The relationship between glenoid inclination and instability following primary reverse shoulder arthroplasty.

Purpose: Despite advances in implant design and surgical technique, instability remains the most common early complication and reason for early revision after reverse shoulder arthroplasty (RSA). The purpose of this study is to evaluate the glenoid implant inclination, as measured by the β-angle, as an independent risk factor for instability after primary RSA.

Methods: A retrospective case-control study was conducted matching cases with instability after primary RSA using a single implant to controls without instability. Controls were matched to age, sex, body mass index, and baseplate type (1:3 ratio of cases to controls). The preoperative, postoperative, and the change in pre- to postoperative glenoid inclination (β-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 (63° to 100°) and pre- to postoperative change (-16.5° to +30.5°) in β-angles collectively. There was no significant difference in the postoperative β-angle (mean, 80.8° vs. 82.7°, P = .19) or the change in β-angle (mean, +1.7° vs. +3.4°, P = .35) between cases and controls, respectively. Regression analysis demonstrated no increased odds of instability with the postoperative β-angle, odds ratio 0.965 (confidence interval [CI] = 0.916-1.02, P = .19). Likewise, for the preoperative to postoperative change in β-angle, there was no significantly increased odds of instability, odds ratio 0.978 (CI = 0.934-1.03, P = .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 = .28).

Conclusions: Neither the final prosthetic glenoid inclination nor the change in glenoid inclination, as measured by the β-angle, significantly influences the risk of prosthetic instability after primary RSA.