Background: Significant bone defects are associated with poor clinical results after surgical stabilization in cases of glenohumeral instability. Although multiple factors are thought to adversely affect enlargement of bipolar bone loss and increased shoulder instability, these factors have not been sufficiently evaluated. The purpose of this study was to identify the factors related to greater bone defects and a higher number of instability episodes in patients with glenohumeral instability. Methods: A total of 120 consecutive patients with symptomatic unilateral instability of the glenohumeral joint were retrospectively reviewed. Three-dimensional surface-rendered/registered models of bilateral glenoids and proximal humeri from computed tomography data were matched by software, and the volumes of bone defects identified in the glenoid and humeral head were assessed. After relationships between objective variables and explanatory variables were evaluated using bivariate analyses, factors related to large bone defects in the glenoid and humeral head and a high number of total instability episodes and self-irreducible dislocations greater than the respective 75th percentiles were evaluated using logistic regression analyses with significant variables on bivariate analyses. Results: Larger humeral head defects (P <.001) and a higher number of total instability episodes (P =.032) were found to be factors related to large glenoid defects. On the other hand, male sex (P =.014), larger glenoid defects (P =.015), and larger number of self-irreducible dislocations (P =.027) were related to large humeral head bone defects. An increased number of total instability episodes was related to longer symptom duration (P =.001) and larger glenoid defects (P =.002), and an increased number of self-irreducible dislocations was related to larger humeral head defects (P =.007). Conclusions: Whereas this study showed that bipolar lesions affect the amount of bone defects reciprocally, factors related to greater bone defects differed between the glenoid and the humeral head. Glenoid defects were related to the number of total instability episodes, whereas humeral head defects were related to the number of self-irreducible dislocations.
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