The relationship between the function of whole body and static occlusion has been reported in many articles; however, it still remains unknown whether clenching —which is one of the dynamic occlusal functions— exerts an influence on the whole body.
In this study, the shifting of the gravity center with mandible at resting position, in occlusion and in clenching with voluntary occlusal force was measured to investigate whether or not the mechanism of physical adjustment of equilibrium was affected by the alteration of mandibular position. The following results were obtained:
1. In the testing with eyes closed, the-distance and velocity of the shifting of the gravity center tended to be relatively smaller with the mandible in occlusion and when clenching in the intercuspal position with 10% and 50% of occlusal force than when the mandible was kept at resting position or clenched with 100% occlusal force.
2. The extent of shifting tended to be smaller —except in one subject— when the mandible was occluded, clenched with 10% occlusal force and cle nched with 50% occlusal force.
3. The center of gravity tended to be displaced on the whole towards the posterior direction.
4. Slight alteration of mandibular position at rest and in occlusion did not produce significant differences in the shifting of the gravity center.

View full abstract

Dentists engaged in prosthodontic practice often encounter cases in which a mandibular first molar tooth is missing. However, not all the patients complain about difficulty in chewing food. This is probably because the rest of the teeth are in place and their investing and supporting tissues are healthy.
The present study was carried out to re-evaluate the functional merits of the fixed prosthodontic treatment usually given to the patients with a missing lower first molar. For this purpose, we used a newly developed software to examine the changes in masticatory function with the lapse of time after loss of the mandibular first molar tooth, based on the analysis of masticatory rhythm, which is regarded as a common method for assessing the physiology of mastication.
At the same time, with the use of the same software, the changes were assessed by analyzing the stability among the components of the masticatory force applied by the muscles attached to the mandible during mastication, because the parameters for masticatory rhythm use the time element only.
The results were as follows:
1. The software made it possible to automatically analyze masticatory rhythm and stability of muscle forces.
2. The normal ranges of both masticatory rhythm and stability of muscle activity values were made clear.
3. Changes in masticatory function after loss of a mandibular first molar were favorably affected with the replacement by the artificial substitute that is not easily removable. Chewing rhythm and stability of muscular activities were improved. This tendency was particularly notable in the latter.

View full abstract

The purposes of occlusal adjustment of a crown are: to make the crown be harmonious to the dental arch in the intercuspal position; to maintain occlusal pattern in mandibular movements; and to build up numbers of occlusal contact points, contact area and contact sections that are similar to the adjacent teeth.
In the ordinary indirect method, the height of the crown is excessively high as a cast, 200-300μm, at intercuspal position. But, its height is proper only at intercuspal position in accuracy of a few μm, by means of occlusal adjustment using burs, abrasive points and articulating paper.
The aim of this investigation is to find out whether there are occlusal contact points with an accuracy of a few 10μm at intercuspal position and those in each cusp of crown, after occlusal adjustment during lateral and protrusive excursions.
Crowns were made on upper and lower molar teeth for each of 10 subjects. The occlusion of the crowns was adjusted in the ordinary clinical method before cementing by means of observation, tactile sensibility and an articulating paper pull-out test and occlusal contact pattern as index. After occlusal adjustment, the silicone-black method was used for estimating the occlusal contact in the intercuspal position. As for three out of ten cases, the occlusal contact of crowns was analyzed. In trial, temporary cementation and cementing of crowns, and after a week of cementation, occlusal contact patterns were taken on the articulating paper and the silicone-black. They were also observed by microscope and visual scaler.
As a result, occlusal contact sections were located on the incline of cusp or marginal ridges, and occlusal contact points were limited in near contact parts of facets shown on the silicone-black, which were made in occlusal adjustment. In the process of the occlusal adjustment, the number of occlusal contact points were gradually increased and contact areas became smaller.
Conclusions are as follows:
1. It is not necessarily easy to make occlusal contact point on each cusp of crown at intercuspal position.
2. Estimation should be done of the near contact part with relation to occlusal contact.

View full abstract

Bite force decreases as bite points change posteroanteriorly along the dentition; this raises questions about how the efficiency of the jaw elevator muscles is affected. The objective of this study was to determine the relation between bite force and the EMG/Force characteristics of the jaw elevator muscles as the bite point is changed posteroanteriorly during maximal (MVBF) and submaximal (30%MVBF) isometric clenching efforts. Seven fully dentate subjects without any history of craniomandibular dysfunction, mean age 30.8±2.4 volunteered for this study. Bilateral surface bipolar EMG activity from the superficial masseter (SM) and anterior temporal (AT) muscles was recorded simultaneously to alternate antagonistic vertical bite force measurements from the right and left first molar, first premolar, canine and anterior incisor teeth. The subjects clenched three times for 2 seconds each time to a requested level (MVC; 30%MVBF) on a force transducer. EMG/Force ratio (E/F) values for working (W) and balancing (B) muscles were calculated and normalized for each task. Statistical differences among unilateral bite points were analyzed with Two-way ANOVA. The magnitude of bite forces were similar to those proposed by other authors. Changing posteroanteriorly the bite point induced a gradual and significant (MVBF: p<0.01; 30%MVBF: p<0.001) increase of the corresponding working and balancing SM E/F values, with the latter being either the same or higher than the working ones. The E/F registered at the MVBF and 30%MVBF molar and anterior bite points were different in a proportion of 1: 2 in all subjects. At MVBF the AT's E/Fs also increased corresponding to the anteroposterior shift of the bite point (p<0.01) ; however, the 1: 2 proportion of the molar vs the anterior bite point was not observed; at 30%MVBF there was not any evident pattern, and the E/F were not significantly different among bite points. From these results it can be concluded that the EMG/Force ratio of the working and balancing superficial masseter and anterior temporal muscles is a posteroanterior increasing function and therefore has an inverse relationship with bite force, condition which may induce a negative effect in the electrical efficiency of the jaw elevator muscles.
The results of this study have been previously presented at the following meetings:
74th General Session of the International Association for Dental Research.
20th Annual Conference of the European Prosthodontic Society.
96th Meeting of the Japan Prosthodontic Society.
44th Annual Meeting of the Japanese Association for Dental Research.
75th General Session of the International Association for Dental Research.
Annual Meeting of the Niigata Dental Association, July 1997.

View full abstract

A thickerner can be prepared in any desired viscosity to provide a foodstuff for training aid and/or examining aid for swallowing impairment, and various products have been introduced on the market. In this study, we designed a test food to be used as an examining aid with videofluorography, by mixing Mousse-Up^® and Baritop^® as the thickener and the contrast medium, respectively. The test food was prepared with various mixture ratios of Mousse-Up^®, water, and Bantop^®. It was then evaluated in such the specificity as texture and contrast as well as its mobility during swallowing.
Results are as follows:
1. The thickerner and contrast medium could be homogenized.
2. Test foods could be prepared in any desired viscosity: liquid, soft, or hard, by changing the ratio between water and the thickerner.
Similar texture of test food to that of yogurt or rice gruel known as a training aid to the swallowing impaired could be prepared. The viscosity did not change when the test food was placed in a refrigerator for hours, thus the test food could be stocked in advance for the case.
3. The contrast efficiency on the videofluorograph was enough to trace the food clearly while the subject swallowed it, and contrast medium added to the test food did not affect the characteristic in viscosity.
4. To evaluate the mobility of test food during swallowing, electrical activities in the suprahyoid muscles were obtained from normal subjects.
When viscosity increased and/or volume of the food increased, the duration between the time when food bolus was formed and the time when the bolus was pushed back to the tongue base increased. Furthermore the bolus became difficult to be swallowed at a stroke. These results suggested that the form and the volume of food should be decided under the consideration of the swallowing function of patients.
The results suggest that the mixture made of Mousse-Up^® and Baritop^® cleared the requirements as a test food for swallowing functions.

View full abstract

The purpose of this study was to investigate the influence of the forced crossbite on the bilateral balance of the masticatory muscle activity in children. The subject examined in the study were 17 children with forced crossbites (7 boys, 10 girls, mean age 7.6 years old) .
The balance of the temporalis and the masseter muscle acivity was assessed by the asymmetry index (AI), and occlusal deviation and occlusal contacts were evaluated by the occlusal detection system “Occlusogram”.
The results obtained were as follows;
1. The mandible was detected to be laterally deviated to the side of the crossbites from the initial occlusal contacts to the intercuspal position (ICP) .
2. The activity of the temporal muscle during clenching in the ICP tended to be more dominant at the side of the deviated mandible than the other side.
3. When the subjects were classified into two groups; deviation side dominant and opposite side dominant group based on the balance of muscle activity of the masseter, AI of the masseter in both groups showed high correlation with the extent of lateral occlusal deviation.
These results suggest that the occlusal deviation due to the forced crossbites in children affected on the bilarteral balance of masticatory muscle activity.

View full abstract

It has been shown in humans that during mastication the head moves rhythmically and coordinately with jaw movements. A neurophysiological explanation for the mechanism of this finding has not been given so far, matter which became the objective of this study. Head movement and EMG of neck (sternomastoid and splenius capitis) muscles were recorded simultaneously in awake unrestrained rabbits during bread and raw rice chewing. Also, EMG activity was recorded in anesthetized rabbits from the sternomastoid and semispinalis capitis muscles during cortically evoked fictive mastication.
In voluntary mastication, the rabbits moved their heads rhythmically in a cycle similar to their chewing cycle and in an opposite direction to the jaw movement. Rhythmical EMG activity was observed in the sternomastoid, which showed biphasic activity in the jaw closing and opening movements during bread chewing; however, during raw rice chewing it was active only in the jaw closing movements. The splenius muscle showed tonic activity and no rhythmic activity was observed regardless of the test food used.
In cortically evoked fictive mastication the sternomastoid was activated only in the jaw closing movements. This activity increased when biting on wooden sticks between opposing molars took place. In the semispinalis capitis tonic activity was observed in both events.
Rhythmical head movement coordinated with jaw movement was observed during mastication not only in humans but also in rabbits; this observation indicates that rabbits are appropriate animal models to study head movement during mastication. From the results of this experiment we suggest that the sternomastoid may be reflexly coactivated with masticatory muscles by peripheral inputs such as periodontal afferents and afferents from propioceptors regulating sense of position during jaw opening and closing movements.

View full abstract