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R M Verbeeck

Publications and source records attributed to R M Verbeeck.

62 records · Page 4Linked to original sources
PubMed1981

Lattice parameters and cation distribution of solid solutions of calcium and lead hydroxyapatite.

Solid solutions of calcium hydroxyapatite (CaOHA) and lead hydroxyapatite (PbOHA) of the formula Ca10-x Pbx (PO4)6 (OH)2 were prepared by coprecipitation followed by heating at 800 degrees C in a steam of CO2-free water vapor of 1 atm. The samples were apatitic in the range 0 less than x less than 6 and contained lead phosphates as a second phase at higher Pb/Ca ratios. Lattice parameters and cation distribution of the apatitic samples were determined by X-ray diffraction. The lattice parameters varied linearly with x in the range considered, whereas all Pb2+ were located in the six-fold position for cations. There was a miscibility gap in the apatite series of solid solutions in the range 1 less than x less than 4, whereas apatites in the range 6 less than x less than 10 were not stable under the conditions of preparation. It is concluded that apatites in the range 4 less than x less than 6 represent a minimum in the free energy of solid solutions between CaOHA and PbOHA.

Calcium↗
PubMed1980

On the physical chemistry of tooth enamel and the caries process.

A model is presented for the pathway of reactions of the mineral of tooth enamel during caries. It is based on a mathematical simulation of the caries process (van Dijk, Borggreven and Driessens, 1979) and knowledge about the variable solubility behaviour of calcium phosphates (Driessens, van Dijk and Borggreven, 1978). According to this model the surface of the crystals in the intact superficial layer of enamel will transform into a relatively pure fluor-hydroxyapatite; in the lesion a mineral with a brushite- or monetite-like composition will be formed, while at the bottom of the lesion an increasing amount of the original carbonatoapatite will be found. The experimental studies of Arends and Davidson (1975) and of Featherstone, Duncan and Cutress (1978) corroborate these results. The model also explains why demineralization take place at some distance under the enamel surface, and predicts that the intact superficial layer of enamel over a lesion becomes thicker with increasing pH of the plaque fluid and with its degree of saturation with respect to fluor- and hydroxyapatite (Larson, 1974 b). Sobel's finding (1960) that high-carbonate teeth are more susceptible to caries than low-carbonate teeth is consistent with the present model.

Apatites↗
PubMed1980

Contribution to the physicochemical rationale for the caries reducing effect of fluoride.

The effect of fluoride on the stability of apatites containing Na+ and CO3(2)- ions is described qualitatively. Also the boundaries of the apatite stability field in the quasi-septary system CaO - P2O5 - H2O - Na2O - CO2 - NaF - MX are presented. At these boundaries brushite (or monetite), octocalciumphosphate, calcite and calcium fluoride can become stable solid phases. As a consequence, certain mechanisms for the caries reducing effect of fluoride gain probability over other proposed mechanisms. First, it is likely that endemic fluoride yields more fully mineralized enamel which in turn results in a lower initial rate of the caries process. Second, after eruption the mineral in the tooth superficial layer is bound to accumulate fluoride on the surface of its crystals. This causes a slower rate of the crystal surface dissolution at that site and, hence, a thicker and denser intact superficial layer over the lesion. Finally, the formation of a precipitate of fluorhydroxyapatite and perhaps also of calcium fluoride is promoted in the plaque during cycles of metabolic activity. Subsequent dissolution of the CaF2 and transformation of the fluorhydroxyapatite sets free Ca+ and F- ions. Both of these ions promote the remineralization of the lesion during periods of metabolic rest whereby brushite or monetite are transformed back into apatite.

Apatites↗
PubMed1979

Lattice parameters and cation distribution of solid solutions of calcium and strontium hydroxyapatite.

Solid solutions of strontium and calcium hydroxyapatite were synthesized by solid-state reaction. Lattice parameters of these compounds were determined using two types of Guinier cameras. They vary linearly with the molar percentage of strontium hydroxyapatite. The distribution of Ca and Sr ions over the fourfold and sixfold positions in the apatite structure was determined by comparing experimental and calculated values for the intensity ratios of suitable reflections. A slight, although significant, preference of Sr for the sixfold position was found. An ideal behavior is predicted for these solid solutions.

Calcium↗
PubMedDate not supplied

Evidence for intermediate metastable states during equilibration of bone and dental tissues.

Experimental data were collected on the solubility equilibrium of the mineral of bone, tooth enamel and calcium phosphate renal stones. Evaluation in the form of potential diagrams of Ca(OH)2 versus H3PO4 shows that Ca/P ratios of 1, 4/3, 10/7, 3/2 and 5/3, related to phases like brushite, octocalcium phosphate, whitlockite, defective apatite and hydroxyapatite respectively can be important. These facts allow the interpretation that many of these calcium phosphates are present simultaneously in biominerals or that they are formed during the equilibration.

Adult↗
PubMedDate not supplied

Degree of saturation of blood plasma in vertebrates with octocalcium phosphate.

In previous papers it has been shown that octocalcium phospate OCP occurs in bone mineral of vertebrates. Although this compound is not stable, there is a continuous new-formation of OCP due to bone turnover. Literature data of the calcium and phosphate concentrations in the blood plasma of vertebrates were collected and the degree of saturation with OCP was calculated. The results show that blood plasma of vertebrates is almost saturated with OCP. This fact indicates that OCP is the solubility controlling phase in the mineral of vertebrates. Further it verifies the expectation based on physicochemical theory that the interaction between body fluids and bone mineral is important in the calcium and phosphate homeostasis.

Animals↗