Effect of divalent metal cations on hydroxyapatite dissolution kinetics relevant to dental caries and erosion.
In recent years there has been an increasing awareness of the influence of various trace elements on reducing the progression of dental caries and of erosion. However, there are few clinical and even fewer in-vitro studies of the cariostatic effect of some trace elements on the progression of dental caries. Further, there is currently no consensus on the underlying physico-chemical mechanism on the influence of trace elements on these processes. The aim of this study was to investigate the effect of three divalent cations; zinc (Zn2+), strontium (Sr2+) and copper (Cu2+), on the physical-chemistry influencing hydroxyapatite (HAp) dissolution kinetics, using scanning microradiography (SMR), under simulated cariogenic and erosive conditions relevant to the oral environment. Compressed and sintered porous HAp discs were used as model systems for dental enamel. These discs were exposed to demineralising solutions containing a range of concentrations of Zn2+, Sr2+ and Cu2+, and either 0.1% acetic acid at pH 4.0 resembling dental caries, or 0.3% citric acid at pH 2.8 resembling erosion conditions. SMR is a development of the photographic microradiography technique of mineral quantification by means of X-ray absorption, but allows real-time quantification measurement of the rate of HAp mineral loss (RDHAp). Sequential SMR experiments during which the HAp disc was exposed to demineralising solution, containing each cation in either increasing or decreasing concentration order (separated by 30 minutes of washing with de-ionised water) allowed evaluation of the persistence of the influence of the divalent cations being investigated. The results showed that all three divalent cations decreased RDHAp significantly under both investigated conditions but via two different mechanisms. It was proposed that Zn2+ and Cu2+ decrease the RDHAp through a surface controlled mechanism whereas Sr2+ decreases the RDHAp through a solid phase change. This information will be useful as part of the development of therapeutic products which include these ions for the prevention of dental caries and erosion
AuthorsLingawi, Hanadi Saud
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