[The basic human thumb joint--an egg-shaped joint].

Abstract

The metacarpophalangeal joint of the human thumb (Articulatio metacarpophalangea pollicis) is an ellipsoidal (condyloid) joint. The head of the first metacarpal bone is shaped like a spindle polled at either end. Its form relevant to the mobility is determined by the correlation of two flexures, expressed by the quotient R/r, where R = semi-diameter of the radio-ulnar flexure and r = semi-diameter of the dorso-palmar flexure in the middle of the head. Only the substantial kind of motion (flexion-extension) maintains the perfect contiguity of the articular surfaces. Owing to the deformability of the articular cartilage, the actual littleness of the articular surfaces as well as the relative smallness of the cavity with respect to the head permits both transverse motions (adduction-abduction) and axial motions (rotation inward and outward). The amplitude of the transverse motion unexpectedly does not correlate to the intensity of the radio-ulnar flexure of the head.