Oral Behaviours and Masseter Activity in Patients with Masticatory Muscle Pain

Background: Myogenous pain of the chewing muscles is a subgroup of temporomandibular disorder (TMD). It is a complex condition not fully understood at this time. One possible causal factor of myogenous TMD may be overloading of the jaw joint and muscles as a result of prolonged low-level clenching. This includes behaviours such as clenching and grinding, which may be observed during awake hours. The use of wireless surface electromyography (EMG) allows for the collection of objective data regarding jaw muscle activity in the habitual environment. Methods: Female patients (N = 27, mean age 27.0 ± 6.3 years) diagnosed with myalgia or myofascial pain with referral were age matched with TMD-free controls (N = 26, mean age 28.0yrs ± 6.5 years). A single examiner completed a standardised TMD examination on all participants to confirm eligibility for the study. Participants were fitted with a minimally invasive wireless EMG sensor on the skin surface overlying the masseter muscle on their preferred chewing side. Participants wore the EMG sensors while awake, over two consecutive days. Maximum voluntary contraction (MVC) was identified as peak bite force. Contraction episodes were detected at four thresholds: 3 x minimum root mean square (RMS) at rest, 3% MVC, 5% MVC, 10% MVC. Results: The vast majority of the participants in the patient group had myofascial pain affecting the masseter muscles. Maximum opening was decreased in the patients compared to controls. The Oral Behaviour Checklist (OBC) summary score was higher in patients than controls (p < 0.01). MVC was 1124.7 ± 550.8 μV in the patient group and 1202.5 ± 424.5 μV in the control group (p = 0.559). The frequency, duration and amplitude of masseter contraction episodes were calculated. Most masseter contraction episodes of both patients and controls were of relatively low amplitude (< 10% MVC) and short duration (< 10 seconds). There was no significant difference in the number of episodes per hour between groups. A significant difference in total contraction time % was found between groups (p = 0.039) with a tendency to longer contractions in the patient group. No significant association was found between self-reported parafunction and masticatory muscle activity. Conclusion: Patients with myogenous TMD reported a higher level of parafunction. Patients and controls have a similar level of number of contractions during waking hours but the contractions are longer. The OBC may not be a reliable tool for assessing wake-time parafunction