Lower shoulder abduction during throwing motion may cause forceful internal impingement and decreased anterior stability

Background: Internal impingement and decreased anterior stability, which result from shoulder capsular loosening, are common shoulder pathologies in throwing athletes. The purpose of this study was to assess the effect of shoulder abduction angle on shoulder internal impingement and anterior shoulder stability during the simulated throwing motion. Methods: Eight cadaveric shoulders were tested by simulating the late-cocking and acceleration phases of the throwing motion for intact and thrower's shoulder conditions. The maximal glenohumeral external rotation, anterior translation, location of the rotator cuff insertion with respect to the glenoid, length and site of internal impingement, and glenohumeral contact pressure were measured. All data were compared between shoulder abduction angles of 80° 90° and 100°. Results: Decreasing shoulder abduction in the simulated late-cocking phase shifted the humeral head posteriorly (P <.03) and superiorly (P <.001), decreasing the total internal impingement area between the greater tuberosity and glenoid (P =.04) and increasing the glenohumeral contact pressure during internal impingement (P =.02). In the simulated acceleration phase, anterior glenohumeral translation significantly increased as the shoulder abduction angle decreased (P <.001). Conclusion: Decreasing shoulder abduction significantly increased the contact pressure during internal impingement in the simulated late-cocking phase of the throwing motion. During the simulated acceleration phase of the throwing motion, anterior glenohumeral translation significantly increased as shoulder abduction decreased.