Response of masticatory muscles to passive stretch stimulus - from perspectives of functional appliances

Objective: The aims of this study were to examine whether a passive stretch stimulus by means of a functional appliance induces changes in the fiber composition of masticatory muscles and whether these changes are similar to the changes in stretched limb muscle fi bers by using RT-PCR, western blot, and immunohistochemical assays. Methods: Five male New Zealand White rabbits were fi tted with a prefabricated inclined plane on the maxillary central incisors to force the mandible forward (- 2 mm) and downward (- 4 mm). Further, 1 hind limb was extended and constrained with a cast so that the extensor digitorum longus (EDL) was stretched when the animal used the limb. The animals were sacrifi ced aft er 1 week and the masseter, lateral pterygoid, and EDL were processed and compared with those from control animals (n = 3). Results: The stretched EDL had a significantly higher percentage of slow fibers, whereas the stretched masticatory muscles did not show changes in the composition of the major contractile proteins aft er 7 days. Conclusions: Th e transition of fi ber phenotypes in response to a stretch stimulus may take longer in the masticatory muscles than in the limb muscles. [Korean J Orthod 2012;42(2):64-72]