Purpose: To help frame long-range programs for education, research, and clinics on bridges and dentures, we estimated the numbers of these prosthetic appliances that will be needed in the future.
Methods: People of age 65 and above, including dependent elderly, were investigated. The average numbers of these appliances per person were estimated by regression analysis based on survey reports of dental diseases. Several regression lines were corrected so that mutual intersections would not occur between age groups and the estimated value would not exceed each upper limit. The total numbers of these appliances in the future were calculated from the estimated average numbers and population projections.
Results: It was projected that the numbers of bridges and dentures used by the elderly would increase 2.0 times (2.2-1.1 times, by 95% confidence interval) and 1.5 times (1.8-1.0 times) in 20 years, respectively, and plateau during the subsequent 10 years. The older the age group was, the higher these rates of increase would become. The numbers of bridges and dentures for dependent elderly people were also estimated to increase 2.7 times (3.2-1.0 times) and 1.8 times (2.2-1.0 times) in 25 years, respectively.
Conclusions: We will be faced with the importance of dental education, research, and clinics on bridges and dentures for the elderly and dependent elderly people.

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Purpose: It has been reported that removable partial dentures with retainers that are rigidly supported had many troubles about nonvital abutment teeth, especially displacement of the prosthesis and fracture of abutment teeth, than removable partial dentures with retainers that are unrigidly supported. The purpose of this study was twofold: 1) to examine the relationship between the rigidity of a retainer and root fracture, and 2) to clarify the effect of the remaining quantity on root fracture.
Methods: Two kinds of retainers with different rigidity (Akers clasp and Konuskronen-Telescope), and three kinds of abutment teeth with different heights of remaining dentin (2 mm, 1 mm, and 0 mm), were used. Strains of the crown-margin and post-surface on abutment teeth under two different types of load were measured. The loads were a static load and a cyclic load.
Results: 1. Under a static load, strains of the crown-margin and post-surface increased as the load increased, and the strains on the Konuskronen-Telescope were significantly larger than those on the Akers clasp. 2. Under a static load, strains on the Konuskronen-Telescope decreased when the height of remaining dentin exceeded 1 mm. 3. Under a cyclic load, preliminary failure (failure of the cement layer occurred between the crown or the cast core with a post and the teeth) occurred when the heights of remaining dentin of abutment teeth were less than 1 mm, in the case of a Konuskronen-Telescope, and 0 mm, in the case of an Akers clasp.
Conclusions: It was showed that the necessary heights of remaining dentin of abutment teeth were more than 2 mm in the case of a Konuskronen-Telescope, and more than 1 mm in the case of an Akers clasp.

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Purpose: Currently, magnetic attachments are used widely in clinical dentistry, because they have strong power of absorption to support dentures, despite their small size. However, it is not clear whether magnetism leakage from the magnetic attachments has an effect on the oral environment. Therefore, as a first step to evaluate this problem, we examined the effect of magnetism leakage on several oral microorganisms, by an adhesion assay.
Methods: Two kinds of magnetic assemblies were used in this study: one with magnetic fields and another without magnetic fields, as a control. Microorganisms were grown to the late log phase, and cultures were treated to adjust to appropriate cell densities. The magnetic assemblies were immersed in the cell suspension, for microbial adhesion; then they were incubated for a short time, followed by washing, to remove unattached cells. It was then briefly sonicated in a solution, and then a portion of the solution was spread on agar plates, to count the number of attached cells. Then, we compared the results between those with a magnetic field and the control.
Results: There was no significant difference in the viable adhering cells to the magnetic assembly between the magnetic fields and the control, though there was a small difference between different species.
Conclusions: We concluded that there is no significant effect on oral microbial adhesion in magnetism leakage from magnetic attachments, and we suggest that magnetism leakage from magnetic attachments has no detectable influence on oral microorganisms.

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Purpose: The purpose of this study was to compare contact patterns of tongue-to-palate and alveolar or maxillary dentures in edentulous patients with and without their dentures, during the production of [sa] and [∫i] sounds.
Methods: The subjects were 10 edentulous adults (3 men and 7 women). Artificial palates, made of black vinyl covering the palate and alveolar ridge of the maxilla, were coated with white alginate powder, and were then inserted into the mouths of the subjects. After pronunciation of the test sounds, the tongue contact areas on the artificial palates were demonstrated by wetting of the powder and assesing the change in color from white to black. An image processor system was employed for averaging five samples of the same sound for each subject. The averaged pattern of each subject's palatograms was then converted to the standardized dental arch form, added, and stored. Seventy percent of each subject's common parts were extracted from the combined palatograms of the same sound.
Results and Conclusions: On comparing the contact patterns of the edentulous stage (E) with the contact patterns of the denture-wearing stage (D), in both [s] and [c] sounds, the contact area of E was found to be wider than that of D, and also the distance between the right and left contact region of E was shorter than that of D.

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Purpose: To clarify the peripheral feedback mechanism of chewing various types of food, we analyzed the movement of the mandibular incisal point and the masseter muscular activity for the initial few cycles of mastication.
Method: Ten healthy subjects were asked to chew boiled fish paste, chewing gum, peanuts, and beef jerky, on their habitual chewing side, and for the first ten cycles of chewing, the gape, masticatory width, cycle time, and cumulative value of masseter muscular activities were compared among subjects, cycles, and the tested foods.
Results: Indicators were all largest at the first cycle, and thereafter decreased in the order of cycles. Though there were large variations among subjects and among cycles, the variations became small after the third cycle for chewing gum, after the fourth cycle for fish paste and peanuts, and after theseventh cycle for beef jerky. Comparing cycles, a significant difference was found between the first cycle and all the other cycles for indicators. Comparing the foods tested, no significant difference was found between any two foods up to the fourth cycle, and after the fifth cycle, significant differences were found between almost any two foods.
Conclusion: For the initial few cycles of mastication, in order to determine the amount of movement and muscular activity needed for the food, the masticatory movement changed with large variation. But once the amount has been determined, masticatory movement adapted to the texture of food takes place accompanied by minute changes.

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Purpose: To clarify the reliability of evaluating masticatory function, the stability of the masticatory movement of healthy subjects and TMD (Temporomandibular disorders) patients were analyzed.
Methods: The indicators used were SD/OD (standard deviation/opening distance) of lateral components of opening and closing, and of the vertical component, which show stability of path; the coefficient variation of opening, closing, occluding, and cycle time, which shows stability of rhythm, and SD/OD of the velocity component of opening and closing, which shows stability of velocity. A normal range was established from data of 50 healthy subjects (group A). Then another 50 healthy subjects (group B) and 50 TMD patients (TMD group) were used, to analyze the degree of specificity and sensitivity.
Results: The TMD group had much higher values than group A and group B (p<0.01). Because it was possible to make all subjects of group A fall into the normal range by taking at least 2 indicators for path, at least 3 indicators for rhythm, and at least 1 indicator for velocity, these numbers of indicators were used as the standard for determining the stability of masticatory movement. Evaluating the specificity and sensitivity of group B and the TMD group by path, rhythm, and velocity, each of them individually showed a value greater than 72%, except for sensitivity of velocity stability. Evaluating by any two indices, the value was greater than 80%, and when all three indices were used together, the value was greater than 90%.
Conclusions: It was concluded that it is more reliable to evaluate masticatory function with all of the indicators for stability of movement path, movement rhythm, and movement velocity, than with just one or two of the indicators.

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Purpose: The aim of this study was to clarify the effect of a change of oral sensory input on brain function, and to confirm if it can be detected electroencephalographically (EEG).
Method: Fourteen healthy dentate subjects were selected. A palatal plate and occlusal interference were used as experimental conditions for changing the oral cavity environment. Condition 1 (C1) subjects wore a palatal plate, and Condition 2 (C2) subjects wore both a palatal plate and an occlusal interference. Control subjects wore neither. Electromyographs (EMG) from the masseter and the posterior temporal muscles were taken simultaneously with brain waves, before and after mastication of Kamaboko. In the EMGs, the swallowing threshold, grouping voltage analysis, and bilateral power coordination patterns were analyzed. In EEGs, total power in, μV² for each frequency band, and the percentage power fraction (%α, %β), were analyzed, before and after mastication. The data for all three groups were compared statistically.
Results: In C 1, the effect due to an occlusal change was little for all parameters. On the other hand, the swallowing threshold significantly increased in C 2. In EMGs after mastication, mean voltages of the working side masseter muscles decreased significantly, and a tendency of corticalized control was seen in the grouping voltage. In EEGs after mastication, %α decreased significantly, and the power value of β rhythm and %β increased significantly.
Conclusion: The brain waves responded sensitively to a change of the oral cavity environment. The specific reciprocal activity of α rhythm decreasing and β rhythm increasing was seen, with coincidence to an occlusal sensory disorder and/or a depression of masticatory function. In conclusion, somatosensor information from occlusal sensation might be recognized in the higher center of the brain, and the possibility of detecting a change might be indicated electroencephalographically.

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Purpose: It is believed that occlusal interference has an effect on normal masticatory function. Yet, the relationship between masticatory movement and higher brain function during occlusal interference is not clearly understood. The aim of this study was to investigate the physiological effects of experimental occlusal interference during mastication of kamaboko on the interference side.
Method: Eight healthy dentate dental students were selected as subjects. Simultaneous recordings were made with electromyograms and electroencephalograms, and analyzed, before and after mastication of kamaboko. Evaluation by visual analogue scale questionnaires was also done. Twelve-session experiments were carried out over a period of 12 days. Each session included periods before and after application of an occlusal interference, the interference period, removal of the interference, and subsequent observation of interference. Daily changes of masticatory muscle activity, six comfort parameters, and changes of electroencephalograms and electromyograms caused by premature contact, were analyzed.
Results: Discomfort, which was described by the visual analogue scale, did not disappear during the interference period. Masticatory movement tended toward corticalization in analysis of grouping voltage, but recovered the following day. On the other hand, in electromyograms of masticatory muscles, the bilateral coordination pattern indicated a tendency toward the collapse of predominantly working masseter muscle activity, and it was not recovered even after the second day following removal. No daily change was observed in electroencephalograms before mastication; however %α, in electroencephalograms after mastication, decreased significantly compared with the control during the interference period, as verified by visual analogue scale evaluation.
Conclusion: It is suggested that the decrease of %α in electroencephalograms after mastication may be caused by premature contact influence on the masticatory system, reflected in the presence of discomfort during occlusion.

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Purpose: Dental casting silver alloy containing Ag, Sn, and Zn is frequently used for metal core or partial veneer restoration, and tarnish is observed in approximately 89% of such cases. However, there have been few reported observations of silver alloy's corrosive reaction to organic acids other than lactic acid. The purpose of this experiment was to evaluate the chemical stability of silver alloy in physiological saline and organic acids, which are mainly contained in saliva and dental plaque.
Methods: The dissolution of the constituent elements of silver alloy, changes in the weight and surface colors, and the composition of the corrosive product were analyzed using an immersion test. Results: The dissolution and insoluble deposition of Zn and Sn were recognized in all solutions tested, while those of Ag were little. Weight loss and discoloration of the specimens were also confirmed. Low discoloration was observed in the specimens in which the dissolution and weight loss were high. Silver alloy became more corrosive in organic acids, such as lactic acid and formic acid, than in physiological saline, and the dynamic state of dissolvgd ions differed depending on the types of the organic acids.
Conclusions: Silver alloy can dissolve constituent elements in the oral cavity. The progress of corrosion should not be evaluated only by changes in colors.

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Purpose: The purpose of this study was to investigate the conditions for getting good dimensional accuracy of calcium phosphate crystalline ceramic bridges in laboratory operation.
Methods: Internal gaps between bridges fabricated at different conditions, and master die and arc-shaped deformation of bridges, were measured using a three-dimensional measurement system.
Factors influenced dimensional changes were examined from the signal-to-noise ratio, sensitivity, and axis angle of retainers. The optimum condition that made the minimum amount of change for those factors was estimated by analysis of variance and confirmed.
Result and Conclusion: 1. The dispersion of gaps between bridges and a master die was narrowed by the use of ring liners. 2. Arc-shaped deformation of bridge was smaller by ac asting ring of 45 millimeters in diameter. 3. A gap between bridge and master die was adjusted by “Expansion of investment” and “Liquid concentration, ” which did not influence dispersion and arc-shaped deformation significantly. 4. The optimum conditions for getting good dimensional accuracy of bridges in this study were to use a casting ring of 45 mm in diameter, a ring liner, an investment of medium expansion, and 50% liquid concentration.

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