Mechanical properties of the dentinoenamel junction: AFM studies of nanohardness, elastic modulus, and fracture

The dentinoenamel junction (DEJ) is a complex and poorly defined structure that unites the brittle overlying enamel with the dentin that forms the bul k of the tooth. In addition, this structure appears to confer excellent tou ghness and crack deflecting properties to the tooth, and has drawn consider able interest as a biomimetic model of a structure uniting dissimilar mater ials. This work sought to characterize the nanomechanical properties in the region of the DEJ using modified AFM based nanoindentation to determine na nohardness and elastic modulus. Lines of indentations traversing the DEJ we re made at 1-2 mum intervals from the dentin to enamel along three directio ns on polished sagittal sections from three third molars. Nanohardness and elastic modulus rose steadily across the DEJ from bulk dentin to enamel. DE J width was estimated by local polynomial regression fits for each sample a nd location of the mechanical property curves for the data gradient from en amel to dentin, and gave a mean value of 11.8 mum, which did not vary signi ficantly with intratooth location or among teeth. Nanoindentation was also used to initiate cracks in the DEJ region. In agreement with prior work, it was difficult to initiate cracks that traversed the DEJ, or to produce cra cks in the dentin. The fracture toughness values for enamel of 0.6-0.9 MPa. m(1/2) were in good agreement with recent microindentation fracture results . Our results suggest that the DEJ displays a gradient in structure and tha t nanoindenation methods show promise for further understanding its structu re and function. (C) 2000 John Wiley & Sons, Inc.