Critical shoulder angle: a “combined shoulder angle”—balanced contributions of glenoid inclination and acromial angle in shoulder arthritis progression

Background The Critical Shoulder Angle (CSA) has been described as a morphometric value that could be predisposing to the evolution of a shoulder towards a primary glenohumeral osteoarthritis (PGHOA) or massive rotator cuff tear (MRCT). CSA is described as the sum of the acromial angle (AA) and the glenoid inclination (GI). The aim of this study is to evaluate the impact of GI and AA measurements on CSA value in PGHOA and MRCT. Methods CT-scans of 186 patients were extracted from our database and assigned into 3 groups: control group (48 healthy shoulders) and 2 pathological shoulder groups with no glenoid erosion: 66 MRCT (E0 glenoids) and 72 PGHOA (A1 or B1 glenoids). CT-scans were analyzed using an automatic software. The GI, AA and CSA variables were compared knowing that AA+GI=CSA. Results there was a significant difference (p<0.001) in CSA between the MRCT, PGHOA and control groups, respectively (33±4°, 29±4° and 32±4°). There was a significant difference between the MRCT and PGHOA for GI (8±5° and 6±6° respectively, p=0.026), and no difference with the control group (7±5°, p>0.05). The AA was not statistically different among the three groups (respectively 25±6°, 23±6° and 24±6, p=0.063). Within each group, GI and AA displayed a negative linear correlation (R 2 =0.5591 for PGHOA, R 2 =0.4984 for MRCT and R 2 =0.5829 for control group). Conclusion CSA is a reliable parameter for predicting progression to PGHOA or MRCT, with a notable effect size. No single component of CSA predominates, and the linear correlation between GI and AA underscores a balance between these angles. Thus, CSA should indeed be considered as a "Combined Shoulder Angle."