Background

Orthodontic treatment is a common clinical method of malocclusion. Studies have found that neurons in the sensorimotor cortex of the brain undergo adaptive remodeling in response to changes in oral behavior or occlusion.

Objective

To explore whether orthodontic treatment could be sufficient to cause neuroplastic changes in the corticomotor excitability of the masseter muscle.

Methods

Fifteen Angle Class II malocclusion patients who were receiving the orthodontic treatment participated in the study. Cortical excitability was assessed by electromyographic activity changes evoked by transcranial magnetic stimulation. Four orthodontic time points were recorded, including baseline, day 1, day 7, and day 30. Motor evoked potentials (MEPs) were recorded in the masseter muscle and the first dorsal interosseous muscle (FDI) serving as a control. The data were analyzed by stimulus-response curves and corticomotor mapping. Statistical analyses involved repeated measures analysis of variance, two-way ANOVA and post-hoc Tukey tests.

Results

Motor evoked potentials (MEPs) of the masseter muscle were significantly decreased during orthodontic treatment compared with baseline (P<0.001). MEPs of masseter muscle were dependent on session and stimulus intensity (P<0.001), whereas MEPs of FDI were only dependent on stimulus intensity (P=0.091). Finally, Post hoc Tukey tests demonstrated that MEPs of masseter muscle on days 1 and 7, with 70% to 90% stimulus intensities, were higher compared with baseline values. (P<0.001)

Conclusions

The present study suggested that orthodontic treatment can lead to neuroplastic changes in the corticomotor control of the masseter muscle, which may add to our understanding of the adaptive response of subjects to changes of oral environment during the orthodontic treatment.