KING, J. D.: Beit Memorial Research Fellow, University Field Laboratories, Sheffield, England (Dec. 29, 1938).

Tooth structure, and the nutritional factors that control it, are of great importance in determining liability to caries. Other agencies may also be associated. Tooth structure and caries susceptibility are closely related. No controlled investigations have negatived this hypothesis, if the Mellanby cri­teria for tooth structure are carefully followed, but the meaning of ” hypo­plasia ” must be clearly defined.

In the hypoplasia of text-books (” gross ” hypoplasia) teeth are structurally defective, as evidenced clinically by obvious pitting, grooving and deficiency of enamel substance, but the abnormalities are usually very localized, other areas of the tooth often being of fairly good structure. In the dentin also, localization of defects, as in the case of Owen’s lines, is apparent in microscopic preparations. A much more common type of hypoplasia is that in which defective structure of enamel is less evident to the naked eye, although by means of a probe rubbed over the surface, or with magnifying glass, its various grades are easily detected. In this second type, the abnormalities are not localized but are more or less evenly distributed over much of the enamel surface. Similarly with the dentin—under

the microscope, structural deficiencies are more diffuse. The importance of dis­tinguishing between these two types of hypoplasia becomes obvious when their relationship to caries is considered. For example : in the rural districts of the Isle of Lewis, the incidence of caries and the Mellanby type of hypoplasia are relatively low, but in the urban population of the island the incidence and severity of both conditions are much greater. In town as well as country districts, however, the proportion of teeth affected by ” gross ” hypoplasia is small and bears no relation­ship to caries susceptibility.

References: J. Physiol., 1936; Lancet, 1938.

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