Characterization and modification of permselective properties of apatite membranes.

Compressed apatite disks were used as models for tooth enamel. Their permselective properties were studied as functions of the pH of the gradient solutions, the composition of the apatites, and the pretreatments with anionic phosphate compounds and cationic proteins. The results are discussed in relation to the caries mechanisms.

Characterization and modification of electrochemical properties of teeth.

Permselectivity of teeth was studied by membrane potential measurements. The enamel was found to have a preferential affinity for calcium and hydrogen ions. Its permselectivity could be modified by anionic compounds, such as phytate and alginate; by cationic compounds, such as protamine, polyarginine, and polylysine; or by an alternating coating of the above compounds.

The effect of supersaturation on apatite crystal formation in aqueous solutions at physiologic pH and temperature.

The importance of supersaturation in the dynamics of apatite precipitation from aqueous solutions is well-established. To determine whether this parameter has a comparable impact on the concomitant development of the textural properties of this phase, such as crystal size and shape, we investigated mineral accretion in synthetic solutions seeded with 0.67 g/L apatite over a range of supersaturations at pH 7.4 and 37 degrees C. A dual specific-ion electrode-controlled titration method was used to maintain the seeded reactions under the following solution conditions: 1.0 to 1.8 mmol/L Ca2+, 0.67 to 1.2 mmol/L total phosphate (PO4), Ca/PO4 (initial) = 1.5, 143 mmol/L KNO3, and 10 mmol/L HEPES. Samples were collected for chemical and textural analyses when the seed apatite was reduced by new accretions to 1/2, 1/4, 1/8, 1/16, and 1/32 of the total solids in suspension. All new accretions were found to be apatitic. At the lowest supersaturation, accretion occurred primarily by growth of the seed crystals. However, at the highest supersaturation examined, the crystals at the end of the experiments were actually smaller, on average, than the original seeds, even though the total mass increased 32-fold. The results suggest that proliferation of new crystals supplanted growth of the seed crystals as supersaturation was increased. The results also suggest that differences in tissue fluid supersaturation may contribute to the large disparity in dimensions between dentin and enamel apatite crystals.

Hydrolysis of dicalcium phosphate dihydrate in the presence or absence of calcium fluoride.

Effects of temperature (25 and 37 degrees C), pH (4.9-10.5), and CaF2 on CaHPO4.2H2O (DCPD) hydrolysis were studied in a pH-stat. Octa-calcium phosphate (OCP) was the product at pH 6.2-6.8 and 25-37 degrees C; thermodynamically stable apatitic compounds were formed at higher pH and/or higher temperature. In the presence of CaF2, apatite was the product, its crystallinity improved, and the fluoride content increased as pH of the reaction decreased. The results demonstrate the remarkable ability of fluoride to promote the hydrolysis of an acidic calcium phosphate, DCPD, to apatite.

Effects of calcium phosphate solutions on dentin permeability.

Calcium phosphate solutions at various concentrations and pH levels were used to obstruct the dentinal tubules. The effects were evaluated by measurements of permeability through dentin discs and by scanning electron microscopy. Precipitation kinetics were followed by pH changes in the solutions and products were determined by X-ray powder diffraction. The solutions were applied in two ways: (a) calcium and phosphate solutions were mixed before application and (b) one solution (calcium or phosphate) was applied first followed by the other solution. Three kinds of human dentin discs were used; one with smear layer and the other two with tubules exposed by sonication or etched by acid. The high concentration calcium phosphate solutions at pH = 9.5 rapidly precipitated amorphous calcium phosphates that obstructed the dentinal tubules and decreased dentin permeability by 85% or more. At pH = 5.6, the calcium phosphate solutions precipitated large crystals of dicalcium phosphate dihydrate. In this case, the effectiveness in obstructing dentinal tubules was found to be procedure sensitive.

The effects of magnesium and fluoride on the hydrolysis of octacalcium phosphate.

The adsorption of Mg ions on octacalcium phosphate (OCP) and its effect on OCP hydrolysis, with and without F, were studied. The Mg adsorption isotherm was fitted by the Langmuir model with an affinity constant of 0.74 ml/mumol and maximum number of sites, 31.19 mumol/g. The hydrolysis rates were measured in a pH stat by titration of base and were strongly temperature dependent. The products were examined by X-ray diffraction and chemical analysis. OCP hydrolysis takes place in two stages: the fast initial process, which is attributed to the surface topotactical conversion, followed by the main, slower process, which involves the nucleation and crystal growth. Mg ions, as 1 mmol/l MgCl2, prevented the initial surface reaction and decreased the nucleation rate dramatically and the growth rate slightly; F increased the rates of surface reaction and both the nucleation and crystal growth processes. The Ca/P ratio (1.53) and the line broadening in the X-ray diffraction patterns of the apatitic products were not significantly affected by the F. Mg also did not affect the Ca/P ratio and the line broadening at (002) diffraction, but decreased the line broadening at (310) diffraction.

Dentine and enamel bonding agents.

Previous studies have shown that sequential use of aqueous FO (ferric oxalate containing a small concentration of HNO3), acetone solutions of NPG (N-phenylglycine), and PMDM (the reaction product of pyromellitic dianhydride and 2-hydroxyethylmethacrylate) yields strong adhesive bonding of composite resins to both dentine and enamel. The purpose of this study was to determine if aluminum ions could be substituted for ferric ions and if the procedure could be simplified. Aqueous solutions containing aluminum oxalate and aluminum nitrate, followed in sequence by acetone solutions of NPG and PMDM, gave strong tensile adhesive bond strengths between a composite and extracted human teeth. Comparable values have been obtained with FO, NPG and PMDM. Aluminum oxalate solutions containing no nitrate gave lower bond strengths, as was the case with FO. Aqueous solutions of acidified aluminum oxalate can dissolve NPG, thereby allowing a simplification of the procedure. Tested for comparison, commercially available dentine bonding agents gave lower average bond strengths on dentine than did some of the experimental materials.

Dental applications of amorphous calcium phosphates.

Certain commercial materials and equipment are identified in this paper to specify the experimental procedure. In no instance does such identification imply recognition or endorsement by the National Institute of Standards and Technology or the ADA Health Foundation, or that the material or equipment identified is necessarily the best available for the purpose.

The role of octacalcium phosphate in subcutaneous heterotopic calcification.

Comparison of lattice parameters and morphology of some of the microcrystallites in a subcutaneous heterotopic calcification reported by Daculsi et al. [1] with those of heat-treated octacalcium phosphate (OCP) suggests that OCP is one of the mineral phases in dense globules and one of the precursors for the apatite.

An intermediate state in hydrolysis of amorphous calcium phosphate.

The hydrolysis of previously prepared amorphous calcium phosphate (ACP) was studied in a solution "saturated" with ACP; this eliminated the initial consumption of acid due to ACP dissolution. The procedure established that conversion of high-concentration ACP slurry to an apatite involves two processes: the first process consumes acid and indicates the formation of a more acidic calcium phosphate intermediary with the solubility of octacalcium phosphate (OCP); the second process consumes base and indicates the conversion of the intermediary to apatite and, possibly, direct conversion of ACP to apatite. The thermodynamic analysis of the solution composition data suggests that ACP converts into a nonstoichiometric apatite when the OCP-like intermediary is formed, and a stoichiometric apatite is formed when no OCP-like intermediary is involved.

A mechanism for incorporation of carbonate into apatite.

Octacalcium phosphate (Ca8H2(PO4)6 . 5H2O) is considered to be precursor in the formation of apatite in bones and teeth; a crucial step for incorporation of impurities appears to occur during its hydrolysis. The present study examines the role that octacalcium phosphate plays in the process of incorporation of carbonate into apatite. Chemical, X-ray diffraction, and infrared techniques were used. When octacalcium phosphate is hydrolyzed in the presence of sodium and carbonate ions in aqueous media, approximately one sodium and one carbonate ion seem to substitute for a calcium and phosphate ion, respectively, in forming apatite, and the a axis is shortened. The infrared spectrum of the product indicates that the carbonate is in the type B site, which is presumed to be a phosphate site. This mechanism is of particular importance since the presence of carbonate in human enamel appears to be related to caries susceptibility. A structural mechanism for the incorporation of impurities during hydrolysis of octacalcium phosphate is presented.