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Dissertation zugänglich unter
Structural aspects of bleaching and fluoride application on dental enamel
Dokument 1.pdf (8.746 KB)
Bismayer, Ulrich (Prof. Dr.)
Tag der mündlichen Prüfung:
Kurzfassung auf Englisch:
Tooth bleaching has become a popular modality to whiten discolored teeth. It occurs via the oxidation reactions between oxidizing agents and chromogenes deposited into dental hard tissues. Bleaching products are divided into two groups according to the type of the active oxidizing agent: peroxide-based and non-peroxide-based substances. Although various products have been widely used, the impact of bleaching on the structural aspects of dental enamel remains unclear so far. In addition, contradictory findings on the effect of fluoride application on enamel erosion have been reported. Therefore, this in vitro study focused on the effect of different whitening agents and fluoride reagents on the inorganic crystal chemistry of enamel using attenuated total reflectance infrared spectroscopy (ATR IR), combined with scanning electron microscopy (SEM), Raman spectroscopy, x-ray diffraction (XRD), electron probe microanalysis (EPMA), flame atomic absorption spectroscopy (FAAS) and total reflection x-ray fluorescence (TXRF).
In the first part of this thesis, three peroxide-based bleaching products: Opalescence Xtra Boost, Opalescence PF 20% and sodium perborate were studied. No significant difference was observed between unbleached and bleached samples by different analysis techniques, which indicated no structural and chemical changes in enamel apatite due to peroxide-based bleaching treatment. However, a comparison between neutral and acidic aqueous hydrogen peroxide (HP) solutions suggested that a low pH value can modify the dental enamel apatite.
Secondly, enamel alteration caused by a non-peroxide-based, “over-the-counter” bleaching product (Rapid White) was studied. Rapid White consists of a sodium chlorite-containing “accelerator” and a citric acid-containing “whitening gel”. The separate investigation of each component of Rapid White revealed that the citric acid-containing “whitening gel” rather than the sodium chlorite-containing “accelerator” substantially impacted on dental enamel. Enamel was affected at several levels: (i) the organic component was removed from superficial and deeper enamel layers and remnants of the bleaching gel were embedded in the emptied voids; (ii) cracks and chemical inhomogeneities with respect to Ca and P occurred on the surface; (iii) within a submicron layer of enamel, the Ca-O bond strength in apatite decreased, thus enhancing calcium leaching from the bleached dental enamel. Additional studies on aqueous citric acid solutions indicated that the structural modification of enamel apatite increased with the increase of the citric acid concentration and the number of treatments.
To study the protective potential of fluoride reagents against citric acid-induced erosion, enamel and, for comparison, geological hydroxyapatite samples were treated with 0.1 mol/l citric acid aqueous solutions and sodium fluoride of different concentrations ranging from 0.5 to 2.0%, respectively. The two chemical agents were applied either simultaneously or consecutively. The application of sodium fluoride alone did not suppress the atomic-level changes in apatite exposed to acidic agents. The admixture solutions containing citric acid and sodium fluoride led to the formation of CaF2 and considerably reduced the changes in the apatite P-O-Ca framework. However, the CaF2 globules deposited on the enamel surface seem to be unable to prevent the alteration of the apatite structure during further exposure to acidic agents. No evidence of fluoride-induced recovery of the modified apatite structure was found.