Chewing-induced Increase of Brain Blood Flow in Mandibular Prognathism Was Less Compared to Normal Occlusion

Masticatory function is closely related to brain function, and normal occlusion is an essential factor for favorable brain function. Mandibular prognathism （MP） shows diminished occlusal function. Jaw deformity is presumed to adversely affect brain function due to decreased occlusal function. In this study, functional near-infrared spectroscopy （fNIRS） was used to examine brain blood flow to investigate the relationship between MP and brain function. Seventeen subjects with normal occlusion and 69 patients with MP participated in this study. The number of occlusal contacts was counted using silicone materials. Electromyography （EMG） of the masseter muscles during clenching was also recorded. Brain blood flow was measured with fNIRS during a calculation task and chewing tasks of preferential chewing of paraffin wax and hard gummy. For parametric data, Student’s t-test was used for statistical significance. The Mann-Whitney U test was used for the statistical significance of nonparametric data. The case of P＜0.05 was considered to show a statistically significant difference. As a result, the number of occlusal contacts in MP was smaller than that in normal occlusion. Masseter muscle activity during the clenching task was weaker in MP than normal occlusion. The calculation task increased oxy-Hb in both groups. There was no significant difference in oxy-Hb between groups in the calculation task. The task of preferential chewing of paraffin wax increased oxy-Hb in both groups. This increase was significant in the bilateral inferior frontal gyrus, and interestingly, the increase was lower in MP patients than in normal occlusion. The task of chewing hard gummy also induced an increase in oxy-Hb in the bilateral inferior frontal gyrus in both groups, though the increase was lower in MP patients than in normal occlusion. Correlation analysis revealed quite a weak correlation between the number of occlusal contacts and oxy-Hb, though EMG exhibited a stronger correlation to oxy-Hb than the number of occlusal contacts. In conclusion, compared with normal occlusion, MP patients had decreased brain blood flow during mastication in the bilateral inferior frontal gyrus, which is considered to be particularly involved in cognitive function in the prefrontal cortex. This suggests that MP may influence not only oral function but also systemic function such as brain blood flow.