Atrophy of the masseter muscle during preoperative orthodontic treatment was observed greater in patients with increased open bite due to improved dental compensation in patients with skeletal class III dentofacial deformities with maxillary retraction. Masticatory muscles have been evaluated using various imaging modalities such as CT, magnetic resonance imaging, and ultrasonography [
1,
2,
5,
7‐
16]. In the current study, CT was performed before the start of preoperative orthodontic treatment and before orthognathic surgery for planning treatment. We selected multi-slice CT images for evaluating the masseter muscle pre- and posttreatment, because reproducible image evaluation was possible not only of soft tissues and muscles but also of hard tissue landmarks [
14]. With regard to masseter muscle cross-sectional area determined via magnetic resonance imaging and CT in participants with normal craniofacial morphologies, reported values range from 363 to 500 mm
2 [
8,
10,
11,
13]. In the present study, cross-sectional area of the masseter muscle of both sides of almost patients were within this previously reported range before the start of preoperative orthodontic treatment. However, immediately before surgery, they were below this range. The correlations of masseter muscle cross-sectional area with maximum occlusal force and with masticatory function have been reported [
6,
10]. According to these reports, decreased posttreatment masseter muscle cross-sectional area could lead to decreasing masticatory function. Katsumata et al. [
12] reported that masseter muscle cross-sectional area was lower in skeletal class III patients with dentofacial deformities who underwent sagittal split ramus osteotomy and intraoral vertical ramus osteotomy using three-dimensional CT imaging. Kikuta et al. [
17] reported that occlusal force was decreased 3 months after orthognathic surgery, but increased 6 months after the surgery. The results of the present study suggest that particular attention should be paid to masseter muscle atrophy in patients with worse open bite after preoperative orthodontic treatment and in those with maxillary undergrowth. However, it is not clear if masticatory ability would be compromised by masseter muscle atrophy immediately after the surgery. Decreased maximum occlusal force in patients with open bite has been reported [
18], which supports our result that increased open bite led to decreased masseter muscle cross-sectional area. In this study, smaller SNA angle was related to smaller masseter muscle cross-sectional area, which indicated the association between mandibular prognathism and maxillary retrusion. Further investigations analyzing the relationships between skeletal morphology and changes in masticatory muscles are required.
The cells forming skeletal muscles are muscle cells, and they enclose myofibrils. Myofibrils are composed of myosin and actin filaments, which have two heavy chains and four light chains. Myosin heavy chains are generally classified into slow muscle fiber type I and fast muscle fiber type II [
19]. Although there are individual differences in the composition of human masseter muscle fibers [
20‐
22], more than half of the muscle fibers are type I fibers. Furthermore, myofibrils are affected by maxillomandibular skeletal morphology [
23‐
25]. Rowlerson et al. [
24] reported that the proportion of type II fibers increases in overcapped cases and decreases in open bite cases. Human masseter muscle may contain embryonic myosin heavy chains and fetal (neonatal) myosin light chains, which are specific myosin isoforms evident during the early development of muscles of the trunk and extremities [
26]. It has been reported that the masseter muscle has excellent regenerative capacity [
27]. In the masseter muscle after orthognathic surgery, type I fibers were reduced and type II fibers were increased [
28], indicating that the masticatory muscle may be affected by environmental factors. Fiber type properties are closely associated with variations in vertical growth of the face, statistically significantly with respect to overall comparisons. Increases in masseter muscle type II fiber areas and percentages of tissue are reportedly inversely related to increases in vertical facial dimensions [
20]. Facial biotype characteristics that define vertical facial skeletal pattern affect the cortical bone thickness of mandibular condyle [
29]. Type II fibers may be especially reduced in reduced over bite, because in the present study, the atrophy of masticatory muscle cross-sectional area was greater in cases wherein open bite progressed due to preoperative orthodontic treatment. Notably however, pathological examination is needed to confirm this. The potentiality of 3D imaging technology applied to CBCT for the analysis of the skeletal component in this kind of studies was reported [
30,
31]. We plan to follow the patients enrolled in the present study and monitor the changes in masseter muscle cross-sectional area after orthognathic surgery using CBCT.