The influence of humeral neck shaft angle and glenoid lateralization on range of motion in reverse shoulder arthroplasty

Background: Recent developments in reverse shoulder arthroplasty (RSA) have focused on changes in several design-related parameters, including humeral component design, to allow for easier convertibility. Alterations in humeral inclination and offset on shoulder kinematics may have a relevant influence on postoperative outcome. This study used a virtual computer simulation to evaluate the influence of humeral neck shaft angle and glenoid lateralization on range of motion in onlay design RSA. Methods: Three-dimensional RSA computer templating was created from computed tomography (CT) scans in 20 patients undergoing primary total shoulder arthroplasty for concentric osteoarthritis (Walch A1). Two concurrent factors were tested for impingement-free range of motion: humeral inclination (135 degrees vs. 145 degrees) and glenoid lateralization (0 mm vs. 5 mm). Results: Decreasing the humeral neck shaft angle demonstrated a significant increase in impingementfree range of motion. Compared to the 145 degrees configuration, extension was increased by 42.3 degrees (-8.5 degrees to 73.5 degrees), adduction by 15 degrees (10 degrees to 23 degrees), and external rotation with the arm at side by 15.1 degrees (8.5 degrees to 26.5 degrees); however, abduction was decreased by 6.5 degrees (-1 degrees to 12.5 degrees). Glenoid lateralization led to comparable results, but an additional increase in abduction of 7.6 degrees (-1 degrees to 16.5 degrees) and forward flexion of 26.6 degrees (6.5 degrees to 62 degrees) was observed. Conclusion: Lower humeral neck shaft angle and glenoid lateralization are effective for improvement in range of motion after RSA. The use of the 135 degrees model with 5 mm of glenoid lateralization provided the best results in impingement-free range of motion, except for abduction.