Joints of Carpus Anatomy
The carpal bones are arranged in two rows: a proximal row, consisting of, from lateral to medial, scaphoid, lunate, triquetral and pisiform; and a distal row consisting of trapezium, trapezoid, capitate and hamate bones (Fig. 3.96).
The joints between the carpal bones are supported by anterior, posterior and interosseous ligaments. The cavities of these joints usually communicate (Fig. 3.96) and function as a single unit called the midcarpal joint (Fig. 3.97). The joints are most stable in full extension when the anterior ligaments are taut.
The pisiform, a sesamoid bone in the tendon of flexor carpi ulnaris, has a separate joint with the triquetral (Fig. 3.98) and is firmly anchored distally to the hook of the hamate and base of the fifth metacarpal bone by pisohamate and pisometacarpal ligaments (Fig. 3.47). The relations of the inter- carpal joints are illustrated in Figure 3.98.
Movements at the radiocarpal and intercarpal joints are complementary, allowing flexion, extension, adduction and abduction of the hand on the forearm. Although the long flexors and extensors of the digits act on the radiocarpal and intercarpal joints, flexion is due principally to the two carpal flexors, and extension to the three carpal extensors. Adduction is produced mainly by the simultaneous contraction of the flexors and extensors on the ulnar side of the forearm and abduction by contraction of the muscles on the radial side.
The radiocarpal and intercarpal joints are stabilized by the extensors and flexors of the wrist during action of the long flexors and/or extensors of the digits. In many activities, movement between the hand and forearm combines extension and abduction, achieved by the two radial carpal extensors.
The carpal tunnel (canal) is a fibro-osseous passage linking the anterior compartment of the forearm with the palm of the hand. The walls of the tunnel consist anteriorly of the flexor retinaculum and posteriorly of the two rows of carpal bones which form a deep groove on their flexor surfaces. The retinaculum (Fig. 3.99) lies transversely across the anterior aspect of the wrist, attaching to the trapezium and scaphoid laterally and to the pisiform and hook of the hamate medially. The retinaculum lies in the hand, its proximal border level with the distal skin crease. The tendon of palmaris longus gains partial attachment to the retinaculum and enters the hand in front of the carpal tunnel (Fig. 3.39). The tendon is accompanied on its medial side by the ulnar artery and nerve, which pass lateral to the pisiform and the flexor carpi ulnaris tendon, but medial to the hook of the hamate. However, the median nerve and the other tendons entering the palm pass deep to the flexor retinaculum and traverse the carpal tunnel.
Within the tunnel (Fig. 3.98) the tendons of flexor digitorum superficialis lie anterior to those of flexor digitorum profundus. These tendons all possess a common synovial sheath, which is usually in continuity with the digital synovial sheath of the little finger but not with those of the other fingers. The tendon of flexor pollicis longus also traverses the tunnel, invested by a separate synovial sheath which continues into the thumb. The tendon of flexor carpi radialis lies laterally in a groove on the trapezium, isolated from the main part of the carpal tunnel.
The median nerve traverses the tunnel immediately deep to the flexor retinaculum, lying approximately at the midpoint of the wrist close to the tendon of palmaris longus, anterior to the tendon of flexor pollicis longus and medial to the flexor carpi radialis tendon. Compression of the median nerve may occur within the carpal tunnel, giving rise to a condition called the carpal tunnel syndrome, which may result in weakness of the thenar muscles and altered sensation (paresthesiae or ‘pins and needles’) felt in the thumb, index and middle fingers. Since skin on the lateral side of the palm is supplied by a branch of the median nerve which crosses superficial to the retinaculum, sensation in this area usually remains intact (p. 99). The syndrome may be treated operatively by dividing the flexor retinaculum to decompress the tunnel.