MRI-based anatomical and biomechanical characterization of the human masticatory muscles

Biomechanical models of the masticatory muscles quantify the complex force and movement patterns of the masticatory system and are essential for evidence-based diagnosis, treatment planning, and prosthetic care in the craniomandibular region. This study provides comprehensive, sex-specific parameters for assessing the morphology of the masticatory muscles in young adult males and females, based on high-resolution magnetic resonance imaging (MRI) images and semi-automatic three-dimensional (3D) segmentation. Parameters such as volume, length, anatomical cross-sectional area and surface area of the masseter muscle, pterygoid muscle and temporalis muscle are evaluated bilaterally. Their force application points on the skull and mandible are determined using a new center-of-mass-based segmentation method for biomechanical model interpretation. Statistical analysis indicates that 66 of 68 parameters show significant sex-specific differences, mostly with large effect sizes: male participants generally showed higher volumes, lengths, and cross-sectional areas, except for two cross-sectional areas of the superficialis masseter muscle, resulting in higher surface-to-volume ratios in women. Although minor asymmetrical effects are observed, no significant lateral differences are found within each sex, suggesting bilateral symmetry of the masticatory muscles in the study cohort. Comparing volume data with existing literature shows that this study's data generally exceeds historical cadaver measurements but is consistent with recent digital studies, underscoring the influence of age, methodology, and sex-specific differences on the interpretation of morphometric muscle parameters. Overall, this research provides a valuable foundation for the morphometric characterization and sex-specific biomechanical modeling of the masticatory musculature, while also highlighting the limitations of small, unbalanced, and mixed-sex reference groups.