Rotator cuff muscle imbalance associates with shoulder instability direction

• Published in: Journal of shoulder and elbow surgery 2023-01, Vol.32 (1), p.33-40
• Main Authors: Ishikawa, Hiroaki, Smith, Karch M., Wheelwright, J. Cade, Christensen, Garrett V., Henninger, Heath B., Tashjian, Robert Z., Chalmers, Peter N.
• Format: Article
• Language: English
• Subjects: Bankart lesion; cross-sectional area; deltoid; Humans; Joint Instability; Magnetic resonance imaging; muscle size; rotator cuff; Rotator Cuff - diagnostic imaging; Rotator Cuff - pathology; Rotator Cuff - surgery; Rotator Cuff Injuries - diagnostic imaging; Rotator Cuff Injuries - pathology; Rotator Cuff Injuries - surgery; Shoulder; shoulder dislocation; Shoulder Joint - diagnostic imaging; Shoulder Joint - physiology; Shoulder Joint - surgery
• ISSN: 1058-2746; 1532-6500
• DOI: 10.1016/j.jse.2022.06.022

Although muscle weakness and/or imbalance of the rotator cuff are thought to contribute to the development of shoulder instability, the association between muscular dysfunction and shoulder instability is not completely understood. The purpose of this study was to evaluate rotator cuff and deltoid muscle cross-sectional areas in different types of shoulder instability (anterior, posterior, and multidirectional instability [MDI]) and to determine the associations between muscular imbalance and shoulder instability direction. Preoperative magnetic resonance images of patients with shoulder instability who subsequently underwent arthroscopic glenohumeral labral repair or capsular plication were evaluated. Shoulder instability was classified into 3 categories by direction: (1) anterior, (2) posterior, and (3) MDI. The rotator cuff (supraspinatus, subscapularis, and infraspinatus + teres minor) and deltoid (anterior and posterior portions, and total) muscle areas were measured on T1 sagittal and axial slices, respectively. The ratios of the subscapularis to infraspinatus + teres minor area and the anterior deltoid to posterior deltoid area were calculated to quantify the transverse force couple imbalance. A total of 189 patients were included, where each group consisted of 63 patients. The infraspinatus + teres minor muscle area was smaller than the subscapularis muscle area in the anterior instability group (P = .007). The subscapularis muscle area was smaller than the infraspinatus + teres minor muscle area in the posterior instability and MDI groups (P ≤ .003). The anterior deltoid muscle area was smaller than the posterior deltoid muscle area in all groups (P ≤ .001). The subscapularis–to–infraspinatus + teres minor area ratio in the anterior instability group (1.18 ± 0.40) was higher than that in the posterior instability and MDI groups (0.79 ± 0.31 and 0.93 ± 0.33, respectively; P