Radiographic comparison of humeral and glenoid lateralization and distalization between onlay and inlay reverse total shoulder arthroplasty designs in Japanese shoulders.

Background: The lateralization shoulder angle (LSA) and modified distalization shoulder angle (mDSA) are radiographic parameters used to assess lateralization and distalization after reverse total shoulder arthroplasty. However, studies comparing implant designs using these parameters are scarce, particularly in smaller Asian populations. We evaluated post-operative distalization and lateralization in Japanese patients and compared 2 implant types. Methods: We analyzed 139 shoulders of 139 Japanese patients who underwent reverse total shoulder arthroplasty at a single institution. The Aequalis Ascend Flex (Group A, onlay type; Stryker, USA) and Tornier Perform (Group P, inlay type; Stryker, USA) were compared. Group A used a 145° neck shaft angle stem; Group P used a 135° stem with a 10° insert, yielding a 145° total neck shaft angle. All cases used a 36-mm glenosphere with an increased bony offset. Post-operative radiographs measured LSA, mDSA, and Schippers indices: glenoid lateralization angle/humeral lateralization angle and glenoid distalization angle/humeral distalization angle. Stem alignment was assessed, and correlations with range of motion and clinical scores (Constant and American Shoulder and Elbow Surgeons scores) were analyzed. Results: Groups A and P included 98 and 41 shoulders, respectively. LSA (79 ± 7.2° vs. 85 ± 9.2°) and humeral lateralization angle (26 ± 5.2° vs. 32 ± 6.5°) were significantly higher in Group P ( Conclusion: Inlay implants demonstrated greater lateralization parameters, mainly due to increased humeral-side lateralization parameters. Implant design and alignment should be considered when interpreting post-operative geometry, especially in smaller Asian/Japanese shoulders.