[Dissolution kinetics of synthetic hydroxyapatite and human enamel crystals]

A comparative study of the dissolution of synthetic hydroxyapatite and human enamel crystals has been realized in a buffered (sodium citrate - citric acid) solution in presence of variable quantities of sodium fluoride. The pH of the solutions used was 3.7 and 5.6. The dissolution curves of the synthetic hydroxyapatite and the human enamel crystals were similar. In all cases, the dissolution mechanism was characterized by a first step with a rapid kinetic corresponding to the formation of a central cavity extending parallel to the c axis of the apatite crystal and observed with the electron microscope. In a second stage, the dissolution kinetic was much slower corresponding to a saturation of the buffered solution and an extension of the crystal destruction towards its lateral external surfaces. The dissolution kinetic was faster in a more acidic solution and the total dissolution at a given moment varied inversely to the fluoride amount.