicipital tuberosity (radial tuberosity), both the proximal radius and ulna, the radius with a tendinous band joining it to the coronoid process of the ulna, fascia of the forearm, or muscles of the forearm arising from the medial epicondyle and from the flexor muscle origin (11). The brachiofascialis muscle of Wood denotes an anomalous insertion portion of the brachialis into the forearm fascia (11,18). A slip from the brachialis may insert into the bicipital aponeurosis. A slip of the brachialis may also insert into the capsule of the elbow joint, and is known as the capsularis brachialis muscle. The brachialis may coalesce with several muscles, including the brachioradialis, pronator teres, or biceps. The brachialis may also be absent. Variations in innervation may exist. The brachialis usually is innervated by the musculocutaneous nerve. The radial nerve usually sends a small branch into the distal lateral portion of the muscle. The median nerve also may innervate a small portion of the brachialis, sending a small branch into the medial side of the distal muscle near the elbow joint (11). The musculocutaneous nerve may be absent. The brachialis can receive its innervation directly from the median nerve (32). Clinical Correlations: Brachialis Although rupture of the proximal or distal tendons of the biceps is a relatively common injury, isolated rupture of the brachialis has been noted only rarely (102). It is well established that the median nerve can be compressed in the forearm by several structures, including the lacertus fibrosus, pronator teres, and flexor digitorum superficialis (FDS). In addition, an accessory slip of the brachialis tendon distal in the forearm has been noted to cause median nerve compression (49). Weakness of the biceps brachii and brachialis due to isolated palsy of the musculocutaneous has been reported. It can follow heavy exercise (36–39) or can occur atraumatically (35). Bilateral palsy also has been noted (40). Violent extension of the forearm may be a factor. The syndrome fea-

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tures painless weakness of the biceps and brachialis, sensory loss in the distal lateral forearm, and a history of recent vigorous upper extremity resistive exercise. Loss of contour of the biceps may be noted (38). The syndrome usually resolves with rest, but may take weeks or months (37). The musculocutaneous nerve is injured distal to the innervation of the coracobrachialis. It has been postulated that nerve entrapment or stretching occurs where the nerve passes through the coracobrachialis (38). The condition should not be confused with C5 to C6 radiculopathy, brachial plexopathy, or rupture of the biceps brachii muscle belly or tendon. With 6 weeks of heavy isometric strength training, the strength of the elbow flexors can be increased by 14%, with a mean increase in cross-sectional area of 5.4% (99). Male and female percentage increases in strength and muscle size are similar (no significant differences) (99). TRICEPS BRACHII Derivation and Terminology. Triceps is derived from the Latin and Greek tri meaning “three,” and the Latin caput, meaning “head.” Triceps thus refers to “three heads.” Brachii is derived from the Latin and Greek brachialis and brachion, respectively, which designate or pertain to the arm (1,2). Note that brachi and brachial pertain to “arm,” and should not be confused with brachy (from Greek brachys), which refers to “short” (i.e., brachydactyly for short digits) (1,2). Origin. From three heads. Long head: from the infraglenoid tubercle of the scapula. Lateral head: from a narrow, linear or oblique ridge on the posterolateral surface of the proximal humeral shaft and from the lateral intermuscular septum. Medial head: from an extensive area including the posterior surface of the humeral shaft, distal to the radial groove from the insertion of the teres major to the distal humerus (3,4,11). Insertion. The olecranon process of the ulna. Innervation. Radial nerve (C6, C7, C8), with separate branches to each head. Vascular Supply. The triceps is supplied by the axillary artery through branches of the posterior humeral artery, branches from the profunda brachial artery (including deltoid and middle collateral branches), and from the superior and inferior ulnar collateral arteries and interosseous recurrent artery (3,4,11,103). Principal Action. Extension of the forearm. The long head may assist with adduction of the abducted humerus, or extension of the forward-flexed humerus. Gross Anatomic Description: Triceps Brachii The triceps is a wide, powerful muscle that comprises the entire posterior muscle compartment of the arm (Appendix 2.2). The muscle is complex, with three heads and an exten-

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sive, complex origin principally from the posterior humerus (3,4,8,9,11,13). The long head of the triceps originates from the infraglenoid tubercle of the scapula (see Fig. 2.1). It occasionally may extend along the axillary border of the scapula to varying distances. The long head initially is a broad, flat tendon, with attachments that blend with the inferior aspect of the shoulder capsule (104). The long head extends distally and somewhat laterally to join the lateral head. The long head initially passes superficial to the medial head. In the midportion of the humerus, the long head joins with the muscle bellies of the lateral and medial head to from a large, single muscle belly. To some degree, the fibers and course of the long head can be traced from the insertion to the origin. From the origin of the long head, the tendon splits into two layers, one located inferiorly and one superficially (11). The muscle fibers from the two layers extend distally in a parallel fashion and then twist as they descend distally. At the insertion level, the original anterior surface of the origin becomes the dorsomedial portion of the tendon at the insertion. The fibers of the long head of the muscle are found on the medial side of the tendon, and terminate at approximately the distal fourth of the arm as the myotendinous junction is formed. The long head of the triceps contributes to the formation of the well known quadrangular space and the triangular space of the axillary region. From its origin, the long head extends distally anterior to the teres minor and posterior to the teres major, dividing the wedge-shaped interval between them into the triangular and quadrangular spaces (3,4,11). The triangular space is bordered by the teres minor (superiorly), the long head of the triceps (laterally), and the teres major (inferiorly). Branches of the circumflex scapular artery cross through the triangular space. The more anatomically significant quadrangular space is bordered by the teres minor and subscapularis (superiorly), the long head of the triceps (medially), the teres major (inferiorly), and the humeral neck (laterally). The axillary nerve and posterior humeral circumflex artery pass through the quadrangular space (3,4,8,13,68). The lateral head of the triceps originates as a flattened tendon from a narrow, linear, oblique ridge on the posterior surface of the proximal humeral shaft, just distal to the neck (see Fig. 2.2B). The origin is medial to the insertion of the teres minor, and is anterior and lateral to the proximal portion of the radial groove. The distal portion of the origin of the lateral head is located just posterior to the insertion of the deltoid. In addition, part of the lateral head originates from the lateral intermuscular septum. The fibers of the lateral head extend distally to coalesce with the fibers of the long and medial heads. The superior fibers of the lateral head pass vertically and the inferior fibers pass obliquely to insert into the dorsal and ventral surfaces of the proximal lateral margin of the common insertional tendon (3,4).

The lateral head of the triceps is visible as a prominence in the posterolateral aspect of the proximal arm, most apparent in athletic individuals. The prominence is parallel and medial to the posterior border of the deltoid. The muscle head becomes most prominent when the elbow is actively extended. The mass that is located medial to the lateral head is the long head (3,4). The medial head of the triceps has an extensive origin from the distal half of the posterior humeral shaft (see Fig. 2.2B). It is located posterior and medial to the radial groove of the humerus. The origin extends from the vicinity of the insertion of the teres major (proximal on the humerus) to the distal portion of the humerus, to within 2.5 cm of the trochlea. A portion of the medial head also originates from the medial intermuscular septum and the lower part of the lateral intermuscular septum. The medial head lies deep to the long head, and when the muscles coalesce, the medial fibers remain in the deeper parts of the muscle. Some of the fibers attach directly to the olecranon, although most first coalesce with the other heads to form the common tendon of insertion (3,4,11). Once the long, medial, and lateral heads have coalesced, the fibers continue distally to converge into a thick, stout tendon. The myotendinous junction is relatively large and begins in the middle third of the muscle. Operative exposure of the distal half of the muscle often exposes only a large tendinous portion. The tendon has two layers, one superficial and one deep. The layers unite to form the common tendon, which extends distally to attach to the olecranon (see Fig. 2.3B). Some of the muscle fibers or a portion of the tendon on the lateral side form a band of fibers that inserts into the articular capsule of the elbow or continues distally over the anconeus to coalesce with the antebrachial fascia. A part of muscle slip that inserts into the articular capsule is referred to as the subanconeus muscle or articularis cubiti (3,11). The triceps muscle is innervated by the radial nerve (C6, C7, C8). Each head receives a separate branch or branches. The branch to the long head is the most proximal branch. It arises in the axilla and enters the lateral margin of the proximal muscle. The nerve may penetrate the muscle as several small branches. The radial nerve continues distally along the radial groove of the humerus, between the lateral and medial heads. The radial head gives off two or three small branches to supply the medial head, followed by separate branches to the lateral head. Actions and Biomechanics: Triceps Brachii The principal action of the triceps muscle is to extend the forearm. The long head, which originates proximal to the shoulder on the infraglenoid tubercle of the scapula, also functions to assist with humeral adduction. When the humerus is in a forward-flexed position, the long head can assist with extending the humerus back to the neutral posi-

2 Muscle Anatomy

tion. The lateral head is the strongest and contributes most to elbow extension. The long head has more effect on the shoulder joint then at the elbow (104). Electromyographic studies indicate that the medial head is active in all forms of extension of the forearm. The long and lateral heads, however, are minimally active except in extension of the forearm against resistance (105). This occurs as in pushing or supporting body weight on the hands with the elbows in mid-flexion. The long head appears to give support to the lower part of the shoulder capsule, especially when the arm is raised (104). The triceps has an important function in stabilization of the elbow during forceful supination of the forearm with the elbow flexed. In forceful forearm supination, there is strong contraction of both the supinator and biceps brachii. The triceps contracts synergistically to maintain the flexed or semiflexed position of the elbow. Otherwise, without this triceps cocontraction, it would be difficult forcefully to supinate the forearm without simultaneously flexing the elbow (3,4,11,68). Anomalies and Variations: Triceps Brachii The three heads of the triceps may coalesce with the neighboring muscles (11). A fourth muscle head has been noted to occur with the triceps (106). This head has been noted to arise from the humerus, axillary margin of the scapula, capsule of the shoulder joint, coracoid process, or tendon of the latissimus dorsi (11,106). The radial nerve is rarely noted to be absent. The triceps is then innervated by the musculocutaneous or ulnar nerve (11). The radial nerve rarely passes through the quadrangular space, along with the axillary nerve. The radial nerve still innervates the three heads of the triceps (11). The patella cubiti is a sesamoid bone in the triceps tendon, located near the insertion (107). It also is referred to as the sesamum cubiti or elbow disc (11). Its presence has been noted to be associated with a rupture of the distal triceps tendon (see later) (107–116). The latissimocondyloideus or dorsoepitrochlearis is an anomalous muscle found in approximately 5% of individuals. The muscle extends from the tendon of the latissimus dorsi to the brachial fascia, triceps brachii, shaft of the humerus, lateral epicondyle, olecranon, or fascia of the forearm (11). When absent (95% of individuals), the muscle normally is represented by a fascial slip from the tendon of the latissimus dorsi to the long head of the triceps of from the brachial fascia. The muscle is innervated by the radial nerve (11). Clinical Correlations: Triceps Brachii Ulnar neuropathy or neuritis at the elbow in conjunction with an abnormal triceps muscle slip or an aberrant muscle belly is well documented (117–125). This type of cubital

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tunnel syndrome has been related to either a separate or prominent medial head of the triceps (119,123), an unstable, dislocating medial triceps tendon (117,118,124,125), or an abnormal insertion or subanconeus muscle (an auxiliary extension of the medial portion of the medial triceps that inserts into the joint capsule, fascia, or medial epicondyle) (120,122). Radial nerve entrapment by the lateral head of the triceps has also been noted (126). Complete avulsion or incomplete rupture of the triceps tendon is well documented (107–116). It usually involves rupture at the distal tendon, but may occur at the musculotendinous junction (110). Rupture has been associated with patients on hemodialysis (112,113,115) and with those with secondary hyperparathyroidism (114), seizure disorders (115), hypertension (110), or diabetes mellitus (110). Spontaneous rupture also has been reported in association with a patella cubiti, a sesamoid bone in the triceps tendon (107). In a rare case, it also has occurred in association with radial neuropathy (111). Similar to rupture of the biceps tendon, operative repair for complete rupture usually is indicated (108–110,114,116). With incomplete rupture, conservative management has been used successfully (112). In arthrogryposis, the elbow is often in a fixed position in varying degrees of extension. Triceps lengthening, in conjunction with capsulotomy or tendon transfer, often is performed to gain elbow motion (127,128). ANCONEUS Derivation and Terminology. The word anconeus is derived from the Greek ankon, which means “elbow” (1,2). Origin. The posterior surface of the distal aspect of the lateral epicondyle. Insertion. The lateral aspect of the olecranon and the proximal fourth of the posterior surface of the shaft of the ulna. Innervation. Radial nerve (C6, C7, C8). Vascular Supply. The interosseous recurrent artery, middle collateral (posterior descending) branch of the profunda brachii (3,4,11). Principal Action. Extension of the forearm. The anconeus may have a secondary role in stabilizing the ulna, especially during rotation of the forearm. Gross Anatomic Description: Anconeus The anconeus is a small, triangular or quadrangular muscle of the posterolateral elbow. It is often partially blended with the distal portion of the triceps, and is thought morphologically and physiologically to belong to the triceps. It has a similar function of elbow extension and is supplied by the same (radial) nerve. In some primates, the anconeus in not distinguishable from the triceps (3).

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The anconeus originates from the distal aspect of the posterior lateral epicondyle of the humerus (see Fig. 2.2B). The origin consists of a short tendon, often covered with muscle. The tendon extends on the deep surface and lateral margin of the muscle. A portion of the muscle also originates from the adjacent portion of the posterior elbow joint capsule. The fibers of the anconeus diverge medially toward the ulna, with the more proximal fibers extending transversely directly to the ulna, and the more distal and lateral fibers extending more obliquely. The muscle covers the posterior aspect of the annular ligament. The anconeus inserts onto the lateral aspect of the olecranon and on the adjacent lateral aspect of the proximal ulna (see Fig. 2.3B). The superior part of the muscle usually is continuous with the medial head of the triceps brachii. The insertional area extends distally to stretch along the proximal quarter of the ulna. The anconeus is innervated by the radial nerve (C6, C7, C8). The motor branch arises from the radial nerve trunk in the radial groove of the humerus. This motor branch passes through the medial head of the triceps, supplying the triceps and continuing distally to enter the proximal border of the anconeus (3,4,11). Actions and Biomechanics: Anconeus The anconeus assists the triceps with extension of the elbow. The major function of the anconeus may not be fully recognized. The anconeus may have a secondary role in stabilizing the ulna, especially during rotation of the forearm. During pronation of the forearm, it has been postulated that the anconeus moves the ulna laterally at the ulnohumeral joint. In this way, the anconeus allows the forearm to turn over the hand without translating it medially (3,4,11,13). Anomalies and Variations: Anconeus The anconeus may be coalesced to the medial head of the triceps to varying degrees. It also may blend with the extensor carpi ulnaris (ECU) (11). The subanconeus (articularis cubiti) is a small muscle extension formed from fibers from the deep surface of the distal part of the medial head of the triceps. It is a separate muscle from the anconeus. The subanconeus crosses or covers a portion of the anconeus, attaching to the posterior aspect of the elbow capsule or blending with the antebrachial fascia (3,11). The epitrochleoolecranonis anconeus epitrochlearis (epitrochleoanconeus, epitrochleocubital, or anconeus sextus) is a muscle distinct from the anconeus and the triceps (129). It extends from the medial epicondyle of the humerus, arches across the groove for the ulnar nerve, and inserts onto the olecranon process of the ulna. It is thought to occur in 25% of individuals and takes the place of a

fibrous arch that usually passes between the epicondylar and ulnar heads of the flexor carpi ulnaris (FCU) (11). The anconeus may coalesce with the epitrochleoolecranonis. Clinical Correlations: Anconeus The anomalous muscles associated with the anconeus (the epitrochleoolecranonis anconeus epitrochlearis, epitrochleoanconeus, epitrochleocubital, or anconeus sextus) may be associated with cubital tunnel syndrome (120,121,130, 131). The muscles extend from the medial epicondyle and cross superficial to the cubital tunnel to reach the olecranon. There is thus a potential compression of the ulnar nerve. BRACHIORADIALIS Derivation and Terminology. Brachioradialis is derived from the Latin and Greek brachialis and brachion, respectively, which designate or pertain to the arm. Radialis is from the Latin radii, which means “spoke” (used to describe the radius of the forearm) (1,2). Note that brachi and brachial pertain to “arm,” and should not be confused with brachy (from Greek brachys), which refers to “short” (i.e., brachydactyly for short digits). Origin. From the proximal two-thirds of the lateral ridge of the humeral epicondyle and from the anterior surface of the lateral intermuscular septum. Insertion. The lateral aspect of the base of the styloid process of the radius. Innervation. Radial nerve (C5, C6). Vascular Supply. The radial collateral branch of the profunda brachii, the radial artery, and the radial recurrent artery from the radial artery (3,4,132,133). Principal Action. Flexion of the forearm. It may assist in rotating the forearm to the neutral rotation position from a position of full pronation or full supination. Gross Anatomic Description: Brachioradialis The brachioradialis consists of muscle fibers in its proximal half and a long, strong tendon in its distal half. Positioned on the lateral aspect of the forearm, it forms the lateral margin of the cubital fossa. The brachioradialis, along with the ECRL and ECRB, occupies the muscle compartment known as the mobile wad compartment of the forearm (Appendix 2.2) (12). The muscle originates mostly from the proximal two-thirds of the lateral epicondylar ridge of the humerus (see Fig. 2.2). Additional fibers originate from the anterior aspect of the lateral intermuscular septum. The muscle fibers extend distally and volarly to terminate in a penniform manner on the tendon. The muscle belly twists slightly as it extends from proximal to distal. At the origin,

2 Muscle Anatomy

its broad surface faces laterally; in the forearm, the broad surface faces anteriorly; and in the distal forearm, the tendon twists so that it again faces laterally. The muscle may have extensive fascial attachments or attachments to the bellies of the neighboring muscles. The muscle fibers usually end proximal to the mid-forearm level, and appear to form a short, abrupt myotendinous junction. The tendon, however, usually extends quite proximally on the deep surface of the muscle. The brachioradialis tendon is oval or flat, and extends distally along the radial margin of the radius to reach the insertion point just proximal to the styloid. Along its course, the tendon tapers and becomes narrower, and winds around the radius from the volar to the lateral surface. It widens proximal to the insertion point. Near the insertion point of the tendon, the brachioradialis is crossed by the abductor pollicis longus (APL) and extensor pollicis brevis (EPB). The tendon inserts into the lateral aspect of the base of the styloid process of the radius (see Fig. 2.3A). Vascular studies have been performed on the brachioradialis because of its potential use as a rotation musculocutaneous flap for local soft tissue reconstruction (132,133).

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Sanger and colleagues found that the dominant perforator to perfuse the muscle arose from the brachial artery in 27%, from the radial recurrent artery in 33%, or from the radial artery in 39% (132). Additional studies by Leversedge et al. confirm the brachioradialis is perfused (partly) by the radial recurrent artery [which perfuses an average of 41% (range, 20% to 60%) of the muscle length]. Injection studies of combined radial artery and radial recurrent arteries show that the two arteries combined account for perfusion of 80% (range, 59% to 100%) of the muscle length. This corresponds to 90% of the muscle volume (133). Muscle function and design can be evaluated by the results of tendon transfers from studies on muscle architecture (15,134–142). Architectural features of a muscle include the physiologic cross-sectional area of the muscle, the fiber bundle length, muscle length, muscle mass, and pennation angle (angle of the muscle fibers from the line representing the longitudinal vector of its tendon). Skeletal muscle architectural studies by Lieber, Friden, and colleagues provide the data for the brachioradialis (135–139) (Table 2.1 and Fig. 2.4). The brachioradialis has relatively long fibers arranged at a small

TABLE 2.1. ARCHITECTURAL FEATURES OF SELECTED MUSCLES OF THE UPPER EXTREMITY

Muscle BR (n = 8) PT (n = 8) PQ (n = 8) EDC I (n = 8) EDC M (n = 5) EDC R (n = 7) EDC S (n = 6) EDQ (n = 7) EIP (n = 6) EPL (n = 7) PL (n = 6) FDS I(P) (n = 6) FDS I(D) (n = 9) FDS I(C) (n = 6) FDS M (n = 9) FDS R (n = 9) FDS S (n = 9) FDP I (n = 9) FDP M (n = 9) FDP R (n = 9) FDP S (n = 9) FPL (n = 9)

Muscle Mass (g)

Muscle Length (mm)

Fiber Length (mm)

Pennation Angle (Degrees)

Cross-Sectional Area (cm2)

Fiber Length/ Muscle Length Ratio

17 ± 2.8 16 ± 1.7 5 ± 1.0 3 ± .45 6 ± 1.2 5 ± .75 2 ± .32 4 ± .70 3 ± .61 5 ± .68 4 ± .82 6 ± 1.1 7 ± 0.8 12 ± 2.1 16 ± 2.2 10 ± 1.1 2 ± 0.3 12 ± 1.2 16 ± 1.7 12 ± 1.4 14 ± 1.5 10 ± 1.1

175 ± 8.3 130 ± 4.7 39.3 ± 2.3 114 ± 3.4 112 ± 4.7 125 ± 10.7 121 ± 8.0 152 ± 9.2 105 ± 6.6 138 ± 7.2 134 ± 11.5 93 ± 8.4 119 ± 6.1 207 ± 10.7 183 ± 11.5 155 ± 7.7 103 ± 6.3 149 ± 3.8 200 ± 8.2 194 ± 7.0 150 ± 4.7 168 ± 10.0

121 ± 8.3 36 ± 1.3 23 ± 2.0 57 ± 3.6 59 ± 3.5 51 ± 1.8 53 ± 5.2 55 ± 3.7 48 ± 2.3 44 ± 2.6 52 ± 3.1 32 ± 3.0 38 ± 3.0 68 ± 2.8 61 ± 3.9 60 ± 2.7 42 ± 2.2 61 ± 2.4 68 ± 2.7 65 ± 2.6 61 ± 3.9 45 ± 2.1

2 ± 0.6 10 ± 0.8 10 ± 0.3 3 ± 0.5 3 ± 1.0 3 ± 0.5 2 ± 0.7 3 ± 0.6 6 ± 0.8 6 ± 1.3 4 ± 1.2 5 ± 0.2 7 ± 0.3 6 ± 0.2 7 ± 0.7 4 ± 0.6 5 ± 0.7 7 ± 0.7 6 ± 0.3 7 ± 0.5 8 ± 0.9 7 ± 0.2

1.33 ± 0.22 4.13 ± 0.52 2.07 ± 0.33 0.52 ± 0.08 1.02 ± 0.20 0.86 ± 0.13 0.40 ± 0.06 0.64 ± 0.10 0.56 ± 0.11 0.98 ± 0.13 0.69 ± 0.17 1.81 ± 0.83 1.63 ± .22 1.71 ± .28 2.53 ± .34 1.61 ± .18 0.40 ± .05 1.77 ± .16 2.23 ± .22 1.72 ± .18 2.20 ± .30 2.08 ± .22

0.69 ± 0.062 0.28 ± 0.012 0.58 ± 0.021 0.49 ± 0.024 0.50 ± 0.014 0.42 ± 0.023 0.43 ± 0.029 0.36 ± 0.012 0.46 ± 0.023 0.31 ± 0.020 0.40 ± 0.032 0.34 ± 0.022 0.32 ± 0.013 0.33 ± 0.025 0.34 ± 0.014 0.39 ± 0.023 0.42 ± 0.014 0.41 ± 0.018 0.34 ± 0.011 0.33 ± 0.009 0.40 ± 0.015 0.24 ± 0.010

BR, brachioradialis; PT, pronator teres; PQ, pronator quadratus; EDC I, extensor digitorum communis (index finger); EDC M, extensor digitorum communis (middle finger); EDC R, extensor digitorum communis (ring finger); EDC S, extensor digitorum communis (small finger); EDQ, extensor digiti quinti; EIP, extensor indicis proprius; EPL, extensor pollicis longus; PL, palmaris longus; FDS I (P), flexor digitorum superficialis of index finger, proximal belly; FDS I (D), flexor digitorum superficialis of index finger, distal belly; FDS I (C), flexor digitorum superficialis of index finger, combined properties of the proximal and distal bellies; FDS M, flexor digitorum superficialis (middle finger); FDS R, flexor digitorum superficialis (ring finger); FDS S, flexor digitorum superficialis (small finger); FDP I, flexor digitorum profundus (index finger); FDP M, flexor digitorum profundus (middle finger); FDP R, flexor digitorum profundus (ring finger); FDP S, flexor digitorum profundus (small finger); FPL, flexor pollicis longus. Reproduced from Lieber RL, Jacobson MD, Fazeli BM, et al. Architecture of selected muscles of the arm and forearm: anatomy and implications for tendon transfer. J Hand Surg Am 17:787–798, 1992, with permission.

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Systems Anatomy FIGURE 2.4. Architectural features of selected upper extremity muscles. A: Muscle fiber lengths of selected upper extremity muscles: bar graph of the fiber lengths from several studied muscles of the upper extremity. Note that the flexors and extensors are similar to one another and that the brachioradialis differs significantly. B: Physiologic cross-sectional areas of selected upper extremity muscles: bar graph of the physiologic cross-sectional areas from several studied muscles of the upper extremity. Note that the flexors and extensors are similar to one another and that the BR and the PT differ significantly. C: Cross-sectional area versus fiber length: scatterplot of fiber lengths versus physiologic cross-sectional area of selected upper extremity muscles. Fiber length value (in millimeters) for the BR is listed in parentheses next to it on the chart because it would actually place off the graph. Similarly, the physiologic cross-sectional area for the combined FDP and FDS muscles also is shown in parentheses. Muscles that cluster together in this graph are architecturally similar. Because fiber length is proportional to muscle excursion (or velocity), and physiologic cross-sectional area is proportional to force generation, the location of each muscle indicates its design characteristics and specialization. (Muscles with higher fiber lengths are designed more for excursion or velocity; muscles with higher physiologic cross-sectional areas are designed more for force generation.) Each bar represents mean ± standard deviation (SEM). FCR, flexor carpi radialis; FCU, flexor carpi ulnaris; PL, palmaris longus; ECRB, extensor carpi radialis brevis; ECRL, extensor carpi radialis longus; ECU, extensor carpi ulnaris; FDS (I), flexor digitorum superficialis (index finger); FDS (M), flexor digitorum superficialis (middle finger); FDS (R), flexor digitorum superficialis (ring finger); FDS (S), flexor digitorum superficialis (small finger); FDP (I), flexor digitorum profundus (index finger); FDP (M), flexor digitorum profundus (middle finger); FDP (R), flexor digitorum profundus (ring finger); FDP (S), flexor digitorum profundus (small finger); FPL, flexor pollicis longus; EDC (I), extensor digitorum communis (index finger); EDC (M), extensor digitorum communis (middle finger); EDC (R), extensor digitorum communis (ring finger); EDC (S), extensor digitorum communis (small finger); EDQ, extensor digiti quinti; EIP, extensor indicis proprius; EPL, extensor pollicis longus; PT, pronator teres; PQ, pronator quadratus; BR, brachioradialis. (A–C from Lieber RL, Jacobson MD, Fazeli BM, et al. Architecture of selected muscles of the arm and forearm: anatomy and implications for tendon transfer. J Hand Surg [Am] 17:787–798, 1992, with permission.)

A

B

C

pennation angle, with a relatively small physiologic crosssectional area. This indicates that the brachioradialis is designed more for excursion and velocity than for force generation (135). Its relative difference index values compare it with other upper extremity muscles, based on architectural features. These values are listed in Appendix 2.3. The brachioradialis is innervated by the radial nerve (C5, C6). This innervation is anatomically unusual because the brachioradialis is a flexor of the elbow; the same radial nerve also innervates the extensors (triceps) of the elbow. The motor nerve branch to the brachioradialis exits from

the radial nerve trunk proximal to the level of the elbow, as the radial nerve descends between the brachialis and brachioradialis. The nerve branch continues distally and enters the muscle in its proximal third. Actions and Biomechanics: Brachioradialis The primary function of the brachioradialis is elbow flexion. It has maximal mechanical advantage when the forearm is in 0 degrees of pronation or supination, or in slight

2 Muscle Anatomy

pronation. With the forearm in full pronation or full supination, it may assist in bringing the forearm back to the neutral position of 0 degrees of pronation or supination. The brachioradialis can thus act as a supinator when the forearm is extended and pronated (139). It can act as a forearm pronator when the forearm is extended and supinated. Based on electromyographic studies, the brachioradialis is minimally active with slow flexion movements of the elbow or with the forearm supine. It does, however, generate increased activity when movements are rapid (105). The brachioradialis also may function to help stabilize the elbow during forearm rotation (3,4). Based on cross-sectional analysis of the major elbow muscle flexors, the biceps brachii appears to contribute 34% of flexion torque, with the brachialis contributing 47% and the brachioradialis 19% (20). Anomalies and Variations: Brachioradialis The muscle belly of the brachioradialis may be divided, doubled, or multiple. The tendon may be doubled along its

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course (11) (Fig. 2.5). An accessory brachioradialis may exist, and may cause proximal radial nerve compression at the level of the elbow (143). In approximately 7% of individuals, the tendon of the brachioradialis may divide into two or three separate slips that insert into the radial styloid (11). A slip may insert into the forearm fascia. A second belly may attach distally to the radius near the radial tuberosity, or to the ulna (11). When two slips of the brachioradialis tendon are present, the radial sensory nerve may pass between them. The nerve is at risk for compression if it penetrates between the slips (144–146) (see Fig. 2.5). The supinator longus accessories or brachioradialis brevis is an accessory brachioradialis. It arises adjacent to the brachioradialis and inserts onto the radial tuberosity or into the supinator (see Fig. 2.5). It acts as a supinator of the forearm. The brachioradialis brevis also may insert into the pronator teres or into the ulna (11). The brachioradialis may be coalesced or tethered with other muscles, most commonly the brachialis (near the origin of the brachioradialis) as well as the ECRL, pronator

FIGURE 2.5. The normal brachioradialis (left) and some of its clinically relevant variations. The split or duplicated muscle may cause confusion during harvest for tendon transfer. The split tendon may be responsible for neuropathy of the superficial branch of the radial nerve, if the nerve passes through the split tendon. The brachioradialis brevis is an anomalous muscle that inserts into the radial tuberosity or the biceps tendon. It can function as a supinator of the forearm, as well as an elbow flexor (11).

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Systems Anatomy

teres, and FCR (147). The brachioradialis may send slips to the deltoid (see later), supinator, or APL (11). The origin of the brachioradialis may extend proximally as far as the mid-humerus, at the level of the deltoid insertion (11). The insertion point may be located more proximally or more distally than the styloid. The brachioradialis may insert as far proximal as the middle third of the radial shaft. It may insert as far distally as the scaphoid, trapezium, or base of the index metacarpal (11,148). The brachioradialis muscle or tendon may be absent. If the tendon is absent, the brachioradialis muscle may insert onto the radius more proximally along the lateral diaphysis (11). The brachioradialis usually is innervated by the radial nerve. Anomalous innervation by the musculocutaneous nerve has been reported as an unusual variation (149). Clinical Implications: Brachioradialis Sensory radial neuropathy may be caused by a split brachioradialis tendon or muscle, resulting in compression of the superficial branch of the radial nerve passing through the split tendon. Because the brachioradialis is relatively expendable, it is used as a donor muscle for several reconstructive procedures, including tendon transfer (134,139,150,151), as a myocutaneous or rotation muscular flap for soft tissue reconstruction (152,153), or for retinacular reconstruction (154). Freehafer and associates studied the anatomy, properties, and value of the brachioradialis for tendon transfer in the tetraplegic patient. The relatively large excursion and adequate muscle force measurements of the brachioradialis support its use as a donor for tendon transfer (134). Friden et al. studied the architectural properties of the brachioradialis and further emphasized the muscle’s value in tendon transfers (139) (see Fig. 2.4). Its relatively high fiber length indicates its design for excursion and velocity. The brachioradialis does, however, have limitations as to excursion secondary to extrinsic soft tissue constraints and interconnections, which may limit its potential true excursion when used in reconstructive procedures. These constraints include presence of an internal tendon, as well as substantial fascial interconnections to the bellies of the neighboring muscles and associated fascia (see earlier, under Anomalies and Variations). Mobilization of the muscle and release of these soft tissue constraints should increase the functional range of excursion (135). When using the brachioradialis as a donor for tendon transfer, it is optimal to mobilize and free the muscle belly quite proximally in the forearm. Awareness of the possible split muscle belly (and other anomalies as described previously) avoids confusion if it is encountered during harvest of the brachioradialis for tendon transfer (Fig. 2.5). The brachioradialis may be a major participant in spastic flexion of the elbow in patients with acquired spasticity. Selective denervation or recession (proximal release) of the

brachioradialis in selected patients can help relieve the flexion attitude of the elbow (155). PRONATOR TERES Derivation and Terminology. Pronator is derived from the Latin pronus, meaning “inclined forward” (the Latin pronatio denotes the act of assuming the prone position or a state of being prone). Teres is derived from the Latin indicating “long and round” (1,2). Origin. Two heads exist. The humeral (principal) head originates from the anterior surface of the medial epicondyle (common flexor origin) and from the intermuscular septum. The ulnar (deep) head originates from the medial border of the coronoid process (3,4). Insertion. The middle third of the lateral surface of the radius. Innervation. Median nerve (C6, C7). Vascular Supply. The ulnar artery, by direct muscular arterial branches (3,4). Principal Action. Pronation of the forearm, through rotation of the radius on the ulna. Gross Anatomic Description: Pronator Teres The pronator teres is the most radial muscle of the superficial flexors of the forearm (which also include the FCR, palmaris longus, FDS, and FCU). The pronator teres lies in the superficial volar muscle compartment of the forearm (Appendix 2.2). As the name implies, the pronator teres is a long, round, and somewhat cylindrical muscle. The pronator consists of two heads: a larger, more superficial humeral head (often designated as the principal or primary head ) , and a smaller, deeper ulnar head (also referred to as the accessory or deep head ). The humeral head has been found to be consistently present. The ulnar head, however, may be absent in approximately 22% of specimens (156,157). The humeral head arises from the common tendon of the flexor–pronator muscles (see Fig. 2.2A). This tendon of origin attaches to the medial epicondyle, arising from a point of attachment on the proximal half of the anterior surface of the epicondyle. The humeral head also arises from the overlying antebrachial fascia, and from the intermuscular septum that separates the pronator teres from the medial head of the triceps and the FCR. The ulnar head is smaller, and positioned deeper. It arises from an aponeurotic band attached to the medial border of the coronoid process, located medial to the tendon of the brachialis (see Fig. 2.3A). The origin is distal to the attachment of the FDS. The ulnar head joins the humeral head at an acute angle. The morphology of the ulnar head is variable. In 11 of 60 limbs it was found to be muscular; in 6 of

2 Muscle Anatomy

60 it was predominantly tendinous, and in 30 of 60, it was found to be mixed (156,157). A fibrous arch is formed by the humeral and ulnar heads. The arch is located within 3 to 7.5 cm of the arch created by the origin of the FDS muscle (158). In 83% of arms, the median nerve passes between the pronator muscle heads. The median nerve is at risk for compression as it passes through this arch (147,156–165). The nerve is separated from the ulnar artery by the ulnar head of the pronator (3,4,13). The humeral and ulnar head join to form a common muscle belly. The muscle passes obliquely across the proximal volar forearm in a medial-to-lateral direction. The muscle fibers converge to end in a flat tendon that attaches to a rough area on the lateral surface of the radial shaft (see Fig. 2.3B). The point of insertion is roughly at the junction of the proximal third and distal two-thirds of the radius, at the “summit” of the lateral curve of the radius (3,4). The lateral border of the muscle forms the medial border of the cubital fossa. At the point of insertion, the tendon of the pronator teres becomes broader and winds around the anterior surface of the radius, finally attaching to the cortex. Most of the insertional tendon is continuous with muscle fibers from the humeral head. The muscle fibers of the ulnar head extend distally along the lateral border of the fibers from the humeral head. Much of the ulnar head inserts or blends into the radial side of the deep surface of the humeral head (3,4,8,11). Architectural features of the pronator teres include the physiologic cross-sectional area of the muscle, the fiber bundle length, muscle length, muscle mass, and pennation angle (angle of the muscle fibers from the line representing the longitudinal vector of its tendon). Skeletal muscle architectural studies by Lieber, Friden, and colleagues provide the data for the pronator teres (135–139) (see Table 2.1 and Fig. 2.4). The pronator teres has a relatively large physiologic cross-sectional area, indicating that its design is more optimal for force generation. It has a relatively short muscle fiber length, indicating that it is not specifically designed for excursion or velocity. Its relative difference index values compare it with other upper extremity muscles, based on architectural features. These values are listed in Appendix 2.3. The pronator teres is innervated by a branch or branches from the median nerve (C6, C7). Each head receives a separate branch. The branches usually exit the median nerve trunk before the median nerve passes between the two heads of the pronator. The nerve branch to the humeral head enters the proximal part of the middle third of the belly of the muscle, on its deep surface near the radial border. The branch to the ulnar head usually enters the muscle proximal to the point where the two bellies join (11). Actions and Biomechanics: Pronator Teres The pronator teres pronates the forearm and acts with cocontraction of the pronator quadratus. With full flexion

111

of the elbow, the fibers of the muscle are short and unable to produce maximal force. The pronator teres also functions as a weak elbow flexor (3,4,11). Anomalies and Variations: Pronator Teres A supracondylar process is a small, curved, hook-shaped process of the distal humerus, several centimeters proximal to the elbow, and usually located on the medial side. It often is associated with a ligament (or muscle) slip that extends distally to the medial epicondyle. The ligament, known as the ligament of Struthers, is thought to be an extension of the pronator teres. The median nerve may pass deep to the ligament, and may thus be at risk for compression (166–168). The brachial artery also may pass deep to a ligament of Struthers, and brachial artery entrapment (presenting as ischemia during extension of the elbow) may occur (169). Accessory slips may attach from the pronator teres to the biceps brachii, brachialis, or to the median intermuscular septum. Nebot-Cegarra et al. studied 60 upper extremities and found slips to the biceps brachii in 3.3%, to the brachialis in 5.0%, to the FDS muscle in 1.6%, and to Gantzer’s muscle in 1.6%. In all cases, the accessory slips were connected to the deep (humeral) head, and were in the vicinity of the median nerve, possibly producing a risk for nerve encroachment (156). Clinical Correlations: Pronator Teres The median nerve may become compressed as it passes between the humeral and ulnar heads of the pronator teres, referred to as pronator syndrome (147,156–165). The median nerve (and brachial artery) may become compressed if passing deep to the anomalous ligament of Struthers. The ligament of Struthers, which is thought to be an extension of the pronator teres, originates from a supracondylar process of the humerus and attaches to the medial epicondyle (166–169). FLEXOR CARPI RADIALIS Derivation and Terminology. Flexor is derived from the Latin flexus, indicating “bent” (and flexor, which indicates “that which bends,” or “bending”). Carpi is from the Latin carpalis and Greek karpos, both of which indicate “wrist” (the carpus). Radialis is from the Latin radii, which means “spoke” (used to describe the radius of the forearm) (1,2). Origin. Medical epicondyle through the common flexor origin. Insertion. To the volar base of the index finger metacarpal. An accessory slip may attach to the adjacent volar base of the long finger metacarpal. Innervation. Median nerve (C6, C7).

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Vascular Supply. The ulnar aspect by direct intramuscular branches; superior and inferior ulnar collateral arteries; to a variable degree, contributions from the anterior and posterior ulnar recurrent arteries; in the distal aspect, superficial palmar branch of the radial artery; at the insertion, the palmar metacarpal arteries and perforating branches from the deep palmar arch. The anterior interosseous artery also may supply the FCR (3,4,8,11). Principal Action. Flexion of the wrist. Working with the radial wrist extensor, the FCR can assist with wrist radial deviation. Gross Anatomic Description: Flexor Carpi Radialis The FCR comprises one of the more radially located muscles of the superficial flexors of the forearm (along with the

pronator teres, palmaris longus, FDS, and FCU). The FCR lies in the superficial volar muscle compartment of the forearm (Appendix 2.2). The muscle is positioned between the pronator teres (medially) and the palmaris longus (laterally) (170). It originates from the common flexor origin of the medial epicondyle (see Fig 2.2A). Additional sites of attachment include the adjacent intermuscular septum and the adjacent fascia of neighboring muscles. The muscle belly is relatively large and fusiform, and usually extends to at least the mid-portion of the forearm halfway to the wrist. The muscle fibers from the epicondyle extend distally in a vertical fashion to the anterior and sides of the tendon. The fibers that originate from the intermuscular septa tend to extend in an oblique fashion to the deep surface of the tendon. The mid-portion of the muscle belly lies in the central portion of the proximal forearm. The myotendinous junction spans several centimeters and gives rise to a long ten-

A FIGURE 2.6. Anterior (A) and posterior (B) views of the skeleton hand, showing muscle origins (red) and insertions (blue).

2 Muscle Anatomy

don. Studies by Bishop et al. have shown the myotendinous portion of the muscle begins an average of 15 cm (range, 12 to 17 cm) proximal to the radiocarpal joint. The muscular fibers end an average of 8 cm (range, 6 to 9 cm) proximal to the wrist (171). The tendon is initially flat, but becomes rounder as it continues distally. The tendon passes across the distal half of the forearm, coursing distally and radially to the wrist. There is a torsional component of the tendon as it passes distally (172). The radial artery usually is located radial to the tendon of the FCR, situated between it and the brachioradialis. The tendon passes radial to the carpal tunnel, and travels through its own fibroosseous tunnel formed in part by a groove in the trapezium and overlying fibrous arch. The tendon occupies 90% of the space in the fibroosseous tunnel and is in direct contact with the slightly roughened surface of the trapezium (171). The tendon does not pass through the carpal tunnel. In this distal portion of

113

its course, the tendon often has a synovial sheath. The tendon dives deep, deep to the oblique head of the adductor pollicis, to reach the proximal aspect of the base of index metacarpal (Fig. 2.6A). The tendon inserts into the proximovolar aspect of the index metacarpal, and also commonly sends a slip to the adjacent base of the long finger metacarpal (173). A small slip often attaches to the trapezial crest or tuberosity (171). The insertion tendon of the FCR extends out from the muscle mass a distance equivalent to approximately 75% of the muscle length. Architectural features of the FCR include the physiologic cross-sectional area of the muscle, the fiber bundle length, muscle length, muscle mass, and pennation angle (angle of the muscle fibers from the line representing the longitudinal vector of its tendon). Skeletal muscle architectural studies by Lieber, Friden, and colleagues provide the data for the FCR (135–139,174) (Table 2.2; see Fig. 2.4).

B FIGURE 2.6. (continued)

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TABLE 2.2. ARCHITECTURAL FEATURES OF WRIST EXTENSOR AND FLEXOR MUSCLES One-Way ANOVA Significance Levelb

Measured Properties of Muscles and Tendonsa Parameter Muscle properties Muscle length (mm) Fiber length (mm) Physiological CSA (mm2) Predicted maximum tetanic tension (N) Tendon properties Aponeurosis length (mm) External tendon length (mm) Total tendon length (mm) Tendon length: fiber length ratio Tendon CSA (mm2) Tendon stress at P0 (MPa) Tendon strain at P0 (%) Modulus at P0 (MPa) Ultimate stress (MPa) Tangent modulus (MPa) Safety factor (× P0) Biochemical properties Hydration (% dry mass) Collagen (% dry mass)

ECRB

ECRL

ECU

FCR

FCU

186.4 70.8 240.1 58.8

± ± ± ±

4.5 1.7 20.5 5.0

155.3 127.3 130.0 31.9

± ± ± ±

6.9 5.6 11.1 2.7

209.9 58.8 210.0 51.5

± ± ± ±

6.0 1.7 14.1 3.4

192.8 59.8 211.9 51.9

± ± ± ±

4.8 1.5 15.4 3.7

220.6 41.9 363.6 89.0

± ± ± ±

8.6 1.6 34.3 8.4

p p p p

< < <
>
.3

ECRB, extensor carpi radialis brevis; ECRL, extensor carpi radialis longus; ECU, extensor carpi ulnaris; FCR, flexor carpi radialis; FCU, flexor carpi ulnaris; CSA, cross-sectional area; P0, muscle maximum tetanic tension. aValues shown are mean ± standard error of n = 5 independent measurements. bSignificance level from one-way analysis of variance (ANOVA). cSignifies n = 4. Reproduced from Loren GJ, Lieber RL. Tendon biomechanical properties enhance human wrist muscle specialization. J Biomech 28:791–799, 1995, with permission.

The FCR has a moderate fiber length and physiologic crosssectional area, indicating that its design is moderate for both excursion and force generation. Its relative difference index values compare it with other upper extremity muscles, based on architectural features. These values are listed in Appendix 2.3. In comparing the architectural features of the FCR with the FCU, the FCR muscle length is shorter than the FCU, although the muscle fibers of the FCR are longer (136,174). The relatively longer fiber length indicates that the FCR is designed more for excursion and velocity of contraction (because excursion and velocity are proportional to fiber length) compared with the FCU. The FCU, in contrast, has a higher pennation angle with a larger physiologic cross-sectional area. This indicates that the FCU is designed more for force production and less for excursion and velocity compared with the FCR (because cross-sectional area is proportional to force production) (174,175) (see Table 2.2 and Fig. 2.4). The FCR is innervated by the median nerve (C6, C7, C8). It usually is supplied by a direct branch that divides into smaller branches before entering the muscle. The nerve branches usually enter the muscle near the junction of its

proximal and middle third, and enter on the deep surface (3,4,176). Actions and Biomechanics: Flexor Carpi Radialis The FCR functions mainly to flex the wrist. It works with the FCU and the digital flexors during strong wrist flexion. In addition, in working with the ECRL (and ECRB), the FCR may assist with radial deviation of the wrist. The FCR also can assist with elbow flexion, and can act as a relatively weak pronator of the forearm. As noted previously, from an architectural standpoint in comparison with the FCU, the relatively longer fiber length of the FCR indicates that it is designed more for excursion and velocity than for force production (135,174). Anomalies and Variations: Flexor Carpi Radialis The FCR may be absent (11,177). The FCR may exist as a double or split muscle (11,178,179). Several accessory slips

2 Muscle Anatomy

of the FCR may exist in the proximal forearm, including slips to or from the biceps tendon, brachialis, bicipital aponeurosis, coronoid process, or radius. In the distal forearm, the FCR may have slips that attach to the trapezium, scaphoid, flexor retinaculum, or fourth metacarpal. Partial or total insertion into the trapezium is the more common insertional anomaly (11). An FCR brevis has been described as a small muscle arising from the radius and usually inserts into the fibrous sheath of the tendon of the FCR. It was noted in 6 of 70 limbs by Wood, and in 1 of 400 limbs by Gruber (11), as well as in a more recent case report by Effendy (180). An additional, different FCR brevis muscle was described as an anomalous muscle that originates from the anterior surface of the radius and forms a tendon at the radiocarpal joint. It enters the carpal tunnel and the tendon extends between the bases of the index and long finger metacarpals to interconnect with the tendon of the ECRB. The muscle is innervated by the anterior interosseous nerve (181). In addition, it was noted that the ECRB had split into two tendons, one inserted normally into the radial part of the base of the long finger metacarpal and the other connected to the anomalous FCR brevis. It was postulated that this anomaly may cause restricted wrist flexion or extension (11). Clinical Correlations: Flexor Carpi Radialis The FCR, innervated by the median nerve, is a common muscle used for transfer to the extensor digitorum communis (EDC) to provide digital extension in patients with radial nerve palsy (182–185). From an architectural standpoint, its design for greater excursion makes it (architecturally) a better choice than the FCU, which is designed more for force generation (see earlier, and Fig. 2.4C). Attritional rupture of the FCR has been noted to occur in association with scaphotrapezial osteoarthritis (186). PALMARIS LONGUS Derivation and Terminology. Palmaris is derived from the Latin palma, which means “pertaining to the palm.” Longus is the Latin for “long” (1,2). Origin. Medical epicondyle through the common flexor origin. Insertion. The palmar fascia of the hand. Innervation. Median nerve (C7, C8). Vascular Supply. Muscle belly: the ulnar artery, brachial artery, superior and inferior ulnar collateral arteries, anterior interosseous artery, and variable contributions from the anterior and posterior ulnar recurrent arteries. Distal tendon: rami from the ends of the superficial arch (3,4,8,11, 187,188).

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Principal Action. Flexion of the wrist. It also contributes to anchoring of the palmar fascia to resist horizontal shearing forces moving distally in the hand. It can assist with weak pronation of the forearm. Gross Anatomic Description: Palmaris Longus The palmaris longus comprises one of the central muscles of the superficial flexors of the forearm (along with the pronator teres, FCR, FDS, and FCU). It lies in the superficial volar muscle compartment of the forearm (Appendix 2.2). The palmaris longus is small but clinically important (189–194). It also is well documented as one of the most variable, in terms of presence (or absence) (195–206) as well as muscle variations and anomalies (207–262). It is clinically important because of its value as a free tendon graft. Because absence is relatively common, this variation is discussed here instead of under Variations and Anomalies. Its absence has been the subject of several anatomic investigations (197–206). The incidence sometimes is given in terms of patients (or cadavers), or in terms of limbs. The frequency of absence in one or both limbs has been noted from 6% (197) to as high as 31% to 64% (187,194,198). Most studies indicate an absence in one or both limbs in approximately 12% to 25% of patients (or cadavers) (11,196,199), or 5% to 15% of individual limbs (187,194,231). In 2001, in a relatively large study, Thompson et al. examined 300 caucasian subjects (150 male, 150 female) and found unilateral absence of the palmaris longus in 49 subjects (16%), and bilateral absence in 26 (9%) (199). The rate of absence of the tendon may be different in different ethnicities. Reporting in the Indian Journal of Medical Sciences, Ceyhan and Mavt in 1997 evaluated 7,000 students of the Graduate School at Gaziantep University for absence of the palmaris longus (198). Findings included, in women, unilateral absence in 23% and bilateral absence in 45.3%. In men, unilateral absence was found in 19.5% and bilateral absence in 42.1%. The overall percentage of absence was 63.9%. This is among the highest reported absence rates (198). One of the lowest rates of absence was reported by Troha and colleagues in 1990 (197). In 200 caucasian patients (100 men, 100 women), the tendon was absent in one extremity in only 3% of patients. Bilateral absence was seen in 2.5%, for a 5.5% rate of total overall absence (197). In addition, the frequency of absence has been as low as 3.5% in the Japanese population and 2% in the Chinese population (11). There is disagreement as to the frequency of unilateral versus bilateral absence. Several studies and authors have noted a more common occurrence of bilateral absence (11,198). However, studies do not consistently support this (197,199). If a patient has a tendon absence on one side, it

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was shown that there is a 67% chance that the contralateral tendon also will be absent (196). Although some suggest that the palmaris longus is absent more often in women, and more often on the left side (11), Thompson et al., in a study of 300 caucasian subjects, showed no statistical differences between the sexes or in absence in the right versus the left extremity (199). It has been suggested that there may a higher incidence of Dupuytren’s disease in patients with a present palmaris longus tendon (200). Additional investigations with larger populations are needed to substantiate this association. From an anatomic standpoint, the tendon arises with the other superficial flexors (including the pronator, FCR, FDS, and FCU) from the common flexor origin of the medial epicondyle of the humerus (see Fig. 2.2A). It is slender, usually fusiform or slightly triangular, and located ulnar to the FCR and superficial and radial to the FDS. Besides the medial epicondyle, the palmaris longus has proximal attachments to the neighboring superficial muscle fascia as well as from the intermuscular septa and deep antebrachial fascia. The muscle fibers are aligned in a nearly parallel course to the tendon. The muscle usually has a fairly abrupt myotendinous junction located in the mid-portion of the forearm, giving rise to a long, slender tendon. The tendon extends distally, superficial to the flexor retinaculum. It becomes broad and flat to form a sheet that connects or is continuous with the palmar fascia (palmar aponeurosis) of the hand. A few connections may interweave with the transverse fibers of the retinaculum, although most of the fibers are oriented longitudinally in a proximal-to-distal direction. The radiating fiber bundles on the radial and ulnar aspects extend distally to attach to the overlying fascia of the thenar and hypothenar muscles. The more central bundles usually are more developed and constitute the more substantial portion of the palmar fascia (3,4,8). Fahrer has shown that the proximal end of the palmar fascia receives two important contingents of fibers from the FCU. A superficial component blends with the fibers of the palmaris longus; a deep component runs on the surface of the pisohamate ligament and connects the flexor retinaculum to the palmar fascia (193). The tendon and palmar fascia continue distally to form a diverging sheet that splits longitudinally to send thickenings of the fascia to each of the four rays, with variable fiber bundles extending toward the thumb (3,4). These diverging fiber bundles form a triangular connective tissue sheet in the midpalm with the apex proximal. The palmar fascia has interconnections with the fibroosseous tendon sheaths, with the skin, and in the fascia of the distal palm and digital webs. Although the palmaris longus often is absent, absence of the palmar fascia has not been noted (194). From gross and microscopic observations, as well as staining properties, the palmaris longus tendon and palmar fascia appear as tendon and fascia, respectively. These observations support the idea

that the palmaris longus and palmar fascia are separate anatomic structures that develop independently and are associated only by anatomic proximity (194). The morphology and biomechanical aspects of the palmaris longus tendon have been evaluated in terms of its use as a tendon graft, and in comparison with other tendons used as grafts (191). The palmaris longus mean tendon length is 161 mm, its mean cross-sectional area 3.1 mm2, and its mean volume 529 mm3. The tendon is among the stiffest at 42.0 N/mm (191). The average width of the palmaris tendon is approximately 3 mm (189,191). The arterial supply has been studied in detail by Wafae and associates (187). Most muscles received one or two arterial branches from the ulnar artery (86%), and less frequently from the brachial artery (23%). The arterial branches penetrate the muscle through the posterior surface, 63% in the proximal third and 34% in the middle third of the muscle. The most frequent patterns observed included one or two branches of the ulnar artery penetrating the proximal third of the muscle (29%), and two branches of the ulnar artery, one entering the proximal third and one entering the middle third of the muscle belly (187). The architectural properties of the palmaris longus are listed in Table 2.1 and shown in Fig. 2.4A and B. The palmaris longus is innervated by the median nerve (C7 and C8). The nerve branch usually is a common branch from the median nerve that also supplies the FCR. It often courses along with the branch supplying the proximal part of the FDS. The nerve to the palmaris longus usually enters in the middle third of the muscle (3,4,8,11). Actions and Biomechanics: Palmaris Longus The palmaris longus is a weak flexor of the wrist. The muscle also may assist with a relatively weak contribution to forearm pronation. It may represent an evolutionary remnant of a flexor of the metacarpophalangeal (MCP) joints (188) because it appears that the palmar fascia extends to that level. In addition, the palmaris longus plays a role in the stabilization of the palmar fascia. A purpose of the palmar fascia is to anchor the skin on the palm to resist shearing forces (compared with the loose skin on the dorsum of the hand, the palmar skin is relatively immobile). This anchoring of the skin assists with grasp functions, so that objects do not move or shift during tight grasp. The palmaris longus, which has power to apply force to the palmar fascia, contributes to this anchoring of the palmar fascia to resist horizontal shearing forces moving distally in the hand. It has been postulated by Fahrer that, in congenital absence of the palmaris, the FCU takes over as the longitudinal tensor of the palmar fascia through interconnecting fibers of the tendon and the palmar fascia (192,193).

2 Muscle Anatomy

Fahrer and Tubiana suggest that the palmaris longus contribute to opposition and pronation of the thumb under some circumstances (192). The palmaris longus, however, is restricted in this motion because it is tethered by its tendon’s medial slip and terminal insertion that attaches to the palmar fascia (192). Kaplan and Smith also give credit to the palmaris longus as a synergist in thumb opposition (195). The tendon becomes tense when opposition of the thumb is attempted or maintained. The contraction is thought to produce synergistic tension of the transverse carpal ligament to provide better fixation at the origins of the thenar muscles (195). In addition, the palmaris longus tendon often has a slip that inserts into the abductor pollicis brevis (APB) and can therefore act directly on the muscle during opposition. It was concluded by Kaplan and Smith that the palmaris is an unimportant flexor of the wrist but a strong synergist of abduction and opposition of the thumb. In paralysis of the other flexors of the wrist, the palmaris longus may become a fairly important wrist flexor if it has a firm insertion into the transverse carpal ligament or the carpal bones (195). Anomalies and Variations: Palmaris Longus The palmaris longus is one of the most variable muscles in the upper extremity (195). The presence (or absence) of the palmaris longus is quite variable. In general, there is an absence in one or both limbs in approximately 12% to 25% of patients (or cadavers) (196,199), or absence in individual limbs in 15% to 31% (187,194). Because absence is relatively common, the incidences are discussed in more detail earlier, under Gross Anatomic Description. Several variations have been reported (207–262) (Fig. 2.7). These have clinical implications because of the value of the palmaris as a free graft or transfer. An awareness of the variability of the palmaris may help avoid difficulty or confusion in the harvest of the free graft. In addition, many of the anomalous muscles cause problems with nerve compression, including the median nerve in the forearm (207–210,224) and the carpal tunnel (229–231), the palmar cutaneous branch of the median nerve (230–232), and the ulnar nerve (225,233–235). The more common variations and anomalies are as follows: Distal Belly (Reverse Belly, Palmaris Longus Inversus) The palmaris longus can have a distal or reverse muscle belly (see Fig. 2.7). In the reversed form, the tendon is proximal and the muscle is distal. Variations of this form have been referred to as the palmaris longus inversus (11). The distal muscle can cause median neuropathy in the forearm (208,209,211). If the muscle reaches or enters the carpal tunnel, carpal tunnel syndrome can result (212,

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219). A reversed muscle also can lead to ulnar nerve compression (234). Digastric Head The palmaris longus may have a digastric head (two heads, one proximal and one distal, separated by an intercalary tendon) (see Fig. 2.7). The distal muscle belly may cause median neuropathy in the forearm or, if it reaches or enters the carpal tunnel, can result in carpal tunnel syndrome (227–232). Split or Double Belly Tendon The muscle may be split along its course, presenting as two separate muscle bellies (see Fig. 2.7). When two bellies are present, there may be several variations of the origin and insertion attachments (198,237). The tendon itself may be split or doubled (11,231). Dowdy and colleagues identified 2 specimens of 52 with a split palmaris longus tendon (231). The palmar cutaneous branch of the median nerve passed through the split at 1 to 1.5 cm proximal to the insertion into the palmar fascia. In the presence of this anomaly, the nerve is at risk for injury in the harvest of the tendon. The authors recommend transecting the tendon 2 cm proximal to its insertion into the palmar fascia to avoid possible nerve injury (231). In addition, this split may place the nerve at risk for compression neuropathy. Palmaris Longus Profundus The palmaris profundus is an anomalous palmaris longus that arises from the lateral edge of the radius, in its middle third, external to the FDS and deep to the pronator teres. The tendon passes deep to the flexor retinaculum (to the radial side of the median nerve) and broadens in the palm to insert into the deep side of the palmar aponeurosis (11,215,221,228). It can be noted as an incidental operative finding without any clinical consequences. However, as it enters the carpal canal, it can result in carpal tunnel syndrome (221). It has been reported to occur bilaterally (215,221,228). The muscle also can cause ulnar nerve compression at the wrist (261). Palmaris Bitendinous The palmaris bitendinous is an anomalous muscle that is located deep to the palmaris longus and has a distal insertion on the deep surface of the palmar aponeurosis, similar to the palmaris profundus. It can result in median neuropathy in the forearm and hand (210). Continuous Muscle The palmaris longus may have one continuous muscle from origin to insertion. The distal muscle extension can

118 FIGURE 2.7. The normal palmaris longus and some of its clinically relevant variations. The palmaris longus with a distal muscle belly may be responsible for median or ulnar nerve compression. The median nerve can be compressed either in the distal forearm or in the carpal tunnel if an anomalous portion or slips extend into the canal. The split or duplicated muscle belly of the palmaris longus and the digastric variation (with a distal belly) may cause difficulty or confusion during harvest for transfer or free graft if these possible variations are not appreciated or recognized. The digastric form also may contribute to median and ulnar nerve compression in the forearm (11).

2 Muscle Anatomy

cause median neuropathy in the forearm or carpal tunnel (195). Central Belly The muscle belly may be located centrally between two tendons, so that the origin and insertion are both tendinous (195,237). Continuous Tendon The palmaris longus may exist only as a tendon from origin to insertion (11,195). Triple Muscle Bellies The muscle may exist as three distinct muscle bellies (195). The tendon also may be split or triplicated (11).

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noted to split from the palmaris longus tendon and enter the ulnar tunnel to cause ulnar tunnel syndrome (225,233). The ulnar artery also may be compressed by an anomalous palmaris longus slip that enters the ulnar tunnel (11). Intrapalmar Muscle An intrapalmar accessory head of the muscle has been identified in the carpal tunnel, causing carpal tunnel syndrome (223). Palmaris Longus and the Accessories Ad Flexoram Digiti Minimi The tendon of the palmaris longus may give origin to an additional muscle, the accessories ad flexorum digiti minimi. This muscle usually inserts on the body and head of the fifth metacarpal between the abductor digiti minimi and flexor digiti minim brevis (11).

Variable Origin The site of origin is variable, and has been noted to include attachments to the fascia of most of the muscles of the ulnar side of the forearm (including the biceps, brachialis, and FDS), from the medial intermuscular septum, from the coronoid process of the ulna, and from the proximal radius (11,195). With a double muscle belly, one can arise in a normal fashion from the medial epicondyle, and the other from the aforementioned muscles, fascia, intermuscular septum, proximal ulna, or proximal radius (11,195). Variable Insertion and Accessory Distal Slips The site of insertion is equally as variable as the site of origin. It may have abnormal extensions, anomalous slips, an abnormal split, and associated anomalous muscle bellies (11,195,215,216,222–225,250). The palmaris longus may insert into the tendon of the FCU, transverse carpal ligament, antebrachial fascia, scaphoid, pisiform, or APB (195). It commonly has fascial extensions to the fascia of the base of the thenar and hypothenar muscles (and attachments to these muscles are so common they may be considered part of the normal insertion). The tendon can insert onto the deep surface of the palmar fascia (260). Several accessory slips or anomalous muscle heads at the insertional area have been identified. The accessory slips may attach to various flexor tendons and extend distally as far as the MCP joint (11). Median nerve compression in the forearm and carpal tunnel has been associated with the accessory slips, especially if the anomalous tendon or muscle enters the carpal tunnel (215,216,222,224,250). An accessory muscle inserting into the base of the hypothenar muscles has been shown to cause carpal tunnel syndrome (213). An ulnarsided palmar accessory muscle was noted to cause ulnar tunnel syndrome (234,236). An accessory slip has been

Palmaris Longus Substituting for Digital Flexors The palmaris longus can substitute for the ring finger FDS. In the absence of the FDS, a palmaris longus was found to extend to the middle phalanx of the ring finger and function as a digital flexor of the proximal interphalangeal joint (PIP) (259). Clinical Correlations: Palmaris Longus The most important anatomic clinical considerations with the palmaris longus include its variable presence and the common anomalies. The specific anatomic forms are discussed in detail previously. The possible variations and anomalies are important both from the standpoint of free tendon harvest or transfer, as well as with regard to the many associated nerve compression syndromes caused by an anomalous palmaris longus tendon. Problems associated with anomalous muscles include median compression in the forearm (208–210,212,224) and the carpal tunnel (131–229), compression of the palmar cutaneous branch of the median nerve (230–232), and compression of the ulnar nerve in the forearm or ulnar tunnel (233–236,261) (discussed in detail earlier, under Variations and Anomalies). The possibility of absence is of clinical significance because of the common use of the palmaris longus as a free graft or tendon transfer (263–267). Its presence always should be tested by having the patient place the pulp of the thumb in opposition to the pulp of the small finger. When the wrist is flexed, the tendon of the palmaris becomes prominent. In general, there is an absence in one or both limbs in approximately 12% to 25% of patients (or cadavers) (11,196,199) and absence in individual limbs in 15%

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to 31% (187,199) (discussed in detail earlier, under Gross Anatomic Description). Magnetic resonance imaging or ultrasound (UTZ) have been shown to be capable of detecting the absence of the palmaris longus or the presence of anomalies (225,250). Hypertrophy of a normal palmaris longus tendon can result in median neuropathy simulating carpal tunnel syndrome (11,212,218). For low median neuropathy, such as with severe, longstanding carpal tunnel syndrome, the Camitz transfer is a type of opponensplasty used to provide thumb palmar abduction and opposition. It was popularized by Braun and uses the palmaris longus, extended by a strip of palmar fascia, to transfer to the thenar muscles (266–272). FLEXOR DIGITORUM SUPERFICIALIS (FLEXOR DIGITORUM SUBLIMIS) Derivation and Terminology. Flexor is derived from the Latin flexus, indicating “bent” (and flexor, which indicates “that which bends,” or “bending”). Digitorum is from the Latin digitus or digitorum, indicating the digits. Superficialis denotes its superficial location in the forearm. The term sublimis sometimes is used. This is derived from Latin sublimis, indicating “superficial” (1,2). Origin. There are two heads with separate origins. Humeroulnar head: from the medical epicondyle of the humerus and from the proximal medial ulna. Radial head: from a long, oblique, linear attachment from the volar proximal radial shaft, along the proximal third of the diaphysis. Insertion. To the medial and lateral margins of the volar shaft of the middle phalanges of the index, long, ring, and small fingers. Innervation. Median nerve (C7, C8, T1). Vascular Supply. The ulnar artery, superior and inferior ulnar collateral arteries, anterior and posterior ulnar recurrent arteries, superficial palmar arch, common and proper palmar digital arteries (3,4,8). Principal Action. Flexion of the PIPs of the index, long, ring, and small fingers. It also contributes to flexion of the digital MCP joints, and flexion of the wrist. Gross Anatomic Description: Flexor Digitorum Superficialis The FDS is one of the central muscles of the superficial flexors of the forearm (along with the pronator teres, FCR, palmaris longus, and FCU). It lies in the superficial volar muscle compartment of the forearm (Appendix 2.2). The FDS is located medial and deep to the palmaris longus and FCR. The FCU lies ulnar and superficial to the FDS. It is an important flexor of the digits, and is one of the largest of the superficial flexor muscles of the forearm (3,4,8,13).

The FDS has two main heads, the humeroulnar head and the radial head. The humeroulnar head has several origin sites. It arises, in part, from the medial epicondyle through the common flexor origin (see Fig. 2.2A). The muscle has additional origin attachments from the anterior band of the ulnar collateral ligament, from adjacent intermuscular septa, and from the medial side of the coronoid process proximal to the ulnar origin of the pronator teres (Fig. 2.3A). Additional origin attachments may connect to the fascia of the brachialis. The radial head is a long, thin, flat muscular sheet. It arises from the oblique line of the radius, which is a long, linear, oblique attachment area from the volar radial shaft in its proximal third (see Fig. 2.3A). The origin extends distally from the anterior lateral border of the radius, just proximal to the insertion of the pronator teres. The origin of the FDS extends along the anterior diaphysis proximally and medially to reach the medial side of the radial tuberosity. The two heads form a muscular arch, through which the median nerve and ulnar artery pass. The muscular arch formed by the FDS is a well known site for potential median nerve compression, especially in forearm compartment syndromes or ischemic contracture (3,4,11,13,18,273–276). The muscular fibers extend distally, with the fiber bundles of the ulnar head and the upper part of the radial head converging. The ulnar fiber bundles extend distally in a vertical fashion. The fibers from the radial head extend distally obliquely to form a common belly. The deep surface of the FDS on the ulnar side usually is covered by a dense tendinous or fibrous sheet (3,4). The muscle belly of the FDS forms two separate submuscle bellies (273–284). These resemble planes or sheets of muscle fibers (4), referred to as strata by Williams (3). There is a deep and superficial plane of fibers. The superficial plane of fibers further divides into two parts that end in the tendons for the long and ring fingers. Similarly, the deep plane of fibers further divides into two parts, which end in the tendons for the index and small fingers (4). Of these muscles, the FDS belly to the long finger may arise more independently than the others (277). Before dividing, the deep plane gives off a muscular slip to join the portion of the superficial plane associated with the tendon of the ring finger. The arrangement of deep and superficial muscle planes is retained at the wrist level. As the four tendons continue distally in the forearm and pass deep to the flexor retinaculum, they still are arranged in pairs, the superficial and deep. The superficial pair, located superficial and the central of the four tendons, continues to the long and ring fingers. The deep pair, located deep and at the radial and ulnar margins of the four tendons, continues to the index and small fingers, respectively. (Note: This arrangement of the tendons at the distal forearm and in the carpal tunnel can be simulated on one’s own hand. If one touches the index and small fingers behind the ring and long fingers, the pattern of tendons is roughly simulated, with the ring and long tendons located superficial and central, and the index and small finger tendons located deep and to the radial

2 Muscle Anatomy

and ulnar margins, respectively.) The tendons diverge from one another in the palm and extend distally deep to the superficial palmar arterial arch and the digital branches of the median and ulnar nerves. At the level of the base of the proximal phalanges, each tendon divides into two slips. The divergence of the two slips forms an interval through which the associated tendon of the FDP passes. The two slips of the FDS then rotate 90 to 180 degrees, flattened against the profundus tendon. The slips thus encircle the profundus tendon. At the side of the profundus tendon, the spiraling, flat bands of the FDS tendon have rotated such that the fibers that were nearest to the midline in the undivided tendon become the most volar at the sides of the middle phalanx. These anterior fibers continue on the same side of the profundus tendon attached to the proximal part of the ridge on the margin of the middle phalanx. The posterior fibers sweep around the profundus tendon to reunite dorsal to the profundus. The two portions of the FDS reunite at Camper’s chiasma. In this area, the FDS slips form a grooved channel for passage of the profundus. At the level of Camper’s chiasma, the FDS slips decussate in an “X” pattern (behind the profundus, on the volar surface of the middle phalanx) and pass distally to attach to the distal part of the ridge on the opposite margins of the middle phalanx (282). Each slip of the tendon of the FDS inserts into the medial and lateral aspects of the volar shaft of the associated digit (see Fig. 2.6A). The chiasma can be variable in terms of anatomy and morphology (3,4,284). The vascular supply to the tendons comes from several sources. These include the longitudinal vessels (some of which may originate in the muscle belly) that enter in the palm and extend down intratendinous channels; vessels that enter at the level of the proximal synovial fold in the palm; segmental branches from the paired digital arteries that enter in the tendon sheaths by means of the long and short vincula; and the vessels that enter the FDS and FDP tendons at their osseous insertions (285–299). In the digital sheath, the segmental vascular supply to the flexor tendons is through long and short vincular connections. These include the vinculum brevis superficialis, the vinculum brevis profundus, the vinculum longum superficialis, and the vinculum longum profundus. The vincula often are variable in presence and configuration (299). In addition to the vascular supply, the tendons in the synovial sheath receive nutrition through synovial fluid diffusion. The vinculum longum superficialis arises at the level of the base of the proximal phalanx. Here, the digital arteries give rise to branches on either side of the tendons that interconnect anterior to the phalanx, but deep (dorsal) to the tendons. These branches form the vinculum longum superficialis that connects to the FDS at the floor of the digital sheath. The vinculum longum superficialis supplies the FDS at the level of the proximal phalanx (285–299). The vinculum brevis superficialis and the vinculum brevis profundus consist of small triangular mesenteries near the insertion of the FDS and FDP tendons, respectively.

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The vinculum brevis superficialis arises from the digital artery, at the level of the distal part of the proximal phalanx. It supplies the FDS tendon near its insertion into the middle phalanx. A portion of the vinculum brevis superficialis continues anteriorly, at the level of the PIP joint toward the FDP to form the vinculum longum profundus (285–299). Because the vincula enter the tendon on the dorsal surface, the vascularity of the dorsal half of the tendon in the digits is richer than the palmar half. Architectural features of the FDS include the physiologic cross-sectional area of the muscle, the fiber bundle length, muscle length, muscle mass, and pennation angle. Skeletal muscle architectural studies by Lieber, Friden, and colleagues provide the data for the FDS to each digit (135–139,174) (see Table 2.1 and Fig. 2.4). As can be seen in the figures, the digital extrinsic flexor and extensor muscles have similar architectural features (see Fig. 2.4A and B). The relative difference index values compare the FDS with other upper extremity muscles, based on architectural features. These values are listed in Appendix 2.3. The FDS is innervated by a branch from the median nerve (C7, C8, T1). The nerve branch usually exits the median nerve trunk proximal to the pronator teres and accompanies the median nerve trunk through the two heads of the pronator teres. The branch then divides into multiple smaller motor branches that supply the radial head of the muscle. The muscle portions that ultimately form the tendons to the index and small fingers each may receive a separate motor branch. On occasion, the motor branches may exit the median nerve more distally, in the distal third of the forearm, to supply the FDS (3,4,11,18). Actions and Biomechanics: Flexor Digitorum Superficialis The FDS functions to flex the PIP joints of the index, long, ring, and small fingers. It also contributes to flexion of the digital MCP joints and flexion of the wrist. During flexion, there is a slight adduction component as the FDS draws the digits together, as in making a fist. The tendon of the small finger has a minor rotatory (opposing) action at the carpometacarpal joint (11). The FDS has independent muscle components to each of the four digits. It therefore can flex each PIP independently (unlike the FDP, which has a common muscle group to the middle, ring, and small fingers). The ability of the FDS to flex one PIP at a time is useful in assessing tendon lacerations. Anomalies and Variations: Flexor Digitorum Superficialis Among the many described muscle variations and anomalies of the FDS (300–333), the more common involve

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muscle slips that interconnect the FDS with the other forearm flexors. These include slips to the flexor pollicis longus (FPL), the palmaris longus, or the brachioradialis (324,325). The variations seem to be more common in the index and small fingers (11,311,313,319,326,332). As much as 10% of 70 cadaver hands showed an anatomic variation of the small finger that would preclude its independent function (284). Some anomalies have been noted to occur repeatedly in families or in different generations (326), or to occur bilaterally (312,321,333). A muscle slip, the radiopalmaris, may arise directly from the radius deep to the FDS and attach to the palmar aponeurosis or to the common sheath of the flexor tendons (11,319). Several variations of the radial head of the muscle have been noted. These include complete absence of the radial head (3,4), or absence of one or more of the distal divisions (to form specific tendons) (300). The entire muscle may originate from the radius (11). The muscle belly and tendon to the small finger may be absent (301,326). A rare anomaly is a digastric FDS, consisting of an additional distal muscle belly separated from the main muscle belly by an intercalary tendon (11,302–304,329). The muscle may occur as an accessory FDS, in the presence of a normal FDS (305,306). An anomalous muscle, the palmar FDS accessories, may arise from the palmar fascia and distal border of the transverse carpal ligament and end in a tendon that joins the flexor tendon of the index finger at the level of the MCP (11,309,319). In a literature review by Elias and SchulterEllis, the muscle was shown to be more common in women than men in a 13:2 ratio, and involved the right hand in 12 of 13 cases (309). It was seen bilaterally in 4 of 13 cases. The muscle involved the index finger in all cases; however, a somewhat similar anomalous muscle involving the small finger has been reported (312). This anomaly may present as a painful palmar wrist mass (310,311,313). The muscle usually can be identified with magnetic resonance imaging (311,313). An additional variation of this anomaly includes a palmar muscle belly that originates from the FDS to the index finger by way of an accessory tendon (314). An “accessory” FDS also has been noted, causing a volar soft tissue wrist mass and ulnar neuropathy (307). The FDS may be associated with Gantzer’s muscle (330). Clinical Implications: Flexor Digitorum Superficialis The median nerve passes deep to the arch formed by the heads of the FDS. This is a potential site of nerve compression, and should be considered in compartment syndrome decompression or nerve exploration in ischemic contracture (273,274). Because the muscle bellies for each FDS tendon usually are separate, it is possible independently to flex each of the

PIP joints. By holding the other three digits in extension, the function of the remaining FDS can independently be tested by having the patient attempt to flex the digit at the PIP joint. Note that because the FDP muscle belly usually consists of one belly supplying the four tendons (instead of the four separate muscle bellies supplying the four tendons of the FDS), holding the digits in extension helps to eliminate function of the FDP. Thus, any flexion of the digit at the PIP joint is performed by the FDS, and each digit can be evaluated independently (3). Carpal tunnel syndrome can be precipitated by anomalies and variations of the FDS. The anomalous muscles bellies in the forearm can cause direct encroachment of the median nerve. In addition, an anomalous muscle belly or a belly from a normal muscle that extends abnormally distally into the carpal tunnel can contribute to carpal tunnel syndrome (303,316–320,322,327,331). Holtzhausen and colleagues have shown the prevalence of the FDS and FDP muscle bellies that extend into the carpal tunnel to be as high as 46% in women and 7.8% in men (323). Intermuscular slips that pass between the FDS and the palmaris longus can cause carpal tunnel symptoms (324). Bilateral occurrence of carpal tunnel syndrome due to an anomalous FDS has been reported (331). Ulnar neuropathy has been reported by Robinson, due to an accessory FDS that produced a palpable mass in the volar forearm as well as ulnar nerve encroachment (307). A painful mass in the palm along the tendon course to the index finger may represent an anomalous muscle, the palmar FDS accessories. This muscle may arise from the palmar fascia and distal border of the transverse carpal ligament and end in a tendon that joins the flexor tendon of the index finger at the level of the MCP (309–313,319). The mass usually can be identified as muscle by magnetic resonance imaging (311,313). A fibroma in association with an anomalous FDS tendon also has been the cause of a painful palmar mass (315). Agee and colleagues have studied the FDS, and note that the muscle to the long finger may be anatomically the most independent, arising separately. Therefore, this tendon may be the most suitable for nonsynergistic tendon transfers (277). Progressive flexion contracture of the PIP (resembling camptodactyly) of the right ring finger has been noted to occur from an anomalous origin of the FDS. Operative excision of the aberrant tendon restored normal range of motion at the PIP joint (333). In the absence of the FDS, a palmaris longus has been found to extend to the middle phalanx of the ring finger and function as a digital flexor of the PIP joint (259). Because the vincula enter the tendon on the dorsal surface, the vascularity of the dorsal half of the tendon in the digits is richer than the palmar half. This has implications for placement of sutures in the repair of lacerated tendons. Sutures placed in the palmar half of the tendon should dis-

2 Muscle Anatomy

rupt the intratendinous vascularity to a lesser degree than those in the dorsal half. The vincular system should be appreciated and protected as much as possible in the exploration or repair of the flexor tendons. FLEXOR CARPI ULNARIS Derivation and Terminology. Flexor is derived from the Latin flexus, indicating “bent” (and flexor, which indicates “that which bends,” or “bending”). Carpi is from the Latin carpalis and the Greek karpos, both of which indicate “wrist” (the carpus). Ulnaris is derived from Latin ulna, indicating “arm” (1,2). Origin. From two heads. Humeral head: from the medical epicondyle through the common flexor origin. Ulnar head: extensive origin from the medial margin of the olecranon and proximal two-thirds of the posterior border of the ulna by an aponeurosis shared with the ECU and FDP, and from the adjacent intermuscular septum. Insertion. To the pisiform; a few fibers may attach to the flexor retinaculum. Innervation. Ulnar nerve (C7, C8, T1). Vascular Supply. The ulnar artery, superior and inferior ulnar collateral arteries, anterior and posterior ulnar recurrent arteries, ulnar end of the superficial palmar arch (3,4,334,335). Principal Action. Flexion and ulnar deviation of the wrist. Gross Anatomic Description: Flexor Carpi Ulnaris The FCU is the most medial muscle of the superficial flexors of the forearm (along with, from radial to ulnar, the pronator teres, FCR, palmaris longus, and FDS) (3,4,8,11,13). It lies in the superficial volar muscle compartment of the forearm (Appendix 2.2). The FCU is located medial and superficial to the FDS. It has two heads of origin (336). A smaller humeral head originates from the distal part of the medial epicondyle through the common flexor origin (see Fig. 2.2A). There also are fascial attachments from the humeral head to the adjacent intermuscular septum and deep fascia of the forearm. The larger ulnar head has a more extensive origin, arising from the medial margin of the olecranon and proximal two-thirds of the posterior border of the ulna by a fascial sheet or aponeurosis (see Fig. 2.3B). It shares this aponeurotic origin with the ECU and FDP. The FCU also has attachments to the neighboring intermuscular septum between it and the FDS. The two heads of the FCU create a muscular arch extending from the olecranon to the medial epicondyle. The ulnar nerve and posterior ulnar recurrent artery pass through this fibromuscular arch. The muscle belly from the humeral head extends distally in a nearly longitudinal fashion. The

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muscle fibers from the larger ulnar head, however, extend distally obliquely and anteriorly. This muscle belly, which is highly pennated, may continue nearly the entire length of the muscle–tendon unit, almost to the insertion site. (This is very different from the FCR, which has a fairly abrupt myotendinous junction in the central portion of the forearm, and a long solitary tendon that extends distally without attaching muscle fibers.) The FCU has a long, thick tendon that forms along the anterolateral border of the muscle in its distal half. The tendon usually is more than 10 mm long (337). As the tendon extends distally, it usually retains muscle fibers to the distal portion of the forearm almost to the level of its insertion onto the pisiform (337). Rarely, there is a discrete tendon without accompanying muscle fibers (337). At the level of insertion, all the muscle fibers insert in a penniform manner. The pisiform is a sesamoid bone, and therefore is within a tendon (or ligament). The FCU thus inserts primarily into the pisiform (see Fig. 2.6A), but is also, to an extent, extended distally through the pisiform to the hamate through the pisohamate and pisometacarpal ligaments. In addition, a few fibers attach to the flexor retinaculum and to the palmar aponeurosis, and, possibly, to the base of the third, fourth, and fifth metacarpals (338). As the muscle inserts into the pisiform, the ulnar nerve and artery are located deep and radial to the tendon. Architectural features of the FCU include the physiologic cross-sectional area of the muscle, the fiber bundle length, muscle length, muscle mass, and pennation angle (angle of the muscle fibers from the line representing the longitudinal vector of its tendon). Skeletal muscle architectural studies by Lieber, Friden, and colleagues provide the data for the FCU (135–139,174) (see Table 2.2 and Fig. 2.4). The FCU has a relatively small fiber length and relatively large physiologic cross-sectional area. This indicates that its design is more optimal for force generation (proportional to cross-sectional area) than for excursion or velocity (proportional to fiber length). Its relative difference index values compare it with other upper extremity muscles, based on architectural features. These values are listed in Appendix 2.3. In comparing the architectural features of the FCU with the FCR, the FCR muscle length is shorter than the FCU, but the muscle fibers of the FCR are longer (136,174). The relatively longer fiber length indicates that the FCR is designed more for excursion and velocity of contraction (because excursion and velocity are proportional to fiber length) compared with the FCU. The FCU, in contrast, has a higher pennation angle, with a larger physiologic cross-sectional area. This indicates the FCU is designed more for force production and less for excursion and velocity, compared with the FCR (because cross-sectional area is proportional to force production) (174,175) (see Table 2.2 and Fig. 2.4). The FCU is innervated by the ulnar nerve (C7, C8, T1). The muscle usually receives two to three muscular branches

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in its proximal portion, although there may be up to six separate branches (11,339). These branches usually leave the ulnar nerve near the level of the elbow joint or in the distal portion of the cubital tunnel. Rarely, a branch can exit the ulnar nerve proximal to the elbow (339). The motor branches often are visualized during cubital tunnel decompression or ulnar nerve transposition. Each head of the FCU receives a separate motor branch (336). There occasionally is a single branch that leaves the ulnar nerve trunk, enters the proximal FCU on the deep surface, and then branches in the muscle to send long, slender motor branches through the muscle to reach the middle third (3,4,11,13,340). Actions and Biomechanics: Flexor Carpi Ulnaris The FCU functions primarily to flex the wrist, and usually works with the FCR. It also ulnarly deviates the wrist, especially working with the ECU. The FCU takes an important role in stabilizing the wrist during strong power grip, as in the tight grasp of a hammer. The wrist usually is held in slight ulnar deviation during these functions because the wrist is stabilized, in large part, by the FCU. The FCU also helps stabilize the pisiform, and thus can assist the abductor digiti minimi, which has its origin on the pisiform. The FCU therefore can assist indirectly with abduction of the small digit. From its insertion on the medial epicondyle, and from its course that positions the muscle directly over the medial collateral ligament, it has been postulated that the FCU (along with the FDS) functions to support or stabilize the medial elbow joint (170). As stated earlier, the FCU is architecturally designed more for force generation than for excursion or velocity compared with its radial counter part, the FCR. This is due to the FCU having a larger physiologic cross-sectional area (proportional to force generation), being highly pennated (which helps increase the physiologic cross-sectional area), and having a shorter fiber length (which is proportional to excursion or velocity) (135–138,174,341,342). Anomalies and Variations: Flexor Carpi Ulnaris The muscle and tendon arrangement of the FCU occurs in three general types. The most common is a large muscle belly that runs distally almost to the insertion on the pisiform. The next most common is a muscle belly that ends more proximally, with some large muscle fibers that run parallel to the tendon and almost reach the pisiform. Rarely, the musculotendinous junction ends more proximally, with only single muscle fibers that continue distally. These different muscle–tendon patterns should be kept in mind when interpreting magnetic resonance images, during gen-

eral operative exploration for penetrating trauma, or when performing muscle–tendon lengthening procedures (135–138,174,341–344). Among the most common variations of the FCU is an accessory tendon or muscle slip that extends from the coronoid process and joins the muscle belly in the proximal third of the muscle (3,11). An accessory muscle may extend the entire length of the FCU and resemble a duplicated muscle (345). Distally, there are several possible variations of the insertion of the tendon. It may send tendinous slips to the flexor retinaculum. It may have extensions to the metacarpals of the small, ring, or long fingers, or to the capsules of the carpometacarpal joints (3,4,11). A distal slip inserting into the proximal phalanx of the ring finger has been described (346). A distal anomalous muscle belly and a “reversed” muscle belly located predominantly distally have been noted and associated with ulnar nerve compression, either in the forearm or in the ulnar tunnel (346–350). These anomalous muscles entering the ulnar tunnel also have been associated with ulnar artery thrombosis (351). The insertional tendon may extend to the proximal portion of the abductor digiti minimi. The epitrochleoanconeus [epitrochleoolecranonis or anconeus sextus of Gruber (11)] is a small anomalous muscle closely associated with the FCU. It originates from the posterior surface of the medial epicondyle of the humerus and inserts into the olecranon process. It is superficial to the ulnar nerve (from which it is innervated), and takes the place of the fibrous arch of the deep fascia usually found in the same location. The muscle has a frequency of approximately 25% in cadaver dissections (11). The muscle restricts mobility of the ulnar nerve in the forearm, thereby contributing to the development of neuropathy (especially with trauma as a precipitating factor) (347). A split FCU tendon has been noted, with the ulnar nerve passing between the split. Ulnar nerve compression symptoms were produced with wrist hyperextension (352–354). Clinical Implications: Flexor Carpi Ulnaris Because the FCU is designed most optimally for force generation and less for excursion or velocity, it may be a less optimal tendon transfer for use in radial nerve palsy. The FCR, which is designed more for excursion, may be a more appropriate transfer to achieve digital extension. (In radial nerve transfers, great tendon motor power strength usually is not as important as excursion because the antigravity function of the transfer usually is sufficient to achieve good functional results) (135–138,174,341–343). The ulnar nerve and artery lie deep and radial to the FCU tendon in the distal forearm (the artery is radial to the nerve). This is a reasonable site for ulnar nerve local anesthetic block, by infiltration of the nerve deep to the palpable FCU tendon. For complete block of the ulnar portion

2 Muscle Anatomy

of the hand, the dorsal branch of the ulnar nerve, which leaves the ulnar nerve trunk proximal to the wrist, should be blocked as well. The dorsal branch of the ulnar nerve can be blocked by a wheal of subcutaneous local anesthetic injected circumferentially along the ulnar and dorsal borders of the wrist in the area just distal to the ulnar head. As noted previously, variations and anomalies of the FCU, either in the forearm with accessory slips, fibrous bands, or muscles, or distally with extended muscle bellies or anomalous bellies extending into the ulnar tunnel, can result in ulnar neuropathy (346–350,355,356). In addition, a split FCU tendon pierced by the ulnar nerve or one of its branches can lead to neuropathy (352–354). An anomalous muscle extending into the ulnar tunnel also has been associated with ulnar artery thrombosis (351). Sarcomere length of a muscle can be measured using intraoperative laser diffraction techniques. With these techniques, it is possible to show and measure the change of sarcomere length after muscle transfer. When the FCU was transferred to the EDC (to restore digital extension), the absolute sarcomere length and sarcomere length operating range of the FCU increased. It also was shown that despite good clinical results, a more desirable result could be obtained if the FCU sarcomere length was increased (by approximately 5 µm) by further stretching of the muscle during the transfer. The authors were able to quantify the relationship between the passive tension chosen for transfer, sarcomere length, and the estimated active tension that could be generated by the muscle. These findings demonstrate the feasibility of using intraoperative laser diffraction techniques during tendon transfer as a guide for setting tension and the optimal placement and sarcomere length of the transferred muscle (341–343). FLEXOR DIGITORUM PROFUNDUS Derivation and Terminology. Flexor is derived from the Latin flexus, indicating “bent” (and flexor, which indicates “that which bends,” or “bending”). Digitorum is from the Latin digitus or digitorum, indicating the digits. Profundus is from the Latin profundus, indicating “deep,” and refers to the muscle’s location deep in the forearm (1,2). Origin. From the median and anterior surface of the ulna, interosseous membrane, and deep fascia of the forearm. Insertion. To the base of the distal phalanges. Innervation. Anterior interosseous nerve (from the median nerve) to the index and long finger; ulnar nerve to the ring and small fingers. Vascular Supply. Posterior ulnar recurrent artery, posterior and anterior interosseous arteries, palmar carpal arch, palmar metacarpal arteries, common and proper digital palmar arteries. In addition, the lateral portion is supplied by the ulnar collaterals and the deep palmar arch,

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and the medial part is supplied by the ulnar artery (3,4,11,13,68). Principal Action. Flexion of the distal and interphalangeal joints and flexion of the MCP joints. The FDP also contributes to wrist flexion and functions as the origin for the lumbrical muscles (3,4,11). Gross Anatomic Description: Flexor Digitorum Profundus The FDP, with the FPL, is one of the deep flexors of the forearm and lies in the deep volar muscle compartment of the forearm (Appendix 2.2). The muscle is situated deep in the forearm, lying against the ulnar portion of the interosseous membrane. The FDP is covered anteriorly by the FCU and the FDS. The median nerve courses between the deep flexor muscle group and the superficial flexor muscle group of the forearm. It is a strong, broad, somewhat flat muscle. The FDP arises deep to the superficial flexors from an extensive origin (see Fig. 2.3). The origin includes attachments to the proximal two-thirds of the anterior and medial surface of the ulna. There also are attachments of origin to a depression on the medial side of the coronoid process. Some of its origin extends medially and posteriorly around the ulna to reach the posterior surface of the ulna, and there are connections through an aponeurosis shared with the flexor carpi ulnaris and ECU. In addition, the FDP has attachments from the ulnar half of the anterior surface of the interosseous membrane. There also may be an inconsistent origin from a small area on the radius distal to the bicipital tuberosity. The extensive origin then forms what resembles a single large muscle belly, although the belly to the index finger usually is separate and may be discernible. The muscle then divides into four parts that are more distinct. The myotendinous junction usually is in the central third of the forearm. At the junction, the muscle attaches to the dorsal surface of the tendon, so that more of the tendon is visible on the volar aspect. The myotendinous junction gives rise to four separate tendons usually aligned parallel to each other, from radial to ulnar, to extend distally to the index, long, ring, and small fingers, respectively. This is in contrast to the FDS tendons, which, at the level of the wrist, have a “stacked” pattern, with the FDS tendons to the long and ring fingers located palmar and central to the FDS tendons of the index and small fingers, which are located dorsal and radial (for the index) or dorsal and ulnar (for the small finger) (3,4,11) (see earlier, under Gross Anatomic Description: Flexor Digitorum Superficialis). The muscle belly to the long, ring, and small fingers remain interconnected to some extent from the forearm to the palm through areolar tissue and tendinous slips. The muscle and tendon to the index finger usually remain separate and distinct throughout their course from the muscle belly to the palm. In some, the FDP tendon to the small finger may be more

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independent, and resemble that of the index finger. The tendons to the long and ring finger are the least independent and more often are connected by areolar tissue. The tendons then extend distally, deep to the tendons of the FDS, to cross through the carpal tunnel. At the distal extent of the carpal tunnel, the tendons diverge to cross the palm in the direction of each digit. Just proximal to the MCP joints, the FDP tendons enter the A1 pulley of the fibroosseous tunnel. In the digits, at the level of the proximal phalanx, the FDS tendons split and the associated FDP tendon passes through the split. Each tendon continues distally to insert on the base of each of the distal phalanges (3,4,11,357) (see Fig. 2.6A). At the level of the distal margin of the carpal tunnel, the lumbricals arise from the radial aspect of each FDP tendon. As discussed earlier (see under Gross Anatomic Description: Flexor Digitorum Superficialis), the vascular supply to the FDP and FDS tendons comes from several sources. These include the longitudinal vessels (some of which may originate in the muscle belly) that enter in the palm and extend down intratendinous channels. There also are vessels that enter at the level of the proximal synovial fold in the palm to supply the tendons. In addition, there is the vincular supply, supplied by segmental branches from the paired digital arteries that enter into the tendon sheaths. The most distal vascular supply to the flexor tendons includes vessels that enter the FDS and FDP tendons at their osseous insertions (285–299). The vinculum longum superficialis arises at the level of the base of the proximal phalanx. Here, the digital arteries give rise to branches on either side of the tendons that interconnect anterior to the phalanx, but deep (dorsal) to the tendons. These branches form the vinculum longum superficialis that connects to the superficialis at the floor of the digital sheath. The vinculum longum superficialis supplies the FDS, at the level of the proximal phalanx. In the digital sheath, the segmental vascular supply to the flexor tendons is by means of long and short vincular connections. These include the vinculum brevis superficialis, the vinculum brevis profundus, the vinculum longum superficialis, and the vinculum longum profundus. The vincula often are variable in presence and configuration (285–299). In addition to vascular sources, the tendons in the synovial sheath receive nutrition through synovial fluid diffusion. The vinculum brevis superficialis and the vinculum brevis profundus consist of small, triangular mesenteries near the insertion of the FDS and FDP tendons, respectively. The vinculum brevis superficialis arises from the digital artery, at the level of the distal part of the proximal phalanx. It supplies the FDS tendon near its insertion into the middle phalanx. A portion of the vinculum brevis superficialis continues anteriorly, at the level of the PIP joint, toward the FDP to form the vinculum longum profundus (299).

Because the vincula enter the tendon on the dorsal surface, the vascularity of the dorsal half of the tendon in the digits is richer than that of the palmar half. Architectural features of the FDP include the physiologic cross-sectional area of the muscle, the fiber bundle length, muscle length, muscle mass, and pennation angle. Skeletal muscle architectural studies by Lieber, Friden, and colleagues provide the data for the FDP to each digit (135–139,174) (see Table 2.1 and Fig. 2.4). The digital extrinsic flexor and extensor muscles have similar architectural features. The relative difference index values compare the FDP with other upper extremity muscles, based on architectural features. These values are listed in Appendix 2.3. The FDP is innervated by both the median nerve (through the anterior interosseous nerve to supply the belly of the index and long fingers) and by the ulnar nerve (to supply the bellies of the ring and small fingers). The anterior interosseous nerve usually exits the median nerve trunk proximal to the nerve trunk entering the interval between the heads of the pronator teres. The anterior interosseous nerve branch usually accompanies the main median nerve trunk through the interval between the humeral and ulnar heads of the pronator teres, then through the interval created by the fibromuscular arch of the origins of the FDS. The anterior interosseous nerve then divides into several motor branches to supply the muscle portions of the FDP to the index and long fingers. The nerve branches enter the muscle bellies on the radial border in the middle third of the muscle. A branch of the anterior interosseous nerve continues distally along the anterior surface of the interosseous ligament to reach and enter the proximal border of the pronator quadratus. The ulnar nerve innervation of the FDP is from a motor branch that arises approximately the level of the elbow joint. The nerve branch enters the anterior surface of the muscle in the region of the junction of the proximal and middle thirds. This branch supplies the part of the muscle that provides tendons to the ring and small fingers. Considerable variation exists as to the innervation of the muscle bellies of the FDP. In only approximately 50% of extremities do the median nerve and ulnar nerve specifically innervate the index and long, and the ring and small finger muscle bellies, respectively (3,4,11,358,359). Actions and Biomechanics: Flexor Digitorum Profundus The FDP functions mainly to flex the digits. Through its insertion onto the distal phalanx, it exerts powerful flexion on the distal phalanx at the distal interphalangeal (DIP) joint. However, by passing across the PIP and MCP joints, the FDP tendons assist the FDS to flex the PIP joints, and the FDP assists both the FDS and the interossei and lumbricals to flex the MCP joints. The FDP also assists with flexion of the wrist. The FDP provides the origins for the

2 Muscle Anatomy

lumbricals muscles. When the FDP contracts and moves proximally, there is a dynamic action on the lumbricals. Anomalies and Variations: Flexor Digitorum Profundus There commonly are accessory muscles or tendinous slips from the FDP to the radius, to the FDS, FPL, the medial epicondyle, or to the coronoid process (3,4,11,360–365). Flexor indicis profundus or flexor digitorum profundus indicis. There may be more than four muscle bellies of the FDP, and the separation between the tendons can occur to varying degrees. The separation to the index finger usually is the greatest, but also is variable. If the FDP to the index exists as a separate muscle and tendon, it has been referred to as the flexor indicis profundus or flexor digitorum profundus indicis (11). An anomalous accessory FDP tendon may exist as a separate muscle–tendon unit lying ulnar to the main flexor digitorum profundus indicis. It has been noted then to join the main tendon at the level of the distal palmar crease (363). Other rare described anomalies of the FDP include an anomalous muscle in association with a fibroma of a tendon sheath causing triggering of the wrist (364), and a rare congenital abnormality of the FDP causing a flexion deformity of the long and ring fingers (365). Clinical Implications: Flexor Digitorum Profundus Flexor tendon rupture can occur in the carpal tunnel from several causes, including chronic abrasion against a hook of the hamate fracture or nonunion, attrition against the radial side of the pisiform affected by osteoarthritis of the pisotriquetral joint (366), and in the patient with rheumatoid arthritis. Avulsion of the FDP most commonly involves the ring finger. This is due to its relatively greater length during grasp. During grip, the ring fingertip becomes 5 mm more prominent than any other digit in 90% of subjects, and it absorbs more force than any other finger during pull-away testing (367). The anterior interosseous nerve syndrome involves paresis or palsy of the FDP to the index and long (and occasionally the ring) fingers, as well as paresis of the FPL and pronator quadratus. The syndrome often is associated with trauma, tight-fitting casts, neuritis, or anatomic structures that impinge on the anterior interosseous nerve, including fascial bands, adhesions, and muscle impingement (i.e., fibrous bands of the pronator teres) (368–376). In 1979, Linburg and Comstock described an anomalous tendon slip from the FPL to the FDP to the index finger (360). It appears that the anomaly is present in at least one extremity of 25% to 31% of individuals, and in both

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extremities in 6% to 14%. If present it can be demonstrated when a patient attempts to independently flex the interphalangeal joint of the thumb, and there is coexisting flexion at the DIP joint of the index finger (360,361), called Linburg’s sign. This anomaly may be associated with chronic tenosynovitis or carpal tunnel symptoms. Because the vincula enter the tendon on the dorsal surface, the vascularity of the dorsal half of the tendon in the digits is richer than in the palmar half. This has implications for placement of sutures in the repair of lacerated tendons. Sutures placed in the palmar half of the tendon should disrupt the intratendinous vascularity to a lesser degree than those in the dorsal half. The vincular system should be appreciated and protected as much as possible in the exploration or repair of the flexor tendons. Tendon excursion of the FDP relative to the tendon sheath has been shown to be greatest in zone II during PIP joint rotation. This suggests that PIP joint motion may be most effective in reducing adhesions after tendon repair in zone II (377). After laceration of the FDP distal to the superficialis insertion, tendon advancement of the proximal cut end of the tendon to the insertion has been used as a means of repair. Anatomic studies suggest that 1 cm is approximately the maximum amount that the tendon can be safely advanced, without causing problematic shortening (378). FLEXOR POLLICIS LONGUS Derivation and Terminology. Flexor is derived from the Latin flexus, indicating “bent” (and flexor, which indicates “that which bends,” or “bending”). Pollicis is from the Latin pollex, indicating “thumb.” Longus is the Latin for “long.” It is the longest flexor of the thumb (1,2). Origin. From the anterior surface of the middle third of the radius, the anterior interosseous ligament. Insertion. To the base of the distal phalanx of the thumb. Innervation. Median nerve through anterior interosseous branch (C6, C7, C8) (3,4). Vascular Supply. From the radial artery through direct muscular branches, anterior interosseous artery, princeps pollicis artery, and palmar carpal arch. The tendon receives vascularity, in part, through a vincular system, originating from the digital arteries (3,4,11,379–384). Principal Action. Flexion of the thumb interphalangeal joint and MCP. Gross Anatomic Description: Flexor Pollicis Longus The FPL, with the FDP, is one of the deep flexors of the forearm and lies in the deep volar muscle compartment of the forearm (Appendix 2.2). The FPL is located radial to the FDP, roughly in the same deep plane. Like the FDP, it

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is a relatively large and flat muscle. It has a large area of origin, arising from an obliquely oriented groove on the anterior surface of the radius that extends from just below the tuberosity to the proximal attachment of the pronator quadratus (see Fig. 2.3A). The origin often extends as far proximal as to within approximately 5 cm of the wrist joint. The muscle belly thus attaches and covers the middle third of the anterior surface of the radial diaphysis. It also has attachments from the adjacent interosseous ligament, and there often is an attachment by a variable slip from either the lateral or medial border of the coronoid process. There also can be attachments from the medial epicondyle of the humerus (385). The muscle fibers extend distally and obliquely to attach in a penniform manner on the tendon at the myotendinous junction. The muscle has a relatively long and variable myotendinous junction. At this junction, there usually is more tendon that extends along the ulnar border of the muscle, on its anterior surface. The muscle blends with its broad, flat tendon, usually in the distal third of the forearm. The tendon extends distally, usually in the plane of the tendons of the FDP. The adjacent anterior interosseous nerve also continues distally, between the FPL and the FDP. The FPL then enters the carpal tunnel. Some muscle fibers may accompany the tendon to the level of the proximal edge of the flexor retinaculum. As the tendon passes through the carpal tunnel, it is located radial to the tendons of the FDP and median nerve. It passes deep to the superficial head of the flexor pollicis brevis (FPB). After passing through the carpal canal, the tendon emerges deep to the superficial palmar arch, between the opponens pollicis and the oblique head of the adductor pollicis. It continues between the thumb sesamoid bones, entering its own synovial sheath. The tendon enters the fibroosseous tunnel of the thumb through the A1 pulley at the level of the MCP joint (386). The tendon continues distally to insert onto the palmar surface of the base of the distal phalanx of the thumb (see Fig. 2.6A). Architectural features of the FPL include the physiologic cross-sectional area of the muscle, the fiber bundle length, muscle length, muscle mass, and pennation angle (angle of the muscle fibers from the line representing the longitudinal vector of its tendon). Skeletal muscle architectural studies by Lieber, Friden, and colleagues provide the data for the FPL (135–139,174) (see Table 2.1 and Fig. 2.4). The digital extrinsic flexor and extensor muscles have similar architectural features. The relative difference index values compare the FPL with other upper extremity muscles, based on architectural features. These values are listed in Appendix 2.3 (15). The FPL is innervated by the anterior interosseous nerve from the median nerve (C6, C7, C8) (387–395). There usually are at least two motor branches that enter the proximal half of the muscle at its ulnar aspect.

Actions and Biomechanics: Flexor Pollicis Longus The FPL is the only muscle that flexes the thumb interphalangeal joint (396,397). It assists the thenar muscles with flexion of the thumb at the MCP joint. In addition, the FPL assists with flexion and adduction at the carpometacarpal joint. If a load is applied to the FPL, the moment arm of the tendon in the carpal tunnel can change as the tendon shifts its position in the carpal tunnel (398). Anomalies and Variations: Flexor Pollicis Longus Several anomalies of the FPL have been described (399–427). The FPL can have interconnections of tendon slips or muscle extensions with the FDS, the FDP, or the pronator teres (360,361,399–401). The FPL actually may coalesce and blend with the muscle belly of the FDP, FDS, or pronator teres (402). The origin may extend proximally to the medial epicondyle of the humerus. This anomalous belly is the epitrochlear bundle of the FPL (11). The best documented accessory head of the FPL is Gantzer’s muscle. It has been noted in up to 52% to 66% of limbs and is supplied by the anterior interosseous nerve (403,404). It usually arises from either the medial humeral epicondyle (in 85%) or from a dual origin from the epicondyle and coronoid process (15%). The muscle usually inserts into the ulnar aspect of the FPL and its tendon. Gantzer’s muscle usually is posterior to the median nerve and either anterior or posterior to the anterior interosseous nerves. Anatomic variations of Gantzer’s muscle have contributed to median nerve compression in the forearm (403–407). Most commonly, there can be a tendon slip that connects the tendons of the FPL to the FDP. Attempts at independent flexion of the thumb interphalangeal produce concurrent flexion of the distal phalanx of the index finger. This is referred to as Linburg’s sign, or the Linburg syndrome (360,361) and may be associated with tendonitis or carpal tunnel syndrome. The original portion of the FPL that arises from the interosseous ligament may be absent. The entire FPL may be absent (11,408–417). Congenital absence of the FPL often is associated with a hypoplastic thumb (408,416), and has been noted bilaterally (413). The FPL may exist as a double tendon or malpositioned tendon, or may have an accessory tendon accompanying the normal tendon (420,421). This has been associated with triggering of the thumb (422). Various anomalous insertions of the FPL have been noted and usually result in poor flexor power of the distal phalanx (423–425). Of clinical significance, the FPL may insert onto the proximal as well as the distal phalanx of the

2 Muscle Anatomy

thumb. This may appear to be congenital absence of the FPL because of the lack of flexion on the distal phalanx (423). This insertion can be bilateral. The FPL also may insert into the soft tissue of the carpal tunnel, with the muscle power diverted to flex the wrist. Inadequate flexion power of the thumb will then be present (424). The FPL may be conjoined to the extensor pollicis longus (EPL) (425–427). Clinical Implications: Flexor Pollicis Longus Neuropathy of the anterior interosseous nerve (anterior interosseous nerve syndrome) results in paresis or palsy of the FPL and the FDP to the index and long (and occasionally the ring) fingers, as well as paresis of the pronator quadratus. The syndrome may be caused by trauma, tight-fitting casts, neuritis, or anatomic structures that impinge on the anterior interosseous nerve, including fascial bands, adhesions, or normal or anomalous muscle impingement (i.e., fibrous bands of the pronator teres, Gantzer’s muscle) (368–377). The anomalous tendon slip from the FPL to the FDP to the index finger appears to be present in at least one extremity of 25% to 31% of individuals, and in both extremities in 6% to 14% (360). It can be demonstrated when a patient attempts independently to flex the interphalangeal joint of the thumb, and there is coexisting flexion at the DIP joint of the index finger (360,361), (Linburg’s sign). This anomaly may be associated with chronic tenosynovitis or carpal tunnel symptoms. PRONATOR QUADRATUS Derivation and Terminology. Pronator is derived from the Latin pronus, meaning “inclined forward” (the Latin pronatio refers to the act of assuming the prone position or the state of being prone). Quadratus is a Latin term indicating “squared” or “four sided” (based on the muscle’s shape) (1,2). Origin. There are two heads. The superficial head and deep head originate from the anterior distal ulnar diaphysis. Insertion. The superficial head inserts onto the anterior distal radial diaphysis and anterior metaphysis. The deep head inserts proximal to the ulnar notch of the distal radius. Innervation. Anterior interosseous nerve of the median nerve. Vascular Supply. The radial artery, anterior interosseous artery, anterior descending branch, recurrent branches of the palmar carpal arch (3,4,11,13). Principal Action. Pronation of the forearm. It usually works with the pronator teres. The pronator quadratus may be the principal pronator of the forearm; the pronator teres

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appears to function more during rapid or forceful pronation. Gross Anatomic Description: Pronator Quadratus The pronator quadratus is a flat, quadrangular muscle that covers the distal 25% of the palmar surface of the radius and ulna. In textbooks, it usually is grouped with or discussed under the section on deep flexors of the forearm. The muscle more accurately belongs in its own section. It is now considered to occupy a separate compartment of the forearms, and should be addressed as such with compartment syndromes (273,274,428–432) (Appendix 2.2). The origin of the pronator quadratus is along a relatively narrow, oblique ridge on the anterior surface of the distal ulnar diaphysis (see Fig. 2.3A). Some muscle fibers also originate from the medial surface of the distal ulna and from a thick aponeurosis that attaches to the medial third of the muscle. The muscle fibers pass from medial to lateral, and slightly distally, to reach the radius. The muscle fibers are roughly transverse to the axis of the forearm. The muscle inserts onto the palmar 20% of the distal radius, covering a portion of the distal diaphysis and a portion of the metaphysis (see Fig. 2.3A). The deep (dorsal) fibers insert into a triangular area proximal to the ulnar notch of the radius. Both heads also have fibers that insert into the capsule of the distal radioulnar joint (433). The pronator teres appears to have two distinct heads: a superficial oblique head and a deep head. The superficial head originates from the ulna and passes transversely to an insertion into the radius. It averages 5.1 cm in length, 4.5 cm in width, and 0.2 cm in thickness, and has a mean crosssectional area of 0.95 cm2. The superficial head has a contractile volume of 2.6 cm3. The superficial head entirely covers the deep head, whose muscle fibers are oblique from their ulnar origin to the distal volar surface of the radius. The deep head runs obliquely from a more proximal origin on the ulna to a distal insertion on the radius. It has an average length of 4.0 cm, average width of 3.2 cm, and a thickness of 0.4 cm. Its mean cross-sectional area is 1.64 cm2 and its contractile volume is 2.5 cm3 (434). A group of fibers occasionally has been noted deep to both heads, running at right angles to them and paralleling the direction of the fibers of the interosseous membrane (434). The fibers of both heads are somewhat oblique to the axis of rotation. From this orientation, both heads, by contracting, develop a rotatory and a stabilizing force. The superficial head is thought to provide the major force for rotation in supination and pronation. The deep head functions more to provide maintenance of transverse forces at the distal radioulnar joint. The deep head coapts the joint surfaces and stabilizes the joint (431,433,434).

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The pronator quadratus, located in the distal palmar forearm, has been shown to occupy a functionally separate fascial compartment (428–430,432). The muscle is enclosed anteriorly by a well defined fascial sheath that measures 0.4 to 0.5 mm in thickness. This sheath, along with the relatively rigid posterior boundaries of the interosseous ligament and distal radius and ulna, forms a distinct fascial space. Experimentally injected dye into this compartment does not communicate with the other forearm compartments (430,432). Clinical correlations of compartment syndrome involving the pronator quadratus support the concept of the muscle occupying its own compartment (428–430,434). The architectural features of the pronator quadratus, including the fiber length and physiologic cross-sectional area, are listed in Table 2.1 and depicted in Fig. 2.4. The pronator quadratus is innervated by the anterior interosseous nerve and receives its blood supply from the anterior interosseous artery. The anterior interosseous nerve extends distally along the anterior surface of the interosseous ligament, passes dorsal (deep) to the middle of the proximal margin of the muscle, and gives off several branches to the muscle in its substance. The nerve fibers are derived from C8 (mostly) and C7 (3,4,11,13). Actions and Biomechanics: Pronator Quadratus The pronator quadratus appears to be the principal pronator of the forearm. It usually works with the pronator teres. The pronator teres appears to function more during rapid or forceful pronation. The deeper fibers of the pronator stabilize the distal ulna and radius by preventing or opposing separation of their distal ends, especially during loading of the carpus (3,4,434). Anomalies and Variations: Pronator Quadratus The deep and superficial heads may exist as separate muscle bellies (completely separated) (11). The pronator quadratus may be absent (11). An anomalous head may extend proximally, either to the radial shaft, pronator quadratus, or to the FCR brevis (11). An anomalous head may extend distal to the carpus, either to the radiocarpal or ulnocarpal capsule, to the base of the thenar muscles, or to the adductor pollicis (11). Clinical Implications: Pronator Quadratus The pronator quadratus, although situated in the volar forearm, is considered to occupy a separate compartment. Anatomic dye injection studies by Sotereanos and colleagues have demonstrated a distinct fascial space without

communications to the deep or superficial volar compartment of the forearm (273,274,430–433). Decompression of the volar compartment of the forearm without specifically addressing the pronator quadratus may not consistently decompress the muscle (429,430). The pronator quadratus is a potential pedicle flap, either with or without a portion of attached, vascularized bone; it also can serve as a free muscle flap (435–442). From the standpoint of the use of the pronator as a muscle–bone flap, the vascular anatomy has been studied in detail (442). The anterior interosseous artery divides into a muscular branch and a dorsal branch 1 to 3.5 cm from the proximal margin of the pronator quadratus. There is a rich periosteal plexus to which the anterior interosseous artery also contributes. Both the anterior interosseous artery and the dorsal branch can perfuse the muscle and the portion of radial cortex used for the transfer. The dorsal branch, which provides good perfusion of the distal radius, allows the pedicle muscle flap to be mobilized a farther distance if the dorsal branch is left intact (432). A muscle–bone pedicle graft with a portion of the anteromedial cortex of the distal radius that is mobilized with an intact anterior interosseous artery can be mobilized less than 2 cm. After ligating and dividing the anterior interosseous artery, blood supply to the distal radius bone flap relies on flow through the dorsal branch, and a bone flap can then be mobilized distally up to 4 to 6 cm (442). The pronator quadratus has been used successfully to receive a relocated sensory nerve of the palm after resection of a painful end-neuroma (443). To test pronation strength of the pronator quadratus, the elbow can be flexed past 90 degrees. Pronation strength is then tested. This flexed elbow position helps isolate the pronator strength of the pronator quadratus by eliminating the contribution of the pronator teres (which is lax when the elbow is passively flexed). After stroke or brain injury, the forearm often is held in spastic pronation by both the pronator teres and the pronator quadratus. For correction, operative recession of the pronator quadratus (along with the pronator teres) can be performed by releasing the muscle off the insertion on the distal anterior radius. This usually is performed in combination with digital and wrist flexor lengthening. EXTENSOR CARPI RADIALIS LONGUS Derivation and Terminology. The ECRL derives its name from several sources. Extensor is from the Greek and Latin ex, which indicates out of, and the Latin tendere, “to stretch,” thus extension indicates a motion to stretch out, and extensor usually is applied to a force or muscle that is involved in the “stretching out or straightening out” of a joint. Carpi is derived from the Latin carpalis or the Greek

2 Muscle Anatomy

karpos, both of which indicate “wrist” (the carpus). Radialis is from the Latin radii, which means “spoke” (used to describe the radius of the forearm). Longus is the Latin for “long.” Therefore, extensor carpi radialis longus indicates a long radial wrist extensor (1,2). Origin. From the lateral epicondylar ridge, just proximal to the lateral epicondyle. Additional areas of origin include the lateral intermuscular septum, and the anterior fascia of the muscles that arise from the common extensor origin at the lateral epicondyle. Insertion. To the dorsal base of the index metacarpal. Innervation. Radial nerve (C6, C7). Vascular Supply. The radial recurrent artery, interosseous recurrent artery, posterior interosseous artery, and radial collateral continuation of the profunda brachii artery (3,4,11,13). Principal Action. Extension and radial deviation of the wrist. Assistance with weak flexion of the elbow. The ECRL also helps stabilize the wrist (with cocontractions of the wrist flexors) during powerful grasp functions. Gross Anatomic Description: Extensor Carpi Radialis Longus The ECRL arises from the lateral epicondylar ridge, just proximal to the lateral epicondyle (see Fig. 2.2A). It comprises part of the mobile wad muscle compartment, along with the ECRB and the brachioradialis (Appendix 2.2) (12). Its origin includes the distal third of the lateral supracondylar ridge of the humerus, and the muscle is partly overlapped by the brachioradialis. The ECRL also has attachments of origin that include the common extensor origin of the lateral epicondyle, the lateral intermuscular septum, and the anterior fascia of the ECRB and EDC (both of which arise from the common extensor origin at the lateral epicondyle). The superficial surface of the muscle at first faces radially in the proximal portion near its origin. The muscle then twists slightly so that the superficial surface faces dorsally. The muscle belly extends approximately one-third to one-half the way down the forearm to reach the myotendinous junction, usually noted at the junction of the proximal third and distal twothirds. In this area, the tendinous portion first appears on the lateral and deep surface of the muscle. It then forms a stout, flat, thick tendon that usually is devoid of muscle tissue the entire length. The tendon of the ECRL travels along the lateral surface of the radius, located radial and adjacent to the ECRB. The ECRL and ECRB pass deep to the APL and EPB in the distal third of the forearm to reach its own tunnel as a part of the extensor retinaculum. The tendon lies in a groove on the dorsal surface of the radius just proximal to the styloid process. The ECRL, along with the ECRB, forms the second dorsal compartment. [Editor’s note: The dorsal compartments of the wrist

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are as follows: the APL and EPB comprise the first dorsal compartment; the ECRL and ECRB form the second; the EPL forms the third; the EDC and extensor indicis proprius (EIP) form the fourth; the extensor digiti minimi (EDM, also called extensor digiti quinti [EDQ]) forms the fifth; and the ECU forms the sixth (6).] The tendon of the ECRL continues distally deep to the tendon of the EPL as the tendons exit the extensor retinaculum. The tendon of the ECRL then inserts onto the base of the dorsal surface of the index metacarpal (see Fig. 2.6B). The tendon is not centralized on the metacarpal, but rather attaches off center on the radial aspect of the dorsal surface of the metacarpal base. The insertion may have slips that extend to the metacarpals of the thumb, index, or long fingers, as well as possible slips to the intermetacarpal ligaments (3,4,11,13). Architectural features of the ECRL include the physiologic cross-sectional area of the muscle and the fiber bundle length. Skeletal muscle architectural studies by Lieber and colleagues provide the data for the ECRL (135–139,174) (see Table 2.2 and Fig. 2.4). The relative difference index values compare the ECRL with other upper extremity muscles, based on architectural features. These values are listed in Appendix 2.3 (15). The ECRL is innervated by the radial nerve. The branch leaves the radial nerve trunk proximal to the elbow joint. There may be two nerve branches to the muscle. The motor branches enter the muscle on the deep surface of the proximal third of the muscle belly. The nerve fibers are derived from C6 (mostly) and C7. Actions and Biomechanics: Extensor Carpi Radialis Longus The ECRL functions mainly to provide extension of the wrist. It works in conjunction with the ECRB and ECU. The ECRL, by its insertion onto the radial aspect of the hand, also provides radial deviation of the wrist. In addition, the ECRL gives assistance with weak flexion of the elbow because the muscle’s origin is proximal to the elbow. The ECRL (along with the ECRB and ECU) also helps stabilize the wrist (with cocontractions of the wrist flexors) during powerful grasp functions or heavy lifting (3,4,11, 13). Anomalies and Variations: Extensor Carpi Radialis Longus The ECRL may coalesce with the ECRB, or have several variations where muscle fibers are interconnected between the two muscles. Muscle interconnections also may exist between the APL or to the interosseous muscles (11,444). The ECRL may have a split tendon or multiple tendons that insert into the index metacarpal. There may be an anom-

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alous insertion into the long finger metacarpal, or even to the ring finger metacarpal or to the adjacent carpal bones. The extensor carpi radialis intermedius is an anomalous muscle situated between the ECRL and ECRB (Fig. 2.8). It is a rare muscle that may arise independently from either the lateral epicondyle of the humerus or more proximally on the distal humeral diaphysis. It inserts into the index or long finger metacarpal. The muscle also may present as a muscle slip of variable size that arises from either the ECRL or ECRB and inserts into the index or long finger metacarpal, or both (445–447). The extensor carpi radialis accessorius is an anomalous muscle that arises from the humerus adjacent to the origin of the ECRL. The muscle lies deep to the ECRL and

extends the length of the forearm. It usually inserts onto either the base of the thumb metacarpal, the proximal phalanx of the thumb, or into the tendon of the APB. It also may originate as a muscle slip from the tendon of the ECRL to insert as noted previously (11,446). Clinical Implications: Extensor Carpi Radialis Longus Injury to the posterior interosseous nerve, including compression at the arcade of Frohse (at the proximal edge of the supinator muscle) does not effect the ECRL because the motor nerve of the ECRL leaves the radial nerve trunk proper, usually proximal to the elbow (and therefore proxi-

A

B FIGURE 2.8. The anomalous muscle, the extensor carpi radialis intermedius. It is situated between the extensor carpi radialis longus and extensor carpi radialis brevis. It originates from the lateral epicondylar region (A), or more proximally, on the lateral aspect of the distal humeral diaphysis (B). The muscle inserts into the base of either the index or long finger metacarpal, or both.

2 Muscle Anatomy

mal to the branching of the posterior interosseous nerve). Complete laceration or dense neuropathy of the posterior interosseous nerve usually presents clinically with loss of digital and thumb extension, and weak wrist extension. Residual wrist extension, produced by the intact ECRL, is possible, but the wrist also deviates radially during extension because of the ECRL insertion into the index metacarpal on the radial side of the hand. ECRB function may be preserved because its motor branch usually exits the radial nerve trunk or off of the posterior interosseous nerve proximal to the arcade of Frohse (448–450). Intersection syndrome is a condition of pain and swelling in the region of the muscle bellies of the APL and EPB. As noted by Wolfe, this area lies approximately 4 cm proximal to the wrist joint, and may show increased swelling of a normally prominent area (451). In severe cases, redness and crepitus have been noted. The syndrome originally was thought to be due to friction and inflammation between the APL and EPB muscle bellies and the muscle bellies of the ECRL and ECRB (451–455). More recently, Grundberg and Reagan have demonstrated that the basic pathologic process appears to be tenosynovitis of the ECRL and ECRB (455).

EXTENSOR CARPI RADIALIS BREVIS Derivation and Terminology. The ECRB derives its name from several sources. Extensor is from the Greek and Latin ex, which indicates “out of,” and from the Latin tendere, “to stretch”; thus, extension indicates a motion to stretch out, and extensor usually is applied to a force or muscle that is involved in the “stretching out or straightening out” of a joint. Carpi is derived from the Latin carpalis and the Greek karpos, both of which indicate “wrist” (the carpus). Radialis is from the Latin radii, which means “spoke” (used to describe the radius of the forearm). Brevis is the Latin for “short.” Therefore, extensor carpi radialis brevis indicates a short radial wrist extensor (1,2). Origin. From the lateral epicondyle of the humerus through the common extensor origin (additional attachments to the radial collateral ligament of the elbow, surrounding intermuscular septum; see later). Insertion. To the dorsal base of the long finger metacarpal. Innervation. Posterior interosseous nerve or directly from the radial nerve (C7, C8). Vascular Supply. The radial recurrent artery, interosseous recurrent artery, posterior interosseous artery, radial collateral continuation of the profunda brachii artery (3,4,11, 13). Principal Action. Extension of the wrist. Assistance with weak flexion of the elbow. The ECRB, along with the ECRL and ECU, also helps stabilize the wrist (with cocon-

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tractions of the wrist flexors) during powerful grasp functions (3,4,11). Gross Anatomic Description: Extensor Carpi Radialis Brevis The ECRB originates from the lateral epicondyle of the humerus, as part of the common extensor origin (see Fig. 2.2A). It comprises part of the mobile wad muscle compartment of the forearm (12) (Appendix 2.2). The muscle origin also includes attachments to the intermuscular septum, to the radial collateral ligament of the elbow joint, and to a strong aponeurosis that covers the surface of the muscle. The muscle is shorter than the ECRL and is in part covered by it. The muscle belly, lying adjacent to that of the ECRL, extends to the mid-portion of the forearm. At the myotendinous junction, the tendinous portion is seen first at the dorsolateral surface of the muscle. The myotendinous junction also is in close proximity to that of the ECRL. The tendon of the ECRL is a strong, flat tendon, similar in size to that of the ECRL, and travels with it to the wrist. The ECRB, along with the ECRL, passes deep to the APL and EPB, and then enters the second dorsal extensor compartment of the extensor retinaculum. [Editor’s note: The dorsal compartments of the wrist are as follows: the APL and EPB comprise the first dorsal compartment; the ECRL and ECRB form the second; the EPL forms the third; the EDC and EIP form the fourth; the EDM forms the fifth; and the ECU forms the sixth (6).] As the tendon extends through the second compartment, it lies in a shallow groove on the dorsal surface of the radius, medial to the tendon of the ECRL, and separated from it by a low ridge. The tendon of the ECRB continues distally to reach the base of the long finger metacarpal (see Fig. 2.6B). Similar to the ECRL, the tendon does not insert centrally on the metacarpal, but rather attaches off center on the radial aspect of the dorsal surface of the metacarpal base. The insertion may have slips that extend to the base of the adjacent index metacarpal (3,4,11,13). Architectural features of the ECRB include the physiologic cross-sectional area of the muscle and the fiber bundle length. Skeletal muscle architectural studies by Lieber, Friden, and colleagues provide the data for the ECRB (135–139,174) (see Table 2.2 and Fig. 2.4). The relative difference index values compare the ECRB with other upper extremity muscles, based on architectural features. These values are listed in Appendix 2.3 (15, 456–458). The ECRB is innervated by either the posterior interosseous nerve or by branches directly from the radial nerve. The muscle may receive several motor branches, several of which enter the muscle at the medial margin of the central third. The nerve fibers usually are derived from C6 (mostly), C7, and occasionally C5 (3,4,11,13).

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Actions and Biomechanics: Extensor Carpi Radialis Brevis The ECRB functions mainly to provide extension of the wrist. It works in conjunction with the ECRL and ECU. It may provide some radial deviation of the wrist, working with the ECRL. In addition, the ECRB gives assistance with weak flexion of the elbow because the muscle’s origin is proximal to the elbow. The ECRB (along with the ECRL and ECU) also helps stabilize the wrist (with cocontractions of the wrist flexors) during powerful grasp functions or heavy lifting (3,4,11,13,68). Anomalies and Variations: Extensor Carpi Radialis Brevis The ECRB may coalesce with the ECRL, or have several variations where muscle fibers are interconnected between the two muscles (11). The ECRB may have a split tendon or multiple tendons that insert into the long finger metacarpal. There may be an anomalous insertion into the adjacent metacarpal bases, or to the adjacent carpal bones (11). The extensor carpi radialis intermedius is an anomalous muscle situated between the ECRL and ECRB (see Fig. 2.8). It is a rare muscle that may arise independently from the lateral epicondyle of the humerus, and inserts into the index or long finger metacarpal. The muscle also may present as a muscle slip of variable size that arises from either the ECRL or ECRB and inserts into the index or long finger metacarpal, or both (445). The FCR brevis muscle is a rare anomalous muscle associated with the ECRB. The FCR brevis originates from the anterior surface of the radius and forms a tendon at the radiocarpal joint. The muscle is innervated by the anterior interosseous nerve (181). It enters the carpal tunnel and the tendon extends between the bases of the index and long finger metacarpals to interconnect with the tendon of the ECRB. The ECRB, in addition, splits into two tendons, one that inserts normally into the radial part of the base of the long finger metacarpal, and the other connected to the anomalous FCR brevis. It has been postulated that this anomaly causes restricted wrist flexion or extension (11). Clinical Implications: Extensor Carpi Radialis Brevis Because of the central location of its insertion on the wrist (between the ECRL and ECU), the ECRB often is used as a recipient muscle for transfers to restore wrist extension after nerve or spinal injury. In lateral epicondylitis (tennis elbow), the ECRB is usually implicated as the principal muscle affected. Several methods for operative management have been described, including muscle release, lengthening or debridement of its

tendinous origin (459–463), or lengthening of the muscle at the musculotendinous junction (462,463). Friden and Lieber have studied the physiologic consequences of surgical lengthening of the ECRB at the tendon junction. The authors found that the ECRB develops near-maximal isometric force at full wrist extension. This decreases to 20% maximum at full wrist flexion. Operative lengthening of the tendon by 9.1 mm results in a mean 10% passive shortening of the fibers, and ECRB sarcomere shortening of 0.3 µm. This 0.3-µm sarcomere shortening, in turn, was predicted to have two primary biomechanical effects: (a) a 25% decrease in muscle passive tension that could lead to reduced insertional tension and decrease pain; and (b) a 25% increase in active muscle force, which is in opposition to the notion that tendon lengthening necessarily results in muscle weakness (457,458). Intersection syndrome is a condition of pain and swelling in the region of the muscle bellies of the APL and EPB. As noted by Wolfe, this area lies approximately 4 cm proximal to the wrist joint, and may show increased swelling of a normally prominent area (451). In severe cases, redness and crepitus have been noted. The syndrome originally was thought to be due to friction and inflammation between the APL and EPB muscle bellies and the muscle bellies of the ECRL and ECRB (451–454). More recently, Grundberg and Reagan have demonstrated that the basic pathologic process appears to be tenosynovitis of the ECRL and ECRB (455). EXTENSOR DIGITORUM COMMUNIS Derivation and Terminology. Extensor is from the Greek and Latin ex, which indicates “out of,” and from the Latin tendere, “to stretch”; thus, extension indicates a motion to stretch out, and extensor usually is applied to a force or muscle that is involved in the “stretching out or straightening out” of a joint. Digitorum is from the Latin digitus or digitorum, indicating the digits. Communis is derived from the Latin communis, meaning “common,” and is used to indicate a structure serving or involving several branches or sections (1,2). Origin. From the lateral epicondyle as part of the common extensor origin. Insertion. To the base of the phalanges of the index, long, ring, and small fingers. Innervation. The posterior interosseous nerve, from the radial nerve (C7, C8). Vascular Supply. Posterior interosseous artery (which is a branch of the common interosseous artery); interosseous recurrent artery and the surrounding anastomoses; the distal continuation of the anterior interosseous artery after it passes through the interosseous ligament to reach the dorsal aspect of the forearm; the dorsal carpal arch; dorsal metacarpal, digital, and perforating arteries (3,4,11,13).

2 Muscle Anatomy

Principal Action. Extension of the digits, primarily at the MCP joints. The EDC also assists with extension of the PIP and DIP joints, working with the interossei and lumbricals. The tendons can assist with wrist extension. Gross Anatomic Description: Extensor Digitorum Communis The EDC, with its associated extensor mechanism, juncturae, and anatomic variability, is a complex structure and the subject of many investigations (464–498). It lies in the dorsal muscle compartment of the forearm (Appendix 2.2). It arises from the common extensor origin at the lateral epicondyle of the humerus (see Fig. 2.2A). The muscle also has attachments that arise from the adjacent intermuscular septa and from the fascia of the neighboring forearm muscles (3,4,11,13). It is a relatively large muscle, and its muscle belly is close to the muscle of the EDM. At the junction of the proximal two-thirds and the distal one-third of the forearm, the myotendinous junction arises and four separate tendons are formed. The tendons may be partially attached in the forearm, but more distally, at the level of the extensor retinaculum, four discrete tendons are present. The tendons pass deep to the extensor retinaculum in a tunnel with the EIP. The tendons of the EDC and EIP form the fourth dorsal compartment. [Editor’s note: The dorsal compartments of the wrist are as follows: the APL and EPB comprise the first dorsal compartment; the ECRL and ECRB form the second; the EPL forms the third; the EDC and EIP form the fourth; the EDM forms the fifth, and the ECU forms the sixth (6).] The tunnel also provides a synovial sheath. The tendons exit the retinaculum and diverge on the dorsum of the hand, one or more tendon of the EDC to each digit. The tendon of the EIP extends to the index finger, along the ulnar margin of the EDC to the index. Juncturae tendinum interconnect the tendons, with fewer and thinner juncturae located on the radial aspect of the hand. The ulnar tendons tend to have more, and thicker juncturae (discussed later) (492–498). The tendons then continue into the digits to form the extensor mechanism of each digit. The EDC tendon, through the extensor mechanism, inserts into the base of each distal phalanx (see Fig. 2.6B), the base of each middle phalanx (through the central slip), and, to varying degrees, into the bases of the proximal phalanges. Substantial tendon variability and multiplicity exits with the extensor tendons (Table 2.3). The extensor mechanism is complex, and is referred to as the extensor aponeurosis, dorsal aponeurosis, or extensor expansion (Fig. 2.9). Each of the four digits has a similar extensor mechanism, and it intimately involves the intrinsic muscles of the hand as well. Smith and von Schroeder and Botte have described the mechanism in detail (484,494). Each extrinsic extensor tendon enters the dorsal aponeurosis at the level of the MCP joint. The tendon is joined by the sagittal bands from the medial and lateral aspects. The

135

transverse lamina of the sagittal bands arise from the palmar aspect of the MCP joint, attaching to the volar plate, to intermetacarpal ligaments at the neck of the metacarpals, and to a portion of the fibroosseous tunnel. The sagittal bands extend over the medial and lateral aspects of the MCP joint to envelop the EDC (and EIP) tendons. The sagittal bands help stabilize and centralize the extrinsic extensor tendons. (Injury to the sagittal bands may result in extensor tendon subluxation.) The tendon continues distally, and in the fibrous expansion, the tendon divides into a central slip and two lateral slips. The central slip inserts into the base of the middle phalanx and provides extension of the middle phalanx partially through the central slip. The intrinsic tendons from the lumbricals and interosseous muscles join the extensor mechanism at the level of the proximal and mid-portion of the proximal phalanx. A portion of the lateral band extends dorsally to join the central slip. It is through this portion of the extensor tendon that the intrinsic muscles contribute to PIP joint extension. A portion of the lateral bands also continues distally to join the terminal tendon, to insert onto the base of the distal phalanx. The lateral slips join the tendons of the intrinsic muscles to form the conjoined lateral bands, which continue distally to form the terminal tendon. The lumbricals to the index and long fingers arise from the radial sides of the associated profundus tendons (467,468). The lumbricals to the ring and small finger arise from the adjacent sides of the profundus tendons to the long, ring, and small fingers. Variation of the lumbricals is common, and similar to the extrinsic extensor tendons there is more variability on the ulnar side of the hand. The lumbricals and interosseous muscles are discussed in greater detail under their respective muscle sections (464,465). As mentioned previously, the extensor tendons are interconnected on the dorsum of the hand by the juncturae tendinum and intertendinous fascia. These structures have been studied in detail and classified by Wehbe and von Schroeder et al. (489,493). The juncturae tendinum consist of narrow connective tissue bands or slips that extend between the EDC tendons as well as to the EDM. Very rarely does the EIP have a connecting junctura (493). The function of the junctura remains not entirely understood. The juncturae may assist with spacing of the EDC tendons or with force redistribution (486,487), or may help with coordination of extension or stabilization of the MCP joints (488). The junctura prevent independent extension of the digits and are clinically important because they may bridge and therefore mask tendon lacerations. Juncturae also may cause snapping by subluxating across the metacarpal head. The juncturae also may aid in the surgical identification of the tendons of the hand and has been used in repair of the dorsal aponeurosis. Complete transection of a juncturae and the intertendinous fascia may lead to subluxation of the EDC tendon over a flexed MCP joint (494). The juncturae tendinum are variable, and become progressively thicker

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FIGURE 2.9. The extensor aponeurosis (see text).

from the radial to the ulnar side of the hand. Three distinct type of juncturae tendinum have been identified (493) (Fig. 2.10). A thin filamentous junctura is defined as type 1, and is found primarily between the EDC tendons to the index and middle fingers and between the tendons to the middle and ring fingers. Type 2 juncturae are thicker and well defined and are present between extensor tendons to the long and ring fingers and between the tendons to the ring and small fingers. Type 3 juncturae consist of a thick, tendon-like slip between the extensor tendons to the middle and ring fingers and between the tendons to the ring and small fingers. Two subtypes of type 3 juncturae have been identified, a “y” and an “r” type, based on the interconnec-

tions. The presence of certain juncturae appears to be associated with the presence or absence of tendons. For instance, the type of juncturae in the fourth intermetacarpal space depends on the presence of an EDC tendon to the small finger. Absence of the EDC small finger tendon has been found to be associated with a double EDC ring finger tendon and a thick type 3 junctura that substitutes for the absent EDC small finger tendon (492,493). Although multiple EDC ring finger tendons usually are present, the ulnar portion of the double EDC ring finger tendon and, as mentioned, the type 3 junctura may represent a developmental remnant of the EDC small finger tendon. The presence of juncturae between the extension tendons and adjacent ten-

2 Muscle Anatomy

A

137

B

C

D FIGURE 2.10. Juncturae tendinum of the extensor tendons. A: Type 1. The type 1 junctura is a thin, filamentous connection between the extensor digitorum communis (EDC) of the long and index fingers. It sometimes is present between the EDC of the ring and long fingers. B: Type 2. The type 2 junctura has morphologic features between types 1 and 3. It typically is present between the EDC tendons of the long and ring fingers and sometimes between the tendons of the ring and small fingers. C: Type 3y. The type 3y junctura is a tendon slip most commonly present between the EDC tendons of the small and ring fingers. D: Type 3r. The type 3r junctura is a tendon slip most commonly present between the EDC of the ring finger and the extensor digiti quinti. Its presence is associated with an absent EDC to the small finger. (Adapted from von Schroeder HP, Botte MJ, Gellman H. Anatomy of the juncturae tendinum of the hand. J Hand Surg [Am] 15:595–602, 1990, with permission.)

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dons should be appreciated when tendon transfer or harvesting is used (492,493). Architectural features of the EDC include the physiologic cross-sectional area of the muscle, the fiber bundle length, muscle length, muscle mass, and pennation angle (angle of the muscle fibers from the line representing the longitudinal vector of its tendon). Skeletal muscle architectural studies by Lieber, Friden, and colleagues provide the data for the EDC (135–139,174) (see Table 2.1 and Fig. 2.4). The digital extrinsic extensor and flexor muscles have similar architectural features. In general, the EDC muscles do have smaller physiologic cross-sectional areas compared with the extrinsic flexors, indicating that the EDC is not optimally designed for force generation. The relative difference index values compare the EDC with other upper extremity muscles, based on architectural features. These values are listed in Appendix 2.3 (15). The EDC is innervated by the posterior interosseous nerve, derived mostly from C7 as well as from C6 and C8. The posterior interosseous nerve passes through the supinator muscle, and branches into several motor branches that

enter the deep surface of the middle third of the muscle. There is variation among the motor branches, and there may be a common branch or branches that also innervate the EDM or ECU. The EDC muscle may receive a variable number of branches.

Actions and Biomechanics: Extensor Digitorum Communis and the Associated Extensor Mechanism The EDC functions mainly to extend the digits, primarily at the MCP joints (494). The EDC also assists with extension of the PIP and DIP joints, working with the interossei and lumbricals. The tendons also can assist with wrist extension. Extension of the digits is a complex function, involving simultaneous actions of the intrinsic and extrinsic extensor muscles (464,465,467,468,474,494). The interossei and lumbricals extend the PIP and DIP joints and flex the MCP joints. The extrinsic digital extensor

TABLE 2.3. EXTENSOR TENDON VARIATIONS AND MULTIPLICITY Incidence as %

Tendon EIP

EIP to middle EMP EDC—index EDC—long

EDC—ring

EDC—small

EDQ

EDQ to ring EDBM

Tendons or Slips

von Schroeder/ Botte (492)

Absent 1 2 3 Present Present 1 2 1 2 3 4 1 2 3 4

0 77 16 7 5 12 98 2 51 28 16 5 12 63 16 9

Absent 1 2 3 1 2 3 4 Present Present

54 19 26 2 2 84 7 7 2 0

Schenk

Mestdagh et al. (479)

Leslie (478)

1 93 6

0 96 4

3

Ogura et al. (500)

2 5 95 5 61 39

63 31 5

56 44

7 84 7 2

5 91 9

16 84

97 3 4 94 2 2

0

3

EIP, extensor indicis proprius; EMP, extensor medii proprius; EDC, extensor digitorum communis; EDQ, extensor digiti quinti; EDBM, extensor digitorum brevis manus. Reprinted from von Schroeder HP, Botte MJ. Functional anatomy of the extensor tendons of the digits. Hand Clin 13:51–62, 1997, with permission.

2 Muscle Anatomy

muscles, including the EDC, EIP, and EDM, function primarily to extend the MCP joints, but do have extensor function at the PIP and DIP joints. The flexor muscles and respective tendons on the palmar aspect of the hand are important in stabilizing and balancing the phalangeal joints during extension. Despite all the separate tendons involved in finger extension, complete independent extension of each finger is not always possible. This is due in part to the juncturae tendinum and intertendinous fascia between the extrinsic tendons on the dorsum of the hand (491–496). As noted previously, the EDC muscles have smaller physiologic cross-sectional areas than the extrinsic flexors, indicating that the EDC is not optimally designed for force generation (466) (see Table 2.1 and Fig. 2.4). Although the EDC appears as a single muscle belly that forms four tendons, each tendon usually can be traced back to a muscle belly that can be separated from the remaining EDC muscle. Each of these four muscle bellies are similar, however. The EDCs to the long and ring fingers have a relatively larger cross-sectional area than the EDCs to the index and small fingers. The cross-sectional area of the EDC muscles to the long and ring fingers also are larger that those of the EIP or EDM. At the level of the extensor retinaculum, the EDC usually exits as four tendons. Distal to the wrist, many of the tendons divide into double or triple tendons. These anatomic variations as well as their arrangement and incidences have been recognized in clinical and anatomic studies (462,475,478,479,481) (Table 2.3). Because these variations are so common, it is difficult to label these as anomalies; they perhaps are best considered as normal variations. In a study of 43 hands, the most common pattern on the dorsum of the hand was a single EIP tendon (77%) that inserted ulnar to the index finger EDC on the dorsal aponeurosis of the index finger; a single index finger EDC (98%); a single long finger EDC (51%); a double ring finger EDC tendon (63%) with a single insertion; an absent small finger EDC (54%); and a double EDQ tendon (84%) with a double insertion into the dorsal aponeurosis of the small finger (492). The extensor tendons typically have longitudinal fissures or striae, but tendons that can be readily divisible along fissures without sharp dissection are defined as tendon slips (492). Anomalies and Variations: Extensor Digitorum Communis The EDC tendons are extremely variable as to number and presence (see Table 2.3). Double and triple tendons exist. An EDC tendon may be absent (in 54% to 56% of hands) (481,492). Absence of the EDC to the small finger often is associated with a double EDM to the small finger (492). There commonly are thick juncturae from the ring finger EDC to the small finger (493).

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The most common extensor tendon pattern is as follows: a single EIP tendon (77%), a single index finger EDC (98%), a single long finger EDC (51%), a double ring finger EDC (63%), an absent small finger EDC (54%), and a double EDM (84%) (492). Additional frequent variations include a double EIP (16%), a double (28%) or triple (16%) long finger EDC, a single (12%) or triple (16%) ring finger EDC, and a single (19%) or double (26%) small finger EDC (492). The juncturae tendinum of the EDC are variable, with fewer and thinner juncturae on the radial side of the hand compared with the ulnar (493). There is more tendon variability and multiplicity (along with more juncturae) toward the ulnar side of the hand (492). The muscle belly of the EDC may exist as a single or double muscle, or as four separate bellies (11). The EDC may have a tendon slip or a junctura that extends to the extensor tendon of the thumb (11). The extensor medii proprius (EMP), also known as the extensor medii digiti or extensor medii communis, is a deeply situated anomalous muscle that is analogous to the EIP but inserts into the ulnar aspect of the dorsal aponeurosis of the long finger (Fig. 2.11A). The EMP and EIP muscles usually have a common origin on the distal ulna and adjacent interosseous ligament. The EMP is encountered in 0.8% to 10.3% of hands (11,479,485,490), but is rarely described or noted (478). The EMP is commonly found in Old World monkeys, whereas the EMP is variably present in the chimpanzee and gorilla, as it is in humans. Because of these findings, von Schroeder and Botte speculate that the EMP is an evolutionary remnant and not a variation of a normal arrangement (494). The extensor indicis et medii communis (EIMC) muscle is an anomalous muscle similar to the EIP muscle, except that it splits to insert into both the index and long fingers (see Fig. 2.11B). It has been studied in detail by von Schroeder and Botte, who observed an incidence of 3.4% (490). Similar to the EMP, the EIMC commonly is found in Old World monkeys, whereas the EIMC is variably present in the chimpanzee and gorilla, as it is in humans. Because of these findings, the EIMC (like the EMP) may be an evolutionary remnant and not a variation of a normal arrangement (494). The extensor medii et annularis communis is an anomalous EIP muscle that splits to insert into both the long and ring fingers (490). The extensor digitorum brevis manus is an anomalous muscle that originates from the distal radius, radiocarpal ligament, or the distal ulna (Fig. 2.12). The tendon inserts into the index finger or, less commonly, into the long finger. It is innervated by a branch of the posterior interosseous nerve. Most of the muscle belly is located on the dorsum of the hand, and can cause local discomfort or tendon dysfunction. It can be mistaken for a ganglion or other tumor. The muscle may become symptomatic

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Systems Anatomy

A

B FIGURE 2.11. Anomalous extensor tendons of the hand and forearm. A: Schematic illustration of the extensor medii proprius (EMP). The EMP originates in the forearm and inserts into the dorsal aponeurosis of the long finger. The EMP is similar to the extensor indicis proprius (EIP); however, the EMP inserts into the aponeurosis of the long finger, not the index finger. The insertions (cut) of the extensor digitorum communis (arrows) to the index and long fingers also are shown. B: Schematic illustration of the extensor indicis et medii communis (EIMC). The EIMC consists of one muscle belly and two tendons that insert into the index and long fingers. The EIP is absent. The insertions (cut) of the extensor digitorum communis (arrows) to the index and long fingers also are shown. (From von Schroeder HP, Botte MJ. The extensor medii proprius and anomalous extensor tendons to the long finger. J Hand Surg [Am] 16:1141–1145, 1991, with permission.)

FIGURE 2.12. The extensor digitorum brevis manus is an anomalous muscle that originates from the distal radius, radiocarpal ligament, or the distal ulna. It can resemble a dorsal wrist ganglion.

2 Muscle Anatomy

deep to the extensor retinaculum. Excision of this anomalous muscle or decompression under the extensor retinaculum may be performed if symptoms warrant (494, 499–504). Clinical Implications: Extensor Digitorum Communis Their frequent multiplicity and variability and the possible presence of many anomalous extensor tendons should be appreciated during extensor tendon exploration for trauma repair or tendon transfer. The index finger has the greatest independent motion in extension. It has two independent tendons (index finger EDC and EIP) that are the least variable of the extensor tendons. It also has the lowest frequency of interconnecting juncturae tendinum. These anatomic findings help explain its relatively independent extension capabilities compared with the more ulnarly located digits (e.g., the ring finger). Occasionally, an anomalous junctura tendinum may cross between the EDC and the EPL tendons. This junctura restricts digital motion, making it impossible actively to extend the digits fully while maintaining the interphalangeal joint of the thumb in flexion (483). The extensor digitorum brevis manus (see earlier) can be mistaken for a ganglion or other tumor. It may become symptomatic deep to the extensor retinaculum. Excision of this anomalous muscle or decompression under the extensor retinaculum may be performed if symptoms warrant (494,499–504) (see Fig. 2.12). EXTENSOR INDICIS PROPRIUS Derivation and Terminology. Extensor is derived from the Greek and Latin ex, which indicates “out of,” and from the Latin tendere, “to stretch”; thus, extension indicates a motion to stretch out, and extensor usually is applied to a force or muscle that is involved in the “stretching out or straightening out” of a joint. Indicis is from the Latin to indicate the index finger (1,2). Origin. The dorsal surface of the distal ulna and adjacent interosseous ligament. Insertion. The extensor hood of the index finger. Innervation. Posterior interosseous nerve (C7, C8). Vascular Supply. The posterior interosseous artery, interosseous recurrent artery and its communicating vessels, continuation of the anterior interosseous artery after it passes through the interosseous ligament; the dorsal carpal arch; dorsal metacarpal, digital and perforating arteries (3,4,11,13). Principal Action. Extension of the index finger. As with the index finger EDC, the principal action is on the MCP joint.

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Gross Anatomic Description: Extensor Indicis Proprius The EIP is a relatively small and short extensor located deep to the EDC, EDM, and ECU. It lies in the dorsal muscle compartment of the forearm (Appendix 2.2). The EIP originates from a diagonally oriented origin on the ulnar aspect of the distal forearm (see Fig. 2.3B). The muscle arises from the dorsal surface of the distal ulna and from a portion of the adjacent interosseous ligament. Additional attachments include the fascia or septum between the EIP and EPL. The muscle belly of the EIP lies next to and ulnar to the muscle belly of the EPL. The tendon of the EIP passes deep to the EDM and EDC tendons in an oblique fashion as it extends distally toward the index finger. It joins the tendons of the EDC in the fourth dorsal compartment as it passes deep to the extensor retinaculum. As the EIP enters the extensor retinaculum, it is located on the ulnar margin of the retinaculum, and positioned ulnar and deep to the EDC. In the extensor retinaculum, the EIP tendon continues in a diagonal course to cross deep to the EDC tendons, so that when the EIP emerges from the extensor retinaculum, it is on the lateral aspect of the retinaculum. The tendon continues distally and laterally toward the dorsal aspect of the index finger, and remains in close proximity and ulnar to the EDC to the index finger. (This ulnar position of the tendon is important in identification of the tendon for harvest for tendon transfer during such procedures as opponensplasty.) At the level of the index metacarpal head and neck, the tendon of the EIP joins the tendon of the index EDC to form a continuous extensor hood (3,4,11,13,68) (Fig. 2.6B). Architectural features, including the physiologic crosssectional area and the muscle fiber length of the EIP, are listed in Table 2.1 and depicted in Fig. 2.4. The EIP is innervated by the posterior interosseous nerve, predominantly C7, as well as C8. Anatomic studies have shown that the branch to the EIP usually is the last or terminal motor branch of the posterior interosseous nerve (505). Actions and Biomechanics: Extensor Indicis Proprius The EIP assists with extension of the index finger. It also assists with wrist extension. The separate muscle of the EIP provided to the index finger assists with the strong independent motion of the index finger. Principal action is on the MCP joint. Anomalies and Variations: Extensor Indicis Proprius See also Anomalies and Variations: Extensor Digitorum Communis. Despite the variability and multiplicity of the extensor tendons (see Table 2.3), the EIP usually exists as a single

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tendon. In a study of 43 hands, the most common pattern on the dorsum of the hand was a single EIP tendon (77%) that inserted ulnar to the index finger EDC on the dorsal aponeurosis of the index finger, and a single index finger EDC (98%) (492). The EIP may be absent (11). The muscle or tendon of the EIP may be doubled (11). Muscle or tendon slips can pass to the thumb or adjacent digits, including additional anomalous insertions into the base of the long finger metacarpal or base of the long finger proximal phalanx. The extensor medii proprius (EMP) and the extensor indicis et medii communis (EIMC) are anomalous muscles similar to the EIP that attach to the index or long fingers, and are seen in 2% to 6.5% of hands (478,490,506–508) (see also under Anomalies and Variations: Extensor Digitorum Communis, and Fig. 2.10). The EIP tendons usually insert ulnar to the index finger EDC tendon (81% to 87% of specimens). However, they may be located or insert directly palmar to the index finger EDC in 10% to 11%, and radial to the index finger EDC in 3% to 8% (11,479,490). Clinical Implications: Extensor Indicis Proprius Their frequent multiplicity and variability, and the possible presence of many anomalous extensor tendons should be appreciated during extensor tendon exploration for trauma or for tendon transfer (492,493,509). The EIP usually is located along the ulnar aspect of the index finger EDC tendon (3,4,11,13,492). This positioning helps identify the tendon for repair or for harvest for transfer (i.e., for opponensplasty). EXTENSOR DIGITI MINIMI (EXTENSOR DIGITI QUINTI) Derivation and Terminology. Extensor is from the Greek and Latin ex, which indicates “out of,” and from Latin tendere, “to stretch”; thus, extension indicates a motion to stretch out, and extensor usually is applied to a force or muscle that is involved in the “stretching out or straightening out” of a joint. Digiti is the plural of the Latin digitus, “digit.” Minimi is from the Latin minima, “the minimum,” referring to the small finger (1,2). Origin. From the lateral epicondyle through the common extensor origin, as well as from the adjacent intermuscular septum (between it and the ECU), and from the overlying fascia. Insertion. To the extensor mechanism of the small finger. Innervation. Posterior interosseous nerve (C7, C8). Vascular Supply. The posterior interosseous artery; interosseous recurrent artery and its communicating branches; from the continuation of the anterior interosseous artery after it passes through the interosseous

ligament; the dorsal carpal arch; dorsal metacarpal, digital, and perforating arteries (3,4,11,13,68). Principal Action. Extension of the MCP joint of the small finger, extension of the PIP and DIP joints. The EDM also assists with wrist extension. Gross Anatomic Description: Extensor Digiti Minimi The EDM is a relatively small, slender muscle. It lies in the dorsal muscle compartment of the forearm (Appendix 2.2). It originates from the lateral epicondyle of the humerus as part of the common extensor origin tendon (3,4,11,13) (Fig. 2.2A). In addition, fibers arise from the adjacent intermuscular septum (between the EDM and the EDC) as well as from the overlying deep antebrachial fascia. The narrow muscle is formed and blends to some extent to that of the EDC. The tendon forms in a manner similar to those of the EDC in the distal third of the forearm. The tendon passes deep to the extensor retinaculum, comprising the fifth dorsal compartment. [Editor’s note: The dorsal compartments of the wrist are as follows: the APL and EPB comprise the first dorsal compartment; the ECRL and ECRB form the second; the EPL forms the third; the EDC and EIP form the fourth; the EDM forms the fifth; and the ECU forms the sixth (6).] The fifth dorsal compartment is located dorsal to the distal radioulnar joint. The tendon continues distally to reach the dorsal surface of the small finger metacarpal. It remains on the ulnar side of the EDC tendon to the small finger. The EDM inserts, in part, into the base of the proximal phalanx of the small finger (Fig. 2.6B). The tendon also is joined by the slip from the EDC to the small finger. The tendon often is split or doubled, and exhibits variability, as do the EDC tendons (492) (see Table 2.3). The architectural features of the EDM are listed in Table 2.1 and depicted in Fig. 2.4. The EDM is innervated by the posterior interosseous nerve, mostly from C7 and C8. The nerve branch or branches enter the muscle belly of the EDM in the middle third of the muscle on the deep surface. Actions and Biomechanics: Extensor Digiti Minimi The EDM provides extension of the MCP joint of the small finger, as well as extension of the PIP and DIP joints. The EDM also assists with wrist extension. It works with the small finger EDC, and may be the only digital extensor of the small finger if the small finger EDC tendon is absent (3,4,11,13). Anomalies and Variations: Extensor Digiti Minimi See also Anomalies and Variations: Extensor Digitorum Communis.

2 Muscle Anatomy

The EDM exhibits variability similar to that of the EDC tendons (see Table 2.3). The tendon may be absent, or exist as a double or triple tendon. Its most common pattern is that of a double tendon, seen in 84% (492,495,497,498). The muscle belly may be doubled, or have an accessory head. An accessory head may originate from the ulna (11). The muscle belly may blend or coalesce with the EDC muscle belly (11). Several variations in the insertion can exist. A tendon slip to the base of the ring finger proximal phalanx has been noted in 6% to 10% (11,446). An ulnar slip has been noted to insert onto the base of the small finger metacarpal (11). Clinical Implications: Extensor Digiti Minimi The EDM may provide the principal digital extension for the small finger in the absence of the EDC to the small finger. The most common pattern of the extensor tendons actually is an absent small finger EDC, and a double tendon of the EDM (492,497,498) (Table 2.3). EXTENSOR CARPI ULNARIS Derivation and Terminology. Extensor is derived from the Greek and Latin ex, which indicates “out of,” and from the Latin tendere, “to stretch”; thus, extension indicates a motion to stretch out, and extensor usually is applied to a force or muscle that is involved in the “stretching out or straightening out” of a joint. Carpi is from the Latin carpalis or the Greek karpos, both of which indicate “wrist” (the carpus). Ulnaris is derived from the Latin ulna, “arm,” and ulnaris, “pertaining to the arm” (1,2). Origin. The lateral epicondyle of the humerus through the common extensor origin. Additional attachments include the posterior border of the ulna by an aponeurosis that wraps around the ulna and is shared with the FCU and FDP. The ECU also has attachments of origin from the overlying fascia. Insertion. Base of the small finger metacarpal, dorsal aspect. Innervation. Posterior interosseous nerve (C6, C7, C8). Vascular Supply. The posterior interosseous artery; interosseous recurrent artery (3,4,11,13). Principal Action. Extension of the wrist. It contributes to ulnar deviation of the wrist. The ECU also helps stabilize the wrist during forceful grip or lifting, or production of a clenched fist. Gross Anatomic Description: Extensor Carpi Ulnaris The ECU originates mainly from the lateral epicondyle of the humerus through the common extensor origin (see Fig. 2.2A). It lies in the dorsal muscle compartment of the forearm

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(Appendix 2.2). In addition, there may be several other sites of origin (3,4,11,13). The ECU usually also has attachments to the posterior border of the ulna that connect to an aponeurosis that wraps around the ulna and is shared with both the FCU and FDP (see Fig. 2.3B). The ECU also has attachments of origin from the overlying fascia of the forearm muscles. Two heads may be present. One head originates from the distal dorsal portion of the lateral epicondyle of the humerus and from the investing fascia and septa between the ECU and EDM, anconeus, and supinator. The other head originates from the proximal dorsal border of the ulna. The muscle fibers extend distally along the dorsal ulnar portion of the forearm in an osteofascial compartment consisting of the dorsal surface of the ulna, the fascia of the forearm, dense fascia lying on the ulnar origin of the muscles of the thumb, and the origin of the extensor indicis. The muscle usually extends the distal threefourths of the forearm to end in a thick tendon. The tendon first appears on the dorsal surface of the muscle or deep in the muscle on the radial border of the middle third of the posterior surface of its belly (3,4,11). The tendon reaches the extensor retinaculum to form the sixth dorsal compartment. [Editor’s note: The dorsal compartments of the wrist are as follows: the APL and EPB comprise the first dorsal compartment; the ECRL and ECRB form the second; the EPL forms the third; the EDC and EIP form the fourth; the EDM forms the fifth; and the ECU forms the sixth (6).] In the sixth compartment, the tendon is stabilized by traversing a groove in the distal ulna. The groove is located lateral to the styloid process of the ulna, but medial to the head of the ulna. The dorsal retinaculum holds the tendon in place. The tendon extends distally in close proximity to the dorsomedial portion of the triangular fibrocartilage (510,511). The tendon continues across the ulnar carpus to reach the base of the fifth metacarpal (see Fig. 2.6B). It inserts onto a tubercle located on the medial aspect of the dorsal base of the metacarpal (3,4,11,13) (Fig. 2.6B). Architectural features of the ECU include the physiologic cross-sectional area of the muscle and the fiber length. Skeletal muscle architectural studies by Lieber and colleagues provide the data for the ECU (135–139,174) (see Table 2.2 and Fig. 2.4). The relative difference index values compare the ECU with other upper extremity muscles, based on architectural features. These values are listed in Appendix 2.3 (15). The ECU is innervated by the posterior interosseous nerve, comprising contributions from the C6, C7, and C8 nerve roots. The branch to the ECU usually leaves the posterior interosseous nerve just distal to the distal edge of the supinator muscle. The nerve may branch into several smaller branches that enter the middle third of the muscle belly on its deep surface. Actions and Biomechanics: Extensor Carpi Ulnaris The main function of the ECU is extension of the wrist. It also contributes to ulnar deviation of the wrist. The ECU

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helps stabilize the wrist during forceful gripping or lifting, or producing a clenched fist. It is a dynamic stabilizer of the distal radioulnar joint and distal ulna. In stabilizing the distal radioulnar joint complex, the ECU works with the interosseous ligament, the extensor retinaculum and competence of the sigmoid notch of the distal radius, and the dynamic forces of the pronator quadratus (510–515).

Vascular Supply. The radial artery; posterior interosseous artery; radial recurrent artery; interosseous recurrent artery; middle collateral artery (3,4,11,13). Principal Action. Supination of the forearm (lateral rotation of the forearm so that the palm faces anteriorly, or superiorly if the elbow is flexed). Gross Anatomic Description: Supinator

Anomalies and Variations: Extensor Carpi Ulnaris The ECU may consists of a double muscle belly, or terminate in a double tendon (516,517). With a double tendon, one slip may insert onto the base of the fourth metacarpal (517). The ulnaris digiti minimi (or ulnaris digiti quinti) is an anomalous muscle closely associated with the ECU. It arises distally in the forearm from the dorsal surface of the distal ulna. This small muscle extends distally along the ulnar wrist and hand to insert into the base of the distal phalanx of the small finger. The ulnaris digiti minimi may represent an extension or accessory belly of the ECU. It may be a separate tendon slip arising from the tendon of the ECU. The ulnaris digiti minimi also may have insertions into the dorsal fascia of the fifth metacarpal, capsule of the MCP joint, or proximal phalanx of the small finger (11). The ECU may be absent (518). This is rare, occurring in 0.55% (11). Absence has been noted to be bilateral (518). Clinical Implications: Extensor Carpi Ulnaris Duplication of the ECU tendon, or a double tendon that extends to the base of the small finger distal phalanx, may impair simultaneous extension of the wrist and the small finger. Synovitis has been associated with this anomaly (517). Dislocation, subluxation, and stenosing tenosynovitis are potential problems of the ECU tendon as it passes to and through the dorsal retinaculum (519–522). SUPINATOR Derivation and Terminology. Supinator is derived from the Latin supinatio, which denotes the act of assuming the supine position, or the state of being supine. Applied to the hand, it is the act of turning the palm forward (anteriorly) or upward, performed by lateral rotation of the forearm (1,2). Origin. From the lateral epicondyle and the lateroposterior ulna. Insertion. To the proximal radius, along the lateral, posterior, and anterior surface. Innervation. Posterior interosseous nerve (C6, C7).

The supinator is a relatively broad and flat muscle of the proximal deep forearm. It comprises one of the many muscles of the dorsal muscle compartment of the forearm (Appendix 2.2). It arises from two main areas: the lateral epicondyle and the proximal lateral ulna (3,4,11,13) (see Fig. 2.3). From the lateral epicondyle, it arises from the dorsal aspect from a tendinous band that joins the deep surface of the tendons of origin of the ECRL, ECRB, and EDC. It also has attachments to the radial collateral ligament of the elbow joint. The other main area of origin is from the proximal ulna, on its lateral aspect. Some fibers arise from a depression distal to the radial notch and others from a crest on the proximal ulna known as the supinator crest. The fibers extend radially and slightly distally to the radius, to insert onto the proximal radius (see Fig. 2.3). The insertion area surrounds the proximal third of the radius, from the radial tuberosity to the attachment of the pronator teres, or to the upper part of the radius between the anterior and posterior oblique lines. The muscle has two layers, a superficial and a deep layer. These layers are separated by a connective tissue septum through which the posterior interosseous nerve courses. The two layers arise together, the superficial by tendinous origin and the deep by muscular fibers from the lateral epicondyle of the humerus, from the radial collateral ligament of the elbow joint and the annular ligament of the superior radioulnar joint, from the supinator crest of the ulna, and from the posterior aponeurosis covering the muscle (523). The proximal portion of the muscle contains an opening in the superficial layer, the arcade of Frohse. The arcade of Frohse allows the passage of the posterior interosseous nerve as it enters between the two heads. There is variability of the anatomy pertaining to the tendinous or membranous nature of the rim of the arcade of Frohse (524–529). Thomas and colleagues noted that the arcade of Frohse was lined by a tendinous rim in 32% and a membranous rim in 68% (523). Conversely, Ozkan and colleagues reported that the rim of the arcade was fibrous in 80% and membranous in 20% of specimens (524). In addition, Debouck and Rooze noted that the arcade was tendinous in 64% (525) and Papadopoulos et al. noted a tendinous arcade in 90% (528). The arcade of Frohse is a well known area of possible nerve impingement resulting in posterior interosseous neuropathy (526). It remains unclear if a fibrous rim of the arcade predisposes the posterior interosseous nerve to impingement (529). After the nerve enters the supinator, it continues obliquely through the

2 Muscle Anatomy

muscle, with the direction of the nerve roughly perpendicular to the fibers of the muscle. The nerve often branches within the muscle, and several branches often are seen exiting the distal edge of the muscle. The nerve also may be compressed at the distal edge of the supinator (530). The supinator is innervated by the branches of the posterior interosseous nerve before the nerve passes through the arcade of Frohse. Theses branches usually carry contributions from C5, C6, and C7 (3,4,11,13,505,531).

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Insertion. The base of the thumb metacarpal, dorsal aspect. Innervation. Posterior interosseous nerve (C7, C8). Vascular Supply. The posterior interosseous artery; perforating arteries and continuation of the anterior interosseous artery; radial artery in the anatomic snuff-box; first dorsal metacarpal artery; dorsal carpal arch (3,4,11,13). Principal Action. Abduction of the thumb metacarpal (abduction of the thumb in the radial direction in the plane of the palm).

Actions and Biomechanics: Supinator The supinator functions mainly for supination of the forearm (lateral rotation of the forearm so that the palm faces anteriorly, or superiorly if the elbow is flexed). It works in conjunction with the biceps for forearm supination, and is thought to provide approximately half the power of the biceps muscle for supination (11). It may act alone in slow, unopposed supination and together with the biceps in fast or forceful supination (3,4,11,13). Anomalies and Variations: Supinator The supinator may exist as only one muscle head, without a superficial and deep layer (11). Accessory slips of muscle or tendon may interconnect the supinator with the biceps tendon, annular ligament of the elbow, tuberosity of the radius, and neighboring areas (11,532). The tensor ligamenti anularis anterior muscle is an anomalous muscle that connects the supinator to the annular ligament in 5% of individuals (11). Clinical Implications: Supinator The arcade of Frohse is the opening of the superficial layer of the supinator. It often is lined by a fibrous rim, and provides the opening of the muscle through which the posterior interosseous nerve passes. The arcade of Frohse is a well known area of possible nerve impingement resulting in posterior interosseous neuropathy (526). ABDUCTOR POLLICIS LONGUS Derivation and Terminology. Abductor is derived form the Latin ab, meaning “away from,” and from ducere, which means “to draw”; therefore, abductor is “that which draws away from.” Pollicis is from the Latin pollex, indicating “thumb.” Longus is derived from the Latin longus, indicating “long.” The APL is the longest abductor of the thumb (1,2). Origin. The mid-dorsal radial diaphysis and adjacent portion of the interosseous ligament, and from the lateral edge of the middle third of the ulnar diaphysis.

Gross Anatomic Description: Abductor Pollicis Longus The APL is located in the deep layer of the posterior forearm. The muscle comprises one of the many muscles of the dorsal muscle compartment of the forearm (Appendix 2.2). It arises from the lateral edge of the dorsal radial diaphysis, from a portion of the interosseous ligament, and from the proximal part of the middle third of the ulna (3,4,11,13,68). Its origin from the radius is distal and central to the supinator, but proximal to the origins of the EPL and EPB (see Fig. 2.3B). Additional areas of origin include the septa between the APL and the supinator, the ECU, and the EPL. The muscle fibers converge in a penniform manner to join in a muscle belly that extends distally in an oblique fashion, coursing radially in the direction of the thumb. The muscle then forms the myotendinous junction in the distal third of the forearm, joined by the tendon of the EPB, which lies immediately ulnar to the APL. The tendon becomes more superficial in the distal third of the forearm. The tendon of the APL is round and thick. At the level of the extensor retinaculum, the APL and EPB enter their own fibroosseous tunnel to comprise the first dorsal compartment (3,4,533). [Editor’s note: The dorsal compartments of the wrist are as follows: the APL and EPB comprise the first dorsal compartment; the ECRL and ECRB form the second; the EPL forms the third; the EDC and EIP form the fourth; the EDM forms the fifth; and the ECU forms the sixth (6).] The first dorsal compartment is located on the dorsolateral surface of the distal radius, just lateral to the tendons of the ECRL and ECRB of the second dorsal compartment. The APL exits the first dorsal compartment, remaining on the lateral side of the EPB, and continues toward the base of the thumb to insert onto the base of the thumb metacarpal on its radial surface (see Fig. 2.6A). The tendon often splits into two slips, one attaching to the radial side of the thumb metacarpal base and the other to the trapezium. Variations in the number and course of the tendon are so numerous that the normal pattern of a single APL and EPB occurs less than 20% of the time (451,534). This variability has implications for the etiology and treatment of de Quervain’s tenosynovitis (534–553). The first dorsal compartment may have more variations in tendon structure and organization than most

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other muscles in the upper extremity (451). This is discussed in detail later, under Anomalies and Variations: Abductor Pollicis Longus. The APL is innervated by the posterior interosseous nerve, usually by one or more branches. The branches enter the muscle just after the nerve exits the supinator muscle. The branches then enter the proximal third of the muscle belly, usually on the superficial surface. The motor branches usually have contributions mainly from C7, but also from C6 and C8 (3,4,11,13,68). Actions and Biomechanics: Abductor Pollicis Longus The APL functions mainly to abduct the thumb metacarpal from the hand in the radial direction and in the plane of the palm. During maximal contraction, it also may contribute to flexion of the wrist or radial deviation of the wrist. It is considered an antagonist to the opponens pollicis (11). The APL works in conjunction with the APB to abduct the thumb; it works in conjunction with the EPL and EPB to assist with extension at the thumb carpometacarpal joint. Anomalies and Variations: Abductor Pollicis Longus The tendon of the APL often is doubled. It may have multiple tendons. With double tendons, both often still insert to the base of the thumb metacarpal. In several studies, a double tendon was more common that a single tendon, with the single APL and EPL pattern occurring less than 20% of the time (451,534,537,538,540,542,544,546,549, 550,554,555). Failure to recognize these variations potentially leads to persistence or recurrence of pain after operative decompression because of incomplete surgical release of the tendon sheath (535,536,545). The muscle belly may be split or doubled, or there may be multiple bellies or slips (11). Multiple accessory muscles or tendon slips have been noted, including those that extend to the trapezium, scaphoid, opponens pollicis, proximal phalanx of the thumb, flexor retinaculum (volar carpal ligament), superficial muscles on the thenar eminence, other areas of the thumb metacarpal, APB, or FPB (451,544,549,550). The septum in the first dorsal compartment may have several variations as well. In 24% to 34% of specimens in anatomic studies, the first compartment was found to be subdivided by a longitudinal ridge and septum into two distinct osteofibrous tunnels, an ulnar one for the EPB and a radial one containing one or more slips of the APL (451,541–543). The reported incidence of separate compartments at surgery is higher than that seen in anatomic specimens in several series (539,548,550–552,554), which, as noted by Wolfe, raises the possibility that septationw increases the probability that nonsurgical treatment will fail (451).

The abductor pollicis tertius (extensor atque abductor pollicis accessorius) is a rare anomalous muscle that arises from the dorsal aspect of the radius with the APL and inserts, after coalescing with the APB, onto the thumb metacarpal (11). Clinical Implications: Abductor Pollicis Longus The APL and EPB often are afflicted with tendonitis, resulting in the well known de Quervain’s tenosynovitis (534–553). The disease often is referred to as stenosing tenovaginitis of the first dorsal compartment. As noted earlier under Anomalies and Variations, several studies have shown a double tendon was more common that a single tendon, with the single APL and EPL pattern occurring less than 20% of the time (451,534,537,538,540,542,544,546,549, 550,554,555). The number of variations in tendon structure and organization in the first dorsal compartment are among the greatest of the upper extremity muscles. Failure to recognize these variations potentially leads to persistence or recurrence of pain after operative procedures because of incomplete surgical release of the tendon sheath (535,536, 545). The variability of the septa in the first dorsal compartment may also be related to the incidence of stenosing tenosynovitis. The reported incidence of separate compartments at surgery is higher than that seen in anatomic specimens in several series (539,548–552,554). Wolfe has noted that this raises the possibility that septation of the EPB increases the probability that nonsurgical treatment will fail. Harvey et al. reported success with one or two steroid injections in 80% of patients and found separate compartments for the APL and EPB in 10 of 11 wrists that failed injection and required surgical release (539). It also has been noted that observations at surgical release suggest that either one of both subdivisions of the first dorsal compartment may be stenotic (544). The radial nerve and, to a lesser extent, the radial artery are at risk for injury during surgical release of the first dorsal compartment (451). The radial artery passes diagonally across the anatomic snuffbox from the volar aspect of the wrist to the dorsum of the web space deep to the APL, EPB, and EPL. It is separated from the first dorsal compartment by areolar tissue, and usually is not at risk if the floor of the compartment sheath is not perforated distal to the radial styloid. The radial nerve, however, has two or three terminal divisions that lie superficial to the first dorsal compartment and must be identified and protected during the surgical procedure (451,534,547). Radial neuroma is a not uncommon complication, and can result in failure of treatment. Intersection syndrome is a condition of pain and swelling in the region of the muscle bellies of the APL and EPB. As noted by Wolfe, this area lies approximately 4 cm

2 Muscle Anatomy

proximal to the wrist joint, and may show increased swelling of a normally prominent area (451). In severe cases, redness and crepitus have been noted. The syndrome originally was thought to be due to friction and inflammation between the APL and EPB muscle bellies and the muscle bellies of the ECRL and ECRB (451–455). More recently, Grundberg and Reagan have demonstrated that the basic pathologic process appears to be tenosynovitis of the ECRL and ECRB (455). EXTENSOR POLLICIS BREVIS Derivation and Terminology. Extensor is derived from the Greek and Latin ex, which indicates “out of,” and the Latin tendere, “to stretch”; thus, extension indicates a motion to stretch out, and extensor is usually applied to a force or muscle that is involved in the “stretching out or straightening out” of a joint. Pollicis is derived from the Latin pollex, “thumb.” Brevis is the Latin for “short.” Therefore, extensor pollicis brevis indicates a short thumb extensor (1,2). Origin. The distal end of the middle third of the radius, on the medial portion of the posterior surface of the radius and adjacent interosseous ligament. This origin is distal to the origins of the APL and EPL. The muscle also may have origin attachments to the ulna. Insertion. The base of the proximal phalanx of the thumb. Innervation. Posterior interosseous nerve (C7, C8). Vascular Supply. The posterior interosseous artery, continuation and the perforating branches of the anterior interosseous artery. The tendon receives vascularity from the radial artery in the anatomic snuffbox from branches to the radial side of the thumb, and from the first dorsal metacarpal artery and dorsal carpal arch (3,4,11,13). Principal Action. Extension of the proximal phalanx of the thumb. It also assists with extension of the thumb metacarpal. Gross Anatomic Description: Extensor Pollicis Brevis The EPB lies close to the APL, and takes origin from the radial diaphysis and adjacent interosseous ligament just distal to that of the APL (see Fig. 2.3B) (3,4,11,13,68). It comprises one of the many muscles of the dorsal muscle compartment of the forearm (Appendix 2.2). The area on the radius includes a portion of the distal part of the middle third, along the medial border of the dorsal surface. Approximately half of the origin is also from the adjacent interosseous ligament. There may be rare attachments to the adjacent ulna. The muscle fibers converge in a radial direction toward the thumb, just distal to and adjacent to the path of the APL. The EPB usually is thinner than the APL. The EPB along with the APL crosses obliquely and

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superficially to the ECRB and ECRL. In the distal forearm, the EPB and APL are superficial to the most distal portion of the brachioradialis. The myotendinous junction of the EPB forms just proximal to the extensor retinaculum. The EPL enters the extensor retinaculum with the APL to comprise the first dorsal compartment. [Editor’s note: The dorsal compartments of the wrist are as follows: the APL and EPB comprise the first dorsal compartment; the ECRL and ECRB form the second; the EPL forms the third; the EDC and EIP form the fourth; the EDM forms the fifth; and the ECU forms the sixth (6).] The tendon anatomy and presence of septa in the first dorsal compartment commonly show anatomic variations and anomalies (see earlier, under Abductor Pollicis Longus, Gross Anatomic Description and Anomalies and Variations). In general, in approximately 24% to 34% of specimens in anatomic studies, the first compartment has been found to be subdivided by a longitudinal ridge and septum into two distinct osteofibrous tunnels, the ulnar one for the EPB and the radial one containing one or more slips of the APL (451,541–543). Muscle fibers often extend to the proximal edge of the extensor retinaculum. In the first dorsal compartment, the tendon is located on the radial side of the radial metaphysis. The tendon is parallel with the ulnar border of the APL tendon, and together the tendons pass through the fibroosseous compartment. The EPL then crosses the dorsoradial carpus to extend distally on the dorsal aspect of the thumb metacarpal (see Fig. 2.6B). It remains radial to the EPL tendon. The EPB then inserts into the base of the proximal phalanx of the thumb. It also may send slips to the capsule of the MCP joint (3,4,11,13). The EPB is innervated by the posterior interosseous nerve, mostly from C7 with additional contributions from C8. There usually is a single motor branch that supplies the EPB. The nerve branch usually arises in common with or near the nerve to the APL. The nerve may cross the APL to reach the EPB. The motor nerve to the EPB enters the muscle in the proximal third, usually along the radial border (11). Actions and Biomechanics: Extensor Pollicis Brevis The EPB functions mainly to extend the proximal phalanx of the thumb. Because it crosses the thumb carpometacarpal joint, the tendon also assists with extension of the thumb metacarpal. In addition, at extremes of contraction, it assists with radial deviation of the wrist (3,4). Anomalies and Variations: Extensor Pollicis Brevis Several variations of the septa and tendon slips in the first dorsal compartment exist (see earlier, under Anomalies and Variations: Abductor Pollicis Longus).

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The EPB is absent in 5% to 7% of individuals (544,549,550). The EPB may have an anomalous tendon slip that extends to the base of the thumb distal phalanx as well the normal insertion into the base of the proximal phalanx. Rarely, it inserts only onto the distal phalanx. The muscle also may have a tendon slip to the thumb metacarpal (11,556,557). The EPB may coalesce with the APL, forming one muscle, and inserts into the thumb metacarpal (11). The EPB may exist as a double tendon (11). Rarely, the tendon coalesces with the EPL (3). Clinical Implications: Extensor Pollicis Brevis See also Clinical Implications: Abductor Pollicis Longus. The EPB and APL are the tendons involved with de Quervain’s tenosynovitis (see earlier, under Clinical Implications: Abductor Pollicis Longus). Yuasa and Kiyoshige have suggested that the EPB is the main tendon involved, and have demonstrated successful resolution of symptoms after decompression of the EPB alone (558). Intersection syndrome is a condition of pain and swelling in the region of the muscle bellies of the APL and EPB. As noted by Wolfe, this area lies approximately 4 cm proximal to the wrist joint, and may show increased swelling of a normally prominent area (451). In severe cases, redness and crepitus have been noted. The syndrome originally was thought to be due to friction and inflammation between the APL and EPB muscle bellies and the muscle bellies of the ECRL and ECRB (451–455). More recently, Grundberg and Reagan have demonstrated that the basic pathologic process appears to be tenosynovitis of the ECRL and ECRB (455). EXTENSOR POLLICIS LONGUS Derivation and Terminology. Extensor derived is from the Greek and Latin ex, which indicates “out of,” and the Latin tendere, “to stretch”; thus, extension indicates a motion to stretch out, and extensor usually is applied to a force or muscle that is involved in the “stretching out or straightening out” of a joint. Pollicis is derived from the Latin pollex, “thumb.” Longus is derived from the Latin longus, indicating “long.” Therefore, extensor pollicis longus indicates the long extensor of the thumb (1,2). Origin. The dorsal middle third of the ulna and adjacent interosseous ligament. Insertion. The base of the distal phalanx of the thumb. Innervation. Posterior interosseous nerve (C7, C8). Vascular Supply. The posterior interosseous artery, continuation and the perforating branches of the anterior interosseous artery. The tendon receives vascularity from the radial artery in the anatomic snuffbox from branches to the radial side of the thumb, and from the

first dorsal metacarpal artery and dorsal carpal arch (3,4, 11,13,559). Principal Action. Extension of the distal phalanx of the thumb. Also contributes to extension of the proximal phalanx and the thumb metacarpal through the MCP and carpometacarpal joints, respectively. Gross Anatomic Description: Extensor Pollicis Longus The EPL is a deep extensor of the dorsal forearm situated between the EIP (ulnarly) and the EPB (radially) (3,4,11,14). It is one of the many muscles that comprise the dorsal muscle compartment of the forearm (Appendix 2.2). It is much larger than the EPB. The EPL arises from the dorsal middle third of the ulna, chiefly on its radial border (see Fig. 2.3B). In addition, at least half of the muscle takes origin from the adjacent interosseous ligament. Portions of the muscle also arise from the septa between the EPL and the EIP and ECU. The muscle courses obliquely in a radial direction as it extends distally, in the direction of the thumb. The muscle fibers converge in a bipenniform manner on the two sides of a flattened tendon that first appears proximally on the dorsal surface of the muscle. The EPL is initially deep to the EDC, crossing obliquely toward the thumb to emerge from the EDC and enter the extensor retinaculum just radial to the EDC. The muscle belly usually is fusiform (7,8). The myotendinous junction is located deep to the EDC, proximal to the extensor retinaculum, and muscle fibers may continue with the tendon as far distally as the extensor retinaculum. The EPL then enters its own fibroosseous tunnel at the extensor retinaculum to form the third dorsal compartment. [Editor’s note: The dorsal compartments of the wrist are as follows: the APL and EPB comprise the first dorsal compartment; the ECRL and ECRB form the second; the EPL forms the third; the EDC and EIP form the fourth; the EDM forms the fifth; and the ECU forms the sixth (6).] The path through the third compartment continues in an oblique direction toward the thumb. It is stabilized in part by a narrow groove in the distal radius, and passes ulnar to Lister’s tubercle before taking a more oblique direction. The tendon in this region appears to have a slightly smaller cross-sectional area (560), and is relatively poorly vascularized (561). This area also coincides with an area commonly affected by closed rupture (see later, under Clinical Implications: Extensor Pollicis Longus). The tendon exits the third compartment over the distal radius or radiocarpal joint. It passes across the dorsal surface of the carpus, superficial to the tendon of the ECRL and ECRB, to the dorsum of the thumb metacarpal. It is located ulnar to the EPB, and, in the region of the radial styloid and scaphoid, the EPL and EPB form a triangular depression (when the thumb is in full extension). This depression, referred to as the anatomic snuffbox, lies over scaphoid, and point tenderness in this area usually indicates injury to the scaphoid (or possibly the radial styloid). The EPL remains ulnar to the EPB but becomes

2 Muscle Anatomy

adjacent to the EPB just proximal to the MCP joint. The EPL tendon continues distally on the dorsal surface of the proximal phalanx and expands to insert onto the base of the distal phalanx (see Fig. 2.6B). The tendon becomes an aponeurosis as it is joined by the tendon of the APB laterally and the first palmar interosseous and adductor pollicis medially. Together, the EPL with the thumb intrinsic muscles form the aponeurosis that comprises the extensor mechanism of the thumb (3,4,11,562). The EPL is innervated by the posterior interosseous nerve, chiefly from C7, but also from C8 and C6. There usually initially is one branch to the EPL that may divide before entering the muscle belly. The motor branches usually enter the muscle in the proximal third, usually into the radial border. Actions and Biomechanics: Extensor Pollicis Longus The EPL functions mainly for extension of the distal phalanx of the thumb. It also contributes to extension of the proximal phalanx (working with the EPB) and to extension of the thumb metacarpal (working with the APL) (563). In extremes of contraction, it can contribute to radial deviation of the wrist. When the thumb is in full extension, the EPL also can contribute to adducting the thumb toward the index metacarpal. Anomalies and Variations: Extensor Pollicis Longus Most of the variations of the EPL involve variations in the distal tendon. There may be an accessory slip to the base of the carpal bones (especially the capitate), to the index finger (distal phalanx), to the EPB, or to the extensor retinaculum (11,564–569). A double tendon or double muscle belly may exist. An accessory EPL in the third dorsal compartment has caused dorsal wrist pain that resolved after excision of the accessory EPL (570). Extensor communis pollicis et indicis is an anomalous muscle found in approximately 6% of dissected specimens. It crosses between the EIP and the EPL. The muscle may have two tendons that insert into the distal phalanges of the thumb and the index finger. The muscle may replace the EPL or EIP (11,564). Clinical Implications: Extensor Pollicis Longus Closed rupture of the EPL is well documented (571–588), and has been associated with tendon injury after fractures of the distal radius (589–605), or inflammatory conditions such as rheumatoid arthritis (590,606,607). Nonunion also has been associated with EPL ruptures. Ruptures often occur even after nondisplaced fractures, usually in the

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region of the distal radius. Engkvist and Lundborg have shown that in the common area of the rupture, there is a relatively poorly vascularized portion of tendon (561). In addition, Wilhelm and Qvick have shown that the crosssectional area of the tendon in this area is slightly smaller (560). These factors may play a role in the closed or delayed ruptures of the EPL (especially those associated with nondisplaced fractures, where tendon injury or attrition from uneven bone edges is unlikely). In patients with rheumatoid arthritis, the EPL is at risk for rupture at the level of Lister’s tubercle, due to either chronic tenosynovitis (590) at the dorsal wrist or local attrition against the friction point at the tubercle (especially if there is bony irregularity from chronic arthritis). Closed rupture also has occurred after use of anabolic steroids (608). The EPL also is subject to subluxation or dislocation, usually associated with rupture or damage to the radial side of the extensor hood on the dorsum of the MCP joint of the thumb. The EPL subluxates to the ulnar side (609,610). Dislocation also can occur after fracture of the distal radius (611,612). The EPL can be affected by tenosynovitis, or triggering, as it courses through the third dorsal compartment (613–616). ABDUCTOR POLLICIS BREVIS Derivation and Terminology. Abductor is derived from the Latin ab, meaning “away from,” and ducere, which means “to draw”; therefore, abductor is “that which draws away from.” Pollicis is derived from the Latin pollex, “thumb.” Brevis is Latin for “short” (1,2). Therefore, abductor pollicis brevis indicates a short thumb abductor. Origin. From the flexor retinaculum, scaphoid tubercle, trapezial ridge or tubercle. Insertion. To the base of the thumb proximal phalanx, palmar surface. Innervation. Recurrent branch of the median nerve (C8, T1). Vascular Supply. The radial artery and superficial palmar arch. Principal Action. Palmar abduction of the thumb (pulling the thumb away from the palm) at right angles to the palm. In addition, the APB contributes to flexion of the proximal phalanx of the thumb. Through the superficial layer of the APB that continues distally and dorsally to reach the EPL, the APB contributes to extension of the thumb distal phalanx as part of the extensor mechanism (3,4,68). Gross Anatomic Description: Abductor Pollicis Brevis The APB, along with the opponens pollicis, FPB, and adductor pollicis, comprises one of the thenar muscles (3,4,

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11,13,68). In terms of muscle compartments, it is one of the three muscles that comprise the thenar muscle compartment of the hand. (The adductor pollicis has a separate compartment, Appendix 2.2). The APB is located subcutaneously on the radial aspect of the thenar eminence, and constitutes the shape and contour of the radial border of the thenar eminence. The muscle is flat and broad, and covers the opponens pollicis and approximately 30% of the FPB. The ABP arises mostly from the flexor retinaculum (see Fig. 2.6A). Fibers also arise from the scaphoid tubercle, the trapezial tubercle, and possibly from the terminal tendon or tendon sheath of the APL, as the APL inserts onto the base of the thumb metacarpal (496,617). The muscle courses distally and radially toward the thumb, located as a superficial thenar muscle, in line with the thumb metacarpal. The muscle fibers converge into a flat tendon. It joins the fibers of the FPB. The muscle of the APB often consists of two layers or bellies, a deep (or medial layer) and a superficial (or lateral) layer. The deep layer inserts onto the radial sesamoid and radial side of the base of the proximal phalanx of the thumb (see Fig. 2.6A). The superficial layer continues radially and dorsally to join the aponeurosis of the EPL as part of the extensor mechanism of the thumb. The APB is innervated by the recurrent branch of the median nerve. This usually is the first branch from the lateral side of the median nerve in the hand. The nerve

receives contributions mostly from T1 and C8. The nerve takes a recurrent course proximally and laterally superficial to or through the superficial division of the FPB and enters the deep surface of the APB in the middle third near its ulnar border (11). Actions and Biomechanics: Abductor Pollicis Brevis The APB functions mainly to provide palmar abduction of the thumb (pulling the thumb away from the palm, at right angles to the palm. The APB also contributes to flexion of the proximal phalanx of the thumb. A superficial layer of the distal tendon of the APB continues radially and dorsally past the MCP joint of the thumb to reach and attach to the tendon of the EPL. Through this aponeurosis, the APB becomes part of the extensor mechanism of the thumb and contributes to extension of the distal phalanx of the thumb. Its extensor function of the distal phalanx is relatively weak (496). From architectural studies on the muscle’s physiologic cross-sectional area, muscle length, muscle fiber length, and muscle mass, it can be seen that the muscle architecture is fairly close to that of the other thenar muscles (466). It therefore would have similar relative abilities for force generation, velocity, and excursion (466) (Table 2.4 and Fig. 2.13).

TABLE 2.4. ARCHITECTURAL FEATURES OF INTRINSIC MUSCLES OF THE HAND Muscle (n = 9) ADM APB APL AP DI 1 DI 2 DI 3 DI 4 EPB FDM FPB Lum 1 Lum 2 Lum 3 Lum 4 ODM OP PI 2 PI 3 PI 4

Muscle Mass (g) 3.32 2.61 9.96 6.78 4.67 2.65 2.01 1.90 2.25 1.54 2.58 0.57 0.39 0.37 0.23 1.94 3.51 1.56 1.28 1.19

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

1.67 1.19 2.01 1.84 1.17 1.01 0.60 0.62 1.36 0.44 0.56 0.19 0.22 0.16 0.11 0.98 0.89 0.22 0.28 0.33

Muscle Length (mm) 68.4 ± 6.5 60.4 ± 6.6 160.4 ± 15.0 54.6 ± 8.9 61.9 ± 2.5 62.8 ± 8.1 54.9 ± 4.6 50.1 ± 5.3 105.6 ± 22.5 59.2 ± 10.4 57.2 ± 3.7 64.9 ± 10.0 61.2 ± 17.8 64.3 ± 8.9 53.8 ± 11.5 47.2 ± 3.6 55.5 ± 5.0 55.1 ± 5.0 48.2 ± 2.9 45.3 ± 5.8

Fiber Length (mm) 46.2 41.6 58.1 34.0 31.7 25.1 25.8 25.8 55.0 40.6 41.5 55.4 55.5 56.2 50.1 19.5 35.5 25.0 26.0 23.6

± 7.2 ± 5.6 ± 7.4 ± 7.5 ± 2.8 ± 6.3 ± 3.4 ± 3.4 ± 7.5 ± 13.7 ± 5.2 ± 10.2 ± 17.7 ± 10.7 ± 8.4 ± 4.1 ± 5.1 ± 5.0 ± 4.3 ± 2.6

Pennation Angle (Degrees)

Cross-Sectional Area (cm2)

Fiber Length/ Muscle Length Ratio

3.9 ± 1.3 4.6 ± 1.9 7.5 ± 2.0 17.3 ± 3.4 9.2 ± 2.6 8.2 ± 3.1 9.8 ± 2.8 9.4 ± 4.2 7.2 ± 4.4 3.6 ± 1.0 6.2 ± 4.5 1.2 ± 0.9 1.6 ± 1.3 1.1 ± 0.8 0.7 ± 1.0 7.7 ± 2.9 4.9 ± 2.5 6.3 ± 2.2 7.7 ± 3.9 8.2 ± 3.5

0.89 ± 0.49 0.68 ± 0.28 1.93 ± 0.59 1.94 ± 0.39 1.50 ± 0.40 1.34 ± 0.77 0.95 ± 0.45 0.91 ± 0.38 0.47 ± 0.32 0.54 ± 0.36 0.66 ± 0.20 0.11 ± 0.03 0.08 ± 0.04 0.08 ± 0.04 0.06 ± 0.03 1.10 ± 0.43 1.02 ± 0.35 0.75 ± 0.25 0.65 ± 0.26 0.61 ± 0.23

0.68 ± 0.10 0.69 ± 0.09 0.36 ± 0.05 0.63 ± 0.15 0.51 ± 0.05 0.41 ± 0.13 0.47 ± 0.07 0.52 ± 0.11 0.54 ± 0.13 0.67 ± 0.17 0.73 ± 0.08 0.85 ± 0.03 0.90 ± 0.05 0.87 ± 0.07 0.90 ± 0.05 0.41 ± 0.09 0.64 ± 0.07 0.45 ± 0.08 0.54 ± 0.08 0.52 ± 0.10

ADM, abductor digiti minimi; APB, abductor pollicis brevis; APL, abductor pollicis longus; AP, adductor pollicis; DI 1–4, dorsal interosseous muscles; EPB, extensor pollicis brevis; FDM, flexor digiti minimi; FPB, flexor pollicis brevis; Lum 1–4, lumbrical muscles; ODM, opponens digiti minimi; OP, opponens pollicis; PI 2–4, palmar interosseous muscles. Values represent mean ± standard deviation. Reproduced from Jacobson MD, Raab R, Fazeli BM, et al. Architectural design of the human intrinsic hand muscles. J Hand Surg [Am] 17:804–809, 1992, with permission.

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A

B

C

D

E FIGURE 2.13. Architectural features of the intrinsic muscles of the hand. A: Intrinsic muscle lengths. Note the short, uniform lengths. B: Intrinsic muscle fiber lengths. There is more disparity in fiber length than in muscle length. This illustrates the relatively large excursions of the relatively short intrinsic muscles. C: Intrinsic muscle masses. The intrinsic muscles have low masses, with the exception of the first dorsal interosseous (DI1) and the AddP. D: Intrinsic muscle crosssectional areas. The interossei have greater cross-sectional areas than the smaller lumbrical muscles, and in general the lumbrical fibers are longer. This would indicate that the lumbricals are designed more for excursion or velocity and less for force generation. E: Intrinsic muscle fiber length/muscle length (FL/ML) ratios. Note the high FL/ML ratio of the intrinsic muscles, especially the lumbricals, demonstrating their relative design for excursion and velocity. The lumbricals have among the highest FL/ML ratios of all muscles studied (both extrinsic and intrinsic), and this indicates their specialization for excursion (and velocity) and their relatively poor design for force production. Bars represent mean ± standard deviation (SEM). AbDM, abductor digiti minimi; AbPB, abductor pollicis brevis; AbPL, abductor pollicis longus; AddP, adductor pollicis; DI1–DI4, dorsal interosseous muscles 1–4; EPB, extensor pollicis brevis; FDM, flexor digiti minimi; FPB, flexor pollicis brevis; L1–L4, lumbrical muscles 1–4; ODM, opponens digiti minimi; OpP, opponens pollicis; PI2–PI4, palmar interosseous muscles 2–4. (From Jacobson MD, Raab R, Fazeli BM, et al. Architectural design of the human intrinsic hand muscles. J Hand Surg [Am] 17:804–809, 1992, with permission.)

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Anomalies and Variations: Abductor Pollicis Brevis The APB may have two separate heads (besides the two distal layers, as discussed previously) (11). The APB may be absent (11,618). The muscle may have attachments to several other neighboring structures. These include the scaphoid, the radial styloid, the adductor pollicis, the EPL or EPB, opponens pollicis, palmaris longus, ECRL (accessory ECR), or FPL (11,619,620). An entire third head may arise from the opponens pollicis (11). Clinical Implications: Abductor Pollicis Brevis Paralysis or laceration of the distal median nerve usually results in thenar paralysis (as well as loss of sensibility on the radiopalmar hand). Loss of thenar function results in difficulty with attempted palmar abduction of the thumb (bringing the thumb out of the palm). Therefore, despite functioning of the adductor pollicis and FPL, thumb opposition with the digits remains difficult. To restore thumb opposition, several opponensplasty procedures have been described. Muscles used for transfer for opponensplasty include the EIP, the FDS to the ring finger, the abductor digiti minimi (Huber transfer), or the palmaris longus elongated by a strip of the palmar fascia (Camitz or Braun transfer, more commonly used with severe carpal tunnel syndrome and thenar dysfunction) (266–272, 621–624). FLEXOR POLLICIS BREVIS Derivation and Terminology. Flexor is derived from the Latin flexus, indicating “bent” (and flexor, which indicates “that which bends,” or “bending”). Pollicis is derived from the Latin pollex, “thumb.” Brevis is the Latin for “short.” Therefore, flexor pollicis brevis indicates a short thumb flexor (1,2). Origin. From two heads, superficial and deep. Superficial head: from the trapezium, adjacent flexor retinaculum, and the tendon sheath of the FCR. Deep head: from the trapezoid and capitate, and from the palmar ligament from the distal carpal row. Insertion. Superficial head: to the radial side of the anterior aspect of the proximal thumb phalanx. Deep head: inserts into a tendon that connects with the superficial head. Innervation. Variable; classically, the recurrent branch of the median nerve supplies the superficial head; the terminal branch of the ulnar nerve supplies the deep head. Either head may be supplied by either the recurrent branch of the median nerve or by the ulnar nerve (see later).

Vascular Supply. The radial artery, superficial palmar branch, branches from the opponens pollicis, and the radialis indicis (3,4,11). Principal Action. Flexion of the MCP joint of the thumb. Gross Anatomic Description: Flexor Pollicis Brevis The FPB lies medial and slightly deep to the APB (3,4,7,8). It helps comprise the thenar muscle compartment of the hand (Appendix 2.2). It has two heads, a superficial and a deep (625). The superficial head arises from the distal border of the flexor retinaculum and the distal part of the tubercle of the trapezium (see Fig. 2.6A). The superficial head also may have origin attachments to the tendon sheath of the FCR. The superficial head courses obliquely toward the base of the thumb to reach the radial side of the base of the proximal phalanx (Fig. 2.6A). The deep head arises from the trapezoid and capitate and from the palmar ligaments of the distal row of the carpus (see Fig. 2.6A). The deep head passes deep to the tendon of the FPL and joins the superficial head on the sesamoid bone and base of the first phalanx. An additional muscle head or fascicle has been described by Tountas and Bergman (11). It arises from the ulnar side of the base of the thumb metacarpal and the adjacent carpal ligaments. It inserts onto the ulnar side of the base of the proximal phalanx (see Fig. 2.6A). This fascicle sometimes is considered to be the deep head of the FPB. It is closely joined to the carpal head of the adductor pollicis, and the two muscles share a common tendon. Some fibers of the medial division of the tendon may be traced into the aponeurosis of the extensor tendon. It has been suggested that this portion of the muscle represents a first palmar interosseous. This component of the FPB remains controversial (11). The architectural features of the muscle are listed in Table 2.4. The innervation of the FPB appears to be quite variable (625). Classic descriptions suggest that the superficial head usually is supplied by the lateral terminal branch of the median nerve, and the deep head by the deep branch of the ulnar nerve (3,4,68). More recently, the variable innervation has been described, and various combinations exist. The muscle usually is supplied chiefly by branches that originate from the recurrent branch of the median nerve. The branch penetrates the muscle in the region of the carpal tunnel. Additional branches derived from the ulnar nerve also often are found, and usually supply the deep portion. Contributions from both the median and ulnar nerve were found in 19 of 29 cases. In 5 cases, the median nerve alone supplied FPB, and in 5 the ulnar nerve alone supplied the FPB muscles. In addition, when evaluating

2 Muscle Anatomy

innervation specifically of the deep head, the deep head was supplied by the ulnar nerve in 16 of 24 cases, by the median nerve in 3 of 24 cases, and by both nerves in 5 of 24 cases (11,625). Actions and Biomechanics: Flexor Pollicis Brevis The FPB functions primarily to provide flexion of the MCP joint of the thumb, as well as flexion of the carpometacarpal joint of the thumb. It also contributes to rotation of the thumb in the medial direction (in preparation for opposition). From its contributions into the extensor mechanism of the thumb, the FPB contributes to extension of the distal phalanx of the thumb (3,4,68). Anomalies and Variations: Flexor Pollicis Brevis A relatively common observation is the coalescing of the superficial head with the opponens pollicis. The deep head is variable in size and may be absent. The entire FPB may be absent (11). Clinical Implications: Flexor Pollicis Brevis Paralysis or laceration of the distal median nerve usually results in thenar paralysis (as well as loss of sensibility on the radiopalmar hand). Loss of thenar function results in difficulty with attempted palmar abduction of the thumb (bringing the thumb out of the palm). Therefore, despite functioning of the adductor pollicis and FPL, thumb opposition with the digits remains difficult. To restore thumb opposition, several opponensplasty procedures have been described. Muscles used for transfer for opponensplasty include the EIP, the FDS to the ring finger, the abductor digiti minimi (Huber transfer), or the palmaris longus elongated by a strip of the palmar fascia (Camitz or Braun transfer, more commonly used with severe carpal tunnel syndrome and thenar dysfunction) (266–272, 621–624).

OPPONENS POLLICIS Derivation and Terminology. Opponens is the Latin indicating the movement against or toward an opposing structure. Pollicis is derived from the Latin pollex, “thumb” (1,2). Origin. From the tubercle of the trapezium and from the flexor retinaculum. Insertion. To the radial and palmar aspect of the thumb metacarpal.

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Innervation. Recurrent branch of the median nerve (T1 and C8). A branch from the deep branch of the ulnar nerve also may contribute. Vascular Supply. The radial artery, superficial palmar branch, first palmar metacarpal artery, arteria princeps pollicis, arteria radialis indicis, deep palmar arch (3,4,11,13,14). Principal Action. Flexion, adduction, and median rotation of the thumb metacarpal (contributing to the motion of opposition). Gross Anatomic Description: Opponens Pollicis The opponens pollicis is a deep thenar muscle covered anteriorly by the APB (Appendix 2.2). It originates from the tubercle of the trapezium and from the flexor retinaculum (see Fig. 2.6A). It courses obliquely toward the thumb metacarpal to insert onto the lateral and anterior aspects of the diaphysis of the thumb metacarpal (see Fig. 2.6A). The muscle usually covers the entire lateral part of the palmar surface of the shaft (3,4). The architectural features of the muscle are listed in Table 2.4. The opponens pollicis is innervated by the recurrent branch of the median nerve. The branch takes a recurrent course proximally and laterally, superficial to or through the superficial divisions of the FPB near its origin. The nerve provides one or two branches that enter the palmar surface of the proximal third of the opponens pollicis near its ulnar border (11). The nerve arises from C6, C7, and C8. As with the FPB, the deep branch of the ulnar nerve can provide various contributions. A double innervation of both the recurrent branch of the median nerve and the deep branch of the ulnar nerve was noted in 92 of 120 hands (625–627). Because of the frequent duel innervation, it has been suggested that double innervation with the median and ulnar nerves be considered the normal (3). Actions and Biomechanics: Opponens Pollicis The opponens pollicis functions mainly to provide flexion, adduction, and medical rotation of the thumb metacarpal (contributing to the motion of opposition) (3,4). Opposition occurs when the thumb is flexed, palmarly abducted, and rotated medially so that the palmar surface of the thumb opposes the palmar surface of the digits. The opponens pollicis does not cross the MCP joint (as does the APB and FPB), and therefore does not contribute to flexion of the proximal phalanx of the thumb. Anomalies and Variations: Opponens Pollicis The opponens pollicis may coalesce with the FPB (11). Two heads of the opponens pollicis may be present (11). Complete absence has been reported, but is rare (11).

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Clinical Implications: Opponens Pollicis Paralysis or laceration of the distal median nerve usually results in thenar paralysis (as well as loss of sensibility on the radiopalmar hand). Loss of thenar function results in difficulty with attempted palmar abduction of the thumb (bringing the thumb out of the palm). Therefore, despite functioning of the adductor pollicis and FPL, thumb opposition with the digits remains difficult. To restore thumb opposition, several opponensplasty procedures have been described. Muscles used for transfer for opponensplasty include the EIP, the FDS to the ring finger, the abductor digiti minimi (Huber transfer), or the palmaris longus elongated by a strip of the palmar fascia (Camitz or Braun transfer, more commonly used with severe carpal tunnel syndrome and thenar dysfunction) (266–272,621–624).

ADDUCTOR POLLICIS

usually contains a sesamoid bone. The tendon inserts into the ulnar side of the base of the proximal phalanx of the thumb (see Fig. 2.6A). Additional fibers may pass more obliquely deep to the tendon of the FPL to attach to the lateral portion of the FPB and the APB (3,4). The transverse head (deep head, metacarpal head) arises from the long finger metacarpal. Its origin is a broad attachment that includes the distal two-thirds of the palmar surface of the long metacarpal along the palmar ridge. It also may arise from the deep palmar fascia of the third interspace and, occasionally, from the deep fascia of the fourth interspace and from the capsules of the second, third, and fourth MCP joints. It is more deeply situated than the thenar muscles. The transverse head is triangular and converges in a radial direction toward the base of the proximal phalanx of the thumb. Its distal border usually lies transverse to the axis of the upper limb. The tendon continues toward the proximal thumb phalanx to join the tendon of the oblique head. The common tendon inserts onto the ulnar side of the base of the proximal phalanx of the thumb (3,4,7,8,11,13,14) (Fig. 2.6A). A sesamoid bone usually is found in the tendon, just proximal to the MCP joint. The architectural features of the muscle are listed in Table 2.4. The adductor pollicis is innervated by the deep branch of the ulnar nerve, from T1 and C8. The deep branch of the ulnar nerve, along with the deep palmar arterial arch, passes through the interval created between the oblique and transverse heads of the muscle (3,4).

Derivation and Terminology. Adductor is derived from the Latin adducere, which means “to draw toward.” Pollicis is derived from the Latin pollex, “thumb” (1,2). Origin. Two heads. Oblique head: arises from the capitate, bases of the second and third metacarpals, intercarpal ligaments, and sheath of the FCR. Transverse head: arises from the distal two-thirds of the palmar surface of the third metacarpal. Insertion. Oblique and transverse heads unite to insert into ulnar side of the base of the proximal phalanx of the thumb. Innervation. Deep branch of the ulnar nerve (C8, T1). Vascular Supply. Arteria princeps pollicis, arteria radialis indicis, or combined artery as the first palmar metacarpal artery, deep palmar arch (3,4,11). Principal Action. Moves the thumb proximal phalanx from an abducted position toward the palm of the hand. It therefore adducts the thumb proximal phalanx. It also assists with adduction of the thumb metacarpal.

The two heads usually work together. The muscle moves the thumb proximal phalanx from an abducted position toward the palm of the hand. It therefore adducts the thumb proximal phalanx. It also assists with adduction of the thumb metacarpal. The adductor pollicis works with greatest advantage when the thumb is abducted (3,4,11).

Gross Anatomic Description: Adductor Pollicis

Anomalies and Variations: Adductor Pollicis

The adductor pollicis lies deep to the extrinsic flexor tendons and radial lumbricals. It occupies its own muscle compartment (Appendix 2.2). The muscle consists of two heads, an oblique and a transverse. The oblique head (carpal head) takes origin from several slips, including the palmar capitate, the base of the second and third metacarpals, the intercarpal ligaments, the sheath of the FCR, and possibly from a slip from the flexor retinaculum (3,4,7,8,11,13,14) (see Fig. 2.6A). From this origin, the muscle fibers converge and pass distally and radially toward the base of the proximal phalanx of the thumb. The fibers converge into a common tendon (joined by the transverse head). The tendon

The two heads of the adductor pollicis vary in size. The two heads can be coalesced to various degrees. The muscle also can be split into additional bellies (11). The transversus manum muscle is an anomalous muscle closely related to the adductor pollicis. It arises from the palmar MCP ligaments and connects to the base of the thumb proximal phalanx, or in its vicinity (11).

Actions and Biomechanics: Adductor Pollicis

Clinical Implications: Adductor Pollicis The adductor pollicis may contribute to thumb-in-palm deformity in patients with muscle spasticity (cerebral palsy,

2 Muscle Anatomy

traumatic brain injury, stroke). Release of the origin of the adductor pollicis (muscle recession) often is incorporated in muscles lengthened or released to help correct the deformity. Care must be taken to protect the deep palmar arterial arch and the deep branch of the ulnar nerve, both of which pass through the interval created by the two heads of the muscle. PALMARIS BREVIS Derivation and Terminology. Palmaris is derived from the Latin palma, which means “pertaining to the palm.” Brevis is the Latin for “short” (1,2). Origin. From the flexor retinaculum and medial border of the central part of the palmar fascia. Insertion. Inserts into dermis on the ulnar border of the hand. Vascular Supply. The superficial palmar arch. Principal Action. The palmaris brevis wrinkles the skin on the ulnar side of the palm of the hand. It deepens the hollow of the palm by accentuating the hypothenar eminence. Gross Anatomic Description: Palmaris Brevis The palmaris brevis is a small, thin muscle located in the skin and subcutaneous tissue of the ulnar palm. It is quadrangular and arises from the flexor retinaculum and medial border of the central part of the palmar aponeurosis. The fibers are perpendicular to the axis of the upper extremity, and insert into the dermis on the ulnar border of the hand. This muscle is superficial to the ulnar artery and terminal branches of the ulnar nerve (3,4,11). The palmaris brevis is innervated by the superficial branch of the ulnar nerve, from C8 and T1. Actions and Biomechanics: Palmaris Brevis In wrinkling the skin on the ulnar side of the palm of the hand and deepening the hollow of the palm, the palmaris brevis may assist with cupping the hands for holding water and may contribute to the security of the palmar grip (3). ABDUCTOR DIGITI MINIMI (ABDUCTOR DIGITI QUINTI) Derivation and Terminology. Abductor is derived form the Latin ab, meaning “away from,” and ducere, which means “to draw”; therefore, abductor is “that which draws away from.” Digiti is the plural of the Latin digitus, “digit.”

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Minimi is from the Latin minima or minimum, indicating the smallest. Abductor digiti minimi therefore indicates the abductor of the smallest digit(s). Quinti is from the Latin quintus, indicating “fifth.” Therefore, the abductor digiti quinti is the abductor of the fifth digit (1,2). Origin. From the pisiform, terminal tendon of the FCU, and the pisohamate ligament. Insertion. Two slips: one slip to the ulnar side of the base of the proximal phalanx of the small finger. The other slip continues dorsally to the ulnar border of the dorsal digital aponeurosis of the EDM. Innervation. Deep branch of the ulnar nerve (C8, T1). Vascular Supply. The ulnar artery, deep palmar branch, ulnar end of the superficial palmar arch, palmar digital artery (3,4,7,8,11,13,14). Principal Action. Abduction of the small finger (proximal phalanx) from the ring finger (thus spreading the fourth web space when the digits are extended). Through its contribution to the extensor mechanism, the abductor digiti minimi may contribute to extension of the middle phalanx (and possibly of the distal phalanx) of the small finger. Gross Anatomic Description: Abductor Digiti Minimi The abductor digiti minimi is the most medial of the three hypothenar muscles (which also include the flexor digiti minimi and opponens digiti minimi; Appendix 2.2). The abductor digiti minimi lies on the ulnar border of the palm. The muscle arises from the pisiform, from the FCU (at the FCU insertion), and from the pisohamate ligament (496) (see Fig. 2.6A). The muscle extends distally along the ulnar palm and splits into two slips. One slip inserts into the ulnar side of the base of the proximal phalanx of the small finger (see Fig. 2.6A). The other slip continues distally and dorsally to join the ulnar border of the EDM (in the dorsal digital aponeurosis) so that it contributes to the extensor mechanism of the digits (3,4,7,8,11,13,14). The architectural features of the muscle are listed in Table 2.4. Actions and Biomechanics: Abductor Digiti Minimi The abductor digiti minimi functions mainly to provide abduction of the small finger (proximal phalanx) from the ring finger (thus spreading the fourth web space when the digits are extended). It also provides some abduction when the digits are tightly adducted in flexion and extension. Through its connection to the extensor mechanism (through the ulnar dorsal slip), the abductor digiti minimi may contribute to extension of the middle phalanx (and possibly of the distal phalanx) of the small finger (3,4,7,8, 11,13,14).

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Anomalies and Variations: Abductor Digiti Minimi Accessory slips may join the muscle from the tendon of the FCU, the flexor retinaculum, the fascia of the distal forearm, or the tendon of the palmaris longus (11). A part of the muscle may insert onto the metacarpal of the small finger (11). Clinical Implications: Abductor Digiti Minimi The abductor digiti minimi can be used to help restore thumb opposition as a donor muscle for opponensplasty. This transfer often is referred to as the Huber transfer, described in 1921 (621–624).

FLEXOR DIGITI MINIMI (FLEXOR DIGITI MINIMI BREVIS) Derivation and Terminology. Flexor is derived from the Latin flexus, indicating “bent” (and flexor, which indicates “that which bends,” or “bending”). Digiti is the plural of the Latin digitus, “digit.” Minimi is from the Latin minima or minimum, indicating “the smallest.” Brevis is the Latin for “short.” Flexor digiti minimi therefore indicates the short flexor of the smallest digit(s) (1,2). Origin. From the hook of the hamate and flexor retinaculum. Insertion. To the ulnar aspect of the base of the proximal phalanx of the small finger. Innervation. Deep branch of the ulnar nerve (T1, C8). Vascular Supply. The ulnar artery, deep palmar branch, ulnar end of the superficial palmar arch, palmar digital artery (3,4,11). Principal Action. Flexion of the proximal phalanx of the small finger. Gross Anatomic Description: Flexor Digiti Minimi The flexor digiti minimi, along with the abductor digiti minimi and opponens digiti minimi, helps form the hypothenar muscles (Appendix 2.2). The muscle lies deep and adjacent to the abductor digiti minimi, along the radial border of the abductor and coursing in the same direction. The muscle takes origin from the convex surface of the hook of the hamate and the palmar surface of the flexor retinaculum (see Fig. 2.6A). The point of origin is slightly more distal than that of the abductor digiti minimi. The muscle extends distally in the same direction and plane as the abductor digiti minimi to reach the insertion at the ulnar side of the base of the proximal phalanx of the small finger. The muscle inserts onto the lateral tubercle of the proximal phalanx (see Fig. 2.6A). The insertion also is adja-

cent to that of the abductor digiti minimi, but located slightly palmar. By this more palmar insertion point, the muscle exerts a flexor force on the proximal phalanx. The flexor digiti minimi is separated from the abductor digiti minimi at its origin by the deep branches of the ulnar nerve and ulnar artery (3,4,7,8,11,13,14). The architectural features of the muscle are listed in Table 2.4. Actions and Biomechanics: Flexor Digiti Minimi The flexor digiti minimi functions mainly to provide flexion of the proximal phalanx at the MCP joint. It may assist with lateral rotation of the proximal phalanx (3,4,11,13, 14). As noted earlier, because the flexor digit minimi inserts onto the proximal phalanx at a point adjacent to but more palmar than that of the abductor digiti minimi, the flexor digiti minimi is able to exert a flexor force on the proximal phalanx. Anomalies and Variations: Flexor Digiti Minimi The flexor digiti minimi may be very small. If so, the abductor digiti minimi usually is larger than normal (11). The flexor digiti minimi may be absent (11). The flexor digiti minimi may coalesce with the abductor digiti minimi (11). The flexor digiti minimi may have a tendinous slip that attaches to the metacarpal of the small finger (11). OPPONENS DIGITI MINIMI Derivation and Terminology. Opponens is the Latin term indicating movement against or toward an opposing structure. Digiti is the plural of the Latin digitus, “digit.” Minimi is from the Latin minima or minimum, indicating “the smallest” (1,2). Origin. The hook of the hamate and adjacent flexor retinaculum. Insertion. The ulnar and anterior margin of the metacarpal of the small finger. Innervation. Deep branch of the ulnar nerve. Vascular Supply. Ulnar artery, deep palmar branch, medial end of the deep palmar arch (3,4). Principal Action. Opposition of the small finger to the thumb. This is a combination movement of abduction, flexion, and lateral rotation of the metacarpal of the small finger. It thereby brings the small finger in opposition to the thumb. Gross Anatomic Description: Opponens Digiti Minimi The opponens digiti minimi, along with the abductor digiti minimi, and flexor digiti minimi, form the hypothenar

2 Muscle Anatomy

muscles (Appendix 2.2). The opponens digiti minimi lies deep to the flexor digiti minimi and abductor digiti minimi (3,4,7,8,11,13,14). It is triangular, broad at its base and tapering to an apex distally. The muscle arises from the convex surface of the hook of the hamate, the adjacent pisohamate ligament, and the adjacent part of the palmar surface of the flexor retinaculum (496) (see Fig. 2.6A). The muscle becomes wider distally, to form a wide expansion for its insertion. The muscle inserts along most of the ulnopalmar surface of the diaphysis of the small finger metacarpal (see Fig. 2.6A). The architectural features of the muscle are listed in Table 2.4. The opponens digiti minimi is innervated by the deep branch of the ulnar nerve, containing fibers from T1 and from C8. Actions and Biomechanics: Opponens Digiti Minimi The opponens digiti minimi permits opposition of the small finger to the thumb. This is a combination movement of abduction, flexion, and lateral rotation of the metacarpal of the small finger. It thereby brings the small finger in opposition to the thumb. This motion also is referred to as supination of the small finger (496). Unlike the flexor digiti minimi and abductor digiti minimi, the opponens digiti minimi does not normally cross the MCP joint, and therefore does not act on the proximal phalanx of the small finger (3,4,7,8,11,13,14). Anomalies and Variations: Opponens Digiti Minimi The opponens digiti minimi may be divided into two layers by the deep branches of the ulnar artery and ulnar nerve (11). The opponens digiti minimi may coalesce with the abductor digiti minimi or the flexor digiti minimi (11). LUMBRICALS Derivation and Terminology. Lumbrical is derived from the Greek lumbricus, which means “earthworm.” The lumbrical muscles resemble the earthworm in shape, size, and color (1,2). Origin. From the FDP tendon. Insertion. To the tendinous expansion of the EDC (into the extensor hood). Innervation. The first and second lumbricals are innervated by the median nerve (C8, T1). The third and fourth are innervated by the deep branch of the ulnar nerve (C8, T1). The third may receive variable innervation from the median or ulnar nerve (3,4,628). Vascular Supply. First and second lumbricals: first and second dorsal metacarpal and dorsal digital arteries; arteria

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radialis indicis, first common palmar digital artery. Third and fourth lumbricals: second and third common palmar digital arteries, third and fourth dorsal digital arteries and their anastomoses with the palmar digital arteries (3,4). Principal Action. Through the extensor mechanism, the lumbricals function to provide extension at the PIP and DIP joints. In addition, they provide assistance with flexion of the MCP joint (629–634). Gross Anatomic Description: Lumbricals The lumbricals consist of four small, somewhat cylindrical muscle bellies. They arise from the FDP tendons and insert into the extensor hood. The muscles lie in the central palmar compartment of the hand (Appendix 2.2; see Table 2.4). The first and second lumbricals take origin from the radial sides and palmar surfaces of the FDP tendons of the index and long finger, respectively (3,4,7,8,11,13,14). The third lumbrical arises from the adjacent sides of the FDP tendons of the long and ring fingers. The fourth lumbrical arises from the adjacent sides of the FDP tendons of the ring and small fingers. The muscles pass volar to the deep transverse metacarpal ligament. Each lumbrical passes to the radial side of the corresponding digit. At the level of the MCP joint, the tendon of each lumbrical passes in a dorsal direction to reach the radial lateral bands of the extensor mechanism. The tendon of each muscle approaches the digit at approximately a 40-degree angle before insertion into the radial lateral band (484) (see Fig. 2.9). The lumbricals are unique in that they originate from a flexor tendon in the palm and insert into the dorsal aponeurosis on the radial side of the four digits. These functions have been studied and discussed in detail by von Schroeder and Botte and Lieber and colleagues (466,496). Because the lumbricals originate on the flexor side and insert into the extensor side of the fingers, they provide unique proprioceptive sensory information. Each lumbrical muscle also is unique in that, by originating from the FDP tendon, it is the only muscle that is able to relax the tendon of its own antagonist (484). Smith has recommended that when considering lumbrical action, it is best not to focus on its origin and insertion, but rather on its two attachments—to the profundus tendon and to the lateral band. Thus, if the profundus contracts and the lumbrical relaxes, the interphalangeal joints of the fingers flex. If the profundus is relaxed, contraction of the lumbrical pulls the lateral band proximally and the profundus tendon distally. Thus, the flexion or tension of the profundus is lessened, and the lumbrical is able to extend the proximal and interphalangeal joints (484). Hence, the lumbrical has relaxed its own antagonist. When both the profundus and the lumbrical contract, the interphalangeal joints and MCP flex simultaneously (484,617,631,632,635–637). In addition, the lumbricals have a unique architectural design. Their muscle fibers extend 85% to 90% of the length of the muscle (466) and are designed for excursion

Systems Anatomy

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(Table 2.4, Fig. 2.13). The actual length of the muscle fibers is similar to that of the extrinsic extensors on the dorsum of the forearm, but the lumbricals have a very small pennation angle and cross-sectional area and are ideally suited for creating an even contractile force (466,496). The lumbricals of the index and long fingers arise from their respective FDP tendons, which allows a greater independent motion compared with the lumbrical of the ring finger, which originates from the adjacent sides of the two FDP tendons (long and ring), or the lumbrical to the small finger, which originates from the adjacent sides of the FDP tendons to the ring and small fingers. Variation of the lumbricals is common (638) and, as with the extensor tendons; more variability is observed on the ulnar side of the hand (492,497,498). All lumbricals insert into the lateral band on the radial side of their respective fingers (Table 2.5). The architectural features of the lumbricals are listed in Table 2.4 (466).

The innervation of the lumbricals is split. The median nerve innervates the index and long finger lumbricals, which corresponds to the innervation of the FDP to these two fingers (496). The ring and small finger lumbricals are innervated by the ulnar nerve, which also innervates the FDP to the same fingers (496). Actions and Biomechanics: Lumbricals The function of the lumbricals is complex and has been discussed in detail by Smith and von Schroeder and Botte (484,496). Roughly stated, the lumbricals provide extension of the proximal and interphalangeal joints and flexion of the MCP joint. From origin to insertion, the lumbricals pass volar to the deep transverse metacarpal ligaments. As such, they are volar to the axis of rotation of the MCP joint and therefore can act as MCP flexors (3,4,13,14,617).

TABLE 2.5. INTRINSIC MUSCLES OF THE HAND: ORIGIN, INSERTION, AND FUNCTION OF THE DEEP AND SUPERFICIAL BELLIES OF THE DORSAL INTEROSSEI, THE VOLAR INTEROSSEI, AND THE LUMBRICALS Muscle Group Interossei (7)a Dorsal (4) Deep belly (3)

Origin

Index and long MC Long and ring MC Ring and small MC

Superficial belly (3)

Index MC

Index and long MC

Ring and small MC

Volar (3)

Index MC Ring MC Small MC

Lumbricals (4)

FDP index FDP long FDP long and ring FDP ring and small

Insertion

Lat tendon to lat band of DA, radial side of long finger Lat tendon to lat band of DA, ulnar side of long finger Lat tendon to lat band of DA, ulnar side of ring finger Med tendon to lat tubercle of prox phalanx, radial side of index finger Med tendon to lat tubercle of prox phalanx, radial side of long finger Med tendon to lat tubercle of prox phalanx, ulnar side of ring finger Lat band of DA, ulnar side of index finger Lat band of DA, radial side of ring finger Lat band of DA, radial side of small finger Lat band of DA, radial side of index finger Lat band of DA, radial side of long finger Lat band of DA, radial side of ring finger Lat band of DA, radial side of small finger

Function

Abduct and flex MCP joint, extend IP joints long finger Abduct and flex MCP joint, extend IP joints long finger Abduct and flex MCP joint, extend IP joints ring finger (abduction of small finger by ADQ) Abduct and weak flexion MCP joint, index finger Abduct and weak flexion MCP joint, long finger Abduct and weak flexion MCP joint, ring finger Adduct and flex MCP joint, extend IP joints index finger Adduct and flex MCP joint, extend IP joints ring finger Adduct and flex MCP joint, extend IP joints small finger Extension IP joints, weak flexion MCP joint index finger Extension IP joints, weak flexion MCP joint long finger Extension IP joints, weak flexion MCP joint ring finger Extension IP joints, weak flexion MCP joint small finger

aNumbers in parentheses denote number of muscles. ADQ, abductor digitorum quiti; DA, dorsal aponeurosis; FDP, flexor digitorum profundus tendon; IP, interphalangeal; lat, lateral; MC, metacarpal bone; MCP, metacarpophalangeal; med, medial; prox, proximal. Reprinted from von Schroeder HP, Botte MJ. The dorsal aponeurosis, intrinsic, hypothenar and thenar musculature of the hand. Clin Orthop 383:97–107, 2001, with permission.

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However, as noted by several authors, the interossei and the FDP and FDS tendons are primary flexors of the MCP joints, whereas the lumbricals function primarily to extend the interphalangeal joints through the dorsal aponeurosis (496,629,630,633,638–642). The origins, insertions, and functions of the lumbricals are summarized in Table 2.5 (496). The role of the lumbricals in interphalangeal joint extension has been emphasized by Smith and others, who have credited the lumbricals as the “workhorse of the extensor apparatus” (484,633,634,640). Electromyography of the lumbricals reveals high levels of activity whenever there is active extension of the interphalangeal joints. In addition, strong electrical stimulation of the lumbrical produces interphalangeal joint extension followed by MCP joint flexion. Low levels of electrical stimulation produce only interphalangeal joint extension (629,630). Although the lumbricals are located on the radial side of the fingers, they apparently do not function as abductors or adductors of the MCP joints because of their relatively parallel paths along the axis of the fingers (496). There is no radial deviation of the digits when the lumbricals contract (484,631). Although interphalangeal joint extension is an important part of lumbrical function, the lumbrical contributes relatively less or little to flexion of the proximal phalanx (484). This may seem at first inherently somewhat odd because the lumbrical tendon passes volar to the axis of the MCP joint (and volar to the interossei). However, electromyographic studies performed by Long and Brown indicated that under normal circumstances, the lumbrical contributes little to MCP joint flexion (633). When the interossei are paralyzed, however, the lumbrical can initiate flexion at this joint. Flexion of the proximal phalanx also may be achieved through contraction of the FDS and FDP. When these muscle contract, they first flex the interphalangeal joints. After full interphalangeal joint flexion is achieved, the long flexors flex the MCP joint until the digit is completely flexed (484,642). If finger flexion were performed solely by the FDP and FDS, MCP joint flexion would occur only after interphalangeal joint flexion was complete (643–654). The fact that the lumbricals originate from the FDP tendons but antagonize FDP flexion at the interphalangeal joint is an interesting phenomenon. Although it seems to contradict the respective functions of the muscle units, the lumbricals can relax the FDP tendons and thereby enhance their own function toward interphalangeal extension (496). When the FDP and lumbricals contract simultaneously, flexion of the interphalangeal and MCP joints occurs. This cocontraction enhances stability and occurs in power grip. The end result is simultaneous MCP and interphalangeal joint flexion (496,633,635), compared with a sequential contraction (DIP to PIP, then MCP contraction) that occurs with FDP and FDS contraction (636). The interossei also contribute to flexion of the MCP joints.

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Anomalies and Variations: Lumbricals Variations in sites of attachments of the lumbricals are relatively common. Each muscle may originate by varying amounts from the adjacent FDP tendons. The first lumbrical may have attachments that extend to the FPL tendon. Accessory tendon slips that attach to the adjacent FDS tendon may be present (11). Clinical Correlations: Lumbricals The lumbricals and interossei work together to provide flexion of the MCP joints and simultaneous extension of the PIP and DIP joints (see earlier, under Actions and Biomechanics: Lumbricals, and later, under Actions and Biomechanics: Dorsal Interossei, for specific differences and nuances of function of these muscles). Both muscles often are grouped together and referred to as the intrinsics or intrinsic muscles of the hand. In a spastic deformity or inflammatory condition with chronic spasm, with relative overactivity of the intrinsic muscles, the hand assumes a position dictated by these muscles—that is, flexion of the MCP joints and extension of the PIP and DIP joints. This position often is referred to as the intrinsic plus position, indicating overactivity of these intrinsic muscles. In contrast, with paralysis of the intrinsics (due to ulnar nerve laceration or neuropathy), the hand assumes a position opposite to what the muscles would provide (secondary to muscle imbalance of the functioning muscles). This results in a position of extension of the MCP joints and flexion of the PIP and DIP joints. This often is referred to as the intrinsic minus position, indicating lack of intrinsic function. Intrinsic minus also can occur with relative overpull of the extrinsic flexors and extensors, in conditions such as ischemic contractures after severe compartment syndrome (643– 654). Although the thenar and hypothenar muscle are true intrinsic muscles of the hand, the terms intrinsic plus and intrinsic minus do not pertain to these muscles. Dysfunction of the thenar muscles is referred to simply as thenar paralysis or (if present) thenar atrophy. After amputation of the distal phalanx (or untreated distal FDP tendon laceration or rupture), the detached FDP tendon may migrate proximally along with its lumbrical. This initially may increase tension of the lumbrical on the intrinsic extensor mechanism. If active flexion of the digit is attempted, the detached FDP tendon migrates proximally and pulls the lumbrical with it. Instead of digital flexion, the tension of the lumbrical on the extensor apparatus results in PIP joint extension. The hand is considered to have a lumbrical plus digit. The undesired PIP extension often is referred to as a paradoxical extension (because the person actually is attempting to flex the digit). The lumbrical plus digit does not occur consistently. If it does develop, elective operative resection of the lumbrical eliminates the

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paradoxical extension and allows the FDS to assume flexion control of the PIP joint (641). DORSAL INTEROSSEI Derivation and Terminology. Dorsal is derived from the Latin dorsalis or dorsum, which indicates “the back.” Dorsal usually is used to indicate the same side as the back, or the “back side.” Interossei is derived from the Latin inter, which indicates “between” or “among”; ossei is derived from ossis, which means “bone.” The dorsal interossei are the muscles between the bones, on the back side of the hand (1,2). Origin. There are four dorsal interossei. The first arises from adjacent sides of the thumb and index metacarpal, the second from the adjacent sides of the index and long metacarpal; the third from the adjacent sides of the long and ring metacarpals, and the fourth from the adjacent sides of the ring and small metacarpals (3,4,6,7,11,13). Insertion. The first dorsal interosseous inserts into the radial side of the base of the index proximal phalanx and into the dorsal aponeurosis of the extensor hood of the index finger. The second inserts into the radial side of the base of the long finger proximal phalanx and into the dorsal aponeurosis of the extensor hood of the long finger. The third inserts into the ulnar side of the base of the proximal phalanx of the long finger and into the dorsal aponeurosis of the extensor hood of the long finger. The fourth inserts into the ulnar side of the base of the proximal phalanx of the ring finger and into the dorsal aponeurosis of the extensor hood of the ring finger. The relative amounts of insertion into the associated proximal phalanx versus the amount reaching the extensor are not the same for each digit. The first dorsal interosseous inserts mainly into the proximal phalanx, with a lesser component inserting into the extensor hood. The second, third, and fourth have variable insertions, but, in general, the second and fourth have substantial contributions to both the associated proximal phalanx and to the dorsal aponeurosis. The third dorsal interosseous inserts mainly into the dorsal aponeurosis of the long finger, with a minimal component inserting into the base of the proximal phalanx (484,631,635) (for additional details, see later, under Gross Anatomic Description). Innervation. Deep branch of the ulnar nerve (C8, T1). Vascular Supply. Dorsal metacarpal arteries, second to fourth palmar metacarpal arteries; small branches of the radial artery; arteria princeps pollicis; arteria radialis indicis; perforating branches from the deep palmar arch (proximal perforating arteries); three distal perforating arteries; dorsal digital arteries (3,4,6,7,11,13). Principal Action. The dorsal interossei draw the index, long, and ring finger proximal phalanges away from the mid-axis of the long finger. The muscles also flex the MCP joints. Through the extensor hood, the dorsal interossei help to extend the PIP and DIP joints (475,484,496).

Because each dorsal interosseous muscle varies in the relative amounts of insertion into the proximal phalanx or into the dorsal aponeurosis, the functions of the interossei vary among the digits. The first dorsal interosseous inserts mainly into the proximal phalanx of the index finger (usually nearly 100%); it tends to function more for abduction of the proximal phalanx than it does for extension of the PIP or DIP joints. Conversely, the third interosseous usually inserts more into the extensor hood (approximately 94%), and therefore functions more for interphalangeal joint extension of the long finger. The second and forth dorsal interosseous have variable but substantial insertions into both the associated proximal phalanx and the dorsal aponeurosis, and therefore the second and fourth dorsal interossei contribute both to abduction of the associated proximal phalanx and extension of the proximal and interphalangeal joints. There also is a component of flexion of the MCP joint provided by the dorsal interossei (see later, under Actions and Biomechanics). The first dorsal interosseous also adducts the thumb metacarpal toward the index metacarpal during key pinch functions. This is combined with simultaneous abduction of the index proximal phalanx, which helps stabilize the MCP joint during forceful pinch. This provides simultaneous adduction of the thumb (metacarpal) toward the index finger, and allows the index finger (proximal phalanx) to oppose the force of the thumb. Thus, a strong key pinch can be generated (3,4,6,7,11,13,475,484,496,655). Gross Anatomic Description: Dorsal Interossei There are four dorsal interossei and three palmar interossei. The palmar interossei are described later in a separate section. In general, the dorsal interossei are larger and have a more complex anatomic arrangement than the palmar interossei (484,496). The four dorsal interossei also comprise four separate dorsal interosseous muscle compartments of the hand (Appendix 2.2). The dorsal interossei originate from and lie between the metacarpals (see Fig. 2.6B). Cross-sections of the hand in this area show the muscles occupying the space from the dorsal to palmar extent of the metacarpal, although the space is shared by the palmar interossei, which take origin more from the palmar portion of the metacarpal shaft (656). Each dorsal interosseous muscle is bipennate, with two muscle heads, each of which arises from the adjacent metacarpal. The two bellies join in a central longitudinal septum and the fibers course distally toward the associated digit. Three of the four dorsal interossei then form a deep muscle belly and three have a superficial muscle belly (484,496,655). The first and second dorsal interossei pass the radial side of the associated MCP joints to reach their respective digits; the third and forth dorsal interossei pass the ulnar side of the associated MCP joints to reach their respective digits (496) (see Table 2.5).

2 Muscle Anatomy

The muscle bellies of the dorsal interosseous should not be confused with the two heads of each muscle. Each muscle head arises from the adjacent metacarpal and joins to its associated partner head at the septum to form a bipennate muscle. In contrast, the deep and superficial bellies are more distally located divisions of the muscle. The superficial and deep head of each muscle usually form just proximal to the MCP joint and are the terminal divisions of each muscle. The deep and superficial muscle bellies have different final destinations for insertion, either into the associated proximal phalanx (superficial belly) or into the associated extensor hood (deep belly; see Fig. 2.9). The size, insertions, and amount of muscle fibers of the deep and superficial bellies ultimately determine the function of the specific dorsal interosseous. The superficial and deep muscle bellies have been studied and discussed in detail by Smith, Kaplan, von Schroeder and Botte, Landsmeer, and others (475,484,496, 642,655–657). The origin, insertion, and function of the deep and superficial bellies of the interossei and lumbricals are summarized in Table 2.5 (496). The deep belly of each dorsal interosseous muscle is the portion of the muscle that continues to join the lateral bands to reach dorsal aponeurosis and become part of the extensor mechanism of the associated index, long, and ring finger. Like the superficial belly, the deep belly arises as part of the main dorsal interosseous muscle from the adjacent surfaces of the midshafts of the adjacent metacarpals. Just proximal to the MCP joint, the dorsal interosseous splits into a deep and superficial belly. The deep belly continues distally to form or terminate into the lateral tendon. This lateral tendon of the deep belly is potentially larger than the medial tendon (which is derived from the superficial belly). The lateral tendon continues distally to pass superficial to the sagittal bands. The lateral tendon passes the MCP joint and continues distally and dorsally to become part of the extensor aponeurosis. The lateral tendon of the deep belly forms part of the transverse fibers of the dorsal aponeurosis (of the intrinsic muscle apparatus; see Fig. 2.9). The lumbrical tendon joins the extensor aponeurosis just distal to the joining point of the lateral tendon of the dorsal interosseous. The lumbricals help form the oblique fibers of the extensor aponeurosis. Through the deep belly and its insertion into the extensor apparatus, the dorsal interosseous assists interphalangeal joint extension. This muscle also provides flexion and assists with abduction of the proximal phalanges. When the MCP joint is flexed to approximately 90%, no significant abduction can be performed by the deep belly (496). The deep and superficial bellies of each dorsal interosseous muscle are of different sizes; and, the relative insertion into the extensor mechanism versus insertion into the proximal phalanx differs among the interossei. These differences, in turn, influence their respective functions of interphalangeal joint extension versus proximal phalanx abduction. These issues are discussed later.

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The superficial belly of each dorsal interosseous muscle is the portion that inserts into the base of the associated proximal phalanx and functions mostly for digital abduction. Although the superficial belly is a terminal division of the dorsal interossei, the muscle belly arises from the adjacent surfaces of the midshafts of the contiguous metacarpals as part of the main dorsal interosseous muscle. The fibers form a bipennate muscle that continues distally to converge into either a deep belly (described previously) or a superficial belly. The superficial belly splits from the main dorsal interosseous muscle just proximal to the MCP joint. It then forms or terminates into the medial tendon. The medial tendon is a small tendon that continues distally and passes deep to the sagittal bands of the MCP joint. The medial tendon continues past the MCP joint to insert onto the lateral tubercle at the base of the proximal phalanx. Through this osseous insertion, the muscle belly functions primarily as an abductor of the proximal phalanx. It also is a weak flexor of the proximal phalanx (484). This weak flexion component increases in power as the MCP joint is increasingly flexed because the tendon passes volar to the axis of rotation of the joint, and increasing flexion increases its flexion moment arm. The superficial belly has no direct effect on interphalangeal joint extension (655). The first dorsal interosseous also is known as the abductor indicis, and is the largest of the dorsal interossei (3,4). The first dorsal interosseous is triangular, thick, and flat. As described earlier, there are two heads, each arising from the adjacent metacarpal. The radial (lateral) head of the first dorsal interosseous arises from the proximal half or threefourths of the ulnar border of the thumb metacarpal. The ulnar (medial) head arises from the major portion of the radial border of the second metacarpal. The origin from the index metacarpal usually is slightly larger that that from the thumb, but each covers approximately two-thirds to threefourths of the associated sides of the metacarpals (11). As a bipennate muscle, there is a septum that separates the two heads, in which the muscle fibers converge in an oblique and distal direction. There also is a fibrous arch in the proximal aspect of the first dorsal interosseous that forms an interval through which the radial artery passes from the dorsal aspect of the hand to form the deep palmar arterial arch. The muscle fibers converge toward the septum, running centrally and longitudinally through the muscle. Just proximal to the MCP joint, on the radial side of the joint, the first dorsal interosseous muscle divides into the superficial and deep bellies, which in turn give rise to the medial and lateral tendons, respectively (484,496). The first dorsal interosseous is unique in that most of the muscle consists of the superficial belly, which gives rise to a median tendon that inserts into the base of the proximal phalanx. The deep belly is small or inconsistent, and few, if any, fibers form this deep belly to give rise to a lateral tendon to insert into the dorsal aponeurosis (635). Therefore, the first dorsal interosseous inserts almost entirely into the proximal pha-

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lanx of the index finger. The first dorsal interosseous thus functions largely in abduction of the index finger proximal phalanx. Through the proximal phalanx insertion, the first dorsal interosseous also contributes to flexion of the MCP joint. The first dorsal interosseous provides little, if any, contribution toward PIP or DIP joint extension. The abduction of the index proximal phalanx helps stabilize the MCP joint, especially during key pinch function, where index finger abduction action helps oppose the force of the thumb. The first dorsal interosseous also provides an important function for the thumb metacarpal. The muscle adducts the thumb metacarpal toward the index metacarpal. This function is used constantly during the pinch function, especially in key pinch, where the thumb metacarpal is pulled toward the index metacarpal in the plane of the palm. The simultaneous abduction of the index proximal phalanx helps stabilize the index finger during the key pinch maneuver. The second dorsal interosseous, like the other dorsal interossei, has two heads. The radial (lateral) head arises from the ulnar side of the index metacarpal. The ulnar (medial) head arises from the radial side of the long metacarpal. Each of these muscle origins covers approximately the proximal two-thirds to three-fourths of the sides of the shafts of each associated metacarpal. The origin from the long finger usually is slightly larger than that from the index metacarpal (11). The fibers converge into a central septum, with the fibers oriented obliquely distally and toward the central septum, forming the bipennate muscle. Proximal to the MCP joint, on the radial aspect of the joint, the fibers of the second dorsal interosseous divide into superficial and deep bellies (described previously). Approximately 60% of the fibers insert into the proximal phalanx of the radial aspect of the base of the long finger. The remaining 40% of the fibers reach the extensor hood (634). (Thus, functionally, the muscle’s contribution to abduction of the long finger is approximately equal or slightly greater compared with its function in extension of the PIP and DIP joints.) Through the dorsal hood, the second dorsal interosseous also contributes to flexion of the MCP joint. The third dorsal interosseous also has two heads. The radial (lateral) head arises from the ulnar side of the long metacarpal. The ulnar (medial) head arises from the radial side of the ring metacarpal. As with the second dorsal interosseous, the muscle origins of the third dorsal interosseous attach to the proximal two-thirds to threefourths of the sides of the shafts of each associated metacarpal. The origin from the long metacarpal usually is slightly larger than that from the ring metacarpal (11). The fibers converge into a central septum, with the fibers oriented obliquely distally and toward the central septum, forming the bipennate muscle. Proximal to the MCP joint, on the ulnar aspect of the joint, the fibers of the third dorsal interosseous divide into superficial and deep bellies

(described previously). Approximately 6% of the fibers insert into the proximal phalanx of the ulnar aspect of the base of the long finger. The remaining 94% of the fibers reach the extensor hood (635). (Thus, functionally, the muscle’s contribution to abduction of the long finger is minimal compared with its major function of extension of the PIP and DIP joints.) Through the dorsal hood, the third dorsal interosseous also contributes to flexion of the MCP joint. The fourth dorsal interosseous also has two heads. The radial (lateral) head arises from the ulnar side of the ring metacarpal. The ulnar (medial) head arises from the radial side of the small finger metacarpal. As with the other dorsal interossei, the muscle origin covers the proximal twothirds to three-fourths of the sides of the shafts of each associated metacarpal. The origin from the ring metacarpal usually is slightly larger than that from the small finger metacarpal (11). Similar to the other dorsal interossei, the fibers of the fourth dorsal interosseous converge into a central septum, with the fibers oriented obliquely distally and toward the central septum, forming the bipennate muscle. Proximal to the MCP joint, on the ulnar aspect of joint, the fibers of the fourth dorsal interosseous divide into superficial and deep bellies (described previously). Approximately 40% of the fibers insert into the proximal phalanx of the ulnar aspect of the base of the long finger. The remaining 60% of the fibers reach the extensor hood (635). (Thus, functionally, the muscle’s contribution to abduction of the long finger is slightly less than its function in extension of the PIP and DIP joints.) Through the dorsal hood, the fourth dorsal interosseous also contributes to flexion of the MCP joint. It also may contribute to adduction of the small finger metacarpal if the ring finger metacarpal is fixed. The dorsal interossei usually all are innervated by the deep branch of the ulnar nerve. For each of the muscles, the deep and superficial bellies are separately innervated by distinct small nerve branches (484). It therefore is possible to contract the deep belly of a dorsal interosseous without contracting the superficial belly, or vice versa (484). Several variations in innervation are possible. The first dorsal interosseous may be innervated by either the median nerve, radial nerve, or musculocutaneous nerve. Median nerve innervation is through the Martin-Gruber or Riche-Cannieu anastomosis (see later, under Anomalies and Variations). Actions and Biomechanics: Dorsal Interossei In general, the dorsal interossei usually are credited with the function of abduction of the associated digit (as well as flexion of the MCP joint), along with and extension of the PIP and DIP joints. The function of each dorsal interosseous is different and depends on the relative amounts of insertion

2 Muscle Anatomy

into bone (the associated proximal phalanx), which provide digital abduction, compared with the relative amounts of insertion into the dorsal aponeurosis of the extensor hood, which provide flexion of the MCP joint and extension of the PIP and DIP joints (475,484). Studies have investigated the relative insertions of each dorsal interosseous into the proximal phalanx versus the extensor aponeurosis. Eyler and Markee noted the following insertion ratios: first dorsal interosseous, 100% proximal phalanx, 0% extensor aponeurosis; second dorsal interosseous, 60% proximal phalanx, 40% extensor aponeurosis; third dorsal interosseous, 6% proximal phalanx, 94% extensor aponeurosis; forth dorsal interosseous, 40% proximal phalanx, 60% extensor aponeurosis (635). Given these relative amounts of insertion into the proximal phalanx versus the dorsal aponeurosis, the relative amounts of digital abduction versus interphalangeal joint extension provided by the muscle can be extrapolated (475,484,496,655) When the function of abduction of the digits is examined, it is understood that abduction refers to “a drawing away from the midline.” In the digits, this refers to the midline of the hand, and the mid-axis of the long finger usually is used as the reference line. The first dorsal interosseous functions to abduct the index finger, or draw it away from the mid-axis of the long finger in the radial direction. The second dorsal interosseous abducts the long finger, drawing it away from the midline in a radial direction. The third dorsal interosseous abduction component (although relatively weak) abducts the long finger, drawing it away from the midline in the ulnar direction. The fourth dorsal interosseous abducts the ring finger, drawing it away from the mid-axis of the long finger in the ulnar direction (496). The long finger only abducts from the mid-axis, and therefore there are two abductors present on either side. There is no such movement of adduction of the long finger when it is in a normal resting position. It is, however, possible for the long finger to adduct back to a normal position from a position of abduction (radial or ulnar deviation). Returning back to the normal position can, in a sense, be considered as adduction of the long finger. Abduction of the small finger is performed by the abductor digiti minimi (quinti). Abduction of the thumb is performed primarily by the APL and APB (496). The first dorsal interosseous also functions to adduct the thumb metacarpal toward the index metacarpal in the plane of the palm. Based on muscle architecture, the dorsal and palmar interossei (and lumbricals as well) are all highly specialized muscles with similar architectural features (see Table 2.4 and Fig. 2.13). These muscles, with their relatively long fiber length and relatively small physiologic cross-sectional areas, are designed more optimally for excursion (and velocity) than force generation. The small finger has no dorsal interosseous muscle inserting into it. Abduction of the small finger is performed by the abductor digiti minimi.

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Anomalies and Variations: Dorsal Interossei The deep branch of the ulnar nerve normally innervates all of the dorsal interosseous muscles. Infrequently, the median nerve may innervate the first dorsal interosseous (in 3% of limbs) (11,484,628,639). This variation may be associated with the Martin-Gruber anastomosis, which is the medianto-ulnar nerve crossover in the forearm (658,659), or may be associated with the Riche-Cannieu anastomosis, which is the median-to-ulnar nerve crossover in the palm (475,660). These anomalies are not uncommon, and their presence explains continued function of the interosseous muscle(s) in the presence of ulnar nerve laceration or severe neuropathy. Rarely, the dorsal interosseous may be innervated by the radial nerve or, more infrequently, there may be intercommunication between the musculocutaneous and median nerves (628). The presence of these anomalies also explains continued function of the interosseous muscle in the presence of ulnar nerve laceration or severe neuropathy. The interossei may have additional muscle bellies or may be completely absent in one or two of the interspaces (11). Clinical Correlations: Dorsal Interossei Because the first dorsal interosseous inserts mainly into the proximal phalanx of the index finger, its principal function is to abduct the proximal phalanx of the index finger (compared with its contribution to extension of the PIP or DIP joints). By abducting the index proximal phalanx away from the long finger, the first dorsal interosseous is able to help stabilize the index MCP joint by opposing the thumb during key pinch. During key pinch, the first dorsal interosseous can visibly be seen and felt contracting. The second dorsal interosseous also has a substantial insertion into the proximal phalanx (60%), and therefore this muscle probably also contributes to opposing the force of the thumb or stabilizing the long finger MCP joint. This is functionally advantageous when the long finger participates in pinch, such as in three-jaw chuck-type pinch (484,635). As opposed to the first dorsal interosseous, most of the fibers of the third dorsal interosseous continue to the dorsal aponeurosis to reach the extensor hood. Thus, functionally, the third dorsal interosseous contributes much more to extension of the PIP and DIP joint, compared with its minimal contribution toward abduction of the long finger. From a functional standpoint, this is advantageous because abduction of the long finger (in the ulnar direction) is relatively unimportant. However, extension of the long finger PIP and DIP joints and flexion at the MCP joint are useful and important movements provided by the dorsal aponeurosis. The interossei and lumbricals work together to provide simultaneous extension of the PIP and DIP joints and flexion of the MCP joints (475,484,496) (see earlier, under

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Actions and Biomechanics, for specific differences and nuances of function of these muscles). This is known as intrinsic function, and is a complex and important component of hand movement required for everyday tasks. At the initiation of a grasping maneuver, simultaneous extension of the interphalangeal joints and flexion of the MCP joint allows the digits to “wrap around” a relatively large object such as a milk carton, doorknob, or orange-sized object. Without the intrinsics providing the initial extension of the interphalangeal joints, extrinsic tendon flexion function of the digits results in flexion at the MCP, PIP, and DIP joints. The flexion of the digits often starts at the DIP joint, followed by the PIP and MCP. The digits flex and tend to “roll up” onto themselves and into the palm, similar to the way a party blower toy roles up on itself after it is blown out and inflated into a straight position and allowed passively to roll back up. When the fingers flex or “role up” into the palm, grasping of large objects is impossible. The digits are unable to wrap around the object (which requires interphalangeal joint extension at the initiation of the maneuver). This is demonstrated when the intrinsic minus hand (or claw hand) attempts to grasp a large object, and is a major functional problem of the intrinsic minus hand. Function of an intrinsic minus hand can be roughly simulated in a cadaver. Flexion of the digits by the extrinsic muscle in the absence of intrinsic muscle can be created in a cadaver by grasping an extrinsic FDP tendon in the forearm and pulling proximally. This produces extrinsic flexion without intrinsic function. The digit flexes at the DIP, PIP, and MCP joints, but tends to roll up onto itself, as described previously. The difficulties of the intrinsic minus hand in grasp can thus be demonstrated. Both the interossei and lumbrical muscles often are grouped together and referred to as the intrinsics or intrinsic muscles of the hand (484,496,661). In a spastic deformity, or an inflammatory condition with chronic spasm, with relative overactivity of the intrinsic muscles, the hand assumes a position that the muscles normally produce or provide, that is, flexion of the MCP joints and extension of the PIP and DIP joints. This position often is referred to as the intrinsic plus position, indicating overactivity of these intrinsic muscles. In contrast, with paralysis of the intrinsics (due to ulnar nerve laceration or neuropathy), the hand assumes a position opposite to that which the muscles would provide (secondary to imbalance of the functioning muscles). This results in a position of extension of the MCP joint and flexion of the PIP and DIP joints. This often is referred to as the intrinsic minus position, indicating lack of intrinsic function. The intrinsic minus position also can be produced by relative overactivity or contracture of the extrinsic muscles. This can be seen with ischemic contracture after compartment syndrome of the forearm (662–664). Although the thenar and hypothenar muscles are true muscles intrinsic to the hand, the terms intrinsic plus and intrinsic minus do not pertain to these muscles. Dysfunc-

tion of the thenar muscles is referred to simply as thenar paralysis or (if present) thenar atrophy. PALMAR INTEROSSEI Derivation and Terminology. Palmar is derived from the Latin palma, which means “palm,” or palmaris, which means “pertaining to the palm.” Interossei is derived from the Latin inter, which indicates “between” or “among”; ossei is derived from ossis, which means “bone.” The palmar interossei are the muscles between the bones, on the palm side of the hand (1,2). Origin. There are usually three palmar interossei attached to the index, ring, and small fingers. Four palmar interossei are sometimes described (see Anomalies and Variations, Palmar Interossei). The first arises from the ulnar side of the index metacarpal. The second arises from the radial side of the ring metacarpal. The third arises from the radial side of the small finger metacarpal. The origins are located palmar to the dorsal interossei, and both sets of muscles share the metacarpals for their origins. Insertion. The palmar interossei insert into the dorsal aponeurosis of the associated digit. The first inserts into the dorsal aponeurosis on the ulnar side of the index finger. The second inserts into the dorsal aponeurosis on the radial side of the ring finger. The third inserts into the dorsal aponeurosis on the radial side of the small finger (3,4,484,496). Innervation. From the deep branch of the ulnar nerve (C8, T1). Vascular Supply. Deep palmar arch, arteria princeps pollicis, arteria radialis indicis, palmar metacarpal arteries, proximal and distal perforating arteries, common and proper digital (palmar) arteries, common and proper palmar digital arteries, dorsal digital arteries (3,4,6,7,11,13). Principal Action. The first, second, and third palmar interossei adduct the proximal phalanx of the index, ring, and small finger, respectively. Gross Anatomic Description: Palmar Interossei The three palmar interossei are smaller and more uniform than the dorsal interossei, and occupy the palmar portion of the intermetacarpal spaces, also shared with the dorsal interossei.The three palmar interossei comprise three separate palmar interosseous compartments of the hand (Appendix 2.2). Each palmar interosseous arises form the associated side of its metacarpal, covering the base to the head and neck region of the bone (see Fig. 2.6A). The second and third palmar interossei tend to arise from the entire surface, whereas the first originates from and covers a slightly smaller area (3,4,11,13). Each belly converges to a tendon at the level of the MCP joint and passes the joint on the adductor side (which corresponds to the ulnar side of

2 Muscle Anatomy

the joint for the first, and the radial side for the second and third palmar interossei). In classic anatomy textbooks and descriptions of the insertions of the palmar interossei, the muscles have been described as inserting into both the lateral bands of the extensor aponeurosis as well as into the base of the proximal phalanx (3,4,13,14,662). From the studies of Eyler and Markee, and as emphasized by Smith and von Schroeder and Botte, it appears that the palmar interossei have few, if any, significant insertions into the proximal phalanx (484,496,635). Eyler and Markee studied the relative insertions of each palmar interosseous into the proximal phalanx versus into the extensor aponeurosis. The relative ratios of muscle insertion for each palmar interosseous were as follows: first palmar dorsal interosseous, 0% proximal phalanx (of index finger), 100% dorsal aponeurosis; second palmar interosseous, 0% proximal phalanx (of ring finger), 100% dorsal aponeurosis; third palmar interosseous, 10% proximal phalanx (of small finger), 90% dorsal aponeurosis (634). Smith has emphasized that the palmar interossei have no distinct deep and superficial bellies (as do the dorsal interossei), and thus none is inserted onto the proximal phalanx. Each of the palmar interossei can still adduct and flex the proximal phalanx and can extend the distal two phalanges of the finger. But these functions are performed through insertions into the lateral bands of the dorsal aponeurosis (and not through bone insertions into the proximal phalanges) (484,496). The origin, insertion, and function of the interossei and lumbricals are summarized in Table 2.5 (496). Architectural features are shown in Table 2.4 and Figure 2.13. The first palmar interosseous arises from the ulnar side of the second metacarpal diaphysis. The fibers converge into its tendon at the level of the MCP joint, on its ulnar aspect. The tendon then inserts into the lateral band of the dorsal aponeurosis on the ulnar side of the proximal phalanx of the index finger (3,4,484,496). The second palmar interosseous arises from the radial side of the ring metacarpal diaphysis. The fibers converge into its tendon at the level of the MCP joint, on its radial aspect. The tendon then inserts into the lateral band of the dorsal aponeurosis on the radial side of the proximal phalanx of the ring finger (3,4,484,496). The third palmar interosseous arises from the radial side of the small finger metacarpal diaphysis. The fibers converge into its tendon at the level of the MCP joint, on its radial aspect. The tendon then inserts into the lateral band of the dorsal aponeurosis on the radial side of the proximal phalanx of the small finger. According to Eyler and Markee, a small amount, approximately 10% of the muscle, of the third palmar interosseous also may insert into the base of the proximal phalanx of the small finger (635). Some authors describe four palmar interossei (3,13). In usual descriptions, however, this muscle is considered as part of the adductor pollicis (see later, under Anomalies and Variations).

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Actions and Biomechanics: Palmar Interossei Each palmar interosseous adducts and flexes the proximal phalanx of the associated digit, and extends the middle and distal phalanges (484). The first, second, and third palmar interossei act on the proximal phalanx of the index, ring, and small finger, respectively. Adduction of the digits refers to drawing the digit toward the midline of the hand (toward the mid-axis of the long finger). This movement is performed by the muscles’ insertion into the dorsal aponeurosis (3,4). Anomalies and Variations: Palmar Interossei Variations of the palmar interosseous muscles are rare. A muscle can be duplicated. Most of the variations are related to innervation, such as with the median nerve (see earlier, under Anomalies and Variations: Dorsal Interossei). Although three palmar interossei usually are present, occasionally a fourth palmar interosseous is present or described (13). This may represent an alternative description of basically normal anatomy, or may be a variant of the adductor pollicis. The authors who describe a fourth palmar interosseous usually attach the term first palmar interosseous to a muscle or fibers that passes from the base of the thumb metacarpal to the base of the thumb proximal phalanx. This muscle usually inserts with the adductor pollicis. In their description, the remaining palmar interossei (as described previously) become the second, third, and fourth palmar interossei, respectively. Because the thumb has a large adductor muscle of its own, these fibers have been considered as part of that muscle in most descriptions (3,13,14). Clinical Correlations: Palmar Interossei The thumb and the long finger do not have or need a palmar interosseous muscle. The long finger lies in the midline of the hand, and therefore does not need to be “adducted.” If it is in a position of abduction in the ulnar or radial direction, it can be brought back to the midline (adducted, in a sense) by the second or third dorsal interossei, respectively. The thumb does not require a palmar interosseous because it has the adductor pollicis (3,13,14).

REFERENCES 1. Dorland’s illustrated medical dictionary, 28th ed. Philadelphia: WB Saunders, 1994. 2. Stedman’s medical dictionary, 23rd ed. Baltimore: Williams & Wilkins, 1976. 3. Williams PW. Gray’s anatomy: the anatomical basis of medicine and surgery, 38th ed. New York: Churchill Livingstone, 1995:737–900.

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APPENDIX 2.1. MUSCLES OF THE HAND AND FOREARM AND ARM: ORIGIN, INSERTION, ACTION, INNERVATION Muscle Deltoid

Coracobrachialis Biceps brachii

Brachialis

Triceps brachii

Anconeus

Origin

Insertion

Lateral one-third clavicle, acromion, spine of scapula Coracoid process of scapula Short head from coracoid process, long head from supraglenoid tuberosity Distal two-thirds of anterior humerus

Deltoid tuberosity of humerus

Long head from infraglenoid tuberosity of scapula, lateral head from posterolateral humerus, medial head from distal posterior humerus Lateral epicondyle of humerus, posterior capsule of elbow

Axillary n. (C5, C6)

Coronoid process of ulna

Flexion of forearm

Olecranon, deep fascia of forearm

Extension of forearm, adduction of arm (long head)

Musculocutaneous, (and occasionally radial) n. (C5, C6, C7) Radial n. (C6, C7)

Lateral side of olecranon and posterior surface of ulna Lateral, distal radius, styloid process

Extension of forearm

Radial n. (C7, C8)

Flexion or forearm, assistance of pronation of forearm (when forearm is supinated), assistance of forearm supination (when forearm is pronated) Pronation of forearm, assistance of flexion of forearm

Radial n. (C5, C6)

Flexion, radial deviation of wrist, assistance with flexion and pronation of forearm Flexion of wrist, assists flexion, pronation of forearm Flexion, ulnar deviation of wrist, assistance with flexion of forearm

Median n. (C6, C7)

Medial humeral diaphysis Radial tuberosity, lacertus fibrosis

Lateral supracondylar ridge of humerus, lateral intermuscular septum

Pronator teres

Humeral head from medial epicondylar ridge of humerus, ulnar head from medial side of coronoid process of ulna Medial epicondyle of humerus (common flexor origin)

Central lateral radial diaphysis

Medial epicondyle of humerus (common flexor origin) Humeral head from medial epicondyle of humerus (common flexor origin), ulnar head from proximal dorsal ulna Humeral head from medial epicondyle of humerus (common flexor origin), ulnar head from coronoid process of ulna, radial head from oblique line of radial diaphysis

Palmar fascia (aponeurosis)

Palmaris longus

Flexor carpi ulnaris

Flexor digitorum superficialis

Innervation (Nerve Roots)

Abduction of humerus, forward flexion or extension of humerus Forward flexion, adduction of humerus Flexion, supination of forearm

Brachioradialis

Flexor carpi radialis

Action

Base of metacarpals of index and long fingers

Pisiform (possible extensions to hamate and base metacarpal of little finger)

Palmar middle phalanges of digits

Flexion of middle and proximal phalanges, assistance with forearm and wrist flexion

Musculocutaneous n. (C5, C6) Musculocutaneous n. (C5, C6)

Median n. (C6, C7)

Median n. (C6, C7)

Ulnar n. (C8, T1)

Median n. (C7, C8)

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APPENDIX 2.1. (continued) Muscle Flexor digitorum profundus

Flexor pollicis longus

Pronator quadratus Extensor carpi radialis longus

Extension carpi radialis brevis

Extensor digitorum communis

Extensor digiti minimi

Extensor carpi ulnaris

Supinator

Abductor pollicis longus

Extensor pollicis brevis

Extensor pollicis longus

Extensor indicis proprius

Origin Medial anterior surface of ulna, interosseous membrane, deep fascia of forearm Palmar surface of radius, interosseous membrane, medial border of coronoid process Distal palmar ulna Lateral supracondylar ridge of humerus, lateral intermuscular septum Common extensor origin from lateral epicondyle of humerus, radial collateral ligament of elbow joint, intermuscular septum Common extensor origin from lateral epicondyle of humerus, intermuscular septum Common extensor origin from lateral epicondyle of humerus, intermuscular septum Common extensor origin from lateral epicondyle of humerus, posterior border of ulna Lateral epicondyle of humerus, lateral capsule of elbow, supinator crest and fossa of ulna Dorsal surface of mid-diaphysis of radius and ulna, interosseous membrane Dorsal surface of radial diaphysis, interosseous membrane Dorsal surface of ulnar diaphysis, interosseous membrane Dorsal distal ulnar diaphysis, interosseous membrane

Insertion

Action

Palmar distal phalanges

Flexion of distal (and middle and proximal) phalanges, assistance with wrist flexion

Median n. to radial 2 digits, ulnar n. to ulnar 2 digits (C7, C8)

Base, palmar distal phalanx of thumb

Flexion of distal (and proximal) phalanx of thumb

Median n. (C8, T1)

Distal palmar radius

Pronation of forearm

Median n. (C8, T1)

Dorsal base of index metacarpal

Extension, radial deviation of wrist

Radial n. (C6, C7)

Dorsal base of long finger metacarpal

Extension, radial deviation of wrist

Posterior interosseous or radial n. (C6, C7)

Dorsal bases of middle and distal phalanges

Extension of digits, assistance with wrist extension

Posterior interosseous of radial n. (C6, C7)

Dorsal base of distal phalanx of little finger

Extension of little finger

Posterior interosseous of radial n. (C7, C8)

Dorsomedial base of little finger metacarpal

Extension, ulnar deviation of wrist

Posterior interosseous of radial n. (C6, C7)

Radiopalmar surface of proximal radius

Supination of forearm

Radial n. (deep branch) (C6, C7)

Radial base of thumb metacarpal

Abduction of thumb, assistance of wrist abduction

Posterior interosseous or radial n. (C6, C7)

Base, proximal phalanx of thumb

Extension of proximal phalanx (and metacarpal) of thumb

Posterior interosseous or radial n. (C6, C7)

Dorsal base, distal phalanx of thumb

Extension of distal phalanx of thumb, assists extension of proximal phalanx and metacarpal of thumb Extension of proximal phalanx of index finger

Posterior interosseous or radial n. (C6, C7)

Dorsal proximal phalanx of index finger

Innervation (Nerve Roots)

Posterior interosseous or radial n. (C6, C7)

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182

APPENDIX 2.1. (continued) Muscle Abductor pollicis brevis

Opponens pollicis

Flexor pollicis brevis Adductor pollicis

Palmaris brevis

Adductor digiti minimi

Flexor digiti minimi Opponens digiti minimi

Origin Transverse carpal ligament, scaphoid tubercle, palmar trapezium Transverse carpal ligament, palmar trapezium metacarpal Transverse carpal ligament, palmar trapezium Oblique head from palmar trapezium, trapezoid, and capitate Transverse head from palmar surface of long finger metacarpal Ulnar side of transverse carpal ligament, palmar aponeurosis Pisiform, tendon of flexor carpi ulnaris

Transverse carpal ligament, hook of hamate Transverse carpal ligament, hook of hamate

Lumbricals

Four lumbricals arise from tendons of flexor digitorum profundus

Dorsal interossei

Four dorsal interossei each from sides of adjacent two metacarpals

Insertion

Action

Innervation (Nerve Roots)

Radial side, base of proximal phalanx of thumb

Palmar abduction of proximal phalanx of thumb

Recurrent branch of median n. (C8, T1)

Radiopalmar surface of thumb

Recurrent branch of median n. (C8, T1)

Base proximal phalanx of thumb

Opposition of thumb to digits (palmar abduction, pronation of thumb) Flexion of proximal phalanx of thumb

Ulnar side, base of proximal phalanx of thumb

Adduction of thumb, assistance with opposition

Deep branch of ulnar n. (C8, T1)

Skin on ulnar border of palm

Corrugation of skin on ulnar palm (deepening of palm)

Superficial branch of ulnar n. (C8, T1)

Ulnar side, base of proximal phalanx of little finger, aponeurosis of extensor digiti minimi Ulnar side, base of proximal phalanx of little finger Ulnar side of metacarpal of little finger

Abduction of little finger from palm

Deep branch of ulnar n. (C8, T1)

Flexion of proximal phalanx of little finger Opposition of little finger to thumb, flexion of metacarpal of little finger anteriorly out of palm Extension of the middle phalanges, flexion of the proximal phalanges

Deep branch of ulnar n. (C8, T1)

Abduction of index, long, ring fingers from midline of hand, flexion of proximal phalanges, extension of middle phalanges

Deep branch ulnar n. (C8, T1)

Join with interossei to form lateral bands that become dorsal hood with the extensor digitorum communis tendons; ultimate insertions include base of the middle phalanx (central slip) and base of distal phalanx First into radial side of proximal phalanx of index finger; second into radial side of proximal phalanx of long finger; third into ulnar side of proximal phalanx of long finger; fourth into ulnar side of proximal phalanx of ring finger All interossei also with variable contributions to lateral bands to form part of the dorsal hood

Recurrent branch median n. (C8, T1)

Deep branch of ulnar n. (C8, T1)

Median n. to radial two lumbricals, ulnar n. to ulnar two lumbricals (C8, T1)

2 Muscle Anatomy

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APPENDIX 2.1. (continued) Muscle Palmar interossei

Origin Three palmar interossei: First from ulnar side of index metacarpal, second from radial side of ring metacarpal, third from radial side of little finger metacarpal

Insertion First into ulnar side of proximal phalanx of index; second into radial side of proximal phalanx of ring finger; third into radial side of proximal phalanx of little finger

Action

Innervation (Nerve Roots)

Adduction of digits

Deep branch ulnar n. (C8, T1)

APPENDIX 2.2. MUSCLE COMPARTMENTS AND FASCIAL SPACES OF THE UPPER EXTREMITY Compartment Deltoid compartment Anterior compartment of the arm

Posterior compartment of the arm Mobile wad compartment of the forearm

Superficial volar compartment of the forearm

Deep volar compartment of the forearm Pronator quadratus compartment Dorsal compartment of the forearm

Carpal tunnela Central palmar compartment of the hand Thenar compartment

Hypothenar compartment

Adductor compartment of the hand Interosseous compartments of hand

Principal Muscles Deltoids Coracobrachialis Biceps brachii Brachialis Triceps muscle (three heads) Brachioradialis Extensor carpi radialis longus Extensor carpi radialis brevis Pronator teres Flexor carpi radialis Palmaris longus Flexor digitorum superficialis Flexor carpi ulnaris Flexor digitorum profundus Flexor pollicis longus Pronator quadratus Extensor digitorum communis Extensor indicis proprius Extensor carpi ulnaris Extensor digiti quinti Extensor pollicis longus Supinator Abductor pollicis longus Extensor pollicis brevis Extrinsic digital flexor tendons Extrinsic flexor tendons Lumbricals Abductor pollicis brevis Flexor pollicis brevis Opponens pollicis Abductor digiti minimi Flexor digiti minimi Opponens digiti minimi Adductor pollicis Dorsal interossei (four) Palmar interossei (three)

a Although not a true muscle compartment, the carpal tunnel is listed here because it can have the physiologic properties of a closed compartment in the presence of compartment syndrome.

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0.00 0.63 0.63 0.36 0.94 0.27 0.31 0.42 0.20 0.84 0.22 0.46 0.24 0.27 0.15 0.77 0.59 0.58 0.78 0.61 0.77 0.53 0.71 0.92 1.03

0.00 1.23 0.65 1.40 0.39 0.62 0.46 0.80 1.44 0.77 0.51 0.63 0.66 0.61 1.38 1.21 1.20 1.39 1.19 1.39 1.11 0.57 1.42 1.30

FCU

0.00 0.87 0.94 0.90 0.78 1.02 0.52 0.25 0.62 1.02 0.71 0.79 0.65 0.20 0.28 0.10 0.16 0.12 0.20 0.19 1.26 0.75 1.24

PL

0.00 0.86 0.33 0.56 0.38 0.43 1.03 0.35 0.49 0.50 0.23 0.44 0.96 0.73 0.80 1.01 0.89 0.96 0.79 0.45 0.87 1.02

ECRB

0.00 1.06 0.99 1.00 0.78 1.03 0.73 1.02 0.95 0.78 1.07 0.86 0.68 0.87 0.98 1.01 0.91 1.04 1.24 0.86 0.74

ECRL

0.00 0.34 0.23 0.43 1.10 0.39 0.31 0.32 0.29 0.30 1.03 0.84 0.85 1.05 0.87 1.04 0.79 0.54 1.10 1.05

ECU

0.00 0.37 0.34 1.01 0.34 0.30 0.08 0.37 0.39 0.91 0.74 0.75 0.92 0.72 0.94 0.72 0.87 1.18 0.86

FDSI

0.00 0.51 1.23 0.43 0.14 0.38 0.28 0.50 1.13 0.92 0.97 1.17 1.00 1.15 0.95 0.58 1.20 0.89

FDSM

0.00 0.73 0.12 0.52 0.26 0.29 0.32 0.63 0.43 0.46 0.66 0.51 0.65 0.48 0.84 0.86 0.91

FDSR

0.00 0.82 1.25 0.95 0.99 0.84 0.21 0.40 0.27 0.15 0.34 0.12 0.35 1.41 0.69 1.43

FDSS

0.00 0.45 0.27 0.18 0.36 0.71 0.49 0.56 0.76 0.62 0.74 0.58 0.77 0.87 0.84

FDPI

0.00 0.33 0.34 0.54 1.14 0.93 0.98 1.17 0.99 1.17 0.96 0.72 1.28 0.82

FDPM

0.00 0.32 0.32 0.84 0.67 0.68 0.86 0.67 0.87 0.65 0.83 1.11 0.88

FDPR

0.00 0.40 0.88 0.65 0.73 0.93 0.79 0.90 0.74 0.63 0.95 0.84

FDPS

0.00 0.81 0.65 0.61 0.80 0.62 0.80 0.51 0.72 0.95 1.16

FPL

0.00 0.25 0.20 0.13 0.30 0.11 0.38 1.36 0.70 1.23

EDCI

0.00 0.20 0.34 0.36 0.28 0.38 1.15 0.62 1.06

EDCM

0.00 0.21 0.20 0.20 0.21 1.19 0.68 1.19

EDCR

0.00 0.24 0.12 0.32 1.39 0.76 1.32

EDCS

0.00 0.31 0.19 1.27 0.86 1.23

EDQ

0.00 0.34 1.34 0.64 1.31

EIP.

0.00 1.15 0.77 1.30

EPL

0.00 1.14 1.34

PT

0.00 1.48

PQ

0.00

BR

FCR, flexor carpi radialis; FCU, flexor carpi ulnaris; PL, palmaris longus; ECRB, extensor carpi radialis brevis; ECRL, extensor carpi radialis longus; ECU, extensor carpi ulnaris; FDSI, FDSM, FDSR, FDSS, flexor digitorum superficialis to the index, middle, ring, and small fingers; FDPI, FDPM, FDPR, FDPS, flexor digitorum profundus to the index, middle, ring, and small fingers; FPL, flexor pollicis longus; EDCI, EDCM, EDCR, EDCS, extensor digitorum communis to the index, middle, ring, and small fingers; EDQ, extensor digiti quinti; EIP, extensor indicis proprius; EPL, extensor pollicis longus; PT, pronator teres; PQ, pronator quadratus; BR, brachioradialis. a Editors Note: Architectural features of a muscle include the physiologic cross-sectional area of the muscle, fiber bundle length, muscle length, muscle mass, and pennation angle (angle of the muscle fibers from the line representing the longitudinal vector of its tendon.) This table lists each of the difference index values, which is a number that compares a pair of muscles. The difference index is the amount that the two muscles differ from each other, and has been determined based on the architectural features (15). A lower number (index value) indicates a lesser difference, a larger index value indicates a greater difference. The mean architectural difference index among the upper extremity muscles is 0.74. Reproduced from Lieber RL, Brown CG. Quantitative method for comparison of skeletal muscle architectural properties. J Biomech 25:557–560, 1992, with permission.

FCR FCU PL ECRB ECRL ECU FDSI FDSM FDSR FDSS FDPI FDPM FDPR FDPS FPL EDCI EDCM EDCR EDCS EDQ EIP EPL PT PQ BR

FCR

APPENDIX 2.3. HUMAN FOREARM MUSCLE DIFFERENCE INDEX VALUES: A COMPARISON OF ARCHITECTURAL FEATURES OF SELECTED SKELETAL MUSCLES OF THE UPPER EXTREMITYA

3 NERVE ANATOMY MICHAEL J. BOTTE

The gross anatomy of the upper extremity peripheral nerves is described in the following sections. The physical course of each nerve and its associated branches is outlined, followed by descriptions of nerve anomalies or variations, and clinical correlations. For descriptive purposes, each nerve discussion is divided into the regions of the arm, forearm, and wrist and hand, if applicable. Sensory nerve organelles are discussed at the end of the chapter. The dermatomes of the upper extremity are depicted for reference in Appendix 3.1. MEDIAN NERVE Origin of the Median Nerve The median nerve arises from the lateral and medial cords of the brachial plexus, and comprises fibers from the anterior rami of C5, C6, C7, C8, and T1 (Fig. 3.1). The median nerve originates from two branches, one each from the lateral and medial cords of the brachial plexus. The two branches, referred to as the lateral and medial roots, unite adjacent to and anterior or anterolateral to the third portion of the axillary artery, in the vicinity of the medial border of the coracobrachialis. This occurs approximately at the longitudinal level of the surgical neck of the humerus with the shoulder abducted 90 degrees (1–5). Median Nerve in the Axilla and Arm The median nerve continues distally in the arm, posterior to the pectoralis major, anterior to the coracobrachialis, lateral to the brachial artery, and medial to the biceps brachii. In the arm, and along most of its course, it lies anteromedial to the brachialis muscle and posteromedial to the biceps brachii muscle. The median nerve does not normally supply motor branches to any muscle in the arm. In the mid-portion of the arm, in the vicinity of the insertion of the coracobrachialis muscle, the median nerve crosses anterior to the brachial artery to lie on the medial side of the artery. The nerve continues to the cubital fossa, remaining medial to the brachial artery. Both the nerve and artery remain close to the biceps tendon just proximal to the lacertus fibrosus. The mnemonic, MAT, helps in remembering the relationship (from medial to lateral) of the median nerve, brachial artery, and the biceps tendon in this area (6). The nerve usually gives off several small vascular

branches, but does not provide innervation to muscles in the arm (7,8) (Fig. 3.2). In the distal third of the arm, the brachial artery gives off several muscular arteries, including the supratrochlear artery (inferior ulnar collateral arteries). These branches cross anteriorly or posteriorly to the nerve, often in close proximity. Adjacent to the brachial artery are venae comitantes, two to three of which lie between the artery and the median nerve (6). Throughout the course of the median nerve in the arm, the ulnar nerve remains posterior and somewhat parallel to the median nerve, diverging slightly from the median nerve as the two nerves descend along the arm. The ulnar nerve continues distally to reach the cubital fossa. Anomalies and Variations: Median Nerve in the Axilla and Arm Although the median nerve usually is formed by the union of the lateral and medial cords anterior or lateral to the axillary artery, the nerve also has been noted rarely to be formed by the branches of these cords uniting posterior to the axillary artery (7). The median nerve usually is formed at the level of the third portion of the axillary artery. The nerve also can originate from the union of the lateral and medial cords more distally, in the proximal third of the arm (7,9,10). Fibers from C7 may leave the lateral root in the distal part of the axilla and pass distomedially posterior to the medial branch from the medial cord. The nerve usually passes anterior to the axillary artery, to join the ulnar nerve. These fibers are believed to be mainly motor fibers to the flexor carpi ulnaris (3,11). If the lateral cord is small, the musculocutaneous nerve (C5, C6, and C7), which usually arises from the lateral cord, can arise directly from the median nerve (1,3,11). A branch from the musculocutaneous nerve occasionally joins the median nerve after the musculocutaneous nerve pierces the coracobrachialis muscle. This variation has been reported in 8% to 36% of dissected specimens (12). The fibers enter the musculocutaneous nerve from the lateral cord rather than passing into the lateral root of the median nerve. The communicating branch leaves the musculocutaneous nerve, descends from lateral to medial between the brachialis and biceps muscles, and joins the median nerve

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Systems Anatomy

FIGURE 3.1. Schematic illustration of the brachial plexus and associated major branches. A, nerve to subclavius; B, lateral pectoral nerve; C, subscapular nerves; D, thoracodorsal nerve; E, medial antebrachial cutaneous nerve; F, medial brachial cutaneous nerve; G, medial pectoral nerve.

in the mid-portion of the arm. When this anomaly occurs, the branch (or branches) of the lateral cord that joins the medial cord is smaller than normal. Fibers may cross from the median to musculocutaneous nerve. This anomaly is rare. A nerve to the pronator teres muscle may leave the main median nerve trunk in the arm as high as 7 cm proximal to the epicondyles (7,13). Clinical Correlations: Median Nerve in the Axilla and Arm The median nerve may be compressed at several points in the upper extremity. These are well described by Siegal and Gelberman (7), and include the following areas:

At the level of the coracoid process, the nerve (or lateral cord) may be compressed by the pectoralis minor muscle. The muscle lies on the anterior surface of the nerve, and can cause nerve compression, especially when the arm is hyperabducted (7,14). An anomalous muscle known as Langer’s muscle can cause median nerve compression. This muscle arises from the latissimus dorsi tendon, crosses the axillary neurovascular bundle, and inserts on the pectoralis major (7,15). Median nerve compression can occur in the axilla and arm from anomalous vascular arches, or perforations of the nerve by anomalous vessels. The vascular anomalies may be arterial or venous in origin. An 8% incidence of abnormal relationships between the vascular and neural elements in the axilla has been reported (7,16).

3 Nerve Anatomy

FIGURE 3.2. Schematic illustration of the median nerve and the musculocutaneous nerve, with associated branches and innervated muscles.

187

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Systems Anatomy

The deltopectoral fascia, when thickened and fibrotic, may occasionally compress the median nerve at its distal edge. This has been noted after blunt trauma to the shoulder (7,17). The supracondylar process and associated ligament of Struthers may compress the median nerve in the distal arm (7,18–23). The supracondylar process, a hook-shaped projection from the medial aspect of the distal humerus, usually is located 3 to 5 cm proximal to the medial epicondyle. This anomalous protrusion provides attachment for an anomalous ligament, the ligament of Struthers. The ligament spans between the supracondylar process and medial epicondyle, forming a fibroosseous tunnel, which is present in 1% of limbs. It may represent an accessory origin of the pronator teres muscle. The nerve passes through the tunnel with either the ulnar or brachial artery and veins, medially to the vessels. Nerve compression may be caused by either the supracondylar process itself or by the ligament (7,24). Just proximal to the elbow, in the area of the medial epicondyle, there is a constant relationship of the median nerve, brachial artery, and the biceps tendon. The mnemonic, MAT, describes this relationship (from medial to lateral) of the median nerve, brachial artery, and biceps tendon (6). Median Nerve in the Forearm In the cubital fossa, the nerve dives deep to the lacertus fibrosus, lying anterior to the brachialis muscle and medial to the brachial artery. As the nerve crosses the level of the elbow joint, one to two articular branches are given off to supply the proximal radioulnar joint (25) (see Fig. 3.2). The median nerve in the proximal third of the forearm supplies the flexor pronator group of muscles that arise from the medial epicondyle. These include the pronator teres, the flexor carpi radialis, and the palmaris longus. The proximal portion of the flexor superficialis, which arises from the medial epicondyle and the thickened fascia (raphe) in the proximal third of the forearm, obtains its motor supply from the motor branches supplying the flexor carpi radialis and the palmaris longus. The motor branches supplying the medial portion of the flexor pronator mass usually enter the muscles on their deep (posterior) surface (6). When the anterior surface of the antecubital region is exposed, these branches usually are not readily visible because of their deep course. On deeper exposure and inspection, three to four motor branches can be found traversing deep to the muscles to innervate the pronator teres, flexor carpi radialis, palmaris longus, and the humeral portion of the flexor digitorum superficialis (see Fig. 3.2). The nerve enters the forearm between the superficial (humeral) and deep (ulnar) heads of the pronator teres muscle. The nerve passes deep to the humeral head even when there is a congenital absence of the ulnar head, as noted in 6% of cases (7,26). As the nerve passes through the muscle bellies, it crosses the ulnar artery anteriorly, from medial to

lateral, separated from the artery by the deep head of the pronator teres. Most commonly, the pronator teres motor nerve has a common branch with nerve branches to the superficial and deep heads (60% of specimens). Alternatively, two separate nerve branches may be found, one going to the superficial head and one going to the deep head of the pronator teres (7,26,27). After emerging from the pronator teres, the median nerve passes deep to an arch created by the two heads of the flexor digitorum superficialis. In the region of the superficialis arch, the median nerve usually provides three motor branches to the flexor digitorum superficialis. These branches are located on the deep surface of the muscle (6). The nerve continues distally in the forearm between the flexor digitorum superficialis and flexor digitorum profundus (28). The nerve usually is in the fascia of the flexor digitorum superficialis, or may occasionally lie in the substance of the muscle (7,29). The nerve usually becomes superficial approximately 5 cm proximal to the wrist, emerging between the flexor digitorum superficialis and flexor carpi radialis, dorsal and slightly radial to the tendon of the palmaris longus (7,30). In the proximal forearm, the median nerve innervates the pronator teres, flexor carpi radialis, palmaris longus, and flexor digitorum superficialis (see Fig. 3.2). The branch to the pronator teres arises from 7 cm above the medial epicondyle to 2.3 cm distal to the medial epicondyle (31). In 45% of studied specimens, Sunderland and Ran noted two branches to the pronator teres, in 30% one branch, and in 25%, three or four branches (32). The anterior interosseous nerve usually branches from the dorsoradial surface of the median nerve trunk, usually arising immediately distal to the flexor digitorum superficialis arch, 5 cm distal to the medial epicondyle (see later). Proximal to the anterior interosseous nerve branch, the median nerve supplies the flexor carpi radialis, palmaris longus, and flexor digitorum superficialis. There usually is only a single nerve to the flexor carpi radialis and only one to the palmaris longus, but often from two to seven branches to the flexor digitorum superficialis. The branch to the index finger portion of the flexor digitorum superficialis arises in the midportion of the forearm, up to 20 cm distal to the medial epicondyle. (7). The muscular branches of the median nerve arise primarily from its medial surface (7,33). The median nerve and its branches supply the sympathetic fibers to the portions of the vascular structures of the forearm and hand in a segmental fashion. At the elbow, the median nerve provides a branch to the region of the bifurcation of the brachial artery. The nerve arborizes in the proximal few centimeters of the radial and ulnar arteries. The anterior interosseous nerve provides fibers to the anterior interosseous artery (see later). The sympathetic branches from the median nerve continue distally to provide sympathetic fibers into the palm to supply the superficial palmar arch, and, with the ulnar nerve, partially supply the deep palmar arch of the hand (see later) (6).

3 Nerve Anatomy

Anterior Interosseous Nerve The anterior interosseous nerve is the largest muscular branch that originates from the median nerve. The anterior interosseous nerve provides innervation to the flexor digitorum profundus to the index and long fingers (i.e., the radial half of the muscle), the flexor pollicis longus, and the pronator quadratus (34) (see Fig. 3.2). The terminal portion of the nerve also provides sensory fibers to the carpal joints. The nerve typically arises from the trunk of the median nerve on the dorsoradial surface at a level of approximately 5 to 8 cm distal to the medial epicondyle. Sunderland has demonstrated that the interosseous nerve actually becomes a separate group of fascicles at a point approximately 2.5 cm proximal to its branching from the median nerve trunk and at approximately 22 to 23 cm proximal to the radial styloid process (35). After leaving the median nerve, the anterior interosseous nerve initially lies between the flexor digitorum superficialis and flexor digitorum profundus. The nerve passes dorsally, in the interval between the flexor pollicis longus and the flexor digitorum profundus, providing two to six branches to each of these muscles. The nerve reaches the anterior surface of the interosseous ligament (interosseous membrane) and continues distally, usually close to the anterior interosseous artery. The nerve eventually reaches the pronator quadratus, where it penetrates the muscle proximally and passes deep to the belly to innervate the muscle. The nerve continues distally to the wrist, containing sensory afferent fibers for the intercarpal, radiocarpal, and distal radioulnar joints (6). The anterior interosseous nerve also supplies sympathetic nerve fibers to the proximal forearm. The sympathetic nerve fibers exit the anterior interosseous nerve and join with the anterior interosseous artery to continue distally (6). Palmar Cutaneous Branch of the Median Nerve The palmar cutaneous branch of the median nerve is the last major branch of the median nerve in the forearm (see Fig. 3.2). This nerve provides sensory fibers to the base of the thenar eminence. It contains no motor fibers. The nerve usually arises from the anteroradial aspect of the median nerve trunk, 5 to 7 cm proximal to the wrist (6,36). This is in the vicinity of the radial margin of the flexor digitorum superficialis (37). The palmar cutaneous nerve usually consists initially of only one nerve branch as it exits the main median nerve trunk, and usually can be identified approximately 5.5 cm proximal to the radial styloid. Before branching, the nerve usually continues in or adjacent to the epineurium of the median nerve trunk for 16 to 25 mm before separating from the median nerve. The nerve courses distally in the very distal forearm along the ulnar side of the flexor carpi radialis tendon, adherent to the undersurface of the antebrachial fascia. At the proximal edge of the trans-

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verse carpal ligament, the nerve deviates ulnarly and enters its own short fibrous tunnel in the ligament. The tunnel through the transverse carpal ligament is usually 9 to 16 mm long (6,37). The nerve pierces the transverse carpal ligament in line with the ring finger and enters the ligament, dividing into ulnar and medial branches. These branches supply the skin of the proximal two-fifths of the palm on the radial side and the thenar eminence (7,24,38). Anomalies and Variations: Median Nerve in the Forearm The most common nerve anomalies in the forearm are connections between the median and ulnar nerves. A connection often exists between the anterior interosseous and ulnar nerves in the substance of the flexor digitorum profundus. This intramuscular communication leads to multiple variations in patterns of innervation of the muscle. Dual innervation is most common in the long finger flexor, but may occur in all the digits. The median nerve, or rarely the ulnar nerve, may innervate the entire flexor digitorum profundus (7,39). When the median nerve supplies the entire flexor digitorum profundus, it usually is through fibers from the anterior interosseous nerve. (The anterior interosseous nerve normally supplies the flexor digitorum profundus to the index and long finger, but in the “all median nerve hand,” the anterior interosseous nerve also supplies the flexor digitorum profundus to the ring and small fingers.) The complete median- and complete ulnar-innervated hand: There are several described clinical situations where the hand appears to be completely innervated by the median or ulnar nerve. Within these described conditions, there are several variations of reported findings. These variations probably are due to gradations between median and ulnar innervations, representing individual differences in anatomic arrangements. Fibers may pass between the ulnar and median nerves in the forearm or hand. Their terminal branches may send communicating fibers within the hand. The median nerve sometimes innervates the interosseous muscles, particularly the first dorsal interosseous, either alone or jointly with the ulnar nerve (40,41). In the extreme “all-median hand,” the anterior interosseous nerve (from the median nerve) supplies the flexor digitorum profundus to the ring and small fingers (which normally are supplied by the ulnar nerve) (6). The ulnar nerve more often provides dual or replacement innervation to muscles usually innervated by the median nerve (36,41–44). Less often, the median nerve innervates muscles that usually are innervated by the ulnar nerve (44). Each of the lumbrical muscles can have dual innervation from both the median and ulnar nerves (45,46). Double innervation of the flexor pollicis brevis is relatively common. Several patterns with ulnar innervation of the thenar muscles have been noted (43,45,47). The Martin-Gruber anastomosis: The Martin-Gruber anastomosis is an anomalous or variant communication that

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contains motor, sensory, or mixed fibers from the median or anterior interosseous nerve to the ulnar nerve in the proximal forearm (6,48,49). This anastomosis has been found in approximately 15% (range, 10% to 44%) of dissected forearms (39,50–52). Several variations of this anastomosis are recognized, although most of the communications consist of a communication branch that originates from either the trunk of the median nerve or from the anterior interosseous nerve and crosses ulnarly to reach the ulnar nerve in the proximal, middle, or distal forearm. Approximately half of the communications are recognized to arise from the anterior interosseous nerve (6). Mannerfelt cites the earliest known description of the anomaly by Martin in 1763 (46,53). Gruber made similar findings in 1870 (51). The connections usually pass distally and ulnarly, dorsal and adjacent to the ulnar artery, in the plane between the flexor digitorum superficialis and flexor digitorum profundus muscle bellies. In addition, a variant of the Martin-Gruber anastomosis consists of motor fibers from the motor branches of the flexor digitorum profundus crossing over to the ulnar nerve in the muscle of the flexor digitorum profundus. The Martin-Gruber communication occasionally sends branches to the flexor digitorum profundus or the flexor digitorum superficialis (54). There may be a loop-shaped connection, with convexity distally, that contains motor fibers. Straight connections usually are sensory (7). Electrophysiologic and electrodiagnostic studies have supported these anatomic findings, where investigators have identified Martin-Gruber communications carrying median nerve fibers to the hand through the ulnar nerve (55–59). There is an increased incidence of the Martin-Gruber communication in some families, and an autosomal dominant inheritance pattern of median–ulnar connections has been observed (60). Comparative anatomy studies have shown that a communication between the median and ulnar nerves exists in the proximal forearm in all baboons, rhesus monkeys, and certain (cynomolgus) monkeys (27,61). The Martin-Gruber communication presents several distinctly different types of anomalous motor innervation of the hand muscles. These have been studied and outlined by Meals, Spinner, and others (6,34,36,41,62). Of 226 ulnar or median nerve–injured patients, Rowntree found evidence of anomalous innervation of hands in 20% (41). These included cases where the median nerve innervated the first dorsal interosseous muscle, and where the ulnar nerve innervated the abductor pollicis brevis. He also noted cases of the “complete median” or “complete ulnar” innervation of the hand. The so-called all-ulnar or all-median hand probably is represented in situations where one or the other nerve is cut without evident functional impairment of the hand (36,41). There is a pattern of variation that consists of motor fibers that pass from the median to ulnar nerve, proceeding

to innervate muscles of the hand usually innervated directly by median nerve branches (44,46,47). In this case, an additional crossover occurs in the palm for these fibers to reach the thenar muscles. There is a pattern of variation where fibers pass from the median to ulnar nerve, eventually terminating in muscles that usually are ulnar nerve innervated (6,34,46,47,58). Here, the Martin-Gruber communication provides a pathway for redirecting nerve fibers that were not completely sorted in the brachial plexus. There is a pattern of variation where ulnar nerve–derived fibers targeted for muscles normally innervated by the ulnar nerve sometimes cross over into the median nerve (ulnar to median). This is a variation of the Martin-Gruber communication, and the fibers therefore must cross over again in the palm to reach their targets (6,41). Nerve anastomoses from the ulnar nerve to the median nerve also are observed, but are much more infrequent than from the median nerve to ulnar nerve. When present, the connections usually are located in the distal forearm, palmar to the flexor digitorum profundus (12). Overlapping of territory in the innervation of the flexor digitorum profundus by the median and ulnar nerves has been noted in up to 50% of specimens. It is twice as common for the median nerve to encroach on the ulnar nerve compared with ulnar encroachment on median-innervated muscles (63,64). The portion of the flexor digitorum profundus to the index finger is the only part of that muscle constantly supplied by one nerve, the median nerve (63,64). In most specimens, the flexor digitorum profundus and the lumbrical of a particular digit are innervated by the same nerve. Encroachment of the median on the ulnar nerve is less common for the lumbricals than for the flexor digitorum profundus (63,64). In 16% of specimens studied, the relation of the median nerve to the two heads of the pronator teres varies from that traditionally described (65,66). Some of these variations have been found to be associated with congenital absence of the ulnar head of the pronator teres. When the ulnar head is absent, the nerve (which usually passes between the ulnar and humeral heads) has been found to pass either deep to the humeral head in 6% or through the humeral head in 2% (26). Variations of the Anterior Interosseous Nerve Several variations of the anterior interosseous nerve have been described. Anterior Interosseous Nerve Innervation to the Flexor Digitorum Superficialis Sunderland has noted that in 30% of 20 specimens studied, the anterior interosseous nerve supplied a branch to the flexor digitorum superficialis (35,63). The specimens also had separate nerve innervation from the main trunk of the median nerve supplying the flexor digitorum superficialis. Thus, in a

3 Nerve Anatomy

dense anterior interosseous syndrome, there may be some variable weakness of the flexor digitorum superficialis (6). Anterior Interosseous Nerve Innervation to Gantzer’s Muscle Gantzer’s muscle is an accessory head to the flexor pollicis longus (67–69). Its presence is variable, but it has been noted in up to two-thirds of limbs. It is innervated by the anterior interosseous nerve in most specimens (69). Gantzer’s muscle is of clinical significance because it may be a causative factor in anterior interosseous nerve syndrome by muscle/fibrous entrapment; in addition, fibrosis of the muscle with secondary contraction can produce a flexion contracture of the thumb distal phalanx (69). High Division of the Median Nerve and Bifid Median Nerve in the Forearm The median nerve may aberrantly divide into two components at the level of the wrist or forearm. Subsequently, two separate nerve “branches,” a medial and a lateral component, extend down the forearm and enter the carpal tunnel. The two branches can be of equal or unequal size. Early descriptions of this anomaly, as noted by Sunderland, were by Gruber, who described four cases in which the median nerve branch to the third web space originated in the proximal forearm (6,44). In addition, Amadio found high branching of the median nerve in 3% of cases (70). Hartmann and Winkelman and Spinner also have reported a similar high branching of the median nerve in the forearm (71,72). In most of the cases studied by Amadio, the bifid median nerve had two branches that remained independent of one another. However, two of nine cases had a loop communication in which one or the other median nerve branch received a communicating branch from the other in or just distal to the carpal canal (70). This communicating loop was also noted in 3 of 29 cases reported in the literature at the time of Amadio’s study (70). The variant branch of the nerve may pass through the muscle mass or anterior to the flexor digitorum superficialis (instead of its usual course deep to the muscle) (6). At the level of the division, a small or large ellipse or opening can occur, in which a tendon, muscle, or vascular structure can pass (6,71). The high division of the median nerve can be accompanied with multiple other variants, including the Martin-Gruber anastomosis, a communication between the ulnar and median nerves distal to the flexor retinaculum, and two components to the median nerve crossing the distal half of the forearm and carpal canal (6). The high division of the median nerve is a true division of the nerve into two separate components. It therefore probably is incorrect to describe this variant as a “duplication” of the median nerve, as it is sometime referred to in the literature (see also later discussion of bifid median nerve, under Anomalies and Variations: Median Nerve in the Wrist and Hand).

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Accessory Motor Supply to the Flexor Digitorum Superficialis Spinner has noted several variations to the flexor digitorum superficialis (6). An accessory nerve to the flexor superficialis can arise from the motor branches to the flexor carpi radialis or palmaris longus. The accessory branch usually crosses between the superficial and deep head of the pronator teres. This branch then crosses deep to the flexor digitorum superficialis arch to innervate a portion of the flexor digitorum superficialis muscle. Similarly, the anterior interosseous nerve, which supplies the flexor pollicis longus and flexor digitorum profundus to the index and long fingers, also may at times supply a portion of the flexor digitorum superficialis. Variations of the Palmar Cutaneous Branch of the Median Nerve The palmar cutaneous branch of the median nerve usually divides from the median nerve trunk approximately 5 to 7 cm proximal to the wrist (approximately 5.5 cm proximal to the styloid) and traverses the transverse carpal ligament through its own fibrous tunnel (6,17). Several variations of the palmar cutaneous branch have been noted. Two Separate Branches of the Palmar Cutaneous Branch Two separate nerves of the palmar cutaneous branch may exist. One can arise at the usual location. The other can arise more proximally, from 9 cm or more proximal from the median nerve (44). In addition, two palmar cutaneous nerves may exit the median nerve trunk at the normal location, approximately 5.5 cm proximal to the styloid (73,74). Distal Exit of the Palmar Cutaneous Branch of the Median Nerve The palmar cutaneous branch of the median nerve may exit the median nerve trunk more distally than usual. It may continue with the median nerve trunk to the very distal forearm flexor compartment before crossing the transverse carpal ligament (70). It also has been observed to arise from the median nerve at the radial styloid or in the proximal end of the carpal tunnel. It penetrates the transverse flexor retinaculum and palmar fascia to reach the skin at the base of the thenar muscles. Absence of the Palmar Cutaneous Branch of the Median Nerve Complete absence of the palmar cutaneous branch of the median nerve has been noted (6,44). In its absence, it has been replaced with either an anterior division of the musculocutaneous nerve, a branch of the superficial radial nerve, a branch of the palmar cutaneous nerve from the ulnar nerve, or a combination of these branches (75). Palmar Cutaneous Nerve Deep to the Palmaris Longus The palmar cutaneous branch of the median nerve may lie deep to the tendon of the palmaris longus, especially if the

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palmaris longus is abnormal. At the level of the wrist crease, the palmaris longus tendon may have a broad insertion into the palmar fascia or a variant muscular attachment. In these cases, the palmar cutaneous nerve has been noted to be deep to or adjacent to the palmaris longus tendon (6). Clinical Correlations: Median Nerve in the Forearm

findings from electrodiagnostic studies, when evaluation for nerve compression is sought at specific sites. A patient with carpal tunnel syndrome with median-to-ulnar nerve communication may have normal latency from the elbow to the thenar muscles, but prolonged latency across the wrist (36). Because the incidence of the Martin-Gruber connection is high (10% to 44%), it is not surprising that inconsistencies occur between the clinical examination and electrodiagnostic studies (39,50–52).

Martin-Gruber Anastomosis The Martin-Gruber anastomosis consists of an anomalous communication that carries motor fibers from the median nerve to the ulnar nerve in the forearm (6,49) (see earlier, under Anomalies and Variations: Median Nerve in the Forearm). The motor fibers from the median nerve cross from either the median nerve trunk or from the anterior interosseous nerve, and travel to reach the ulnar nerve in the proximal, middle, or distal forearm. The Martin-Gruber communicating fibers from the median nerve often carry the motor innervation of several of the intrinsic muscles of the hand. These muscles include the first dorsal interosseous, the adductor pollicis, the abductor digiti quinti, and, less commonly, the second and third dorsal interosseous muscles (46). Both anatomic and electrical studies have noted these findings (6,46). If a high ulnar nerve laceration (at or proximal to the proximal forearm) is accompanied with preservation of intrinsic muscle function, along with loss of function of the flexor carpi ulnaris and flexor digitorum profundus to the little finger, a Martin-Gruber communication should be suspected distal to the area of nerve injury. If a high median nerve laceration (at or proximal to the proximal forearm) is accompanied with loss of some of the intrinsic muscles (usually innervated by the ulnar nerve), a Martin-Gruber communication should be suspected distal to the area of nerve injury. Additional support for this occurrence is provided if normal sensibility to the ring and little fingers remains (innervated by the ulnar nerve). Spinner has reported a patient with a complete ulnar nerve laceration at the wrist that did not develop clawing. That same patient did develop transient clawing only after blocking the ulnar nerve at the elbow with local anesthetic (6). A Martin-Gruber communication distally may have been the pathway through which ulnar nerve–derived fibers reached the intrinsic muscles (36). Electrophysiologic Studies and the MartinGruber Anastomosis Electrophysiologic studies have been used to evaluate and confirm the presence of Martin-Gruber connections (55–59). When the Martin-Gruber connection carries median nerve fibers to the hand through the ulnar nerve, this can result in varying degrees of anomalous innervation of the intrinsic muscles. This also effects or confuses the

Compression of the Median Nerve in the Forearm The median nerve is at risk for compression at several sites in the forearm. These include the lacertus fibrosus, the two heads (humeral and ulnar heads) of the pronator teres muscle, and the proximal origin or deep fascia of the flexor digitorum superficialis (17,76–80). Pronator Syndrome The pronator syndrome is a result of median nerve compression in the proximal forearm, most often caused by or related to the pronator teres muscle (6,77,81–84). The clinical syndrome includes several findings: pain in the proximal volar forearm that is increased with pronation against resistance; paresthesias or numbness in the palmar thumb, index, long, and radial ring finger; negative Phalen’s test (wrist flexion does not produce median nerve paresthesias); variable weakness of the median-innervated intrinsic muscles (thenar muscles and radial lumbricals); normal extrinsic function of muscles innervated by the anterior interosseous nerve (flexor pollicis longus, flexor digitorum profundus to the index and long, and pronator quadratus); and electrodiagnostic studies suggestive of localized sensory and motor conduction delay in the proximal forearm (and absence of generalized polyneuropathy). (Electrodiagnostic studies may be variable and unreliable.) Although the pronator teres muscle most often is the site of compression of the median nerve, compression at two other adjacent sites also has been included in the pronator syndrome (6). These include compression by the lacertus fibrosus and by the fibrous arch of the flexor digitorum superficialis. Reproduction of forearm pain with elbow flexion and forearm supination against resistance suggests involvement of the lacertus fibrosus. Forearm pain reproduced by flexion of the long finger proximal interphalangeal joint (flexor digitorum superficialis) suggests a site of compression at the arch of the flexor digitorum superficialis. Causes of Pronator Syndrome Anatomic abnormalities and related problems that have been observed with the pronator syndrome include (6,66, 81,85–92): n Hypertrophied pronator teres n Fibrous bands in the pronator teres or associated tendons (93)

3 Nerve Anatomy

n Median nerve passing posterior to both heads of the pronator teres n Thickened lacertus fibrosus (94) n Hematoma deep to the lacertus fibrosus, resulting from blood sample drawn from antecubital fossa with difficulty in patient on renal dialysis or anticoagulant therapy n Thickened flexor digitorum superficialis arch n An accessory tendinous origin of the flexor carpi radialis from the ulna n Tightness of the lacertus fibrosus from serial casting to correct elbow flexion contractures Anterior Interosseous Nerve Syndrome Compression or injury causing neuropathy of the anterior interosseous nerve usually is associated with a classic clinical presentation referred to as the anterior interosseous syndrome (6,95–113). Because of Kiloh and Nevin’s early description of neuritis of the anterior interosseous nerve (114), the syndrome also has been referred to as the Kiloh-Nevin syndrome, especially in the international literature (115–119). The clinical findings consist of paralysis or weakness of the flexor pollicis longus, flexor digitorum profundus to the index and long fingers, and pronator quadratus. An episode of pain in the proximal forearm may precede the clinical paresis. When the patient attempts to perform a thumb-to-index pulp pinch or a three-jaw chuck pinch, the interphalangeal joint of the thumb and the distal interphalangeal joints of the index and long collapse into extension (owing to weakness of the associated flexor muscles to the distal joints). Forearm pronation may be weak because of involvement of the pronator quadratus, although the pronator teres is intact and still provides some pronation. There is no detectable sensibility abnormality or involvement of other muscles supplied by the median nerve. Variations in clinical presentation can exist depending on the extent of the nerve lesion, whether partial or complete, and the specific site of involvement along the course of the nerve. In addition, specific anatomic variations in a particular limb may contribute to variations in clinical presentation. Spinner has noted that in the extreme all-median hand, the anterior interosseous nerve supplies all of the flexor profundus muscles. Thus, in this variant, there would be weakness of flexion of the distal phalanx of the ring and small fingers as well (6). Conversely, in variations where the ulnar nerve innervates more of the profundi, the flexor digitorum profundus of the long finger may be unaffected or only partially weakened by loss of function of the anterior interosseous nerve (6). To test for insolated function of the pronator quadratus in the presence of anterior interosseous nerve syndrome, the pronation power of the pronator teres must be eliminated. This can be accomplished by testing for forearm pronation strength with the elbow fully flexed. In this position, most of the pronation strength of the pronator teres is eliminated as the muscle is shortened and slack. This can by corroborated by direct electrodiagnostic studies.

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Causes and Sites of Anterior Interosseous Nerve Compression or Injury Several causes of anterior interosseous nerve compression or injury have been recognized, including injury by penetrating trauma, external compression, intrinsic compression by either muscle/tendon structures or vascular structures, and iatrogenic causes (6,120,121). Penetrating injuries of the proximal forearm have included glass and metal lacerations, stab wounds, injections by drug abusers, and gunshot injuries. Fractures also have been known to result in anterior interosseous syndrome (122), and usually consist of either supracondylar fractures in children or forearm fractures treated in either an open or closed fashion (6,123, 124). Iatrogenic injury also has been reported after cutdown catheterization in the forearm (125) and from the flexor pronator slide procedure (126). Causes of external compression include tight-fitting casts, especially the proximal rim of the short arm cast. Several causes of intrinsic compression have been noted. Those involving compression by muscle or tendon structures include (6): n A tendinous origin of the deep head of the pronator teres (a tendinous loop encircling the median nerve at the level of the origin of the anterior interosseous nerve) (6) n A tendinous origin of the flexor superficialis to the long finger n An accessory head of the flexor pollicis longus (Gantzer’s muscle) n An accessory muscle and tendon from the flexor superficialis to the flexor pollicis longus n A tendinous origin of anomalous muscles such as the palmaris profundus or the flexor carpi radialis brevis (127) n An enlarged bicipital bursa encroaching on the median nerve near the origin of the anterior interosseous nerve n Vascular structures such as thrombosis or dilation of crossing ulnar collateral vessels, and an aberrant radial artery Anterior Interosseous Nerve and the Martin-Gruber Anastomosis The Martin-Gruber anastomosis (between the median and ulnar nerves) occurs in 15% of limbs (54). In approximately half of these anastomoses, the communication branch arises from the anterior interosseous nerve. The communicating branch from either the median nerve or anterior interosseous nerve often carries fibers to various intrinsic muscles, including the first dorsal interosseous, adductor pollicis, abductor digiti minimi, and, less commonly, the second and third dorsal interosseous. Therefore, as noted by Spinner, in the presence of a Martin-Gruber communication, a patient with dense anterior interosseous nerve syndrome also may show some dysfunction of the intrinsic muscles of the hand (6). Anterior Interosseous Nerve and the Flexor Digitorum Superficialis Sunderland has noted that in 30% of 20 specimens studied, the anterior interosseous nerve supplied a branch to the

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flexor digitorum superficialis (35,63). The specimens also had separate innervation from the main trunk of the median nerve supplying the flexor digitorum superficialis. Spinner thus has pointed out that in a complete anterior interosseous nerve syndrome, there also may be some variable weakness of the flexor digitorum superficialis muscles (6). Differential Diagnosis in Anterior Interosseous Nerve Syndrome Several clinical conditions can produce loss of flexion of the distal joints of the thumb, index, and long finger. These include brachial plexus compression, traumatic lesions, or neuritis (Parsonage-Turner syndrome; see later), compartment syndrome or Volkmann’s contracture, attritional rupture of the radial flexor tendons, and congenital absence of the flexor tendons (128,129). Chronic inflammatory conditions such as rheumatoid arthritis can produce carpal subluxation or tendon-damaging irregularities involving the scaphoid or lunate. These can produce attritional ruptures of the radial digital flexors of the hand. Congenital absence of the deep flexors of the hand can involve the flexor pollicis longus and the flexor digitorum profundus, thus resulting in a pinch similar to that seen in anterior interosseous syndrome. A history of weakness since birth, along with electrodiagnostic studies, helps confirm the diagnosis of the congenital condition (6). Anterior Interosseous Nerve Palsy and the Neuritis of Parsonage and Turner In the patient presenting with weakness of flexion of the interphalangeal joint of the thumb and the distal interphalangeal joints of the index and long fingers, the differential diagnosis includes, besides the anterior interosseous syndrome, the neuritis described by Parsonage and Turner (129). In the Parsonage-Turner syndrome, there often is weakness of the distal phalanges of the thumb and index fingers. However, there usually is an associated variable weakness of the scapular muscles, which distinguishes this form of brachial plexopathy from anterior interosseous nerve palsy. High Division of the Median Nerve (Bifid Median Nerve) High division of the median nerve can subject the nerve to potential injury during forearm dissection, especially if one of the two branches is not recognized. The variant branch of the nerve may pass through the muscle mass or anterior to the flexor digitorum superficialis (instead of its usual course deep to the muscle) (6). If unrecognized, the anomalous nerve branch is at additional risk for injury during operative procedures in the region. High Division of the Median Nerve (Bifid Median Nerve) and Forearm Lacerations Laceration of the forearm associated with numbness of the third web space and accompanying loss of sensibility in the

ulnar half of the long finger and radial half of the ring finger suggests the occurrence of a bifid median nerve with laceration to the ulnar component (or perhaps a partial laceration of a normal median nerve) (6). Conversely, forearm laceration with sparing of sensibility to the third web space suggests either incomplete median nerve laceration (in a normal nerve) or laceration to the radial component of a bifid median nerve. Injury to the Palmar Cutaneous Branch of the Median Nerve Surgery adjacent to or along the ulnar border of the flexor carpi radialis must be performed with caution to avoid injury to the palmar cutaneous branch of the median nerve. The flexor carpi radialis and the radial styloid can be used to help identify the palmar cutaneous branch of the median nerve. The nerve usually exits the median nerve trunk as one branch, approximately 5.5 cm proximal to the radial styloid. The exit point is along the radial margin of the flexor digitorum superficialis and continues along the ulnar margin of the flexor carpi radialis longus tendon. If the nerve is injured, the resulting loss of sensibility may be of secondary concern compared with problems associated with a painful neuroma (37,130). A painful neuroma can be disabling. For this reason, if the palmar cutaneous branch of the median nerve is inadvertently injured, or if the nerve is found injured from penetrating trauma, nerve repair, if possible, usually is warranted (more from the standpoint of neuroma prevention than from that of sensibility restoration). If the nerve is not reparable, it can be transected cleanly at its point of exit from the nerve trunk, or can be placed in an area of protection deep to or inside a muscle belly (37,130). Isolated Compression of the Palmar Cutaneous Branch of the Median Nerve Entrapment of the palmar cutaneous nerve has been reported, caused by or associated with an abnormal palmaris longus tendon. Associated symptoms included localized pain, and numbness at the base of the thenar muscles. Nerve decompression may be indicated (6,131). Absence of the Palmar Cutaneous Branch of the Median Nerve With absence of the palmar cutaneous branch of the median nerve, sensibility at the base of the thenar muscles usually is provided by the anterior division of the musculocutaneous nerve, a branch of the superficial branch of radial nerve, a branch of the palmar cutaneous nerve from the ulnar nerve, or a combination of these branches (75). In these situations, lacerations of any of these nerves results in anesthesia at the base of the thenar muscles.

3 Nerve Anatomy

Peripheral Block of the Palmar Cutaneous Branch of the Median Nerve To provide adequate local anesthesia for procedures in the region of the palmar thenar muscles, block of the palmar cutaneous branch must be included along with block of the median nerve (unless the median nerve is blocked proximal to the origin point of the palmar cutaneous nerve). Usually, infiltration of anesthetic solution along the ulnar border of the flexor carpi radialis anesthetizes the palmar cutaneous branch of the median nerve.

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The median nerve usually passes through the carpal tunnel as the most palmar structure (volar to the flexor tendons), with the transverse carpal ligament lying immediately against the palmar surface of the nerve. The median nerve then divides into three common palmar digital nerves (discussed later). In general, the common digital nerves divide at the junction of the middle and distal third of the metacarpal shafts to form the proper digital nerves. This branch point usually is approximately 1 cm distal to the superficial palmar arch.

Median Nerve in the Wrist and Hand

Common Palmar Digital Nerves

The median nerve becomes superficial in the distal forearm approximately 5 cm proximal to the wrist, surfacing from the radial border of the flexor digitorum superficialis. The nerve continues distally, deep and slightly radial to the palmaris longus (if present). The nerve is ulnar to the flexor carpi radialis and anterior and ulnar to the flexor pollicis longus. In the very distal forearm or at the level of the wrist, the median nerve comes to lie palmar to the flexor digitorum superficialis, and continues into the carpal region by entering deep to the transverse carpal ligament (flexor retinaculum). The median nerve enters the carpal tunnel at a level that corresponds to the volar flexion crease of the wrist. The carpal tunnel boundaries comprise the transverse carpal ligament on the palmar aspect, the scaphoid and trapezium on the radial aspect, the hook of the hamate and pisiform on the ulnar aspect, and the palmar radiocarpal ligaments on the dorsal aspect. The median nerve usually enters the carpal tunnel as one nerve trunk. At this level, the internal topography of the nerve is well organized and consistent. Within the epineurium, the groups of fascicles are arranged linearly according to their destination. The motor fibers are anterior. The sensory fascicles for each of the web spaces and the radial three and one-half digits are located from lateral to medial in progressive sequence in the nerve (6,35,44).

The first common palmar digital nerve divides into three proper palmar digital nerves, two of which supply sensibility to the palmar aspects of the thumb and one that continues as the proper palmar digital nerve for the radial aspect of the index finger (after supplying a small nerve branch to the first lumbrical) (1,2,4,11). This branch to the first lumbrical branches off just distal to the edge of the transverse carpal ligament, in the proximal or middle palm (Fig. 3.2). The second common palmar digital nerve supplies a small nerve branch to the second lumbrical, and continues to the web between the index and long fingers. The nerve splits into proper digital nerves for the ulnar aspect of the index finger and the radial aspect of the long finger (1–4,11) (Fig. 3.2). The third common palmar digital nerve occasionally gives a small branch to the third lumbrical (in which the muscle receives double innervation from both the ulnar and median nerves). The third common palmar digital nerve also often communicates with a branch of the ulnar nerve, and continues to the web space between the long and ring fingers. The nerve then splits into proper digital nerves to supply the ulnar aspect of the long finger and radial aspect of the ring finger (Fig. 3.2).

Recurrent Motor Branch

The proper digital nerves of the median nerve supply the skin of the palmar surface and the dorsal surface of the distal phalanx of the respective digits. At the end of each digit, the nerve terminates in two or three branches. One branch usually innervates the pulp of the digit, another usually supplies the tissue deep to the nail. These nerves often communicate with the dorsal digital branches of the superficial radial nerve. In the palm, the median nerve branches usually are located deep to the associated arterial structures, but superficial (palmar) to the flexor tendons. These branches pass deep to the superficial palmar arch and usually cross deep to the common digital arteries as the nerves and arteries course distally. The division of the common digital nerves into proper digital nerves usually occurs at the level of the metacarpal necks. At this level, the proper digital nerves course more palmarly, to come to lie palmar (superficial) to the digital arteries. The nerves enter the digits between the

After passing through the carpal tunnel, the recurrent motor branch to the thenar muscle arises from the radial surface of the median nerve (132–134). Variations of the point of branching are well appreciated (see later, under Anomalies and Variations: Median Nerve in the Wrist and Hand). Most commonly, an extraligamentous recurrent branch leaves the main nerve trunk at the distal margin of the transverse carpal ligament. The nerve branch curves proximally and radially to enter the thenar muscles. This pattern has been noted in 46% of studied specimens. The first muscle branch usually is to the flexor pollicis brevis, followed by a branch to the abductor pollicis brevis. The nerve then passes deeply to innervate the opponens pollicis from the ulnar border of the muscle. The motor branch of the median nerve rarely may supply innervation to the first dorsal interosseous muscle (7).

Proper Digital Nerves

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deep and superficial transverse metacarpal ligaments, maintaining their palmar relationship to the digital arteries (7). Anomalies and Variations: Median Nerve in the Wrist and Hand Because of its clinical relevance, the anatomy of the median nerve has received substantial attention in anatomic studies. As a result, several variations and anomalies have been noted (36,44,134–139). In general, the anomalies usually are one of the various patterns of the median nerve in the carpal tunnel, or involve the median-to-ulnar or ulnar-tomedian nerve anastomosis in the palm. Median Nerve Variations in the Carpal Tunnel In the carpal tunnel, several variations of median nerve anatomy have been described (6,41,71,72,137,138, 140–149) (Table 3.1). Lanz has described eight patterns (138). These variations also have been classified by Spinner (6) and by Amadio, based on evaluation of 275 carpal tunnel releases (70) (see Table 3.1). The variations described by Lanz (138) include the following: Among the most common patterns is the usual form and course where the recurrent motor branch exits from the radial aspect of the median nerve trunk just distal to the transverse carpal ligament. This is a relatively safe pattern when performing carpal tunnel release because the recurrent motor branch courses distal to the area of ligament transection. Although this is the most common pattern of the recurrent branch of the median nerve, Amadio found an overall 19% incidence of variations in a study of 275 patients undergoing carpal tunnel release (70) (see Table 3.1).

TABLE 3.1. CLASSIFICATION OF MEDIAN NERVE ANOMALIES IN THE CARPAL TUNNEL High division Open branching Closed loop Motor branch Transretinacular Multiple Multiple and transretinacular Palmar cutaneous branch Transretinacular Multiple Multiple and transretinacular Median–ulnar sensory ramus (Arising on median nerve proximal to superficial arch) Unclassified From Amadio PC. Anatomic variation of the median nerve within the carpal tunnel. Clin Anat 1:23–31, 1988.

A Transligamentous Passage of the Recurrent Motor Branch A transligamentous (transretinacular) passage of the recurrent motor branch is a pattern where the recurrent branch penetrates the transverse carpal ligament, usually in the distal half. This pattern is the second most common type, and is potentially problematic because the motor branch may travel in the ligament and is at risk for injury when the ligament is transected during carpal tunnel release (70,134, 135–139). The relatively high frequency of this transligamentous course of the recurrent branch has been well documented by several authors (134,135–139). Spinner describes a separate tunnel for the nerve in its transligamentous course, where the nerve passes through the transverse retinaculum 2 to 6 mm from the distal margin of the ligament (6,137,149). The length of the transligamentous tunnel is 15 to 30 mm (134,135–139). When the transligamentous pattern is encountered during carpal tunnel release, the nerve branch should be decompressed throughout its tunnel through the ligament. Subligamentous Origin of the Recurrent Motor Branch Subligamentous origin of the recurrent motor branch is a pattern where the recurrent motor branch leaves the median nerve trunk more proximally, within the carpal tunnel, but continues in a distal direction to the distal edge of the transverse carpal ligament and curves back to the thenar muscles in a retrograde fashion. The nerve branch does not penetrate the transverse carpal ligament. Multiple Recurrent Motor Branches Multiple recurrent motor branches is a pattern where the nerves originate from the median nerve trunk in the more common site just distal to the transverse carpal ligament, but more than one branch is present (70,138,139,143, 146,147). This anomaly was found in 4% of patients undergoing carpal tunnel release (70). When there are multiple branches present, it is not uncommon for some branches to pass through the ligament (70,143,147). The nerve branches also may course either in their usual recurrent course or through different aberrant paths. On occasion, an accessory motor branch can arise in the distal forearm or proximal wrist. It can pass through the carpal tunnel or through the flexor retinaculum (138,146–148). In Amadio’s study, when multiple recurrent branches were present, approximately half of the branches were found to pass through the retinaculum (70). Mumford et al. found 2 branches in 1 of 10 dissections; one of the branches passed through the retinaculum (134). An accessory thenar nerve arising from the first common digital nerve or the radial proper digital nerve was noted and reported by Mumford et al. These findings were seen in 15 of 20 hands dissected (134). The accessory thenar nerve was the only median nerve supply to the flexor pollicis brevis in eight specimens.

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Distal Branching of the Median Nerve Distal branching of the median nerve is a pattern where the recurrent motor branch leaves the median nerve more distally in the palm, distal to the carpal tunnel. The nerve branch then loops back proximally to reach the thenar muscles, extending in a retrograde fashion. Recurrent Motor Branch Arising from the Ulnar Aspect of the Median Nerve The recurrent motor branch arises in or distal to the carpal tunnel, but the branch point usually is on the ulnar aspect of the median nerve trunk. In addition, the motor branch can arise from the central, anterior surface of the median nerve, then pass ulnarly and distally until it clears the transverse carpal ligament, where it turns and passes radially and somewhat retrograde over the ligament to reach the thenar muscle mass (36,150). A variation of this anomaly was reported by Papathanassiou, who noted one clinical case and one dissection specimen in which the motor branch arose from the ulnar, anterior aspect of the radial division of the median nerve (149). This anomaly was found in 16 of 20 dissections by Mumford et al. (134). The anomaly also was encountered once by Lanz (138,146). Ulnar-sided Exit of the Recurrent Motor Branch with Hypertrophy of the Flexor Pollicis Brevis or Palmaris Brevis An associated concomitant hypertrophy of the flexor pollicis brevis or palmaris brevis has been noted to occur commonly with the aberrant origin of the recurrent motor branch arising from the ulnar side of the nerve rather than from the radial aspect (150). The hypertrophied flexor pollicis brevis lies anterior to the flexor retinaculum. Spinner emphasizes that when this muscle variant is found, it is safer to identify the median nerve in the carpal tunnel, and locate the motor branch by opening the carpal tunnel on the medial side. The motor branch can then be traced distally as it recurs through the superficial hypertrophied muscle (6,150). Recurrent Motor Branch Arising Anteriorly Recurrent motor branch can arise anteriorly, then pass over the surface of the transverse carpal ligament. The recurrent motor branch arises in the carpal tunnel, more proximally than normal, originating from the palmar aspect of the nerve. The nerve extends distally, around the distal edge of the transverse carpal ligament, and loops back proximally to reach the thenar muscles in a retrograde fashion. Recurrent Motor Branch and Median Nerve Passing Anterior to the Transverse Carpal Ligament A rare pattern noted by Sunderland involves the entire median nerve passing superficial to the transverse carpal ligament (44).

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Absence of the Recurrent Motor Branch to the Thenar Muscles Complete absence of the recurrent motor branch to the thenar muscles has been described (6,41,140). This is observed in the all-ulnar hand, in which all of the thenar muscles are innervated by the ulnar nerve through various communicating branches (41,140). High Division of the Median Nerve (Bifid Median Nerve) Branching of the median nerve proximal to the wrist is well described, and often presents as a bifid median nerve (135,141,144). The bifid median nerve can be discovered in the carpal canal during carpal tunnel release or in the forearm during operative exploration (138,141,144,146). There usually is a larger, more radial component and a smaller ulnar component that travels parallel to the larger component. This anomaly has been described by Gruber, who noted four cases in which the median nerve branch to the third web space originated in the proximal forearm. Amadio found high branching of the median nerve in 3% of cases (70). Hartmann and Winkelman and Spinner also have reported high branching of the median nerve in the forearm (71,72). In most of the cases studied by Amadio, the bifid median nerve had two branches that remained independent of one another. However, two of nine cases had a loop communication in which one or the other median nerve branch received a communicating branch from the other in or just distal to the carpal canal (70). This communicating loop also was noted in 3 of 29 cases reported in the literature at the time of Amadio’s study (70). A median artery also may be present with the bifid median nerve. The median artery is an anomalous artery that is a persistent extension of the anterior interosseous artery. The median artery can result from persistence of an embryonic artery known as the forearm axis artery. Anomalous muscles such as aberrant flexor digitorum superficialis or lumbricals also have been associated with a high division of the median nerve. Riche-Cannieu Anastomosis Nerve communication between the median nerve recurrent motor branch and the ulnar nerve deep branch is referred to as a Riche-Cannieu communication or anastomosis. In 1897, Riche and Cannieu independently described a connection between these nerves occurring between the fibers of the median nerve recurrent motor branch traveling to the superficial head of the flexor pollicis brevis and the fibers of the deep ulnar branch going to the deep head of the flexor pollicis brevis (151,152). Mannerfelt drew additional attention to this important anastomosis (46,150). The communicating fibers pass radially from the deep ulnar branch between the heads of the adductor pollicis, then pass deep to the flexor

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pollicis longus tendon. The fibers continue proximally to the radial side of the flexor pollicis longus tendon as they approach the median nerve recurrent motor branch. This communication was found in 77% of cadaver specimens studied, and was found in virtually all fresh cadaver hands (153). Riche described two other anatomic median–ulnar nerve communications. In one, the communication occurred between a thumb digital nerve (derived from the median nerve) and fibers en route to the adductor pollicis (derived from the deep ulnar nerve branch). The communicating fibers were found in the adductor muscle on the medial side of the flexor pollicis longus tendon. In another pattern, the communicating fibers passed through the first lumbrical, which was innervated by the ulnar nerve (36,152). It is now assumed that the Riche-Cannieu connection usually carries motor fibers only (36,150,153), although early investigators thought it carried sensory fibers (154). Foerster, as a result of war-injury studies, and Harness and Sekeles, as a result of anatomic dissections, believed that the anastomosis was of the motor type (153,155,156). Because Harness and Sekeles found that most of the preserved specimens studied (77%) and virtually all of the fresh specimens contained the Riche-Cannieu communication, they concluded that this nerve anastomosis is common and normal, and may represent the more usual innervation pattern of the thenar muscles (153). Additional clinical and electromyographic studies have supported this consideration (36,153). However, Mannerfelt has noted that the nature (sensory, motor, or mixed), incidence, and direction of the fiber passage (i.e., median to ulnar nerve, or ulnar to median nerve) remain unresolved (36,150). Either way, the communication provides a potential pathway for double innervation of the intrinsic muscles anywhere in the hand. A variation of the RicheCannieu anastomosis has been noted by Harness and Sekeles and by Hovelacque, in which a branch from the deep ulnar nerve communicates with a thumb digital nerve. This presents the possibility that median motor fibers destined for the thenar muscles were traveling in the digital nerve (36,157). Basic Patterns of the Riche-Cannieu Anastomosis Spinner has summarized the basic patterns of the RicheCannieu anastomosis (6): n An anastomosis in the substance of the adductor pollicis between the median and ulnar nerves n A communicating branch from the motor branch of the median nerve coursing anterior to the radial head of the flexor pollicis brevis and the ulnar component passing deep to the ulnar head of this muscle n Anastomosis between the two motor nerves across the first lumbrical

n Anastomosis between the branch of the deep ulnar nerve to the adductor pollicis or flexor pollicis brevis and the median nerve digital branch to the thumb or index finger Palmar Ulnar–Median Communicating Branch of Berrettini As noted previously, the Riche-Cannieu anastomosis usually carries motor fibers and occurs in the region of the adductor pollicis and thenar muscles. However, a distal communicating branch between the ulnar and median sensory nerves is not uncommon; in fact, the presence of a communicating branch may be the most common (and normal) nerve pattern. Classically, palmar sensation in the fingers is described as divided between ulnar and median nerves at the midline of the ring finger. Berrettini described and illustrated this communicating branch in 1741 (158). More recently, Meals and Shaner found a communicating branch between the ulnar and median nerves in the palm in 40 of 50 dissected specimens. Several studies have confirmed the common presence of this communicating branch (44,46,52,159). The communicating branch usually passes immediately deep to the superficial palmar arch; however, in some specimens the branch courses just distal to the transverse carpal ligament (70,157). Innervation of the Lumbricals and Associated Flexor Digitorum Profundus In general, the belly of the flexor digitorum profundus of the index finger and the first lumbrical muscle nearly always are supplied by branches of the median nerve. However, innervation of the other flexor digitorum profundus muscle bellies and their corresponding lumbricals is quite variable. The lumbrical usually is supplied by the same major nerve (median or ulnar) that supplies the corresponding belly of the flexor digitorum profundus. However, in 50% of cases, there are variations from the classic pattern of innervation (in which the median nerve innervates the radial two bellies and the ulnar nerve innervates the ulnar two bellies) (36). The variation usually involves the median nerve encroaching on the ulnar nerve distribution. However, the ulnar nerve also can encroach laterally to innervate the long finger belly partially or exclusively (36). Clinical Correlations: Median Nerve in the Wrist and Hand As the median nerve passes through the carpal tunnel, it is the most palmarly located structure, with the transverse carpal ligament adjacent to its palmar surface. The median nerve is therefore at inherent risk for injury during carpal

3 Nerve Anatomy

tunnel release. Scarring or adhesions add to the risk of injury if the median nerve is adherent to the ligament. This risk is especially significant when repeat or revision carpal tunnel release is performed (160,161). Anatomic Aspects of Carpal Tunnel Syndrome Several causes of carpal tunnel syndrome have been recognized (6,142,160–177). Specific anatomic abnormalities that can be factors in carpal tunnel syndrome include the following: n A palmaris profundus muscle. The palmaris profundus is a muscle that originates from the radius, ulna, and interosseous ligament in the forearm, and passes through the carpal tunnel to insert onto the dorsal surface of the palmar fascia. It can produce symptoms if its tendon is large or if the musculotendinous junction extends into the carpal tunnel (6,127) n An anomalous flexor digitorum superficialis, especially that with a muscle belly that extends distally into the carpal tunnel (178–184) n Anomalous lumbrical muscles that extend proximally into the carpal tunnel (185) n An enlarged, inflamed, thrombosed, or calcified median artery in the carpal tunnel (186,187) n A hypertrophied palmaris longus (160) High Division of the Median Nerve (Bifid Median Nerve) The high division of the median nerve results in two nerves entering the carpal tunnel. This variant can subject the nerve to potential injury during carpal tunnel release, especially if one of the two branches is not recognized. An unrecognized branch is particularly vulnerable during flexor tenosynovectomy or flexor tendon repair in the carpal tunnel. Spinner notes that in carpal tunnel syndrome with atypical findings such as sensibility abnormalities isolated only to the third web space or only to the more lateral aspect of the hand (sparing the third web space), the examiner should consider the bifid median nerve as a potential finding (6). Similarly, laceration of the forearm associated with numbness of the third web space and its accompanying digital manifestation in the ulnar half of the long finger and radial half of the ring finger suggest the occurrence of a bifid median nerve (or perhaps a partial laceration of a normal median nerve) (6). When a bifid median nerve is encountered or a median nerve found with high branches originating in the forearm, special care is required during carpal tunnel release or median nerve exploration, both for nerve protection and for adequate decompression. Release of the median nerve branches from separate fascial channels in the transverse carpal ligament may be needed (36,135).

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The Recurrent Motor Branch of the Median Nerve The most common pattern of the recurrent branch of the median nerve is the course where the nerve exits the nerve trunk distal to the transverse carpal ligament, then curves back proximally in a retrograde fashion to reach the thenar muscles. This common pattern also is relatively safe because the nerve branch does not penetrate or lie within the ligament that is transected. The presence of variations in number and patterns of the recurrent branch of the median nerve should be kept in mind during operative exploration of hand lacerations with loss of thenar muscle function. The Transretinacular Pattern The transretinacular pattern of the recurrent motor branch, in which the recurrent branch penetrates the transverse carpal ligament, is the second most common pattern, and is potentially problematic. The motor branch that travels in the ligament is at risk for injury when the ligament is transected during carpal tunnel release. Injury to the nerve with this pattern can be minimized by an appreciation of the anatomy, as well as by transection of the transverse carpal ligament carried out toward the ulnar side of the canal. When the transligamentous pattern is encountered during carpal tunnel release, the nerve branch should be decompressed throughout its tunnel through the ligament. This pattern has been thought to be potentially responsible for carpal tunnel syndrome that presents with more motor or even pure motor dysfunction, compared with sensory abnormalities (6,188). Palmar Ulnar–Median Communicating Branch of Berrettini The communicating sensory branch between the ulnar and median nerves (palmar ulnar–median communicating branch of Berrettini; see earlier) may course between the nerves just distal to the transverse carpal ligament (70,157). It is vulnerable to injury during carpal tunnel release or palmar exploration for operative procedures such as flexor tendon repair or partial palmar fasciectomy for Dupuytren’s contracture, especially along the axis of ring finger ray (36). Common and Proper Digital Nerves and Arteries During nerve and artery exploration in the palm or digits, an appreciation of the relationship between the common and proper digital nerves and arteries is emphasized. In the palm, the median nerve branches usually are located deep to the associated arterial structures. These nerve branches pass

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deep to the superficial palmar arterial arch and usually pass deep to the common digital arteries as the nerves and arteries course distally. At the approximate level of the metacarpal necks, the nerves course more palmarly, and come to lie palmarly at the base and along the digits. In the digits, the digital nerves are palmar to the digital arteries. Thus, it is possible (and not uncommon) to encounter a clinical situation where both digital nerves are lacerated in the digit, but the digit remains vascularized. The deeperlying arteries are more protected, and can therefore more often survive penetrating trauma. All Ulnar Nerve–Innervated Hand In the all ulnar nerve–innervated hand, there is absence of a thenar branch from the median nerve. With a complete median nerve laceration at the wrist, operative exploration reveals only a small median nerve in the carpal tunnel. The only deficit noted may be loss of sensibility to the palmar aspect of the index finger. The ulnar nerve provides the remaining motor and sensory fibers (6). ULNAR NERVE Origin of the Ulnar Nerve The ulnar nerve arises from the medial cord of the brachial plexus, and is composed of fibers from the anterior rami of C8, and T1 (1–4,11) (see Fig. 3.1).

TABLE 3.2. ORDER OF INNERVATION OF MUSCLES SUPPLIED BY THE ULNAR NERVE Muscle Flexor carpi ulnaris Flexor digitorum profundus Abductor digiti minimi Flexor digiti minimi Opponens digiti minimi Fourth web space interossei Third web space interossei Second web space interossei Fourth lumbrical Third lumbrical Adductor pollicis (oblique head) Adductor pollicis (transverse head) First web space interosseous From Sunderland S, Ran LJ. Metrical and non-metrical features of the muscular branches of the median nerve. J Comp Neurol 85:191, 1946.

Ulnar Nerve in the Axilla and Arm At the level of the pectoralis minor muscle, the medial cord divides into two branches. One branch courses slightly laterally to join a branch from the lateral cord to form the median nerve. The other branch of the medial cord continues distally to form the ulnar nerve (see Fig. 3.1). In the axilla and arm, the ulnar nerve remains the most medially positioned major nerve. In the axilla, the ulnar nerve is medial and adjacent to the axillary artery. The axillary vein is located medial to the ulnar nerve. At the inferior border of the subscapularis muscle, the ulnar nerve may receive additional fibers of the C7 nerve root through the “lateral root of the ulnar nerve” (189). This supplemental nerve arises from either the lateral cord or middle trunk (8). The ulnar nerve continues distally from the medial cord deep (posterior) to the pectoralis minor and pectoralis major and anterior to the subscapularis, latissimus dorsi, and teres major. Along this course, it remains medial or posteromedial to the axillary artery and subsequent brachial artery. At the inferior border of the pectoralis major, the ulnar nerve continues and diverges medially from the brachial artery (as the artery courses slightly anteriorly). The ulnar nerve pierces the medial intermuscular septum approximately 8 cm proximal to the medial epicondyle (13). As the nerve passes from the anterior compartment to the posterior compartment through the medial intermuscular septum, it passes deep to the arcade of Struthers, if present (see later, under Anomalies and Variations: Ulnar Nerve in the Axilla and Arm). In this vicinity, the brachial artery gives off the superior ulnar collateral artery, which also pierces the medial intermuscular septum and continues distally along with the nerve. The nerve remains to the medial aspect of the superior ulnar collateral artery. Both nerve and artery continue distally and medially on the anterior surface of the medial head of the triceps muscle. The artery is then joined by a branch of the inferior ulnar collateral artery at the medial supracondylar ridge. These arteries continue in close proximity to the nerve as the nerve enters the interval between the medial epicondyle of the humerus and the olecranon. The nerve passes into the ulnar groove on the dorsal aspect of the medial epicondyle. The ulnar nerve does not normally innervate any muscles of the arm, although a muscular branch to the flexor carpi ulnaris may branch from the ulnar nerve proper 1 cm proximal to the medial epicondyle (189) (Table 3.2 and Fig. 3.3). The Medial Antebrachial Cutaneous Nerve The medial antebrachial cutaneous nerve (medial cutaneous nerve of the forearm) is a sensory nerve with several branches that innervates the medial forearm (discussed in detail later, under Medial Antebrachial Cutaneous Nerve; Fig. 3.4). It is mentioned here because of its close anatomic

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FIGURE 3.3. Schematic illustration of the ulnar nerve and associated branches and innervated muscles.

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A FIGURE 3.4. Cutaneous nerves of the upper extremity. A: Anterior aspect.

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B FIGURE 3.4 (continued). B: Posterior aspect.

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proximity to the ulnar nerve. The medial antebrachial cutaneous nerve originates from the lower trunk or medial cord of the brachial plexus, just proximal to the actual origin of the ulnar nerve (190). It contains fibers from C8 and T1. In the axilla, the nerve runs with the ulnar nerve between the axillary artery and vein. A small branch leaves the nerve to supply the skin over the biceps muscle and the elbow flexion crease (along with branches of the medial cutaneous nerve of the arm, discussed in further detail later) (191). The medial antebrachial cutaneous nerve descends along the medial surface of the brachial artery. It pierces the antebrachial fascia in the middle third of the arm with the basilic vein. The nerve divides into anterior and posterior branches approximately 15 cm proximal to the medial epicondyle. The anterior branch passes anterior to the median cubital vein between the medial epicondyle and biceps tendon, and innervates the mediopalmar skin of the forearm (Fig. 3.4A-B). The terminal branches join the palmar cutaneous branches of the ulnar and median nerves in the hand. The posterior branch of the medial cutaneous nerve of the forearm often crosses the ulnar nerve from approximately 6 cm distal to the medial epicondyle. It descends along the medial side of the basilic vein, supplying the dorsomedial skin of the forearm. Distally, the nerve joins the dorsal cutaneous branch of the ulnar nerve (189,190) (Fig. 3.4A-B). Anomalies and Variations: Ulnar Nerve in the Axilla and Arm The ulnar nerve normally originates from the medial cord of the brachial plexus. It may, however, receive fibers from several other sources, including the lateral cord, the middle trunk, and the anterior division of the middle trunk. These neural elements are collectively referred to as the lateral root of the ulnar nerve. The lateral root of the ulnar nerve joins the ulnar nerve proper at or distal to the inferior border of the subscapularis muscle. The lateral root nerve fibers may provide innervation to the flexor carpi ulnaris (8). Arcade of Struthers As the ulnar nerve passes from the anterior to the posterior muscle compartment of the arm, it may encounter a myofibrous or fasciomyofibrous tunnel, the arcade of Struthers. This common structure, first described by Struthers in 1854 (18), should not be confused with the rare (1%) unrelated anatomic structure, the ligament of Struthers (which is seen in association with a supracondylar process and can result in median neuropathy in the arm; see earlier, under Median Nerve in the Axilla and Arm). The arcade of Struthers is common, and has been shown to occur in 70% of specimens (192,193). The arcade of Struthers is a fibrous or fascial sheet located in the distal third of the medial aspect of the humerus. When the arm is in the anatomic position, the roof of the arcade faces medially. It is formed

by a thickening of the deep investing fascia of the distal part of the arm, by superficial muscular fibers of the medial head of the triceps, and by attachments of the internal brachial ligament (6). (The internal brachial ligament is a relatively long, longitudinal ligament originating from the region of the coracobrachialis tendon.) The anterior border of the arcade of Struthers is the medial intermuscular septum. The lateral border of the arcade is formed by the medial aspect of the humerus covered by deep muscular fibers of the medial head of the triceps. Spinner has noted that the presence of the arcade of Struthers should be suspected if, at the time of operative exposure of the proximal portion of the ulnar nerve, the muscle fibers of the medial head of the triceps are seen crossing obliquely, superficial to the nerve. This is in the area where the nerve traverses from the anterior to posterior compartment. When no muscular fibers can be seen crossing the ulnar nerve approximately 5 to 7 cm proximal to the medial epicondyle, the arcade probably is not present (193). The arcade of Struthers may be a potential area of ulnar nerve compression. If decompression or transposition of the ulnar nerve is performed, awareness of this structure is important for through decompression. Compression of the ulnar nerve can occur above the elbow at the arcade at the level of the medial epicondylar groove, or distally as the nerve passes between the ulnar and humeral heads of the flexor carpi ulnaris (17,78,193). The First Branch The first branch of the ulnar nerve usually originates in the cubital tunnel. However, variation in the articular and first muscular branches is common. The articular branch, normally the first branch of the nerve, exits from the main trunk in the ulnar groove and passes horizontally into the joint. One or several articular branches may originate in the arm, up to approximately 1 cm proximal to the medial epicondyle. The first muscular branch, usually to the flexor carpi ulnaris, usually exits immediately distal to the articular branch. However, division as high as 4 cm proximal to the medial epicondyle has been reported (189,194). The Medial Antebrachial Cutaneous Nerve and the Ulnar Nerve The medial antebrachial cutaneous nerve may arise from several slightly different points. It usually arises from the medial cord of the brachial plexus, just proximal to the origin of the ulnar nerve. It usually arises just distal to the origin of the medial brachial cutaneous nerve, which is the smallest branch of the brachial plexus (194) (see Fig. 3.1). The medial antebrachial cutaneous nerve also may arise from the lower trunk of the brachial plexus, from the first thoracic nerve root (T1), or from the ulnar nerve itself. The medial antebrachial cutaneous nerve commonly communi-

3 Nerve Anatomy

cates with the intercostobrachial nerve in the axilla and the medial cutaneous nerve of the arm proximally (195). Clinical Correlations: Ulnar Nerve in the Axilla and Arm Arcade of Struthers During exploration of the ulnar nerve at the elbow for neuropathy, awareness of the possible presence of an arcade of Struthers is important because this may be a potential area of nerve compression (see earlier). The nerve should be explored proximally to the level of where the nerve passes from the anterior to posterior compartments. Muscle fibers of the medial head of the triceps that cross obliquely superficial to the nerve usually indicate the presence of an arcade of Struthers. If present, the fascial sheet of the arcade of Struthers should be incised. If the nerve is transposed anteriorly, it should be confirmed that an arcade of Struthers is not present or is not causing tethering or compression of the proximal aspect of the transposed nerve. The Arcade and the Ligament of Struthers The arcade of Struthers should not be confused with the ligament of Struthers. The arcade of Struthers, present in approximately 70% of studied specimens, is located at the medial intermuscular septum, and can cause compression of the ulnar nerve. The ligament of Struthers, in contrast, is rare, occurring in only 1%, and consists of a ligament or extension of the pronator teres muscle from the medial epicondyle to an (anomalous) supracondylar process. The ligament of Struthers is a possible site of compression of the medial nerve (6,17,18,20–22,78,192). Ulnar Nerve in the Elbow and Forearm Ulnar Nerve in the Cubital Tunnel The cubital tunnel at the elbow is a fibroosseous tunnel (189,196,197). The lateral border consists of the humerus, ulna, and elbow joint. The medial and inferior border consists of a fascial sheath confluent with the brachial and antebrachial fascia of the adjacent muscles. The distal medial border consists of the aponeurosis or fascia between the two heads of the flexor carpi ulnaris (6,17,78). As noted by Siegel and Gelberman, the tunnel can be divided geographically into three parts (189). Ulnar Nerve in the First Part of the Cubital Tunnel The first part of the cubital tunnel is the entrance of the tunnel, formed by the ulnar groove in the medial epicondyle. At this entrance, the ulnar nerve lies in the extensor side of the arm. In the first part, the ulnar nerve usually provides one branch or several small articular branches to the elbow joint.

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These branches usually are proximal to the branches given off to innervate the flexor carpi ulnaris (189). Ulnar Nerve in the Second Part of the Cubital Tunnel The second and middle part of the tunnel consists of a fascial arcade. (This arcade should not be confused with the arcade of Struthers, which is a separate fascial arcade located more proximally in the arm; see earlier.) The fascial arcade of the second part of the cubital tunnel attaches to the medial epicondyle and to the olecranon. It connects the ulnar and humeral heads of the origin of the flexor carpi ulnaris muscle. In this area, the nerve crosses the medial surface of the elbow. It lies on the posterior and oblique portions of the ulnar collateral ligament. The nerve usually gives off two branches to innervate the flexor carpi ulnaris. One branch usually supplies the humeral head and one supplies the ulnar head. The first branch exits the main nerve trunk horizontally. The second branch continues distally for several centimeters before entering the flexor carpi ulnaris. Up to four motor branches to the flexor carpi ulnaris may be given off, exiting the main nerve at a point between 4 cm proximal and 10 cm distal to the medial epicondyle (13). The motor branches enter the flexor carpi ulnaris on its deep surface. The first motor branch of the flexor carpi ulnaris divides in 5% of limbs to supply the flexor digitorum profundus as well (63) (see Table 3.2). In the second portion of the cubital tunnel, the distance between the medial humeral epicondyle and the olecranon is shortest with elbow extension. This distance increases with elbow flexion (198). The roof of the cubital tunnel is formed by the fascial arcade, which becomes taut with elbow flexion (189). Ulnar Nerve in the Third Part of the Cubital Tunnel The third and most distal part of the tunnel consists of the muscle bellies of the flexor carpi ulnaris. The flexor carpi ulnaris provides a portion of the roof in this area. Although the ulnar nerve enters the cubital tunnel on the extensor side of the arm (in the first part of the tunnel), it comes to lie on the flexor surface on exiting the tunnel in the third part. The nerve courses through the interval between the humeral and ulnar heads of the flexor carpi ulnaris or between the flexor carpi ulnaris and flexor digitorum profundus muscles (189). The volume of the tunnel decreases with elbow flexion, and the pressure within it increases, even in the normal elbow when the aponeurotic arch or surrounding soft tissues are not thickened. The nerve then continues distally in the forearm between the flexor digitorum profundus, located dorsally and laterally to the nerve, and the flexor carpi ulnaris, located anteriorly and medially. The nerve maintains this relationship with the muscles through the proximal to middle forearm. In general, the nerve runs a straight course through the forearm from the level of the medial epicondyle of the distal humerus to the pisiform–hamate groove in the

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carpus. In the distal third of the forearm, the ulnar nerve courses more superficially, lying just radial and deep (dorsal) to the flexor carpi ulnaris muscle (6,189). Motor Branches of the Ulnar Nerve in the Forearm In the forearm, and distal to the exit of the motor branches to the flexor carpi ulnaris, the ulnar nerve usually has three additional main branches. These are (a) the motor branch to the flexor digitorum profundus (to the ring and small fingers), (b) the palmar cutaneous portion of the ulnar nerve, and (c) the dorsal branch of the ulnar nerve (189,199). Motor Branch to the Flexor Digitorum Profundus (to the Ring and Small Fingers) The motor branch to the flexor digitorum profundus from the ulnar nerve usually innervates the ulnar half of the muscle, which includes the muscle bellies to the ring and small fingers. (The anterior interosseous nerve from the median nerve usually innervates the radial half of the flexor digitorum profundus, including the muscle bellies to the long and index fingers, as well as the flexor pollicis longus.) The motor branch from the ulnar nerve is located proximally in the forearm. It arises approximately 3 cm distal to the medial epicondyle and usually exits the ulnar nerve trunk just distal to the branches to the flexor carpi ulnaris. The motor branch passes distally for approximately 2.5 cm, usually lying on the anterior surface of the flexor digitorum profundus (1–4,11,13,189,191). It then enters the muscle at approximately 6 cm distal to the medial epicondyle (6), whereas the anterior interosseous nerve enters the flexor digitorum profundus to the index and long fingers approximately 4 to 7 cm more distally (6). In 80% of upper limbs, a single branch from the ulnar nerve supplies the flexor digitorum profundus. In approximately 20%, two or more branches supply the muscle. There may not be a direct branch from the main ulnar nerve trunk that supplies the flexor digitorum profundus. In these specimens, the flexor digitorum profundus may by innervated by the branch of the ulnar nerve to the flexor carpi ulnaris or by a branch from the median nerve. In the forearm, the ulnar nerve lies medial and adjacent to ulnar artery. Traditionally, the ulnar nerve is described as innervating the flexor digitorum profundus to the ring and small fingers, and the anterior interosseous nerve from median nerve is described as innervating the flexor digitorum profundus to the index and long fingers. This pattern, however, has been noted actually to comprise only 50% of upper limbs (32). In several studied specimens, the median nerve or derived branches was found to innervate the flexor digitorum to the ring and little fingers. In addition, the ulnar

nerve was found occasionally to supply the flexor digitorum profundus to the long finger (32,189). The flexor digitorum profundus to the index finger, however, does seem to be innervated consistently by the median nerve. Sympathetic Fibers from the Ulnar Nerve in the Forearm In the middle forearm, the ulnar nerve supplies the accompanying ulnar artery with a segmental sympathetic nerve. This is the nerve of Henle (200–202) (Fig. 3.3). Palmar Cutaneous Branch of the Ulnar Nerve The palmar cutaneous branch of the ulnar nerve is not as consistent as its median nerve counterpart, the palmar cutaneous branch of the median nerve. When present, the palmar cutaneous branch of the ulnar nerve arises at variable levels from the ulnar nerve in the distal forearm, usually in the vicinity of the junction of the middle and distal thirds of the forearm. It courses distally on or in the epineurium of the ulnar nerve on the palmar surface of the ulnar artery. The nerve then perforates the antebrachial fascia just proximal to the distal wrist flexion crease, and innervates the skin in the hypothenar eminence, the ulnar artery, and, occasionally, the palmaris brevis muscle (6) (see Figs. 3.3 and 3.4A). Dorsal Cutaneous Branch of the Ulnar Nerve The dorsal cutaneous branch of the ulnar nerve arises from the medial aspect of main ulnar nerve trunk in the distal forearm and curves dorsally to supply cutaneous innervation to the dorsal aspect of the small finger and ulnar ring finger (199,203,204) (see Fig. 3.4B). Its point of origin is an average of 6.4 cm from the distal aspect of the head of the ulna and 8.3 cm from the proximal border of the pisiform. The cross-sectional shape of the nerve at its origin usually is round or slightly oval, with a mean diameter of approximately 2.4 mm. The point of nerve origin corresponds to a point located at the distal 26% of the total length of the ulna (199). The nerve extends distally and medially, passing dorsal to the flexor carpi ulnaris, and pierces the deep antebrachial fascia. The nerve emerges at the dorsomedial border of the flexor carpi ulnaris at a mean distance of 5 cm from the proximal edge of the pisiform. At this point, the nerve pierces the deep antebrachial fascia to become subcutaneous on the medial aspect of the distal forearm. Proximal to the wrist, the nerve provides two to three branches. A branch piercing the capsule of the ulnocarpal joint usually is present. With the forearm in supination, the nerve branch passes along and close to the medial aspect of the head of the ulna near the widest diameter of the ulnar head (equator of the ulnar head). With the fore-

3 Nerve Anatomy

arm pronated, the nerve branches displace slightly palmarly to pass along the palmoulnar aspect of the ulnar head. In the hand, an additional one or two branches usually are given off. The total number of branches averages five, with a range from three to nine. Two branches typically extend to the small finger, one to the dorsoulnar aspect of the ring finger, and one or two branches to the dorsoulnar aspect of the carpus and hand. The diameters of the branches range from 0.7 to 2.2 mm (199). The branches of the dorsal branch of the ulnar nerve continue to the level of the proximal interphalangeal joints, where the nerves arborize and become difficult to trace. There are no apparent further communications between the dorsal branch of the ulnar nerve and the ulnar nerve proper, with the palmar cutaneous branch of the ulnar nerve, or with the nerve of Henle (200). In the proximal forearm, the posterior ulnar recurrent artery, which arises from the ulnar artery close to the bifurcation of the radial artery, courses ulnarly and proximally to continue in proximity to the ulnar nerve and motor branches to the flexor digitorum profundus, along the ulnar border of the nerves (1–4,11). The superior ulnar artery accompanies the ulnar nerve into the cubital tunnel. In the cubital tunnel, the superior ulnar collateral artery joins the posterior ulnar recurrent artery to form one of the vascular collateral pathways around the elbow and bypassing the distal portion of the brachial artery (1,4). In the region of the junction of the proximal and middle thirds of the forearm, the ulnar artery joins the ulnar nerve and continues on the radial aspect of the nerve. This relationship is maintained as the nerve and artery emerge from the radial edge of the flexor carpi ulnaris tendon, coursing slightly radial to pass radial to the pisiform and enter Guyon’s canal at the wrist. Anomalies and Variations: Ulnar Nerve in the Elbow and Forearm Anomalous Connections between the Ulnar and Median Nerve In the distal forearm, a crossing of nerve fibers from the ulnar nerve to the median nerve can occur, although with less frequency than the more common crossing of fibers in the opposite direction from median nerve or anterior interosseous nerve to ulnar nerve (the Martin-Gruber anastomosis). These anomalous connections between the ulnar nerve and median nerve in the forearm are discussed in detail earlier, under Nerve Anomalies and Variations: Median Nerve in the Forearm. In general, from the elbow to the wrist, the ulnar nerve shows relatively few anomalies or deviations from its normal course. The division of its branches is relatively consistent.

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Variations in Innervation of the Flexor Carpi Ulnaris The flexor carpi ulnaris usually receives two or three motor branches. Up to five branches have been noted (6). In isolated case reports, the flexor carpi ulnaris was found to have a motor branch from the median nerve (6). Variations in Innervation of the Flexor Digitorum Profundus Muscle Variations in the innervation of the flexor digitorum profundus muscle have been reported (205). Traditionally, the ulnar nerve is thought to innervate the flexor digitorum profundus to the ring and small fingers, and the median nerve innervates the index and long fingers. However, this pattern was found in only 50% of upper limbs (32). In several specimens, the median nerve was found to innervate the ring and little fingers and the ulnar nerve was found to supply the long finger (32,189). It is more common for the median nerve to innervate muscles traditionally supplied by the ulnar nerve than for the ulnar nerve to innervate muscles usually supplied by the median nerve (32,35,63). This may occur in the all–median nerve hand. Many of the variations in branching occur in the muscle belly of the flexor digitorum profundus, and therefore are difficult to identify by superficial visualization and examination of the muscle. The flexor digitorum profundus to the index finger, however, does seem to be innervated most consistently by the median nerve. Sunderland has noted only one case in which the flexor digitorum profundus to the index finger was innervated by the ulnar nerve (44). Sensory Variations of the Dorsal Branch of the Ulnar Nerve in the Forearm The dorsal branch of the ulnar nerve usually arises from the ulnar nerve trunk at approximately 6 to 8 cm from the wrist joint (mean distance of 6.4 cm from the distal aspect of the head of the ulna and 8.3 cm from the proximal border of the pisiform) (199). Several variations can occur. The branch may arise from the ulnar nerve as far proximal as the elbow and continue subcutaneously along the entire length of the forearm (206). Alternatively, an entire nerve loop has been noted to form around the pisiform between the ulnar nerve and a branch from the dorsal cutaneous nerve. This branch of the dorsal cutaneous nerve appeared to contribute additional fibers to the ulnar digital nerve to the small finger (207). Absence of the Dorsal Cutaneous Branch of the Ulnar Nerve In 1 of 24 specimens, the dorsal branch of the ulnar nerve was found to be absent (199). With complete absence of the

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dorsal cutaneous branch of the ulnar nerve, sensibility to the dorsum of the ulnar hand can be supplied by the superficial radial nerve (208), the musculocutaneous nerve (6), or the posterior cutaneous nerve of the forearm. Ulnar Nerve Compression by Anomalous Anconeus Epitrochlearis The ulnar nerve may be compressed at the elbow by an anomalous muscle, the anconeus epitrochlearis. The anconeus epitrochlearis originates from the medial border of the olecranon and adjacent triceps tendon and inserts into the medial epicondyle of the elbow. The muscle appears as an auxiliary extension of the medial portion of the triceps. The muscle crosses the ulnar nerve posterior to the cubital tunnel. When present, it forms a portion of the cubital tunnel, reinforcing the aponeurosis of the two heads of the origin of the flexor carpi ulnaris (6). The Posterior Cutaneous Nerve The posterior cutaneous nerve of the forearm usually is a branch of the radial nerve. Rarely, the posterior cutaneous nerve may arise from the ulnar nerve (189). Clinical Correlations: Ulnar Nerve in the Elbow and Forearm The ulnar nerve is at risk for compression or stretch at the cubital tunnel of the elbow. Panas in 1878 described a condition now known as tardy ulnar palsy (209). Several anatomic and mechanical etiologic factors have been described (6,17,78,189,208,210–218) (Table 3.3). Neuropathy of the ulnar nerve as it passes through the cubital tunnel posterior to the medial epicondyle of the humerus may be associated with recurrent dislocation of

TABLE 3.3. ANATOMIC AND MECHANICAL FACTORS CONTRIBUTING TO CUBITAL TUNNEL SYNDROME Idiopathic Ganglion Anomalous muscle (anconeus epitrochlearis) Arcade of Struthers Hypertrophic arthritis Fracture malunion, nonunion Fracture callus Traumatic heterotopic ossification Neurogenic heterotopic ossification Cubitus valgus Rheumatoid synovitis of elbow joint Supracondylar process Translocation, subluxation, or snapping of the triceps Translocation, subluxation, or dislocation of ulnar nerve Trauma (contusion, stretch, friction, repetitive traction)

the nerve. This condition was described by Collinet in 1896, followed by reports by Cobb and Momberg (both in 1903) (219–221). In 1926, Platt discussed the pathogenesis of neuritis of the ulnar nerve in the cubital tunnel, specifically in the postcondylar groove (79,215). The ulnar nerve also is subject to compression in the cubital tunnel by the overlying fascia at the level of the medial condyle, as well as by the fascia between the heads of the flexor carpi ulnaris and in the muscle itself (222–225). Spinner has suggested that the most common cause for an idiopathic type of ulnar nerve paralysis is entrapment of the nerve at the distal cubital tunnel where the ulnar nerve enters the forearm posteriorly between the two heads of the flexor carpi ulnaris. A fascial connection is present between the two, and the proximal edge may at times be thickened and act as a compressing band (6). In the cubital tunnel, an articular branch (or branches) is (are) usually given off by the ulnar nerve, followed by a motor branch to the flexor carpi ulnaris (which exits the nerve trunk just distal to the articular branch). Appreciation of these two nerves and their respective functions and destinations is relevant for ulnar nerve exploration in the cubital tunnel. In performing an anterior transposition of the ulnar nerve, the articular branch in the cubital tunnel may tether the nerve trunk and prevent mobilizing the ulnar nerve for transposition. This branch often is sacrificed to allow anterior mobilization of the nerve, and causes minimal morbidity. Occasionally, a branch to the flexor carpi ulnaris also is a limiting structure to anterior transposition. Obviously, protection and preservation of this nerve is optimal because morbidity may be substantial if the flexor carpi ulnaris has no additional motor nerves and becomes denervated by sacrifice of the motor branch. To mobilize the ulnar nerve, distal nerve dissection and mobilization to allow transposition is preferred over sacrifice of the motor branch of the flexor carpi ulnaris. With elbow flexion, the cubital tunnel decreases in volume and the aponeurosis becomes taut over the ulnar nerve (196,198,213,214,226). During elbow flexion, the nerve stretches and elongates approximately 4.7 mm. During flexion, the medial head of the triceps has been noted to push the ulnar nerve anteromedially 0.73 cm (227). When there is fixation of the nerve, a traction neuritis can develop (6). The ulnar nerve may be compressed at the elbow by an anomalous muscle, the anconeus epitrochlearis (6). The anconeus epitrochlearis is a muscle variant that originates from the medial border of the olecranon and adjacent triceps tendon and inserts into the medial epicondyle of the elbow. The muscle appears as an auxiliary extension of the medial portion of the triceps. The muscle crosses the ulnar nerve posterior to the cubital tunnel. When present, it forms a portion of the cubital tunnel, reinforcing the aponeurosis of the two heads of the origin of the flexor carpi ulnaris (6). It has been found to be a factor in producing

3 Nerve Anatomy

ulnar compressive neuritis posterior to the elbow. Excision of the muscle mass without translocation of the nerve has relieved symptoms when it was the single factor in the pathogenesis (6). The flexor carpi ulnaris was found, in an isolated case, to have a motor branch from the median nerve (6). With this variant, weak action of the muscle could be observed when a complete high ulnar lesion was present (6). The flexor carpi ulnaris sometimes may receive an additional inconsistent motor branch from the ulnar nerve in the mid-forearm. Compression of the Dorsal Cutaneous Branch of the Ulnar Nerve The dorsal cutaneous branch of the ulnar nerve is vulnerable to compression by external pressure in individuals who write with their left hand. Often, these individuals write with the ulnar border of the wrist against the firm writing surface. If the dorsal cutaneous branch of the ulnar nerve passes from its volar position to the dorsum of the hand over the prominence of the distal ulna, external pressure can cause symptoms of pain in the wrist and numbness of the dorsoulnar aspect of the hand (6). Absence of the Dorsal Cutaneous Branch of the Ulnar Nerve Complete absence of the dorsal cutaneous branch of the ulnar nerve can occur (see earlier, under Anomalies and Variations: Ulnar Nerve in the Elbow and Forearm). Sensibility to the dorsoulnar hand can then be supplied by the superficial radial nerve (208), by a dorsal division of the musculocutaneous nerve (6), or by the posterior cutaneous nerve of the forearm. With this variation, an injury or lesion of the ulnar nerve at the elbow does not produce sensory loss of the dorsum of the hand, but presents with sensory findings similar to those of a low ulnar nerve lesion. This variation should be suspected if electromyographic localization of the nerve lesion is at the elbow when clinical findings suggest a lesion at the wrist (6). The presence of this variation can be evaluated by local anesthetic block of the superficial radial nerve or the musculocutaneous nerve, which produces anesthesia over the dorsoulnar hand. Ulnar Nerve at the Wrist and Hand Ulnar Nerve in the Ulnar Tunnel The ulnar nerve and ulnar artery enter the ulnar tunnel (Guyon’s canal) at the wrist. The artery usually is located radial to the nerve (228). The nerve and artery pass radial to the pisiform, anterior (superficial) to the transverse carpal ligament (flexor retinaculum), and dorsal to the superficial palmar carpal ligament. The ulnar nerve divides

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into deep terminal and superficial palmar branches at the base of the hypothenar eminence. The ulnar nerve extends approximately 4 cm in its path through the ulnar tunnel. The tunnel originates at the proximal edge of the palmar carpal ligament and extends distally to the fibrous arch of the hypothenar muscles. The tunnel has been described in terms of having a floor (dorsal surface), a roof (palmar surface), and two walls (medial and lateral). The boundaries change from proximal to distal, and the four walls are not distinct through the entire course. The roof of the tunnel is composed of the palmar carpal ligament, the palmaris brevis, and hypothenar fat and fibrous tissue. The floor of the tunnel consists of tendons of the flexor digitorum profundus, the transverse carpal ligament, the pisohamate and pisometacarpal ligaments, and the opponens digiti minimi. The medial wall consists of the flexor carpi ulnaris, the pisiform, and the abductor digiti minimi. The lateral wall is composed of the tendons of the extrinsic flexors, the transverse carpal ligaments, and the hook of the hamate (229,230). The distal ulnar tunnel has been divided in three zones based on topography of the nerve and its relationship to the surrounding structures (230). Zone I consists of the portion of the tunnel proximal to the bifurcation of the ulnar nerve. Zone II encompasses the deep motor branch of the nerve and surrounding structures. Zone III includes the superficial branch and adjacent distal and lateral tissues. Ulnar Nerve in Zone I of the Ulnar Tunnel In zone I, the nerve continues for approximately 3 cm, stretching from the proximal edge of the palmar carpal ligament to the nerve’s bifurcation. The palmar carpal ligament, lying superficial (anterior to the ulnar nerve), is actually a thickening of the superficial forearm fascia that becomes distinct approximately 2 cm proximal to the pisiform. The ligament arises ulnarly from the tendon of the flexor carpi ulnaris and inserts radially on the palmaris longus tendon and the transverse carpal ligament, forming the roof (palmar surface of the proximal part of zone I). The ulnar nerve, along with the ulnar artery, passes deep to the palmar carpal ligament to enter the ulnar tunnel. At this level, the ulnar artery lies slightly superficial and radial to the nerve. The deep (dorsal) surface of zone I consists of tendons of the flexor digitorum profundus and the ulnar portion of the transverse carpal ligament. The lateral wall is formed by the most distal fibers of the palmar carpal ligament, which curve radially and posteriorly to wrap around the neurovascular bundle and merge with the fibers of the transverse carpal ligament. The pisiform and tendon of the flexor carpi ulnaris comprise the medial wall of the tunnel at this level (229,230). Distal to the palmar carpal ligament, the roof of the ulnar tunnel consists of the palmaris brevis muscle. This muscle originates from the distal palmar aspect of the pisiform and hypothenar muscle fascia and inserts on the ligament. The length of the palmaris brevis

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from the proximal to distal border is approximately 2.5 cm (229,230). In this area, deep to the palmaris brevis, the ulnar nerve bifurcates into the deep motor branch and the superficial branch of the ulnar nerve. The point of nerve branching is approximately 1 cm distal to the proximal edge of the pisiform. Three to 7 mm distal to the bifurcation of the nerve, the ulnar artery divides into two branches. The larger of the arterial branches accompanies the superficial branch of the nerve and becomes the superficial palmar arch. The smaller arterial branch continues with the motor branch into the deep space of the palm and terminates in the deep palmar arch. Both arteries remain superficial and radial to the nerves they accompany (230). The distal extent of zone I terminates at the level of the bifurcation of the ulnar nerve. At this level, the roof of the tunnel is formed by the palmaris brevis and the floor formed by the pisohamate and pisometacarpal ligaments. The pisohamate ligament arises from the distal, radial, and dorsal aspects of the pisiform and inserts on the proximal, ulnar, and palmar aspects of the hook of the hamate. Ulnar to the pisohamate ligament, the pisometacarpal ligament arises from the distal aspect of the pisiform and inserts on the palmar radial aspect of the base of the fifth metacarpal. The divergence of these ligaments leaves an opening in the floor of the tunnel that is filled with fibrofatty tissue overlying the capsule of the triquetrohamate joint (229,230). In zone I, the ulnar nerve carries both motor and sensory fibers. The nerve fibers are arranged in two distinct groups of fascicles. The palmar-radial fibers contain the fascicles that become the superficial branch of the ulnar nerve, whereas the dorsal-ulnar fibers become the deep motor branch. Thus, in zone I, the ulnar nerve actually is two nerves contained in a common epineurial sheath (229–231). Ulnar Nerve in Zone II of the Ulnar Tunnel Zone II encompasses the portion of the ulnar tunnel distal to the bifurcation, in the region where the deep (motor) branch of the ulnar nerve passes. This zone usually is located in the dorsoradial portion of the ulnar tunnel. The palmar (superficial) aspect of zone II is bordered by the palmaris brevis and the superficial branch of the ulnar nerve. The lateral border of zone II consists of transverse carpal ligament, which forms a wall that merges with the floor of the tunnel. The floor of zone II consists of the pisohamate and the pisometacarpal ligaments. At the distal extent of zone II, the fibrous arch of the hypothenar muscles lies palmar to the nerve, the opponens digiti minimi lies posterior, the hook of the hamate and flexor digiti minimi are located laterally, and the abductor digiti minimi lies on the medial aspect (230). The deep branch of the ulnar nerve passes deep to the fibrous arch and between the muscles as it exits the tunnel. The nerve to the abductor digiti minimi usually is given off just proximal to its entrance into these muscles. The motor branch innervates the opponens digiti minimi as

it continues radially and posteriorly around the hook of the hamate. The nerve then continues deeply across the palm (229,230). The ulnar artery enters zone II radially and palmarly, just distal to the level of the bifurcation of the nerve. The artery follows the nerve, lying palmar and slightly radial. Both structures continue distally and pass deep to the arch of the origin of the hypothenar muscles. In zone II, the deep branch of the ulnar carries motor fibers. Ulnar Nerve in Zone III of the Ulnar Tunnel Zone III encompasses the portion of the ulnar tunnel distal to the bifurcation, in the region of the superficial branch of the ulnar nerve, also referred to as the superficial palmar branch (189). At the entrance to zone III, the palmaris brevis comprises the palmar boundary, the abductor digiti minimi comprises the medial border, and the pisometacarpal ligament and capsule of the triquetrohamate joint comprise the dorsal border. The lateral and dorsal borders are formed by zone II. As the superficial branch of the ulnar nerve continues distally, it gives off two small branches that innervate the palmaris brevis. This occurs either in the ulnar tunnel or just distal to exiting it (189,230). Distal to this point, the nerve usually contains only sensory fibers. The nerve emerges from zone III by passing over the fibrous arch of the hypothenar muscles. The ulnar artery continues with the nerve throughout zone III, remaining superficial and radial to the nerve. At the distal end of the zone, the superficial palmar branch of the ulnar nerve lies between the hypothenar fascia posteriorly and the artery and a fibrofatty layer deep to the subcutaneous tissues palmarly (229,230). The superficial palmar branch in zone III contains mostly sensory fibers along with motor fibers to the hypothenar muscles. Lesions in this zone should produce primarily sensory deficits with possible motor weakness of the hypothenar muscles. Superficial Palmar Branch of the Ulnar Nerve The superficial palmar branch exits the distal ulnar tunnel with the superficial terminal branch of the ulnar artery. The nerve then provides several small twigs to innervate the skin on the medial side of the hand. The motor branches to the palmaris brevis may leave the nerve at this point (if not branched more proximally in the ulnar tunnel). The nerve continues distally and radially and divides into the proper digital nerve to the ulnar side of the little finger and the common palmar digital nerve to the fourth web space. At the level of the metacarpal shafts, the common digital nerve divides into two proper digital nerves, one each to supply adjacent aspects of the fourth web space between the small and ring fingers (see Fig. 3.3). In the palm, the nerves lie dorsal to the superficial palmar arch and palmar to the flexor tendons. Immediately after division, in the region of the metacarpal necks, the proper digital nerves course anteriorly to lie palmar (superficial) to the digital arteries. The

3 Nerve Anatomy

neurovascular bundles are stabilized in the digits by the retaining skin ligaments, Cleland’s ligaments located dorsal to the neurovascular bundle, and Grayson’s ligaments located palmarly. The proper palmar digital nerves supply the palmar skin of the digits, and the skin distal to the distal interphalangeal joints on the dorsal surface (189). Deep Terminal Branch of the Ulnar Nerve The deep terminal branch of the ulnar nerve exits from zone II of the ulnar tunnel dorsoulnar to the deep terminal branch of the ulnar artery (1,3,159,232). The nerve passes medial to the hook of the hamate, deep to the fibrous arch of the hypothenar muscle origin. The nerve continues between the abductor digiti minimi and flexor digiti minimi muscles, supplying motor branches to each. The nerve then pierces and innervates the opponens digiti minimi (41). The deep branch then crosses the palm with the ulnar artery (which now forms the deep palmar arch). Along its course, the nerve is deep to the extrinsic flexor tendons and deep to the mid-palmar and thenar fascial clefts, but palmar to the interossei (11). At the level of the third metacarpal, the deep branch of the ulnar nerve and the deep palmar arch cross between the oblique and transverse heads of the adductor pollicis. Along its deep course, the nerve innervates each of the seven interossei, the third and fourth lumbricals, the adductor pollicis, the flexor pollicis brevis and the hypothenar muscles (see Table 3.2 and Fig. 3.3). The deep terminal branch provides sensory afferent nerves to the ulnocarpal, intercarpal, and carpometacarpal joints (191). Sympathetic Fibers from the Ulnar Nerve in the Hand At the wrist, sympathetic fibers arise from the distal ulnar nerve and supply the proximal ulnar portions of the superficial and deep vascular arches of the hand. The deep vascular arch is segmentally innervated by fibers from the ulnar nerve and the superficial radial nerve (the median nerve and the superficial radial nerve also give segmental supply to the superficial vascular arch in the palm of the hand) (6). Anomalies and Variations: Ulnar Nerve in the Wrist and Hand The Riche-Cannieu Communication The Riche-Cannieu communication consists of a communication between the deep terminal branch of the ulnar nerve and the motor branch of the median nerve (see earlier, under Anomalies and Variations: Median Nerve in the Wrist and Hand). Because it occurs in 50% to 77% of hands (194), it can be argued whether this is a normal pat-

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tern or a variation. The communication occurs at the terminal portion of the deep branch of the ulnar nerve in the radial aspect of the palm (41,233). The communicating fibers pass radially from the deep ulnar branch between the heads of the adductor pollicis, then pass deep to the flexor pollicis longus tendon. The fibers continue proximally to the radial side of the flexor pollicis longus tendon as they approach the median motor branch. The communication often occurs in the substance of the flexor pollicis brevis (41). Through the Riche-Cannieu communication, the median nerve may innervate the third lumbrical, or, rarely, all of the lumbrical muscles (35,63). Conversely, the second lumbrical may be innervated by the ulnar nerve (see earlier under Anomalies and Variations: Median Nerve in the Wrist and Hand). There is some question as to whether there is a crossing of sensory fibers as well (39). Variations of Innervation of the Flexor Pollicis Brevis Considerable variation exists as to the innervation of the flexor pollicis brevis. Reports have suggested the muscle is innervated by the ulnar nerve in 50%, the median nerve in 35%, and both in 15%. Each head may receive a different contribution, with the deep head more commonly innervated by the ulnar nerve and the superficial head more commonly innervated by the median nerve (11). Variations of Innervation of the Abductor Pollicis Brevis The abductor pollicis brevis is innervated by the median nerve in 95%, the ulnar nerve in 2.5%, and by both nerves in 2.5% (9,41,189). Variation of Innervation of the Opponens Pollicis The opponens pollicis muscle is innervated by the median nerve alone in 83%, the ulnar nerve in 10%, and by both nerves in 7% (153). Variations in Sensory Innervation of the Ulnar Nerve Proper in the Hand Several variations in the sensory innervation of the ulnar nerve have been noted. Distal to the wrist, the ulnar nerve proper usually innervates the palmar aspect of the small finger and ulnar aspect of the ring finger. The pattern is variable, and the area of ulnar innervation includes the volar aspect of the entire ring finger, the ulnar aspect of the long, or the entire long finger. Conversely, the ulnar nerve may innervate only the volar aspect of the small finger. The ulnar supply to the fourth web space (to the space between the ring and small finger), instead of arising in its usual location

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at the distal end of the ulnar tunnel, has been observed to arise in the mid-forearm and continue on an aberrant course superficial to the transverse carpal ligament and the palmar aponeurosis (234). A communicating branch may exist between the superficial branch of the ulnar nerve and the common digital nerve of the third web space (common digital nerve of the median nerve to supply adjacent sides of the long and ring finger). This is a relatively common finding (189,191) and leads to dual innervation to the adjacent sides of the long and ring fingers. Variations in Sensory Innervation of the Dorsal Cutaneous Branch of the Ulnar Nerve in the Hand The dorsal aspect of the hand usually is innervated by the dorsal branch of the ulnar nerve. However, this area may be supplied partially or entirely by the radial nerve or by the posterior cutaneous nerve of the forearm. Complete absence of the dorsal branch of the ulnar nerve has been found in 1 of 24 specimens (199). In these cases, the radial nerve or posterior cutaneous nerve of the forearm supplies the dorsoulnar hand sensibility. The dorsal branch of the ulnar nerve may deviate palmarly at the pisiform, join the superficial (sensory) branch, and supply the palmar surface of the little finger. A nerve connection may exist between the dorsal sensory branches of the ulnar nerve and the superficial radial nerves. This communication between the dorsal branch of the ulnar nerve and a subcutaneous branch from the superficial branch of the radial nerve was observed in 1 of 24 specimens. The communication was noted on the dorsal aspect of the hand (199). An additional variation is the presence of a third dorsal digital branch from the ulnar nerve. When present, this branch from the ulnar nerve supplies the third web space in conjunction with the radial digital nerve, providing dual innervation (6). Variations of Division and Recommunication of the Ulnar Nerve into Deep and Superficial Branches Variations exist as to the point of division of the ulnar nerve into its deep and superficial branches. The deep motor branch may divide proximal to the hook of the hamate. The radial division may enter the carpal tunnel (radial to the hook of the hamate) and rejoin the ulnar division distal to the hamate (235,236). Less commonly, the deep motor branch may divide proximal to the pisiform, communicate with the dorsal sensory branch, or rejoin the nerve distal to the pisiform. In the event of nerve injury distal to an anomalous division, function is partially preserved. The ulnar digital nerve to the ring finger may arise in the forearm, passing superficial to the ulnar tunnel. Similarly, the dorsal

cutaneous branch may arise near the elbow, passing distally in the subcutaneous tissue to reach the hand (41,189). An anomalous terminal branch of the ulnar nerve has been observed at the distal end of Guyon’s canal, which joined the digital sensory branch to the medial aspect of the small finger (237,238). The Ulnar Palmar Cutaneous Nerve The ulnar palmar cutaneous nerve is not a consistent branch, as is its adjacent counterpart, the median palmar cutaneous nerve (6,239) (see Fig. 3.4A). When present, it arises at variable levels from the ulnar nerve in the distal half of the forearm. Clinical Correlations: Ulnar Nerve in the Wrist and Hand In zone I of the ulnar tunnel, the ulnar nerve carries both motor and sensory fibers. A compression or traumatic lesion in zone I has a high likelihood of producing both motor and sensory deficits. If the lesion is in zone I, or in the area just proximal to the entrance of the ulnar tunnel, the dorsal sensor branch (which exits the ulnar nerve more proximally in the distal forearm) is spared. Therefore, sensibility to the dorsal aspect of the small and ulnar side of the ring finger is spared. These findings, of palmar sensibility loss (to the small and ulnar side of the ring) with intrinsic motor loss and with sparing of dorsal sensibility, help localize the area of compression or dysfunction (229,240–242). In zone I of the ulnar tunnel, the ulnar nerve fibers are arranged in two distinct groups of fascicles, with the palmar-radial fibers containing fascicles that become the superficial branch of the ulnar nerve (mostly sensory fibers), whereas the dorsal-ulnar fibers become the deep branch (motor branch). A lesion in zone I that involves the palmarradial aspect or the dorsal-ulnar aspect of the nerve may involve mostly sensory or mostly motor fibers, respectively, and thus produce an associated clinical presentation (229–231). In zone II of the ulnar tunnel, the deep branch of the ulnar nerve carries motor fibers. A lesion in zone II should produce only motor deficits. Conversely, if an occult lesion or penetrating injury produces only motor loss, zone II should be suspected as a site of the lesion. In zone III of the ulnar tunnel, the superficial branch of the ulnar nerve carries mostly sensory fibers, along with motor fibers to the palmaris brevis and hypothenar muscles. Therefore, it is technically incorrect to refer to this branch at this point as the sensory branch of the ulnar nerve. The superficial branch of the ulnar nerve is preferred. In zone III of the ulnar tunnel, the superficial branch contains mostly sensory fibers along with motor fibers to the hypothenar muscles. Lesions in this zone should produce primarily sensory deficits with possible motor weak-

3 Nerve Anatomy

ness of the hypothenar muscles. Conversely, if an occult lesion or penetrating injury produces mostly sensory loss (or concomitant weakness of the hypothenar muscles), zone II should be suspected as the site of the lesion. In carpal tunnel syndrome, the etiology often is unknown, and it is attributed to an idiopathic cause. However, in ulnar nerve compression in the ulnar tunnel, a cause more commonly is found. These include tumors in the ulnar tunnel (ganglions, lipomas, giant cell tumor, desmoid tumors, rheumatoid synovial cysts), anatomic abnormalities that encroach on the ulnar nerve (anomalous muscles, thickened ligaments, anomalous hamulus), trauma with associated inflammation, edema, or hematoma (fractures, repetitive trauma, edema after burns), vascular pathology, or inflammatory conditions (rheumatoid arthritis or degenerative arthritis) (6,243–267) (Table 3.4). Ganglions are the most common tumor related to ulnar tunnel syndrome, accounting for 29% to 45% of reported caused of ulnar tunnel syndrome. Other more common related factors include anomalous muscles (see later), fractures, and vascular abnormalities (230,268–273). Anomalous muscles reported to cause ulnar tunnel syndrome include the several variations of the palmaris longus (274–276), an accessory flexor digiti minimi (262,277), an accessory abductor digiti minimi (198,278), an accessory muscle from the flexor carpi ulnaris tendon, and various anomalous muscles located in the canal (see later) (279, 280). TABLE 3.4. COMMON CAUSES OF ULNAR NERVE COMPRESSION AT THE WRIST BASED ON 135 REPORTED CASES Cause Tumors Ganglion Lipoma Giant cell tumor Desmoid tumor Anatomic abnormalities Anomalous muscles Thickened ligaments Anomalous hamulus Trauma Fractures Repetitive trauma Edema after burns Other trauma Vascular pathology Arthritis Rheumatoid Degenerative Other Dupuytren’s contracture

Number

46 3 2 1 22 4 3 19 8 10 3 9 4 1 1 136 total

From Botte MJ, Gelberman RH. Ulnar nerve compression at the wrist. In: Szabo RM, ed. Nerve compression syndromes: diagnosis and treatment. Thorofare, NJ: Slack, 1989:121–136.

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Several variations of the palmaris longus have been related to ulnar variations in Guyon’s canal and to ulnar tunnel syndrome. These include a reversed muscle–tendon relationship with a distal muscle belly and proximal tendon (275), an anomalous extension into Guyon’s canal, an accessory palmaris longus, and a duplicated palmaris longus (6,9,274,275). An anomalous palmaris longus may have a reversal of its normal muscle relationship, with the tendon arising proximally from the medial epicondyle, and the muscle belly attaching distally to the flexor retinaculum at the wrist. There may be an associated accessory musculotendinous slip, approximately 1 cm thick, which inserts into the pisiform (275). This anomalous palmaris can create an arch that reinforces the roof of the tunnel. However, the ulnar nerve and artery must penetrate through this arch to reach the wrist, and thus are more vulnerable to compression. The nerve and artery run their normal course deep to the palmaris brevis (275). Spinner has referred to this anatomic arrangement as the variant canal of Guyon (6). An anomalous accessory palmar longus has been noted in the ulnar tunnel. Thomas described a 1-cm-wide muscle arising from the palmaris longus tendon. The muscle inserted into the soft tissues of the region of the hypothenar muscles and into the pisiform. This muscle passed through the ulnar tunnel, and was thought to be responsible for clinical symptoms of fatigability of the hand (274). King and O’Rahilly reported a duplication of the palmaris longus with either a separate muscular slip (accessory palmaris) or a separate tendon that originated from the duplicated palmaris and extended to the abductor digiti quinti or the flexor digiti quinti. The accessory muscle passed volar to the ulnar nerve and ulnar artery. The muscle appeared to form part of the roof of the ulnar tunnel. An associated tendinous slip that extended between the ulnar artery and nerve also was noted to occur. The artery crossed anterior to the slip. As early as 1864, anomalies of the palmaris longus were noted, and associated with variations of the ulnar tunnel (276). A palmaris longus with a double origin was described by Wood. From this palmaris longus tendon, there was an associated anomalous flexor digiti quinti with a high origin from the palmaris longus. Besides the palmaris longus, other aberrant muscles have been noted in the ulnar tunnel or its vicinity that place the ulnar nerve at risk for compression. Schjelderup described an anomalous muscle 4 mm wide that extended in the canal and crossed over the ulnar nerve before the nerve divided (279). Turner and Caird also noted an anomalous muscle in the ulnar tunnel. The muscle originated from the pisiform, crossed through the ulnar tunnel passing between the deep and superficial branches, and inserted into the transverse carpal ligament. This muscle passed between the motor and sensory branches of the ulnar nerve (280).

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Jeffery described an accessory hypertrophied abductor digiti quinti that arose from the fascia of the distal forearm. The muscle was thought responsible for isolated paralysis of the intrinsic muscles without sensibility loss. The patient’s symptoms improved after excision of the abnormal muscle (6,278). An accessory muscle arising from the tendon of the flexor carpi ulnaris was noted by Kaplan. This muscles inserted into the volar carpal ligament. It formed a thickened roof of the ulnar tunnel, possibly increasing the vulnerability to the ulnar nerve (6,9; personal communication to Spinner). Swanson identified an accessory flexor digiti quinti arising from the forearm fascia. The muscle inserted into the flexor digitorum brevis and caused symptoms of ulnar nerve compression (6,277). Hayes et al. described a ligamentous band that attached to the pisiform and extended to the hook of the hamate. The band was located anterior to the deep branch of the ulnar nerve (6,281). The flexor and abductor digiti minimi muscles arose in part from the ligamentous band. In the vicinity of the ulnar tunnel, Lipscomb reported a case of duplication of the hypothenar muscles (282). The duplicated muscle simulated a tumor of the hand. The muscle originated from the pisiform and the hook of the hamate. The palmaris brevis was noted to be six times the normal size. Proximally, these anomalous muscles formed part of the ulnar tunnel (6), and potentially increased the risk of nerve compression. Harrelson and Newman described ulnar tunnel syndrome caused by a hypertrophied flexor carpi ulnaris muscle in close proximity to the ulnar tunnel (283). Most ganglia that cause ulnar tunnel syndrome arise from the palmar aspect of the carpus and present in zone I or II. Although the deep terminal branch of the ulnar nerve consists mostly of motor fibers, it also contains sensory afferent nerves to the ulnocarpal, intercarpal, and carpometacarpal joints. It is thus not a purely motor nerve, although it sometimes incorrectly is referred to as the deep motor branch of the ulnar nerve. The correct names include deep branch of the ulnar nerve and deep terminal branch of the ulnar nerve (189,229,284). The deep branch of the ulnar nerve and the deep palmar arch cross between the interval between the oblique and transverse heads of the adductor pollicis at the level of the third metacarpal. This interval is useful in identifying the neurovascular bundle during exploration for deep or severe trauma. The neurovascular bundle also requires isolation and protection in adductor pollicis recession, as often is performed for correction of thumb-in-palm deformities in spastic muscle disorders. Compression of the deep branch of the ulnar nerve by the adductor pollicis also has been noted (285).

Because the ulnar nerve on occasion may innervate the third lumbrical muscle, a high ulnar nerve lesion can produce clawing in three fingers instead of two. Although ulnar neuropathy is a relatively common cause of intrinsic muscle atrophy, several other etiologies are possible: Charcot-Marie-Tooth disease, thoracic outlet syndrome, C8 to T1 root level impingement, anterior horn cell disorders, and even compression at the foramen magnum level (foramen magnum meningioma) (45,46,196,213,240, 241,273,286–289). The ulnar supply to the fourth web space (to the space between the ring and small fingers), instead of arising in its usual location at the distal end of the ulnar tunnel, has been observed to arise in the mid-forearm and continue on an aberrant course superficial to the transverse carpal ligament and the palmar aponeurosis (280). When present, it can be vulnerable to injury during carpal tunnel decompression (6). RADIAL NERVE Origin of the Radial Nerve The radial nerve arises from the posterior cord of the brachial plexus, posterior to the third portion of the axillary artery (1–4,11) (see Fig. 3.1). It contains fibers from C5 through C8 (and occasionally T1) and is the largest terminal branch of the brachial plexus. The lower trunk contributes fibers from T1 in 8% of upper limbs (13). Radial Nerve in the Axilla and Arm In the proximal portion of the arm, the radial nerve courses posterior to the brachial artery, anterior to the subscapularis muscle, the teres major and latissimus dorsi muscle tendons, and the long head of the triceps. At the junction of the proximal and middle thirds of the humerus, the nerve courses dorsolaterally, passing posterior to the medial head of the triceps and anterior to the long head. The radial nerve is accompanied by the profunda brachii artery, and continues distally close to the posterior cortex of the humerus (290). The nerve and artery pass through the extensor compartment of the arm, between the medial and lateral heads of the triceps muscle. The nerve continues distally, coursing slightly anteriorly as it spirals around the humerus to reach the lateral intermuscular septum. The nerve is separated from the humeral cortex by the medial head of the triceps, which lies adjacent to but not in the spiral groove of the humerus (291,292). The radial nerve leaves the extensor compartment of the arm at the lateral border of the medial head of the triceps muscle, sequentially providing motor branches to the triceps long head, medial head, and lateral head (Table 3.5 and Fig. 3.5). The nerve enters the flexor compartment of the arm, piercing the lat-

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TABLE 3.5. LEVEL AND ORDER OF INNERVATION OF MUSCLES SUPPLIED BY THE RADIAL NERVE

Muscle Triceps Long head Medial head Lateral head Anconeus

Range in cm from Tip of Acromion (Shortest to Longest)

7.1 9.5–11.2 10.1

Range in cm from Humerus (from 10 cm above Lateral Epicondyle) Brachioradialis Extensor carpi radialis longus Extensor carpi radialis brevis

8.2–10.0 10.5–12.3 14.7–16.5 Range in cm from Lateral Epicondyle

Extensor carpi ulnaris Extensor digitorum communis Extensor digiti minimi Abductor pollicis longus Extensor pollicis longus Extensor pollicis brevis Extensor indicis proprius

10.2–10.6 10.2–12.5 11.7–12.0 11.4–14.2 13.9–17.6 15.9–16.4 16.9–18.0

From Sunderland S, Hughes ESR. Metrical and non-metrical features of the muscular branches of the ulnar nerve. J Comp Neurol 85:113–120, 1946; and Linnell EA. The distribution of nerves in the upper limb, with reference to variabilities and their clinical significance. J Anat 55:79, 1921.

eral intermuscular septum approximately 10 cm proximal to the lateral humeral epicondyle (6). The radial collateral artery (the terminal branch of the profunda brachii artery) accompanies the radial nerve in this area. The radial nerve continues deep in the intermuscular interval between the brachialis and brachioradialis muscles. It continues distally, and extends in the interval between the extensor carpi radialis longus muscle and brachialis. The nerve exits the arm anterior to the tip of the lateral epicondyle, dividing into the superficial and deep terminal branches as it enters the forearm (13,291,292). In the arm, the radial nerve sequentially innervates the three heads of the triceps and the anconeus. In the distal third of the arm proximal to the elbow epicondylar line, the radial nerve innervates the brachioradialis and extensor carpi radialis longus (see Table 3.5 and Fig. 3.5). Occasionally, the radial nerve provides a motor branch to the radial portion of the brachialis (6,293), which usually is supplied by the musculocutaneous nerve. The motor branch to the extensor carpi radialis brevis can have a variable source. In most limbs (58%), motor innervation to the extensor carpi radialis brevis arises from the sensory division of the radial nerve in the forearm, the superficial radial nerve (294).

FIGURE 3.5. Schematic illustration of the radial nerve and associated branches and innervated muscles.

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Clinical Correlations: Radial Nerve in the Axilla and Arm Holstein-Lewis Fracture The close proximity of the radial nerve to the surface of the humeral diaphysis places the nerve at risk for injury with humeral fractures (295–301). Transient nerve injury is the most common type of complication associated with humeral shaft fractures. Most nerve injuries are associated with transverse or short oblique fractures. Transection of the radial nerve is rare and associated most commonly with open fractures, penetrating injuries, or spiral oblique fractures (301). Radial nerve compression in the arm has been attributed to impingement by the triceps muscle (302,303). Radial Nerve in the Forearm and Hand The radial nerve passes anterior to the lateral epicondyle to enter the forearm. At approximately the level of the elbow, the radial nerve divides into the superficial and deep terminal branches deep to the brachioradialis and extensor carpi radialis longus and brevis (6,291) (see Fig. 3.5). The point of bifurcation usually is at the level of the radiocapitellar joint, but it may divide 2 to 5 cm proximal or distal to this joint (6,13,304). The superficial branch passes anterior (superior) to the supinator muscle in the proximal third of the forearm and continues along the deep surface of the brachioradialis muscle. Proximally, the nerve is adjacent to the anterior third of the brachioradialis, but as it descends distally, it courses laterally and anteriorly. The radial artery passes palmar to the insertion of the pronator teres muscle and comes to lie on the ulnar border of the brachioradialis muscle in the middle third of the forearm. The superficial branch, which descends more laterally, is lateral to the radial artery, palmar to the origins of the radial head of the flexor digitorum superficialis and flexor pollicis longus muscle. The superficial branch continues distally on the deep surface of the brachioradialis, crossing and descending along

the radius. The superficial branch pierces the antebrachial fascia on the ulnar side of the brachioradialis tendon, (between the tendons of the brachioradialis and extensor carpi radialis longus). The nerve thus becomes subcutaneous at approximately 9 cm proximal to the wrist (291). Superficial Branch of the Radial Nerve Several patterns of the superficial branch of the radial nerve have been noted (305,306). The superficial branch of the radial nerve arose from the radial nerve at the level of the lateral humeral epicondyle in 8 of 20 specimens, and within 2.1 cm of the lateral epicondyle in the remaining 12. The superficial branch courses distally deep to the brachioradialis muscle until it emerges between the tendons of the brachioradialis and extensor carpi radialis longus to pierce the antebrachial fascia. In 10% of specimens, the superficial branch became subcutaneous by actually piercing the tendon of the brachioradialis. Table 3.6 shows relationships of the superficial branch of the radial nerve to specific landmarks. The superficial branch of the radial nerve becomes subcutaneous at a mean of 9 cm proximal to the radial styloid [range, 7 to 10.8 cm, standard deviation (SD) 1.4 cm]. When the nerve initially enters the subcutaneous tissue, its mean width is 3 mm (SD 0.5 mm). The superficial branch of the radial nerve continues distally and usually divides into two branches (85% of specimens) or three branches (15% of specimens). The first major branch point occurs at a mean distance of 5.1 cm (range, 3.2 to 7.1 cm, SD 1.8 cm) proximal to the radial styloid. The point at which the superficial branch of the radial nerve becomes subcutaneous is, on average, the distal 36% of the distance from the lateral humeral epicondyle to the radial styloid. The first branch point of the superficial branch of the radial nerve after it enters the subcutaneous tissue is, on average, the distal 20% of that distance. At the level of the extensor retinaculum, the width of the palmar and dorsal major branches averages 2 mm (SD 0.4 mm) and 2 mm (SD 0.2 mm), respectively. The nearest branch to the center of the

TABLE 3.6. RELATIONSHIPS OF THE SUPERFICIAL BRANCH OF THE RADIAL NERVE TO SPECIFIC LANDMARKS

Mean Min. Max.

Forearm Length (cm)

SBRN-SQa to RS (cm/% Forearmb)

Branch to RSc (cm/% Forearm)

Distance to Center of First DC (cm)

Distance of Closest Branch to Lister’s Tubercle (cm)

25.5 21.5

9.0/36% 6.1/25% 11.6/40%

5.1/20% 2.7/11% 10.5/38%

0.4 0.0 1.6

1.6 0.5 2.9

DC, dorsal compartment; RS, radial styloid. aSBRN-SQ is the distance from the RS to where the superficial branch of the radial nerve (SBRN) became subcutaneous. b% forearm indicates the percentage of the distal forearm length at which the SBRN became subcutaneous or had its first major branch point. cBranch to RS is the distance from the RS to the first major branch point. From Abrams RA, Brown RA, Botte MJ. The superficial branch of the radial nerve: an anatomic study with surgical implication. J Hand Surg [Am] 17:1037–1041, 1992.

3 Nerve Anatomy

first dorsal wrist compartment is within a mean transverse distance of 0.4 cm (SD, 0.4 cm), and in 35% of specimens, there is a branch lying directly over the center of the first dorsal wrist compartment. All branches pass radial to Lister’s tubercle by a mean distance of 1.6 cm (SD 0.05 cm). No branches pass closer than 0.5 cm to the tubercle (305). In all specimens studied, the major palmar branch continues distally to become the dorsoradial digital nerve of the thumb. In half of the specimens, before it reached the thumb, the palmar branch divides into other smaller cutaneous branches that extend to the palmar radial thenar eminence. In 35%, there were connections between these branches of the superficial branch of the radial nerve and branches from the lateral antebrachial cutaneous nerve. The major dorsal branch, with numerous branching configurations, continues distally, branching into the dorsoulnar digital nerve to the thumb and the dorsoradial digital nerve to the index finger, and a third branch continues distally to become the dorsoulnar and dorsoradial digital nerves of the index and long fingers, respectively. The dorsoulnar digital nerve to the long finger arises from the dorsal sensory branch of the ulnar nerve in 90% of specimens (305). The dorsoulnar digital nerve to the thumb parallels the thumb metacarpal running superficial to the first dorsal interosseous muscle, passing dorsoulnar to the metacarpophalangeal joint. The widths of the dorsoradial and dorsoulnar digital nerves to the thumb at the level of the metacarpophalangeal joints are 1.5 mm (SD 0.5 mm) and 1.4 mm (SD 0.3 mm), respectively (305). Despite pattern variations, discernible features were as follows: The palmar branch from the first major branch point always became the dorsoradial digital nerve to the thumb. In 65%, the dorsoulnar digital nerve to the thumb and the dorsoradial digital nerve to the index finger came from the same branch, which emanated from the first main dorsal branch. In 30%, the dorsoulnar nerve to the thumb and the dorsoradial nerve to the index finger came from different branches off the main dorsal branch, and in 1 specimen of 20, the dorsoulnar nerve to the thumb was noted to arise from a trifurcating branch at the first major branch paint. In all specimens, the continuation of the main dorsal branch bifurcated distally, usually near the metacarpal heads, into the dorsoulnar digital nerve to the index finger and the dorsoradial digital nerve to the long finger (305). Posterior Interosseous Nerve The posterior interosseous nerve, the deep terminal branch of the radial nerve, innervates the extensor muscles of the forearm and contains sensory afferent fibers to the wrist joint (307,308) (Table 3.7, and see Fig. 3.5). The posterior interosseous nerve is one of the main continuing branches after the bifurcation of the radial nerve (291,307,309). The bifurcation usually occurs at approximately the level of the radiocapitellar joint. The posterior interosseous nerve con-

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TABLE 3.7. THE POSTERIOR INTEROSSEOUS NERVE: ORDER OF MUSCLE INNERVATION AND DISTANCE FROM THE DISTAL EDGE OF THE SUPINATOR TO THE POINT OF MUSCLE PENETRATION OF INNERVATED MUSCLE Extensor carpi ulnaris Extensor digitorum communis Extensor digiti quinti Abductor pollicis longus Extensor pollicis brevis Extensor indicis proprius Extensor pollicis longus

1.25 cm 1.23–1.8 cm 1.8 cm 5.6 cm 6.5 cm 6.8 cm 7.5 cm

From Spinner M. Injuries to the major branches of peripheral nerves of the forearm, 2nd ed. Philadelphia: WB Saunders, 1978.

tinues a few centimeters to enter the supinator muscle. Just before entering the supinator, the motor branch to the extensor carpi radialis brevis is given off. The motor branch to the extensor carpi radialis brevis usually exits off the lateral aspect of the posterior interosseous nerve. The extensor carpi radialis brevis usually receives its innervation at the level of the radial head or distal to it (6). The supinator muscle, arising from the lateral epicondyle, radial collateral ligament, and the proximal ulna, is divided into deep and superficial heads. The muscle is approximately 5 cm broad. The posterior interosseous nerve gives off one or more branches to the supinator muscle before entering it; however, additional fibers may remain within the epineurium of the main trunk for several centimeters, supplying the muscle between its two heads. The posterior interosseous nerve enters the supinator muscle at the muscle’s proximal end, through a teardrop-shaped opening in the superficial head of the muscle. The opening leads the plane between the deep and superficial heads. The opening in the superficial head contains a fibrous or muscular thickening along its margin, referred to as the arcade of Frohse (Frohse, 1908). The nerve enters the arcade of the Frohse and continues distally to pass obliquely between the superficial and deep muscle bellies. In its course through the supinator, the nerve usually is somewhat perpendicular to the direction of the line of the muscle fibers. The nerve continues dorsolaterally around the neck of the radius and innervates the supinator while coursing through it. The nerve is separated from the radius by the deep head of the supinator muscle, but may come into contact with the bone, especially when the fibers of the deep head parallel the course of the nerve (291,310). The nerve crosses the proximal radius to exit the distal portion of the supinator approximately 8 cm distal to the elbow joint (6). The nerve thus emerges dorsally to enter the extensor compartment of the forearm. As the nerve emerges from the supinator, it divides into multiple branches, dividing in a somewhat radial pattern resembling a cauda equina. There is a basic pattern to the multiple branches, consisting of two major components. These include those branches that supply the

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superficial layer of muscles (extensor digitorum communis, extensor digiti quinti, and extensor carpi ulnaris) and those branches coursing deep to the outcropping muscles (abductor pollicis longus, extensor pollicis longus and brevis, and extensor indicis proprius). The branch pattern may be quite variable. After leaving the supinator muscle, the nerve lies between the abductor pollicis longus muscle (located deeply) and the extensor carpi ulnaris, extensor digiti minimi, and extensor digitorum communis muscles (all located superficially). The posterior interosseous nerve is joined on the extensor surface of the forearm by the posterior interosseous artery, a branch of the common interosseous artery. Coursing distally in the forearm, the nerve passes superficial to the extensor pollicis brevis and deep to the extensor pollicis longus muscles (291). It penetrates deeply, either over or through the extensor pollicis brevis muscle, and comes to lie on the interosseous membrane between the radius and ulna. Continuing distally on the interosseous membrane, it divides into terminal branches that provide sensory innervation to the wrist (291). The extensor carpi radialis brevis muscle may be innervated by the radial nerve, its superficial branch, or the posterior interosseous nerve. Branches to this muscle most commonly originate 2 cm distal to the tip of the lateral epicondyle, but may arise between 2 and 5 cm distal to it (35,291). As noted previously, the branch pattern of the posterior interosseous nerve is variable after it exits the supinator, and variations exist as to the order and distance that muscles are innervated (44) (see Fig. 3.5). In general, the nerve gives off three short and two long motor branches after it leaves the muscle (291). The general order of muscle innervation and the distance from the distal edge of the supinator to the point of innervation of the associated muscle is as follows: extensor carpi ulnaris, innervated approximately 1.25 cm distal to the supinator; extensor digitorum communis, innervated approximately 1.25 to 1.8 cm distal to the supinator; extensor digiti quinti, innervated approximately 1.8 cm distal to the supinator; abductor pollicis longus, innervated approximately 5.6 cm distal to the supinator; extensor pollicis brevis, innervated approximately 6.5 cm distal to the supinator; extensor indicis proprius, innervated approximately 6.8 cm distal to the supinator; and extensor pollicis longus, innervated approximately 7.5 cm distal to the supinator (6,291,311) (see Tables 3.5 and 3.7). There are three short branches given off after the posterior interosseous nerve exits the supinator. These innervate the extensor digitorum communis, followed by the extensor digiti minimi and the extensor carpi ulnaris muscles, and arise in close succession and travel a variable distance before entering their respective muscles (see distances above, Fig. 3.5). Although variation exists, there is a relatively constant pattern in that the extensor carpi ulnaris and extensor digitorum communis muscles are innervated proximal to the abductor pollicis longus and extensor pollicis brevis. One to three terminal branches of the posterior interosseous nerve

supply the extensor carpi ulnaris. These branches pass horizontally in a medial direction to reach the muscle. These branches arise from the posterior interosseous nerve at approximately the level just distal to the most distal portion of the insertion of the anconeus (6). The branches then run proximally and distally within the muscle. The extensor digiti minimi is supplied by a branch of the posterior interosseous nerve just radial to the innervation of the extensor carpi ulnaris. These motor branches are vulnerable to injury if the interval between the extensor carpi ulnaris and the extensor digiti minimi, or between the extensor digiti minimi and extensor digitorum communis in the midforearm, is explored (6). A long lateral branch supplies the abductor pollicis longus 5.6 cm distal to the division and ends in the extensor pollicis brevis, 6.8 cm distal to the division (311). Multiple branches to these muscles are common (291) (Fig. 3.5). A final long medial muscular branch provides innervation to the extensor indicis proprius 6.8 cm distal to the nerve division and to the extensor pollicis longus 7.5 cm distal to the division (see earlier). This medial branch may divide and innervate both the extensor pollicis longus and the extensor indicis proprius, or two separate nerves can exist that each exit the posterior interosseous nerve, with each muscle receiving its separate nerve (291). After innervation of the extensor pollicis longus, the nerve exits from the muscle belly or from its course superficial to this muscle. The nerve comes to lie on the dorsal aspect of the interosseous membrane between the radius and ulna. The nerve continues distally on the interosseous membrane, where it divides into terminal branches that provide sensory innervation to the wrist (291). Specific branches innervate the ligaments of the radiocarpal, intercarpal, and carpometacarpal joints (291,312). The radial nerve and its branches also carry sympathetic nerve fibers. The main trunk of the radial nerve, which divides into several branches in the proximal forearm, supplies sympathetic fibers to the radial artery at the elbow or in the proximal forearm. More distally in the forearm, the radial artery is supplied segmentally in the middle and distal portions by sympathetic nerve fibers from the superficial radial nerve (313). Anomalies and Variations: Radial Nerve in the Forearm and Hand Three patterns of variability are recognized in the course of the radial nerve in the forearm. The first pattern concerns the terminal branching of the radial nerve trunk. Most commonly, the nerve bifurcates into superficial and deep branches at the level of the tip of the lateral epicondyle. The level of division may vary from 4.5 cm proximal to 4 cm distal to the epicondyle; the distal division is more common (205). A second pattern of variability concerns the level of innervation of the forearm muscles. The extensor carpi radi-

3 Nerve Anatomy

alis brevis muscle may be innervated directly from the radial nerve trunk, from its bifurcation, from the posterior interosseous nerve, or from the superficial branches. The supinator muscle usually receives a single branch from the posterior interosseous nerve before it enters the muscle and several short branches within the muscle. However, several branches have been noted to divide proximally to supply the supinator muscle (17,35,44). As the posterior interosseous nerve leaves the supinator, several branches arise to supply the superficial and deep forearm extensor muscles. Although the level of innervation and branching described usually is adhered to, significant variation exists among individuals. All of the branches may arise from one common nerve, or may divide much like the cauda equina (2,3,6,11,13,25,44,191,291). Rarely, as noted by Linell, the motor branch to the extensor carpi radialis longus can arise from the posterior interosseous nerve and penetrate the supinator muscle to reach its destination (205). In this situation, a lesion or compression of the posterior interosseous nerve may present not only with loss of digital extension, but also with complete loss of wrist extension. The hand has no sensory abnormalities, and there is no dysfunction of the brachioradialis muscle. The posterior interosseous nerve has been shown to have variable patterns. The nerve may pass superficial to the supinator, rather than through it. Distally, the nerve may pass under, over, or through the extensor pollicis brevis muscle before coming in contact with the interosseous membrane. Krause and von Luschka have described the motor branch to the abductor pollicis longus and extensor pollicis brevis, extensor pollicis longus, and extensor indicis proprius passing superficial to the superficial head of the supinator, while the remaining major portion of the posterior interosseous nerve, supplying the extensor digitorum communis, extensor digiti quinti proprius, and extensor carpi ulnaris, follows its usual course (6,314,315). Froment-Rauber Nerve The posterior interosseous nerve rarely may continue distally to innervate the first, second, and third dorsal interosseous muscles. This was first described by Froment in 1846 (316), and further noted by Rauber in 1865 (317,318), and by Shevkunenko in 1949 (312). Spinner has referred to the anomaly as the Froment-Rauber nerve (6). Froment-Rauber Anastomosis An anastomosis may exist between the terminal branches of the posterior interosseous nerve and the deep branch of the ulnar nerve in the dorsal interosseous muscles of the hand. Although originally described by Bichat in 1802 (319) and again by Hovelacque in 1927 (73,74), the anastomosis usually is referred to as the Froment-Rauber anastomosis. Spin-

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ner suggests that the name of Bichat should be added to the eponym because of Bichat’s early description. Anterior Interosseous Nerve to Posterior Interosseous Nerve Anastomosis Rauber described a communication between the anterior interosseous nerve and the posterior interosseous, passing through a foramen in the interosseous ligament. The anterior interosseous nerve usually is divided into three long branches. The main branch supplies the flexor pollicis longus, the flexor profundus muscles to the index and long fingers, and the pronator quadratus. The other two branches pass adjacent to the interosseous membrane, where they innervate the interosseous ligament and the periosteum of the radius and ulna (6). Some of the branches that travel along the interosseous ligament penetrate the ligament to communicate with terminal branches of the posterior nerve. In the distal forearm, a terminal branch of the main anterior interosseous nerve branches posterior to the pronator quadratus and passes through a foramen in the interosseous ligament to anastomose with branches of the posterior interosseous nerve. The latter communication can occur at the distal border of the interosseous ligament. This is a potential pathway for communication of nerve fibers between the median nerve and radial nerve. It also is possible that the median nerve fibers that join the posterior nerve actually may continue to reach the intrinsic muscles of the hand (6). Spinner notes that this is an example of neural plexification that occurs throughout the entire peripheral nervous system. The Superficial Branch of the Radial Nerve The superficial branch of the radial nerve may wind around the brachioradialis and continue on the superficial surface of the muscle, rather than along the deep surface. It can thus course from the elbow to the hand in the subcutaneous tissue on the dorsolateral surface of the forearm (6,194). Rarely, the brachioradialis and extensor carpi radialis longus muscles share a common muscle belly, or have a conjoined muscle. In these cases, the superficial branch of the radial nerve has been reported to perforate a conjoined tendon that is shared by the two muscles (6). Absence of the Superficial Branch of the Radial Nerve Complete absence of the superficial branch of the radial nerve has been described (320). In this case, the area normally supplied by the radial nerve was supplied by the musculocutaneous nerve (which extended more distally than normal), and an enlarged ulnar dorsal cutaneous nerve (320) was found to supply the autonomous zone of the thumb.

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The superficial branch of the radial nerve may supply sensibility to the thenar eminence in the region (normally innervated by the palmar cutaneous branch of the median nerve), and to the palmar aspect of the thumb (normally innervated by the common digital nerves of the thumb originating from the median nerve). Thus, it is possible for an injury to the superficial branches of the radial nerve to produce numbness or anesthesia of both the dorsal and palmar aspects of the thumb. The superficial branch of the radial nerve may supply the entire dorsum of the hand. Learmonth has reported an anatomic specimen in which the entire dorsal cutaneous branch of the ulnar nerve was absent. The region normally supplied by the ulnar nerve was supplied by an enlarged superficial radial nerve, which had additional branches (208). The Musculocutaneous Nerve Spinner and colleagues noted several specimens and clinical cases where the musculocutaneous nerve extended more distally than traditionally depicted. The nerve can continue into the hand to supply the anterior palmar aspect of the thumb or thenar eminence (in the region of the thumb metacarpal between the sensory region of the medial palmar cutaneous nerve area and the more dorsal superficial radial autonomous zone) (6). The musculocutaneous nerve also may supply sensibility to the dorsum of the thumb in the area usually supplied by the superficial branch of the radial nerve (6). It is not uncommon for there to be a communicating branch between the superficial branch of the radial nerve and the musculocutaneous nerve (205). Spinner has pointed out that communicating branches between the median and musculocutaneous nerves in the arm and between the median and ulnar nerves in the intrinsic muscles probably pass distally through the posterior cord to the posterior interosseous nerve rather than through the usual medial cord to the ulnar nerve path (6). Clinical Correlations: Radial Nerve in the Forearm and Hand Radial Tunnel Syndrome The radial nerve may be compressed or develop neuritis along its course in the radial tunnel, frequently between the head or neck of the radius and the supinator muscle (321,322). The radial nerve is particularly at risk at its entrance into the supinator, at the arcade of Frohse. The arcade of Frohse is a fibrous or fascial band resembling an oval-shaped window at the proximal aspect of the supinator muscle. The nerve also may be compressed in the muscle itself (see discussion of paralysis of the posterior interosseous nerve, later). Mass lesions such as synovial cysts, synovitis, or lipomas also can impinge on the radial nerve and associated branches (323–326).

The radial nerve is at risk in the radial tunnel during radial head excision or fixation of fractures (327–344). During operative exposure of the radial head and neck, rotation of the forearm in pronation rotates the nerve away from and slightly more distal to the operative site, and provides additional safety. With the elbow in supination, the posterior interosseous nerve passes the neck of the radius with a minimal distance of approximately 2.2 cm (mean, 3.3 cm) distal to the radiocapitellar articulation. With the elbow pronated, this minimal distance increases to 3.8 cm (mean 5.2 cm), thus moving the nerve away from the operative area (345). The Presence or Absence of the Wrist Extensors The presence or absence of the active wrist extension (extensor carpi radialis longus and brevis and extensor carpi ulnaris) is helpful in determining the level of nerve injury or dysfunction. A high radial nerve injury that is above the elbow usually results in loss of wrist and digital extension. If wrist extension and radial deviation are present (indicating function of the extensor carpi radialis longus), the lesion is distal to the branching of this nerve. A lesion of the posterior interosseous nerve usually preserves the branch to the extensor carpi radialis brevis, which branches from the superficial branch of the radial nerve or from its own branch proximal to the supinator muscle. Posterior Interosseous Nerve Paralysis A hand with a posterior interosseous nerve paralysis usually dorsiflexes in a radial direction because of preservation of the extensor carpi radialis longus (and brevis). On occasion, the wrist may dorsiflex more neutrally. This can be due to variation of the insertion of the radial extensors of the wrist. The extensor carpi radialis longus can have a tendinous attachment to the brevis tendon. The extensor carpi radialis longus also can insert not only to the base of the index metacarpal, but to the base of the long metacarpal. Either of these conditions helps produce a more neutral wrist extension with complete paralysis of the posterior interosseous nerve. Spontaneous Neuropathy of the Posterior Interosseous Nerve The most frequent cause of spontaneous neuropathy of the posterior interosseous nerve probably is entrapment of the nerve as it enters the supinator muscle at the arcade of Frohse (346,347). Spontaneous neuropathy is well documented in the historical literature (73,111,208,316,327, 346,348–361). Two clinical pictures are described. The first is a complete paralysis of all innervated muscles (the extensor carpi radialis brevis often is spared because it often arises

3 Nerve Anatomy

separately from the superficial branch of the radial nerve, or from the posterior interosseous nerve proximal to the arcade of Frohse, and does not penetrate the muscle). The second clinical picture is a slow, progressive paralysis of the posterior interosseous nerve, usually commencing with paralysis of one or several muscles. If untreated, it frequently progresses to a complete paralysis. Pseudoulnar Claw Hand When there is an incomplete, spontaneous neuropathy of the posterior interosseous nerve, the ring and small fingers initially may be involved. There is lack of extension of these digits, which assume a position of flexion at the metacarpophalangeal joints and the proximal and distal interphalangeal joints. The hand with these flexed digits may resemble a claw hand (similar to ulnar neuropathy, without the extension at the metacarpophalangeal joints). This partial, spontaneous neuropathy of the posterior interosseous has been described as a pseudoulnar claw hand (362). Additional partial paralysis of the posterior interosseous nerve includes loss of extension at the metacarpophalangeal joints of single digits, combinations of digits, or the thumb (350,351,354,356,357,363). Differential Diagnosis in Loss of Digital Extension Loss of digital extension can occur from several etiologies, especially in the patient with inflammatory arthritis. The causes of digital extensor function loss include posterior interosseous nerve paralysis, spontaneous rupture of extrinsic extensor tendon(s), extensor tendon subluxation into the valley between metacarpal heads (such as can occur with inflammatory arthritis that results in incompetence of the sagittal bands for tendon centralization and stabilization), and metacarpophalangeal joint subluxation (in inflammatory arthritis). Partial posterior interosseous nerve paralysis can be distinguished from the other causes by clinical examination, as follows: Neuropathy of the Posterior Interosseous Nerve Partial or complete posterior interosseous nerve paralysis results in loss of active digital extension specifically at the metacarpophalangeal joint. Active digital extension function remains intact at proximal and distal interphalangeal joints because of ulnar nerve–innervated intrinsic muscles. The tenodesis effect is intact (with digital extension occurring when the wrist is passively flexed), and thus helps rule out extensor tendon rupture. Radiographs help determine if metacarpophalangeal joint subluxation is present. Extensor Tendon Rupture There is loss of active digital extension at the metacarpal joints. The tenodesis effect is absent (showing no digital

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extension occurring when the wrist is passively flexed). The patient is unable to maintain digital extension at the metacarpophalangeal joint when the joint is passively placed in an extended position (helping to rule out extensor tendon subluxation between the metacarpal heads). Radiographs help determine if metacarpophalangeal joint subluxation is present (364). Extensor Tendon Subluxation With extensor tendon subluxation, there is weakness or inability actively to extend the digit at the metacarpophalangeal joint. However, the patient is able to maintain digital extension when the digits are passively placed in extension. This is possible because the tendon often centralizes when the metacarpophalangeal joint is passively placed in extension. The patient is able momentarily to maintain the extended position. However, when the digit is flexed, the tendon resubluxates, and digital extension no longer is possible. Metacarpophalangeal Joint Subluxation With metacarpophalangeal joint subluxation, as can develop with rheumatoid arthritis, the patient is unable fully to extend the digits. Passive extension of the digit may not be possible, and this helps distinguish the condition from tendon subluxation. Radiographs show metacarpophalangeal joint subluxation, and help distinguish the condition from nerve palsy. Innervation of the Posterior Interosseous Nerve There is clinical relevance to the order and distance of innervation of the posterior interosseous nerve (see Table 3.7). These can be used in identifying the portion or level of nerve injured from penetrating trauma. The order and distances also have predictive usefulness post-nerve repair in the evaluation of nerve regeneration success and expectations. After successful neurorrhaphy or neurolysis of the posterior interosseous nerve, the earliest clinical sign of impending recovery is the ability of the wrist to dorsiflex in a neutral, or even ulnar, direction. This indicates recovery of function of the extensor carpi ulnaris (and, to some extent, of the extensor digitorum communis). Safe and Unsafe Internervous Planes Because of the transverse or horizontal branching of the posterior interosseous nerve in the mid-forearm, motor branches are vulnerable to injury if the intervals between the extensor carpi ulnaris and the extensor digiti minimi, or between the extensor digiti minimi and extensor digitorum communis, are explored (6). Relatively safe internervous planes in this area are between the anconeus and the extensor carpi ulnaris and between the extensor digitorum communis and the extensor carpi radialis brevis.

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Communication Between the Anterior Interosseous Nerve and the Posterior Interosseous As noted earlier, Rauber described a communication between the anterior interosseous nerve and the posterior interosseous, passing through a foramen in the interosseous ligament. When present, this explains the retained function of the intrinsic muscles in a hand when the ulnar nerve has been severed (6,317,318). The Superficial Branch of the Radial Nerve The superficial branch of the radial nerve may be compressed distal to its exit from the radial tunnel and along its course in the forearm and wrist. It may be impinged at its passage from the subfascial to the subcutaneous level, where its exits into the subcutaneous tissues between the brachioradialis and extensor carpi radialis longus. Dysfunction of the superficial branch of the radial was described by Wartenberg in 1932, and his name often is used in association with the clinical syndrome (365,366). The superficial branch of the radial nerve usually passes dorsally from the deep surface of the brachioradialis to become subcutaneous approximately 10 cm proximal to the radial styloid. The nerve is especially vulnerable to external injury or compression from this point distally. The nerve has been compressed by external objects, such as tight wristwatches, bracelets, handcuffs, gloves, and casts (6,367). The nerve also may be injured from iatrogenic causes, including laceration from release of the first dorsal compartment in De Quervain’s disease, from injury from a cutdown procedure of a vein in the distal forearm, or from laceration from tendon lengthening procedures involving the extensor carpi radialis or longus (6). Injury to the superficial branch of the radial nerve can result in considerable pain and disability, and a full causalgia syndrome can develop (368). Neuromas or associated regional pain syndromes from sympathetic-mediated nerve dysfunction are particularly troublesome. A communication branch between the superficial branch of the radial nerve and the musculocutaneous nerve in the distal forearm is not uncommon (205). Because of this, laceration of the superficial branch of the radial nerve in the proximal forearm (proximal to the communicating branch) may not present clinically with the classic sensory loss expected for superficial radial nerve injury.

MUSCULOCUTANEOUS NERVE Origin of the Musculocutaneous Nerve The musculocutaneous nerve originates from the lateral cord of the brachial plexus and is derived from the ventral rami of C5, C6, and C7. It branches from the lateral cord at the level of and deep to the pectoralis minor (see Fig. 3.1).

Musculocutaneous Nerve in the Axilla and Arm The nerve extends distally on a course lateral to the remaining brachial plexus and medial to the proximal humerus. The nerve pierces the coracobrachialis and continues distally, in a lateral course between the biceps and brachialis to the lateral side of the arm. The course of the nerve in this part of the arm has been delineated by Williams and Latarjet et al., noting that the nerve projects along a line drawn from the lateral side of the third part of the axillary artery across the coracobrachialis and biceps to the lateral side of the biceps tendon (3). The course is varied by its point of entry into the coracobrachialis (369). The musculocutaneous nerve supplies the coracobrachialis, both heads of the biceps, and most of the brachialis (see Fig. 3.2). The branch to the coracobrachialis exits the musculocutaneous nerve before it enters the muscle. The fibers from this branch (to the coracobrachialis) are derived from the ventral ramus of C7. This nerve may branch directly from the lateral cord. The branches to the biceps and brachialis leave the musculocutaneous after the nerve pierces the coracobrachialis. The nerve branch to the brachialis also sends a branch to the elbow joint for innervation. The nerve also supplies a small branch to the humerus, where it enters the cortex with the nutrient artery. At a point just distal to the elbow, the musculocutaneous nerve pierces the deep fascia lateral to the tendon of the biceps. From this point, it continues as the lateral antebrachial cutaneous nerve (lateral cutaneous nerve of the forearm). Lateral Antebrachial Cutaneous Nerve (Lateral Cutaneous Nerve of the Forearm) The lateral antebrachial cutaneous nerve originates as a continuing branch of the musculocutaneous nerve (see Figs. 3.2 and 3.4). The musculocutaneous nerve in the arm passes deep to the biceps and superficial to the brachialis, in a medial-tolateral direction. As the musculocutaneous nerve passes distally and laterally, it reaches the approximate level of the elbow joint, and exits from the deep surface of the biceps to become cutaneous. At this point, the musculocutaneous nerve becomes the lateral antebrachial cutaneous nerve. The lateral antebrachial cutaneous nerve continues distally in the forearm, deep to the cephalic vein, and descends along the radial border of the forearm to reach the wrist. In the forearm, the nerve sends out small cutaneous branches to provide sensibility to the skin of the anterolateral forearm. The nerve may have anastomoses distally with either the posterior cutaneous nerve of the forearm or with the superficial branch of the radial nerve (3). The nerve may give rise to a slender recurrent branch that extends along the cephalic vein as far as the middle third of the arm, giving off several small branches to provide sensibility to the skin over the distal third of the anterolateral surface of the upper arm (370,371). This recurrent

3 Nerve Anatomy

branch rarely is mentioned in most descriptions of the nervous anatomy in the upper extremity (3). At the level of the wrist joint, the lateral antebrachial cutaneous nerve is located anterior to the radial artery and may have several small branches that pierce the deep fascia and accompany the radial artery to the dorsum of the wrist. The nerve then passes to the base of the thenar eminence and ends in multiple small cutaneous rami. The nerve often connects with the superficial branch of the radial nerve and the palmar cutaneous branch of the median nerve. Anomalies and Variations: Musculocutaneous Nerve and Lateral Antebrachial Cutaneous Nerve Several variations of the lateral antebrachial cutaneous nerve have been described. n The musculocutaneous nerve may pass behind the coracobrachialis (instead of passing through the muscle) (3). n The musculocutaneous nerve may accompany or actually adhere to the median nerve in its course in the arm. n The musculocutaneous usually supplies motor innervation to the coracobrachialis. The muscle, however, may be innervated by its own nerve, and branch directly from the lateral cord of the brachial plexus. n Small branches of the median nerve may pass to the musculocutaneous nerve and continue with the musculocutaneous nerve. Conversely, small branches of the musculocutaneous nerve may pass to the median nerve, and continue with the median nerve. n The distal branches of the lateral antebrachial cutaneous nerve may have anastomoses with the superficial branch of the radial nerve or with the palmar cutaneous branch of the median nerve. n The lateral antebrachial cutaneous nerve may, through these small distal branches, innervate or help innervate the pronator teres. n The lateral antebrachial cutaneous nerve may have small branches that extend to the dorsum of the thumb and supply sensibility to the overlying skin (replacing the innervation of the terminal portion of the superficial branch of the radial nerve) (3). Clinical Correlations: Musculocutaneous Nerve and Lateral Antebrachial Cutaneous Nerve Injury to the musculocutaneous nerve can occur from fractures of the proximal humerus. Clinical findings include weakness of elbow flexion (from paresis of the biceps and brachialis) and sensory loss on the lateral aspect of the forearm. Pain and paresthesia may be aggravated by elbow extension, which can stretch the musculocutaneous nerve. The lateral antebrachial cutaneous nerve can be used as a donor nerve for nerve grafting. However, because of

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donor site morbidity with numbness on the lateral aspect of the forearm, other donor nerves (such as the sural nerve) usually are selected. MEDIAL BRACHIAL CUTANEOUS NERVE (MEDIAL CUTANEOUS NERVE OF THE ARM, NERVE OF WRISBERG) The medial brachial cutaneous nerve, often referred to as the medial cutaneous nerve of the arm, or as the nerve of Wrisberg (3,11), is a sensory nerve that supplies the medial aspect of the arm from the axilla to the medial elbow. It is considered the smallest true nerve branch that originates from the brachial plexus. Origin of the Medial Brachial Cutaneous Nerve The medial brachial cutaneous nerve originates from the medial cord of the brachial plexus. It comprises mostly fibers from the ventral rami of C8 and T1 (see Fig. 3.1). The nerve branches from the medial cord at a point slightly proximal to the point of origin of the medial antebrachial cutaneous nerve. Medial Brachial Cutaneous Nerve in the Axilla and Arm From its origin from the medial cord, the medial brachial cutaneous nerve passes through the axilla deep to the pectoralis insertion and anterior to the latissimus dorsi. In its proximal course, it is located dorsal to the axillary artery and vein. As it continues distally, it comes to lie medial to these vessels. The nerve may pass posterior to the axillary vein. In the axilla, it may anastomose with the intercostal nerves. The medial brachial nerve may branch early and consist of several branches as it exits the axilla. The nerve and associated branches continue distally medial to the brachial artery and basilic vein. The nerve descends distally along the medial aspect of the arm and pierces the deep brachial fascia to become cutaneous in the mid-portion of the arm. It continues to branch and provides sensibility to the medial aspect of the arm as far distally as the medial epicondyle and olecranon (11) (see Fig. 3.4). Anomalies and Variations: Medial Brachial Cutaneous Nerve n The medial brachial cutaneous nerve may communicate with the medial antebrachial cutaneous nerve through the ulnar branch of the latter nerve. n The medial brachial cutaneous nerve may originate as a branch of the medial antebrachial cutaneous nerve (11). n The anastomoses with the intercostal nerve in the proximal axilla may have so many branches that the connections assume a plexiform pattern in the axilla.

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n The intercostobrachial nerve communication may be large (between the medial brachial cutaneous nerve and the intercostal nerves) and may be reinforced by a part of the lateral cutaneous branch of the third intercostal nerve. When there is a large contribution or component from the lateral cutaneous branch of the third intercostal nerve, it may replace the medial cutaneous nerve of the arm (3). Clinical Correlations: Medial Brachial Cutaneous Nerve The medial brachial cutaneous nerve contains only sensory fibers. Injury to the medial brachial cutaneous nerve results in loss of sensibility to the medial aspect of the arm. MEDIAL ANTEBRACHIAL CUTANEOUS NERVE (MEDIAL CUTANEOUS NERVE OF THE FOREARM) The medial antebrachial cutaneous nerve, often referred to as the medial cutaneous nerve of the forearm, is a sensory nerve that supplies the medial aspect of the forearm from the elbow to the wrist. It also supplies sensibility to the skin overlying a portion of the anterior arm anterior to the biceps muscle. Origin of the Medial Antebrachial Cutaneous Nerve The medial antebrachial cutaneous nerve originates from the medial cord of the brachial plexus. It comprises mostly fibers from the ventral rami of C8 and T1 (see Fig. 3.1 and Appendix 3.1). The nerve branches from the medial cord at a point slightly distal to the point of origin of the medial brachial cutaneous nerve. Medial Antebrachial Cutaneous Nerve in the Axilla, Arm, and Forearm From its origin from the medial cord, the medial antebrachial cutaneous nerve passes through the axilla deep to the pectoralis insertion and anterior to the latissimus dorsi. In its proximal course, it lies medial to the axillary artery, much closer to the artery than the medial brachial cutaneous nerve. It often is situated between the axillary artery and vein. In the proximal portion, just distal to the axilla, the nerve gives off a small branch that pierces the fascia over the proximal and anterior aspect of the biceps muscle. This branch supplies sensibility to the skin overlying the anterior biceps muscle from the axilla to the level of the elbow. The main nerve continues distally along the medial aspect of the arm medial to the brachial artery. It pierces the deep fascia with the basilic vein to become cutaneous in the mid-por-

tion of the arm. The nerve divides into an anterior and a posterior (ulnar) branch (Fig. 3.4). Anterior Branch of the Medial Antebrachial Cutaneous Nerve The anterior branch of the medial antebrachial cutaneous nerve usually is a larger branch than the posterior (ulnar) branch of the medial antebrachial nerve. The anterior branch continues distally along the anteromedial aspect of the forearm. Proximally in the forearm, it usually passes superficial to the median basilic vein. The nerve then continues on the anterior part of the ulnar forearm, supplying the skin of the anteromedial forearm as far distally as the wrist. It often has an anastomosis with the palmar cutaneous branch of the ulnar nerve (3,11) (see Fig. 3.4). Posterior (Ulnar) Branch of the Medial Antebrachial Cutaneous Nerve The posterior (ulnar) branch of the medial antebrachial cutaneous nerve continues obliquely distally along the medial side of the basilic vein, anterior to the medial epicondyle of the humerus but curving posteriorly, and spiraling around the ulnar aspect of the forearm to reach the dorsal portion of the medial forearm. It continues distally along the ulnar aspect of the forearm as far distal as the wrist, supplying the overlying skin as it extends distally (see Fig. 3.4). It often has anastomoses with the medial brachial cutaneous nerve (in the proximal forearm), with the dorsal antebrachial cutaneous nerve, and with the dorsal branch of the ulnar nerve (3,11). Anomalies and Variations: Medial Antebrachial Cutaneous Nerve The anterior branch of the medial antebrachial nerve descends anteromedially in the forearm to reach the wrist. In this area it often has an anastomosis with the palmar cutaneous branch of the ulnar nerve. The posterior branch of the medial antebrachial nerve descends distally and posterior to the dorsal aspect of the forearm, to reach the medial border of the wrist. Along its course, it may have several anastomoses, including those with the medial brachial cutaneous (in the proximal forearm), or with the posterior cutaneous nerve of the forearm or the dorsal branch of the ulnar nerve. Clinical Correlations: Medial Antebrachial Cutaneous Nerve The medial antebrachial cutaneous nerve contains only sensory fibers. Injury to the medial antebrachial cutaneous nerve results in loss of sensibility to the medial aspect of the

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forearm and a portion of the anterior arm overlying the anterior biceps. Injury to the medial cord or to the C8 or T1 nerve roots results in dysfunction of the medial antebrachial cutaneous nerve (as well as ulnar neuropathy), and is associated with numbness along the medial aspect of the forearm and a portion of the anterior arm overlying the anterior biceps. SENSORY ORGANELLES Several sensory nerve endings (organelles) terminate in the skin, usually in relatively high concentrations in the hand. These are innervated by the sensory nerve endings of the median, ulnar, and radial nerves. The nerve endings are encapsulated and exhibit considerable variety in size, shape, and distribution, but all share in common the feature of an axon terminal encapsulated by nonexcitable cells. The

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major end organelles include the pacinian corpuscles, Meissner corpuscles, Ruffini nerve endings, and Merkel receptors (3,372,373) (Fig. 3.6). Pacinian Corpuscles Pacinian corpuscles (corpuscles of Vater-Pacini) are relatively large, lamellated structures located in the subcutaneous tissue. They occur in high concentrations on the palmar surface of the hand and digits (as well as in the plantar foot, periostea, interosseous membranes, and periarticular areas). These are rapidly adapting receptors, and their function usually is considered to be detection of vibration, pressure, or coarse touch (3,373). They are oval, spherical, or irregular firm masses, smooth and glistening white or yellow in color, up to 2 to 4 mm in size (approximately 100 to 500 µm across) (373), and are easily seen with (or without) loupe magnification during operative procedures on the palmar surface of the hand or digits. Each has a capsule, an intermediate growth zone, and a

FIGURE 3.6. Sensory organelles.

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central core containing an axon terminal. The capsule is formed by approximately 30 concentrically arranged lamellae of flat cells. The axon terminal consists of an unbranched terminal of a peripheral nerve, and is in contact with the innermost core lamellae (3,372,373). Meissner Corpuscles Meissner corpuscles (tactile corpuscles of Meissner) are found in the dermis, usually in the superficial layers very close to the epidermis. They are in relatively high concentrations in all parts of the hand (and foot), especially in the distal digits (373). They also are rapidly adapting and highly sensitive to fluctuating mechanical forces acting on the surface of the skin. Meissner corpuscles are particularly sensitive to vibration at certain frequencies. The structures are somewhat cylindrical in shape, with their long axes perpendicular to the skin surface. They are much smaller than the pacinian corpuscles, measuring approximately 80 µm long and 30 µm across. The organelle has a connective tissue capsule and a central core, the capsule being loosely attached to the core. Like the pacinian corpuscle, the Meissner corpuscle has an axon terminal ending inside of the capsule (3) (Fig. 3.6).

Ruffini Nerve Endings Ruffini endings (type II slowly adapting cutaneous mechanoreceptors) occur in the dermis of hairy skin. These are slowly adapting (compared with the rapidly adapting pacinian and Meissner corpuscles) and responsive to continuous forces such as maintained stress or stretch of the skin. They consist of highly branched nerve endings that are distributed among bundles of collagen fibers in a spindle-shaped structure. The structure is enclosed partly by a fibrocellular sheath derived from the perineurium of the nerve (3) (Fig. 3.6). Merkel Receptors Merkel receptors basically are nerve endings (type I slowly adapting cutaneous mechanoreceptors), and occur in the skin in the vicinity of the dermal–epidermal junction. The nerve ending is located in the basement membrane and keratinocytes of the epidermis, or near the hair follicle. The Merkel receptors are sensitive to perpendicular pressure or indentation of the skin, or to the bending of the hair follicle (3,373).

APPENDIX 3.1. DERMATOMES OF THE UPPER EXTREMITY

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