Alterations to mandibular form following motor denervation of the masseter muscle. An experimental study in the rat.

The form of the mandible in young rats was studied following denervation of the masseter muscle. Wistar-derived male rats aged 27 d were randomly assigned either to experimental, sham-operated or control groups. The main trunk and initial branches of the masseteric nerve were resected on one side in each experimental animal. The nerve was exposed, but not resected, in the sham-operated group. No surgery was performed on the control group. All animals were killed at 69 d of age, examined and standardised contact radiographs taken of the cleaned and dried mandibles. Radiographic measurements describing the form and size of the mandibles were compared. On the operated side in the experimental animals the masseter muscle was smaller and the temporalis muscle larger as compared with the unoperated side. The angular process and the subcondylar incisure were also smaller and the coronoid process was positioned more rostrally. The most striking feature was that relatively little change accompanied denervation of masseter in the growing experimental rats. It is concluded that some of the changes previously attributed to muscle denervation may be due to other causes.