Anterior shoulder instability: diagnostic criteria determined from prospective analysis of 121 MR arthrograms.

PURPOSE: To determine magnetic resonance (MR) arthrographic criteria in the diagnosis of anterior glenohumeral instability. MATERIALS AND METHODS: In 121 patients with diagnoses proved surgically, the labrum, glenohumeral ligaments, and capsular insertion types were assessed prospectively with gadolinium-enhanced MR arthrography. Findings were compared in stable and unstable shoulders. RESULTS: Operative results showed 59 normal, 57 torn, and five deficient labra. In 37 unstable shoulders, 31 had discrete inferior labral-ligamentous lesions and six had capsular laxity. MR arthrograms showed labral abnormalities with 92% sensitivity, 92% specificity. Inferior labral-ligamentous lesions enabled prediction of anterior instability with 76% sensitivity (capsular laxity was missed in all shoulders), 98% specificity. Inferior labral-ligamentous abnormalities were strongly associated with unstable shoulders (P .8). CONCLUSION: On MR arthrograms, inferior labral-ligamentous abnormalities were most closely correlated with anterior glenohumeral instability. Capsular insertion sites had no role in the prediction of shoulder instability.