A functional evaluation of the rotator cuff length after reverse total shoulder arthroplasty.

Background: A biomechanical result of the reverse total shoulder arthroplasty (rTSA) design is the medialization and inferiorization of the greater tuberosity, which influences the length of the cuff muscles. A well-known complication after rTSA is a lack of external rotation force. The purpose of this study was to investigate the difference in the length of the cuff muscles in a native shoulder and in shoulders treated with six commercial rTSA designs. Methods: Six implant systems were implanted on identical sawbones. A robotic setup was used to perform and control the shoulder's position and measurements. The muscle lengths were measured by draw wire encoders. Results: In the three functional positions, the length of the cuff muscles was significantly lower in the Delta Xtend. In all measured positions, there was a strong negative correlation between the medialization of the humerus and the length of the cuff muscles. A lower position of the humerus after rTSA had a positive impact on the length of the infraspinatus and subscapularis. Discussion: This study found a distinct difference in the slackening of the cuff muscles. Still, in the commercial reverse shoulder arthroplasty designs studied, this slackening never exceeded 15% in the above-mentioned maneuvers, which is functional and safe for the remnants of the muscles.