Dissolution Of Hydroxyapatite Research Articles

A hypothesis for the local control of osteoclast function by Ca2+, nitric oxide and free radicals.

Several important conclusions have recently emerged from in vitro studies on the resorptive cell of bone, the osteoclast. First, it has been established that osteoclast function is modulated locally, by changes in the local concentration of Ca2+ caused by hydroxyapatite dissolution. It is thought that activation by Ca2+ of a surface membrane Ca2+ receptor mediates these effects, hence providing a feedback control. Second, a number of molecules produced locally by the endothelial cell, with which the osteoclast is in intimate contact, have been found to affect bone resorption profoundly. For instance, the autocoid nitric oxide strongly inhibits bone resorption. Finally, reactive oxygen species have been found to aid bone resorption and enhance osteoclastic activity directly. Here, we will attempt to integrate these control mechanisms into a unified hypothesis for the local control of bone resorption.

Mechanical failure of hydroxyapatite- and polysulfone-coated titanium rods in a weight-bearing canine model

Three types of material that have shown potential as coatings for orthopaedic implants were studied. Using a weight-bearing canine model, Ti-6A1-4V femoral intramedullary rods coated with (1) sintered titanium beads, (2) plasma-sprayed hydroxyapatite, and (3) silyl coupled polysulfone beads were evaluated for mechanical strength and bone ingrowth. The model was designed to secure optimal prosthetic stability by obtaining maximal bony ingrowth during an initial non-weight-bearing phase, then stressing the implant during a full-weight-bearing phase. None of the rods coated with titanium beads failed. All 17 polysulfone-coated rods failed, 13 of them at the interface between the polysulfone coating and the titanium core. Of 18 rods coated with hydroxyapatite, 15 suffered implant breakdown at the interface between the hydroxyapatite coating and the titanium core. This may be due to dissolution of the plasma-sprayed hydroxyapatite in vivo. Testing of retrieved specimens from both hydroxyapatite- and polysulfone-coated implants showed that the shear strength at the coating-rod interface had decreased to less than 40% of the shear strength at manufacture. Despite mechanical failure, histologic study showed extensive bone ingrowth or apposition onto both the polysulfone and hydroxyapatite coatings.

Kinetics of hydroxyapatite dissolution in acetic, lactic, and phosphoric acid solutions.

The present study was undertaken in an attempt to relate the kinetics of hydroxyapatite dissolution to solution parameters, under experimental conditions relevant to the dental caries process. Thus, the dissolution of hydroxyapatite was studied in acetic, lactic, and dilute phosphoric acid solutions having initial pH values from 4 to 6. Rates of dissolution and the corresponding degree of saturation with respect to hydroxyapatite were determined at various times throughout the dissolution process. Rates of dissolution of all solutions were found to decrease with increasing degree of solution saturation and were greater in solutions with lower initial values of pH. However, rates of dissolution in partially saturated phosphoric acid solutions (without added organic acid) were at least one order of magnitude lower than those observed in the organic acid buffers with the same initial pH, over the same range of saturation values. The data obtained are consistent with a surface-controlled dissolution model in which the rate of dissolution is dependent upon the degree of saturation and the sum of the activities of the acidic species in solution, i.e., phosphoric and organic acids. These results suggest that in order to assess the cariogenic potential of a given medium (e.g., plaque fluid), one must determine both the degree of saturation with respect to the dissolving mineral and the activities of acidic species in solution.

Bisacylphosphonates inhibit hydroxyapatite formation and dissolution in vitro and dystrophic calcification in vivo.

Some geminal bisphosphonates are used clinically for a number of important bone/calcium related diseases; however, side effects and lack of selectivity impede their wide use. This work reports the synthesis and evaluation of bisacylphosphonates (e.g., adipoyl- and suberoylbisphosphonate). These compounds were found to inhibit significantly hydroxyapatite formation and dissolution in vitro and the calcification of bioprosthetic tissue implanted subdermally in rats. These are the first instances of nongeminal bisphosphonates [P-(C)n-P, n greater than or equal to 2] that have been reported to be active in calcium-related disorders. The reported bisacylphosphonates possess apparent lower toxicity, and their calcium complexes/salts have improved solubility properties. Therefore, they are of potential importance for clinical applications.

Dissolution of hydroxyapatite by calcium complexing agents

Hydroxyapatite (HAP) dissolution effected by calcium complexing ions was studied. The efficiencies of various complexants on the dissolution of HAP at 37°C and pH=7.4 were compared. This effect was also examined in correlation with parameters such as: the concentration of the complexant solution, the volume of the complexant solution, dissolution times and the perfection of crystals structure. EDTA has been found to be superior complexant to others. The significant dissolution time occurs between 1 and 3 min and as the volume and concentration of the complexant solution increase, there is a significant increase in the efficiency of dissolution of HAP.

Incongruent dissolution of hydroxyapatite in the presence of phosphoserine

Concentrations of phosphate and calcium ions, liberated from the surface of hydroxyapatite (HAP) during the adsorption of phosphoserine (PSer), were determined at 30°C. HAP showed a marked incongruent dissolution behavior in the presence of PSer. That is, the concentration of phosphate ion in solution increases with the addition of PSer due to the ion-exchange between PSer and phosphate ion on HAP (molar ratio of the former to the latter=3∶2), whereas the concentration of calcium ion decreases with this release of phosphate ion, because the solubility product of HAP restricts the concentrations of both ions in solution (calculated values of — log (Ca2+)10 (PO43−)6 (OH−)2 were 115.8±1.0). The affinity of PSer to HAP was highest at pH 5.8 where the PSer and the HAP surface had the opposite charges. This electrostatic attraction force between PSer and HAP was shielded to some extent by the addition of KCl.

Diffusion coefficients for the ternary system Ca(OH)2–H3PO4–water

A conductimetric method has been used to measure the ternary diffusion coefficients of calcium hydroxide–phosphoric acid in dilute aqueous solution at 25 °C. Diffusion coefficients for concentrated solutions of the electrolytes are evaluated from previously reported Gouy optical data for the related system Ca(H2PO4)2–H3PO4–water. The experimental results indicate strong electrostatic coupling between diffusive flows of aqueous Ca(OH)2 and H3PO4. The implications for dissolution of calcium hydroxyapatite, the main constituent of bone and teeth, are discussed.

Diffusion and solid dissolution/precipitation in permeable media

AbstractWe have developed an analytical solution which describes mineral zonation caused by diffusion in permeable media. For a semiinfinite domain, the species conservation equations transform into ordinary differential equations that yield a closed‐form solution. The solution exhibits shock dissolution/precipitation fronts and gradual (nonshock) precipitation fronts. The solution can exhibit regions (gaps) containing no reactive solids which separate moving dissolution and precipitation fronts. The analysis is, in principle, extendable to include intraaqueous reactions, although the mathematics quickly becomes intractable.Numerical simulation exhibits all of the features of the more restricted analytical solution and is in good agreement with the data on hydroxyapatite dissolution taken by Kim and Cussler (1987).

The influence of EHDP on the dissolution rate behavior of heat-treated and non-heat-treated hydroxyapatites

Ethane-1-hydroxy-1,1-diphosphonic acid (EHDP), a known inhibitor of hydroxyapatite (HAP) dissolution in aqueous acid media, was employed as a probe to investigate the difference in dissolution characteristics between a heat-treated HAP preparation and a non-heat-treated HAP preparation. The primary HAP sample was prepared by precipitation and digestion at 105°C (UU-5). Preparation UU-5HT was obtained by heat-treating UU-5 at 1200°C for 30 min in a stream of nitrogen and water vapor. All dissolution rates were determined using the crystal suspension method in 0.10 M acetate buffer at pH = 4.5 and at various levels of EHDP. Significant dissolution rate retarding effects were found with all HAP samples at EHDP levels as low as 1 × 10 −5 M. At 1 × 10 −4 M EHDP, the initial dissolution rates (IDRs) for UU-5 (non-heat-treated) were found to be around 30% of the control (zero EHDP) rates. However, with this HAP preparation further decrease in the IDRs with increase in the EHDP levels was gradual and, even at millimolar levels of EHDP, the IDRs were still of the order of 10% of the control rates. In contrast, the heat-treated HAP preparation, UU-5HT, gave a much more rapid decline in the IDRs with increasing EHDP. For example, with UU-5HT the IDR values were essentially zero at 1 × 10 −4 M EHDP. These results are consistent with the interpretation that the non-heat-treated HAP samples exhibit two-site dissolution kinetics and that heat treatment anneals away the more active of the two sites.

Crystal dissolution of biological and ceramic apatites

High resolution transmission electron microscopy (Hr TEM) studies on biological and synthetic calcium phosphate have provided information on the dissolution process at the crystal level. The purpose of this study was to investigate the dissolution of ceramic hydroxyapatite (HA) after implantation using Hr TEM. Recovered HA ceramic implanted in bony and nonbony sites in animals and in periodontal pockets in humans were used for the study. For comparison, sections of human fluorotic enamel with caries and sections of shark enameloid previously exposed to 0.1 HCl were similarly investigated. Hr TEM studies demonstrated that in both the biological and ceramic apatites, the lattice and atomic defects were the starting points in the dissolution process. However, significant differences in the process of dissolution were observed: (1) biological apatite crystals showed preferential core dissolution whereas ceramic apatite crystals showed nonspecific dissolution at the cores and at the surfaces; (2) the dissolution of biological apatites appeared to consistently extend along the crystal's c-axis whereas dissolution of the ceramic HA did not appear to be correlated with the crystal's c-axis. The observed differences in crystal dissolution between biological and ceramic apatites may be attributed to the following: (1) the unique crystal/protein interaction present with biological apatites but absent in ceramic HA; (2) differences in defect distribution between biological and ceramic apatites which are due to the differences in the original of these defects; and (3) the longer morphological c-axis of biological apatites compared with that of ceramic apatites.(ABSTRACT TRUNCATED AT 250 WORDS)

Dissolution kinetics of human enamel powder: I. Stirring effects and surface calcium accumulation

The kinetics of dissolution of human enamel powder were studied at 37°C and constant pH in the range 3.7–6.5 by continuous recording of proton uptake and calcium release. The effects of stirring and the behavior of the enamel powder during the first stages of dissolution were analyzed. It was shown that the dissolution rate was controlled at low stirring speed by a diffusive process which became a solid-like controlled process at higher stirring speed. Moreover, in any case, a tendency of interfacial calcium accumulation was observed in parallel with a progressive slowing of the dissolution rate. The higher the pH, the more important the accumulation. These observations suggest that an interfacial calcium-and/ or phosphate-rich layer is rapidly formed on the crystal surface. This “by-product” layer reduces considerably the rate of dissolution of hydroxyapatite. At high stirring, the dissolution rate is limited by the diffusion of calcium and/or phosphate ions through this layer, whereas at low stirring, the rate is limited by the diffusion of calcium or phosphate in the Nernst layer adjacent to the surface deposited layer.

The influence of high- and low-molecular-weight inhibitors on dissolution kinetics of hydroxyapatite and human enamel in lactate buffers: a constant composition study.

The kinetics of dissolution of hydroxyapatite (HAP) and human enamel (HE) powders in lactate buffers was investigated by means of a Constant Composition technique. The rates were interpreted in terms of a surface-controlled dissolution mechanism. It was shown that the overall dissolution of both HAP and HE crystallites in lactate buffers resulted from an interplay of the calcium chelating action of lactate ion, its interaction with surface calcium sites, and the retardation effects of both ionized (L-) and un-ionized (HL) lactic acid due to adsorption at phosphate surface sites. The presence of lactate in demineralizing solutions also influenced the "acquired crystallite resistance" for dissolution, and the rates decreased as the reaction proceeded. The influence of surface protective agents [methanehydroxydiphosphonate (MHDP) and polyacrylic acid (PAA)] on HAP and HE dissolution rates was also extensively studied in partially saturated lactate buffer (0.05 mol L-1). Although the overall kinetic effects of both additives appeared to be similar, there were considerable mechanistic differences. MHDP might interact with lactate by competition for surface calcium sites. In contrast, PAA appeared to act independently of lactate under the experimental condition used. This would tend to support the recommendation of White (1987), that PAA was more suitable as a surface protective agent for artificial caries lesion preparation.

The effect of lactic and acetic acid on the formation of artificial caries lesions.

Fluoride specifically adsorbed to hydroxyapatite reduces the rate of dissolution of hydroxyapatite. Since specific adsorption is a property which anions of all weak acids have in common, it is expected that the anions of lactic and acetic acid will be specifically adsorbed to hydroxyapatite too and in this way will cause a reduction in the rate of dissolution of hydroxyapatite. In order to investigate this, we used the experimental results from Featherstone and Rodgers (1981). Calculations of the rate of increase of the lesion depth in dental enamel per mmol. L-1 of the un-ionized acid revealed that lesion progress was pH-dependent and was lowest at that pH which corresponds with the pK-value of the relevant acid. It is concluded that the anions of lactic and acetic acid are specifically adsorbed to the enamel mineral and in this way reduce the rate of dissolution of the mineral. This suggests that a description of the development of an artificial caries lesion in mathematical terms should include the effect on the rate of dissolution of the mineral of specific adsorption of the relevant acid anions to the mineral.

The mechanism of dissolution of hydroxyapatite and carbonated apatite in acidic solutions

As potential models for biological minerals, the dissolution of hydroxyapatite (HAP) and carbonated apatite (CAP) has been extensively studied. In the present work, the constant composition method has been used to investigate these dissolution reactions at constant undersaturation. The rates of dissolution of both HAP and CAP show marked decreases as the reactions proceed despite the constancy of undersaturation. In order to examine the influence of trace impurities, both ultrapure and reagent grade chemicals have been used together with traces of methanehydroxydiphosphonate (MHDP) as an “impurity probe”. Interestingly, MHDP accelerated the reaction at very low concentration (∼5×10 -7M) but reduced the rate at higher levels. The results have been analyzed in terms of a polynuclear dissolution mechanism with surface properties expressed in terms of free surface energy which in turn is related to a surface entropy factor characterising the micro-roughness.

Dissolution of Sparingly Soluble Inorganic Compound to Aqueous Suspension with Ion Exchange Resin (Part 1)

Significant solubility of a sintered hydroxyapatite (HAp) tablet was observed by soaking in aqueous suspension with strongly acidic ion exchange resin, H-form (H-R), compared with the conventional chemolysis reagent for renal calculi. The pH value after soaking was nearly 2 for self-exhausted H3PO4 by the ion exchange reaction. The dissolution tendency of an HAp tablet depended on the pH value of H3PO4 solution used. From these facts, the dissolution mechanism of HAp in aqueous suspension with H-R was studied for various suspensions with 0-3.0g of H-R and 0-600mg of HAp powders. Analysis of Ca2+ and PO43- by ion chromatography and measurements of pH and specific conductivity were performed for the filtrate, and Ca/PO4 molar ratio values thus obtained were discussed from the view point of formation of soluble Ca(H2PO4)2. Variations in these parameters can be expressed as contour maps on the plane of HAp (mg) and H-R (g) axes. In conclusion, the dissolution of HAp proceeds by the cation exchange reaction with H-R (Eq. 1) followed by the successive exchange reaction in the presence of sufficient of H-R (Eq. 2).Ca10(PO4)6(OH)2+14(H-R)→3Ca(H2PO4)2+7Ca-R2+2H2O (1) Ca(H2PO4)2+2(H-R)→Ca-R2+2H3PO4 (2)Furthermore, H3PO4 formed by Eq. (2) contributes to successive dissolution of HAp to enhance the solubility.

The influence of incorporated and adsorbed fluoride on the dissolution of powdered and pelletized hydroxyapatite in fluoridated and non-fluoridated acid buffers.

The effects of fluoride (F) incorporated into hydroxyapatite (HA), adsorbed onto the HA surface, and fluoride in solution were studied during HA dissolution. Dissolution rates at pH 5.0 and 25 degrees C were determined in fluoridated (0.1, 5.0 micrograms/mL) and non-fluoridated buffers for the following powdered and pelletized synthesized HAs: non-fluoridated HA, partially-fluoridated (100, 1000, 37,000 micrograms/g) HA, and surface-adsorbed fluoridated (100, 1000 micrograms/g) HA. The dissolution rate curves were used to derive two components we have called 'diffusion' and 'surface chemical reactivity'. With powders, the rate-determining factor was surface chemical reactivity during the first 60 min. Diffusion was reduced by increasing the F levels in the apatite or buffer. In non-fluoridated buffers, diffusion was reduced 29% for apatite with 100 micrograms/g incorporated F and 99% with 37,000 micrograms/g. Diffusion was reduced by 37% and 81% by 0.1 and 5.0 micrograms/mL F, respectively, in the buffer. With pelletized HA, the rate-determining factor during the first 10 min was surface chemical reactivity. Diffusion then became relatively more important, but the diffusion rate was independent of F content. During the dissolution of powdered apatites, F was taken up by the remaining apatite solid, as evidenced by the lower F concentration in solution than that calculated to be released during dissolution. There was also a decrease in the F concentration of the 0.1 microgram/mL buffer as reaction progressed. Pelletized apatites did not release enough F to influence dissolution significantly. Surface-adsorbed F was more effective than incorporated F in reducing HA dissolution.(ABSTRACT TRUNCATED AT 250 WORDS)

The influence of dodecylamine hydrochloride adsorption on the dissolution rate behavior of heat-treated and non-heat-treated hydroxyapatite

The present work has shown the usefulness of combining heat treatment studies of hydroxyapatite (HAP), the use of dodecylamine hydrochloride (DAC) as a probe, and partial saturation studies in the investigation of the complex dissolution behavior of hydroxyapatite in acidic aqueous media. In the context of the two-site theory, the heat treatment (1200°C) of HAP results in the elimination of site (s) 1. This is characterized by a negligibly slow dissolution of heat-treated HAP in solutions supersaturated with respect to K HAP= a Ca 2+ 10 a PO 4 3− 6 a OH− 2 ⋍ 1 × 10 −130 and by the ability of 3.0 m M DAC to completely inhibit the dissolution of heat-treated HAP. For non-heat-treated HAP, which demonstrates two-site kinetics, dissolution proceeds at moderate rates in dissolution media with K HAP greater than 1 × 10 −130. Furthermore, the non-heat-treated HAP exhibits a residual dissolution component in 3.0 m M DAC which is probably associated with a fixed amount of mineral. The removal of this mineral does not, however, alter the ability of the residual HAP to dissolve in media supersaturated with respect to K HAP ⋍ 1 × 10 −130.

Dissolution of Hydroxyapatite in Water Containing a Strongly Acidic Ion Exchange Resin and Its Analysis

Dissolution of Hydroxyapatite in Water Containing a Strongly Acidic Ion Exchange Resin and Its Analysis

The kinetics of dissolution of tooth enamel--a constant composition study.

The kinetics of dissolution of powdered bovine enamel and of human enamel, both untreated and extracted with either hypochlorite or chloroform, has been studied using a constant solution composition technique in undersaturated solutions of calcium phosphate (total molar calcium concentration, TCa = 0.3 to 13.1 X 10(-3) mol L-1, total molar phosphate, Tp = 0.18 to 7.9 X 10(-3) mol L-1) at an ionic strength of 0.15 mol L-1, and pH = 4.5. The kinetic equations describing the dissolution reactions suggest a surface dislocation mechanism, and the presence of fluoride ion markedly retarded the reaction. For human enamel, a fluoride level of only 0.5 ppm reduced the rate of dissolution ten-fold. In contrast, the dissolution of hydroxyapatite, HAP, is best interpreted in terms of a polynucleation process.

The effect of aluminum on the rate of dissolution of calcium hydroxyapatite--a contribution to the understanding of aluminum-induced bone diseases.

Aluminum ions, Al3+, as these ions exist in aqueous solutions at pH approximately equal to 7, adsorb onto calcium hydroxyapatite (HAP) crystals and severely inhibit the dissolution process of these crystals at 0.1 microM concentrations of aluminum. Human bone crystals have also been shown to adsorb these ions or molecules. A mechanism explaining why aluminum, in connection with bone resorption, causes less demineralization in the osteoid/calcified bone region than deeper in the calcified bone, is suggested.