Journal of Dental Research, Vol 78, 1735-1744, Copyright © 1999 by International & American Associations for Dental Research Online Journals
Morphologic and biochemical changes of the masseter muscles induced by occlusal wear: studies in a rat model
D. Bani, T. Bani and M. Bergamini
Department of Anatomy, Histology and Forensic Medicine, School of Dentistry, University of Florence, Italy.
Occlusal alterations may result in changes in the functional performance of
masticatory muscles. In this study, we set up an experimental model in rats
to examine whether masticatory muscle abnormalities occur after a
malocclusion is induced. Rats underwent unilateral amputation of the molar
cusps to simulate an occlusal wear situation. The masseter muscles
ipsilateral and contralateral to the amputated molars were excised at
different experimental times. Sham-operated rats were used as controls. The
tissue samples were studied by light and electron microscopy and
morphometry. Tissue calcium content, a biochemical index of muscle injury,
was also determined. The results show that occlusal dysfunction leads to
microvessel constriction and clear-cut morphologic damage of muscular
fibers and blood capillary endothelium, as well as to elevation of tissue
calcium content, in the ipsilateral masseter muscle. These changes are
likely related to muscle fatigue and ischemia. The early signs of injury do
not involve the entire muscle but are mostly restricted to tissue areas
rich in type I (slow) muscle fibers, which are characterized by a
predominantly aerobic metabolism. The muscle damage becomes more extended
and severe with time. On the other hand, the contralateral muscles show
only slight alterations which are reversible with time, possibly due to an
adaptive response.
