Muscle contribution to elbow joint valgus stability.

Repetitive valgus stress of the elbow can result in excessive strain or rupture of the native medial ulnar collateral ligament (MUCL). The flexor-pronator mass (FPM) may be particularly important for elbow valgus stability in overhead-throwing athletes. The aim of this study was to identify the relative contribution of each muscle of the FPM--that is, the flexor carpi ulnaris (FCU), flexor digitorum superficialis (FDS), flexor carpi radialis (FCR), and pronator teres (PT)--and of the extensor-supinator mass, including the extensor carpi ulnaris (ECU), extensor digitorum communis (EDC), extensor carpi radialis longus and brevus, and brachioradialis, to elbow valgus stability at 45 degrees and 90 degrees of elbow flexion angles. Eight fresh-frozen elbow specimens (mean age at death, 73.75 +/- 14.07 years) were tested. With the skin and subcutaneous tissue removed but all muscles left intact, each individual muscle of the FPM and extensor-supinator mass was loaded at 3 levels of force. During loading, strain on the MUCL and the kinematics of the elbow were measured simultaneously. Kinematic measurements were later repeated when the MUCL was fully cut. At 45 degrees and 90 degrees of elbow flexion, individual loading of the FCU, FDS, and FCR caused significant relief to the MUCL whereas the PT produced no significant change. Furthermore, of these flexor muscles, the FCU provided the greatest MUCL relief at both 45 degrees and 90 degrees . In contrast, loading of the ECU at 45 degrees of elbow flexion produced a significant increase in MUCL strain. All FPM muscles caused significant elbow varus movement at both 45 degrees and 90 degrees when loaded individually. At 90 degrees , the FCU created more motion than both the FCR and PT but not the FDS, and the FDS created more motion than the PT. The EDC and ECU created significant valgus movement at 45 degrees and 90 degrees , which became insignificant when the MUCL was transected. Our study suggested that the FCU, FDS, and FCR may function as dynamic stabilizers, with the FCU being the primary stabilizer for elbow valgus stability, incorporating with the MUCL for all tested joint configurations. Our findings also suggest that the ECU and EDC increased MUCL strain and elbow valgus movement at both 45 degrees and 90 degrees .