Abstract
Eleven fresh cadaver shoulders were studied to determine the static contribution (bulk effect) of the rotator cuff on inferior glenohumeral stability provided by scapular inclination. All musculature, including the rotator cuff, was removed. The position of the humerus relative to the scapula was recorded using an electromagnetic tracking device under conditions of no force and 1.5 kg of inferior translation force applied to the humerus, with the arm in the hanging position (sulcus test) and then in 90° abduction (Abduction-Inferior Stability test=ABIS test), with the scapula inclined referable to the vertical line at −15°, 0°, 15° and 30° in the sulcus test and at 15°, 30°, 45° and 60° in the ABIS test. In the sulcus test without load, all shoulders dislocated at scapular inclination angles of −15° and 0°, whereas no shoulders dislocated at 30°. The angle of scapular inclination had a significant effect on humeral head positions (p<0.0001), with the head position at −15° and 0° being lower than at 15°, which was lower than at 30°. In the ABIS test, none of the shoulders dislocated, although the effect of the angle of scapular inclination was significant (p<0.0001), with the position of the humeral head being higher at 15° than at other angles of inclination. Comparison of these data and previously reported data with the cuff intact showed no significant effect of rotator cuff removal on humeral head position and displacement in both tests. Therefore, we conclude that the static condition of the rotator cuff has no significant effect on the stabilizing function of scapular inclination. The stabilizing mechanism of scapular inclination seems to be associated with the bony configuration and/or anatomy and biomechanical properties of the superior capsuloligamentous structures. © 1993, Tohoku University Medical Press. All rights reserved.