Risk factors for instability after reverse shoulder arthroplasty

BackgroundInstability after primary reverse total shoulder arthroplasty (rTSA) is a rare but serious complication, potentially resulting in revision surgery. The causes of instability after rTSA are multifactorial and sometimes unknown. The goal of this study is to analyze an international database of one-platform shoulder prosthesis and conduct a logistic multivariate regression analysis to identify the factors associated with instability after primary rTSA and quantify the 2-year minimum clinical outcomes of patients with and without instability.MethodsA total of 5631 primary rTSA patients were analyzed from the international database of single rTSA prosthesis to quantify clinical outcomes at 2-year minimum follow-up for patients with and without instability. rTSA patients were divided into 2 cohorts based on if they were stable or unstable, and a subanalysis was conducted for patients who were unstable early (<6 months) and also unstable late (>6 months). For both stable and unstable rTSA patients, univariate and multivariate analyses were performed to quantify the patient, implant, and operative risk factors associated with instability after rTSA.ResultsFifty-five of the 5631 primary rTSA shoulders were reported to be unstable, with an overall instability rate of 0.98%. Female patients had an instability rate of 0.60% (21/3496), which was significantly lower (P < .0001) than the 1.63% instability rate for male patients (34/2085). Patients with subscapularis repair had an instability rate of 0.45% (10/2222), which was significantly lower (P = .0052) than the 1.17% instability rate of patients without a subscapularis repair (37/3161). Multivariate analysis identified numerous risk factors for instability, including younger age at the time of surgery, the use of cemented humeral fixation, larger glenosphere diameters, expanded/lateralized center of rotation glenospheres, and not repairing the subscapularis.DiscussionOur study demonstrated that patients with instability had significantly worse clinical outcomes, more pain, and worse function and range of motion as compared to rTSA patients who were stable. The univariate and multivariate analyses identified numerous patient, implant, and operative risk factors associated with instability. A patient with 1 or more of these identified parameters has an increased risk for instability, and that recognition is useful for patient counseling and consideration of repair of the subscapularis, when possible.