Effect of Scapular Orientation on Shoulder Internal Impingement in a Cadaveric Model of the Cocking Phase of Throwing

Background: Although deviations in scapular orientation are thought to predispose to shoulder injuries in throwing athletes, the biomechanical mechanism underlying shoulder injuries in throwing athletes with an altered scapular orientation remains unclear. Methods: Seven fresh-frozen cadaveric shoulders were evaluated at 90° of abduction, with the humerus externally rotated from 90° to the maximum angle, to simulate the late cocking phase of the throwing motion. Loads were applied to the deltoid, pectoralis major, latissimus dorsi, teres major, and all rotator cuff muscles. Contact pressure in the glenohumeral joint was measured with use of a pressure sensor. The area of internal impingement was calculated on the basis of three-dimensional position data. Glenohumeral contact pressure and the area of impingement were compared between 20°, 30°, and 40° of internal scapular rotation; between 20°, 30°, and 40° of upward scapular rotation; and between 0° and 10° of anterior scapular tilt. Data were analyzed with use of repeated-measures analysis of variance with the Tukey post hoc test. Results: Contact pressure was at its maximum in the posterior aspect of the glenohumeral joint. The glenohumeral contact pressure and internal impingement area increased with increasing internal scapular rotation. The glenohumeral contact pressure at 40° of internal scapular rotation was significantly (43.4%) greater than that at 20° of internal scapular rotation (p < 0.01), and the impingement area at 40° of internal scapular rotation was significantly (43.1%) greater than that at 20° of internal scapular rotation (p < 0.05). Decreasing upward scapular rotation resulted in an increase in internal impingement area. The internal impingement area at 40° of upward motion was 38.1% less than that at 20° of upward rotation (p < 0.001) and 28.9% less than that at 30° of upward rotation (p < 0.01). Conclusions: Increasing internal scapular rotation and decreasing upward scapular rotation significantly increase glenohumeral contact pressure and the area of impingement of the rotator cuff tendon between the greater tuberosity and glenoid during simulated throwing motion. Clinical Relevance: Shoulder internal impingement is accentuated by increasing internal rotation or decreasing upward rotation of the scapula during the throwing motion.