A smaller preoperative-to-prosthetic center of rotation shift is associated with better range of motion after bony increased-offset reverse shoulder arthroplasty: A retrospective cohort study of 97 patients by the FP-UCBM Shoulder Group

Background: Reverse shoulder arthroplasty (RSA) alters the glenohumeral center of rotation (CoR), but the relevance of preoperative-to-prosthetic CoR shift remains unclear. This study evaluated its association with postoperative outcomes after bony increased-offset RSA (BIO-RSA). Methods: We retrospectively reviewed 120 patients undergoing BIO-RSA with a 145° onlay humeral stem and a minimum 24-month follow-up; 97 met inclusion criteria. CoR positions were quantified using angular measurements based on reproducible scapular landmarks. Preoperative measurements were obtained from scapular-plane computed tomography reconstructions and postoperative measurements from 2-year true anteroposterior radiographs. ΔCoR was defined as Pre-CoR angle minus Post-CoR angle. Outcomes included visual analog scale pain, Constant–Murley score, Simple Shoulder Test, and active range of motion; associations were assessed using linear-regression and Spearman correlation analyses. Results: Each 1° increase in ΔCoR was associated with lower postoperative abduction (−1.13°, p = .028, R2 = .055) and forward flexion (−0.82°, p = .037, R2 = .050). Spearman analysis showed a significant association only with abduction (ρ = −0.255, p = .016). No significant associations were observed with pain or functional scores. Conclusion: A smaller preoperative-to-prosthetic CoR shift may be associated with greater elevation after BIO-RSA. Given the weak associations, ΔCoR should be interpreted cautiously as a radiographic parameter rather than an independent predictor.