A FIBER TYPE ASSESSMENT OF MASSETER MUSCLES IN FIVE ASYMMETRY CLASSIFICATIONS

Abstract

ABSTRACT: Objectives: Masseter muscle fast-fiber composition is reportedly increased on the deviant facial asymmetry side in dentofacial deformities patients. Recently, four distinct asymmetry classes have been identified, based on posterior-anterior cephalometrics in this same population, which may have different functional etiologies. Our aim is to relate muscle fiber type properties with specific asymmetric craniofacial growth in these asymmetry groups. Methods: Diagnostic evaluations, radiographs, and masseter specimens were obtained from orthognathic surgery patients at the University of Lille. Immunohistochemical muscle staining and morphometrics determined the mean areas and percent occupancies of slow-I, fast-II, neonatal, atrial and hybrid fiber types. Eighty-three subjects (twenty nine symmetric, fifty-four asymmetric) had at least unilateral (left or right) fiber type information, while twenty-seven had bilateral (left and right) data. Fiber data were compared between symmetry and asymmetry subjects and between the four asymmetry classes. Significant differences between groups were determined by Fisher’s and ANOVA tests. Results: Type-II mean fiber area (p<0.006) and percent occupancy (p<0.018) were significantly greater on shorter ramal sides in asymmetric compared to symmetric subjects, supporting previous data for facial vertical dimension asymmetry. Neonatal- atrial mean fiber area (p<0.017) and percent occupancy (p<0.027) were decreased in 3 asymmetric groups. No significant difference was found amongst the four asymmetry classes (p<0.05). Conclusions: Imbalanced skeletal proportions are associated with similar imbalances in fiber type properties in left versus right masseter muscle biopsies sampled in the same patient at the time of orthognathic surgery. Specific to our findings, there is an association between increase in neo-atrial and decrease in type II fiber type area and percent occupancy in symmetric patients.