Masseter function and skeletal malocclusion

Abstract

The aim of this work is to review the relationship between the function of the masseter muscle and the occurrence of malocclusions. An analysis was made of the masseter muscle samples from subjects who underwent mandibular osteotomies. The size and proportion of type-II fibers (fast) decreases as facial height increases. Patients with mandibular asymmetry have more type-II fibers on the side of their deviation. The insulin-like growth factor and myostatin are expressed differently depending on the sex and fiber diameter. These differences in the distribution of fiber types and gene expression of this growth factor may be involved in long-term postoperative stability and require additional investigations. Muscle strength and bone length are two genetically determined factors in facial growth. Myosin 1H (MYOH1) is associated with prognathia in Caucasians. As future objectives, we propose to characterize genetic variations using "Genome Wide Association Studies" data and their relationships with malocclusions.

Figure 1

Lateral cephalograms taken before orthognathic surgery in two subjects: one from the class III open bite group (A) and one from the class II deep bite group (B). The insert boxes (top left) show examples of immunostaining analysis of the masseter muscle samples of these subjects based on fast myosin isoform content: dark staining indicates type-II fibers and pale staining type-I fibers. The scale bar is 100 micrometers in both cases.

Figure 2

Comparison of mean fiber areas (left) and occupancies (right) of masseter muscle fiber types in male and female subjects by vertical skeletal malocclusion groups. Data from skeletal open bite (14 males and 36 females), normal vertical dimension (16 males and 42 females), and deep-bite malocclusion categories (16 males and 15 females). Bars represent one standard deviation of the mean. Significant differences are indicated by the asterisks as indicated.

Figure 3

Comparison of mean fiber areas of masseter muscle fiber types in male and female subjects by vertical skeletal malocclusion groups. Data for mean fiber area values in μm² for normal vertical dimension (16 males and 42 females) and deep-bite malocclusion categories (16 males and 15 females). Bars represent one standard deviation of the mean, and the significance of the difference between males and females is indicated by the asterisks: *P < 0.05; ***P less than or equal to 0.0004.

Figure 4

Anteroposterior cephalogram showing lateral deviation of the mandible’s midline (Md) towards the right side and compared to the maxilla’s midline (Mx). The right mandibular ramus (Ra) is also “shorter” than the left mandibular ramus (La).

Figure 5

Average relative expression of MYO1H in human muscle RNA. Data was determined from 3 assays, done in triplicate, for RNA from a commercial preparation of skeletal muscle (control), limb muscles (limb; n = 4) and masseter muscles (masseter; n = 3). Thymus RNA was the calibrator used in calculation of relative quantities and by definition had a value of 1.0. Bars represent the standard error of the mean.