Anatomy

The hand consists of 27 bones: 14 phalangeal bones, 5 metacarpal bones, and 8 carpal bones arranged in 5 rays of metacarpals and phalanxes having its base at the carpometacarpal (CMC) articulation (£19.26011).

FIG. 260-1. Bones of the hand.

The carpal bones are made up of two rows of four bones. They are concave on the volar surface and bridged by flexor retinaculum. This forms the carpal tunnel through which pass the median nerve and the nine long flexor tendons of the fingers. The bases of the second and third CMC articulation are fixed. The thumb, ring, and little finger have mobility at the CMC joint and provide movement that allows for grasp and adaptive movement of the hand.

The soft tissue supporting these bones and joints are the capsular ligamentous structures that give stability, the intrinsic muscles of the hand, and the tendinous structures that generate mobility. The collateral ligaments of the MP joints are tightest in flexion ( Fig 2.6.0.-2). The IP collaterals are tight throughout the entire range of motion (Fig 2.6.0.-2). The intrinsic muscles of the hand are those that have their origins and insertions within the hand. They consist of the muscle of the thenar and hypothenar eminences, adductor pollicis, the interossei, and the lumbricals.

FIG. 260-2. Ligament attachments of the metacarpophalangeal and interphalangeal joints.

The thenar muscles cover the thumb metacarpal, originate in the flexor retinaculum and carpal bones, and insert at the base of the first metacarpal and first proximal phalanx. The thenar muscles consist of abductor pollicis brevis, opponens pollicis, and flexor pollicis brevis. The median nerve innervates all three. Adductor pollicis is innervated by the ulnar nerve and originates from the second and third metacarpals and inserts in the first proximal phalanx.

The hypothenar group includes opponens digiti minimi, the flexor digiti minimi, and the abductor digiti minimi. These muscles originate in the flexor retinaculum and carpal bones and insert at the proximal phalanx and metacarpal of the little finger. They are innervated by the ulnar nerve ( Fig 260-3).

There are seven interossei. The three palmar and four dorsal interossei lie between the metacarpal bones and originate from them. The palmar interosseus and the palmar portion of the dorsal interosseus have an insertion into the extensor hood. The palmar interosseus adducts the index, ring, and small finger. The dorsal portion of the dorsal interosseus inserts by tendons into the base of the proximal phalanx. The dorsal interosseous muscles abduct the fingers away from the midline. The interossei are innervated by the ulnar nerve (Fig... , , .260-4).

The lumbricals arise from the flexor digitorum profundus tendons in the palm and course radially to the metacarpophalangeal (MP) joints and reinforce the interosseous lateral band on the radial side of the digit. The lumbricals contribute little to the flexion of the MP joint; however, they contribute to the extension of the interphalangeal (IP) joints. The lumbricals play a critical role coordinating the flexor and extensor system of the digits. The median nerve innervates the radial two lumbricals, and the ulnar nerve innervates the ulnar two.

The extensor tendons course over the dorsal side of the forearm, wrist, and hand. Nine extensor tendons pass under the extensor retinaculum and separate into six compartments. In the dorsum of the hand, the extensores digitorum communis are connected by junctura. Because of this, a complete tendon laceration proximal to the junctura may still result in normal extensor function. In the finger, the extensor expansion divides into a central slip that attaches to the middle phalanx and into two lateral bands that join with the tendons of the lumbricals and interosseous muscles and that attach to the base of the distal phalanx ( Fig 260-5).

The flexor tendons course over the volar side of the forearm, wrist, and hand. Flexor carpi radialis, flexor carpi ulnaris, and palmaris longus primarily flex the wrist. The remaining nine tendons pass through the carpal tunnel. One tendon goes to the base of the distal phalanx of the thumb. The other four digits have two tendons each. The flexor digitorum superficialis (FDS) inserts into the middle phalanx and flexes all the joints it crosses. The flexor digitorum profundus (FDP) runs deep to FDS until the level of the MP joint where FDS bifurcates. FDP inserts at the base of the distal phalanx and acts primarily to flex the distal interphalangeal (DIP) joint as well as all other joints flexed by FDS (Fjg.:.,,260-6). Unlike the extensor tendons, the flexor tendons are enclosed in synovial sheaths, making them prone to deep space infections.

FIG. 260-6. Anatomy of flexor digitorum superficialis (FDS) and profundus (FDP).

The hand and digits have dual blood supplies with contributions from the radial and ulnar arteries. The blood supply of the proximal portion of the hand is composed of a series of deep and superficial arches on the palmar and dorsal side. The blood supply of the fingers is distributed by the digital arteries that arise from the superficial palmar arch (Fig., 260-3 and Fig 260-7).

FIG. 260-7. The dual blood supply to the hands and digits.

The radial, ulnar, and median nerves innervate the hand. In the hand, the median and ulnar nerves have mixed motor and sensory function. The radial nerve (C5-T1) just has sensory function to the dorsal radial aspect of the hand. The ulnar nerve (C7-T1) supplies sensory function to the ulnar one and one-half fingers and motor function to the hypothenar muscles, ulnar two lumbricals, interossei, adductor pollicis, and the deep head of the flexor pollicis longus. The median nerve (C5-T1) supplies sensory function to the thumb and radial two and one-half fingers and motor function to abductor pollicis brevis, superficial head of flexor pollicis brevis, and opponens pollicis (Fig.260-8). As the digital nerves course through the palm, they are superficial structures and are the structures most often injured, so that digital nerve sensation and two-point discrimination should be routinely assessed when evaluating lacerations of the palm ( Fig.:... . . 260-9). The palm is often called "no man's land" because penetrating injuries in this area are so difficult to evaluate and treat. Careful neurovascular testing of the hand and digits is necessary for all palmar injuries that involve more than the skin, and hand consultation is advised if the extent of injury is uncertain. In the digits, digital nerves divide into volar and dorsal branches to supply sensation to the fingers. Knowing their location is important to properly perform a digital block (see Fig; 260-3).

FIG. 260-8. The cutaneous nerve supply in the hand. M = median, R = radial, U = ulnar, PCM = palmar branch of median nerve, DCU = dorsal branch of ulnar nerve.
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