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A Cadaveric Model of the Throwing Shoulder: A Possible Etiology of Superior Labrum Anterior-to-Posterior Lesions
Mark G. Grossman, MD1; James E. Tibone, MD3; Michelle H. McGarry, MS2; David J. Schneider, MD2; Sergio Veneziani, MD2; Thay Q. Lee, PhD2
1 Department of Orthopaedic Surgery, Winthrop University Hospital, 120 Mineola Boulevard, Suite 600, Mineola, NY 11501. E-mail address: mgrossman@winthrop.org
3 Orthopaedic Surgery Associates, University of Southern California, 1510 San Pablo Street, Suite 322, Los Angeles, CA 90033
2 Orthopaedic Biomechanics Laboratory, VA Long Beach Healthcare System (09/151), University of California Irvine, 5901 East 7th Street, Long Beach, CA 90822. E-mail address for T.Q. Lee: tqlee@med.va.gov
View Disclosures and Other Information
In support of their research or preparation of this manuscript, one or more of the authors received grants or outside funding from the Department of Veterans Affairs and Kerlan Jobe Orthopaedic Foundation. None of the authors received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, educational institution, or other charitable or nonprofit organization with which the authors are affiliated or associated.
Investigation performed at Orthopaedic Biomechanics Laboratory, VA Long Beach Healthcare System and University of California Irvine, Long Beach, California

The Journal of Bone and Joint Surgery, Incorporated
J Bone Joint Surg Am, 2005 Apr 01;87(4):824-831. doi: 10.2106/JBJS.D.01972
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Background: It has been speculated that a shift of the throwing arc commonly develops in athletes who perform overhead activities, resulting in greater external rotation and decreased internal rotation caused by anterior capsular laxity and posterior capsular contracture, respectively. Osseous adaptation in the form of increased humeral and glenoid retroversion may provide a protective function in the asymptomatic athlete but cannot explain the pathological changes seen in the shoulder of the throwing athlete. Therefore, the objective of the present study was to examine the biomechanical effects of capsular changes in a cadaveric model.

Methods: Ten cadaveric shoulders were tested with a custom shoulder-testing device. Humeral rotational range of motion, the position of the humerus in maximum external rotation, and glenohumeral translations in the anterior, posterior, superior, and inferior directions were measured with the shoulder in 90° of abduction. Translations were measured with the humerus secured in 90° of external rotation. To simulate anterior laxity due to posterior capsular contracture, the capsule was nondestructively stretched 30% beyond maximum external rotation with the shoulder in 90° of abduction. This was followed by the creation of a 10-mm posterior capsular contracture. Rotational, humeral shift, and translational tests were performed for the intact normal shoulder, after anterior capsular stretching, and after simulated posterior capsular contracture.

Results: Nondestructive capsular stretching resulted in a significant increase in external rotation (average increase, 18.2° ± 2.1°; p < 0.001), and subsequent simulated posterior capsular contracture resulted in a significant decrease in internal rotation (average decrease, 8.8° ± 2.3°; p = 0.02). There also was a significant increase in anterior translation with the application of a 20-N anterior translational force after nondestructive capsular stretching (average increase, 1.7 ± 0.3 mm, p = 0.0006). The humeral head translated posteroinferiorly when the humerus was rotated from neutral to maximum external rotation. This did not change significantly in association with anterior capsular stretching. Following simulated posterior capsular contracture, there was a trend toward a more posterosuperior position of the humeral head with the humerus in maximum external rotation in comparison with the position in the stretched conditions, although these differences were not significant.

Conclusions: A posterior capsular contracture with decreased internal rotation does not allow the humerus to externally rotate into its normal posteroinferior position in the cocking phase of throwing. Instead, the humeral head is forced posterosuperiorly, which may explain the etiology of Type-II superior labrum anterior-to-posterior lesions in overhead athletes.

Clinical Relevance: Understanding the biomechanical effects of capsular changes in a cadaveric model of the throwing shoulder may confirm clinical observations and provide insight into the pathological changes often seen in the shoulder of throwing athletes.

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    Accreditation Statement
    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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