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PubMed2002

Novel synthesis and characterization of an AB-type carbonate-substituted hydroxyapatite.

A novel synthesis route has been developed to produce a high-purity mixed AB-type carbonate-substituted hydroxyapatite (CHA) with a carbonate content that is comparable to the type and level observed in bone mineral. This method involves the aqueous precipitation in the presence of carbonate ions in solution of a calcium phosphate apatite with a Ca/P molar ratio greater than the stoichiometric value of 1.67 for hydroxyapatite (HA). The resulting calcium-rich carbonate-apatite is sintered/heat-treated in a carbon dioxide atmosphere to produce a single-phase, crystalline carbonate-substituted hydroxyapatite. In contrast to previous methods for producing B- or AB-type carbonate-substituted hydroxyapatites, no sodium or ammonium ions, which would be present in the reaction mixture from the sodium or ammonium carbonates commonly used as a source of carbonate ions, were present in the final product. The chemical and phase compositions of the carbonate-substituted hydroxyapatite was characterized by X-ray fluorescence and X-ray diffraction, respectively, and the level and nature of the carbonate substitution were studied using C-H-N analysis and Fourier transform infrared spectroscopy, respectively. The carbonate substitution improves the densification of hydroxyapatite and reduces the sintering temperature required to achieve near-full density by approximately 200 degrees C compared to stoichiometric HA. Initial studies have shown that these carbonate-substituted hydroxyapatites have improved mechanical and biologic properties compared to stoichiometric hydroxyapatite.

Bone Substitutes↗

PubMed2003

Phase evolution during the formation of stoichiometric hydroxyapatite at 37.4 degrees C.

The mechanism of formation of stoichiometric hydroxyapatite in aqueous solution by the acid-base reaction of the precursor calcium phosphates tetracalcium phosphate [Ca4(PO4)2O, TetCP] and anhydrous dicalcium phosphate (CaHPO4, DCPA) was investigated. Phase evolution during the formation of hydroxyapatite was studied, and could be correlated with the kinetics of reaction. Initial dissolution of precursors is followed by hydroxyapatite nucleation and growth. Consumption of DCPA precedes that of TetCP and results in the formation of nonstoichiometric hydroxyapatite. As a result of continued TetCP dissolution, the pH of the solution increases towards a pH at which hydroxyapatite and Ca(OH)2 co-exist in equilibrium. Hydroxyapatite formation takes place in two stages: initially formed, nonstoichiometric hydroxyapatite subsequently reacts with TetCP and is converted to stoichiometric hydroxyapatite.

Biocompatible Materials↗

PubMed1991

Biocompatibility of hydroxyapatite-coated hip prostheses.

In three patients a mechanically well-fixed Mathys Ceros 80 (Ha) hydroxyapatite-coated acetabular component was revised 2, 5 and 13 months after total hip replacement due to component malposition. In each case there was a thin cellular connective tissue membrane between hydroxyapatite and bone, the main cell type being fibroblast with only occasional giant cells. Immunohistological analysis revealed some MHC locus II antigen positive cells that were identified as monocytes. No interleukin-2 receptor positive cells were found. Under clinical cyclic loading conditions there does not seem to be chemical fixation or bony ingrowth into the hydroxyapatite coated prosthesis component. In human lymphocyte cultures, hydroxyapatite (Interpore 200, particle diameters 15-40 microns) did not cause an increase in lymphocyte DNA synthesis as assessed by the 3H-thymidine incorporation method on culture days 1, 3 and 5. As analysed with lymphocyte activation markers, the hydroxyapatite-dependent expression of MHC locus II antigen was modest and differed significantly (P less than 0.05) from that in culture medium only on day 3. Hydroxyapatite induced only a slight interleukin-2 receptor expression that did not differ from culture medium on days 1, 3 and 5. CD4 and CD8 positive lymphocytes as well as monocytes were not seen attached to hydroxyapatite particles during the culture days. Our findings suggest that hydroxyapatite is an immunologically inert implant material.

Acetabulum↗

PubMed2003

[Experience with orbital implants in particular with porous hydroxyapatite materials].

PURPOSE: Various materials have been developed since the first implantation of a hydroxyapatite sphere in 1985. They are similar to the porous basic structure and imitate the biological behavior of coralline implants. This article presents own clinical experience with hydroxyapatite implants from 1993 to 2003 and compares own results and other porous orbital implants in the international scientific literature. RESULTS: The original hydroxyapatite implant is well tolerated and implant extrusion is rare. A retrospective analysis of 357 patients regarding this complication shows an extrusion rate of 2.6% over 10 years. The subjective positive tolerance of 71.2% corresponds to the results of international studies. After several stages of development the synthetic product (FCI3) is now comparable with the original product with regard to operative complications and subjective compatibility. Both orbital implants should be used with a protective covering to avoid premature extrusion and to facilitate suturing the extraocular muscles anterior. When using material from humane donors the material must be guaranteed to be completely sterile. The use of vicryl as an orbital plomb wrapping leads to contradictory reports in the literature. Hydroxyapatite ceramics in combination with silicone india rubber represent an alternative to the materials listed above and in this case a wrapping of the orbital plomb is unnecessary. Spherical orbital implants made of aluminum oxide (bioceramic implant) are an alternative to corraline hydroxyapatite implants. Because of their porous,crystalline structure bioceramic implants vascularize well. Porous polythylene orbital implants,which are not available in Germany, are economical, but due to their porosity and vascularization properties they are not comparable with pure hydroxyapatite or hydroxyapatite ceramics. FUTURE VIEW: The dynamic development of the infant anopthalmus adapted to the size growth of the orbita and the exact volume replenishment of the adult orbita cannot yet be fulfilled with the presently available porous hydroxyapatite materials and is the subject of future research.

Eye Enucleation↗

PubMed2005

An investigation of some food-approved polymers as agents to inhibit hydroxyapatite dissolution.

Dental erosion involves dissolution of the hydroxyapatite fraction of enamel and dentine, so agents that reduce the dissolution rate of hydroxyapatite could find application in food products aimed at reducing erosion. This study was performed to test some common food ingredients and additives for their effect on the dissolution rate of hydroxyapatite in a citric acid solution representative of soft drinks. Pyrophosphate, tripolyphosphate and a linear chain polyphosphate (average 25 phosphate units) significantly reduced the hydroxyapatite dissolution rate by 35, 46 and 64%, respectively. Xanthan gum and carboxymethylcellulose significantly reduced the hydroxyapatite dissolution rate by 29 and 16%, respectively. The protective effect may be ascribed to the binding of condensed phosphate or to the formation of an adsorbed layer of gum at the hydroxyapatite surface. Several other common food additives had no statistically significant effect on the hydroxyapatite dissolution rate. Polyphosphate exhibited a considerable persistence of action, causing a reduction in the dissolution rate for 3 h after treatment. Tripolyphosphate was slightly persistent, and pyrophosphate and xanthan gum did not exhibit a substantial persistence of action. A solution containing polyphosphate and xanthan gum reduced the hydroxyapatite dissolution rate by 70% and exhibited a similar persistence of action to the solution containing only polyphosphate. These compounds are suggested to have potential as erosion-reducing agents in soft drinks.

Buffers↗

PubMed2001

[Wet synthesis and characterization of fluoride-substituted hydroxyapatite].

Hydroxyapatite and fluoride-substituted hydroxyapatite powders are synthesized by wet method, compacted into pellets and subjected to heat treatment at 300, 600, 900 degrees C respectively. Analysis of the fluoride-substituted hydroxyapatites and HA by FTIR spectroscopy indicated that with the increasing of sintering temperatures, the phase-pure fluoride-substituted hydroxyapatite and stoichiometric hydroxyapatite could be got. According to the X-ray diffraction analysis, the fluoride-substituted hydroxyapatite is less stable for decomposing into beta-Ca3(PO4)2 at the temperature of 900 degrees C. The in vitro bioactivity of the fluoride-substituted hydroxyapatites and hydroxyapatite heat treated at 900 degrees C was assessed by immersion testing in simulated body fluid (SBF). Both scanning electron microscopy (SEM) analysis and ionic solution changes (Ca2+) demonstrated that the low fluoride content apatite could induce a surface bonelike apatite layer formation.

Fluorides↗

PubMed1996

Role of different loading conditions on resorption of hydroxyapatite coating evaluated by histomorphometric and stereological methods.

The role of different loading conditions on resorption of plasma-sprayed hydroxyapatite coating was investigated in an experimental study. Resorption of hydroxyapatite was quantified by histomorphometric and stereological methods on backscattered scanning electron images. Hydroxyapatite-coated titanium implants were inserted unilaterally into the medial femoral condyle of the knee in 14 mature dogs. Initially, all implants were subjected to controlled micromotion of 150 microns. After 4 weeks, the dogs were randomly assigned either to have the implant surgically immobilized to prevent further micromovement or to have a sham operation. Sixteen weeks after the first operation, the implants were analyzed. Six noninserted implants served as controls. The surface area and volume of the hydroxyapatite coating were reduced on the immobilized implants by 53 and 67% (p < 0.05), respectively, and were further significantly reduced on the continuously loaded implants by 83 and 87%, respectively, compared with the control implants. The hydroxyapatite coating was significantly thinner on immobilized (15 microns) and continuously loaded (15 microns) implants as compared with control implants (23 microns), but no difference between the inserted implants was found. Areas not covered with hydroxyapatite had 29 and 24% bone coverage on the immobilized and continuously loaded implants (not significant). Resorption of hydroxyapatite coating did occur in vivo. Continuous loading of the implants accelerated resorption significantly compared with immobilization of the implants. It is suggested that completely resorbed hydroxyapatite was partly replaced by bone in direct contact with the metal implant surface.

Animals↗

PubMed1994

Management of exposed hydroxyapatite orbital implants.

BACKGROUND: Porous coralline hydroxyapatite orbital implants have gained significant popularity for use in anophthalmic sockets. Several reports have noted increased rates of early implant exposure. METHODS: Six patients with exposure complications of primary hydroxyapatite implants after enucleation or evisceration were studied. RESULTS: The hydroxyapatite implant exposures occurred 2 to 8 weeks after implantation. Magnetic resonance imaging scans with gadolinium enhancement obtained in two patients demonstrated a "cold" zone anteriorly with failure of fibrovascular ingrowth in the region of anterior exposure. Several different surgical procedures were used to regain implant coverage, including freshening the edge of the conjunctival defect and observing for spontaneous closure, burring away the anterior surface of the hydroxyapatite implant, and placing autogenous free tissue grafts of hard-palate mucosa or thin dermis-fat graft over the exposed hydroxyapatite. Repeated exposure after free graft was noted in several patients, and three of the six patients followed have an intact surface but very thin conjunctiva overlying hydroxyapatite spicules. CONCLUSION: Although small areas of hydroxyapatite exposure (< 3 mm) may close spontaneously, larger defects do not tend to close spontaneously and should be actively managed. Based on our experience, optimal results are obtained with free autogenous tissue grafts combined with burring of the anterior hydroxyapatite surface. Steps should be taken at the time of primary implantation to minimize implant exposure, including choosing appropriately sized implants and obtaining careful coverage with good vascularized tissue under minimal tension.

Adult↗

PubMed1993

Expression of the osteogenic phenotype in porous hydroxyapatite implanted extraskeletally in baboons.

A porous hydroxyapatite was used as a morphogenetic matrix to study early tissue formation preceding the morphogenesis of bone in extraskeletal sites of the baboon (Papio ursinus). Porous hydroxyapatites, obtained by hydrothermal conversion of the calcium carbonate exoskeleton of coral, were implanted extraskeletally in 16 baboons. Specimens were harvested at days 30, 60 and 90, and processed to obtain decalcified sections for histomorphometry, and undecalcified sections for enzyme histochemical demonstration of alkaline phosphatase, immunohistochemical demonstration of laminin and type I collagen, and for comparative histologic analysis. At day 30, the tissue that invaded the porous spaces showed mesenchymal condensations at the hydroxyapatite interface, and prominent vascular penetration. Collagen type I staining was localized within mesenchymal condensations. Bone had not formed in any specimen harvested at day 30. At days 30 and 60, alkaline phosphatase staining was initially localized in the invading vasculature, and subsequently found in cellular condensations prior to their transformation into bone, and in capillaries close to cellular condensations. Laminin staining was localized around invading capillaries adjacent to and within mesenchymal condensations, and in capillaries in direct contact with the hydroxyapatite. Bone had formed by day 60; cartilage, however, was never observed. By day 90, bone formation within the porous spaces was often extensive. Goldner's trichrome stain and fluorescence microscopy of tetracycline-labeled specimens demonstrated nascent mineralization within condensations during initial bone morphogenesis. Coating the hydroxyapatite with collagen type I prepared from baboon bone did not increase the amount of bone formation. In this hydroxyapatite-induced osteogenesis model in primates, vascular invasion and bone differentiation appear to be accompanied by a specific temporal sequence of alkaline phosphatase expression. The differentiation of osteogenic cells in direct apposition to the hydroxyapatite suggests that this substratum may act as a solid state matrix for adsorption and controlled release of endogenously-produced bone morphogenetic proteins. The porous hydroxyapatite, as used in this bioassay in primates, may be an appropriate delivery system for bone morphogenetic proteins for the controlled initiation of therapeutic osteogenesis.

Alkaline Phosphatase↗

PubMed1997

Migration of hydroxyapatite onlays into the mandible and nasal bone and local bone turnover in growing rabbits.

To determine the effects of local bone turnover on the migration of macroporous hydroxyapatite onlays in the nasal bone and mandibular ramus, we performed histomorphometric analyses of the underlying bone area in 41 New Zealand White rabbits from the age of 4 weeks. The hydroxyapatite implants were placed under the periosteum of the right nasal bone (a depository bone onto its periosteal surface and endosteal resorptive) and the mandibular ramus (resorptive onto its outer surface). The corresponding left sides were sham operated. Following fluorescence bone labeling, composite specimens of the hydroxyapatite block including both sides of the nasal bone and mandible were removed at 0 (n = 1), 3, 6, 9, 12, and 16 weeks postoperatively (n = 8, respectively) and processed to yield undecalcified sections. Bone-bone marrow interfaces in the entire area within 200 microns beneath the base of the hydroxyapatite and in the counter-area on the sham-operated side were measured under a light microscope. In all grafted specimens, the hydroxyapatite matrix was directly united with the underlying tissue by bone ingrowth. However, the sinking of the hydroxyapatite graft in the nasal bone was significant at 3 weeks postoperatively and gradually increased thereafter. In the mandible, the sinking became significant at 6 weeks. In the nasal bone, the bone area density beneath the graft showed a time-dependent decrease during the experimental period, but in the mandibular bone, the value was initially decreased at 3 weeks and then recovered to baseline level. In both bones, parameters of bone resorption, such as osteoclast number and osteoclast surface, were significantly increased from 3 weeks. While the parameters of bone formation, such as osteoblast surface and mineralizing surface, were significantly decreased from 3 weeks in the nasal bone, they were significantly increased in the mandible. Mineral apposition rate showed a significant decrease in both bones. Our data indicate that while the bone area density beneath the hydroxyapatite seemed to depend on bone formation, increased bone resorption would be more critical for the remodeling of underlying bony architecture in the migration of the hydroxyapatite graft.

Animals↗

PubMed2001

Induction of bone formation by recombinant human osteogenic protein-1 and sintered porous hydroxyapatite in adult primates.

A critical issue in tissue engineering and morphogenesis of bone is the development of novel biomimetic biomaterials that are capable of optimizing the biological activity of recombinant human bone morphogenetic and osteogenic proteins, which are molecules that initiate bone formation in vivo. From a therapeutic perspective, a carrier matrix is required for the local delivery of these proteins to evoke a desired osteogenic effect. In view of the affinity of these proteins for hydroxyapatite, which may reflect the in vivo supramolecular assembly of bone proteins bound to both the extracellular matrix and the mineral component of bone, we investigated the efficacy of single applications of different doses of human osteogenic protein-1 (hOP-1) adsorbed onto sintered porous hydroxyapatites for bone induction in orthotopic calvarial defects in 12 adult male baboons (Papio ursinus) and heterotopically in the rectus abdominis of four additional baboons. In orthotopic specimens, pretreatment of sintered porous hydroxyapatites with 100 microgram of hOP-1 in 500 microliter of 5 mM hydrochloric acid resulted in rapid and diffuse osteoinduction restricted within the porous spaces of the hydroxyapatite, as evaluated by histology and histomorphometry on day 30. Hydroxyapatites treated with 500 microgram of hOP-1 showed a different pattern of bone formation and distribution on day 30 as compared with the lower dose of the recombinant morphogen. Although bone formation was extensive with the higher dose, it was found on the endocranial and pericranial aspects of the specimens, enveloping the implanted hydroxyapatite carrier, and the internal porous spaces were occupied by a rich vascular network without any bone formation. By 90 and 365 days after the implantation of both doses of hOP-1, however, there was remodelling and complete penetration of the newly induced bone within the available porous spaces. The combination of hOP-1 and hydroxyapatite also showed extensive bone formation in heterotopic specimens harvested from the rectus abdominis muscle of the baboon using doses of 5, 25, and 45 microgram of hOP-1 per implant. These findings in the adult primate demonstrate extensive bone formation by hOP-1 adsorbed onto sintered porous hydroxyapatites and suggest that predictable osteogenesis in clinical contexts for treatment of craniofacial bone defects may be engineered using inorganic, nonimmunogenic, and carvable delivery systems that initiate osteogenesis with relatively low doses of recombinant osteogenic proteins, thus mimicking the macrostructure and microstructure of living bone.

Animals↗

PubMed2005

Arc-deposited hydroxyapatite-coated cups: results at four to seven years.

UNLABELLED: A second-generation arc-deposited hydroxyapatite-coated acetabular component was developed after detailed analysis of a failed first-generation grit-blasted hydroxyapatite-coated cup. In our prospective multicenter study we aimed to compare clinical results and survivorship of the newly designed cup to the previous cup and to a similarly designed porous-coated cup during the same time period. The effect of bearing surface-alumina-on-alumina to polyethylene-on-metal-was analyzed. Consecutive patients were assigned randomly to one of three groups: arc-deposited hydroxyapatite-coated cup with alumina-on-alumina bearing surface; porous-coated cup with alumina-on-alumina bearing surface; or porous-coated cup with polyethylene-on-metal bearing surface. A fourth comparison group with a grit-blasted hydroxyapatite-coated cup and polyethylene-on-metal bearing surface was culled from a previous study. Acetabular mechanical failure rate is 0.0%, 0.0%, 1.9%, and 11.2% for the arc-deposited hydroxyapatite-coated, porous-coated with alumina bearing surface, porous-coated with polyethylene-on-metal bearing, and grit-blasted hydroxyapatite-coated cup, respectively, at 4- to 7-years' followup. There are no cases of acetabular osteolysis or femoral neck scalloping and no radiolucent lines in any zone with the arc-deposited hydroxyapatite-coated cup. The cups with the alumina-on-alumina bearing surface had less scalloping of the femoral neck than those with a polyethylene-on-metal bearing surface. Early results with this second-generation arc-deposited hydroxyapatite-coated cup are promising. LEVEL OF EVIDENCE: Therapeutic study, Level I (high-quality randomized trial with statistically significant difference or no statistically significant difference but narrow confidence intervals). See the Guidelines for Authors for a complete description of levels of evidence.

Adult↗

PubMed1999

The adsorptive properties of hydroxyapatite to albumin, dextran and lipids.

Hydroxyapatite is widely utilized as a component in dental hygiene agents. The ability to adsorb and remove dental plaque adhering to tooth surfaces is recognized as an important property of hydroxyapatite particles. In this study, the adsorptive ability of hydroxyapatite was evaluated based on its adsorptive amount and adsorptive strength. The adsorptive amount was determined by measuring the weight of albumin, dextran and lipids adsorbing to hydroxyapatite. The adsorptive strength was determined by measuring the weights of albumin and dextran remaining adsorbed after rinsing. Similar procedures were performed in control experiments using calcium hydrogen phosphate dihydrate, alumina and sericite. The results showed that the weights of albumin and dextran adsorbed to hydroxyapatite were 1.4 +/- 0.13 and 1.6 +/- 0.30 times higher than that to calcium hydrogen phosphate dihydrate. The weight of lipids adsorbed to hydroxyapatite was much higher than that to alumina and sericite. The percentages of weight loss of albumin and dextran were 2.3 +/- 1.7% and 5.3 +/- 2.0% for hydroxyapatite and 27 +/- 6.8% and 19 +/- 11% for calcium hydrogen phosphate dihydrate after one rinse. It is concluded that hydroxyapatite is a better adsorbent than calcium hydrogen phosphate dihydrate, alumina and sericite with respect to adsorbing albumin, dextran and lipids.

Adsorption↗

PubMed2002

Osseous integration of hydroxyapatite grafts in metaphyseal bone defects of the proximal tibia (CT-study).

PURPOSE OF THE STUDY: The purpose of the study was the examination of the osseous integration of hydroxyapatite grafts used for the filling of metaphyseal bone defects in tibia head fractures. MATERIAL: Four patients with lateral tibia plateau fractures AO-type B3 (12) were included in the study. Patients were treated by arthroscopically assisted reduction and percutaneous screw fixation. The metaphyseal bone defects were filled with prepared solid hydroxyapatite graft blocks (Endobon Fa. Merk Darmstadt, Germany). In all of the patients a CT study for the osseous integration of hydroxyapatite grafts used for the filling of metaphyseal bone defects in tibial head fractures was performed. Measurements of density were performed of the implant region, the periimplant region, the distant periimplant region and the fibula bone. Follow-up CT examinations of these specific regions were performed 6 and 12 months postoperative. RESULTS: In all cases an increase of density of the hydroxyapatite graft after 6 months and 12 months follow-up was detected. The periimplant region showed in all cases a decrease of density. A progressive decrease of the periimplant and the distant cancellous tibial bone region was also detectable after 6 and 12 months post-op. A similar decrease of density was visible in the region of the cancellous bone of the fibula. In the interface region a direct bone formation between the hydroxyapatite graft and the adjected cancellous bone was visible in all cases during follow-up. CONCLUSION: The increase of density of the hydroxyapatite grafts and the direct bone formation in the interface region between the hydroxyapatite graft and the adjacent cancellous bone are clear radiomorphological signs for an osteointegration of hydroxyapatite grafts in the metaphyseal region.

Adult↗

PubMed1994

Light and electron microscopy of hydroxyapatite depositions in the ligamentum flavum.

STUDY DESIGN: Ligamenta flava obtained from 24 patients who had undergone surgery for degenerative spinal diseases were investigated regarding the histopathology of hydroxyapatite crystal depositions. OBJECTIVES: Light and electron microscopy and energy dispersive x-ray microanalysis were done to clarify the cause and process of hydroxyapatite crystal depositions in the ligament. SUMMARY OF BACKGROUND DATA: No reports have been made regarding the cause of calcification in the ligamenta flava induced by hydroxyapatite crystal depositions. METHODS: Samples were fixed in 10% formalin and 2.5% glutaraldehyde for light and electron microscopy. Energy dispersive x-ray microanalysis was done to detect calcium and phosphate hydroxyapatite crystal deposition areas. RESULTS: Hydroxyapatite crystals, which occasionally formed clusters, existed in the connective tissue of the dural side around abundant profiles of growing capillaries. Hydroxyapatite crystals showed needle shaped figures, and the x-ray microanalysis revealed that the ratio of calcium to phosphate was about 1.66:1. A decrease or loss of collagenous and elastic fibers was prominent in these hydroxyapatite deposition areas. CONCLUSIONS: Hydroxyapatite crystal depositions occur around the growing capillaries in the dural side of the ligamenta flava.

Adult↗

PubMed1998

Influence of hydroxyapatite particle size on bone cell activities: an in vitro study.

Over the past decade, a large number of biomaterials have been proposed as artificial bone fillers for repairing bone defects. The material most widely used in clinical medicine is hydroxyapatite. The aim of our investigation was to study the effect of hydroxyapatite size mechanism on osteoblasts. The osteoblasts were cultured in vitro with 0.1% (1 mg/mL) of various sized hydroxyapatite particles (0.5-3.0, 37-63, 177-250, and 420-841 microm) for 1 h, 3 h, 1 day, 3 days, and 7 days. The results showed that adding hydroxyapatite particles to osteoblast cultures can significantly affect osteoblast cell count. Osteoblast populations decreased significantly. Osteoblast mean surface areas also changed significantly. Transforming growth factor-beta1 (TGF-beta1) concentrations in culture medium decreased significantly with the addition of hydroxyapatite particles. Prostaglandin E2 (PGE2) concentrations in medium increased significantly. The changes in TGF-beta1 and PGE2 concentration were more significant and persisted longer in smaller-particle groups. The inhibitory effects of hydroxyapatite particles on osteoblast cell cultures were mediated by the increased synthesis of PGE2. Caution should be exercised before using a hydroxyapatite product which could easily break down into fine particles.

Alkaline Phosphatase↗

PubMed1994

The formation of hydroxyapatite-gelatin composites at 38 degrees C.

Composites comprised of calcium deficient hydroxyapatite (CA/P = 1.5) and gelatin form in about 4 h at 38 degrees C. The solid reactants used are CaHPO4 and Ca4(PO4)2O and the liquid reactant is gelatin solution. The reaction forming hydroxyapatite occurs in several steps. Based on the measurement of heat liberation, there is an initial period where the reaction rate is low. This is followed by a period of constant reaction until the reaction is complete. Analysis of the variation in pH shows that hydroxyapatite formation occurs at a steady state pH near 8.2. Analyses of the evolution of crystalline phases indicates that Ca4(PO4)2O is consumed prior to complete reaction resulting in a phase assemblage of hydroxyapatite (CA/P > 1.5) and CaHPO4. Further reaction results in compositional adjustments in hydroxyapatite as CaHPO4 is consumed. Microstructurally, the composites which form consist of clusters of hydroxyapatite embedded in a gelatin network. The presence of gelatin does not influence the morphology of the hydroxyapatite which forms nor does it retard its rate of formation.

Biocompatible Materials↗

PubMed1995

Initial bone matrix formation at the hydroxyapatite interface in vivo.

Dense, sintered, slip-cast hydroxyapatite rods were implanted transfemorally in young adult rats. The femora were excised after 2 and 4 weeks and, following fixation, either embedded in methyl methacrylate for light microscopy, decalcified and prepared for transmission electron microscopy, or freeze fractured in liquid nitrogen for scanning electron microscopic analysis. The latter was performed on the two tissue fragments that remained after freeze fracturing, from which the first contained the implants and the second comprised tissue that had been immediately adjacent to the hydroxyapatite rods. Undecalcified light microscopic sections revealed extensive bone tissue formation around and in contact with the hydroxyapatite rods. The initial bone matrix apposed to the implant surface, as demonstrated with scanning electron microscopy, was either composed of globular deposits or an organized network of collagen fibers. The deposits, which ranged in size from 0.1-1.1 microns, fused to form a cement-like matrix to which collagen fibers were attached. Degradation of the hydroxyapatite surface resulted in the presence of unidirectionally aligned crystallites, with which the newly formed bone matrix was closely associated. Ultrastructural analysis of the bone-hydroxyapatite interface with transmission electron microscopy revealed a 50-600-nm-wide collagen-free granular zone, comprising one or more 40-100-nm-thick electron-dense layer(s). These structural arrangements most probably partially represent the globular deposits and proteinaceous material adsorbed onto and partially in the degrading hydroxyapatite surface. Although the latter change in surface topography may have enhanced bonding of the cement-like matrix to the hydroxyapatite, the cause for this change in topography and the type of bond formed are, at present, unknown.

Animals↗