New Measurement Method for Lateral Acromial Protrusion as an Alternative to Critical Shoulder Angle, Independent of Sagittal Glenoid Tilt

Background: Despite the widespread use of the critical shoulder angle (CSA) for assessing lateral acromial protrusion, its reliability and consistency remain debatable. Moreover, significant CSA differences were often observed between shoulders in the same patient, even in the absence of severe bony deformity. Therefore, this study aimed to evaluate the reliability of the CSA across different imaging methods and points of view, and to propose and validate a new measurement method, the acromial protrusion ratio (APR), as a potential alternative for assessing lateral acromial protrusion by complementing the limited reliability of the CSA. Methods: This study was conducted in 2 phases: an exploratory phase evaluating the reliability of the CSA and APR and a validation phase assessing their clinical applicability. We retrospectively reviewed 25 patients who underwent reverse total shoulder arthroplasty with a CSA difference > 5° between the shoulders in the exploratory phase and prospectively collected data from 38 shoulders in the validation phase. Measurements were independently performed by 2 assessors using different imaging methods (plain radiography and 3-dimensional [3D] reconstructed computed tomography) and points of view at the measurement (orthogonal [3DO] and neutral [3DN] scapular positions). Inter- and intraclass correlations were calculated to assess the reliability of the measurements. Results: > 0.05). The APR was calculated using the following formula based on the CSA at the 3DO position: APR (%) = [CSA (°) × 0.007 + 0.053] × 100. Conclusions: The CSA differed according to the measurement method and scapular tilt, even in an optimized true anteroposterior view. In contrast, the APR was consistent regardless of the measurement method and scapular tilt. Therefore, the APR may serve as a reliable alternative for assessing lateral acromial protrusion to complement the weak reliability of the CSA. However, its clinical relevance should be further validated in future studies.