The most fundamental and simple pattern of dental caries incidence is shown by using the cohort survey of one kind of teeth by post-eruptive tooth age under constant environmental conditions.
Takeuchi and his group have analysed various kinds of caries data from this point of view since 1960, but changing environmental factors, especially sugar consumption after eruption, (p), masked the fundamental pattern of caries incidence. Efforts were made by them to resolve the Cx pattern with aid of the probability of normal and exponential compound function suggested by Kawakami (1958), supposing that sugar consumption would not change.
Material used in this paper is the data from oral examination of school children, living in Shizuoka City, a typical urban area in Japan, with a fluoride content of 0.06ppm in the drinking water. Oral examination was done by the author himself and used the criteria of the WHO Oral Health Surveys (1971) for caries detection.
The number of teeth used for analysis is given in Table 3. All teeth investigated here were presumed to be exposed to constant annual sugar consumption after their eruption (p=about 27kg per head per year). In Fig. 2, the method of counting ΣCx (cumulative number of carious teeth up to the post-eruptive tooth age x-year per initial 100 teeth) and Cx (number of teeth newly attacked by caries during the post-eruptive tooth age x-year per initial 100 teeth) from the observed figures is illustrated. Both Cx and Mx are induced from ΣCx, applying the following fomula:
Cx=ΣCx-ΣCx_-1: Mx=Cx/ (100-C_x-1) ×100
In Table 4, the analysed indices of caries incidence are shown. In Fig. 3, the method of drawing abridged e^-kx line based on ΣCx and Cx is illustrated.
ΣCx and Cx given in Table 4 were drawn in Fig. 5 with the computed abridged e^-kx line. A fairly good fit of observed polygons and theoretical e^-kx lines of each tooth or tooth surface proves that the Cx curve is composed theoretically of two parts with both normal distribution and an exponential function of probability. Shimamura (1973) also reached the same conclusion from the data of deciduous teeth.
Caries attack has, therefore, a compound mechanism: the first mechanism consists of the distribution of susceptibility for caries per se of the individual tooth or tooth surface, and second mechanism is the probability of being newly attacked by caries, the chance to become carious, according to the level of an environmental factor, sugar consumption (p). This compound mechanism is shown as a model in Fig. 6.