Physical chemical effects of zinc on in vitro enamel demineralization

ObjectivesZinc salts are formulated into oral health products as antibacterial agents, yet their interaction with enamel is not clearly understood. The aim was to investigate the effect of zinc concentration [Zn2+] on the in vitro demineralization of enamel during exposure to caries-simulating conditions. Furthermore, the possible mechanism of zinc's action for reducing demineralization was determined.MethodsEnamel blocks and synthetic hydroxyapatite (HAp) were demineralized in a range of zinc-containing acidic solutions (0-3565 ppm [Zn2+]) at pH 4.0 and 37 °C. Inductively coupled-plasma optical emission spectroscopy (ICP-OES) was used to measure ion release into solution. Enamel blocks were analysed by Attenuated Total Reflectance-Fourier Transform Infrared Spectroscopy (ATR-FTIR), and HAp by X-ray diffraction (XRD) and neutron diffraction (ND).ResultsICP-OES analysis of the acidic solutions showed a decrease in [Ca2+] and [PO43−] release with increasing [Zn2+]. FTIR revealed a α-hopeite (α-Zn3(PO4)2.4H2O)-like phase on the enamel surfaces at >107 ppm [Zn2+]. XRD and ND analysis confirmed a zinc-phosphate phase present alongside the HAp.ConclusionsThis study confirms that zinc reduces enamel demineralization. Under the conditions studied, zinc acts predominantly on enamel surfaces at PO43− sites in the HAp lattice to possibly form an α-hopeite-like phase.Clinical significanceThese results have a significant implication on the understanding of the fundamental chemistry of zinc in toothpastes and demonstrate its therapeutic potential in preventing tooth mineral loss.