Electromyographic evaluation of masseteric activity during maximum opening in patients with temporomandibular disorders and limited mouth opening

This study aimed to identify the differences in masseteric activity during maximum mouth opening among healthy volunteers, patients with limited mouth opening (LMO) and myofascial pain, and patients with LMO and disc displacement. Thirty-seven patients with temporomandibular disorders and LMO (< 37 mm) and 27 healthy volunteers participated in this study. Patients were categorized into three groups based on magnetic resonance imaging (MRI) findings and clinical symptoms: disc displacement (DD), disc displacement with myofascial pain (DD + M), and myofascial pain (M). The DD group included pain-free disc patients with displacement patients, excluding those with myofascial pain or arthralgia. After a five-minute relaxation period, the participants were instructed to perform maximum opening (O_max) and maintain the O_max position for five seconds, and this was repeated three times. Subsequently, the participants were asked to perform maximum voluntary clenching for five seconds, which was repeated twice. At rest, no significant masseter potential differences were observed among the DD, DD + M, M, and healthy groups (p > .05). However, during O_max, the DD + M and M groups exhibited significantly higher masseteric potential than the healthy and DD groups (p < .001). Furthermore, the masseteric potential during O_max correlated with self-reported pain levels on a Visual Analog Scale (VAS). Overall, there are distinct differences in masseteric activity levels between patients with LMO and with or without myofascial pain or disc displacement.