Mastication in jaw muscle pain

Abstract: Background: Integrated Pain Adaptation Models suggest a possible pain-motor interaction. Mechanisms affecting the jaw muscle spindles seem to affect the ability to bite and chew, suggesting that jaw muscle pain may be a potential modifier of mastication in humans. Objectives: The general objective of this doctoral thesis was to investigate the mastication performance in patients with painful temporomandibular disorders (TMD) and, more specifically, clinical chronic pain within the masticatory muscles. Study I investigated the effects of chronic and acute jaw muscle pain on oral motor control during precision biting. Study II focused on the optimisation of excessive gum chewing as an experimental model to induce jaw muscle pain and fatigue similar to that seen in painful TMD. Study III focused on chewing performance in TMD patients with myalgia. Methods and Results: Study I involved a comparison of patients with chronic masseter muscle pain and healthy participants. Experimental acute pain was induced by bilateral, simultaneous sterile hypertonic saline infusions into the healthy masseter muscles. A standardised hold and split biting task was used to assess precision biting. No significant differences were found in the hold forces, split forces or durations of split within or between the pain and pain-free conditions. Study II was a randomised, double-blinded study that included healthy participants of both sexes. A standardised chewing protocol of either 40- or 60-min of chewing was used, with a wash-out period. Subjective fatigue, pain characteristics, and functional measures were all assessed. Significant high subjective fatigue scores were induced in both the 40- and 60-min chewing trials. Significant but mild pain was induced only in the 60-min trial, and only in men. The induced pain area was significantly larger in the 60-min trial. The induced fatigue lasted up to 20 minutes after the end of the chewing while the increase in pain intensity and pain area did not until the first 10-min follow-up. Study III involved a series of chewing tasks involving viscoelastic soft and hard candies as well as a two-coloured gum. Optical imaging and analysis were conducted, and both bite force and the characteristics of pain and fatigue were assessed. Patients with painful TMD chewed the soft candies into particles that were fewer in number and which had a larger minimum Feret’s diameter after standardised chewing as compared to healthy pain-free control individuals. Surprisingly, the two-coloured gum was less mixed in the control cases. However, there were significant differences between the patients and the healthy controls in terms of self-assessed masticatory ability, mainly driven by painrelated issues. There was also obvious agreement between the patients’ self-assessed masticatory ability and the efficiency of their masticatory function. Conclusion: The three studies that form this doctoral thesis suggest that jaw muscle pain does not affect precision biting in humans; however, TMD patients with chronic myalgia exhibit impaired masticatory performance, with less efficiency of food communition, than those in the pain-free healthy control group. However, the excessive chewing model needs further adjustments in order to mimic TMD-pain, especially in women.