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Scientific Articles   |    
Effect of Scapular Orientation on Shoulder Internal Impingement in a Cadaveric Model of the Cocking Phase of Throwing
Teruhisa Mihata, MD, PhD1; Bong Jae Jun, PhD2; Christopher N.H. Bui, BS2; James Hwang, BS2; Michelle H. McGarry, MS2; Mitsuo Kinoshita, MD, PhD1; Thay Q. Lee, PhD2
1 Department of Orthopedic Surgery, Osaka Medical College, 2-7 Daigaku-machi, Takatsuki, Osaka, 569-8686, Japan. E-mail address for T. Mihata: tmihata@poh.osaka-med.ac.jp
2 Orthopaedic Biomechanics Laboratory, Long Beach VA Healthcare System, 5901 East 7th Street (09/151), Long Beach, CA 90822
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Investigation performed at the Orthopaedic Biomechanics Laboratory, VA Long Beach Healthcare System and University of California–Irvine, Irvine, California, and the Department of Orthopedic Surgery, Osaka Medical College, Takatsuki, Osaka, Japan



Disclosure: One or more of the authors received payments or services, either directly or indirectly (i.e., via his or her institution), from a third party in support of an aspect of this work. In addition, one or more of the authors, or his or her institution, has had a financial relationship, in the thirty-six months prior to submission of this work, with an entity in the biomedical arena that could be perceived to influence or have the potential to influence what is written in this work. No author has had any other relationships, or has engaged in any other activities, that could be perceived to influence or have the potential to influence what is written in this work. The complete Disclosures of Potential Conflicts of Interest submitted by authors are always provided with the online version of the article.

Copyright © 2012 by The Journal of Bone and Joint Surgery, Inc.
J Bone Joint Surg Am, 2012 Sep 05;94(17):1576-1583. doi: 10.2106/JBJS.J.01972
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Abstract

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.

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    These activities have been planned and implemented in accordance with the Essential Areas and policies of the Accreditation Council for Continuing Medical Education (ACCME) through the joint sponsorship of the American Academy of Orthopaedic Surgeons and The Journal of Bone and Joint Surgery, Inc. The American Academy of Orthopaedic Surgeons is accredited by the ACCME to provide continuing medical education for physicians.
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