Simultaneous Noxious Stimulation of the Human Anterior Temporalis and Masseter Muscles. Part I: Effects on Jaw Movements

Aims: To test the hypotheses that, in comparison to control (isotonic saline), simultaneous noxious stimulation (hypertonic saline) of the masseter and anterior temporalis muscles would result in (1) reductions in amplitude and velocity of jaw movements during standardized open/close jaw movements and during free and standardized chewing and (2) changes in amplitude and velocity of jaw movements that relate to higher levels of negative mood or pain-related thoughts. Methods: Standardized open/close and free and standardized chewing were recorded in 15 asymptomatic participants in three blocks: block 1 (baseline), block 2 (during 5% hypertonic or 0.9% isotonic saline infusion into the right masseter and anterior temporalis muscles simultaneously), and block 3 (infusion sequence reversed). The Depression, Anxiety, and Stress Scale (DASS-21) and the Pain Catastrophizing Scale (PCS) were completed by the participants before the experiment, and the PCS was completed after the experiment. The amplitude and velocity of opening and closing movements for each task were compared between blocks (repeated-measures analysis of variance). Spearman rank correlation coefficient was used to explore correlations. Statistical significance was considered to be P .05. Results: In comparison to isotonic saline control, hypertonic saline resulted in significantly smaller opening and closing amplitudes and lower velocity during closing in free chewing, but no significant effects in the open/close task or standardized chewing. There were significant correlations between PCS scores and amplitude or velocity during isotonic saline and baseline, but not hypertonic saline. Conclusion: The pain-related reduction in amplitude and/or velocity of free chewing is consistent with the Pain Adaptation Model, but the absence of effects on the open/close task and standardized chewing is not. The few significant correlations between psychologic variables and jaw movement may reflect the low scores. Keywords: facial pain, jaw, mastication, masticatory muscles, movement, pain measurement