The characteristics of surface structures of teeth may play a role in relative immunity or susceptibility to caries. In susceptibility, certain surface peculiarities in structure may constitute (a) predisposing factor(s) for the onset of the caries process. The coronal portions of a human tooth are covered by a structure of epithelial derivation—enamel cuticle. In the author’s studies, enamel cuticles were stained, before removal, by the author’s technic described in McClung’s Handbook of Microscopical Technique, p. 353 (1937). Of the stains employed, Mallory’s triple connective-tissue stain, methylene blue and picro-carmine were most useful. The affinity for stains varied in different cuticles; where it was greatest, cuticles seemed least homogeneous. Outlines of epithelial cells could be demonstrated in some. Cuticles removed from enamel edges of carious lesions—after thorough washing and scrubbing of the edges with a hard brush dipped in tap water and pulverized pumice—were found to be infested with bacteria; other parts were less infested or not at all. Zones of enamel cuticle (striae) corresponding to perikymata on enamel surface were different, in structure, from intervening zones. When the former zones showed great affinity for stain—as was often the case—they contained great numbers of bacteria. For explanatory comment and photomicrograph, see the author’s translation of Meyer’s Normal Histology and Histogenesis of the Human Teeth and Associated Parts, p. 20 (1935). Enamel cuticles of teeth subject to erosion could be removed from only small areas of non-eroded surfaces, and showed only slight affinity for stains. The study is in progress.
The principle(s) of relative immunity and susceptibility of human teeth to caries are not understood. The author’s attempt to clarify these principles—the main results of which up to now are indicated above—has been guided by the following clinical observations : (1) Unerupted, impacted and heterotopic teeth are not subject to caries, (2) the onset of which is limited to that part of a tooth exposed to oral environment. (3) Caries may commence in any part of a tooth surface which has emerged from its retaining tissues. (4) Homozygous twins may show marked similarity in caries susceptibility and immunity, and in the localizations of the process. (5) Developmental grooves are more susceptible to caries than
smooth surfaces. (6) Teeth exposed to unhygienic conditions may be susceptible or immune to caries. (7) Surfaces subject to attrition, (8) hypoplastic teeth, and (9) surfaces subject to erosion, are relatively immune to caries. (10) When a specimen human tooth is inserted into an artificial denture, and exposed to the oral environment, it may or may not become carious.
References: Two; in text, above.