Effect of clenching on T2 and diffusion parameters of the masseter muscle

Background: Persistent muscle contractions during clenching are considered to be one reason for temporomandibular disorders. However, no report has evaluated the effect of clenching on the masticatory muscles, as measured by magnetic resonance imaging (MRI). Purpose: To investigate whether clenching has an effect on either T2 or the coefficients for diffusion of the masseter muscles (MM), and to evaluate the effect of the distribution of bite force on such indices. Material and Methods: Twenty-three subjects were examined. Bite force was measured by a pressure-sensitive sheet, and the force of the right and the left sides was calculated. MRI was used to evaluate T2, the apparent diffusion coefficient (ADC), and the primary (λ1), secondary (λ2), and tertiary eigenvalues (λ3). These indices on the stronger side of the bite force were compared to those on the weaker side. Thereafter, the indices were compared between at rest and during clenching. Results: There was no significant difference in any of the indices (T2, ADC, λ1, λ2, and λ3) between the side of stronger bite force and the side with weaker. T2 increased by clenching, and the difference was significant in the side with stronger bite force (P 0.006). ADC, λ1, λ2, and λ3 increased significantly by clenching (P <0.01, P <0.01, P <0.01, and P <0.01, respectively) on both sides. The percentage of change of λ2 by clenching was 26.2±15.7% on the stronger side and 26.9±18.6% on the weaker side, which was significantly greater than either that of λ1 or λ3. Conclusion: The coefficients for diffusion of the MM were sensitive to change by clenching, and λ2 was the most sensitive. Moreover, the relative distribution of the bite forces had no effect on any of the indices.