The carpometacarpal joints (CMC) are five joints in the wrist that articulate the distal row of carpal bones and the proximal bases of the fivemetacarpal bones.
The CMC of the thumb or the first CMC, also known as the trapeziometacarpal joint (TMC), differs significantly from the other four CMCs.
Articulations of the Other Four Metacarpal Bones with the Carpus (articulationes carpometacarpeæ).—The joints between the carpus and the second, third, fourth, and fifth metacarpal bones are arthrodial. The bones are united by dorsal, volar, and interosseous ligaments.
Movements.—The movements permitted in the carpometacarpal articulations of the fingers are limited to slight gliding of the articular surfaces upon each other, the extent of which varies in the different joints. The metacarpal bone of the little finger is most movable, then that of the ring finger; the metacarpal bones of the index and middle fingers are almost immovable.
Carpometacarpal Articulation of the Thumb (articulatio carpometacarpea pollicis).—This is a joint of reciprocal reception between the first metacarpal and the greater multangular; it enjoys great freedom of movement on account of the configuration of its articular surfaces, which are saddle-shaped. The joint is surrounded by a capsule, which is thick but loose, and passes from the circumference of the base of the metacarpal bone to the rough edge bounding the articular surface of the greater multangular; it is thickest laterally and dorsally, and is lined by synovial membrane.
Movements.—In this articulation the movements permitted are flexion and extension in the plane of the palm of the hand, abduction and adduction in a plane at right angles to the palm, circumduction, and opposition. It is by the movement of opposition that the tip of the thumb is brought into contact with the volar surfaces of the slightly flexed fingers. This movement is effected through the medium of a small sloping facet on the anterior lip of the saddle-shaped articular surface of the greater multangular. The Flexor muscles pull the corresponding part of the articular surface of the metacarpal bone on to this facet, and the movement of opposition is then carried out by the Adductors.
Flexion of this joint is produced by the Flexores pollicis longus and brevis, assisted by the Opponens pollicis and the Adductor pollicis. Extension is effected mainly by the abductor pollicis longus, assisted by the Extensores pollicis longus and brevis. Adduction is carried out by the Adductor; abduction mainly by the Abductores pollicis longus and brevis, assisted by the Extensors.