# Anatomical Characteristics of the Masseter Muscle in
## 1 Introduction  
In the oral and maxillofacial field, mandibular pragnathism is considered to be one of the major issues since it causes functional and esthetic problems, which cannot be treated without surgery in most adults [1, 2]. However, in young patients, it is possible to prevent or relieve mandibular pragnathism by regulating the growth pattern, and predicting mandibular growth and the timing of treatment, which are considered to be the most important factors [3, 4]. Therefore, many studies have been carried out to understand mandibular growth and factors that affect it.  
In 1968, [PERSON] first described the functional units of the mandible to be composed of symphysis, body, angle, coronoid, and dentoalveolus [5]. He proposed the concept of a functional matrix, which is the soft tissue surrounding the bone, and that a functional matrix regulates the growth of bone [6]. Several studies have reported the relationship between the masticatory muscles and craniofacial morphology [7, 8, 9]. [PERSON] et al. showed that wider mandibular shape is associated with larger masticatory muscle force [10]. [PERSON] associated brachycephalic pattern with a strong bite force and the oldichocephalic pattern with a relatively weaker bite force [11]. [PERSON] and [PERSON] described the relationship between brachycephalic morphology and large masticatory muscles [12, 13]. [PERSON] suggested that the combined action of the mandibular muscles can induce a specific stress pattern and mandibular remodeling [14].  
In our previous study, the volumetric characteristics of normal and prognathic mandibles were demonstrated by skeletal unit analysis [15]. The results of the study showed that the mandibular body and condyle of prognathic mandibles had a statistically lower volume/length ratio than that of normal mandibles. As the masseeter muscle attaches to the mandibular body and the lateral pterygoid muscle attaches to the condyle, we assumed that these two muscles are mainly associated with mandibular growth in prognathism.  
There are four bilateral masticatory muscles, and the masseeter muscle is one of them. It originates from the zygomatic arch and inserts into the angle of the mandible. It is derived from a single muscle mass at gestational day 11, matures with the branching of the trigeminal nerve, and establishes complexity before birth [16]. During this period, differentiation occurs during the 22 nd week of gestation, following that of the skeletal muscles of the trunk and limb [17].  
In this study, we compared the masseeter muscle of the normal mandible with that of the prognathic one. To evaluate the force of the muscle, we investigated the volume/length ratio and the orientation to the mandibular plane, the former for the magnitude of the force and the latter for the direction of the force. We compared the orientation of the masseeter muscle to three planes: palatal, occlusal, and mandibular. The palatal and mandibular planes indicate the origin and insertion of the masseeter muscle, respectively. The occlusal plane indicates where upper and lower teeth meet. We assumed that the masseeter muscle of the normal group would be thicker and more vertical with respect to the mandibular plane, which means that the masseeter muscle of the participants in the normal group would have a mechanical advantage over those of the prognathic group.