The Journal of Bone and Joint Surgery (American) 86:1973-1982 (2004)
© 2004 The Journal of Bone and Joint Surgery, Inc.
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 and
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{at}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
Investigation performed at the Department of Orthopaedic Surgery,
University of Zürich, Zürich, Switzerland
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.
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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