Contact patterns in the trapeziometacarpal joint: the role of the palmar beak ligament.

Twenty-three cadaver forearm specimens were mounted on a Plexiglas acrylic plastic base for study of the trapeziometacarpal joint. Extrinsic and intrinsic motors were loaded to simulate lateral pinch, and pressure-sensitive film was used to record joint contact patterns in various functional positions. The palmar compartment of the trapeziometacarpal joint was the primary contact area during flexion adduction of the thumb ray in lateral pinch. Simulation of dynamic pinch and release produced dorsal enlargement of the contact pattern, suggesting physiologic translation of the metacarpal on the trapezium. Detachment of the palmar beak ligament resulted in dorsal translation of the contact area, producing a pattern similar to that of cartilage degeneration seen in the osteoarthritic joint. End-stage osteoarthritic specimens had a nonfunctional beak ligament and demonstrated a pathologic total contact pattern of joint congruity that was unaffected by further manipulations of the beak ligament. This work defines the contact patterns in the normal and diseased trapeziometacarpal joint and establishes the important influence of the palmar beak ligament on those contact areas.