Caries susceptibility increases with the proportion of protein in enamel. There is no connection between degree of hardness of enamel and either percentage of protein or susceptibility to caries. Hardness and specific gravity of enamel decrease from outer surfaces inward, gradually diminishing toward the dentino-enamel junction, and also from incisal areas toward cervical edges. Surface hardness and also specific gravity of enamel, from animals that are practically immune to caries, are considerably lower than for man. In aging —youth (time-range, 10-19 years) to middle age (time-range 40-49 years) —enamel increases in specific gravity and gains about 0.5 percent in weight. Since these increases cannot be caused by precipitations from saliva, they appear to be due to metabolic processes in enamel. Metabolic processes occur in completely formed unerupted enamel, and in enamel during pregnancy, but may be insignificant in the outer portions. In pregnancy these processes are connected with general metabolic conditions. There are no significant correlations between caries-frequency and age, specific gravity and hardness of enamel, for the different kinds of teeth. The old assumption that women’s teeth have a greater liability to caries than those of men is supported by the author’s findings.
The amount of protein in enamel, estimated from findings for nitrogen, ranges between 0.1-0.7 percent. Teeth having high caries-frequency usually contain 50 percent more nitrogen than teeth having low caries-frequency. Experimental data failed to support the opinion that great hardness implies low caries-frequency and vice versa; nor has any connection between hardness of enamel and its percentage of protein been demonstrated. The hardness of teeth of old Northmen (2000 years ago) seems to have been about the same as that for present-day Scandinavians, despite the fact that caries-frequency of old Northmen was very low. The hardness and specific gravity of dental enamel in dog, horse, ox, pig, elephant, and hippopotamus are considerably lower than in man, contradicting the widely held view that low caries-frequency is an attribute of great hardness of enamel.
Scleroscopic study of enamel of permanent unerupted teeth, and comparisons of the resultant hardness curve with the “normal curve ” of the second dentition, indicated that—in completely formed enamel—metabolic processes may occur, although they are very insignificant in the outer layer, but increase toward the inner
parts. The hardness of inner portions of enamel, in teeth from women in childbed, is much lower than that of the same parts for ” normal ” teeth, indicating that metabolic changes take place in a mother’s enamel during pregnancy. Teeth from women in childbed were divided into two groups : (a) from women who, during pregnancy, had been relatively healthy and lived under fairly good dietary conditions; (b) from women whose pregnancy had been disturbed by illness, or who had inadequate diet. The hardness curve for enamel in the ” healthy ” group diverged slightly from the ” normal-curve ” of the second dentition. The curve for the ” abnormal ” group showed that the hardness of inner parts of enamel was markedly diminished, indicating that the metabolic processes in enamel during pregnancy are connected with disturbances in general metabolic conditions. The result for the ” healthy ” group indicates that, during pregnancy, processes in enamel counteract the harmful influences to which the teeth are exposed during that period.
References: Svensk Tand. Tidskr., 1924, 1926, 1927, suppl. 1931 (last two published in English) ; Odon. Tidskr., 1938.