Scientific Article   |    
Biomechanical Comparison of Effects of Supraspinatus Tendon Detachments, Tendon Defects, and Muscle Retractions
A. M. Halder, MD; S. W. O'Driscoll, PhD, MD; G. Heers, MD; N. Mura, MD; M. E. Zobitz, MS; K. N. An, PhD; R. Kreusch-Brinker, MD
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Investigation performed at the Biomechanics Laboratory, Division of Orthopedic Research, Mayo Clinic, Rochester, Minnesota

A.M. Halder, MD
S.W. O'Driscoll, MD, PhD
G. Heers, MD
N. Mura, MD
M.E. Zobitz, MS
K.N. An, PhD
Biomechanics Laboratory, Division of Orthopedic Research, Mayo Clinic, 200 First Street S.W., Rochester, MN 55905. E-mail address for S.W. O'Driscoll: odriscoll.shawn@mayo.edu

R. Kreusch-Brinker, MD
Department of Orthopedics, Asklepios-Klinik, Birkenwerder, Hubertusstrasse 12-22, 16547 Birkenwerder, Germany

In support of their research or preparation of this manuscript, one or more of the authors received grants or outside funding from National Institutes of Health (NIAMS [National Institute of Arthritis and Musculoskeletal and Skin Diseases]) AR41171. 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.

J Bone Joint Surg Am, 2002 May 01;84(5):780-785
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Background: Rotator cuff ruptures are frequently associated with loss of strength of the shoulder. However, the characteristics of the rotator cuff tear that are responsible for the loss of force generation and transmission have not yet been identified. The purpose of this study was to compare the effects of supraspinatus tendon detachments, tendon defects, and muscle retractions on in vitro force transmission by the rotator cuff to the humerus.

Methods: The rotator cuff tendons from ten cadaver shoulders were loaded proportionally to the respective cross-sectional areas of their muscles. A fiberglass rod was cemented into the medullary canal of the humerus and connected to a three-component load cell for the measurement of the forces transmitted by the rotator cuff to the humerus. This study was performed with the humerus in a hanging arm position and with various sizes of supraspinatus tendon detachments, tendon defects, and muscle retractions.

Results: Detachment or creation of a defect involving one-third or two-thirds of the supraspinatus tendon resulted in a minor reduction in the force transmitted by the rotator cuff (≤5%), while detachment or creation of a defect involving the whole tendon resulted in a moderate reduction (11% and 17%, respectively). Simulated muscle retraction involving one-third, two-thirds, and the whole tendon resulted in losses of torque measuring 19%, 36%, and 58%, respectively. Side-to-side repair of the one-third and two-thirds defects nearly restored the force transmission capability, whereas a deficit remained after side-to-side repair following complete resection.

Conclusions: Our results support the rotator cable concept and correspond to the clinical observation that patients with a small rupture of the rotator cuff may present without a loss of shoulder strength. Muscle retraction is potentially an important factor responsible for loss of shoulder strength following large rotator cuff ruptures.

Clinical Relevance: Supraspinatus muscle retraction diminishes glenohumeral abduction torque significantly more than either a defect in the tendon or a simple detachment of the tendon from the tuberosity. In cases of irreparable defects, side-to-side repair may be worthwhile to restore muscle tension and the integrity of the rotator cable.

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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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