Abstract
Background: Excessive anterior capsular laxity (elongation of the anterior capsular ligaments) causes shoulder subluxation during acceleration of the throwing motion, leading to a disabled throwing shoulder. Few biomechanical studies have investigated the relationship between anterior capsular laxity and internal impingement, another cause of the disabled throwing shoulder. Purpose/Hypothesis: The purpose of this study was to assess the effect of anterior capsular laxity on forceful internal impingement during the late cocking phase. The hypothesis was that excessive anterior shoulder laxity caused by elongation of the anterior capsular ligaments will increase the horizontal abduction angle to increase glenohumeral contact pressure. Study Design: Controlled laboratory study. Methods: Eight fresh-frozen cadaveric shoulders were tested with the shoulder abducted to 90° and at maximal external rotation to simulate the late cocking phase of the throwing motion. The angle of external rotation, anterior translation, angle of horizontal abduction, locations of the articular insertion of the rotator cuff tendons (supraspinatus and infraspinatus) on the greater tuberosity relative to the glenoid, and the glenohumeral contact pressure and area during internal impingement were measured. All data were compared between intact and elongated anterior capsule, which was created by repeatedly applying external rotational stretching. Results: Elongation of the anterior capsular ligaments was confirmed by the increase in glenohumeral external rotation and anterior translation after our stretching technique. Location data showed that the posterior half of supraspinatus tendon, the entire infraspinatus tendon, and the posterosuperior labrum were impinged between the greater tuberosity and glenoid. Maximal glenohumeral horizontal abduction (2.2% increase; P = .003) and glenohumeral contact pressure (27.3% increase; P = .04) were significantly increased in the shoulder joint with increased anterior capsular laxity as compared with the intact condition. Conclusion: Increased anterior capsular laxity created by applying repetitive excessive external rotational torque significantly increased horizontal abduction and contact pressure in the glenohumeral joint. Concurrently, the supraspinatus and infraspinatus tendons and posterosuperior labrum were impinged between the greater tuberosity and glenoid. Clinical Relevance: Increased anterior capsular laxity may exacerbate forceful internal impingement during the late cocking phase of the throwing motion. © 2014 The Author(s).