Scapular inclination and inferior stability of the shoulder

Abstract

Eleven fresh-frozen cadaver shoulders were studied to examine the influence of scapular inclination on inferior stability of the glenohumeral joint. All muscles except the rotator cuff were removed, and the capsule was vented. Inferior stability tests in the hanging position (sulcus test) and in 90° abduction (abduction inferior stability [ABIS] test) were simulated by the application of a 1.5 kg load with the scapula inclined at - 15°, 0°, 15°, and 30° in the sulcus test and at 15°, 30°, 45°, and 60° in the ABIS test. An electromagnetic tracking device was used to record the position of the humerus in relation to the glenoid. In the sulcus test all of the shoulders dislocated when the scapula was inclined at - 15°. However, when the scapula was inclined at 30°, no shoulder dislocated before loading, and one shoulder dislocated after loading. As a result both the loaded and unloaded positions of the humeral head shifted significantly to the superior direction as the scapular inclination increased (p < 0.0001). In the ABIS test, however, the positions of the humeral head shifted interiorly with an increase in scapular inclination (p < 0.0001), although none of the shoulders dislocated in any of the inclination angles. We conclude that scapular inclination contributes significantly to inferior stability of the glenohumeral joint. Increased scapular inclination prevents inferior displacement of the humeral head, probably because of a bony cam effect that causes tightening of the superior capsule.