Effect of Rotator Cuff Muscle Imbalance on Forceful Internal Impingement and Peel-Back of the Superior Labrum A Cadaveric Study

Background: Throwing athletes with shoulder pain have been shown to have decreased rotator cuff muscle strength. Shoulder internal impingement and labral peel-back mechanism, as may occur during the late cocking phase of throwing motion, are thought to cause rotator cuff injury and type II superior labrum anterior and posterior lesions. Therefore, the objective of this study was to assess the effect of rotator cuff muscle force on internal impingement and the peel-back of the superior labrum by quantifying maximum external rotation, glenohumeral contact pressure, and position of the cuff insertion relative to the glenoid. Hypothesis: A change in rotator cuff muscle force will lead to increased external rotation, glenohumeral contact pressure, and overlap of the cuff insertion relative to the glenoid. Study Design: Controlled laboratory study. Methods: Eight fresh-frozen cadaveric shoulders were tested at the simulated late cocking position. Glenohumeral contact pressure, location of the cuff insertion relative to the glenoid, and maximum humeral external rotation angle were measured. The forces of the supraspinatus, subscapularis, and infraspinatus muscles were determined based on published clinical electromyographic data. To assess the effect of cuff muscle imbalance, each muscle force was varied. Horizontal abduction positions of 20, 30, and 40 with respect to the scapular plane were tested. Results: Decreased subscapularis strength resulted in a significant increase in maximum external rotation ( P < .001) and increased glenohumeral contact pressure ( P < .01). The cuff insertion overlapped the edge of the glenoid at 30 and 40 of horizontal abduction for all muscle loading conditions. Conclusion: Decreased subscapularis muscle strength in the position simulating the late cocking phase of throwing motion results in increased maximum external rotation and also increased glenohumeral contact pressure.