Enamel and Bleaching or Breaching: Vickers Hardness and Backscattered Electron Imaging

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Abstract
Although healthy teeth are never really white, bleaching agents are used cosmetically to whiten teeth. We studied the in vitro effects of some bleaching agents and acids on the Vickers Hardness (VH) of human dental enamel using a 100 gram [981 mN] load applied for 15 s and examined treated surfaces with backscattered electron mode scanning electron microscopy (BSE SEM; 20 kV, uncoated, chamber pressure 50 Pa). Rows of indents were made in the untreated condition and intervening rows placed after treatment. Thus, the samples served as their own controls. Gel bleaching agents − 35% hydrogen peroxide or carbamide peroxide - and 30% hydrogen peroxide solutions caused substantial reductions in microhardness. Weaker peroxide gels and solutions caused lesser changes. A 5% available chlorine sodium hypochlorite bleach treatment resulted in no significant change. Prism boundary discontinuities in flat polished surfaces are normally thin and dark in BSE-SEM imaging because of the lower mineral content in the region where crystallite orientation changes abruptly – they are also not joined up. Strong peroxide bleached surfaces, however, showed wider, whiter and conjoined boundaries as in acetic acid treated samples. This appearance is the hallmark of subsurface demineralisation and remineralisation, wherein the boundary region is widened and extended by dissolution of mineral, with reprecipitation of dissolved mineral in surrounding partially demineralised tissue and infilling of the original boundary defect. Identical sequences are identified in natural and artificial carious lesions and in acid etched enamel. This appearance was not generated by 6% peroxide gel. The softening of enamel by peroxide bleaching agents is explained both by the removal and redistribution of the hard mineral phase, accounted for by their acidic (i.e. low pH) nature; and the removal of the enamel protein matrix ‘glue’ holding the crystallites together.

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