Motor control of jaw movements: An fMRI study of parafunctional clench and grind behavior.

Jaw-clenching and tooth-grinding associated with bruxism can contribute to abnormal tooth wear and pain in the masticatory system. Clench and tooth-grinding jaw-movement tasks were evaluated in a block-design fMRI study comparing a dental-control (DC) group with a tooth-grinding (TG) group. Group classification was made prior to imaging based upon self-reported parafunctional clench and grind behavior and clinical evidence of abnormal tooth wear. Group differences in brain activation patterns were found for each task compared to the resting baseline. The DC group showed a more widely distributed pattern; more extensive activity in the supplementary motor area (SMA) proper that extended into the pre-SMA; and, for clench, activity in the left inferior parietal lobule (IPL). The DC group activated more than the TG subjects the left IPL for clench, and pre-SMA for grind. Neither task elicited more activity in the TG than DC subjects. Our group findings suggest that jaw-movement tasks executed by the TG group elicited (1) more efficient brain activation pattern consistent with other studies that found less extensive activity with executing "over-learned" tasks; (2) "underactive" SMA activity that underlies reduced motor planning; (3) decreased inferior parietal activity that is associated with lesser motor-attentional demands. Thus orofacial parafunctional habits may influence brain circuits recruited for jaw movements, providing a possible basis for understanding involuntary jaw movements in bruxism and oral movement disorders in general.