The Impact of Critical Shoulder Angle on ACJ Degeneration and Rotator Cuff Tears Severity: A Multivariable Analysis

This study aimed to evaluate the relationship between the critical shoulder angle (CSA) and acromioclavicular joint (ACJ) degeneration, and its effect on supraspinatus, infraspinatus, and subscapularis tendon tear severity.

A retrospective review of 217 patients (mean age 57.1 ± 12.9 years) between June 2019 and June 2024 was conducted. Patients were grouped based on the presence of ACJ degeneration and rotator cuff tears (RCT). Data collected included age, sex, acromion type, CSA, and ACJ degeneration. CSA was measured on anteroposterior (AP) radiographs; tendon tears were evaluated via magnetic resonance imaging (MRI). Associations among CSA, ACJ degeneration, and RCT were analyzed using receiver operating characteristic (ROC) and logistic regression.

Patients with ACJ degeneration had significantly higher CSA (40.0 ± 4.3 vs. 36.6 ± 2.1, p < 0.001). Full-thickness supraspinatus tears were more common with ACJ degeneration (34.2% vs. 1.4%), as were partial infraspinatus (54.1% vs. 2.8%) and subscapularis tears (32.9% vs. 0%) (p < 0.001 for all). CSA was highest in patients with full-thickness supraspinatus tears (42.5 ± 5.0). ROC analysis identified 38.4° as the optimal CSA cutoff. High CSA independently predicted ACJ degeneration. The critical shoulder angle predicted ACJ degeneration with good accuracy, as demonstrated by an area under the curve (AUC) of 0.776 (95% confidence interval (CI): 0.715, 0.836; p < 0.001) in ROC analysis.

CSA is significantly associated with ACJ degeneration and rotator cuff pathology, especially full-thickness supraspinatus and partial infraspinatus tears. CSA may serve as a useful parameter in the assessment and management of shoulder disorders.