Osteoligamentous guidance of the movements of the human thumb

On examination of the thumbs of 20 dissected preparations of ligaments and joints, of ten dry skeletons and of a number of living hands, apoposition (from apo = away from) was distinguished as a position in which the first carpometacarpal joint is fully abducted and laterally rotated and in which one or both distal joints of the thumb are flexed. Apoposition is commonly used in writing and it has a specific osteoligamentous basis for its stability: (1) a Y-shaped intermetacarpal ligament is attached by two crura to the base of the second metacarpal bone and by a common stem to the first metacarpal. Together with the palmar and dorsal oblique ligaments it becomes taut at abduction and establishes thereby a fixed center for the circumduction. Stability is enhanced as the circumduction takes place in the radial flat part of the joint away from the center; (2) of the two palmar prominences of the head of the first metacarpal bone the radial is the larger. At 25-30 degrees flexion in the metacarpophalangeal joint the prominence fits into an excavation on the base of the proximal phalanx in a manner which together with the ulnar collateral ligament locks the joint against mutual abduction and lateral rotation, and (3) the radial part of the trochlea of the interphalangeal joint is larger than the ulnar and secures, together with the ulnar collateral ligament, the joint against a radial luxation. Apoposition does not require activity of the thumb muscles; it is brought about by applying an external force to the ulnar side of the thumb and it is checked by ligaments and the shape of the joints.