Effect of masseter hypofunction on mandibular growth and masticatory muscle coordination: an in vivo study of animal model.

It has been considered that dysfunction of masticatory muscles could affect growth and development of maxillofacial morphology. However, effects of reduced masticatory muscle functions on mandibular growth and changes in the coordination mechanism between individual muscles have not been fully investigated. This study aimed to investigate the association between masticatory function and mandibular growth and then clarify the roles of individual masticatory muscles and their compensation mechanisms. Sixteen general purpose mice received botulinum neurotoxin type A (BoNT⁄A) unilaterally into the right masseter to induce muscle hypofunction, and the left side was used as a control side. Micro-computed tomography was used to evaluate morphometric changes in the mandible at 3 and 9 weeks of age. The amount of growth for each measurement item was obtained by subtracting the measured value at 3 weeks of age from that at 9 weeks of age, and then the results were compared between the BoNT⁄A-injected side and the control side. A paired t-test was used for statistical analysis, and P < 0.05 was considered significant. Growth changes in mandibular body length, condylar width and coronoid process length were significantly smaller on the BoNT⁄A-injected side than on the control side, whereas the change in ramus height was larger on the BoNT⁄A-injected side than on the control side. Mandibular growth was strongly affected by alterations in masticatory function. The present study suggests that the temporalis muscle cannot compensate for hypofunction of the masseter muscle, but the internal pterygoid muscle can.