Anatomy of the Hand

Two characteristics of the normal hand reveal what happens when it is damaged:

1. The hand has a natural resting position. At rest, both the metacarpophalangeal and the interphalangeal joints normally hold a position of slight flexion. The fingers all adopt a slightly different degree of rotation, such that the volar surfaces of the terminal phalanges face progressively more toward the thumb as one moves from the index finger to the little finger. It is critical to appreciate the different degrees of rotation in the finger when assessing displacement in phalangeal or metacarpal fractures. The degree of flexion increases as one passes from the index finger to the little finger. This configuration is a result of muscle balance; if one element is deficient or absent, the resting position of the hand changes. In cases of acute trauma, a cut flexor tendon may leave a finger extended. An abnormal resting position often is indicative of tendon damage.

2. The concept of muscle balance also can be applied to chronic conditions of the hand. In patients with long-standing ulnar nerve lesions, in which the intrinsic muscles of the hand are paralyzed, the hand develops an abnormal attitude because of muscle imbalance. The intrinsic muscles normally flex the fingers at the metacarpophalangeal joints and extend them at the proximal and distal interphalangeal joints. The absence of intrinsic function leads to extension of the metacarpophalangeal joints and flexion of the proximal and distal interphalangeal joints of the affected fingers, resulting in an ulnar claw hand.

Palm

Skin

The skin of the palm and the palmar aspect of the fingers is a tough structure, characterized by flexure creases in the palm and fingerprints in the fingers. The skin has very little laxity because of the series of tough fibrous bands that tie it to the palmar aponeurosis. These bands divide the

subcutaneous fat into small loculi, which are capable of withstanding considerable pressure. The skin’s lack of mobility means that it is difficult to close even small defects in it without resorting to plastic procedures such as V-Y advancement flaps or skin grafting.

The blood supply of the palmar skin is extremely good, and even long, distally based flaps may survive. In an elective incision, however, the angle at the apex of a triangular flap should be more than 60 degrees, and distally based flaps should be avoided, if possible.

To avoid flexion contractures, the flexure creases should not be crossed at 90 degrees. Cutting within a flexure crease itself avoids this problem, but the wound created is difficult to close without inverting the skin edges. That is the reason that many incisions parallel natural flexure creases.

Palmar Aponeurosis

The palmar aponeurosis is a tough fibrous sheath that lies under the skin of the palm (see Fig. 5-37). It is continuous with the tendon of the palmaris longus muscle, spreading distally from the distal border of the flexor retinaculum to cover the central area of the palm between the thenar and hypothenar eminences. At the level of the distal third of the metacarpals, it divides into four bands, one for each finger. At the level of the distal palmar crease, these bands divide into two and run into the fingers to insert into the bases of the proximal phalanges and the fibrous flexor sheaths (see Fig. 5-40).

The nerves and vessels to the palm lie immediately deep to the palmar aponeurosis and actually are in contact with its deep surface. In patients with Dupuytren contracture, the palmar fascia thickens; contracted and fibrous tissue grows all around the digital nerves and vessels to enclose them.46

The fascia over the thenar and hypothenar muscles is thinner than that over the central palm because of the greater mobility required from the first and fifth digits.

The palmar aponeurosis has deep connections to the first and fifth metacarpals at its lateral and medial borders, dividing the hand into three major compartments: The thenar, hypothenar, and palmar compartments. There also are deep connections between the palmar aponeurosis and the metacarpals in the distal part of the hand (see Fig. 5-40).

Thenar Muscles

The thenar eminence consists of three short muscles: The abductor pollicis

brevis, the flexor pollicis brevis, and the opponens pollicis (see Figs. 5-40 and 5-41). All three are supplied by the median nerve via its motor branch, which enters the eminence between the short abductor and the flexor.

The flexor pollicis brevis also receives a nerve supply from the ulnar nerve to its deep head. This dual nerve supply explains the clinical observation that a complete median nerve palsy does not necessarily produce complete flattening of the thenar eminence, because the bulky deep head of the flexor pollicis brevis does not atrophy.

The three short muscles of the thumb lie in two layers. The superficial layer consists of the short abductor and the short flexor, with the abductor lying on the radial side of the flexor. The deep layer consists of the opponens pollicis, which produces rotation of the thumb metacarpal at its saddle-shaped joint with the trapezium. The ability to oppose the thumb and the other fingers is one of the major structural advantages that the human hand has over the ape hand. It is a complex movement requiring several muscles. The abductor pollicis brevis abducts the thumb, rotation is achieved by the opponens pollicis, and the movement is completed by the thumb flexors. The abductor pollicis brevis is the most important muscle in this group. Median nerve paralysis destroys opposition; the resultant hand often is known as a simian (ape-like) hand.

Hypothenar Muscles

The hypothenar eminence consists of three muscles: The abductor digiti minimi, the flexor digiti minimi brevis, and the opponens digiti minimi. These muscles (all of which are supplied by the ulnar nerve) are arranged in the same layering as are those of the thenar eminence. The superficial layer consists of the abductor and flexor, with the abductor lying on the ulnar side; the deep layer consists of the opponens digiti minimi. Together, these muscles help deepen the cup of the palm of the hand. Very little genuine opposition of the fifth finger is possible compared with that of the thumb (see Figs. 5-40 and 5-41).

Lying superficial to the muscles of the hypothenar eminence is the palmaris brevis muscle, the only muscle that is supplied by the superficial branch of the ulnar nerve.

Nerves and Vessels

The second layer of the palm consists of the superficial nerves and vessels (see Figs. 5-39 and 5-40).

The superficial palmar arch is an arterial arcade that is formed largely by the ulnar artery. The arcade is completed by the superficial palmar

branch of the radial artery, but this branch often is missing. When it is, the arch remains incomplete. Four palmar digital arteries arise from the arcade and pass distally. The most ulnar of the arteries supplies the ulnar border of the little fingers; the other three common digital arteries divide in the web space into two vessels that supply adjacent fingers.

Note that this arterial arcade is superficial to the nerves in the palm as opposed to the fingers where the arteries are the superficial structures. Also the thumb and the radial side of the index finger are not supplied by its branches.

The digital nerves lie immediately deep to the superficial palmar arch. The ulnar nerve divides into a superficial and a deep branch at the distal border of the flexor retinaculum. The superficial branch supplies the ulnar 1½ fingers with sensation. The median nerve divides into two sensory branches after giving off its motor branch to the thenar muscles. The medial branch supplies the radial side of the ring finger, the middle finger, and the ulnar side of the index finger. The lateral branch supplies the radial side of the index finger and the whole of the thumb.

Long Flexor Tendons

The third layer of structures in the palm is composed of the long flexor tendons. The tendons of the flexor digitorum superficialis muscle overlie those of the flexor digitorum profundus muscle. Each flexor profundus tendon gives rise to a lumbrical muscle, which passes along the radial side of the metacarpophalangeal joint before inserting into the extensor expansion from the dorsum of the proximal phalanx. Lumbricals that arise by two heads from adjacent profundus tendons (usually the ulnar two) are supplied by the ulnar nerve; lumbricals that arise from one tendon (usually the radial two) are supplied by the median nerve.

Deep Palmar Arch

The deep palmar arch, which is an arterial arcade, lies deep to the long flexor tendons and forms a fourth layer in the palm (see Figs. 5-41 and 5-42). The arterial arch consists of the terminal branch of the radial artery, which enters the palm by passing between the oblique and transverse heads of the adductor pollicis muscle, and the deep branch of the ulnar artery. Running with the ulnar artery is the deep branch of the ulnar nerve, which supplies all the interossei with muscular branches at this level.

Deep Muscles of the Palm

The adductor pollicis muscle and the interossei are the deepest muscles in

the palm (see Fig. 5-42).

The interossei can be divided into two groups, dorsal and palmar. The dorsal interossei arise by two heads from adjacent sides of the metacarpals and insert into the proximal phalanges so that they abduct the fingers away from the line drawn through the center of the third finger.

The three palmar interossei are much smaller. Each arises from only one metacarpal and inserts into the base of the proximal phalanx, adducting the finger toward the middle finger.

All interossei are supplied by the deep branch of the ulnar nerve. (The function of the interossei can be remembered by the mnemonics “PAD” and “DAB”: The palmar interossei adduct, and the dorsal interossei abduct.)47

Other Structures

Two structures in the palm, the deep branch of the ulnar nerve and the radial artery, have courses that do not follow the layering concept. The way in which they run through the wrist and hand ties the rest of the anatomy together.

The radial artery lies on the volar aspect of the distal radius. It reaches the dorsum of the hand under the tendons of the abductor pollicis longus muscle and the extensor pollicis brevis muscle, lying on the scaphoid bone in the anatomic snuff-box. To return to the volar aspect of the palm, it pierces the deepest layer of the palmar structures, passing between the two heads of the first dorsal interosseous muscle. At that point, it gives off two branches, the radialis indicis artery and the princeps pollicis artery, which supply the index finger and thumb, respectively. The main arterial trunk then passes between the two heads of the adductor pollicis and lies superficial to the deepest muscles as it forms the deep palmar arch.

The ulnar nerve enters the hand superficial to the flexor retinaculum within the canal of Guyon. There, it divides into superficial and deep branches (see Fig. 5-38). The superficial branch gives off digital nerves and lies in the same plane as the superficial arterial arcade. The deep branch descends through the layers of the palm, passing between the heads of origin of the opponens digiti minimi to lie on the interossei in the same plane as the radial artery. There, it supplies all the interossei, the two ulnar lumbrical muscles, both heads of the adductor pollicis muscle, the three hypothenar muscles, and the deep head of the flexor pollicis brevis muscle (see Figs. 5-41 and 5-42).

Dorsum of the Hand

The anatomy of the dorsum of the hand is far simpler than that of the palm. The skin is thinner than the palmar skin and is more mobile to allow for finger flexion. The subcutaneous tissue contains very little fat, but a large number of veins. Venous return runs via the dorsum of the hand because the pressure of gripping otherwise would impede it. The blood supply of the dorsal skin is not as good as that of the palmar skin, and distally based skin flaps are less likely to survive.

The backs of the radial 3½ digits are supplied by the terminal branches of the superficial radial nerve as far as the middle of the middle phalanx. The ends of these fingers are supplied by branches of the median nerve that are derived from the volar digital nerves.

The dorsal aspects of the ulnar 1½ digits are supplied by the ulnar nerve. The proximal 1½ phalanges are supplied by dorsal branches of the ulnar nerve, and the distal 1½ phalanges are supplied by volar branches of the ulnar nerve (the volar digital nerves).

The clinical importance of this arrangement is that the terminal phalanx, including the nail bed, can be anesthetized by an injection of local anesthetic around the volar digital nerves.

The only tendons of the dorsum of the hand are those of the common extensors. Just proximal to the metacarpophalangeal joint, these tendons are united by three oblique bands, which limit retraction of the tendon if it is cut. As each long extensor tendon passes over its metacarpophalangeal joint, its deepest part becomes continuous with the dorsal capsule of that joint. The tendon becomes much broader before dividing into three slips over the dorsal surface of the proximal phalanx. The central slip inserts into the base of the middle phalanx and the two marginal slips receive attachments from interossei and lumbrical tendons to form a broad extensor expansion, or hood, which overlies the metacarpal head and the proximal part of the proximal phalanx. The hood is anchored firmly on each side to the volar plate of the metacarpophalangeal joint. Each hood receives some of the insertion of each of two interossei, with the rest going to the proximal phalanx itself. The amount varies considerably from finger to finger. The entire insertion of the lumbrical tendon attaches to the extensor hood (see Figs. 5-11, 5-13, and 5-41).

Over the dorsum of the middle phalanx, the intrinsic tendons are joined to each other by transversely running fibers (the triangular ligament). Initially, the two marginal slips of the long extensor tendon pass outward from the midline to insert into the base of the distal phalanx. By inserting into this extensor expansion from the palmar side, the lumbrical and

interosseous muscles not only can abduct and adduct the fingers at the metacarpophalangeal joint, but also can flex the metacarpophalangeal joint while extending the distal and proximal interphalangeal joints. In this way, each extended finger can be flexed independently.

Disruption of the central slip of the extensor tendon and the triangular ligament may produce a flexion deformity at the proximal interphalangeal joint. The two marginal slips then pass volar to the joint and act as flexors of that joint, and the joint “buttonholes” between these two slips. This deformity is known as a boutonnière (or buttonhole) deformity.

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