Eccentricity and greater size of the glenosphere increase impingement-free range of motion in glenoid lateralized reverse shoulder arthroplasty: A computational study

Introduction: Increasing the impingement-free Range Of Movement (ROM) is crucial for improving patient's functional outcome and reducing the rate of scapular notching. The purpose of this study was to compare, in a virtual model of Reverse Shoulder Arthroplasty with glenoid lateralization (L-RSA): 1) the impingement-free range of movement (ROM) between 155° Grammont-style inlay stem and 135° flushlay stem; 2) the effect of glenosphere size and type (centered vs eccentric) on impingement-free range of movement (ROM) using a 135° Neck Shaft Angle (NSA) flushlay stem. Materials and methods: 200 CT-scans of patients undergoing shoulder replacement for Cuff tear Arthropathy were analyzed in the present study. Virtual implantation of L-RSA was performed using the same glenoid implant and two stems, 155° inlay Grammont-style stem and 135° flushlay stem. For 135° stem, three different glenoid size (36 mm, 39 mm and 42 mm) were tried, as well as two glenoid type (centered and eccentric glenoid), while for 155° stem were tried two different glenoid size (36 mm and 42 mm), as well as two glenoid type (centered and eccentric glenoid). For both stems, two different baseplates (25 mm e 29 mm) were used. Finally, impingement-free ROM for each configuration and each stem was then calculated by the software and collected. Results: Increasing the glenosphere size demonstrated an increase in impingement-free ROM in both 25 mm and 29 mm baseplate groups (p < 0.01). Similarly, using eccentric glenoid type improved impingement-free ROM in the two subgroups (p < 0.01). When comparing inlay and flushlay designs, flushlay shows better total impingement-free ROM as well as better impingement-free ROM in all movements apart from abduction (p < 0.01). Conclusions: This study demonstrated a correlation between glenoid size and glenoid type and impingement-free ROM using 135° flushlay stems. In particular, using a greater glenoid size and eccentric glenoid type allows for more mobility. When comparing 135° flushlay stem with 155° inlay stem, 135° stem allows greater mobility in all movements except for abduction.