The common carpometacarpal joint

The carpometacarpal joints are the sites of articulation between the carpal and metacarpal bones. The bases of the medial four metacarpals form, with the distal row of carpal bones, a common carpometacarpal joint, which has an irregular joint line. The joints are, on the whole, plane synovial joints, the only exception being the slightly beveled joint surfaces between the hamate and the base of the fifth metacarpal.

Articular surfaces

The base of the second metacarpal fits into a recess formed by the medial side of the trapezium, the distal surface of the trapezoid and the anterolateral corner of the capitate. The third metacarpal base articulates only with the distal surface of the capitate. The base of the fourth metacarpal articulates mainly with the anterolateral distal surface of the hamate, but also just catches the anteromedial corner of the capitate. Finally, the base of the fifth metacarpal abuts against the anteromedial part of the distal surface of the hamate.

Joint capsule and synovial membrane

A fibrous capsule surrounds the common joint, in which various capsular thickenings can be identified. Synovial membrane lines the capsule and all non-articular surfaces, attaching to the articular margins. The joint cavity extends proximally between the carpal bones, and usually communicates with the midcarpal joint. Distally the joint space extends between the bones of the medial four metacarpals.

Ligaments

The dorsal and palmar carpometacarpal ligaments are really just thickenings of the joint capsule.

Dorsal carpometacarpal ligaments

These present as a series of bands of fibres which pass from the distal row of carpal bones to the bases of the metacarpals. In general, each metacarpal receives two bands. Those to the second metacarpal come from the trapezium and trapezoid; those to the third from the trapezoid and capitate, and those to the fourth from the capitate and hamate. The base of the fifth metacarpal receives only a single band arising from the hamate.

Palmar carpometacarpal ligaments

The arrangement of the fibrous bands constituting the palmar carpometacarpal ligaments is similar to those for the dorsal ligaments, except that the base of the third metacarpal receives three bands arising from the trapezoid, capitate and hamate.

Interosseus ligament

A short interosseus ligament is usually present passing from the adjacent inferior angles of the capitate and hamate to the base of the third or fourth metacarpal or both. Occasionally, the ligament divides the joint space into medial and lateral compartments.

Blood and nerve supply

The arterial supply to the joint is from the palmar and dorsal carpal networks, while the nerve supply is by twigs from the anterior and posterior interosseus nerves and the deep and dorsal branches of the ulnar nerve.

Relations

The carpometacarpal joints of the fingers lie deep to the tendons of flexor digitorumsuperficialis and profundus. Most laterally on the palmar surface, the tendon of flexor carpi radialis crosses the joint to insert into the base of the second metacarpal, while the tendon of flexor carpi ulnaris passes most medially. Also overlying the joint medially are the muscles of the hypothenar eminence.

On the posterior aspect of the joint are the tendons of the extensor muscles as they pass into the hand. From the lateral to medial these are: extensor carpi radialis longus at brevis, extensor pollicis longus,extensor indicis, extensor digitorum and digiti minimi, and extensor carpi ulnaris.

Stability

The joint is extremely stable, providing a firm base between the joints of the wrist and the hand.

Movements

There is little movement at the carpometacarpal joints of the fingers. The second and third metacarpals are essentially immobile, while a slight amount of gliding may occur between the fourth metacarpal and the hamate. Only the fifth metacarpal shows any appreciable movements as it glides on the hamate, and this is because of the beveled joint surfaces. The movement that occurs is flexion, and is seen during a tight grasp and in opposition of the thumb to the little finger. In addition, there is also a slight rotation during opposition due to the action of opponens digiti minimi.

Accessory movements

A slight degree of anteroposterior gliding can be produced between the base of the metacarpal and the adjacent carpal bone if the appropriate pressure is applied.