So that a possible involvement in the mineralization of
dental plaque could be investigated, the effects of
silicic acid on
calcium phosphate precipitation were assessed in vitro. By measuring the decrease in Ca2+ concentration (by means of
ion-selective electrodes), we determined both spontaneous precipitation and seeded crystal growth from solutions that contained 1 mmol/L
calcium, 7.5 mmol/L
phosphate, 50 mmol/L
Hepes pH 7.2, and various amounts of
silicic acid. Polymerized
silicic acid, but not its monomer, was found both to cause a 60% reduction in the lag period that precedes spontaneous precipitation and to enhance the growth rate of seeded
hydroxyapatite crystals.
Silica suspensions showed effects similar to those of polysilicic
acid. In all cases, the precipitated material was found to be
hydroxyapatite. Whereas seeded
brushite crystals grew slowly without
silicic acid,
hydroxyapatite was the only
mineral detected after crystal growth in the presence of
silicic acid. Apparently, polysilicic
acid acted as a substrate for
hydroxyapatite nucleation, inducing secondary nuclei on both
hydroxyapatite and
brushite crystals. The finding that polysilicic
acid could overcome part of the inhibitory effect of a
phosphoprotein on
calcium phosphate precipitation gave additional support for the idea that polysilicic
acid and
silica may promote the formation of
dental calculus.