Neural Mechanisms Underlying Time-dependent Changes in Orthodontic Pain

Objectives: Discomfort and pain are common experiences during orthodontic treatment. Although various methods for pain relief such as medication or laser irradiation have been studied, the brain mechanism[A1] underlying orthodontic pain has not been revealed to date. Therefore, we performed behavioral and fMRI experiments to clarify the impact of orthodontic pain on brain activity.
Methods: Healthy adult volunteers who had a normal row of teeth, with less than 110-nm contact space between both 2nd bicuspids and the 1st molars of the upper jaw participated in this study. Elastic separators were inserted between both maxillary 2nd bicuspids and 1st molars. Cortical activities (by fMRI) and subjective pain estimations [by visual analog scale (VAS) and magnitude estimation] were measured while the subjects performed tapping and clenching tasks at three time points: before the insertion (Day -0), immediately after the insertion (Day +0), and 1 day after the insertion (Day 1) of the elastic separator.
Results: The pain scores on Day +0 and Day 1 were higher than those on Day -0 in both VAS and magnitude estimation. In addition, the scores on Day 1 were higher than those on Day +0. fMRI data during both tapping and clenching tasks showed remarkable activity in the inferior frontal gyrus and insular cortex area on Day 1, whereas there was no activity on Day +0 in comparison with Day -0.
Conclusions: Previous studies have reported that orthodontic force renders the pain strongest on the day after applying the force on teeth. Moreover, the insular cortex amplifies anxiety and discomfort from cognition of pain. Therefore, the pain experienced 1 day after applying orthodontic force may be associated with emotional aspects.