Caries appears to be dependent on two general kinds of factors : attacking or exciting factors ( food retention, bacteria, saliva) and protective or tooth” resistance ” factors (nutritional conditions, saliva, cellular activity of pulp in individual teeth). If tooth ” resistance ” is the stronger, caries does not occur despite accumulations of food debris on teeth. The hypothesis that natural ” resistance ” of teeth to caries includes localized cellular activity of dental pulps is supported by the data below.
Permeable channels in dentin and enamel: Teeth contain channels (organic matrix) through which nutritional exchanges with the blood occur. Body fluid, when present in dentin, diffuses along organic matrix in enamel. Enamel lamellae may be ” ports of entry ” for caries. Diffusion of fluid from odontoblasts through dentin and enamel: Histological observations and vital staining show presence of fluid in dentin—” dental lymph.” Dry dyes, sealed into experimental cavities in sound teeth of living dogs, sheep, monkeys and men, were dissolved and distributed through ” organized ” channels in dentin and enamel. Such diffusion occurred to greater degrees in recently erupted teeth than in highly matured teeth. Dental pulp regulates diffusion of dental lymph: Removal of pulp temporarily increases permeability of enamel, dentin, and cementum to such an extent that dental lymph is no longer confined to organized channels. Later, this permeability is greatly decreased, reducing caries activity in such teeth accordingly. Irregular distribution
of caries: If caries were due only to environmental factors (food debris, bacteria, saliva) then both surfaces of two contacting teeth should be attacked equally. In 516 sets of full-mouth x-rays, selected at random, 179 cases of unilateral caries were noted, suggesting that teeth offer a varying resistance and that factors other than environmental are influential in causing caries. Physiological changes in structure of teeth: Histological evidence and vital staining show that teeth mature after they take their positions, one of the changes being reduction in permeability of dentin and enamel. The great activity of caries in children and young adults is related to high permeability of teeth at these age-periods ; reduction in rate of acute caries in middle life is related to low permeability. X-ray examination of ground sections revealed that increased calcification occurs in dentin under some carious lesions, suggesting protective ” reaction ” of dentin to irritation by caries. Relation of metabolism to caries: Caries is ” associated both with environmental factors related to food retention and lactobacillus fermentation, and with physiological factors related to calcium-phosphorus metabolism.”
References: numerous, in leading dental journals, since 1906; J. Am. Den. Assoc., 1923; Den. Cosmos, 1934 ; J. Den. Res., 1934, 1937 ; Arch. Clin. Oral Path., 1938.