Os acromiale is associated with higher critical shoulder angle, acromial index, and shoulder disorder

To investigate the impact of os acromiale (OsA) on shoulder morphology and rotator cuff pathology by evaluating the critical shoulder angle (CSA), acromial index (AI), and the prevalence of tendon tears and joint degeneration.

This retrospective single-center study included 247 patients (mean age, 56 ± 10.9 years; 178 women, 69 men), comprising 123 with OsA and 124 matched controls. MRI and radiographs were analyzed to assess CSA, AI, tendon pathology, glenohumeral and acromioclavicular osteoarthritis, and acromioclavicular step-off deformity. Two radiologists independently assessed findings. Group comparisons were performed using parametric or nonparametric tests as appropriate, ROC analysis was used to determine AI cut-off values, and agreement was evaluated using ICC and Cohen's kappa.

CSA and AI were significantly higher in the OsA group on MRI (CSA: 35.13 ± 3.85 vs. 33.96 ± 3.33, p = 0.011; AI: 0.68 ± 0.08 vs. 0.63 ± 0.07, p < 0.001) and radiographs (AI: 0.80 ± 0.11 vs. 0.71 ± 0.07, p < 0.001). Glenohumeral osteoarthritis and step-off deformity were more prevalent in OsA patients (19.5% vs. 2.4%, p < 0.001; 58.5% vs. 12.1%, p < 0.001). Partial and full-thickness infraspinatus tears and partial subscapularis tears were significantly more frequent in OsA, while supraspinatus tear rates did not differ significantly between groups. Agreement was moderate to excellent except for acromioclavicular osteoarthritis.

OsA is linked to higher CSA and AI, a greater prevalence of cuff tears, glenohumeral osteoarthritis, and step-off deformity, suggesting altered scapular morphology and biomechanics.