Biomechanics of massive rotator cuff tears: implications for treatment.

BACKGROUND Some individuals with massive rotator cuff tears maintain active shoulder abduction, and some maintain good postoperative active range of motion despite high rates of repeat tears after repair. We devised a biomechanical rationale for these observations and measured the increases in residual muscle forces necessary to maintain active shoulder motion with rotator cuff tears of various sizes. METHODS A custom cadaver shoulder controller utilizing position and orientation closed-loop feedback control was used. Six cadaver glenohumeral joint specimens were tested in open-chain scapular plane abduction with equivalent upper extremity weight. The shoulder controller limited superior translation of the humeral head to 3.0 mm while maintaining neutral axial rotation by automatically controlling individual rotator cuff forces. Three-dimensional position and orientation and rotator cuff and deltoid force vectors were recorded. Specimens were tested with an intact rotator cuff and with 6, 7, and 8-cm tears. RESULTS All six specimens achieved full abduction with <or=3.0 mm of superior translation of the humeral head for all rotator cuff tear sizes. The effect of rotator cuff tear was significant for all tear sizes (p < 0.01). Compared with the intact condition, the subscapularis force requirements for the 6, 7, and 8-cm tears were increased by 30%, 44%, and 85%, respectively. For the combined infraspinatus and teres minor, the forces were increased by 32%, 45%, and 86%, respectively. The maximum deltoid force for the simulated tear condition never exceeded the deltoid force required at maximum abduction for the intact condition. However, between 10 degrees and 45 degrees of abduction, the average deltoid force requirement increased 22%, 28%, and 45% for the three tear sizes. CONCLUSIONS In the presence of a massive rotator cuff tear, stable glenohumeral abduction without excessive superior humeral head translation requires significantly higher forces in the remaining intact portion of the rotator cuff. These force increases are within the physiologic range of rotator cuff muscles for 6-cm tears and most 7-cm tears. Increases in deltoid force requirements occur in early abduction; however, greater relative increases are required of the rotator cuff, especially in the presence of larger rotator cuff tears.