Three-Part MRI Morphology of the Human Masseter Muscle: A Part-Dominance Classification

Background: A simple MRI-based framework for describing part-specific dominance patterns in the human masseter muscle is lacking. This study aimed to characterize its MRI-visible parts and develop a preliminary part-dominance classification. Methods: This retrospective cross-sectional study analysed 58 clinical MRI examinations, comprising 116 masseter muscles with complete bilateral visualization. The three MRI-visible parts were identified on T2-weighted 3D coronal source images. Thickness and length were measured using a predefined study procedure, and each muscle was assigned a part-dominance type according to which part was thickest. Results: Three distinct parts were identified in all 116 muscles. The inferior part was thickest in 81 muscles (69.8%), the coronoid part in 31 (26.7%), and the middle part in 4 (3.4%). The inferior part was also the longest segment. Conclusions: In this dataset, the human masseter muscle showed a three-part MRI morphology. The findings provide a basis for a preliminary part-dominance classification for anatomical and radiological description. Functional, biomechanical, and clinical correlations require prospective validation.