(23) Relationship Between Temporal and Masseter Muscle Status and Skeletal Patterns with Facial Asymmetry

Abstract: Statements of the problem
Facial morphology is the result of complex integration between genetics and epigenetics. Based on the theory of the functional matrix, the bones do not grow but are grown and regulated by the surrounding soft tissues. Each mandibular unit is related to a different functional matrix, such as the angular unit and the masseter and medial pterygoid muscle, and the coronoid unit and the temporalis muscle. Among the masticatory muscles, the masseter and temporalis muscles not only play roles in regulating the growth pattern, but also serve as reliable structures for diagnosis of temporomandibular disorder (TMD).
In facial asymmetry, differences in jaw muscle characteristics, such as occlusal force, reflex status, and electromyography between the deviated and non-deviated sides have been reported. Based on our knowledge, however, there are few studies concerning whether the masseter and temporalis size and scheme are different among the three facial skeletal types in asymmetric patients. Mandibular asymmetry shows a higher incidence of temporomandibular joint symptoms than other jaw Mandibular asymmetry shows a higher incidence of temporomandibular joint symptoms than other jaw.

The purpose
The purpose of the current study was to evaluate the three-dimensional (3-D) volume and surface area of the masseter and temporalis muscles, including the thickness of the masseter, to determine the relationship between the masseter and temporalis muscle status and facial asymmetry. Moreover, the subjects were sub-classified and differences in scheme compared according to various facial skeletal classifications.

Materials and methods
This retrospective study included 83 patients who underwent facial computed tomography from January 2011 to December 2020. The patients were classified in either the “control” and “asymmetry” group according to the degree of facial asymmetry. Patients that belonged to the asymmetry group were sub-classified as skeletal class I, class II, and class III. The volume and surface area of the temporalis and masseter muscles, including the bilateral thickness of the masseter muscle, were measured twice. Digital Imaging and Communications in Medicine were analyzed using 3-D analysis software (Mimics Research 22.0, Materialise, Belgium). Reliability and comparative analyses were conducted according to groups.

Results of investigation & Outcomes data
There was no significant difference between the bilateral sides for all variables in the control group. In the asymmetry group, all variables on the non-deviated side were significantly larger (p < 0.05), except for masseter muscle thickness. The masseter volume of the non-deviated side was significantly larger than that of the deviated side in skeletal class I (p=0.020) and class III (p=0.011). However, there was no significant difference between the deviated and non-deviated sides in class II (p=0.355). Regarding masseter thickness, there was a significant difference only in class II, which was thicker on the deviated side (p=0.022). (Table 1)

Conclusions relevant to the problem
The different volumes of masseter and temporalis muscles should be considered in facial asymmetry patients while the clinician considers orthognathic surgery to correct the facial deformity.

List of two references
Moss ML. A theoretical analysis of the functional matrix. Acta Biotheoretica. 1968;18(1):195-202. Schiffman E, Ohrbach R, Truelove E, et al. Diagnostic criteria for temporomandibular disorders (DC/TMD) for clinical and research applications: recommendations of the International RDC/TMD Consortium Network and Orofacial Pain Special Interest Group. Journal of oral & facial pain and headache. 2014;28(1):6.

Table 1. Comparison between sides in asymmetry group according to facial skeletal classification