Hardness and Young's modulus of transparent dentin associated with aging and carious disease.

This study investigated the changes in hardness and Young's modulus of the transparent layer of dentin associated with aging and the carious process. Eighteen extracted human molars with or without coronal caries were used in this study. The normal teeth were divided into two groups by age, and the carious teeth were divided into two groups of active or arrested caries. After polishing the specimens parallel to the long axis of the tooth, both hardness and Young's modulus were measured using a nanoindentation tester. The hardness and Young's modulus of the transparent layer in aged dentin were higher than the other portions of aged dentin. The transparent layer under carious lesions had a significantly lower hardness than the underlying normal dentin, whereas its Young's modulus was not significantly reduced. The hardness and Young's modulus of the transparent layer in active carious lesions were lower than those in arrested carious lesions.