Intercrystalline solution ion activity product basis of subsurface dissolution in a hydroxyapatite system.

Recognition of the importance of low level solution fluoride on the propensity of bovine enamel to form an intact surface layer with concomitant subsurface dissolution in an in vitro bovine enamel model hydroxyapatite system has led to the development of a comprehensive physical and mathematical model capable of predicting experimental observations over a wide range of solution chemistries and hydrodynamics. It has been found that as bulk solution fluoride diffuses into the enamel matrix it is adsorbed by the hydroxyapatite crystallites in the surface region. Upon saturation of the available adsorption sites in this surface region the intercrystalline solution ion activity product (K(,FAP) = a(,Ca)('10)(.)a(,PO(,4))('6)(.)a(,OH)('2)) is elevated above a critical value (pK(,FAP) = 115.0 (+OR-) 1.0) and further dissolution of this surface region is inhibited. At the same time because of the adsorption/depletion mechanism, dissolution may proceed at a finite rate beyond the advancing fluoride front resulting in subsurface dissolution with simultaneous preservation of the crystallites in the surface region. Consideration of the appropriate diffusion, reaction and chemical equilibria equations has enabled theoretical computer simulation of the subsurface dissolution phenomenon. Calculations have shown that the theoretical model is semi-quantitatively descriptive of the mineral density changes occurring within the hydroxyapatite matrix as a function of time and position.