Objective: Recent published experimental investigations have shown the relevance of the forearm muscles in providing dynamic stability to the scapholunate joint, in the wrists without osteoarthritis. Some chronic scapholunate advanced collapse (SLAC) wrists remain long time asymptomatic; the reason is unknown. How do forearm muscles influence on SLAC wrists? The kinetic behavior of an experimentally acute sectioned ligament compared with a SLAC wrist has not been determined. The hypothesis of this experimental biomechanical study was that there are no differences in SLAC and recent scapholunate instability wrists kinetics. Methods: The kinetic effects of isometric loading of 5 wrist motor tendons (abductor pollicis longus [APL], extensor carpi radialis longus [ECRL], extensor carpi ulnaris [ECU], flexor carpi ulnaris [FCU], and flexor carpi radialis [FCR]) on 12 fresh normal cadaver arms in which scapholunate ligament was sectioned and in 5 wrists with a SLAC pattern of carpal osteoarthritis were analyzed. A custom-designed testing apparatus was used to hold the forearm and wrist vertical in neutral position. A 6 degree-of-freedom electromagnetic motion tracking device with sensors attached to the scaphoid, triquetrum, capitate, and radius was used to monitor spatial changes in carpal bones alignment. The rotation sustained by the scaphoid in both experimentally acute sectioned scapholunate ligament and chronic SLAC groups were measured and statistically compared. Results: When all tendons were simultaneously loaded, no statistical differences were observed between the 2 groups: The proximal carpal row, as opposed to the normal wrist, tended to pronate in both situations. Under individual muscle loading, the kinetic carpal behavior of both groups was also the same, with no statistical differences between them: The ECU is the only muscle that provokes scaphoid pronation; all other muscles induce its supination. Isometric ECU contraction causes significant radioscaphoid subluxation, with the scaphoid rotating into substantial pronation (average 4.50°; confidence interval [CI], −1.33° to −1.67°) and extension (0.69°; CI, −0.72° to 2.69°). None of the other muscles exhibited such an obvious destabilizing effect. The experimental kinetic behavior of the chronic SLAC wrist is similar to the one with a recent scapholunate dissociation without carpal collapse associated. In both groups (1) the direction of the scaphoid displacement is the same (flexion, pronation and radial deviation), although the magnitude of the scaphoid displacement is greater in the SLAC wrists group, and (2) the ECU muscle load destabilizes the scapholunate joint, although SL joint destabilization is greater in the SLAC wrist group. Conclusions: These findings suggest that, in SLAC wrists, the capsular distension associated to a chronic malfunction of the carpus plays a significant role in the magnitude of the carpal bones displacement under load, and that in patients with scapholunate instability, both with and without carpal collapse associated, ECU muscle contraction should always be avoided.
