THE AXES OF ROTATION OF THE THUMB INTERPHALANGEAL AND METACARPOPHALANGEAL JOINTS

The axes of rotation of the thumb interphalangeal and metacarpophalang eal joints were located using a mechanical method, The interphalangeal joint axis is parallel to the flexion crease of the joint and is not perpendicular to the phalanx, This offset of the axis with respect to the phalanx explains the ulnar deviation and pronation that occurs wit h flexion of the interphalangeal joint. The metacarpophalangeal joint has 2 fixed axes: a fixed flexion-extension axis just distal and volar to the epicondyles, and an abduction-adduction axis related to the pr oximal phalanx passing between the sesamoids. Neither axis is perpendi cular to the phalanges, All physiologic motion for these joints occurs about the axes, These are the mechanical axes of the joints through w hich the muscles and external forces act, Knowledge of their location should help in constructing prosthetic joints and in planning reconstr uctive surgery such as tendon transfers.