Effect of topically applied silver compounds on the demineralisation of hydroxyapatite

ObjectivesThis study investigates the demineralization inhibitory mechanisms of AgNO₃, AgF and silver diamine fluoride (SDF) using a previously used hydroxyapatite (HAP) caries demineralization model system.MethodsHAP discs were allocated into three groups (n = 3) and immersed in demineralization solution (buffered pH 4.0, 0.1 mol/L acetic acid) for 4 h. Each disc was treated topically with either 3.16 M AgNO₃, 3.16 M AgF or 3.16 M SDF using a micro-brush. The discs were then demineralized for a further 4 h. Calcium, silver, and fluoride ion selective electrodes (ISEs) were used to monitor the changes in each ion concentration at 1 min intervals throughout. Demineralization inhibition was calculated as the percentage reduction in the rate of calcium ion loss from HAP (PRCL_HAP). Characterization of similarly treated HAP powder was carried out with Magic Angle Spinning-Nuclear Magnetic ResonanceResultsThe mean PRCL_HAP for each treatment group was; AgF (72.3 + 4.8%), SDF (69.7 + 5.3%) and AgNO₃ (14.9 + 2.7%). Ag₃PO₄ was detected in all HAP powders. CaF₂ and fluorohydroxyapatite (FHA) were detected only in powders treated with either AgF or SDF.The demineralization inhibitory efficacy of topically applied AgNO₃ results from the formation of a Ag₃PO₄ barrier. Whereas, the demineralization inhibitory efficacy of topically applied AgF, and SDF, results from the formation of a barrier composed of Ag₃PO₄, CaF₂, and FHA.SignificanceIn addition to their anti-microbial properties, clinical topical application of silver compounds for caries preventative treatment is due to their ability to form acid-resistant barriers composed of silver phosphate. When fluoride is present, this barrier also contains CaF₂ and FHA, additionally protecting the mineral.