Although there are many studies on bruxism. There is controversy regarding its the epidemiology, mechanism, and treatment. This article summarizes the published experimental and observational studies on bruxism to clarify what is understood to date and to propose the future direction of studies on this issue.
Firstly, bruxism is discussed in relation to the symptoms of temporomandibular disorders, such as masticatory muscle pain and headache, and tooth wear. Secondly, the papers on epidemiology, measuring methods and treatments of bruxism were reviewed. Finally, our studies on “the evaluation of the day to day variation of nocturnal masseter muscle activity” and “the effect of occlusal splint designs on nocturnal masticatory muscle activity” were introduced.
One of the major reasons of controversy is due to the variation of the criteria for bruxers among the studies reviewed. It is also clear that there is a fluctuation in the bruxism activity from night to night, and it may even change in accordance with the stress on measurement. Another reason is due to the difficulty in recording reliable data. It is not easy to record the nocturnal activity of the human subject under a natural environment, and it requires a long period of time for both recording and analysis. Therefore it is necessary to pay attention to the above mentioned issues throughout the study period.

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It is known that sleep associated bruxism is an oral habit during sleep, and it seems to affect periodontal diseases, excessive wear of the teeth and temporomandibular disorders. It is necessary to comprehend the phenomena of bruxism in order to treat it. But there are still many unclear points on the detail. The purpose of this paper is to review the researching problem and methodology of the bruxism refered mainly to our studies. The following 5 items were described in this paper, 1. what is bruxism, 2. the measurement problem of bruxism, 3. the analyzing problem of bruxism, 4. the clinical problem of bruxism, 55 future prospect of bruxism.

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The line of action of a force in relation to the center of resistance of a tooth is an important consideration in determining a set of force and moment acting on the tooth. If the relationship between them could be clarified experimentally, the type of tooth movement would be also estimated, and efficient tooth movements will be achieved through the minimum course without round-tripping of the tooth thereby.
We have developed a system for real-time measurement of two target points' displacements in three dimensions and determined the location of center of resistance of a maxillary central incisor subjected to orthodontic forces in human subjects. As a result, the center of resistance was observed at a point 0.19 times the root length from the alveolar crest and found to be closer to the crown than the location determined from a theoretical consideration. It was suggested that the location of center of resistance depends not simply on the shape of the root but on anatomical morphology of the surrounding tissues such as periodontal ligament and alveolar bone.

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The purpose of this study was to investigate the characteristics of the movement at the lower incisal point around the occlusal phase during gum-chewing movement in children with primary dentition. Mandibular movement was measured using the TRI-MET, which is an opto-electronic analysis system with six degrees-of-freedom at a sampling frequency of 100 Hz. This measurement data was transferred to a graphics workstation via a personal computer and Ethernet. The subjects were categorized according to their age into two groups. The subject group for children with primary dentition consisted of 12 children, 4 boys and 8 girls, ranging in age from 4 to 6 years. These subjects had no dental caries, or had slightly restored teeth, did not have any trouble with, occlusion, or mandibular function, and had no history of orthodontic treatment. The subject group for the adults with permanent dentition consisted of 18 women, ranging in age from 19 to 27 years, and had a normal occlusion with no history of orthodontic treatment. Each subject was instructed to chew the gum freely from maximum intercuspal position for twenty seconds. The entire chewing movement was divided into cycles at the lower position of the movement to prevent from damaging the occlusal phase. Cycles were classified into four patterns according to their closing and opening pathways. Both closing and opening pathways were classified based on their preferred and non-preferred sides.
A three-dimensional velocity curve was estimated by using multilevel model analysis.
As a result, the gum-chewing movement in children with primary dentition was characterized as below:
1) 80% of cycles were chewed on the preferred side,
2) only 35% of cycles used both preferred side and non-preferred side in children, however, 65% of adults used both sides,
3) the speed in children was faster than in adults,
4) the difference of the velocity between types was smaller than that of between groups.
These characteristics might be related to their morphological specific feature, shallow occlusal relationship, flat occlusal plane and/or shallow fossa and flat eminence relationship of the condyle. (334words)

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The process of acquiring masticatory functional stability in the intercuspal position in the treatment course of occlusal guidance for a child patient with posterior crossbite in the left side of the jaw was investigated using the Dental Prescal System. The patient was a male infant in the IIC stage of Hellman's dental age. The crossbite was corrected by means of a framework rapid maxillary expansion appliance. We evaluated, by Dental Prescale, the occlusal contact area, the bite force, the occlusal load center and the asymmetry index, before and after expansion, besides during fixed and natural retention.
1 . Occlusal contact area and Bite force
The occlusal contact areas and the bite force of the left deciduous canine, the left second deciduous molar and the bilateral first molars, were magnified by degrees. The occlusal contact area and the bite force of the total dentition were reduced temporarily after the expansion, but when a lingual arch appliance was used for retention, they increased again.
2. Occlusal load center
After the expansion, the occlusal load center was located forward in the anteroposterior dimension, but during fixed retention it moved backward. Laterally it shifted to the left side, afterwards to the median line.
3. Asymmetry index
The asymmetry index was the least after two months of using the lingual arch appliance as retainer.It increased later on, but it reduced again after the natural retention.
Therefore even if the occlusion had been stable morphologically, the lateral and anteroposterior balances of the bite force distribution were disharmonious; the functional stability of the occlusion was acquired gradually during the natural retention phase.

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