Masseter Muscle Properties Differ between the Left and Right Sides in Mandibular Class III Patients with Asymmetry

Abstract

In this study on muscle characteristics and jawbone morphology, the relationship between masticatory muscle properties and jaw deformity was analysed in skeletal Class III patients with asymmetry. The subjects were 27 patients (21 women and 6 men) who had skeletal mandibular prognathism with mandibular asymmetry. The subjects’ ages ranged from 16 to 51 years, with an average of 23.5 years. Patients with lateral displacement of Me 3 mm or more and an ANB angle of 0 ° or less were included. Morphometric measurements were taken from CT images. Biopsy material was collected during surgery for immunohistological typing of muscle fibres, MyHC1, 2a, and 2x isoforms; measurement of MYH1, 2, 7, 8 mRNA expression. In CT morphometry, there was no difference in muscle thickness or cross-sectional area between the deviated and non-deviated sides, but both sides tended to have thinner muscles than the control group. In the typing of muscle fibres by immunohistochemical staining, type 2 fibres were found to be significantly more numerous on the deviated side than on the non-deviated side. Regarding protein quantities, MyHC1 was more abundant on the deviated side than on the non-deviated side. Regarding the expression of mRNA, MYH7 (type 1) and MYH1 (type 2x) showed no difference between the two sides, and MYH2 (type 2a) had a higher expression level on the deviated side than in the control group. The data suggested a tendency for Class III patients to have characteristics of a short face on the deviated side, and a long face on the non-deviated side. It seems that muscle type switching may be in progress.