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According to the shortened dental arch concept, missing molars should not always be restored with prosthetic treatment. A shortened dental arch with missing molars is associated with a decrease in masticatory function. However, it is not known whether a shortened dental arch influences brain activity during chewing. This study aimed to clarify the effect of posterior arch length of mandibular bilateral distal extension removable partial dentures (RPDs) on brain activity during chewing. Eleven subjects with bilaterally missing mandibular molars (mean age, 66.1 years) participated in the study. RPDs with full dental arch and shortened dental arch were fabricated and brain activity during gum chewing under each dental condition was measured using functional magnetic resonance imaging. Brain activation during gum chewing with the full dental arch was observed in the middle frontal gyrus, primary sensorimotor cortex extending to the premotor cortex, supplementary motor area, putamen, insula and cerebellum. However, activation of the middle frontal gyrus was not observed during gum chewing with the shortened dental arch. The results of this study suggest that human brain activity during chewing in the middle frontal gyrus may be associated with chewing in the presence of the molar region.

Purpose: The degree of crushing and intraoral food flow was evaluated using peanuts by the occlusal forms of a removable bridge as indices to clarify the occlusal formative effect of maxillary molars on masticatory function., Methods: The subject was a 29-year-old male without any gnathostomatic disorders and his removable bridge ([7] 6 [5]) was so made in interocclusal distance as to be 0.5, 1.0 or 1.5 mm from 0 mm standard having occlusal contact to antagonist. Then the subject was asked to chew 3 g of peanuts in a series of unilateral chewing schemes consisting of 5, 10, and 20 masticatory strokes each. The pool rate in the buccal oral vestibule and lingual proper oral cavity as an index of food flow was calculated as the weight of peanut particles accumulated in each side as a percentage of the total volume of recovered peanut particles. Also, the degree of crushing was calculated as the weight of peanut particles that passed through 10-mesh sieves as a percentage of the total weight in each side., Results: As the interocclusal distance to the mandibular cusp increased with abrasion on the inside at maxillary buccal cusps, the pool rate of the buccal oral vestibule was significantly increased and correspondingly that of the lingual proper oral cavity was decreased. The degree of crushing indicated non-significances statistically among 0, 0.5, and 1.0 mm and significantly decreased in 1.5 mm as compared with 0 mm., Conclusion: As the interocclusal distance to mandibular cusps from the maxillary inside of buccal cusps was increased, the pool rate of the crushed peanuts in the buccal side increased and the pool rate of crushed peanuts in the lingual side decreased, and the particles showed a comparatively low degree of crushing. It is clear that maxillary buccal occlusion has an effect on the crushing function and food flow.

This study evaluated the relationship between occlusal force and tooth displacement in function. The three-dimensional tooth displacement transducer Type M-3 was used to measure the tooth displacement path and a three-dimensional small occlusal force sensor was developed to detect the occlusal force. Subjects' teeth were the upper and lower first molars. The three-dimensional tooth displacement and occlusal force of the upper and lower first molars were measured at the same time during light and heavy clenching in vivo. The direction of the tooth displacement and that of the occlusal force of the upper first molar almost coincided and they were along the palato-apical direction. However, in the lower first molar, the direction of occlusal force was in the apical direction, and the tooth displacement path was along the lingual direction. There was no relationship between the direction of occlusal force and clenching intensity at the upper and lower first molars and no relationship was observed between the direction of tooth displacement and occlusal force at the lower first molar. However, the upper molars showed a tendency to move more in the palatal direction as the force increased. In the case of the upper first molar, the energy of force seemed to change with the distortion of the alveolar bone especially around the root surface. However, in the case of the lower first molar, the energy of this occlusal force might be absorbed by the deformation of the mandible.

Purpose: Recently, there have been reports in favor of avoiding prosthodontic intervention as Shortened Dental Arch (SDA) in Northern Europe. However, many of these reports were conducted through interviews and questionnaires, and so the evaluation of the results lacked objectivity. Thus, this research analyzed the clinical status of SDA., Methods: For the selection of subjects, seven patients (average age: 59.3+/-13.2 years old) who had had the free-end missing on both sides of the lower dental arches for over five years were chosen as the SDA group. As the complete dental arch (CDA) group, seven patients (average age: 61.3+/-9.6 years old) who had no clinical defects in their masticatory function, and who had no missing teeth other than the third molar, were selected. Measurement items in this study were occlusal contact areas for the mandible premolar, center of force, occlusion time, and interdentium. The Mann-Whitney U test was used for statistical analysis and the significance level was assumed to be 5%., Results: The results were as follows: 1) The occlusal contact area of the mandible premolar of SDA was significantly large. 2) The center of force of SDA was significantly forward. 3) The occlusion time of SDA had a tendency of prolongation. 4) Interdentium of 3 2 | 2 3, 4 3 | 3 4 and 5 4 | 4 5 of SDA were significantly wide., Conclusions: There were differences of clinical parameters between SDA and CDA in this study.

Purpose: A mouthguard can protect stomatognathic systems from traumatic damage. However, severe occlusal wear of teeth and loss of teeth have often been found in players clinically. These problems might originate in strong clenching during sports. Although it is thought that a mouthguard may be effective for these types of clenching, the relation between mouthguards and clenching has not been sufficiently examined. In this study, the effect of a mouthguard (Drufosoft 3mm, EVA) on tooth distortion caused by clenching was measured and examined at three different clenching strengths., Methods: As a test tooth, a lower first molar was selected. A strain gauge applied to the outer surface of the buccal cusp was used to measure the distortion. A muscle balance monitor (GC) was used to regulate clenching strengths (10, 50, and 100%). The maximum-effort clenching without a mouthguard was assumed to be the 100% clenching strength. Measurements were conducted with or without mouthguard. A maximum value during clenching was assumed to be date of distortion by using analytical software AcquKnowledge (BIOPAC System Inc.). Statistical analysis software SPSS (SPSS Japan Inc.) was used for the Mann-Whitney test., Results: 1. The tooth distortion by clenching, regardless of the presence of the mouthguard, increased as clenching power strengthened, from 10, 50 to 100%. 2. The tooth distortion, regardless of strength of clenching, was decreased by wearing the mouthguard in all subjects. At 50 and 100% clenching, it was decreased significantly by the mouthguard in all subjects., Conclusions: Mouthguards decreaseed the tooth distortion caused by clenching. Therefore, a mouthguard may prevent not only traumatic injuries in contact sports but also damage to teeth and periodontal tissues and so on, which occur due to frequent strong clenching in many sports.

The purpose of this study is to clarify the occlusal contact mode between the upper and lower molars on the working side of group function occlusion during lateral excursion. After the intercuspal position (IP) and two lateral positions (L1, the middle point between IP and L2; L2, the edge-to-edge occlusal position of the molars) on the Gothic arch were defined, occlusal contact relations in these three occlusal positions were recorded, using black silicone. Digital data of real occlusal contacts and visualized data of close (less than 30 microns) occlusal areas, by computer image processing, were analyzed. The conclusions are as follows: 1. Although the numbers of real occlusal contacts and the visualized occlusal area tend to decrease toward L2 during lateral excursion, the former, in some cases, goes up and down. 2. Functional cusps play an important part in occlusal contact at the intercuspal position. 3. Occlusal contact points are on the functional cusps of the upper and lower molars, which can be clinically regarded as certain points, and these points slide on the inclining non-functional cusps of antagonistic teeth during lateral excursion. 4. Each upper and lower molar has 2 to 6 occlusal contact points near the top functional cusps at the intercuspal position, and some of them contact continuously during lateral excursion.

The purpose of this investigation is to detect vertically and buccolingually the occlusal force on the lower first molar in function and to make a two-dimensional vector analysis concerning the occlusal force. The subminiature occlusal force measuring apparatus, which makes use of two-dimension load cell, is developed with the apparatus equipped in the corona on the natural root of the tooth, each occlusal force on the single tooth is detected, when three subjects clench and masticate four kinds of test-foods. Conclusions are as follows: 1. Average occlusal force exerts 3.6 kgf during light clenching and 25.7 kgf during heavy clenching. The average direction of occlusal force against the tooth axis exerts 21.2 degrees during light clenching and 19.5 degrees during heavy clenching. 2. The masticatory force intensity exerts the differences in the phases of mastication and the properties of the test-foods. The maximal masticatory force is 33.1-7.9 kgf during the initial phase and 9.6-5.1 kgf during the last phase of mastication. 3. Average direction of masticatory force against the tooth axis is 19.9 degrees during the initial phase and 27.8 degrees during the last phase of mastication. 4. The operating period of masticatory force per cycle is 0.331 second on the average during the process of mastication. The longest period is in the initial phase of mastication, and as the mastication proceed it gradually decreases.

The study was aimed to investigate the effects of the occlusal contacts on chewing movements according to the changes of the mandibular first molar crown surfaces in 32 young adults. The study includes three components of the experiments as the control, various types of occlusal contact defects and the excessive conditions of occlusal surface contacts on the tooth. The chewing movement consistency was tested in the control study. The effects of the occlusal contact defects to the chewing movements, i.e., a periodontal mechanoreceptive defect (sensory block), a complete defect of coronal contact, partial defects (buccal or lingual cusp and inclination) of coronal surface, were then examined. Third, the effects of the occlusal deflective interference provided for the subjected tooth to the chewing movement was examined as the experiment in the excessive condition. The results obtained were as follows: 1. The consistency of segmental single tooth chewing movements was tested in the control study and revealed highly reliable among parameters of intra-day variance, but not enough for the inter-day variance in each subject. 2. The effects of occlusal contact defects were significant to the path and the rhythm of chewing movements in a poriodontal mechanoreceptive defect, complete coronal defect, cusp and inclination defects. 3. The effects of deflective occlusal interference were significant in the parameters, such as posterior deviation during closing path and inferior deviation of occluding point, prolonged duration in the closing phase, and shortening of duration in the occluding phase. It is suggested that either the occlusal defects or the interference conditions on one side of the first mandibular molar tooth may provide some effects to the chewing movements, and that appropriate occlusal contacts for the tooth may serve to maintain stabilized the path and the rhythm during chewing movements.

The upper first molar trajectory under the loading of extraoral force was studied in human subjects. The five loading directions, that is horizontal, upper, lower, high-pull and cervical were tested. Using a magnet-magnetic sensing system, the rectilinear and rotational movements were traced in the three planes, i.e. sagittal, frontal, and horizontal planes. It was concluded; 1. The molar inclined buccally with the loading in all the applied directions. 2. New evidence was found in the horizontal plane, i.e. the molar rotated mesially with the loading in the lower direction. The rotation may depend on the direction of the loop at the terminal of the inner-bow. Therefore it was suggested that the form of the loop affected the molar movements. 3. With the loading in the horizontal direction, the distal movement of the molar was evident and small displacement was observed in the extruded direction. 4. Loading both in upper and lower directions, the rotation of the molar was more evident than the rectilinear displacement. It was suggested that these loading would not be applicable to control the tooth in the rectilinear movements. 5. With the high-pull loading, the molar inclined mesially and shifted distally a little. On the other hand, it was greatly intruded. The results suggested that effective distal and intrusive movements would be expected by bending the outer-bow downward, since this procedure would prevent the tooth from tipping mesially. 6. The loading in the cervical direction caused large distal movement of the molar. Extrusion was also evident as it was expected from the loading direction.

The biting force of the first permanent molar, from children in the first grade of elementary school to adults in their seventies, was measured in 1959 and in 1987. Comparison between these two sets of data showed no significant difference in each age. The biting force of the second milk molar of kindergarteners aged from 4 to 5 years old, showed a decrease of nearly 5 kilograms on the average and in particular. The number of the children whose biting force was less than 15 kilograms increase. There was a negative correlation between the depth of the periodontal pocket of every kind of tooth and the biting force, and the average of the correlation coefficient was about minus 0.3. Especially, there was negative correlation between the depth of the periodontal pocket of the first permanent molar of patients in their forties and their biting force.