The stabilizing function of passive shoulder restraints.

UNLABELLED: The static restraints of the scapulohumeral joint provide stability for the humeral head in the glenoid cavity, limit extremes of motion of the glenohumeral joint, and guide positioning of the humerus during normal shoulder movement. Eleven fresh-frozen cadaver shoulders of unknown age were attached to a shoulder motion device that allowed measurement of motion in three planes with an accuracy to 0.5 degrees. Four shoulders underwent motion analysis and seven were used for strain gauge analysis of the static scapulohumeral ligamentous restraints. The results of the motion analysis demonstrated that any attempt at simple motion (flexion, extension, abduction, internal or external rotation) resulted in coupled motion in two additional planes. The strain gauge data, expressed as a percent of total tension for each ligament tested, demonstrated a reciprocal tension-sharing relationship among all ligament components and a transference of tension among these components when original and new joint positions were compared. These data provide an in vitro model of shoulder restraint function to explain primary restraint, tension sharing, and transference of tension functions in the in vivo scapulohumeral joint. CLINICAL RELEVANCE: These principles of shoulder function have application in the treatment of instability and frozen shoulder syndrome, and provide an in vitro model to better understand static restraint function in the throwing mechanism.