Skeletal Muscle Fiber Types Vary with Malocclusion: A Case Report from a Severe Class III Malocclusion

A long-standing physiologic question in orthodontics is to what extent fiber type content of masticatory muscles varies with craniofacial morphology. 

Objective:  To determine, in the case of a middle aged male who underwent surgical correction of a severe class III malocclusion, if the fiber type composition of masseter muscle is significantly different in comparison to a group of 40 subjects undergoing orthognathic surgery for treatment of less severe malocclusions (a mixed population of class II, class III, open bite and deep bite morphologies).

Methods:  Orthodontic diagnosis:  The subject was characterized as a severe class III malocclusion using the McNamara cephalometric analysis and Grummons frontal analysis. Craniofacial morphometric data for fiber types in the comparison group have been presented previously in an abstract.¹ Fiber type analysis: Masseter tissue biopsies were excised during surgical repositioning of the mandible.  From serial sections fiber type and average area were determined by immunohistochemical staining and morphometric analysis.¹ Fiber type classes were condensed into the following groups: type I – containing type I myosin heavy chain (MHC), type II – IIA and/or IIX MHC, type I/II hybrid fibers – I and IIA or IIX, and type neonatal/atrial – neonatal and/or α cardiac MHC in addition to other myosins.

Results:  For our subject all four fiber type classes were notably larger.  The mean fiber area (μm²) for comparison group vs. the case subject was as follows: