Cariostatic Influence of Commercial SDF on Hydroxyapatite Disc Demineralisation Kinetics

Objectives: To analyse and understand the synergistic role of silver and fluoride ions on the demineralisation kinetics of hydroxyapatite (HAP) discs.
Methods: HAP discs (PlasmaBiotal, UK; 20% porosity), 14mm (D) X 2mm (H), used as analogues for enamel were demineralised for 4h in 50ml, 1.0 M buffered pH=4.0 acetic acid, 37^oC. Thereafter, the discs were removed and treated with agents (Table 1) including Riva Star (SDI Ltd, Australia), a commercial silver diammine fluoride (SDF) containing oral health care product. The discs were then returned to the demineralisation solution for a further 4h. Throughout the demineralisation period, ISEs (Ion Selective Electrodes) were used to record real-time variations of Ca²⁺, and Ag⁺ concentrations at 1min intervals. The rate of Ca²⁺ release with time was used as a proxy for the rate of demineralisation. Similarly treated HAP powder samples were analysed using ³¹P and ¹⁹F MAS-NMR.
Results: The results of each treatment are shown in Table 1. Riva Star and NaF showed the greatest inhibition of demineralisation. However, AgNO₃ showed an initial increase in release of Ca²⁺ followed by a reduction. Discs treated with AgNO₃ & KI showed an immediate reduction in release of Ca²⁺ without an initial increase. Discs treated with AgNO₃, KI, and NaF showed similar results to those of Riva Star. The MAS-NMR results indicated formation of Ag₃PO₄ in AgNO₃ treated samples, CaF₂ in NaF treated samples, and both Ag₃PO₄ and CaF₂ in the sample treated with Riva Star.
Conclusions: The components of Riva Star work synergistically to inhibit the demineralisation process of hydroxyapatite discs. This may be due to the formation of insoluble Ag₃PO₄ and CaF₂.