Labral morphology does not compensate for reduced bony glenoid concavity in stable shoulders.

Background: Glenoid concavity plays a critical role in shoulder stability via the concavity-compression mechanism. While the bony glenoid concavity, quantified by the bony shoulder stability ratio (BSSR), is a known determinant of stability, the labrum also contributes to the overall glenoid concavity. It remains unclear whether the labrum compensates for reduced bony concavity in stable shoulders. This study aimed to investigate the relationship between labral and bony glenoid concavity implementing the labral shoulder stability ratio (LSSR) and BSSR, respectively. We hypothesized that shoulders with reduced bony concavity (lower BSSR) would demonstrate increased labral concavity (higher LSSR), suggesting a compensatory mechanism. Methods: In this retrospective imaging study, 36 patients (mean age: 26.7 years) undergoing shoulder computed tomography arthrography between January 2020 and December 2024 for noninstability indications were included. BSSR and LSSR were calculated from standardized axial computed tomography images using three-dimensional multiplanar reconstructions. Concavity depth and radius were measured on the bony and chondrolabral contours, and the respective stability ratios were calculated. Inter-rater reliability was assessed using Bland-Altman plots and Pearson correlation. Pearson correlation analysis and subgroup comparisons were conducted to assess the relationship between BSSR and LSSR. Results: The mean BSSR was 28.3% ± 11.1% (range: 10.2%-52.5%), and the mean LSSR was 77.9% ± 10.8% (range: 49.1%-100%). There was no significant correlation between BSSR and LSSR (r = 0.01, Conclusion: This study demonstrates that labral morphology does not compensate for reduced bony glenoid concavity in clinically stable shoulders. Contrary to our hypothesis, lower BSSR was not associated with increased labral concavity, and no inverse relationship was observed between bone and labral curvature.