Masseter muscle volume measured using ultrasonography and its relationship with facial morphology

The aims of this study were to measure the volume, cross-sectional area, thickness, width, and length of contracted masseter muscles in a sample of adults, four males and six females, using three-dimensional (3D) ultrasonography, and to correlate these measurements with the variations in facial morphology of the sample. The scans were carried out bilaterally using a hand-held probe carrying a magnetic positional sensor, which enabled a computer to reconstruct the images into a 3D array of slices. Measurements were made by the computer from the reconstructed images using specially written software. Cephalometric analysis initially involved seven angular, eight linear, and two proportional variables, but as these were strongly inter-dependent, the variables were reduced to four principal components prior to statistical comparison with the mean muscle variables. The values found for muscle volume, cross-sectional area, and thickness were broadly consistent with those of previous investigators. Volume showed a significant, negative correlation with mandibular inclination including gonial angle (P≤0.001), and a significant, positive correlation with total posterior face height and ramus height (P≤0.001), and lower posterior face height percentage (P≤0.01). Weaker correlations were found for length and thickness. While the results support existing evidence that large masticatory muscles are associated with brachycephalism and vice versa, a cautious interpretation is necessary in view of the small sample size. The 3D ultrasonography system is at an experimental stage and requires further development and evaluation.