Role of Subscapularis Repair on Muscle Force Requirements with Reverse Shoulder Arthroplasty.

Concomitant repair of the subscapularis with reverse shoulder arthroplasty (rTSA) is controversial. To evaluate the biomechanical impact of subscapularis repair with rTSA, a cadaveric shoulder controller quantified the muscle forces required to elevate the arm during scapular abduction with the elbow flexed at 90°. The results of this study demonstrate that concomitant subscapularis repair with rTSA creates a biomechanically unfavorable condition during arm elevation. Specifically, repair of the subscapularis significantly increased the force required by the deltoid and posterior rotator cuff and also significantly increased the joint reaction force relative to when the subscapularis was not repaired. These results also demonstrated that both the 42 mm Grammont and 42 mm Equinoxe® rTSA prostheses significantly decreased the mean force required by the posterior rotator cuff and also significantly decreased the mean joint reaction force over the range of motion relative to the native joint with a rotator cuff tear (supraspinatus). As the posterior rotator cuff is often compromised in patients undergoing rTSA, patients may not be able to sustain these elevated forces in the infraspinatus and teres minor required to counteract the adduction and internal rotation moments generated by the subscapularis during activities of daily living. Similarly, the elevated posterior deltoid force and joint reaction loads could be deleterious to the long-term life of the prosthesis and can also increase the risk of loosening and fractures. For all these reasons, rTSA functional outcomes may be compromised if the subscapularis is repaired.