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The dental hard tissues (enamel and dentine) are comprised of the mineral hydroxyapatite with approx 1% protein in enamel and approx 20% protein in dentine. Whilst dental enamel is the hardest material in the body, it remains susceptible to damage caused through dissolution mediated by acids . These acids can be generated either through bacterial fermentation of dietary sugars within oral plaque (caries)  or through direct contact with dietary acids (erosion) . The caries process ultimately results in a subsurface lesion of demineralised tissue, with a relatively well-mineralised surface zone. If this process is allowed to continue, the lesion may cavitate, resulting in catastrophic and extensive tissue loss. Contrastingly, erosion results in the progressive loss of mineral from the surface, with a shallow demineralised surface-softened zone (erosive lesion). Both caries and erosive lesions may also be substantially "repaired" through natural remineralisation processes driven by calcium and phosphate ions present in saliva, and strongly mediated by fluoride from topical toothpastes and mouth rinses.
Whilst the caries process is reasonably well-understood, erosion has received relatively little attention in the literature, and techniques for the characterisation of the surface softened zone remain scant. Measurement of the surface hardness via a Vickers diamond remains the primary technique to quantify the extent of demineralisation, and to assess the efficacy of treatments to remineralise the erosive lesion .
Furthermore, the full roles of chemical agents, such as fluoride and metal ions, have not been systematically investigated, particularly in the case of metal ions. It is generally believed that fluoride is able to somewhat reduce the rate of demineralisation whilst profoundly increasing the rate of remineralisation. By contrast, metal ions can substantially reduce the rate of demineralisation, but general opinion is that they strongly retard remineralisation. The overall beneficial effect of these ions may therefore be considered as the balance of protective effect against demineralisation versus the contradictive retardation of remineralisation. Further these effects are expected to be concentration dependent. Surprisingly, given the prevalence of certain metal ions (in particular Zn2+, Sn2+ and Cu2+) in oral healthcare products, the overall role of metal ions in the erosive dissolution of enamel, particularly when in combination with fluoride, is not well understood.
In a GSK-KU collaborative project (which is nearing completion), a model system was characterised to profile the effect of stirring on acid-based enamel erosion using imaging, surface hardness, profilometry and ion loss measurements. Sample results are shown in Figure 1 - for characterisation of acid-based erosion using scanning electron microscopy and in Figure 2 – for analysis of loss of ions upon treatment with acid.
Figure 1. Effect of duration of acid exposure to erosive loss showing enhanced step formation with prolonged exposure.
Figure 2. Effect of stirring on elemental loss during erosion formation as a measure of level of erosion.
The GSK-KU collaborative project further investigated the effects of a range of metal salts (SnF2, ZnF2, NaF, CuF2) on erosion using confocal laser scanning microscopy (CLSM), back-scattered imaging by scanning electron microscopy (SEM), surface micro-hardness (SMH) measurement by micro indenter and vertical tissue loss analysis by non-contact profilometry (NCP). The preliminary results demonstrated the efficacy of SnF2 and NaF to combat acid erosion in the model developed.
To date, several studies have reported the positive effects of metal ions including stannous salts on enamel erosions in clinical and in vitro studies [4-8]. However, the clear clinical evidence has not been fully supported by in vitro work where discordant outcomes were found [4-8]. None of the in vitro studies have provided an in-depth analysis of the type proposed in this project, which is based on prior preliminary results using the highly characterised erosion model.
I received the opportunity to begin my PhD straight after I completed my MChem degree but have worked in various roles within the NHS throughout school and university ranging from kitchen staff to administrative assistant within surgical offices and A&E receptionist.