Abstract
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.
Highlights
The bone-muscle relationship may be viewed in terms of structure and function (Cianferotti and Brandi, 2014)
We need to consider the simultaneous inhibition of the major masticatory closing muscles, including the masseter, temporal, and medial pterygoid muscles, in order to observe the genuine effect of masticatory muscular inactivity on mandibular growth
To infer how human mandibular growth would be impacted by masticatory muscle hypotrophy, we looked for an animal model with craniofacial growth pattern similar to that of human
Summary
The bone-muscle relationship may be viewed in terms of structure and function (Cianferotti and Brandi, 2014). Some studies have reported aberrant craniofacial structure after experimental muscular function changes (Boyd et al, 1967; Navarro et al, 1995). These growths were influenced by the secondary effects of scar tissues and their subsequent contractures. The inhibition of mandibular growth following treatment of BTX to the masticatory muscles has already been addressed (Sakurai et al, 2007; Tsai et al, 2009; Kun-Darbois et al, 2015), the results of these studies may not be comprehensive in that their muscular environments were unnatural, with singular muscular paralysis and two-dimensional analyses. We need to consider the simultaneous inhibition of the major masticatory closing muscles, including the masseter, temporal, and medial pterygoid muscles, in order to observe the genuine effect of masticatory muscular inactivity on mandibular growth
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