Morphologic and biochemical changes of the masseter muscles induced by occlusal wear: Studies in a rat model

Occlusal alterations may result in changes in the functional performance of masticatory muscles. In this study, we set up an experimental model in rat s to examine whether masticatory muscle abnormalities occur after a maloccl usion is induced. Rats underwent unilateral amputation of the molar cusps t o simulate an occlusal wear situation. The masseter muscles ipsilateral and contralateral to the amputated molars were excised at different experiment al times. Sham-operated rats-were used as controls, The tissue samples were studied by light and electron microscopy and morphometry. Tissue calcium c ontent, a biochemical index of muscle injury, was also determined. The resu lts show that occlusal dysfunction leads to microvessel constriction and cl ear-cut morphologic damage of muscular fibers and blood capillary endotheli um, as well as to elevation of tissue calcium content, in the ipsilateral m asseter muscle. These changes are likely related to muscle fatigue and isch emia. The early signs of injury do not involve the entire muscle but are mo stly restricted to tissue areas rich in type I(slow) muscle fibers, which a re characterized by a predominantly aerobic metabolism. The muscle damage b ecomes more extended and severe with time. On the other hand, the contralat eral muscles show only slight alterations which are reversible with time, p ossibly due to an adaptive response.