DYNAMIC GLENOHUMERAL JOINT STABILITY

Stability of the glenohumeral joint with an anterior, posterior, and i nferior displacement force of 50 N was measured in a dynamic shoulder model. Controlled hydrodynamic actuator Forces were applied to the del toid muscle and to the rotator cuff in seven anatomic specimens. Durin g elevation of the arm the position of the humerus was measured with a six-degree-of-freedom ultrasonic sensor device. The rotational center of the humeral head was used as a reference point For translation. A displacement Force of 50 N led to significant humeral head displacemen t anteriorly and posteriorly but not inferiorly. A 50% reduction of ro tator cuff forces increased anterior displacement by 46% and posterior displacement by 31%. Venting of the glenohumeral joint space and of t he subacromial bursa resulted in a 50% increase of anterior displaceme nt, a 19% increase of posterior displacement, and significant inferior displacement. This study demonstrates that in addition to passive sta bilizers and negative intraarticular pressure, rotator cuff Force sign ificantly contributes to stabilization of the glenohumeral joint durin g arm motion. Muscle strength and coordination should gain more emphas is in the diagnosis and treatment of shoulder instability.