Recent studies show that methacrylate-based composites with amorphous calcium phosphate (ACP) as a filler can release supersaturating levels of calcium and phosphate ions in proportions favorable for apatite formation. These findings suggest that such composites could be effectively used as coatings for remineralizing teeth damaged by tooth decay. To examine this hypothesis, we tested composites in vitro for their efficacy to remineralize artificially formed caries-like lesions in extracted bovine incisors. Single 120-microns-thick sagittal tooth sections were placed in holders that exposed only the carious enamel surface. The exposed surfaces were coated with a 1-mm- to 1.5-mm-thick layer of the composite containing, by mass, 40% apatite, silica, or P2O7(-4)-stabilized ACP and 60% photoactivated resin comprised of Bis-GMA, TEGDMA, HEMA, and ZrM. The photocured composite-coated sections were immersed either in a remineralizing solution for 4 weeks at 37 degrees C (static model) or cyclically immersed in demineralizing (0.5 h) and remineralizing solutions (11.5 h) for 2 weeks (dynamic model). Quantitative digital image analysis of matched 102 microns x 220 microns areas from contact microradiographs taken of the sections before and after immersion showed that lesions coated with ACP-filled composites fractionally recovered 71% +/- 33% of their lost mineral compared with 14% +/- 13% for apatite controls in the static model and 38% +/- 16% compared with -6% +/- 24% in the dynamic model. The results suggest that sealants based on ACP-filled methacrylate composites have the potential to remineralize carious enamel lesions.

The present study examined the effect of anionic liposomes on precipitate formation in supersaturated calcium phosphate solutions. The liposomes were prepared by dispersing 7:2:1 molar mixtures of phosphatidylcholine, dicetyl phosphate, and cholesterol in buffered aqueous solutions containing 0 or 50 mM inorganic phosphate (PI). Unencapsulated PI was removed by gel filtration. The liposomes were then suspended in reaction solutions containing 2.25 mM Ca2+ and either 0, 1.0, or 1.5 mM PI. All experiments were carried out at 22 degrees C, pH 7.4, and 240 mOsm. External solution Ca2+ and PI losses were found to be appreciable only when the membranes of liposomes containing entrapped PI were made permeable to Ca2+ with the addition to the suspension of the ionophore X-537A. The Ca2+ losses, moreover, were up to 3 times as great (1.5 vs 0.5 mM) when accompanied by external PI losses than in PI-free external solutions where Ca2+ alone was involved. Previous studies showed that in this latter situation, decline in external Ca2+ concentration was the result of precipitate formation in the aqueous interiors of the liposomes. The present findings suggest that when the external solution phase was metastable, the apparent coupling of large additional Ca2+ losses with intraliposomal precipitation was due to secondary precipitation brought about by the seeding action of intraliposomal crystals penetrating into the external solution. The results may explain in part the origin of extravesicular mineral deposits in matrix vesicle calcification.

The present study examined calcium phosphate precipitation in aqueous suspensions of multilamellar liposomes as a possible in vitro model for matrix vesicle mineralization. Liposomes were prepared by dispersing CHCl3-evaporated thin films of 7:2:1 and 7:1:1 molar mixtures of phosphatidylcholine, dicetyl phosphate, and cholesterol in aqueous solutions containing 0, 25, or 50 mM PO4 and 0 or 0.8 mM Mg. After removal of unencapsulated PO4 by gel filtration, the liposomes were suspended in 1.33 mM Ca/0.8 mM Mg solutions and made permeable to these cations by the addition of the ionophore X-537A. All experiments were carried out at pH 7.4, 22 degrees C, and 240 mOsm. In the absence of entrapped PO4, Ca2+ taken up by the liposomes was largely bound to inner membrane surfaces. With PO4 present, Ca2+ uptake increased as much as sixfold with maximum accumulations well above values sufficient for solid formation. Precipitated solids appeared to be located predominantly in the aqueous intermembranous spaces of the liposomes. Amorphous calcium phosphate (ACP) precipitated initially in the presence of entrapped Mg2+, then subsequently converted to apatite intermixed with some octacalcium phosphate. The stability of the liposomal ACP was somewhat greater than that observed in bulk solutions under comparable conditions of pH, temperature, and electrolyte makeup. In time, the mineral deposits caused entrapped PO4 to leak from the liposomes. These findings suggest that the precipitation within liposomes is similar to that which occurs in macro-volume synthetic systems but that the precipitated solid eventually impairs the integrity of the surrounding intermembranous space.(ABSTRACT TRUNCATED AT 250 WORDS)

In adult human bone, fluoride uptake is accompanied by an increase in apatite crystal size. This increase, however, is not isotropic but is restricted primarily to growth in width and/or thickness, with no measurable change in length. In the present study, seeded growth experiments were conducted in vitro to determine whether this anisotropic effect is physicochemical in origin, i.e., a direct result of F- selectively enhancing lateral crystal growth, or is an indirect consequence of F--induced alterations in cellular function and matrix development. The growth reactions were maintained at 37 degrees C under physiologic-like solution conditions (1.33 mmol/liter Ca2+, 1.0 mmol/liter total phosphate, 0 or 26 mmol/liter carbonate, 270 mmol/kg osmolality, pH 7.4) using constant-composition methods. When new accretions accumulated to three times the initial seed mass, the solids were collected and net crystal growth was assessed by X-ray diffraction line broadening analysis. The X-ray results revealed that the carbonate constituent in our physiologic-like solutions promoted the proliferation of new crystals at the expense of further growth of the seed apatite. Solution F- concentrations of approximately 2 micromol/liter partially offset the repressive effect that carbonate had on primary crystal growth. Moreover, F- stimulated seed crystal growth in the same anisotropic manner as had been observed for adult human bone apatite, a finding that suggests that the latter growth in vivo was the consequence, in part, of direct F--mineral interactions.