Reverse shoulder arthroplasty leads to significant biomechanical changes in the remaining rotator cuff

Objective: After reverse shoulder arthroplasty (RSA) external and internal rotation will often remain restricted. A postoperative alteration of the biomechanics in the remaining cuff is discussed as a contributing factor to these functional deficits. Methods: In this study, muscle moment arms as well as origin-to-insertion distance (OID) were calculated using three-dimensional models of the shoulder derived from CT scans of seven cadaveric specimens. Results: Moment arms for humeral rotation are significantly smaller for the cranial segments of SSC and all segments of TMIN in abduction angles of 30 degrees and above (p <= 0.05). Abduction moment arms were significantly decreased for all segments (p <= 0.002). OID was significantly smaller for all muscles at the 15 degree position (p <= 0.005), apart from the cranial SSC segment. Conclusions: Reduced rotational moment arms in conjunction with the decrease of OID may be a possible explanation for the clinically observed impaired external and internal rotation.