In reverse shoulder arthroplasty, selectively medializing the glenoid in higher-risk patients is associated with reduced risk of acromial stress fracture

Reverse total shoulder arthroplasty has roughly a 4% risk of acromial stress fracture (ASF) and is associated with poor outcomes. Previous finite element analyses indicated that glenoid center of rotation lateralization significantly increased acromial strain; however, clinical studies have reported conflicting data with regards to implant position parameters on the incidence of ASFs. The purpose of this study was to evaluate whether medializing the glenoid component in patients at higher risk of ASF was associated with a lower incidence of ASF. A retrospective analysis was conducted on all primary reverse total shoulder arthroplasties performed by a single shoulder surgeon between October 2018 and September 2023. Patient factors, including age, sex, body mass index, and bone density, as well as implant factors, including glenoid center of rotation lateralization, humeral lateralization, and humeral distalization, were evaluated. Univariate and multivariate analyses were performed to identify risk factors for ASF. Beginning in the fourth quarter of 2020, the senior author selectively medialized patients at higher risk of ASF. The rate of and risk factors for ASF were then compared between procedures performed before (PRE group) and after (POST group) this time. We included 282 patients, with a follow-up rate of 87%. The rate of ASF was 4.6% overall, and 12 of 13 were in lateralized glenoids, despite less distalization in ASF patients. On multivariate analysis, glenoid lateralization was the only significant factor associated with ASF, with an odds ratio of 14.5 for ASF, P = .013. The ASF rate decreased from 9.2% in the PRE group to 2.3% in the POST group (P = .012), despite 4.5 mm greater humeral distalization in lateralized glenoids in the POST cohort. In the POST group, 3 of 4 ASFs still occurred in lateralized glenoids. Selective glenoid medialization was associated with a lower ASF rate in those with ≥ 2 risk factors by 71%, from 10.7% in the PRE group to 3.1% in the POST group (P = .028), and in those with ≥ 3 risk factors by 76%, from 14.0% in the PRE group to 3.3% in the POST group (P = .034). Of all patient- and implant-related factors analyzed, glenoid lateralization was identified as the most significant predictor of ASF, with an odds ratio of 14.5, despite ASF patients having less distalization. Patients with multiple risk factors have ASF rates of 11%-14%, and selectively medializing the glenoid in these patients was associated with a 71%-76% lower rate of ASF.