The influence of radiographic markers of biomechanical variables on outcomes in reverse shoulder arthroplasty

BACKGROUND: Controversy exists in reverse total shoulder arthroplasty in regard to variability in the center of rotation (COR), which modifies the superior-inferior position of the humerus to affect the acromiohumeral interval (AHI), and its effect on the deltoid lever arm (DLA), acromial index (AI), and critical shoulder angle (CSA). The purpose of this study was to investigate the variation in biomechanics and the association with patient-reported outcomes (PROs) and range of motion (ROM) measurements. METHODS: Radiographs, ROM, and 2-year PRO scores were retrospectively reviewed for 108 patients. RESULTS: There was large variability in preoperative and postoperative biomechanics. The COR was medialized 12.01 ± 4.8 mm. The CSA increased 2.64° ± 12.45°. The AHI increased 20.6 ± 9.80 mm. The DLA lengthened 21.21 ± 10.15 mm. The AI increased 0.009 ± 0.3. Postoperative AI positively correlated with American Shoulder and Elbow Surgeons score and Penn Shoulder Score (P = .03). Specifically, a postoperative AI of 0.62 corresponded to American Shoulder and Elbow Surgeons score (72.5 ± 18.3 vs. 62.3 ± 24.7; P = .02) and Penn Shoulder Score (71.2 ± 21.4 vs. 61.8 ± 25.6; P = .05), an average 10 points higher than AI of <0.6. Also, a smaller postoperative CSA (<25°) correlated with improved forward elevation (P = .02). CONCLUSIONS: This is the first study that evaluates the variability of biomechanical factors and their impact on postoperative ROM and PROs. An increased AI and decreased CSA are associated with improved PROs in this study, and a smaller CSA is associated with better forward elevation. Change in the COR, AHI, or DLA, however, did not affect patient outcomes or ROM. Further study is warranted to determine the optimal position.