Abstract
Background: The objective of this study was to determine the effects of increased horizontal abduction with maximum external rotation, as occurs during the late cocking phase of throwing motion, on shoulder internal impingement. Hypothesis: An increase in glenohumeral horizontal abduction will cause overlap of the rotator cuff insertion with respect to the glenoid and increase pressure between the supraspinatus and infraspinatus tendon insertions on the greater tuberosity and the glenoid. Study Design: Controlled laboratory study. Methods: Eight cadaveric shoulders were tested with a custom shoulder testing system with the specimens in 60° of glenohumeral abduction and maximum external rotation. The amount of internal impingement was evaluated by assessing the location of the supraspinatus and infraspinatus articular insertions on the greater tuberosity relative to the glenoid using a MicroScribe 3DLX. Pressure in the posterior-superior quadrant of the glenoid was measured using Fuji prescale film. Data were obtained with the humerus in the scapular plane and 15°, 30°, and 45° of horizontal abduction from the scapular plane. Results: At 30° and 45° of horizontal abduction, the articular margin of the supraspinatus and infraspinatus tendons was anterior to the posterior edge of the glenoid and less than 2 mm from the glenoid rim in the lateral direction; the contact pressure was also greater than that found in the scapular plane and 15° of horizontal abduction. Conclusion: Horizontal abduction beyond the coronal plane increased the amount of overlap and contact pressure between the supraspinatus and infraspinatus tendons and glenoid. Clinical Relevance: Excessive glenohumeral horizontal abduction beyond the coronal plane may cause internal impingement, which may lead to rotator cuff tears and superior labral anterior to posterior (SLAP) lesions.