BULK EFFECT OF ROTATOR CUFF ON INFERIOR GLENOHUMERAL STABILITY AS FUNCTION OF SCAPULAR INCLINATION ANGLE - A CADAVER STUDY

Eleven fresh cadaver shoulders were studied to determine the static co ntribution (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 t o the scapula was recorded using an electromagnetic tracking device un der conditions of no force and 1.5 kg of inferior translation force ap plied to the humerus, with the arm in the hanging position (sulcus tes t) and then in 90-degrees abduction (Abduction-Inferior Stability test =ABIS test), with the scapula inclined referable to the vertical line at -15-degrees, 0-degrees, 15-degrees and 30-degrees in the sulcus tes t and at 15-degrees, 30-degrees, 45-degrees and 60-degrees in the ABIS test. In the sulcus test without load, all shoulders dislocated at sc apular inclination angles of -15-degrees and 0-degrees, whereas no sho ulders dislocated at 30-degrees. The angle of scapular inclination had a significant effect on humeral head positions (p < 0.0001), with the head position at -15-degrees and 0-degrees being lower than at 15-deg rees, which was lower than at 30-degrees. In the ABIS test, none of th e shoulders dislocated, although the effect of the angle of scapular i nclination was significant (p < 0.0001), with the position of the hume ral head being higher at 15-degrees than at other angles of inclinatio n. Comparison of these data and previously reported data with the cuff intact showed no significant effect of rotator cuff removal on humera l head position and displacement in both tests. Therefore, we conclude that the static condition of the rotator cuff has no significant effe ct on the stabilizing function of scapular inclination. The stabilizin g mechanism of scapular inclination seems to be associated with the bo ny configuration and/or anatomy and biomechanical properties of the su perior capsuloligamentous structures.