COPYRIGHT © 2004 BY THE JOURNAL OF BONE AND JOINT SURGERY, INCORPORATED Effect of Tendon Release and Delayed Repair on the Structure of

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 spontane-ous 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, histologi-cal 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 de