The over-the-top subscapularis repair in reverse shoulder arthroplasty: biomechanical evaluation of a novel technique

BACKGROUND: We evaluate the effect of repairing the upper subscapularis tendon at an alternative location on the anterior greater tuberosity above the center of rotation using a reverse shoulder arthroplasty (RSA) muscle model. METHODS: We compared an innovative subscapularis repair on the anterior aspect of the greater tuberosity with the standard repair on the lesser tuberosity in a previously validated digital linear RSA muscle model. Standard repair vs. a new repair datasets were compared for 3 RSA designs. Each model was run through humeral abduction from 0° to 140° in 2.5° increments; the resulting moment arm measurements (model of tendon efficiency) were recorded in millimeters for 3 sections (superior, middle, inferior). An isolated upper two-thirds subscapularis repair to the anterior greater tuberosity was also evaluated (the over-the-top repair). RESULTS: The new over-the-top subscapularis repair significantly increased the abduction moment arm lengths in the superior, middle, and inferior subscapularis components compared with the standard repair to the lesser tuberosity at all levels of glenohumeral abduction and for all 3 RSA designs. Repair of the upper two-thirds of the subscapularis to the new location was an abductor at a much lower level of abduction compared with the native subscapularis repair. CONCLUSION: By repairing the upper subscapularis tendon above the center of rotation (over-the-top repair) in an RSA muscle model, the subscapularis has an improved movement arm and functions as an abductor through a greater range of motion that may result in clinically increased muscle efficiency and improved strength.