The Critical Shoulder Angle as a Highly Specific Predictor of a Full-Thickness Rotator Cuff Tear: A Case-Control Study.

BACKGROUND The critical shoulder angle (CSA) has become an important topic of study in patients with rotator cuff tears (RCTs). However, there are conflicting data on whether the CSA can differentiate between patients with normal shoulder pathology and full-thickness RCTs on shoulder radiographs. PURPOSE/HYPOTHESIS The purpose of this study was to define the relationship between full-thickness RCTs and the CSA. It was hypothesized that patients with full-thickness RCTs would have an increased CSA compared with matched controls. STUDY DESIGN Cross-sectional study; Level of evidence, 3. METHODS This retrospective case-control study identified patients with magnetic resonance imaging scans showing full-thickness RCTs between 2009 and 2019. A 1 to 1 propensity score match was performed to identify a control group with normal rotator cuffs while controlling for baseline participant characteristics-including age, sex, body mass index, and tobacco use. A total cohort of 532 was identified, with 266 cases and 266 controls. Two independent observers measured CSAs on true anteroposterior shoulder radiographs. RESULTS There was no difference in baseline participant characteristics between the RCT and the non-RCT groups (P > .05). The mean CSA for the entire cohort was 33.6°± 4.2°. The CSA did not significantly vary by sex (P = .088) or tobacco usage (P = .16). The mean CSA for the RCT case group, 36.2°± 3.3°, was significantly different from the mean CSA for the control group, 30.9°± 3.3° (P < .0001). The receiver operating characteristic curve analysis produced an area under the curve of 0.88 (P < .0001). At CSAs ≥35°, there was a 67.7% sensitivity and 89.4% specificity for having a full-thickness RCT. Last, each degree of increase in the CSA increased the risk of having an associated RCT by 1.7 times (OR, 1.7 [95% CI, 1.551-1.852]; P < .0001). CONCLUSION Patients with RCTs had significantly higher CSAs compared with matched controls. Increased CSA was an independent risk factor for RCTs, with an odds ratio of 1.7 per degree. The CSA is an accurate test (area under the curve, 0.88) with good sensitivity (67.7%) and specificity (89.4%) at values ≥35°. The CSA is a simple, reproducible measurement that can assist in clinical decision-making regarding full-thickness RCTs.