The effect of rotator cuff tears on reaction forces at the glenohumeral joint.

The rotator cuff muscles maintain glenohumeral stability by compressing the humeral head into the glenoid. Disruption of the rotator cuff compromises concavity compression and can directly affect the loads on the glenohumeral joint. The purpose of this study was to quantify the effect of rotator cuff tears on the magnitude and direction of glenohumeral joint reaction forces during active shoulder abduction in the scapular plane using nine cadaveric upper extremities. Motion of the full upper extremity was simulated using a dynamic shoulder testing apparatus. Glenohumeral joint reaction forces were measured by a universal force-moment sensor. Five conditions of rotator cuff tears were tested: Intact, Incomplete Supraspinatus Tear, Complete Supraspinatus Tear, Supraspinatus/Infraspinatus Tear, and Global Tear. Reaction forces at the glenohumeral joint were found to steadily increase throughout abduction and peaked at maximum abduction for all conditions tested. There were no significant differences in reaction force magnitude for the intact condition (337 +/- 88 N) or those involving an isolated incomplete tear (296 +/- 83 N) or complete tear (300 +/- 85 N) of the supraspinatus tendon. Extension of tears beyond the supraspinatus tendon into the anterior and posterior aspect of the rotator cuff led to a significant decrease in the magnitude of joint reaction force (126 +/- 31 N). Similarly, such tears resulted in a significant change in the direction of the reaction force at the glenohumeral joint. These results suggest that joint reaction forces are significantly affected by the integrity of the rotator cuff, specifically, by the transverse force couple formed by the anterior and posterior aspects of the cuff. The quantitative data obtained in this study on the effect of rotator cuff tears on magnitude and direction of the reaction force at the glenohumeral joint helps clarify the relationship between joint motion, joint compression and stability.