Purpose: Previous animal experiments showed how reduced masticatory function plays an important role in the craniofacial and mandibular regions in the growing period. However, articles focusing on the effects of reduced masticatory function in adult animals are quite scarce. The purpose of this study was to evaluate the effects of reduced masticatory muscle activity on the jaw bones of adult rats. Materials and Methods: Botulinum neurotoxin type A (BTX-A) was injected into the left masseter muscle to reduce masticatory muscle function, and sequential changes in the jaw bones were investigated. Ten male 60-day-old Sprague-Dawley rats with a mean body weight of about 300 g were used. The following procedures were performed on each sample: 25 U/mL (0.3 mL) of BTX-A was injected into the left masseter muscle, whereas the right masseter muscle received an equal amount of sterilized saline solution. Ninety days after the procedure, the rats were sacrificed, and the weights of the masseter muscles were recorded. Meanwhile, direct anthropometric measurements were taken, and the cortical thickness, trabecular thickness, and bone mineral content of the skull and mandible were measured. All measurements were evaluated with statistical software. Results: There were significant differences in all measurements on the BTX-A side compared with those on the control side. The differences included a decreased ramus height, increased gonial angle, and increased crown height of the posterior teeth on the injection side. Significant decreases in the muscle weight, bone mineral content, cortical thickness, and trabecular thickness were also seen on the paralyzed side compared with the non-injection side. Conclusions: We found that reduced masticatory function in adult rats affected the weight of the masseter muscle and the bony structure and dentition.
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