EFFECTS OF ZINC AND FLUORIDE ON in vitro ENAMEL DEMINERALISATION CONDITIONS RELEVANT TO DENTAL CARIES

Abstract

Zinc has been investigated extensively as an anti-plaque and calculus agent. However, its interaction with enamel and its putative role in demineralisation and remineralisation are considerably less understood. In comparison to zinc, the interactions between fluoride and enamel have been extensively reported in the literature. The overall aim was to understand the effects of zinc ions [Zn2+] on enamel demineralisation (relevant to dental caries), whilst the effect of fluoride ions [F–] were similarly studied and used as a benchmark for the zinc investigations. The independent effects of varying concentrations of [Zn2+] and [F–] on enamel demineralisation kinetics were investigated during in vitro caries-simulating conditions. Human enamel blocks were exposed to series of in vitro demineralising solutions consisting of 0.1 M acetic acid adjusted to pH 4.0, with increasing concentrations of [Zn2+] or [F–]. Scanning Microradiography (SMR) obtained accurate, real-time quantitative measurements of changes in enamel mineral mass, as they were exposed to [Zn2+] or [F–]-containing acid solutions. Further, amongst other characterisation techniques, ultralow Energy Secondary Ion Mass Spectrometry (uleSIMS) and 19F MAS-NMR were used for determination of possible surface physical-chemical mechanisms of [Zn2+] and [F–] on the enamel dissolution processes respectively. This study confirmed that zinc reduces enamel demineralisation during in vitro caries-simulating conditions. An overall log-linear relationship was observed between the reduction in demineralisation and increasing [Zn2+] up to 3565 ppm. Fluoride showed this log-linear relationship up to 135 ppm, however, further reductions in demineralisation were minimal above this concentration. 19F MAS-NMR established that fluorite (CaF2) predominantly formed above 135 ppm [F–], whereas fluorohydroxyapatite (FHAp) largely formed below 135 ppm. uleSIMS indicated that zinc was largely concentrated at the near-surface region of enamel, which suggested that zinc acts predominantly at the enamel crystal surface in its mechanism of reducing demineralisation. Thus, at low concentrations, zinc appeared to act at PO43– sites on enamel surfaces, possibly via an adsorption-type process and was confirmed to form an α-hopeite-like phase, once surface adsorption was complete at higher concentrations. These results have significant implications on the understanding of the fundamental chemical aspects of zinc in toothpastes and demonstrate its therapeutic potential in preventing tooth mineral loss.