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Evaluation of masticatory activity during unilateral single tooth clenching using muscle functional magnetic resonance imaging.

Masticatory muscle activity during teeth clenching is affected by occlusal pattern. However, few studies have performed simultaneous evaluation of all masticatory activities during teeth clenching under various occlusal conditions. The aim of this study was to use muscle functional magnetic resonance imaging (mfMRI) to evaluate the effects of changes in occlusal point on masticatory activity during single tooth clenching. Changes in mean proton transverse relaxation time (∆T2) as an index of activity in all masticatory muscles during left unilateral clenching at the first molar or first premolar for 1 min were examined in nine healthy volunteers. Bite force was maintained at 40% of the maximum voluntary clenching force. The ∆T2 values of the masseter and lateral pterygoid muscles were analysed separately for superficial and deep layers, and for superior and inferior heads. The ∆T2 values for the ipsilateral deep masseter were significantly lower, and for the superior head of the ipsilateral lateral pterygoid muscles were significantly higher, after left first premolar clenching compared to left first molar clenching. These results quantitatively demonstrate a significant increase in activity of the superior head of the ipsilateral lateral pterygoid muscle and a significant decrease in activity of the ipsilateral deep masseter muscle with forward displacement of the occlusal contact point during unilateral tooth clenching.