Quantification of masseter muscle deformation during palpation using MRI and image-analysis procedure

PURPOSE: The aim of this study was to quantify masseter muscle deformation during palpation and to discuss the clinical significance of muscle palpation using magnetic resonance imaging (MRI) and the image-analysis procedure. MATERIALS AND METHODS: Subjects were 10 male volunteers with a mean age of 26.8 years. MRIs were taken under two muscle conditions: compressed and noncompressed (control). By superimposing a compressed image on a noncompressed image, muscle deformation (the cross-sectional area, length of the surrounding area, thickness, circular convolution, compressed point, and anterior and posterior extension) was measured. RESULTS: The cross-sectional area of the masseter muscle was diminished by 14%, circular convolution of the muscle was reduced by 31%, the length of the surrounding area was enlarged by 11%, and the thickness of the muscle was reduced by 6.7 mm because of compressing. Thus, the masseter muscle was compressed. An uneven stretch of the muscle caused by compression was observed at the same time. The anterior portion of the masseter muscles extended on average 10%, with great individual differences. On the other hand, posterior extension (3%) and its individual differences were small. CONCLUSION: The method used made it possible to quantify masseter muscle deformation. By compressing, the masseter muscles were deformed especially in the anterior direction but with great individual differences.