Objectives: Multipennate masseter has functional heterogeneity, which suggests the contraction is not uniform within the muscle. The heterogeneous contraction may yield irregular changes of the muscle outline. This study aimed to elucidate the outline changes of the superficial masseter with clench, by multipoint measurements of sectional ultrasound images. Methods: Serial frontal images (perpendicular to FH-plane, 3 mm interval) of the right superficial masseter were obtained by a real-time ultrasonograph (ALOKA SSD-5500, 13 MHz linear-array transducer) from five healthy men. To define relationship between the transducer and cranium, a 3-D mechanical stage with a face-bow was used. Registrations were made during the muscle relaxation and maximal clenching. Distance between the lateral and medial border of muscle were measured at intervals of 1 mm from the origin to insertion in each image, as thickness at the corresponding measured point. Results: Thickness of the relaxed and contracted muscle, and its difference varied among the measured points. Thickness increased at most measured points with clench, whereas the points where thickness decreased existed. There was strong correlation between thickness at relaxed and contracted (Spearman, r=0.62 - 0.95, n=449 - 602, p<0.001). Correlation between thickness at relaxed and the difference between two conditions was weak (r= -0.18 - 0.25, p<0.001). Maximum thickness at relaxed (R-max) and at contracted (C-max) were 12.8 +- 1.2 mm and 15.7 +- 1.1 mm, respectively. Maximum increase of the thickness (I-max) were 5.3 +- 1.8 mm. Locations of the points with R-max, with C-max, and with I-max didn't coincide. Thickness at the point of R-max did not significantly changed with clench, although that of C-max and I-max significantly increased (p<0.01, Mann-Whitney). Conclusion: Changes of thickness with clench showed great disparity within the superficial masseter, which may result from the complexity of contraction properties due to the multipennate structure and functional heterogeneity.

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