Palm and Digits Anatomy
The hand comprises the wrist (carpus), the palm (metacarpus) and the digits (Figs 3.37 & 3.38). The palm of the hand (Fig. 3.39) contains the palmar aponeurosis, intrinsic muscles, tendons originating from muscles in the anterior compartment of the forearm, and palmar vessels and nerves. The intrinsic muscles comprise the thenar and hypothenar groups, the lumbricals and interossei, and adductor pollicis. The tendons enter the palm through the carpal tunnel (Fig. 3.98) deep to the flexor retinaculum. The skin of the anterior surface of the hand is thick, devoid of hair and contains many sweat glands. It is firmly bound to the underlying deep fascia, producing characteristic creases. Skin over the palmar surface of each distal phalanx is firmly tethered to the periosteum by fibrous septa, which enclose fat-filled spaces. Infection causing increased pressure in the pulp space may result in thrombosis of vessels supplying the distal phalanx causing bone necrosis.
The deep fascia of the palm is thickened centrally to form the triangular palmar aponeurosis (Fig. 3.39) and is thinner at each side where it covers the thenar and hypothenar muscles. Proximally, the palmar aponeurosis attaches to the flexor retinaculum and is continuous with the tendon of palmaris longus. Distally, the aponeurosis gives rise to four pairs of digital slips, which cross the metacar- pophalangeal joints and attach to the proximal phalanges of the fingers via the fibrous flexor sheaths. The aponeurosis covers the superficial palmar arch, the median nerve and the tendons of the long flexors of the digits. Thickening and shortening of the aponeurosis in Dupuytren’s contracture produces fixed flexion of the metacarpophalangeal and proximal interphalangeal joints.
Lateral and medial septa pass from the edges of the palmar aponeurosis to the first and fifth metacarpal bones, respectively.
These septa separate the thenar and hypothenar muscles from a central palmar space, which is traversed by the palmar digital vessels and nerves and by the tendons of the long flexor muscles of the fingers.
The subcutaneous tissue of the medial side of the palm usually contains palmaris brevis (Fig. 3.39), a small muscle attaching to the overlying skin and to the palmar aponeurosis.
The tendons of flexor pollicis longus, flexor digitorum profundus and flexor digitorum superficialis enter the hand deep to the flexor retinaculum (Fig. 3.99). On reaching the appropriate digit, each tendon traverses a tunnel formed by the fibrous flexor sheath and the phalanges (Fig. 3.40). The fibrous sheath is attached to the edges of the anterior surfaces of the phalanges (Fig. 3.41) and continues as far as the distal phalanx (Fig. 3.42). The sheath is thinner and more flexible in front of the interphalangeal joints, allowing flexion of the digit without ‘bowstringing’ of the tendons and thus facilitating gripping. Within the flexor sheaths the tendons are invested by synovial membrane and receive vincula tendinum, small folds of synovium that convey blood vessels to the tendons.
Abductor pollicis brevis, flexor pollicis brevis and opponens pollicis form the thenar eminence on the lateral side of the palm. They attach proximally to the trapezium and scaphoid and to the lateral part of the flexor retinaculum (Fig. 3.37).
Abductor pollicis brevis (Fig. 3.43) lies superficial to the other thenar muscles and passes from the scaphoid to the base of the proximal phalanx of the thumb. The muscle abducts the thumb, moving the digit anteriorly at right angles to the plane of the palm.
Flexor pollicis brevis, lying deep and medial to the abductor, passes from the trapezium to the proximal phalanx of the thumb. The muscle flexes the carpometacarpal and metacarpophalangeal joints, drawing the thumb across the palm (maintaining the thumbnail at right angles to the palmar plane).
Opponens pollicis (Fig. 3.44), the deepest of the thenar muscles, attaches proximally to the trapezium and distally to the shaft of the first metacarpal. The muscle produces opposition of the thumb, allowing pulp-to-pulp contact with the fingers. This movement combines flexion, adduction and rotation of the first metacarpal at its carpometacarpal joint.
The thenar muscles are supplied by the recurrent branch of the median nerve (Figs 3.43 & 3.44). Injury to this nerve may be assessed by palpating the thenar muscles while the subject attempts abduction of the thumb.
Abductor digiti minimi, flexor digiti minimi brevis and opponens digiti minimi (Fig. 3.45) form the hypothenar eminence and attach proximally to the pisiform, the hook of the hamate and the adjacent part of the flexor retinaculum. Abductor digiti minimi passes from the pisiform to the base of the proximal phalanx of the little finger and abducts the digit, moving it medially in the plane of the palm. Flexor digiti minimi brevis attaches between the hamate and base of the proximal phalanx of the little finger and flexes the metacarpal and proximal phalanx of the little finger. Opponens digiti minimi passes from the hamate to the shaft of the fifth metacarpal and on contraction deepens the hollow of the palm.
The hypothenar muscles are supplied by the deep branch of the ulnar nerve (Fig. 3.45).
The deep muscles of the hand consist of adductor pollicis, the palmar and dorsal interossei and the lumbricals.
Adductor pollicis (Fig. 3.46) has transverse and oblique heads. The transverse head attaches to the shaft of the third metacarpal bone, and the oblique head to the trapezoid, capitate and bases of the second and third metacarpals. Both heads pass laterally to attach to the base of the proximal phalanx of the thumb by a common tendon which usually contains a sesamoid bone (Fig. 3.38). The muscle adducts and flexes the thumb at its carpometacarpal and metacarpophalangeal joints and is supplied by the deep branch of the ulnar nerve (Fig. 3.54).
The interossei consist of three palmar and four dorsal muscles (Fig. 3.47). Each palmar interosseous muscle (Fig. 3.48) arises by a single head from the anterior border of the shaft of the second, fourth or fifth metacarpal. Each dorsal interosseous muscle (Fig. 3.49) arises by two heads from the contiguous sides of the shafts of the adjacent metacarpals. Distally the tendon of each interosseous muscle attaches to the base of the proximal phalanx and to the extensor expansion of the appropriate digit (Fig. 3.51). The interossei move the fingers in the plane of the palm (coronal plane); movement of a finger away from the long axis of the middle finger is called abduction while movement towards the middle finger is adduction. The palmar interossei adduct and the dorsal interossei abduct the fingers. In addition, the interossei flex the metacarpophalangeal joints and extend the interphalangeal joints. All the interossei are supplied by the deep branch of the ulnar nerve (Fig. 3.54). The integrity of the nerve and the adducting function of the palmar interossei can be tested by gripping a sheet of paper between the sides of two adjacent fingers.
The lumbricals are four small muscles attaching proximally to the tendons of flexor digitorum profundus (Fig. 3.50). Distally, each lumbrical attaches to the radial side of the extensor expansion of the appropriate finger. The muscles extend the inter-phalangeal joints and flex the metacarpophalangeal joints. The first and second lumbricals usually have only one head each and are supplied by the median nerve, whereas the third and fourth (medial) usually have two heads and are supplied by the ulnar nerve.
Each finger possesses an extensor expansion or hood (Fig. 3.51), which receives the tendons of the appropriate long extensor muscle(s), interossei and lumbricals. The broad proximal part of the expansion overlies the metacarpophalangeal joint. Distally, the expansion tapers and attaches by a central slip to the base of the middle phalanx and by two marginal slips to the base of the distal phalanx.
The arterial supply to the hand is derived from branches of the ulnar and radial arteries, which form superficial and deep palmar arches linking the two main arteries and ensuring a rich blood supply to the palm and fingers.
The ulnar artery enters the hand super-ficial to the flexor retinaculum and gives a deep branch that accompanies the deep branch of the ulnar nerve. The main artery continues as the superficial palmar arch (Fig. 3.52), which passes distally to the level of the thumb web, lying deep to the palmar aponeurosis but anterior to the digital nerves and flexor tendons. The superficial palmar arch gives four palmar digital branches, which supply the adjacent sides of the fingers (Fig. 3.53) and medial side of the little finger, and is completed laterally by a branch of the radial artery.
The radial artery enters the palm from the dorsum of the hand between the two heads of the first dorsal interosseous muscle (Fig. 3.49). The artery gives branches to the thumb and index finger and continues as the deep palmar arch (Fig. 3.54), which lies over the bases of the metacarpal bones, deep to the flexor tendons, and is about 1 cm proximal to the superficial arch. The deep arch provides perforating branches, which anastomose with dorsal metacarpal arteries, and three palmar metacarpal arteries, which anastomose with the palmar digital arteries of the superficial arch. The deep arch is completed medially by the deep branch of the ulnar artery. Other vessels such as dorsal metacarpal arteries from the dorsal carpal arch may provide an important supply of blood to the hand. The anastomoses between branches of arteries supplying the hand usually provide an adequate blood supply even if one artery is blocked.
Most venous blood from the fingers and palm drains into superficial veins on the dorsum of the hand to enter the cephalic or basilic veins.
Innervation of the anterior aspect of the hand is shared by the ulnar and median nerves. The ulnar nerve supplies more intrinsic muscles than the median nerve, which supplies a larger area of skin.
The ulnar nerve (Fig. 3.52) enters the palm superficial to the flexor retinaculum and terminates as superficial and deep branches. The superficial branch supplies digital branches to the skin of the medial one and one-half digits. A corresponding area of the palm is supplied by palmar branches that arise from the ulnar nerve in the forearm. The deep branch of the ulnar nerve (Fig. 3.54) accompanies the deep palmar arch and supplies the three hypothenar muscles, the medial two lumbricals, all the interossei and adductor pollicis. The ulnar nerve also supplies palmaris brevis. Injury to the ulnar nerve produces marked wasting (atrophy) of the muscles between the first and second metacarpal bones. The median nerve traverses the carpal tunnel and terminates as digital and recurrent branches. The digital branches (Fig. 3.52) supply skin of the lateral three-and-one-half digits and usually the lateral two lumbricals. A corresponding area of the palm is supplied by palmar branches arising from the median nerve in the forearm. The recurrent branch of the median nerve (Fig. 3.43) supplies the three thenar muscles.
In the palm, the digital branches of the ulnar and median nerves lie deep to the superficial palmar arch (Fig. 3.52), but in the fingers they lie anterior to the digital arteries arising from the superficial arch (Fig. 3.53). Although there may be variability of innervation of the ring and middle fingers, the skin on the anterior surface of the thumb is always supplied by the median nerve and that of the little finger by the ulnar nerve. The palmar digital branches of the median and ulnar nerves also supply the nail beds of their respective digits.