On the Brittleness of Human Enamel

Cracks and craze lines are typically found in the enamel of both restored and unrestored molars. Although evaluations commonly report the elastic modulus, hardness and fracture toughness of enamel, no study has characterized the brittleness of this hard tissue. The brittleness of a material quantifies its susceptibility to undergo cracking and/or fracture under indentation and/or contact loading. Objectives: To quantify the brittleness of human enamel and to determine spatial variations in this property from the DEJ to the occlusal surface. Methods: Human 3rd molars were sectioned buccolingually, polished using standardized methods and subjected to nanoindentation to measure the hardness and elastic modulus. Indents were made directly beneath the cusps at regularly spaced intervals from the Dentin Enamel Junction (DEJ) to the occlusal surface along the enamel prisms. The “apparent” fracture toughness of the enamel was estimated in terms of the indentation fracture resistance assuming a Palmquist crack configuration and using indents placed at regular intervals from the DEJ to the occlusal surface. The elastic modulus, hardness and apparent fracture toughness were used to estimate the brittleness and its variation with distance from the DEJ. Results: The average hardness and the average elastic modulus of enamel increased with normalized distance from the DEJ from 3.5±0.3 to 4.3±0.5 GPa and from 76.0±5.6 to 86.6±8.7 GPa, respectively. The indentation fracture resistance of the enamel was 0.60±0.03 MPa•m^0.5, and there was no apparent trend in this property as a function of distance from the DEJ. Using the independent measures of hardness, elastic modulus and resistance to fracture, the brittleness numbers of enamel increased from approximately 880 µm^-1 at the DEJ to 1100 µm^-1 near the occlusal surface. Conclusion: The brittleness of human enamel increases with distance from the DEJ to the occlusal surface. (Supported by the National Science Foundation, BES 0521467)