Changes in tooth pulpal detection and pain thresholds in relation to jaw movement in man

The effect of chewing gum on the impact, pain and breakages associated with fixed orthodontic appliances: a randomized clinical trial

OBJECTIVES

To determine whether the use of chewing gum reduced the impact and pain of fixed orthodontic appliances.

SETTING AND SAMPLE POPULATION

The Orthodontic Department of the Charles Clifford Dental Hospital, Sheffield, UK. Fifty-seven patients aged 18 years or younger and who were about to start fixed orthodontic appliance treatment.

SUBJECTS AND METHODS

A randomized clinical trial with two parallel groups either allocated to receive chewing gum after placement of their appliance or who were asked not to chew gum. The patients completed a previously validated Impact of Fixed Appliances questionnaire at 24 h and 1 week following each visit up until the placement of the working archwire. A visual analogue scale (VAS) was used to assess the intensity of pain. Appliance breakages were recorded to the end of treatment.

RESULTS

The difference between the median Total Impact Score of the two groups at 24 h was 16, which was significant (p = 0.031; Mann-Whitney U-test). The difference between the median VAS between the two groups at 24 h was 25 mm, which was significant (p = 0.038; Mann-Whitney U-test). There were no differences at 1 week. None of the risk ratios for appliance breakages were significant.

CONCLUSION

Chewing gum significantly decreased both the impact and pain from the fixed appliances. There was no evidence that chewing gum increased the incidence of appliance breakages.

Threshold for Detection of Incisal Forces Is Increased by Jaw Movement

Current knowledge regarding the sensitivity of the teeth to forces is based on psychophysical experiments that measured touch detection thresholds under static jaw conditions. It is not known whether jaw movements alter the perception of forces applied to the teeth, but, based on limb movement studies, it is hypothesized that the perception of mechanoreceptor outputs will be downwardly modulated by jaw movements. We predicted that, compared with static jaw conditions, rhythmic jaw movements would be associated with significantly higher psychophysical thresholds for the detection of incisally applied forces. In eight participants, mechanical pulses were delivered to an incisor during static jaw holding or during cyclic jaw opening and closing. Analogous to findings in human limbs, the psychophysical salience of periodontal mechanoreceptor feedback was downwardly modulated by physiologically relevant movements; detection thresholds for mechanical pulses applied to a central incisor were significantly higher during jaw-closing movements than during static jaw positioning.

Prolonged rhythmic gum chewing suppresses nociceptive response via serotonergic descending inhibitory pathway in humans

Serotonergic (5-HT) neurons are implicated in modulating nociceptive transmission. It is established that 5-HT neuronal activity is enhanced by rhythmic behaviors such as chewing and locomotion in animals. We thus hypothesized that 5-HT descending inhibitory pathways may be enhanced by rhythmic behavior of gum chewing in humans. To evaluate this idea, we examined nociceptive flexion reflex (NFR), while a subject chewed gum rhythmically for 20 min. NFR was elicited by electrical stimulation of the sural nerve, and the evoked potential was recorded from the biceps femoris muscle. Visual analogue scale (VAS) was also obtained. To assess 5-HT activity, we determined 5-HT levels quantitatively in platelet poor plasma (PPP) and whole blood (WB) using HPLC system. Both NFR area and VAS were significantly decreased at 5 min after the onset of chewing and these reductions persisted until cessation of chewing. There were no significant changes in NFR and VAS while resting without chewing. The PPP 5-HT level increased significantly just after cessation of chewing and had returned to the pre-chewing level by 30 min after cessation of chewing. The WB 5-HT level obtained 30 min after cessation of chewing was significantly greater than the pre-chewing level. Serotonin transporters have recently been discovered at the blood-brain barrier, suggesting that the rise in blood 5-HT may possibly reflect an increase in 5-HT level within the brain. The present results support our hypothesis that the rhythmic behavior of chewing suppresses nociceptive responses via the 5-HT descending inhibitory pathway.

Jaw movement alters the reaction of human jaw muscles to incisor stimulation

The changes in the minimum time to consciously react (reaction time) and the order of jaw muscle recruitment to precisely controlled axial stimulation of the incisors during controlled jaw movements are not known. To this end, ten subjects were recruited to investigate the reaction time of bilateral temporalis and masseter muscles and bite force. Stimuli were delivered axially to the upper central incisors during active jaw closing and opening, and under static conditions. The results showed that the reaction time was increased an average of 35% during both jaw opening and closing movements when compared with static jaw conditions. The left temporalis was recruited approximately 10 ms before the right temporalis, whereas no significant side differences were found between the masseter muscles. The masseter muscles were recruited an average of 20 ms before the temporalis muscles during jaw closing, but no difference existed during opening. Under static conditions the reaction time in the bite force was approximately 16 ms longer than the left temporalis, but was not significantly different from the reaction time of any of the other muscles, indicating that, under the static conditions tested, the left temporalis was more often responsible for initiation of the mechanical reactions in the jaw. Because of active compensation, no force measurements were made during jaw movement. This study is a prerequisite for investigations into the modulation of reflexes during jaw movement, because a response to a stimulus commencing after the minimum reaction time may not be entirely reflex in origin.