Compositional determinants of mechanical properties of enamel

Abstract

Dental enamel is comprised primarily of carbonated apatite, with less than 1% w/w organic matter and 4-5% w/w water. To determine the influence of each component on the microhardness and fracture toughness of rat incisor enamel, we mechanically tested specimens in which water and organic matrix were selectively removed. Tests were performed in mid-sagittal and transverse orientations to assess the effect of the structural organization on enamel micromechanical properties. While removal of organic matrix resulted in up to a 23% increase in microhardness, and as much as a 46% decrease in fracture toughness, water had a significantly lesser effect on these properties. Moreover, removal of organic matrix dramatically weakened the dentino-enamel junction (DEJ). Analysis of our data also showed that the structural organization of enamel affects its micromechanical properties. We anticipate that these findings will help guide the development of bio-inspired nanostructured materials for mineralized tissue repair and regeneration.

Figure 1

Optical micrographs of rat incisors cut in the mid-sagittal (a) and transverse (b) planes, which are normal to each other; black lines outline the dentino-enamel junction (DEJ). SEM-BSE micrographs of enamel polished in the mid-sagittal (c) and transverse (d) planes illustrate the difference in the enamel rod organization in these 2 planes. Note that the crack in (b) is a consequence of specimen preparation. Since the samples were not embedded, but rather were mounted in resin, the pulp cavity remained unfilled; this led to macrocracking of the sample during polishing. However, this crack did not affect the enamel integrity at the microscopic level.

Figure 2

Plots of the hardness (a) and fracture toughness (b) of rat incisor enamel in 4 experimental groups, namely, wet untreated, dry untreated, dry treated, and wet treated, collected in the mid-sagittal (■) and transverse (◆) planes. Error bars represent standard deviation values based on data obtained from 3 incisors per compositional group.

Figure 3

Optical micrographs representing characteristic damage produced by the microindenter in dry-untreated (a) and dry-treated (b) samples in the mid-sagittal plane. SEM-BSE micrographs demonstrating indentation damage in dry-treated samples in the mid-sagittal plane (c), and dry-treated samples in the transverse plane (arrows indicate areas of dentin attached to enamel) (d)