Dento-skeletal adaptation after bite-raising in growing rats with different masticatory muscle capacities.

The aim of this study was to analyse the effects of normal and hypofunctional masticatory muscles on dento-skeletal adaptation to posterior bite blocks in growing rats. Fifty-two young male rats were divided into two groups, fed a hard and soft diet, respectively, to develop different functional capacities in the masticatory muscles. Bone markers were inserted in the mandible on day 0. After two weeks, an appliance that raised the bite by 2 mm was inserted in half of each group. Lateral radiographs were taken on day 0, 14, 28, and 42 of the experiment. Images of the mandible were superimposed on the bone markers. Differences in cephalometric measurements were analysed by two-way ANOVA. The reduced muscle capacity resulted in an upward growth of the snout and a shorter mandibular ramus with less bone apposition on its lower border. Bite blocks induced a more upward growth of the snout and a shorter mandibular ramus, and inhibited the eruption of the upper molars and intruded the lower molars. The rats with weaker masticatory muscles had less inhibitory effect of the posterior bite blocks on upper molar eruption and showed different bone apposition in the ramus, especially during the first two weeks. In conclusion, masticatory muscle capacity seems to influence the effect of the posterior bite blocks on both tooth eruption and skeletal adaptation. The results suggest that the characteristics of the masticatory muscles should be taken into account when predicting the efficiency of a functional appliance.