Predictors of Internal Rotation after Reverse Shoulder Arthroplasty.

Objectives: Internal rotation (IR) remains unpredictable following reverse total shoulder arthroplasty (rTSA). This study aimed to determine if increasing IR limits range of motion in other planes, and to determine pre- and intra-operative factors associated with post-operative IR. Methods: A retrospective analysis of a single surgeon's primary rTSA with a single implant was undertaken, excluding patients with acute fracture or infection. A lesser tuberosity osteotomy (LTO) or subscapularis peel tenotomy was performed and repaired at the surgeon's discretion. One hundred sixty rTSA were included; 142 (88.8%) had documented IR both pre-operatively and at one-year follow-up. Variables were collected to determine their effect on IR at the 1-year follow-up point. A multivariate logistic regression was used to determine independent predictors of sufficient IR. Results: Average age was 69.8 (range: 55-86) years and 55% (88/160) were female. Preoperatively, 20.4% of patients (29/142) had sufficient IR. This improved to 32.4% (46/142) one year following surgery, p<0.001). Factors associated with sufficient post-operative IR were female sex (p=0.05), decreasing body mass index (p=0.04), pre-operative IR (p=0.01), preoperative external rotation (ER) in adduction (p<0.001), radiographic evidence of LTO healing (p=0.02), increased one-year postoperative forward elevation (p<0.001), and increased one-year postoperative ER (p<0.001). Increased postoperative IR did not adversely affect forward elevation or ER. On multivariate analysis, higher preoperative IR and one-year postoperative forward elevation were independently associated with sufficient one-year postoperative IR. Conclusion: IR following rTSA continues to be modest and unpredictable. Independent predictors of sufficient post-operative internal rotation were higher preoperative IR and one-year postoperative forward elevation. In a Grammont-style rTSA system, humeral version, glenosphere lateralization, and glenosphere size do not appear to impact IR. Importantly, achieving sufficient IR does not come at the expense of other planes of motion.