Children with residual brachial plexus birth palsy often develop a fixed internal rotation contracture of the glenohumeral joint. The standard operative procedure to treat this involves release of either the subscapularis tendon, pectoralis major tendon, or capsule. It has been observed that enhanced forearm supination occurs after the shoulder is repositioned in more external rotation. This study attempted to define the relationship between supination within the forearm and rotation of the shoulder. Thirty-nine volunteers underwent assessment of concentric isokinetic supination strength. Performance was measured with the subjects standing and the elbow positioned in 90 degrees of flexion. The humerus was strapped to the side in the anatomic position. The glenohumeral joint was placed at either 45 degrees of internal rotation or 45 degrees of external rotation. Random testing was performed, and the mean and maximum torques were recorded for each arm position. The mean and maximum torques for supination were greater in external rotation by 8.7% and 7.5% respectively. This difference was significant.
Supination of the forearm is performed primarily by the biceps and supinator muscles. Movement of the glenohumeral joint affects the long head of the biceps position as it traverses over the head of the humerus. The improvement in supination after surgery to increase external rotation of the shoulder is thus explained by better position of the biceps tendon for supination. This study does not address the differential effects of the long versus the short head of the biceps during supination, although this may be a secondary cause.
J Bone Joint Surg