title: The Distribution of Carbonate in Enamel and its Correlation with Structure and Mechanical Properties.
abstract: The correlation of carbonate content with enamel microstructure (chemical and crystal structure) and mechanical properties was evaluated via linear mapping analyses using Raman microspectroscopy and nanoindentation. Mappings started at the outer enamel surface and ended in the inner enamel near the dentin-enamel junction (DEJ) in lingual and buccal cervical and cuspal regions. The carbonate peak intensity at 1070 cm-1gradually increased from outer to inner enamel. Moreover, the phosphate peak width, as measured by the full width at half maximum (FWHM) of the peak at 960 cm-1, also increased, going from ~9 cm-1in outer enamel to ~13 cm-1in enamel adjacent to the DEJ, indicating a decrease in the degree of crystallinity of hydroxyapatite from outer to inner enamel. In contrast, Young's modulus decreased from 119±12 to 80±19 GPa across outer to inner enamel with a concomitant decrease in enamel hardness from 5.9±1.4 to 3.5±1.3 GPa. There were also significant correlations between carbonate content and associated crystallinity with mechanical properties. As carbonate content increased, there was an associated decrease in crystallinity and both of these changes correlated with decreased modulus and hardness. Collectively, these results suggest that enamel carbonate content and the associated change in the crystal structure of hydroxyapatite, i.e. degree of crystallinity, may have a direct effect on enamel mechanical properties. The combination of Raman microspectroscopy and nanoindentation proved to be an effective approach for evaluating the microstructure of enamel and its associated properties.
authors.affiliation: Department of Oral Biology, University of Missouri-Kansas City School of Dentistry, Kansas City, MO, USA.
authors.forename: Changqi
authors.initials: C
authors.lastname: Xu
authors.affiliation: Department of Oral Biology, University of Missouri-Kansas City School of Dentistry, Kansas City, MO, USA.
authors.forename: Rachel
authors.initials: R
authors.lastname: Reed
authors.affiliation: Department of Oral Biology, University of Missouri-Kansas City School of Dentistry, Kansas City, MO, USA.
authors.forename: Jeffrey P
authors.initials: JP
authors.lastname: Gorski
authors.affiliation: Department of Oral Biology, University of Missouri-Kansas City School of Dentistry, Kansas City, MO, USA.
authors.forename: Yong
authors.initials: Y
authors.lastname: Wang
authors.affiliation: Department of Oral Biology, University of Missouri-Kansas City School of Dentistry, Kansas City, MO, USA.
authors.forename: Mary P
authors.initials: MP
authors.lastname: Walker
keywords: Raman microspectroscopy; carbonate distribution; enamel; mechanical properties; nanoindentation