Outer shape changes of human masseter with contraction by ultrasound morphometry

Objective: The multipennate masseter has a complex internal architecture, which suggests the contraction is not uniform within the muscle. The heterogeneous contraction may cause inequable changes of the muscle's outer shape. This study aimed to elucidate the outer shape changes of the whole masseter with clenching by multipoint measurement of serial ultrasound images. Design: Serial coronal images (perpendicular to the FH plane, 3 mm interval) of the right masseter of five healthy men were obtained with a real-time ultrasonograph equipped with a 13 MHz linear-array transducer. To define the relationship between the transducer and cranium, we used a 3D mechanical stage with a face-bow. Registrations were made during muscle relaxation and maximal clenching. The distance between the lateral and medial outlines of the muscle was measured at intervals of 1 mm from the origin to insertion in each image as the thickness at the corresponding measured point. Results: The thickness of the relaxed and contracted muscle (R and C) and its difference (Δ) varied among the measured points. Muscle thickness at most measured sites increased with clenching, whereas it decreased at several sites, mainly near the origin and insertion. There were positive correlations in every subject both between R and C, and between C and Δ. On the other hand, the correlation between R and Δ were negative or weak. Conclusion: Changes of thickness with clenching showed great disparity within the masseter, which may result from the complexity of the contraction properties due to the multipennate structure and functional heterogeneity.