Hand Wrist and Forearm

JAMES W. STRICKLAND, MD

I ANATOMY AND KINESIOLOGY OF THE HAND, WRIST, AND FOREARM

One cannot expect to adequately participate in the treatment of disorders of the hand and arm without a solid working knowledge of the intricate anatomic and kinesiologic relationships of the upper extremity. The preparation of externally applied splinting devices to the forearm, wrist, and hand necessitates a thorough understanding of and respect for the underlying anatomic structures. Only through comprehension of the normal anatomy of the human hand can one adequately develop an appreciation for the anatomic alternations that accompany injury and disease. Since it is impossible in this chapter to review in great detail the enormous amount of literature that has been written about the anatomic, kinesiologic, and biome-chanical aspects of the hand, readers are directed to the suggested reading list at the end of this chapter for more extensive reading on these subjects.

The anatomy of the hand must be approached in a systematic fashion with individual consideration of the osseous structures, joints, musculotendinous units, blood supply, nerve supply, and surface anatomy. However, it is obvious that the systems do not function independently, but that the integrated presence of all these structures is required for normal hand function. In presenting this material, I stray into the important mechanical and kinesiologic considerations that result from the unique anatomic arrangement of the hand and briefly try to indicate the problems resulting from various forms of pathologic conditions in certain areas. Surface anatomy and a description of the basic patterns of hand function are also included at the end of the chapter.

Osseous Structures

The unique arrangement and mobility of the bones of the hand (Fig. 2-1) provide a structural basis for its enormous functional adaptability. The osseous skeleton consists of eight carpal bones divided into two rows: the proximal row articulates with the distal radius and ulna (with the exception of the pisiform, which lies palmar to and articulates with the tri-quetrum); the distal four carpal bones in turn articulate with the five metacarpals. Two phalanges complete the first ray, or thumb unit, and three phalanges each comprise the index, long, ring, and small fingers. These 27 bones, together with the intricate arrangement of supportive ligaments and contractile musculotendi-nous units, are arranged to provide both mobility and stability to the various joints of the hand. Although the exact anatomic configuration of the bones of the hand need not be memorized in detail, it is important to develop a knowledge of the position and names of the carpal bones, metacarpals, and phalanges and an understanding of their kinesiologic patterns to proceed with the management of many hand problems.

The bones of the hand are arranged in three arches (Fig. 2-2), two transversely oriented and one that is longitudinal. The proximal transverse arch, the keystone of which is the capitate, lies at the level of the distal part of the carpus and is reasonably fixed, whereas the distal transverse arch passing through the metacarpal heads is more mobile. The two transverse arches are connected by the rigid portion of the longitudinal arch consisting of the second and third metacarpals, the index and long fingers distally, and the central carpus proximally. The longitudinal arch is completed by the individual digital rays, and the mobility of the first, fourth, and fifth rays around the second and third allows the palm to flatten or cup itself to accommodate objects of various sizes and shapes.

To a large extent the intrinsic muscles of the hand are responsible for changes in the configuration of the osseous arches, and collapse in the arch system resulting from injury to the osseous skeleton or paralysis of the intrinsic muscles can contribute to severe disability and deformity. Flatt4-6 has pointed out that grasp is dependent on the integrity of the mobile longitudinal arches and when destruction at the car-pometacarpal joint, metacarpophalangeal joint, or proximal interphalangeal joint interrupts the integrity of these arches, crippling deformity may result.

Joints

The multiple complex articulations between the distal radius and ulna, the eight carpal bones, and the

Distal Transverse Arch

Distal phalanx

Trapezoid ■ Capitate

Distal phalanx

Middle phalanx

-Proximal phalanx-

Hamate-Pisiform-Triquetrum;

■ Metacarpal

Trapezoid ■ Capitate

Trapezium

Middle phalanx

-Proximal phalanx-

■ Metacarpal

Trapezium

Distal Transverse Arch

Hamate Triquetrum

Hamate Triquetrum

Fig. 2-1 Bones of the right hand. A, Palmar surface. B, Dorsal surface.

Distal Transverse Arch

Distal transverse arch

Distal transverse arch

Distal Transverse Arch

Proximal transverse arch

Fig. 2-2 A, Skeletal arches of the hand. The proximal transverse arch passes through the distal carpus; the distal transverse arch, through the metacarpal heads. The longitudinal arch is made up of the four digital rays and the carpus proximally. B, Proximal and distal transverse arches.

Proximal transverse arch

Fig. 2-2 A, Skeletal arches of the hand. The proximal transverse arch passes through the distal carpus; the distal transverse arch, through the metacarpal heads. The longitudinal arch is made up of the four digital rays and the carpus proximally. B, Proximal and distal transverse arches.

metacarpal bases comprise the wrist joint, whose proximal position makes it the functional key to the motion at the more distal hand joints of the hand. Functionally the carpus transmits forces through the hand to the forearm. The proximal carpal row consisting of the scaphoid (navicular), lunate, and triquetrum articulates distally with the trapezium, trapezoid, capitate, and hamate; there is a complex motion pattern which relies both on ligamentous and contact surface constraints. The major ligaments of the wrist (Fig. 2-3) are the palmar and intracapsular ligaments. There are three strong radial palmar ligaments: the radioscaphocapitate or "sling" ligament, which supports the waist of the scaphoid; the radio-lunate ligament, which supports the lunate; and the radioscapholunate ligament, which connects the scapholunate articulation with the palmar portion of the distal radius. This ligament functions as a checkrein for scaphoid flexion and extension. The ulnolunate ligament arises intra-articularly from the triangular articular meniscus of the wrist joint and inserts on the lunate and, to a lesser extent, the tri-quetrum. The radial and ulnar collateral ligaments are capsular ligaments, and V-shaped ligaments from the capitate to the triquetrum and scaphoid have been termed the deltoid ligaments. Dorsally, the radiocarpal ligament connects the radius to the triquetrum and acts as a dorsal sling for the lunate, maintaining the lunate in apposition to the distal radius. Further dorsal carpal support is provided by the dorsal intracarpal ligament. These strong ligaments combine to provide carpal stability while permitting the normal range of wrist motion.

The distal ulna is covered with an articular cartilage (Fig. 2-3, C) over its most dorsal, palmar, and radial aspects, where it articulates with the sigmoid or ulnar notch of the radius. The triangular fibrocartilage complex describes the ligamentous and cartilaginous structure that suspends the distal radius and ulnar carpus from the distal ulna. Blumfield and Champoux (1984) have indicated that the optimal functional wrist motion to accomplish most activities of daily living is from 10° of flexion to 35° of extension.

Taleisnik10,11,13,14 has emphasized the importance of considering the wrist in terms of longitudinal columns (Fig. 2-4). The central, or flexion extension, column consists of the lunate and the entire distal carpal row; the lateral, or mobile, column comprises the scaphoid alone; and the medial, or rotation, column is made up of the triquetrum. Wrist motion is produced by the muscles that attach to the metacarpals, and the liga-mentous control system provides stability only at the extremes of motion. The distal carpal row of the carpal bones is firmly attached to the hand and moves with it. Therefore during dorsiflexion the distal carpal row dorsiflexes, during palmar flexion it palmar flexes, and during radial and ulnar deviation it deviates radially or ulnarly. As the wrist ranges from radial to ulnar deviation, the proximal carpal row rotates in a dorsal direction, and a simultaneous translocation of the proximal carpus occurs in a radial direction at the radiocarpal and midcarpal articulations. This combined motion of the carpal rows has been referred to as the rotational shift of the carpus. It was once taught that palmar flexion takes place to a greater extent at the radiocarpal joint and secondarily in the midcarpal joint, but since dorsiflexion occurs primarily at the midcarpal joint and only secondarily at the radiocarpal articulation, this now appears to be a significant oversimplification. The complex carpal kinematics are beyond the scope of this chapter, and the reader is referred to the works of Weber,15 Taleisnik,12 Lichtman,7 and Cooney3 to gain a thorough understanding of this difficult subject.

The articulation between the base of the first metacarpal and the trapezium (Fig. 2-5) is a highly mobile joint with a configuration thought to be similar to that of a saddle. The base of the first metacarpal is

Lunotriquetral ligament

Vestigial ulnar collateral ligament

Ulnocarpal meniscus homologue

Ulnolunate ligament, (ulnolunate-triquetral)

Radioscapholunate ligament (ligament of Testut and Kuenz)

Deltoid ligaments Space of Poirier

Deltoid ligaments Space of Poirier

Lunotriquetral ligament

Vestigial ulnar collateral ligament

Ulnocarpal meniscus homologue

Ulnolunate ligament, (ulnolunate-triquetral)

Radioscapholunate ligament (ligament of Testut and Kuenz)

Distal Transverse Arch

Radioscaphocapitate ligament

Scapholunate ligament

Radial collateral ligament

IP

Radiolunate

(radiolunotriquetral)

Dorsal intercarpal ligament

Distal Transverse Arch

Fig. 2-3 Ligamentous anatomy of the wrist. A, Palmar wrist ligaments. B, Dorsal wrist ligaments. C, Dorsal view of the flexed wrist, including the triangular fibrocartilage. 1, Ulnar collateral ligament; 2, retinacular sheath; 3, tendon of extensor carpi ulnaris; 4, ulnolunate ligament; 5, triangular fibrocartilage; 6, ulnocarpal meniscus homologue; 7, palmar radioscaphoid lunate ligament. P,Pisiform; H, hamate; C, capitate; Td, trapezoid; Tm, trapezium; Tq, triquetrum; L, lunate; S, scaphoid.

Dorsal intercarpal ligament

Fig. 2-3 Ligamentous anatomy of the wrist. A, Palmar wrist ligaments. B, Dorsal wrist ligaments. C, Dorsal view of the flexed wrist, including the triangular fibrocartilage. 1, Ulnar collateral ligament; 2, retinacular sheath; 3, tendon of extensor carpi ulnaris; 4, ulnolunate ligament; 5, triangular fibrocartilage; 6, ulnocarpal meniscus homologue; 7, palmar radioscaphoid lunate ligament. P,Pisiform; H, hamate; C, capitate; Td, trapezoid; Tm, trapezium; Tq, triquetrum; L, lunate; S, scaphoid.

concave in the anteroposterior plane and convex in the lateral plane, with a reciprocal concavity in the lateral plane and an anteroposterior convexity on the opposing surface of the trapezium. This arrangement allows for the positioning of the thumb in a wide arc of motion (Fig. 2-6), including flexion, palmar and radial abduction, adduction, and opposition. The ligamentous arrangement about this joint, while permitting the wide circumduction, continues to provide stability at the extremes of motion, allowing the thumb to be brought into a variety of positions for pinch and grasp, but maintaining its stability during these functions. The articulations formed by the ulnar half of the hamate and the fourth and fifth metacarpal bases allow a modest amount of motion (15° at the fourth carpometacarpal joint and 25° to 30° of flexion and extension at the fifth carpometacarpal joint). A resulting "palmar descent" of these metacarpals occurs during strong grasp.

The metacarpophalangeal joints of the fingers are diarthrodial joints with motion permitted in three planes and combinations thereof (Fig. 2-7). The

Distal Transverse Arch

Fig. 2-6 A, Multiple planes of motion (arrows) that occur at the carpometacarpal joint of the thumb. B, The thumb moves (arrow) from a position of adduction against the second metacarpal to a position of palmar or radial abduction away from the hand and fingers and can then be rotated into positions of opposition and flexion.

Fig. 2-4 Columnar carpus. The scaphoid is the mobile or lateral column. The central, or flexion extension, column comprises the lunate and the entire distal carpal row. The medial, or rotational, column comprises the triquetrum alone.

Fig. 2-4 Columnar carpus. The scaphoid is the mobile or lateral column. The central, or flexion extension, column comprises the lunate and the entire distal carpal row. The medial, or rotational, column comprises the triquetrum alone.

Distal Transverse Arch

Fig. 2-6 A, Multiple planes of motion (arrows) that occur at the carpometacarpal joint of the thumb. B, The thumb moves (arrow) from a position of adduction against the second metacarpal to a position of palmar or radial abduction away from the hand and fingers and can then be rotated into positions of opposition and flexion.

Distal Transverse Arch

Fig. 2-5 Saddle-shaped carpometacarpal joint of the thumb. A wide range of motion (arrows) is permitted by the configuration of this joint.

cartilaginous surfaces of the metacarpal head and the bases of the proximal phalanges are enclosed in a complex apparatus consisting of the joint capsule, collateral ligaments, and the anterior fibrocartilage or palmar plate (Fig. 2-8). The capsule extends from the borders of the base of the proximal phalanx prox-imally to the head of the metacarpals beyond the cartilaginous joint surface. The collateral ligaments, which reinforce the capsule on each side of the metacarpophalangeal joints, run from the dorsolateral side of the metacarpal head to the palmar lateral side of the proximal phalanges. These ligaments form two bundles, the more central of which is referred to as the cord portion of the collateral ligament and inserts into the side of the proximal phalanx; the more palmar portion joins the palmar plate and is termed the accessory collateral ligament. These collateral ligaments are somewhat loose with the metacarpophalangeal

Fig. 2-7 Joints of the phalanges. The diarthrodial configuration of the metacarpophalangeal joint permits motion in multiple planes, whereas the biconcave-convex hinge configuration of the interpha-langeal joints restricts motion to the anteroposterior plane.

Distal Transverse Arch

Fig. 2-8 Ligamentous structures of the digital joints. The collateral ligaments of the metacarpophalangeal and interdigital joints are composed of a strong cord portion with bony origin and insertion. The more palmarly placed accessory collateral ligaments originate from the proximal bone and insert into the palmar fibrocartilaginous plate. The palmar plates have strong distal attachments to resist extension forces.

Fig. 2-8 Ligamentous structures of the digital joints. The collateral ligaments of the metacarpophalangeal and interdigital joints are composed of a strong cord portion with bony origin and insertion. The more palmarly placed accessory collateral ligaments originate from the proximal bone and insert into the palmar fibrocartilaginous plate. The palmar plates have strong distal attachments to resist extension forces.

joint in extension, allowing for considerable "play" in the side-to-side motion of the digits (Fig. 2-9). With the metacarpophalangeal joints in full flexion, however, the cam configuration of the metacarpal head tightens the collateral ligaments and limits lateral mobility of the digits. This alteration in tension becomes an important factor in immobilization of the metacarpophalangeal joints for any length of time, since the secondary shortening of the lax collateral ligaments that may occur when these joints are immobilized in extension will result in severe limitation of metacarpophalangeal joint flexion by these structures.

The palmar fibrocartilaginous plate on the palmar side of the metacarpophalangeal joint is firmly attached to the base of the proximal phalanx and loosely attached to the anterior surface of the neck of the metacarpal by means of the joint capsule at the neck of the metacarpal. This arrangement allows the palmar plate to slide proximally during metacar-pophalangeal joint flexion. The flexor tendons pass along a groove anterior to the plate. The palmar plates are connected by the transverse inter-metacarpal ligaments, which connect each plate to its neighbor.

The metacarpophalangeal joint of the thumb differs from the others in that the head of the first metacarpal is flatter and its cartilaginous surface does not extend as far laterally or posteriorly. Two small sesamoid bones are also adjacent to this joint, and the liga-mentous structure differs somewhat. A few degrees of abduction and rotation are permitted by the ligament arrangement of the metacarpophalangeal joint at the thumb, which is of considerable functional importance in delicate precision functions. There is considerable variation in the range of motion present at the thumb metacarpophalangeal joints. The amount

Fig. 2-9 At the metacarpophalangeal joint level, the collateral ligaments are loose in extension but become tightened in flexion. The proximal membranous portion of the palmar plate moves proxi-mally to accommodate for flexion. (Modified from Wynn Parry CB, et al.: Rehabilitation of the hand, ed 3, Butterworth, 1973, London.)

of motion varies from as little as 30° to as much as 90°.

The digital interphalangeal joints are hinge joints (Fig. 2-7) and, like the metacarpophalangeal joints, have capsular and ligamentous enclosure. The articular surface of the proximal phalangeal head is convex in the anteroposterior plane with a depression in the middle between the two condyles, which articulates with the phalanx distal to it. The bases of the middle and distal phalanges appear as a concave surface with an elevated ridge dividing two concave depressions. A cord portion of the collateral ligament and an accessory collateral ligament are present, and the collateral ligaments run on each side of the joint from the dorsolateral aspect of the proximal phalanx in a palmar and lateral direction to insert into the distally placed phalanx and its fibrocartilage plate (Fig. 2-10).

Distal Transverse Arch

Fig. 2-10 Strong, three-sided ligamentous support system of the proximal interphalangeal joint with cord and accessory collateral ligaments and the fibrocartilaginous plate, which is anchored prox-imally by the checkrein ligamentous attachment. (Modified from Eaton RG: Joint injuries of the hand, Charles C Thomas, 1971, Springfield, IL.)

Fig. 2-10 Strong, three-sided ligamentous support system of the proximal interphalangeal joint with cord and accessory collateral ligaments and the fibrocartilaginous plate, which is anchored prox-imally by the checkrein ligamentous attachment. (Modified from Eaton RG: Joint injuries of the hand, Charles C Thomas, 1971, Springfield, IL.)

A strong fibrocartilaginous (palmar) plate is also present, and the collateral ligaments of the proximal and distal interphalangeal joints are tightest with the joints in near full extension.

The stability of the proximal interphalangeal joint is ensured by a three-sided supporting cradle produced by the junction of the palmar plate with the base of the middle phalanx and the accessory collateral ligament structures (Fig. 2-10). The confluence of ligaments is strongly anchored by proximal and lateral extensions referred to as the checkrein ligaments. This system has been described as a three-dimensional hinge that results in remarkable palmar and lateral restraint.

A wide range of pathologic conditions may result from the interruption of the supportive ligament system of the intercarpal or digital joints. At the wrist level, interruption of key radiocarpal or intercarpal ligaments may result in occult patterns of wrist instability that are often difficult to diagnose and treat. In the digits, disruption of the collateral ligaments or the fibrocartilaginous palmar plates will produce joint laxity or deformity, which is more obvious. Rupture or attenuation of these supporting structures may result not only from trauma, but may also occur more insidiously with chronic disease processes such as arthritis.

Muscles and Tendons

The muscles acting on the hand can be grouped as extrinsic, when their muscle bellies are in the forearm, or intrinsic, when the muscles originate distal to the wrist joint. It is important to thoroughly understand both systems. Although their contributions to hand function are distinctly different, the integrated function of both systems is important to the satisfactory performance of the hand in a wide variety of tasks. A schematic representation of the origin and insertion of the extrinsic flexor and extensor muscle tendon units of the hand is provided in Figs. 2-11 and 2-15. The important nerve supply to each muscle group is reviewed in this Figure and again when discussing the nerve supply to the upper extremity.

Extrinsic Muscles

The extrinsic flexor muscles (Fig. 2-11) of the forearm form a prominent mass on the medial side of the upper part of the forearm: the most superficial group comprises the pronator teres, the flexor carpi radialis, the flexor carpi ulnaris, and the palmaris longus; the intermediate group the flexor digitorum superficialis; and the deep extrinsics the flexor digitorum profundus and the flexor pollicis longus. The pronator, palmaris, wrist flexors, and superficialis tendons arise from the area about the medial epicondyle, the ulnar collateral ligament of the elbow, and the medial aspect of the coronoid process. The flexor pollicis longus originates from the entire middle third of the palmar surface of the radius and the adjacent interosseous membrane, and the flexor digitorum profundus originates deep to the other muscles of the forearm from the proximal two-thirds of the ulna on the palmar and medial side. The deepest layer of the palmar forearm is completed distally by the pronator quadratus muscle.

The flexor carpi radialis tendon inserts on the base of the second metacarpal, whereas the flexor carpi ulnaris inserts into both the pisiform and fifth metacarpal base. The superficialis tendons lie superficial to the profundus tendons as far as the digital bases, where they bifurcate and wrap around the pro-fundi and rejoin over the distal half of the proximal phalanx as Camper's chiasma (Fig. 2-12). The super-ficialis tendon again splits for a dual insertion on the proximal half of the middle phalanges. The profundi continue through the superficialis decussation to insert on the base of the distal phalanx. The flexor pol-licis longus inserts on the base of the distal phalanx of the thumb.

At the wrist the nine long flexor tendons enter the carpal tunnel beneath the protective roof of the deep transverse carpal ligament in company with the median nerve. In this canal the common profundus tendon to the long, ring, and small fingers divides into the individual tendons that fan out distally and proceed toward the distal phalanges of these digits (Fig. 2-13). At approximately the level of the distal palmar crease the paired profundus and superficialis

Distal Transverse ArchDistal Transverse ArchDistal Transverse Arch

Brachioradialis

Supination Pronation

Distal Transverse Arch

Supination Pronation

Brachioradialis

Pronator teres Nerve: median Action: forearm pronation

Brachioradialis Nerve: radial Action: pronation or supination, depending on position of forearm

Fig. 2-11 Extrinsic flexor muscles of the arm and hand. (Dark areas represent origins and insertions of muscles.) (Modified from Marble HC: The hand, a manual and atlas for the general surgeon, Saunders, 1960, Philadelphia.)

Composite

Distal Transverse Arch

Fig. 2-12 Anatomy of the relationship between the flexor digito-rum superficialis (FDS), flexor digitorum profundus (FDP), and the proximal portion of the flexor tendon sheath. The superficialis tendon divides and passes around the profundus tendon to reunite at Camper's chiasma. The tendon once again divides prior to insertion on the base of the middle phalanx.

Fig. 2-12 Anatomy of the relationship between the flexor digito-rum superficialis (FDS), flexor digitorum profundus (FDP), and the proximal portion of the flexor tendon sheath. The superficialis tendon divides and passes around the profundus tendon to reunite at Camper's chiasma. The tendon once again divides prior to insertion on the base of the middle phalanx.

Deep

Deep

Distal Transverse Arch

Flexor digitorum profundus Nerve: median—index and long ulnar—ring and small Action: flexion of distal interphalangeal, proximal interphalangeal, and metacarpophalangeal joints

Distal Transverse Arch

Flexor pollicis longus Nerve: median Action: flexes interphalangeal and metacarpophalangeal joints of thumb

For legend see opposite page.

Distal Transverse Arch
Fig. 2-13 Flexor tendons in the palm and digits. Fibroosseous digital sheaths with their pulley arrangement are shown, as is a division of the superficialis tendon about the profundus in the proximal portion of the sheath.

tendons to the index, long, ring, and small fingers and the flexor pollicis longus to the thumb enter the individual flexor sheaths that house them throughout the remainder of their digital course. These sheaths with their predictable annular pulley arrangement (Fig. 2-14) serve not only as a protective housing for the flexor tendons, but also provide a smooth gliding surface by virtue of their synovial lining and an efficient mechanism to hold the tendons close to the digital bone and joints. There is an increasing recognition that disruption of this valuable pulley system can produce substantial mechanical

Distal Transverse Arch

Fig. 2-14 Components of the digital flexor sheath. The sturdy annular pulleys (A) are important biomechanically in guaranteeing the efficient digital motion by keeping the tendons closely applied to the phalanges. The thin pliable cruciate pulleys (C) permit the flexor sheath to be flexible while maintaining its integrity. (Modified from Doyle JR, Blythe W: The finger flexor tendon sheath and pulleys: anatomy and reconstruction. In American Academy of Orthopaedic Surgeons: Symposium on tendon surgery in the hand, Mosby, 1975, St. Louis.)

Fig. 2-14 Components of the digital flexor sheath. The sturdy annular pulleys (A) are important biomechanically in guaranteeing the efficient digital motion by keeping the tendons closely applied to the phalanges. The thin pliable cruciate pulleys (C) permit the flexor sheath to be flexible while maintaining its integrity. (Modified from Doyle JR, Blythe W: The finger flexor tendon sheath and pulleys: anatomy and reconstruction. In American Academy of Orthopaedic Surgeons: Symposium on tendon surgery in the hand, Mosby, 1975, St. Louis.)

alterations in digital function, resulting in imbalance and deformity.

Extension of the wrist and fingers is produced by the extrinsic extensor muscle tendon system, which consists of the two radial wrist extensors, the extensor carpi ulnaris, the extensor digitorum communis, extensor indicis proprius and the extensor digiti quinti proprius (extensor digiti minimi) (Fig. 2-15). These muscles originate in common from the lateral epi-condyle and the lateral epicondylar ridge and from a small area posterior to the radial notch of the ulna. The brachioradialis originates from the epicondylar line proximal to the lateral epicondyle and, because it inserts on the distal radius, it does not truly contribute to wrist or digit motion. The extensor carpi radialis longus and brevis insert proximally on the bases of the second and third metacarpals, respectively, and the extensor carpi ulnaris inserts on the base of the fifth metacarpal. The long digital extensors terminate by insertions on the bases of the middle phalanges after receiving and giving fibers to the intrinsic tendons to form the lateral bands that are destined to insert on the bases of the distal phalanx. Digital extension, therefore, results from a combination of the contribution of both the extrinsic and intrinsic extensor systems. The extensor pollicis longus and brevis tendons, together with the abductor pollicis longus, originate from the dorsal forearm and, by virtue of their respective insertions into the distal phalanx, proximal phalanx, and first metacarpal of the thumb, provide extension at all three levels. The extensor pollicis longus approaches the thumb obliquely around a small bony tubercle on the dorsal radius (Lister's tubercle) and therefore functions not only as an extensor but as a strong secondary adductor of the thumb. The extensor indicis proprius also originates more distally than the extensor com-munis tendons from an area near the origin of the thumb extensor and long abductor. It lies on the ulnar aspect of the communis tendon to the index finger and inserts with it in the dorsal approaches of that digit. The extensor digiti quinti proprius arises near the lateral epicondyle to occupy a superficial position on the dorsum of the forearm with its paired tendons lying on the fifth metacarpal ulnar to the communis tendon to the fifth finger. It inserts into the extensor apparatus of that digit.

At the wrist, the extensor tendons are divided into six dorsal compartments (Fig. 2-16). The first compartment consists of the tendons of the abductor pol-licis longus and extensor pollicis brevis and the second compartment houses the two radial wrist extensors, the extensor carpi radialis longus, and brevis. The third compartment is composed of the tendon of the extensor pollicis longus and the fourth compartment allows passage of the four communis extensor tendons and the extensor indicis proprius tendon. The extensor digiti quinti proprius travels through the fifth dorsal compartment and the sixth houses the extensor carpi ulnaris.

Intrinsic Muscles

The important intrinsic musculature of the hand can be divided into muscles comprising the thenar eminence, those comprising the hypothenar eminence, and the remaining muscles between the two groups (Fig. 2-17). The muscles of the thenar eminence consist of the abductor pollicis brevis, the flexor pol-licis brevis, and the opponens pollicis, which originate in common from the transverse carpal ligament and the scaphoid and trapezium bones. The abductor brevis inserts into the radial side of the proximal phalanx and the radial wing tendon of the thumb, as does the flexor pollicis brevis, whereas the opponens inserts into the whole radial side of the first metacarpal.

The flexor pollicis brevis has a superficial portion that is innervated by the median nerve and a deep portion that arises from the ulnar side of the first metacarpal and is often innervated by the ulnar nerve. The hypothenar eminence in a similar manner is made up of the abductor digiti quinti, the flexor digiti quinti brevis, and the opponens digiti quinti, which originate primarily from the pisiform bone and the pisohamate ligament and insert into the joint capsule of the fifth metacarpophalangeal joint, the ulnar side of the base of the proximal phalanx of the fifth finger, and the ulnar border of the aponeurosis of this digit. The strong

Distal Transverse Arch

Extensor indicis proprius Nerve: radial Action: extension of index finger

Abductor pollicis longus Nerve: radial

Action: abduction of thumb

Extensor carpi radialis longus and brevis Nerve: radial Action: extension of wrist and radial deviation of hand

Extensor carpi ulnaris Nerve: radial Action: extension of wrist and ulnar deviation of hand

Extensor indicis proprius Nerve: radial Action: extension of index finger

Extensor pollicis longus Nerve: radial Action: extension of interphalangeal joint and metacarpophalangeal joint of thumb

Distal Transverse Arch

Abductor pollicis longus Nerve: radial

Action: abduction of thumb

Composite

Distal Transverse Arch

Extensor digitorum communis and extensor digiti quinti proprius Nerve: radial Action: extension of fingers

Extensor pollicis brevis Nerve: radial Action: extension of metacarpophalangeal joint of thumb

Extensor pollicis brevis Nerve: radial Action: extension of metacarpophalangeal joint of thumb

Fig. 2-15 Extrinsic extensor muscles of the forearm and hand. (Modified from Marble HC: The hand, a manual and atlas for the general surgeon, Saunders, 1960, Philadelphia.)

thenar musculature is responsible for the ability to position the thumb in opposition so that it may meet the adjacent digits for pinch and grasp functions, whereas the hypothenar group allows a similar but less pronounced rotation of the fifth metacarpal.

Of the seven interosseous muscles, four are considered in the dorsal group (Fig. 2-18, B) and three as palmar interossei (Fig. 2-18, C). The four dorsal interossei originate from the adjacent sides of the metacarpal bones and, because of their bipennate nature with two individual muscle bellies, have separate insertions into the tubercle and the lateral aspect of the proximal phalanges and into the extensor expansion. The more palmarly placed three palmar interossei (Fig. 2-18, C) have similar insertions and origins and are responsible for adducting the digits together, as opposed to the spreading or abducting function of the dorsal interossei. In addition, four lumbrical tendons (Fig. 2-19, A) arising from the radial side of the palmar portion of the flexor digitorum profundus tendons pass through their individual canals on the radial side of the digits to provide an additional contribution to the complex extensor assemblage of the digits. The arrangement of the extensor mechanism, including the transverse sagittal band fibers at the metacarpophalangeal joint and the components of the extensor hood mechanism that gain fibers from both the extrinsic and intrinsic tendons, can be seen in Fig. 2-19, B,C.

An oversimplification of the function of the intrinsic musculature in the digits would be that they provide strong flexion at the metacarpophalangeal joints and extension at the proximal and distal inter-phalangeal joints. The lumbrical tendons, by virtue of their origin from the flexor profundi and insertion into the digital extensor mechanism, function as a governor between the two systems, resulting in a loosening of the antagonistic profundus tendon during inter-phalangeal joint extension. The interossei are further responsible for spreading and closing of the fingers and, together with the extrinsic flexor and extensor tendons, are invaluable to digital balance. A composite, well-integrated pattern of digital flexion and extension is reliant on the smooth performance of both systems, and a loss of intrinsic function will result in severe deformity.

Perhaps the most important intrinsic muscle, the adductor pollicis (Fig. 2-18, A), originates from the third metacarpal and inserts on the ulnar side of the base of the proximal phalanx of the thumb and

First dorsal interosseous^

Extensor indicis proprius

Extensor pollicis brevis

Extensor pollicis longus

Extensor digitorum communis

Extensor digiti quinti proprius

Abductor digiti quinti Extensor carpi ulnaris

Extensor carpi radialis longus and brevis

Abductor pollicis longus

Extensor digitorum communis

Extensor digiti quinti proprius

Abductor digiti quinti Extensor carpi ulnaris

First dorsal interosseous^

Extensor indicis proprius

Extensor pollicis brevis

Extensor pollicis longus

Extensor carpi radialis longus and brevis

Distal Transverse Arch

Abductor pollicis longus

Fig. 2-16 Arrangement of the extensor tendons in the compartments of the wrist.

Fig. 2-16 Arrangement of the extensor tendons in the compartments of the wrist.

Distal Transverse ArchDistal Transverse Arch

Abductor pollicis brevis

Nerve: median

Action: abduction of thumb

Opponens pollicis Nerve: median Action: rotation of first metacarpal toward palm

Distal Transverse ArchDistal Transverse ArchDistal Transverse Arch

Flexor pollicis brevis Nerve: median—superficial ulnar—deep Action: flexion and rotation of thumb

Adductor pollicis Nerve: ulnar Action: adduction of thumb

Distal Transverse Arch

Abductor digiti quinti Nerve: ulnar

Action: abduction of small finger

(flexion of proximal phalanx, extension of proximal and distal interphalangeal joints)

Abductor digiti quinti Nerve: ulnar

Action: abduction of small finger

(flexion of proximal phalanx, extension of proximal and distal interphalangeal joints)

Distal Transverse Arch

Flexor digiti quinti brevis Nerve: ulnar

Action: flexion of proximal phalanx of small finger and forward rotation of fifth metacarpal

Fig. 2-17 Intrinsic muscles of the hand. (Modified from Marble HC: The hand, a manual and atlas for the general surgeon, Saunders, 1960, Philadelphia.)

into the ulnar wing expansion of the extensor mechanism. This muscle, by virtue of its strong adducting influence on the thumb and its stabilizing effect on the first metacarpophalangeal joint, functions together with the first dorsal interosseous to provide strong pinch. The adductor pollicis, deep head of the flexor pollicis brevis, ulnar two lumbricals, and all interos-sei, as well as the hypothenar muscle group, are innervated by the ulnar nerve. Loss of ulnar nerve function has a profound influence on hand function.

Distal Transverse Arch

Lumbricals

Nerve: median—index and long ulnar—ring and small Action: supplements metacarpophalangeal flexion and extension of proximal and distal interphalangeal joints

Lumbricals

Nerve: median—index and long ulnar—ring and small Action: supplements metacarpophalangeal flexion and extension of proximal and distal interphalangeal joints

Distal Transverse Arch
Composite
Distal Transverse Arch

All interossei Nerve: ulnar Action: flexion of metacarpophalangeal joints and extension of proximal and distal interphalangeal joints

All interossei Nerve: ulnar Action: flexion of metacarpophalangeal joints and extension of proximal and distal interphalangeal joints

Distal Transverse Arch
Dorsal interossei
Distal Transverse Arch
Dorsal interossel Nerve: ulnar Action: spread of index and ring fingers away from long finger
Distal Transverse Arch

Palmar interossei

Palmar interossei Nerve: ulnar Action: adduction of index, ring, and fifth fingers toward long finger

Palmar interossei

Palmar interossei Nerve: ulnar Action: adduction of index, ring, and fifth fingers toward long finger

Muscle Balance and Biomechanical Considerations

When there is normal resting tone in the extrinsic and intrinsic muscle groups of the forearm and hand, the wrist and digital joints will be maintained in a balanced position. With the forearm midway between pronation and supination, the wrist dorsiflexed, and the digits in moderate flexion, the hand is in the optimum position from which to function.

It may be seen that muscles are usually arranged about joints in pairs so that each musculotendinous unit has at least one antagonistic muscle to balance the involved joint. To a large extent the wrist is the key joint and has a strong influence on the long extrinsic muscle performance at the digital level. Maximal digital flexion strength is facilitated by dorsiflexion of the wrist, which lessens the effective amplitude of the antagonistic extensor tendons while maximizing the contractural force of the digital flexors. Conversely, a posture of wrist flexion will markedly weaken grasping power.

At the digital level, metacarpophalangeal joint flexion is a combination of extrinsic flexor power supplemented by the contribution of the intrinsic muscles, whereas proximal interphalangeal joint extension results from a combination of extrinsic extensor and intrinsic muscle power. At the distal interphalangeal joint the intrinsic muscles provide a majority of the extensor power necessary to balance the antagonistic flexor digitorum profundus tendon.

The distance that a tendon moves when its muscle contracts is defined as the amplitude of the tendon and has been measured in numerous studies. In actuality the effective amplitude of any muscle will be limited by the motion permitted by the joint or joints on which its tendon acts. It has been suggested that the amplitude of wrist movers (flexor carpi ulnaris, flexor carpi radialis, extensor carpi radialis longus, extensor carpi radialis brevis, and extensor carpi ulnaris) is approximately 30 mm with the amplitude of finger extensors averaging 50 mm; the thumb flexor,

Adductor pollicis

Abductor pollicis brevis

Flexor pollicis brevis Transverse carpal ligament Opponens pollici

Adductor pollicis

Abductor pollicis brevis

Flexor pollicis brevis Transverse carpal ligament Opponens pollici

Distal Transverse Arch

Opponens digiti quinti

Flexor digiti quinti

Abductor digiti quinti

Pronator quadratus

Pronator quadratus

Opponens digiti quinti

Flexor digiti quinti

Abductor digiti quinti

Flexor carpi ulnaris

Ulnar nerve

Ulnar nerve

Distal Transverse Arch
Fig. 2-18 Position and function of the intrinsic muscles of the hand.

50 mm; and the finger flexors 70 mm (Fig. 2-20). Although these amplitudes have been thought to be important considerations when deciding on appropriate tendon transfers, Brand1,2 has shown that the potential excursion of a given tendon such as the extensor carpi radialis longus may be considerably greater than the excursion that was required to produce full motion of the joints on which it acted in its original position.

Efforts have been made to determine the power of individual forearm and hand muscles and a formula based on the physiologic cross section is generally accepted as the best method for determining this value. The number of fibers in cross section determines the absolute muscle power of a given muscle, whereas the force of muscle action times the distance or amplitude of a given muscle determines the work capacity of the muscle. Therefore a large extrinsic muscle with relatively long fibers such as the flexor digitorum profundus is found to be capable of much more work than is a muscle with shorter fibers such as a wrist extensor. Table 2-1 is an indicator of the work capacities of the various forearm muscles. It can be seen that the flexor digitorum profundus and super-ficialis have a significantly greater work capacity than do the remaining extrinsic muscles. The abductor pol-licis longus, palmaris longus, extensor pollicis longus, extensor carpi radialis brevis, and flexor carpi radialis have less than one fourth the capacity of these muscles.

Several mechanical considerations are important in understanding the effect of a muscle on a given

Slip of long extensor to lateral band

Sagittal bands

Long extensor tendon \ Interosseous muscle

Slip of long extensor to lateral band

Sagittal bands

Distal Transverse Arch

Radial

Triangular ligament -Lateral band

Dorsal extensor expansion

-Lumbrical muscle -Long extensor tendon

Interosseous muscle

Radial

Triangular ligament -Lateral band

Dorsal extensor expansion

-Lumbrical muscle -Long extensor tendon

Interosseous muscle

Sagittal bands

I 3 .Dorsal extensor expansion

Central slip of common extensor Lateral band

Long extensor tendon \ Interosseous muscle

Sagittal bands

I 3 .Dorsal extensor expansion

Central slip of common extensor Lateral band

Distal Transverse Arch

Lumbrical muscle

Flexor profundus tendon

Flexor digitorum superficialis

Lumbrical muscle

Flexor profundus tendon

Flexor digitorum superficialis

Long extensor tendon

Sagittal bands

Long extensor tendon

Sagittal bands

Distal Transverse Arch

Interosseous Lumbrical muscle muscle

Lateral band

Fig. 2-19 A, Extensor mechanism of the digits. B,C, Distal movement of the extensor expansion with metacarpophalangeal joint flexion is shown.

Interosseous Lumbrical muscle muscle

Bony insertion of interosseous tendon on proximal phalanx

Distal movement of extensor expansion during flexion

Lateral band

Fig. 2-19 A, Extensor mechanism of the digits. B,C, Distal movement of the extensor expansion with metacarpophalangeal joint flexion is shown.

Mcp Extensor Muscles

Fig. 2-20 Excursion of the flexor and extensor tendons at various levels. The numbers on the dorsum of the extended finger represent the excursion in millimeters required at each level to bring all distal joints from full flexion into full extension. The numbers shown by arrows on the palmar aspect of the flexed digit represent the excursion in millimeters for the superficialis (S) and the profundus (P) required at each level to bring the finger from full extension to full flexion. (From Verdan C: An introduction to tendon surgery. In Verdan C: Tendon surgery of the hand, Churchill Livingstone, 1979, London.)

Fig. 2-20 Excursion of the flexor and extensor tendons at various levels. The numbers on the dorsum of the extended finger represent the excursion in millimeters required at each level to bring all distal joints from full flexion into full extension. The numbers shown by arrows on the palmar aspect of the flexed digit represent the excursion in millimeters for the superficialis (S) and the profundus (P) required at each level to bring the finger from full extension to full flexion. (From Verdan C: An introduction to tendon surgery. In Verdan C: Tendon surgery of the hand, Churchill Livingstone, 1979, London.)

hUmUU^^H Work Capacity of Muscles

Muscle Mkg

Flexor carpi radialis

0.8

Extensor carpi radialis longus

1.1

Extensor carpi radialis brevis

0.9

Extensor carpi ulnaris

1.1

Abductor pollicis longus

0.1

Flexor pollicis longus

1.2

Flexor digitorum profundus

4.5

Flexor digitorum superficialis

4.8

Brachioradialis

1.9

Flexor carpi ulnaris

2.0

Pronator teres

1.2

Palmaris longus

0.1

Extensor pollicis longus

0.1

Extensor digitorum communis

1.7

From Von Lanz T, Wachsmuth W: Praktische anatomie. In Boyes JH: Bunnell's surgery of the hand, ed 5, Lippincott, 1970, Philadelphia.

Fig. 2-21 Biomechanics of the finger flexor pulley system. A. The arrangement of the annular and cruciate pulleys of the flexor tendon sheath. A,B, Normal moment arm (MA), the intraannular pulley distance (IPD) between the A-2 and A-4 pulleys, and the profundus tendon excursion (PTE), which occurs within the intact digital fibroosseous canal as the proximal interphalangeal joint is flexed to 90°. Annular pulleys: A-1, A-2, A-3, A-4, and A-5; cruciate pulleys: C-I, C-2, C-3. C,D, Biomechanical alteration resulting from excision of the distal half of the A-2 pulley together with the C-1, A-3, C-2, and proximal portion of the A-4 pulley. The moment arm is increased, and a greater profundus tendon excursion is required to produce 90° of flexion because of the bowstringing that results from the loss of pulley support. (From Strickland JW: Management of acute flexor tendon injuries, Orthopaedic Clinics of North America, vol 14, Saunders, 1983, Philadelphia.)

joint. The moment arm of a particular muscle is the perpendicular distance between the muscle or its tendon and the axis of the joint. The greater the displacement of an unrestrained tendon from the joint on which it acts, the greater will be the angulatory effect created by the increased length of the moment arm. Therefore a tendon positioned close to a given joint either by position of the joint or by a restraining pulley will have a much shorter moment arm than will a tendon that is allowed to displace away from the joint (Fig. 2-21).

In simplifying the biomechanics of musculotendinous function, Brand1 has emphasized that the "moment" of a given muscle is the power of the muscle to turn a joint on its axis. It is determined by

Distal Transverse Arch
Fig. 2-22 Cutaneous distribution of the nerves of the hand. A, Palmar surface. B, Dorsal surface.

multiplying the strength (tension) of the muscle by the length of the moment arm. Again, it can be seen that the distance of tendon displacement away from the joint is the critical factor and that it does not matter where the tendon insertion lies. The importance of the various anatomic restraints of the extrinsic musculotendinous units at the wrist and in the digits is magnified by these mechanical factors.

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