Biomechanical study of the extensor carpi ulnaris as a dynamic wrist stabilizer.

PURPOSE: To investigate the dynamic stabilizing effect of the extensor carpi ulnaris (ECU) on the distal radioulnar joint (DRUJ) and the ulnocarpal joint in a simulated model of triangular fibrocartilage complex (TFCC) injury.

METHODS: Using 8 fresh-frozen cadavers, we fixed the humerus and ulna at 90° of elbow flexion, and the radius and hand were allowed to rotate freely around the ulna. Passive mobility was tested by translating the radiocarpal unit relative to the ulna along dorsal-palmar directions. Unit displacement was measured by an electromagnetic tracking device in different forearm rotations and under varied loading to the wrist motor tendons. Magnitudes of displacement were compared between different loading patterns of the prime wrist movers in the TFCC-sectioned wrists. The effect of sectioning the ECU subsheath was analyzed.

RESULTS: When physiological loads were applied to all of the prime wrist movers, the magnitude of displacement during passive mobility testing decreased in supination and neutral rotation. After ECU tendon loading was released, mobility increased again in supination and neutral rotation. When the load was applied only to the ECU tendon, mobility decreased in supination and neutral rotation as compared with unloaded. Little change in the mobility was found in pronation regardless of the tendon loading pattern. After sectioning of the ECU subsheath, the stabilizing effect of the ECU decreased in neutral rotation.

CONCLUSIONS: In a neutral wrist position with complete sectioning of the TFCC, the ECU dynamically stabilized the DRUJ and the ulnocarpal joint in supination and neutral forearm rotation. The ECU subsheath assisted ECU tendon stabilization on the ulnar side of the wrist, especially in the neutral rotation.

CLINICAL RELEVANCE: Maintaining the ECU and its subsheath may reduce DRUJ instability in patients with TFCC injuries.