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.
Carpal tunnel
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.
