Age- and sex-related differences in masseter size and its role in oral functions

Background. The masseter muscle plays a key structural and functional role in the stomatognathic system. Researchers' cumulative evidence has suggested that the variation in the size of a person's masseter muscle may be a critical factor related to individual differences in oral functions. However, researchers have not yet investigated systematically the effect of a person's age and sex on masseter muscle size and the association of masseter muscle size with other clinical metrics, including masticatory performance (MP) and salivary flow rate (SFR). Using T1-weighted magnetic resonance imaging (MRI) data provides a noninvasive method for assessing masseter muscle volume (MMV). Methods. Using T1-weighted MRI data, the authors developed a voxel- based method to assess MMV and investigated the associations among MMV, MP, and SFR. Results. The authors acquired T1-weighted MRI data from scans of the heads of 62 healthy adults and assessed MMV by means of using a voxel-based approach. The authors' assessment results had acceptable rates of interrater and intrarater reliability. MMV was significantly lower in the older subgroup and in the female subgroup. In addition, the correlation for MMV was significantly positive with MP and stimulated SFR. Conclusions. The study results revealed evidence that the authors' voxel- based approach, which they designed on the basis of T1-weighted MRI data, would be a reliable method for quantifying MMV. Practical Implications. The findings suggest that the variation in masseter muscle size may be a critical factor to assess individual differences in oral functions.