Analysis of Muscle Fibre Orientation in the Human Medial and Lateral Pterygoid Muscles Using Diffusion Tensor Imaging: A Methodological Study

BACKGROUND: Deep anatomical locations limit functional and morphological evaluation of the medial and lateral pterygoid muscles, which are vital for mandibular kinematics. The lateral pterygoid comprises superior and inferior heads with distinct anatomical and functional characteristics. Diffusion tensor imaging (DTI) tractography non-invasively visualises in vivo muscle fibre orientation. OBJECTIVES: This methodological study aimed to quantify 3D fibre orientation of the medial pterygoid and both lateral pterygoid heads using DTI tractography, comparing simulated mandibular rest and 25-mm open-mouth positions. METHODS: Healthy males (28.4 ± 4.6 years) underwent cephalometric radiography and 3.0-T MRI. T1-weighted and DTI sequences were acquired at both mandibular positions. Target muscles were segmented. MRtrix3 tractograms were reconstructed (fibre orientation distribution [FOD] amplitude cutoffs: 0.02-0.10). Fibre orientations, quantified via vector projection onto coronal and sagittal planes, were statistically compared. RESULTS: Tractography successfully visualised fibre orientation; optimal FOD amplitude cutoffs varied among individuals and muscles. The medial pterygoid showed no significant changes. During mouth opening, seven participants exhibited marked fibre reorientation in the superior head and seven in the inferior head, although the direction and magnitude varied among participants. However, group-level analyses revealed no significant orientation differences for any muscle. CONCLUSION: DTI tractography enabled in vivo evaluation of medial and lateral pterygoid internal fibre architecture. Between simulated rest and 25-mm open-mouth positions, group-level differences were non-significant in either plane despite variable lateral pterygoid reorientation. Inter-individual variability and FOD cutoff dependence highlight the need for participant- and muscle-specific parameter adjustment. These findings provide a methodological basis for future DTI applications in temporomandibular disorders and masticatory dysfunction.