Active and passive factors in inferior glenohumeral stabilization: a biomechanical model.

This study examines the stabilizing factors of the glenohumeral joint against inferior translation over a range of subluxations. Factors examined included the glenohumeral capsular ligaments, the coracohumeral ligament, the rotator cuff forces, and the long head of the biceps force. Simulated muscle forces were applied to eight shoulder specimens with the arm near 0 degrees abduction. Stability was defined as the force required to inferiorly sublux the joint to a specified translation from the centered position and was evaluated under varying configurations of capsule cuts, humeral rotation, and muscle loads. The supraspinatus and biceps muscle forces were found to be important active stabilizers. Thus, tension in the long head of the biceps did not tend to depress the humeral head. The inferior glenohumeral ligament was an important passive stabilizer in external rotation. Understanding the effects of these factors adds insight into the underlying biomechanics of clinical shoulder instability.