Calculus Formation and Inhibition

The formation, development, and dissolution of hard deposits such as calculus are complex processes that involve numerous calcium phosphate phases as well as the interaction of these ions with organic molecules. Although formation is determined by thermodynamic driving forces, kinetic factors are also important determinants for the precipitation of specific calcium phosphate phases. The overall process, therefore, may involve the formation of metastable intermediates which may subsequently transform into the more stable hard deposits observed in vivo. A knowledge of the kinetics of growth of both individual calcium phosphate phases and their mixtures is important for elucidating the mechanism of calculus formation. Although salivary proteins are effective inhibitors of the mineralization reactions that take place in dental plaque, once adsorbed, their conformation may change to present surfaces that catalyze the nucleation of mineral phases. The variable pH conditions in plaque, expressed in terms of free ionic concentrations, will markedly alter the supersaturations with respect to typical calcium phosphate precursor phases such as dicalcium phosphate dihydrate (DCPD) and octacalcium phosphate (OCP). Physical-chemical studies have shown that the mineralization of all the calcium phosphate phases is controlled by reactions at the surface rather than by diffusion of lattice ions through the contacting liquid phase. This makes the rates of reaction very sensitive to ions and molecules in the solution that may adsorb at the active growth sites and, while not significantly incorporating into the precipitated crystal phases, markedly influences the rates of mineralization and demineralization. In addition, the molar ratios of calcium and phosphate lattice ions may also influence the rates of reaction. A complete elucidation of the mechanism of calculus formation must hinge on a physical-chemical understanding of these contributing factors.