Advanced synchrotron SAXS and WAXS analysis of the demineralization of human tooth enamel indicates a crystallographic-orientation-dependent anisotropic dissolution process.Almost everyone suffers at one time or another from tooth decay. Dental caries usually begins in the outer hard mineralized layer of tooth enamel and can undermine its strength and structure, affect appearance, and cause pain. Despite being one of the most common chronic diseases affecting teeth, relatively little is known in detail about how dental caries progresses to cause demineralization including what structural changes take place at the nanoscale in the enamel.
“While the link between a sugar-rich diet, bacterial activity, and the acidic dissolution of the mineralized dental tissue is clear, most practical counter-measures advised by dentists aim to help remove plaque thorough brushing or reduce enamel solubility by replacing hydroxyapatite with its fluorinated counterpart or both,” explains Alexander M. Korsunsky of the University of Oxford.
Efforts to develop substances that control bacterial activity or dissolution have usually relied on empirical observations but now Korsunsky and his team at Oxford, together with colleagues from the Universities of Birmingham and Surrey, have used advanced synchrotron small-angle and wide-angle X-ray scattering (SAXS and WAXS) at the Diamond Light Source at Harwell in the UK to get an understanding of dental caries progression and enamel erosion at very fine scales [Sui et al., Acta Biomaterialia (2018), https://doi.org/10.1016/j.actbio.2018.07.027].
“Our study is part of a large project aimed at collecting insights into the micro- to nanoscale details of demineralization during dental caries with a view to learning to control it in ways that make maximum use of the natural remineralization processes inside human oral cavity,” Korsunsky told Materials Today.
The researchers devised a setup that mimics the natural demineralization process by allowing a controlled amount of lactic acid to be introduced onto a section of a human tooth. SAXS/WAXS analysis during the interaction between the acid and enamel allowed the team to follow one of the key processes in the development of dental caries by monitoring the corresponding changes in enamel microstructure.
Tooth enamel comprises a complex, hierarchical composite structure of interlocking rods and inter-rods in a keyhole-like arrangement. Each rod/inter-rod is made up of bundles of hydroxyapatite (HAp) crystallites, which range in length from a few tens of nanometers to the entire thickness of the enamel. When enamel is exposed to lactic acid, a common agent involved in dental decay, demineralization begins. The acid diffuses into the crystallites, dissolving the HAp and undermining the structure of the enamel.
The researchers found that along with a reduction in mineral volume, the rate and direction of dissolution of HAp crystals depends on the orientation of the crystal faces in the mineral. Moreover, they observed a dramatic initial decrease in the layer containing HAp crystallites with the smallest crystalline dimension. The SAXS/WAXS analysis indicates that the most severe demineralization occurs just beneath the surface of the enamel. When the pH level gradually recovers, remineralization also occurs.
The researchers found that the rate and direction of dissolution of HAp crystals depends on the orientation of the crystal faces in the mineral. The analysis indicated that the most severe demineralization occurs just beneath the surface of the enamel. However, the team also found that remineralization of hydroxyapatite occurs driven by local variations in acidity.
The researchers hope that better understanding of the changes in mineralization associated with dental caries will help lead to more effective treatments or even reversal of decay.