Research publications concerning the prevention and control of an acid erosion of the teeth are relatively scarce. Among the efforts from a localized points of view, mention can be made of the use of alkali as a gargling agent, the gas-mask, the application of fluoride on the tooth surface, the use of anti-acid chewing gum, etc. Of the investigators of anti-acid gum, Baba alone offered his material for a follow-up by other researchers, others simply reporting the effectiveness of this kind of chewing gum in print.
In the present study, the authors were concerned with a series of basic evaluations of the effect of a fluoride used for purpose of caries prevention from the standpoint of anti-acid erosion. With permanent teeth (extracted) as study material, Brudevold's fluoride phosphate acid solution was topically applied to their surfaces and three items were evaluated: i) change in the hardness of dental surfaces, ii) change in the thickness and weight of teeth, and iii) optical microscopical change in the tooth surfaces through immersion in the acid solution. As an experimental method, a scratch ardness tester developed by our Department was used to measure hardness of the tooth surface before and after the application of a fluoride. After 24 hours following the application of the fluoride solution, the teeth were again measured for surface hardness under identical conditions.
The mechanism of dental erosion by an acid is generally thought to start with direct contact of acid fumes on the surface of teeth to resulting in an early stage of decalcification, which is aggravated by the lips and other mechanical action. Therefore, an acid solution was sprayed on the teeth for an hour, and replicas were taken of them after rinsing under tap water (See photographs). Subsequently, foam rubber containing water (5g load) was used to polish the surfaces and, after an hour of polishing, they were rinsed and replicase were taken of them again. These material subjected to observations of 100 hours.
Change in the thickness and weight of teeth was respectively measured by the use of a micrometers and chemical scales after each 10-hour intervals of acid spray. The fluoride solution was applied at intervals of 1-hour or 10-hour sprays, according to Brudevold is first and second methods.
Three solutions of HCl, H₂SO₄ and HNO₃ at pH level of 4.0 were used for immersing the teeth. The teeth were immersed in these solutions for an hour and then rinsed for an hour, and then rinsed for an hour, repeating the same process for as many as 40 hours. Replicas were taken of the teeth at the end of each process and the surfaces were chronologically examined under the optical microscope. Salient findings from these experiments were as follows:
1. As for changes in the surface hardness of teeth, the following measurements were obtained but there was no significant difference between the two sets of values (Brudevold is first and second methods).
First method Second method
Mean of pre-application values 146.2 182.6
Mean of post-application values 172.2 194.2
Differential 26.0 11.6
Evidence could not be found that an increase would occur in the hardness after fluoride application.
2. Thickness and weight of teeth registered a slight reduction in proportion to the passage of time. However, these was not established any significant difference among all the experimental conditions and it was not possible to say that a fluoride was effective in this respect.
3. As for an acid immersion of teeth, the degree of surface decalcification differed a little according to the different acid solutions used. In common to the three acid solutions, a decalcified image began to be observed after about five hours of immersion. Here again it was not possible to confirm the effectivness of a fluoride.

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In view of the spread of periodontal disease, it is important to establish its preventive countermeasures. In order to establish these countermeasures, epidemiological study is necessary, and an index of periodontal disease would be an effective adjunct to this study.
A total of 70 adult females (number of effectives) ranging in age of 18 to 22 in the Tokyo Metropolis were used as subjects. Using 5 indices related to periodontal disease, Debris Index (DI), Calculus Index (CI), Oral Hygiene Index (OHI), PMA Index (PMA-I) and Periodontal Index (PI), this study attempted to measure each index, and to simplify the calculation of each index. The specific character of drugs contained in dentifrices was also studied. The following results were obtained.
(1) The first mass dental examination was carried out in 70 adult females (number of effective strength). They were divided into three groups: 21 in group A, 24 in group B, 25 in group C. Dentifrices containing Allantoin, Ipsilon, and Dihydrocholesterol were used in A group, B group and C group, respectively. The patients were instructed to brush in the morning and evening. Examination was given 4 times every week for 5 weeks. On each examination each index value was determined. The mean of each index in the first examination was 1.92 for DI, 0.35 for CI, 2.26 for OHI, 4.71 for PMA-I and 0.51 for PI.
(2) Correlation between each index of each individual was determined by a coefficient of correlation (r). The following coefficients were obtained: DI and CI was 0.42, DI and OHI was 0.89, DI and PMA-I was 0.25, DI and PI was 0.54, CI and OHI was 0.79, CI and PMA-I was 0.43, CI and PI was 0.54, OHI and PMA-I was 0.39, OHI and PI was 0.62, and PMA-I and PI was 0.54. These coefficients were highly significant.
(3) Among these coefficients, OHI showed the highest correlation with other indices classified by each index. However, since OHI is the sum total of DI and CI, DI, which showed a higher correlation than CI with OHI, was used as a base. The equation of regression to find another index from DI was sought for, and then a highly significant equation was obtained.
(4) Furthermore, coefficients of correlation between each index of each individual, a total of 105 adults in group A, 120 in group B and 125 in group C, were highly significant. From the numbers of each type of examination of the adult females (number of effectives), the difference in each index for each individual for each interval between examinations was sought, and r with each other index was also sought in each group by using DI as a base. As a result, DI and OHI showed more than 0.74 in three groups, noting a highly significant correlation. However, in all 3 groups, DI showed a different correlation with other indices excepting OHI. Therefore, the different correlation between DI and CI, PMA-I, PI was inferred to be due to a difference in efficacy of the three drugs.

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