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Effect of Tendon Release and Delayed Repair on the Structure of the Muscles of the Rotator Cuff: An Experimental Study in Sheep
C. Gerber, MD1; D.C. Meyer, MD1; A.G. Schneeberger, MD1; H. Hoppeler, MD2; B. von Rechenberg, DVM3
1 Department of Orthopaedic Surgery, University of Zürich, Balgrist, Forchstrasse 340, CH-8008 Zürich, Switzerland. E-mail address for C. Gerber: christian.gerber@balgrist.ch
2 Department of Anatomy, University of Bern, Bühlstrasse 26, CH-3012 Bern, Switzerland
3 Musculoskeletal Research Unit, Equine Hospital, University of Zürich, Winterthurerstrasse 260, CH-8057 Zürich, Switzerland
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 ResOrtho Foundation, Zurich, Switzerland, and Swiss National Science Foundation Grant 32-59553.99. 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 the Department of Orthopaedic Surgery, University of Zürich, Zürich, Switzerland

The Journal of Bone and Joint Surgery, Incorporated
J Bone Joint Surg Am, 2004 Sep 01;86(9):1973-1982
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Background: Ruptures of the tendons of the rotator cuff lead to profound and possibly irreversible changes in the structure and physiological properties of the rotator cuff muscles. Muscle atrophy and fatty infiltration are important prognostic factors that affect the natural history and outcome of treatment. The purpose of this study was to examine the amount of muscle atrophy and fatty infiltration in an animal model and to determine whether the repair of a long-standing tendon tear can reverse these changes.

Methods: The infraspinatus tendon in six sheep was released and encased in a silicone tube to prevent spontaneous healing. The musculotendinous unit was allowed to retract for forty weeks. Throughout this period, the muscular changes were studied with use of computed tomography, histological analysis, and electron microscopy. At forty weeks, the elasticity, intramuscular pressure, and perfusion were measured intraoperatively and a tendon repair was carried out. The structural changes of the muscle were studied for thirty-five weeks after the repair. The animals were then killed, and the musculotendinous units were examined macroscopically and by computed tomography, histological analysis, and electron microscopy.

Results: At the time of the tendon release, the infraspinatus showed no fatty changes. The force needed to cause a tendon excursion of 1 cm was a mean (and standard deviation) of 6.8 ± 1 N. The application of tension on the tendon did not alter the perfusion and decreased the intramuscular pressure. After the tendon release, muscular atrophy developed and there was a significant increase (p < 0.001) in interfascicular and intrafascicular fat, representing fatty infiltration rather than fatty degeneration. Furthermore, there was an increase of interstitial connective tissue. At the time of the tendon repair, between forty and forty-two weeks after the release, there was a sevenfold poorer elasticity of the musculotendinous unit but preserved muscle perfusion. The structural changes increased six weeks after the repair and then recovered partially at twelve and thirty-five weeks thereafter but only to the amount demonstrated before the repair.

Conclusions: Musculotendinous retraction induced by tendon release is associated with profound changes in the structure and function of the affected muscle. Vascularization, intramuscular pressure, and individual fiber composition are not markedly affected, and muscle fibers do not appear to degenerate. However, muscle atrophy, infiltration by fat cells, and an increase of interstitial connective tissue lead to impairment of the physiological properties of the muscle. These changes were irreversible under the conditions of this experiment with the repair technique used.

Clinical Relevance: Rotator cuff tendon tears lead to substantial and progressive muscular changes with a severity that is proportional to the amount of musculotendinous retraction. If muscular function is to be preserved, a repair may need to be performed before marked retraction has occurred or new or different techniques for repair need to be developed.

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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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