[Importance of the posteromedial trapezometacarpal ligamentous complex]

Pronation and supination of the human thumb has both a practical and a symbolic importance. In supination the thumb caresses but in pronation it grasps. The articular surfaces of the trapezium and first metacarpal are not congruent enough in themselves to assure pronation and supination, but their reciprocal saddle shape causes them to have two distinct orthogonal "fundamental" axes of rotation, and hence two "privileged" planes of rotation. The one corresponds to the movements of the lateral pinch grip and the other to opposition and counter-opposition of the thumb. The important ligaments are situated on both sides of the trapeziometacarpal joint. The posteromedial ligamentous complex is composed of the posterior oblique ligament, the anterior oblique ligament, the intermetacarpal ligament, and a fourth or more anteromedial ligament which we have called the retinaculometacarpal ligament by reason of its proximal insertion. It plays an essential role in suspending and anchoring the base of the first metacarpal. By its action it induces two types of motion: 1) a discrete translation of the metacarpal along the articular concavity of the trapezium in lateral pinch grip; 2) a more important motion characterised by a prosupination of 90 degrees during its rotation in the plane of opposition. The anterolateral ligament's action is restricted to strengthening the action of the posteromedial ligaments in the extreme positions of closing of lateral pinch and thumb opposition. In cases of trapeziometacarpal subluxation it prevents a complete dislocation. These various observations have significant clinical and therapeutic implications.