Understanding the Hidden Pathways of Human Tooth Decay at the Atomic Scale

Abstract

Dental caries, also known as tooth decay, is the most prevalent human chronic disease worldwide. Understanding the pathways and mechanisms of human tooth decay is central to the development of both prophylaxes and treatments, but only limited information is presently available about the initiation of caries at the nanoscale. In the present thesis, atom probe tomography 3D chemical maps and high-resolution transmission electron microscopy images managed to provide three distinct initial sites for human dental enamel dissolution: a) along the central dark line (CDL) within carbonated apatite nanocrystals, b) at organic-rich precipitates and c) along high-angle grain boundaries (HAGB). In chapter 3, 3D maps of the atoms within hydroxyapatite nanocrystallites in sound and naturally-decayed human dental enamel reveal a higher concentration of Mg and Na in the CDL. The CDL is therefore thought to provide a pathway for the exchange of ions during demineralization and remineralization. Mg and Na enrichment of the CDL also suggests that it is associated with the ribbon-like organic-rich precursor in amelogenesis. In chapter 4, we found that the carious region propagates from the organic-rich precipitates to surrounding crystallites after corrosion. We further showed that the organic C-F bonds have the opposite distribution to the inorganic fluoride, revealed the new insight of the pathways and positions of fluorine in human enamel. In chapter 5, the nanoscale in-situ compression test demonstrated the low-angle grain boundary (LAGB) sliding and the resulting size effect of HAP enamel. We then distinguished that HAGBs are more susceptible to corrosion than LAGBs, independent of whether they are ordinary grain boundaries or triple junction boundaries. The key novelties of this thesis are to provide vital new understanding and the systematic mechanisms of human tooth decay at sub-nanoscale thereby contributing to the prevention, restoration, and early diagnosis of dental caries.