LAUSR

PURPOSE Despite advances in implant design and surgical technique, instability remains the most common early complication and reason for early revision following reverse shoulder arthroplasty (RSA). The purpose of this study is to evaluate the glenoid implant inclination, as measured by the Beta angle, as an independent risk factor for instability following primary RSA. METHODS A retrospective case-control study was conducted matching cases with instability following primary RSA using a single implant to controls without instability. Controls were matched to age, sex, BMI and baseplate type (1:3 ratio of cases to controls). The preoperative, postoperative and the change in pre to postoperative glenoid inclination (Beta angle) were compared between groups. RESULTS Thirty-four cases (mean age 66.2 years) were matched to 102 controls (mean age 67.0 years). There was a wide range of postoperative (63o to 100o) and pre to postoperative change (-16.5o to +30.5o) in Beta angles collectively. There was no significant difference in the postoperative Beta angle (mean 80.8o vs 82.7o, p=0.19) or change in Beta angle (mean +1.7o vs. +3.4o, p=0.35) between cases and controls, respectively. Regression analysis demonstrated no increased odds of instability with postoperative Beta angle, odds ratio 0.965 (CI = 0.916-1.02, p = 0.19). Likewise, for the preoperative to postoperative change in Beta angle, there was no significantly increased odds of instability, odds ratio of 0.978 (CI = 0.934-1.03, p = 0.35). Finally, there was no difference in risk of instability in patients whose implant positioning resulted in a net superior increase in inclination, relative risk 0.85 (95% CI 0.46 - 1.56, p = 0.28). CONCLUSIONS Neither the final prosthetic glenoid inclination nor the change in glenoid inclination, as measured by Beta angle, significantly influences the risk of prosthetic instability following primary RSA. LEVEL OF EVIDENCE Level III; Retrospective Case-Control Design; Prognosis Study.