Accuracy of the critical shoulder angle for predicting rotator cuff tears in patients with nontraumatic shoulder pain

Key points

1. 01The CSA was significantly higher in patients with rotator cuff tears than in those without (38.90 degrees vs 35.99 degrees, P < .001).
2. 02ROC analysis gave the CSA an area under the curve of 70.5%, while the acromion index (AI) showed no predictive value (AUC 47.7%, P = .322).
3. 03A CSA cutoff of 37.52 degrees gave 65.8% sensitivity and 67.8% specificity for a rotator cuff tear.
4. 04Patients with a CSA above 37.52 degrees had nearly 4 times the odds of a tear (adjusted odds ratio 3.92, 95% CI 2.79-5.51).
5. 05This was a retrospective single-center study in a single Asian population, so the exact cutoff may not transfer to other groups.

How it was conducted

Design

Retrospective case-control, cross-sectional study (level of evidence 3), single university hospital, 2014-2018

Participants

601 patients aged 20-80 with nontraumatic shoulder pain; 301 with rotator cuff tears (arthroscopically repaired) and 300 without tears, matched 1:1 by propensity score

Index tests

Critical shoulder angle and acromion index measured on standardized anterior-posterior shoulder radiographs by 2 blinded observers, averaged over 3 measurements

Reference standard

Shoulder ultrasound of the supraspinatus, infraspinatus, and subscapularis tendons to confirm or rule out a tear

Analysis

Independent t test, chi-square test, ROC curve analysis with Youden index cutoff, and stepwise and binary logistic regression

Primary outcome

Accuracy of CSA and AI for predicting the presence of a rotator cuff tear

What they found

• The rotator cuff tear group had a significantly higher CSA than the non-tear group (38.90 degrees plus/minus 3.88 vs 35.99 degrees plus/minus 4.03, P < .001).
• Area under the ROC curve was 70.5% for the CSA (P < .001), compared with 47.7% for the AI (P = .322, not significant).
• Optimal CSA cutoff by Youden index was 37.52 degrees, giving 65.8% sensitivity and 67.8% specificity.
• A CSA above 37.52 degrees carried an adjusted odds ratio of 3.92 (95% CI, 2.79-5.51; P < .001) for a tear.
• Stepwise logistic regression confirmed the CSA as an independent predictor (adjusted odds ratio 1.295 per degree; 95% CI, 1.019-1.571; P = .006).
• The AI did not differ between groups (0.76 plus/minus 0.08 vs 0.77 plus/minus 0.08, P = .497); GA and GH were both higher in the tear group (P < .001).
• The tear group comprised 98 partial-thickness and 203 full-thickness tears.

Limitations

• Only symptomatic patients who had ultrasound were included; asymptomatic people with tears were excluded, which limits generalizability.
• All participants were a single Asian population, so the 37.52 degree cutoff may differ by race or population.
• Retrospective case-control design with level 3 evidence, and contributing factors such as exercise, acromial shape, and daily biomechanical loads were not accounted for.
• Accuracy was moderate (about two-thirds sensitivity and specificity), so the CSA misses and misclassifies a meaningful share of patients.

Why it matters

For patients

If you have ongoing shoulder pain, a wider critical shoulder angle on your X-ray raises the chance of a rotator cuff tear and is a reason to get an ultrasound, but the X-ray alone cannot confirm or rule out a tear.

For clinicians

In a shoulder pain workup, a CSA above roughly 37.5 degrees on a standardized AP radiograph should prompt confirmatory ultrasound, whereas the acromion index added no predictive value here.

For readers

This study supports the critical shoulder angle as a simple radiographic screening signal for rotator cuff tears, with moderate accuracy that warrants confirmation by imaging.