Altered control of submaximal bite force during bruxism in humans

The control of bite force during varying submaximal loads was examined in p atients suffering from bruxism compared to healthy humans not showing these symptoms. The subjects raised a bar (preload) with their incisor teeth and held it between their upper and lower incisors using the minimal bite forc e required to keep the bar in a horizontal position. Further loading was ad ded during the preload phase. A sham load was also used. Depending on the s ession, the teeth were loaded by the experimenter or the subject and in one session the subject did not see the load (no visual feedback). The bite fo rce was measured continuously using a calibrated force transducer. In all t he subjects, the bite force increased with increasing load. Following the a ddition of the load, the level of the tonic bite force was reached rapidly with no marked overshoot. The patients with bruxism used significantly high er bite forces to hold the submaximal loads compared to the control subject s. In the control subjects, the holding forces for each submaximal load wer e identical in the men and the women and were independent of subject maxima l bite force, Sham loading evoked no marked responses in biting force. Whet her the subject or the experimenter added the load or whether the subject h ad visual feedback or not were not significant factors in determining the l evel of bite force. The results indicated that the patients with bruxism us ed excessively large biting forces for each given submaximal load. This stu dy showed no evidence that the inappropriate control of bite force by patie nts with bruxism was due to an abnormality in the higher cortical circuits that regulates the function of trigeminal motoneurons in the brainstem. Thi s was shown by a lack of abnormality in coordination of voluntary hand move ment with biting force, a lack of abnormal anticipation response to a sham load and a lack of any effect of visual feedback. The results were in line with the hypothesis that afferent input from oral (periodontal or masticato ry muscle) tissues does not provide an appropriate control of motor command in bruxism.