Comparison of content of water in the muscles of mastication

Objectives: Edema in the muscles of mastication may be associated with the muscle pain in patients with temporomandibular disorder (TMD). The estimation of content of the water in the muscle may be used for the diagnosis of TMD. However, there is no information about the content of water of the muscles in the normal population. The purpose of this study was to compare the content of water between both sides of muscles of mastication by magnetic resonance imaging (MRI) in patients without TMD symptom. Further, we assessed the associations between content of water, age and sex. Methods: Fifty subjects (mean age ± SD; 52.3 ± 14.1), 24 men and 26 women, participated in this study. None had any history of TMD. All subjects had MRI T2 weighted IDEAL (Iterative Decomposition of water/fat using Echo Asymmetry and Least-squares estimation) examination. MRI signal intensities of the masseter muscle and medial pterygoid muscle in both sides were divided by those of the air near the muscles. These signal ratios were determined as content of water in each muscle. Results: Content of water of any muscles was not associated with the age. Men had higher content of water of the right (P=0.063) and left (P=0.048) masseter muscles than women. No significant difference was observed between both sides of the medial pterygoid muscles. Content of water in the masseter muscle was significantly larger at the right side than the left side (P<0.001). Conclusions: Content of water in the masseter muscle may be higher in the right side in comparison with the left side in the population without TMD symptom. Our results imply that this difference in the masseter muscle should be taken into account in evaluating the edema of the masseter muscles in patients with the muscle pain.