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PubMed1980

Tissue response to dense apatite implants in rats.

Dense (97-99.9%) hydroxylapatite ceramics were implanted in muscle and bone tissue of rats. The aim of this study was to compare apatite implant material made of commercially available powder with laboratory powder prepared in a painstaking manner. Biocompatibility, biostability, and adherence to bone were evaluated. In muscle tissue the implants were found to be encapsulated with a very thin connective tissue layer. Implantation in excavated bone tissue resulted with new bone directly deposited against the implant surface, irrespective of the type of hydroxylapatite used. When the implants were protruding from the bone surface, bone appeared to grow up to the edge of the protruding part of the implant. A very strong bonding developed; push-out tests indicated that the bone fractured but never at the interface. Histologic studies proved that a sleeve of newly formed bone closely encased the implant, regardless of shape. It was concluded that dense apatite ceramics are fully compatible with the tibia of the rat and that no degradation of the implant material occurred for intervals of up to 6 months after implantation. The very strong bonding without mechanical retention indicated continuity between artificial hydroxylapatite and natural bone. No difference was found between the biological behavior of the hydroxylapatites prepared from commercial or laboratory starting powders.

Animals↗

PubMed1999

Metastable Equilibrium Solubility Distribution of Carbonated Apatite as a Function of Solution Composition.

Previous studies have shown that carbonated apatites (CAPs) exhibit the phenomenon of metastable equilibrium solubility (MES) in weak acid media. The purpose of the present investigation was to examine two questions: first, whether the MES concept is applicable to a broader range of solution conditions and, second, whether a driving force function associated with a surface complex having a constant stoichiometry governs the dissolution of CAP and, if so, what is this stoichiometry. CAP preparations with carbonate contents of 1.8-5.7 wt% (synthesized by hydrolysis of dicalcium phosphate anhydrate in solutions of varying bicarbonate levels or by direct precipitation from supersaturated calcium/phosphate/carbonate solutions) were studied as follows. MES distributions for each of the CAP preparations were determined by equilibrating the CAP under stirred conditions in a series of acetate buffers (0.10 M) containing various levels of calcium and phosphate in the pH range 4.5-6.5 and a solution calcium/phosphate ratio in the range 0.1-10. The amount dissolved in each instance was regarded as the fraction of the CAP possessing an MES value greater than that corresponding to the ion activity product (IAP) of the equilibrating solution. The solution IAPs were calculated from the solution compositions using plausible calcium phosphate stoichiometries, viz., dicalcium phosphate dihydrate, octacalcium phosphate, tricalcium phosphate, hydroxyapatite, carbonated apatite (based on the bulk composition of the particular CAP involved in the experiment), and tetracalcium phosphate. The fraction of CAP dissolved was plotted against the solution IAPs for each experimental set using each of the six assumed stoichiometries for the surface complex. The results demonstrated that the MES concept was applicable to all of the CAP preparations in media of various solution compositions and different pH levels. The most important new outcome of this study was that MES profiles for each of the CAP preparations in all of the experiments were found to be superimposable when solution IAPs were calculated using the hydroxyapatite stoichiometry, while such was not the case when other stoichiometries were used to calculate the solution IAPs. Copyright 1999 Academic Press.

Journal Article↗

PubMed2001

Adsorption of O-Phospho-L-Serine and L-Serine onto Poorly Crystalline Apatite.

The adsorption of phosphoserine and serine was studied to determine the effect of amino acid functional groups on the surface reactivity of synthetic poorly crystalline apatite similar to bone mineral. The experimental results for phosphoserine and serine uptake agree respectively with the Langmuir and Freundlich models. Phosphoserine exhibits stronger adsorption capacity and a higher affinity constant for the surface crystals compared to serine molecules. The enhanced adsorption capacity noted for phosphoserine might be related to the presence of phosphate groups in the molecule, which are specific attachment sites. This observation suggests that the strength of phosphate bonds to the solid surface, especially to calcium ions, is higher than that of carboxyl and hydroxyl ones. Spectroscopic observations provide evidence of an adsorption mechanism involving the anionic species of the amino acids and the surface of the crystals. Thus, a change in the position of the band of carboxyl groups occurred for the adsorbed molecules compared to the native amino acids. This revealed that the molecular residues do interact with apatite surface calcium. The shift noted in the frequencies of the bands associated with carboxylate vibrations is more pronounced for phosphoserine, confirming the stronger interaction noted for this molecule. Based on these results, one can conclude that the sorbent and sorbate charged species play an important role in the mechanism of uptake of the amino acids onto crystal surfaces. This may contribute to a better understanding of the mechanism by which phosphoproteins could influence mineralization processes and caries. Copyright 2001 Academic Press.

Journal Article↗

PubMed1992

Polylactide and polyglycolic acid-reinforced coralline hydroxy-apatite for the reconstruction of cranial bone defects in the rabbit.

The possibility of using coralline hydroxy-apatite in combination with polylactide and polyglcycolic acid instead of a bone graft in the skull region is examinated. Coralline hydroxy-apatite blocks strengthened with a membrane made of a combination of polylactide and polyglycolic acid were inserted into bony defects created in 12 rabbit skulls. The blocks were observed during a follow-up of 12 months. They became fixed to the surrounding bone and no adverse effects or harmful reactions in the nearby tissues could be detected.

Animals↗

PubMed1984

Inhibition of apatite crystal growth by the amino-terminal segment of human salivary acidic proline-rich proteins.

Inhibition of seeded apatitic crystal growth by human salivary acidic proline-rich phosphoproteins (PRP) has been related to their adsorption onto the apatite seeds. The amino-terminal 30-residue segment of the PRP makes an important contribution to this adsorption. This peptide (PRP1(T1] and its dephosphorylated analogue from PRP3 (PRP3(T1)DP) were prepared. They have identical sequences, except the phosphates at residues 8 and 22 in PRP1(T1) are absent from PRP3(T1)DP. Adsorption of these peptides onto hydroxyapatite and their effect on crystal growth from a defined supersaturated solution was studied. Adsorption behavior was adequately described by the Langmuir adsorption isotherm. The adsorption affinity constant of PRP1(T1) (K = 20,200 ml/mumol) was more than 10 times the corresponding value for PRP3(T1)DP (1,800 ml/mumol), and similar to that of the parent protein, PRP1 (26,200 ml/mumol). Inhibition of crystal growth by the peptides was interpreted in terms of the fractional coverage of the maximum number of adsorption sites (as derived from the adsorption isotherms), suggesting that the molecules block, by adsorption, specific growth sites on these surfaces. Comparison of precipitation kinetics showed that PRP1(T1) is a more effective inhibitor than PRP3(T1)DP at the same initial concentration (10(-6)-10(-7)M). However, on the basis of per mol adsorbed, PRP3(T1)DP displays a greater inhibitory activity; such a behavior is consistent with a more open molecular structure which blocks more growth sites per mol adsorbed than PRP1(T1).(ABSTRACT TRUNCATED AT 250 WORDS)

Adsorption↗

PubMed1999

Ultrastructural study of apatite precipitation in implanted calcium phosphate ceramic: influence of the implantation site.

Macroporous biphasic calcium phosphate (MBCP) blocks were implanted into rabbit trabecular bone and muscle, recovered 18 weeks later, and then observed and analyzed by transmission electron microscopy (TEM), electron diffraction, and electron microprobes. The results showed that (1) apatitic microcrystals appeared by secondary nucleation in both bone and muscle sites; (2) precipitated microcrystals were aggregated around ceramic crystals in bone sites but distributed randomly and without orientation in micropores in muscle sites; (3) the ratio of calcium to phosphorus was higher for microcrystals in bone than muscle sites; and (4) precipitated microcrystals around beta-tricalcium phosphate (beta-TCP) crystals were less aggregated and dense than those around hydroxyapatite (HA). These findings suggest that microenvironmental parameters such as fluid circulation and the interaction of ceramics with proteins or cells affect the physicochemical dissolution/reprecipitation process. Epitaxic growth of apatitic microcrystals seems more favorable from HA than beta-TCP.

Animals↗

PubMed2005

Studies on calcium deficient apatites structure by means of MAS-NMR spectroscopy.

The development of synthetic apatites that replicate the features and properties of the contained in natural tissues will help to diminish the misfit between artificial implants and natural hostesses but the structure of these compounds is still under discussion. The variability in Ca/P ratio of calcium deficient apatites has been explained through different models: surface adsorption, lattice substitution and intercrystalline mixtures of hydroxyapatite and octacalcium phosphate. This work investigates which of the models mentioned suits better in a range of samples. Hydroxyapatites obtained by precipitation, by hydrolysis of dicalcium phosphate and calcined samples with Ca/P ratio between 1.50 and 1.77 and specific surface area between 7 and 108 m2/g have been analysed. OCP and surface adsorption models suit better for great SSA particles and low Ca/P ratio while for smaller SSA particles the lattice substitution model is more accurate. SSA also plays the main role when the capacity to absorb substances is studied though their chemistry can not be explained solely in terms of surface reactivity.

Absorption↗

PubMed1990

Insolubilized properties of UV-irradiated CO3 apatite-collagen composites.

To examine the response to biological hard tissues, a carbonate-containing hydroxyapatite with chemical composition and crystallinity similar to those of bone was synthesized at pH 7.4 and 60 degrees C. The apatite powder was mixed with collagen solution, whose antigenicity had been removed by enzymatic treatment, and formed into apatite-collagen pellets. After insolubilization by UV-irradiation for 4 h, the composites showed remarkably reduced disintegration and maintained their shape under 3.6 MPa of stress after 1 wk incubation in 0.9% NaCl solution. They showed good biocompatibility when implanted beneath the periosteum cranii of rats. The UV-irradiated sample kept its features well and was packed with newly created material 3 wk after implantation.

Animals↗

PubMed2006

Preparation of SBF with different HCO3- content and its influence on the composition of biomimetic apatites.

The bioactivity of bone and dental implant materials is usually tested in vitro using simulated body fluid (SBF). The composition of common SBF differs from that of blood plasma in that it has a higher Cl- and a lower HCO3- concentration, which affects the composition of in vitro formed bone-like apatite. Five different SBFs with a composition of 142 Na+, 5 K+, 2.5 Ca2+, 1 Mg2+, 1SO4(2-), 1HPO4(2-), and 136 (Cl-+HCO3-) mmol/l were prepared with HCO3- concentrations ranging from 5 to 27 mmol/l. The SBF solutions were prepared by mixing stable concentrated solutions, which increase the reproducibility of in vitro tests due to negligible changes of pH during preparation. The high stability of thus prepared SBF enables the evaluation of hydroxyapatite formation on the surface of bioactive materials without the negative effect of spontaneous precipitation. Furthermore, the use of concentrated solutions offers a facile way to prepare SBF with different ionic contents and thus modify the composition of Ca-P layers precipitated on the surface of the bioactive materials exposed to the SBF solutions. The SBF solutions were shown to be supersaturated with respect to slightly carbonated apatite. The Fourier transform infrared (FT-IR), Raman and X-ray analyses of the precipitated layers indicate that the HCO3- content in SBF influences the composition and structure of the calcium phosphates obtained. It can be supposed that as long as the HCO3- concentration in the testing solutions is lower than 20 mmol/l, only B-type HCA precipitates. At higher HCO3- concentrations, it can be assumed that A-type HCA forms as well considering FT-IR, Raman and X-ray measurements.

Anions↗

PubMed2005

High-strength apatitic cement by modification with superplasticizers.

This study reports on a novel method to improve the strength of apatitic bone cements. The liquid phase of Biocement-H was modified with commercial superplasticizers. The results showed that small additions, i.e. 0.5 vol%, in the aqueous liquid phase improved the maximum compressive strength of Biocement-H (35 MPa) by 71%, i.e. 60 MPa. Moreover, the addition of high amounts of superplasticizers, i.e. 50 vol.%, allowed for a significant reduction of the liquid-to-powder ratio from 0.32 to 0.256 mL/g, without affecting the maximum strength and/or the workability of the cement. These results open up new ways to develop injectable and high-strength apatitic bone cements for load-bearing applications.

Bone Cements↗

PubMed2004

Aspartic acid nucleates the apatite crystallites of bone: a hypothesis.

This review concentrates on the physical state of bone mineral at the nanosize range, where it exists as ultrathin, tiny, platelike particles. They have a distinct X-ray pattern, different from that of apatite crystals, but shared by all bone matter irrespective of source or architecture. The main feature of this diffractogram is a broadened undifferentiated and asymmetric peak at about 32 degrees. The mineral contains several percents of carbonate and also other foreign elements and ions at low concentrations. When new bone is regenerated at a site of damage or resorption, such nanoparticles have to be created as the first step to progressive organizations at higher levels of bone composition. They are generated under cellular control. As they are produced in huge numbers, homogeneous in character, there is no escape from the conclusion that they are nucleated on compounds excreted by the cells and being not foreign to the surrounding. Evidence has been obtained that aspartic acid (AspA), one of the amino acids, which compose all life, has a specific beneficial effect on bone regeneration in an animal model experiment. An analysis of the possible interaction between aspartic acid molecule and the crystallographic unit cell of hydroxyapatite was attempted and is presented in this review. Hydroxyapatite has a large and complex unit cell, and therefore the interaction between such unit and a tiny acid calls for a special array in which the aspartic acid is enclosed inside a carbonated, calcium deficient unit, apparently without changing its external appearance and is able to start assemblage of regular apatite units. The nucleation event depends upon substitution of four carbonate ions in the sites of four phosphate ions. This substitution causes generation of four voids that can accommodate the four terminal oxygen of the aspartic acid. Such unit serves as a nucleating unit.

Animals↗

PubMed2002

Calcium phosphate apatites with variable Ca/P atomic ratio I. Synthesis, characterisation and thermal stability of powders.

Single phased apatitic calcium phosphate powders Ca(10-x)(PO4)(6-x)(HPO4)x(OH)(2-x) with Ca/P molar ratio ranging from 1.5 to 1.667 (0 < or = x < or = 1) were synthesised using wet method. Outside this compositional range the powders were biphasic mixtures composed of a calcium phosphate apatite and CaHPO4 (Ca/P < 1.5) or Ca(OH)2 (Ca/P > 1.667). Temperature and pH of synthesis were the preponderant parameters for the control of the precipitate composition. The precise determination of the chemical composition requires the use of several complementary techniques and thermal treatments of powders. These techniques include high resolution and high temperature X-ray diffractometry and FTIR spectroscopy and show that very small variations of the Ca/P molar ratio of the powder lead to great changes in powder composition and characteristics after thermal treatment.

Biocompatible Materials↗

PubMed2003

Osteoblastic cell response to thin film of poorly crystalline calcium phosphate apatite formed at low temperatures.

The response of osteoblastic cells to a thin film of poorly crystalline calcium phosphate apatite crystals (PCA) was examined in vitro. The PCA thin film was prepared on polystyrene culture dishes using highly metastable calcium phosphate ion solution at low temperatures. The PCA thin film was formed through fusion and transformation of granular calcium phosphate particles, which had initially formed on the surface, into a film of calcium phosphate apatite crystal. The PCA thin film was used for cell culture without additional surface treatment. The osteoblastic cell behaviors including adhesion, proliferation, expression of the marker genes, and calcified matrix formation were examined on the PCA thin film using primary osteoblasts or MC3T3-E1 cells. The cells were well attached and had spread in a slender shape over the PCA thin film. The extent of cell proliferation on the PCA thin film is as much as on the plain dishes. In addition, a much larger number of calcified nodules had formed on the PCA thin film than on the plain dish. The expression of the marker genes such as alkaline phosphatase, osteocalcin, osteopontin, osteonectin was apparent. These results demonstrate that the osteoblasts exhibit a full spectrum of cellular activity including the adequate differentiation on the PCA thin film. Therefore, a PCA thin film can be used as a coating material for biomaterials where the surface is not adequate for inducing the full activity of bone cells.

3T3 Cells↗

PubMed2000

Thin film of low-crystalline calcium phosphate apatite formed at low temperature.

Surface modification of biomaterials to improve biocompatibility without changing their bulk properties is desired for many clinical applications and has become an emerging technology in biomaterial research and industry. In the present study, a simple method of coating the solid surfaces of metals, organic tissue matrices, glasses, inorganic ceramics as well as organic polymers with a thin film of low-crystalline apatite crystals (LCA) was developed. Acidic solution containing calcium and phosphate ions was neutralized with alkaline solution to form calcium phosphate precipitates at low temperature. Precipitates of solid calcium phosphate particles were, then, removed by filtration. Concentration of free ions in the filtered ion solution which were not involved in the formation of calcium phosphate precipitate was high enough to induce the heterogeneous nucleation on the solid surfaces at low temperature. Thin layers of calcium phosphate crystals were formed on the surfaces of metals, glasses, inorganic ceramics, organic polymers including hydrophobic ones, and biological tissue matrices with this solution. The thin layer of crystals consisted of poorly crystalline calcium phosphate apatite crystals which contain high amount of labile ions like bone crystals and did not dissolve in the physiologic solutions. Various cells attached to this crystal layer and proliferated well.

Biocompatible Materials↗

PubMed2001

Continuous crystalline carbonate apatite thin films. A biomimetic approach.

In contrast to extensive studies on hydroxyapatite thin films, very little has been reported on the thin films of carbonated apatite (dahllite). In this report, we describe the synthesis and characterization of a highly crystalline dahllite thin film assembled via a biomimetic pathway. A free-standing continuous precursor film of carbonated calcium phosphate in an amorphous phase was first prepared by a solution-inhibited templating method (template-inhibition) at an air-water interface. A stearic acid surface monolayer acted as the template, while a carbonate-phosphate solution composed a binary inhibition system. The precursor film formed at the air/water interface was heated at 900 degrees C and transformed into a dense crystalline film that retained the overall shape of the precursor. The crystalline phase was characterized by XRD and IR to be a single-phase carbonate apatite, with carbonate substitutions in both type A (OH-) and type B (PO4(3-)) lattice positions.

Journal Article↗

PubMed2006

Highly efficient C-C bond-forming reactions in aqueous media catalyzed by monomeric vanadate species in an apatite framework.

A calcium vanadate apatite (VAp), in which PO4(3-) of hydroxyapatite (HAP), Ca10(PO4)6(OH)2, is completely substituted by VO4(3-) in the apatite framework, was synthesized. Physicochemical analysis of the VAp reveals the presence of isolated VO4 tetrahedron units with a pentavalent oxidation state. The VAp acts as a high-performance heterogeneous base catalyst for various carbon-carbon bond-forming reactions such as Michael and aldol reactions in aqueous media and the H-D exchange reactions using deuterium oxide. For example, a 200-mmol-scale Michael reaction under triphasic conditions proceeded rapidly, with an extremely high turnover number of up to 260 400 and an excellent turnover frequency of 48 s(-1). No vanadium leaching was detected during the above reactions, and the catalyst was readily recycled with no loss of activity.

Journal Article↗

PubMed2005

A molecular dynamics study of the thermodynamic properties of calcium apatites. 1. Hexagonal phases.

Structural and thermodynamic properties of crystal hexagonal calcium apatites, Ca10(PO4)6(X)2 (X = OH, F, Cl, Br), were investigated using an all-atom Born-Huggins-Mayer potential by a molecular dynamics technique. The accuracy of the model at room temperature and atmospheric pressure was checked against crystal structural data, with maximum deviations of ca. 4% for the haloapatites and 8% for hydroxyapatite. The standard molar lattice enthalpy, delta(lat)H298(o), of the apatites was calculated and compared with previously published experimental results, the agreement being better than 2%. The molar heat capacity at constant pressure, C(p,m), in the range 298-1298 K, was estimated from the plot of the molar enthalpy of the crystal as a function of temperature, H(m) = (H(m,298) - 298C(p,m)) + C(p,m)T, yielding C(p,m) = 694 +/- 68 J x mol(-1) x K(-1), C(p,m) = 646 +/- 26 J x mol(-1) x K(-1), C(p,m) = 530 +/- 34 J x mol(-1) x K(-1), and C(p,m) = 811 +/- 42 J x mol(-1) x K(-1) for hydroxy-, fluor-, chlor-, and bromapatite, respectively. High-pressure simulation runs, in the range 0.5-75 kbar, were performed in order to estimate the isothermal compressibility coefficient, kappaT, of those compounds. The deformation of the compressed solids is always elastically anisotropic, with BrAp exhibiting a markedly different behavior from those displayed by HOAp and ClAp. High-pressure p-V data were fitted to the Parsafar-Mason equation of state with an accuracy better than 1%.

Journal Article↗

PubMed2006

Molecular dynamics study of the thermodynamic properties of calcium apatites. 2. Monoclinic phases.

Structural and thermodynamic properties of crystalline monoclinic calcium apatites, Ca10(PO4)6(X)2 (X=OH, Cl), were investigated for the first time using a molecular dynamics (MD) technique under a wide range of temperature and pressure conditions. The accuracy of the model at room temperature and atmospheric pressure was checked against crystal structural data, yielding maximum deviations of ca. 2%. The standard molar lattice enthalpy (DeltalatHo298) of the apatites was also calculated and compared with previously published experimental and MD results for the hexagonal polymorphs. High-temperature simulation runs were used to estimate the isobaric thermal expansivity coefficient and study the behavior of the crystal structure under heating. The heat capacity at constant pressure, Cp, in the range 298-1298 K, was estimated from the plot of the molar enthalpy of the crystal as a function of temperature, Hm=(Hm,298-298Cp,m)+Cp,mT, yielding Cp,m=635+/-7 J.mol-1.K-1 and Cp,m=608+/-14 J.mol-1.K-1 for hydroxy- and chlorapatite, respectively. High-pressure MD experiments, in the 0.5-75 kbar range, were performed to estimate the isothermal compressibility. The Parsafar-Mason equation of state was successfully used to fit the high-pressure p-Vm data, with an accuracy better than 0.03%.

Journal Article↗