# Anatomical Characteristics of the Masseter Muscle in
## 4 Discussion  
In our study, we used a three-dimensional computed tomography (3D CT) to analyze the relationship between the mandibular skeletal units and masseter muscle. Numerous studies in the past have described an association between craniofacial morphology and the surrounding muscles, but these studies were performed using electromyography or ultrasonography [25,26]. The development of 3D CT made it possible to measure the volume and orientation of the muscle and skeletal units. Our previous study analyzed the differences in the mandibular skeletal unit volume/length ratio between a normal group and a prognathic group [15]. According to this study, the total mandible ratio and the condylar part and body part of the mandible ratio, which is the volume divided by length, were significantly smaller in the prognathic group, which implies that the condyle and body may be the key units underlying the etiology of mandibular prognathism. We aimed to evaluate the force of the masseter muscle that attaches to the body of the mandible. Our study revealed significantly greater masseter muscle volume/length ratio in the participants of the normal group than in the prognathic group. As tension develops during muscle contraction, which correlates to muscle thickness, the muscle thickness of the masseter is significantly correlated to the bite force and occlusal tooth contact [25]. Therefore, the difference in masseter thickness may be attributed to the difference in muscle contraction force. This result corresponds to other studies that have elucidated that the \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & & **Normal Group** & **Organathic Group** & \multirow{2}{*}{_p_**Value**} \\ \cline{3-4}  & & **(n = 30)** & **(n = 30)** & \\ \hline \multirow{3}{*}{**Masseter** (Superficial)} & Palatal plane & 68.6 \(\pm\) 11.7 & 78.7 \(\pm\) 8.2 & 0.000 * \\ \cline{2-4}  & Occlusal plane & 74.2 \(\pm\) 8.6 & 80.7 \(\pm\) 7.0 & 0.002 * \\ \cline{2-4}  & Mandibular Plane & 80.4 \(\pm\) 10.2 & 68.0 \(\pm\) 10.1 & 0.000 * \\ \hline \multirow{3}{*}{**Masseter (Deep)**} & Palatal plane & 70.7 \(\pm\) 12.0 & 77.4 \(\pm\) 9.3 & 0.018 * \\ \cline{2-4}  & Occlusal plane & 74.3 \(\pm\) 9.3 & 78.8 \(\pm\) 9.1 & 0.066 \\ \hline \hline \end{tabular} Note: Values are presented as mean \(\pm\) SD; Abbreviation: SD, standard deviation; _p_: _p_-values of the paired _t_-test; *_: \(p\) \(<\) 0.05.  
\end{table} 
Table 2: Angle between the masseter muscle and the reference planes. Figure 6: The distribution of the angle of deep masseter muscles to three planes: palatal, occlusal, and mandibular. Angles to palatal and mandibular planes showed significant differences. The participants of the normal group had a more vertical angle to the mandibular plane than those of the prognathic group. masseter muscle is large in brachycephalic, short- faced, and narrow-mandibular-angled participants [27; 28]. We studied the orientation of the masseter muscle and calculated the angles from the three reference planes. We chose these three planes because the palatal plane is related to the origin of the masseter muscle, the occlusal plane is where the maxillary and mandibular teeth meet, and the mandibular plane is related to the insertion of the masseter muscle. Among them, we focused on the mandibular plane because it is where the force actually acts and is the only plane that moves following the mandibular movement. We found that the angles to the mandibular plane of both the superficial and deep masseeter muscles of participants in the normal group were closer to the right angle than those of the prognathic group. This suggests that the participants in the normal group had a mechanical advantage over those in the prognathic group, since the masseter muscle is orientated vertically along the line of action of the mandible [11; 29]. Additionally, studies revealed that the bite force and the occlusal contact area increases after surgical correction in prognathic participants, which supports our analysis [17; 29; 30].