POSTERIOR GLENOHUMERAL SUBLUXATION - ACTIVE AND PASSIVE STABILIZATIONIN A BIOMECHANICAL MODEL

We examined the role of the glenohumeral and coracohumeral ligaments a s well as the forces provided by the rotator cuff muscles, the long he ad of the biceps, the anterior and middle deltoids, and the pectoralis major in the stabilization of the glenohumeral joint in the posterior direction. Simulated muscle forces were mechanically applied to eight shoulder specimens. The humeroscapular position for testing simulated the 90-degree forward-flexion (humerothoracic) position used clinical ly for the so-called jerk test, which is the most clinically important position with regard to posterior instability of the shoulder. Experi ments were performed with a variety of configurations of ligamentous a nd capsular cuts, humeral rotation, and levels of muscle force. Stabil ity was investigated by measuring the force required to subluxate the humeral head a specified amount from its reduced position. Of the musc les and ligaments tested, the subscapularis muscle contributed the mos t to this subluxation force. The coracohumeral ligament was an effecti ve contributor in neutral humeral rotation, and the inferior glenohume ral ligament was an effective contributor in internal humeral rotation . The long head of the biceps was found to reduce the subluxation forc e in certain positions. CLINICAL RELEVANCE: It is widely agreed that a complex interaction of passive and active stabilizing structures and forces is necessary for clinical stability of the shoulder. The presen t study identified the contributions of ligaments and muscles to poste rior stability of the shoulder in the position of greatest clinical im portance - posterior subluxation with the shoulder in forward flexion.