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Fracture in enamel is anisotropic with respect ot the orientation of the enamel rods, with Wf for fracture parallel to the rods being 0.3 X 10(2) J/meter2 but on the order of 2.0 X 10(2) J/meter2 for fracture perpendicular to the rods. Fractographs of enamel showed that the enamel rods behaved as integral units during controlled fracture. A model was proposed to explain the fracture properties of enamel involving the assumption that the hydroxyapatite crystals did not fracture but that their orientation determined the fracture properties. Dentin was also anisotropic, with fracture parallel to the dentinal tubules being the strong direction - Wf was 5.5 X 10(2) J/meter2 for fracture parallel to the tubules, but 2.7 X 10(2) J/meter2 for fracture perpendicular to the tubules. Wf for dentin was of the same magnitude as the high strain rate Wf found for bone (2.0 X 10(2) J/meter2 [ref 3]). The data obtained in this investigation indicated that dentin and enamel are brittle substances and that forces occasionally imposed during mastication or bruxism probably are capable of initiating fracture when the normal anatomy of the tooth has been altered by cavity preparation. As consequence, the design of cavity preparations should include considerations to reduce stress concentrations and thus improve the practical fracture strength of tooth structure.

SummaryTreatment with ethylene oxide rendered reconstituted acid-soluble collagen readily digestible by combined pure cultures of human oral strains of fusiform bacilli, diphtheroid bacilli, streptococci, and veillonellae. The fact that only a few of these strains individually attacked such altered collagen to a slight extent indicates digestion by synergistic and symbiotic reactions between members of the oral microbiota. Concomitantly with increased digestibility from treatment with ethylene oxide, reconstituted collagen exhibited marked morphological changes as revealed by electron microscopy.

SummaryCalcareous corpuscles isolated from larval Echinococcus granulosus collected from sheep in Chile and New Zealand and from cattle in Lebanon showed only minor differences in their chemical composition and their heat-induced crystallization patterns. The only definite differences found were that the Chilean material lacked manganese, and that it had a somewhat higher magnesium and a lower phosphorus content than the other samples. As a consequence of the relatively low phosphorus content the Chilean material yielded upon heating at most traces of apatite. Previously described much more pronounced differences in chemical composition and crystallization patterns between corpuscles from different tapeworm species are hence probably essentially species-specific.