Exploration of the Association Between Anterior Disc Displacement Without Reduction and Electromyographic Activity of Masseter Muscle Based on Needle Electromyography

Background: Patients with temporomandibular internal derangement, especially anterior disc displacement without reduction (ADDwoR), often present with limitations in masticatory function. The aim of this study was to explore the association between ADDwoR and electromyographic activity of the masseter muscle using needle electromyography. Methods: A total of 137 participants, including 64 controls with normal TMJs and 73 with bilateral ADDwoR, underwent needle electromyography of the masseter muscle. Abnormal electromyographic activity was compared between groups, and logistic regression analyses were performed to examine the associations between TMJ characteristics and electrophysiological abnormalities. Results: Patients with ADDwoR showed a significantly higher prevalence of abnormal masseter electromyographic activity than controls. Compared with the control group, MUPs in the ADDwoR group had longer duration, higher amplitude, and larger area. Multivariable logistic regression showed that ADDwoR was associated with the presence of these abnormalities. Exploratory subgroup analyses suggested possible associations with condylar resorption and disc morphology, though these findings should be interpreted with caution. Conclusions: Bilateral ADDwoR is associated with altered masseter muscle MUPs and a higher prevalence of abnormal needle EMG findings. Exploratory analyses suggest possible associations with condylar resorption and disc morphology, but these results require confirmation in larger studies. These findings suggest that both TMJ structural assessment and masticatory muscle functional evaluation may be useful in the clinical assessment of patients with ADDwoR.