Tooth contact and brain activity

Objectives: The aim of this study was to evaluate the differences among the sensory and motor inputs for individual teeth at the cortical level. 

Methods: Ten healthy subjects participated in this study. We conducted two study protocols with each subject.Study‡@: Functional-magnetic-resonance-imaging (fMRI) was used to determine how the brain reacts. We used a set of 5 mandibular splints for each subject. Each splint restricted contact to a different set of teeth in the maxillary arch (incisors, canines, 1^st-premolars, 1^st-molars, or 2^nd-molars). We asked each subject to grind 4-cycles of 21s with each splint and then to rest 21s rest without any grinding. For control, we conducted the same procedure without a splint. And we scored the abnormality of these splints with a visual-analog-rating-scale (VAS).Study‡A: Near-infrared spectroscopy (NIRS) was used to determine how the brain reacts. Painless vibrotactile stimuli (40Hz) were applied to 10-teeth (left maxillary and mandibular incisors, canines, 1^st-premolars, 1^st-molars, or 2^nd-molars) using a dental handpiece. Each tooth was stimulated with 3 cycles of 20s and 20s resting.

Results: Study‡@: Random-effect group analysis shows the activation of the sensory and motor cortex with the grinding in all conditions. Furthermore, only grinding by 1^st-molars and the 2^nd-molars caused significant BOLD signal increases in the amygdala and the anterior cingulate cortex. The VAS score indicated that the grinding of 1^st-molars and 2^nd-molars was significantly more unpleasant (P<0.05 ANOVA, Tukey’s) compared to the control condition.Study‡A: The somatosensory cortex was activated during all tooth stimulation. Maxillary and mandibular 1^st-molar stimulation elicits the greatest increase of cerebral blood flow in the somatosensory cortex (P<0.05, ANOVA, Tukey’s).

Conclusions: These results indicate that 1^st-molars are the most sensitive at the cortical level. And strong contacts in the molar region disturb normal occlusal function.