Rotator Cuff Muscle Imbalance in Patients with Chronic Anterior Shoulder Instability.

(1) Background: Both intra-articular pathologies and muscle imbalance can be a cause of shoulder instability. The purpose of this study is to examine the cross-sectional areas of the rotator cuff muscle in patients with acute and chronic anterior shoulder instability and to determine the associations between imbalance and chronicity of the rotator cuff. (2) Methods: Patients with confirmed dislocation of the anterior shoulder were included. The patients were divided into two groups according to the time between the initial dislocation event and when MRI imaging was performed Measurements of the rotator cuff muscle areas were performed in the scapular Y view and glenoid face view using MRI. (3) Results: A total of 56 patients were enrolled. In the Y view, a larger area of supraspinatus muscle was observed in the chronic group compared with the acute group (17.2 ± 2.3% vs. 15.6 ± 2.2%, p = 0.006). However, a smaller area of subscapularis muscle was observed in the chronic group (47.1 ± 3.5% vs. 49.6 ± 5.3%, p = 0.044). Using the glenoid face view, a larger area of supraspinatus muscle was observed in the chronic group than in the acute group (18.5 ± 2.5% vs. 15.8 ± 2.2%, p < 0.001). However, a smaller area of subscapularis muscle was observed in the chronic group (41.6 ± 3.2% vs. 45.6 ± 4.4%, p < 0.001). (4) Conclusion: Larger areas of supraspinatus muscle compared with acute instability were observed in patients with chronic anterior shoulder instability. In contrast, a smaller area of subscapularis muscle was observed in the chronic group.