Measurement of the microhardness and Young's modulus of human enamel and dentine using an indentation technique.

Conventional quasi-static and dynamic test methods have a number of limitations when used to measure the mechanical properties of enamel and dentine. These are due to the complex structure of the material and the small specimen size. In this investigation, a microindentation technique was used to measure the hardness and Young's modulus of human enamel and dentine and any variations with location. Freshly extracted molar teeth were sectioned, and the cut surfaces were ground and polished progressively to 1 micron. The polished surfaces were indented at different distances from the surface and amelodentinal junction with a Knoop indentor. Measurements of the length of the long indentation diagonal were used to calculate a value for hardness. It has been shown that the a-value for Young's modulus of a material can be calculated by comparing the ratio of the long and short diagonals on an indented specimen with the actual ratio of the indentor as any changes will be due to elastic recovery in the specimen. Values obtained for the Knoop hardness of enamel and dentine were in good agreement with those of other workers. It was also possible to show that there was a decrease in hardness with depth from the surface in enamel. The hardness of dentine increased with distance from the amelodentinal junction. Values for Young's modulus for dentine were in good agreement with those of other workers, and there was an increase in modulus with depth from the amelodentinal junction from 8.7 to 11.2 GNm-2. Values for Young's modulus of enamel were not as easy to calculate because of surface- and subsurface damage.