Biomechanics of the trapeziometacarpal joint.

The trapeziometacarpal joint is structured to allow simple angular and simultaneous angular or rotational movements of the first metacarpal. The geometry of the joint is partitioned as follows: a saddle-shaped area (the central area of the trapezium and metacarpal ridge), where simple angular movements occur, and a spherical region (the palmar area of the trapezial articular surface and metacarpal slopes), where simultaneous angular movements take place. Rotation of the first metacarpal during opposition (pronation) and retroposition (supination) depends on three factors: muscular activity, ligament tension, and congruence of the spherical portion of the trapeziometacarpal joint.