National Science Foundation
We have used nanoindentation techniques to characterize the hardness and stiffness of enamel as a function of position on cross-sections of human teeth.
Over the last ten years we have used nanoindentation techniques to characterize the hardness and stiffness of enamel as a function of position on cross-sections of human teeth. This work has shown, for the first time, that the mechanical properties of human enamel can vary by more than 100% as one moves from the apex of a human molar down to the enamel/dentin junction. Figure 1 presents a mapping of Young’s Modulus for a maxillary second molar (M2) and similar results were obtained for the hardness of this tooth as well. Both the modulus and hardness maps demonstrate that quantifying variations in mechanical properties offers far more information than the simple averages that had been reported prior to this study. Furthermore, these variations can be correlated with changes in chemistry or organic content .
Utilizing hardness and modulus maps, more recent work has shown that first, second and third molars are different in their mechanical properties and attempts are now being made to correlate these differences in properties to differences in function during mastication. Maps such as the one shown in Figure 1 are being used as inputs to finite element models (FEM) to predict how local stresses during mastication are impacted by these variations in Young’s Modulus with position. Other human teeth, such as incisors, have also been studied and similar correlations will be attempted . We have also begun to compare these results from recently extracted teeth to much older human teeth obtained from the Smithsonian Museum. While the enamel of first, second and third molars from the Smithsonian show similar trends in hardness and modulus with location, far greater location variations in properties have been documented.Being able to explore differences in mechanical properties over smaller (less than 1 square micron) and larger (several square millimeters) areas using smaller and larger probes would be very beneficial and will enable us to link the behavior of individual constituents in enamel’s microstructure with the overall, macroscopic behavior of enamel that we are predicting using FEM. These nano and macro probes are also ideal for exploring the complex composite structure of dentin that support the outer enamel [6-7].
Beyond human teeth we have also studied molars extracted from Howler Monkeys (Alouatta palliata) and the enamel in these teeth show similar variations in hardness and modulus to those of human teeth . The corresponding variations in chemistry are similar as well. The similarities are quite surprising given that Howler Monkeys have far more abrasive diets than humans and are the subject of an ongoing investigation. Lastly, we have also expanded our studies to include vary unusual enamel structures such as that found in the teeth of water voles (Arvicola amphibius) .