Inhibition of enamel erosion and promotion of lesion rehardening by fluoride: a white light interferometry and microindentation study

Abstract

Objective: The primary aim of the present in vitro studies was to investigate fluoride as an inhibitor of citric acid-mediated demineralization of human enamel and promoter of lesion repair using a combination of white light interferometry, scanning electron microscopy, and microindentation. Secondary aims included investigation of the importance of brushing on bulk tissue loss, and comparison of the relative efficacy of commercially available toothpastes on inhibiting enamel surface softening and rehardening of incipient erosive lesions.

Methods: Resin-mounted polished enamel specimens were prepared from extracted human molars and pre-molars. Mean surface roughness (Sa) and bulk tissue loss following exposure to an erosive challenge, or an erosive challenge plus brushing were investigated using a MicroXAM ADE PhaseShift white light interferometer. Surface morphology was determined using a Zeiss Evo 50 scanning electron microscope (SEM). The utility of fluoride-based treatments to protect against subsequent acid demineralization and to promote remineralization of pre-formed incipient lesions was determined using microindentation-based enamel surface softening and enamel lesion rehardening models.

Results: Treating human enamel specimens with Sensodyne Pronamel conferred a clear protective benefit against a subsequent 300-second citric acid challenge as evidenced by the interferometry and SEM data. The increase in Sa and bulk tissue loss caused by an erosive challenge followed by brushing was markedly reduced by pre-treatment with sodium fluoride (NaF) in a concentration-dependent manner. Sensodyne Pronamel statistically outperformed Colgate Sensitive Enamel Protect both in the enamel surface softening model and lesion rehardening model, and conferred statistically superior enamel fluoride uptake. Treatment of erosive lesions with Sensodyne Pronamel resulted in statistically superior rehardening versus two Crest Pro-Health formulations containing stannous fluoride (SnF2) and sodium hexametaphosphate (NaHMP); the latter did not differ significantly from the fluoride-free negative control paste. Sensodyne Pronamel exhibited statistically significant superiority in a human saliva-based lesion rehardening model compared to Zendium Sensitive containing nominally comparable concentrations of NaF, as well as Colgate Sensitive and Colgate Sensitive Multi Protection containing sodium monofluorophosphate (NaMFP).

Conclusion: The utility of NaF, whether delivered from simple solution or toothpaste, to reduce citric acid-mediated surface roughening and bulk tissue loss has been clearly demonstrated. The effectiveness of Sensodyne Pronamel as an anti-erosion toothpaste has also been demonstrated in various microhardness models. Crest Pro-Health toothpastes containing SnF2 and NaHMP were not statistically differentiable from a fluoride-free control paste in the lesion rehardening model. The latter result indicates that the benefit of fluoride to promote mineral formation is outweighed by the effect of NaHMP as a mineralization inhibitor in this model.