Muscle Compensation Strategies to Maintain Glenohumeral Joint Stability in Rotator Cuff Tears

Background: Superior translation of the humeral head is often identified in large and massive rotator cuff (RC) tears. However, the ability of the remaining RC muscles to compensate for the forces causing such superior translation in RC tears remains unclear. The purpose of this study was to investigate the impact of compensatory forces exerted by the remaining RC muscles on humeral head translation using a progressive RC tear model. Methods: Eight fresh-frozen cadaveric shoulders (mean donor age, 57 years) were tested using a custom shoulder testing system. In addition to an intact RC model, 3 RC tear models were created: a supraspinatus tear (Tear I); combined supraspinatus and infraspinatus tears (Tear II); and combined tears of the supraspinatus, infraspinatus, and superior one-third of the subscapularis (Tear III). Compensatory conditions were simulated by increasing the loading of the remaining RC muscles in each RC tear model. Humeral head translation was measured at different abduction and neutral rotation angles in each condition with normal and high deltoid muscle loading. Results: Significant superior translation of the humeral head was observed in Tears II and III (but not Tear I), compared with the intact state, under high loading of the deltoid during abduction and during rotation. In Tear II, compensatory conditions involving increased loading of the teres minor and subscapularis muscles effectively reduced superior translation, so that no significant differences were observed compared with the intact state, even under high deltoid muscle loading. However, in Tear III, significant superior translation was still observed, regardless of the compensatory conditions. Conclusions: Compensation by the remaining RC muscles, particularly the teres minor and subscapularis, effectively reduced superior translation of the humeral head in the posterosuperior RC tear model, whereas this compensatory strategy was insufficient if tears also involved the superior one-third of the subscapularis. Clinical Relevance: Patients with posterosuperior RC tears may find conservative treatment focusing on strengthening the remaining RC muscles, especially the subscapularis and teres minor, to be beneficial. Conversely, patients with repairable massive RC tears also involving the subscapularis tendon may benefit from surgical interventions aimed at primarily repairing the subscapularis tendon to restore the transverse force couple. Massive tears deemed not to be repairable should be evaluated for arthroplasty or other procedures.