LAUSR

Abstract Background/aims Muscular pain often impairs masticatory function in Temporomandibular disorder patients. The specific aim of this study was to investigate how the bite force during mastication is influenced by experimental muscle pain caused by infusion of glutamate into the masseter muscle. Methods 12 healthy adults participated, after providing informed consent. Customized metal frames of the intraoral bite force sensor were manufactured for all subjects, and placed on their preferred chewing side. To induce experimental pain, a sterile solution of glutamate (0.5 M) was infused into the posterior part of the masseter muscle. Isotonic saline (0.9%) was infused as a control (randomized, cross-over design). During chewing three different kinds of test food (two different size carrots and gummy), bite force, electromyographic (EMG) activity of bilateral masseter muscle and anterior temporalis muscle were recorded. The first and last five masticatory cycles were used for analyses. The outcome parameters were as follows; Visual Analog Scale (VAS) for pain, impulse and duration from the bite force signal, duration and amplitude of EMG. Three-way ANOVAs with food (3 levels), sessions (5 levels: baseline, during glutamate, baseline 2, during isotonic saline, follow-up), cycles (2 × 5 levels) were carried out. Results Glutamate caused moderate levels of pain (mean VAS: 2.4 ± 0.9) whereas isotonic saline only caused low levels of pain (Mean VAS: 0.7 ± 0.5). The ANOVA of impulse data, as well as most EMG data, demonstrated a significant effect of session. Post hoc tests showed significantly higher impulse values of both the first and last five cycles in the glutamate session compared to baseline whereas the EMG activity was significantly decreased (P < 0.05). Conclusion Surprisingly, experimental pain induced by glutamate increased the bite force during mastication coupled with a decrease in EMG activity. This indicates a major reorganization of the motor control during painful mastication.