RESUMO
The purpose of this study was to investigate the relationship between masticatory muscular hypotrophy and mandibular growth in juvenile nonhuman primates (cynolmolgus monkeys, Macaca fasicularis). We hypothesized that botulinum toxin (BTX)-induced neuro-muscular junctional block and its resultant hypotrophy of masticatory muscles would produce mandibular growth disturbances in size and shape. Ten male cynomolgus monkeys were divided into three groups: group I (control; n = 3), group II (unilateral BTX; n = 4), and group III (bilateral BTX; n = 3). The unilateral or bilateral muscular hypotrophy of major masticatory closing muscles was induced by synchronous BTX application to masseter, medial pterygoid, and temporal muscle. Mandibular growth was tracked by linear, angular, area and volume measurements using three-dimensional (3D) computed tomography imaging before BTX treatment and after 3 and 6 months. After unilateral hypotrophy of masticatory muscles in group II, vertical growth deficiency was prominent on the BTX side, with compensatory overgrowth on the control side. The bilateral muscular hypotrophy in group III also showed smaller ramal height and width than that of control (group I) and control side (group II). Moreover, ramal sagittal angles (posterior tilt) increased on the BTX side of both groups II and III, but coronal angles (lateral tilt) did so on the BTX side of group II, resulting in asymmetry. The results confirmed our hypothesis that functional activity of masticatory closing muscles is closely related to mandibular growth in size and shape of juvenile nonhuman primates. In addition, the focused growth disturbances on the ramal height and posterior-lateral tilt suggested the possible role of masticatory closing muscles for ramal vertical and angular growth vector of the mandible.
RESUMO
OBJECTIVE: The purpose of this study was to investigate changes in the dentoalveolus and occlusal plane associated with the hypotrophy of unilateral masticatory muscles following botulinum toxin (BTX) treatment in the juvenile period of rats. DESIGN: We hypothesized that the loss of functional loading of masticatory muscles and occlusal force invites compensatory dentoalveolar supraeruption, accelerating occlusal cant and skeletal asymmetry. In order to confirm this hypothesis, six-week-old male rats (N = 5) were treated with BTX simultaneously at the unilateral masseter, temporalis, and medial pterygoid muscles, with a booster injection after six weeks for the experimental group. The control group (N = 6) had saline injections on both sides at the same sites and on the same schedule. RESULTS: After 12 weeks, masseter and medial pterygoid muscles on the BTX side showed hypotrophic change. The mandibular structure was asymmetrical, with decreased size and lateral tilting. The maxillary and mandibular molars were supraerupted from the Frankfort plane or mandibular inferior border with lateral tilt. They accompanied downward occlusal plane cant resulting from the supraerupted maxillary and mandibular molars on the BTX side. The dentoalveolar structural changes included diminished alveolar bone density, narrow periodontal ligament space, and disorganized distribution of periodontal collagen fiber. CONCLUSIONS: Unilateral hypotrophy of masticatory muscles affected the growth, symmetry, and structure of the skeletal jaws and dentoalveolus. Our hypothesis about the dentoalveolar compensation, that muscular hypotrophy was closely integrated with dentoalveolar supraeruption and an inclined occlusal plane, was confirmed.
