Calcium phosphate ceramic coatings on porous titanium: effect of structure and composition on electrophoretic deposition, vacuum sintering and in vitro dissolution

Calcium phosphate coatings for medical implants

In surgical disciplines where bone has to be repaired, augmented or improved, bone substitutes are essential. Although bone banks, such as Eurotransplant, are founded to supply such substitutes, natural bone is not always adequate. For example, frequently these so-called bone grafts resorb after implantation (1). Further, they cannot be used for joint and tooth replacement, and recently worries have been raised about the transfer of infectious diseases. Therefore, interest has dramatically increased in the use of synthetic materials for replacement of lost or damaged bone tissue. The generic name of these tissue alternatives is biomaterials. A special class of these biomaterials is composed of metallic devices with coatings to improve bone bonding. These specialized coatings used to improve the metallic implant are the topic of this paper.

Effect of serum proteins and osteoblasts on the surface transformation of a calcium phosphate coating: a physicochemical and ultrastructural study

Changes occurring at the surface of a calcium phosphate coating when in contact with osteoblasts versus those in acellular solutions were analyzed. The coating studied is one with a well-documented extensive effect on short-term bone growth stimulation. Precipitates associated with original crystals and organized in a weblike structure were observed after a 3-week culture with osteoblasts. The precipitates were identified as carbonated hydroxyapatite (c-HA). In contrast, no significant surface changes were detected after immersion in an acellular serum-containing solution. However, in an acellular serum-free solution simulating the ionic composition of plasma, precipitates, identified as c-HA, were abundantly formed. Dissolution of the original coating preceded precipitation. The data support the hypothesis that dissolution of synthetic calcium phosphate ceramics is an initial step in their transformation to a biologically equivalent apatite, and suggest that both solution-mediated (dissolution-precipitation) and cell-mediated mechanisms are involved in the surface transformation.

Fast precipitation of calcium phosphate layers on titanium induced by simple chemical treatments

A simple two-step chemical treatment, i.e. etching with HCl and H2SO4 followed by immersion in boiling dilute NaOH solution, has been developed by our group to prepare bioactive microporous titanium surfaces allowing fast deposition of a calcium phosphate layer (CPL) from an in vitro supersaturated calcification solution (SCS). In this work, a precalcification (Pre-Ca) procedure was applied by soaking the two-step treated titanium in Na2HPO4 and then saturated Ca(OH)2 solution before immersion in SCS to accelerate further the CPL precipitation. The treated titanium surfaces with Pre-Ca were characterized after 1, 2, 4, 8 and 16 h of immersion in SCS by means of scanning electron microscopy together with energy dispersive X-ray analysis, X-ray diffraction and infrared absorption analysis. It was observed that the CPL precipitation rate with Pre-Ca averaged 1 microm h-1, twice as fast as without Pre-Ca. No precipitation was observed on untreated titanium with Pre-Ca up to day 14 of immersion in the SCS.

Atomic force microscopic study of the surface morphology of apatite films deposited by pulsed laser ablation

Atomic force microscopy (AFM) has been used to study the surface morphology of apatite films deposited on metallic and polyethylene substrates by laser ablation using KrF and transversely excited atmospheric CO2 lasers. The films are found to consist of a smooth apatite coating with macroparticles scattered on the surface. A wide variety of macroparticles, differing in size, shape and roughness, were found and analysed employing the high spatial resolution of AFM (< 1 nm). We have investigated the correlation between the apatite film morphology and the deposition conditions. Of particular importance are laser fluence, gas pressure, the nature of the target and the substrate temperature. We have explained these dependencies on the basis of a theoretical model which includes evaporation and a cluster-type laser ablation mechanism.

Calcium phosphate ceramic coatings as carriers of vancomycin

Infection in the setting of total joint arthroplasty remains a challenging problem. Attention has turned to developing methods of local delivery of antibiotics for prophylaxis. Vancomycin loaded into calcium phosphate ceramic coatings on titanium alloy substrates is a clinically relevant concept in the setting of total joint arthroplasty. Drug loading was accomplished by immersion of ceramic-coated discs in vancomycin-containing simulated physiological solution; in some experiments drug loading by immersion was followed by lipid coating in egg phosphatidylcholine solutions. The kinetics of vancomycin release and the efficacy of drug inhibition of Staphylococcus aureus were determined in vitro in comparison to the release from currently used antibiotic-laden poly(methyl methacrylate) (PMMA). The loading by immersion provided effective release and inhibition at early time points (up to 24 h); however, the lipid-coated samples demonstrated significant release and effective bacterial inhibition up to 72 h. The two-step procedure, i.e. drug loading followed by lipid coating in order to slow antibiotic elution, is more effective than the conventional one-step loading. The study indicated that the osteoconductive calcium phosphate coatings have the potential to serve as drug carriers to prevent infection in the setting of total joint arthroplasty.

Bone tissue reactions to an electrophoretically applied calcium phosphate coating

Oral implants of a threaded design, calcium phosphate (CaP)-coated using an electrophoretic deposition technique, were compared to uncoated commercially pure (c.p.) titanium control in an animal study with 4 weeks and 6 months of follow-up, respectively. The 3D surface roughness of a CaP-coated implant was about three times greater than that of an uncoated control. Histomorphometric analyses of the direct bone-implant contact demonstrated a short-term advantage to the CaP-coated implants, whereas no significant difference to the uncoated titanium was found after 6 months. Comparison of the amount of bone inside or outside the threads showed similar values for test and control after 4 weeks. Significantly higher amounts of bone outside the uncoated c.p. titanium implants were measured after the long-term follow-up.

[Hydroxyapatite ceramics in clinical application. Histological findings in 23 patients]

Based on the histological findings of 23 patients who had received implants of the bovine hydroxyapatite ceramic Endobon for a period of up to 16 months, the biocompatibility, nature and extent of osseointegration as well as the resorption and degradation behaviour of the ceramic were investigated. The investigation material consisted mainly of small fragments that had been retrieved during revision operations that were indicated for other reasons. The results confirm the good tolerability and suitability that have been systematically investigated in experimental studies and described for hydroxyapatite ceramic as bone substitute in a vital cancellous bone bed that is not exposed to excessive strain (due to its brittle character). The importance of fulfilling certain requirements in order to achieve a successful result, such as stable implantation in a well vascularized, infection-free bone bed also with a minimization of the contact with local connective tissue has been further substantiated. Good success has been achieved by simultaneous loading with autogenous bone marrow as is practised by many ceramic users. In some cases a widening of intergrain boundaries as well as partial dissociation of superficial hydroxyapatite crystallites were observed in the implant surface.

Preparation of bioactive Ti and its alloys via simple chemical surface treatment

A simple chemical method was established for inducing bioactivity of Ti and its alloys. When pure Ti, Ti-6A1-4V, Ti-6A1-2Nb-Ta, and Ti-15Mo-5Zr-3A1 substrates were treated with 10M NaOH aqueous solution and subsequently heat-treated at 600 degrees C, a thin sodium titanate layer was formed on their surfaces. Thus, treated substrates formed a dense and uniform bonelike apatite layer on their surfaces in simulated body fluid (SBF) with ion concentrations nearly equal to those of human blood plasma. This indicates that the alkali- and heat-treated metals bond to living bone through the bonelike apatite layer formed on their surfaces in the body. The apatite formation on the surfaces of Ti and its alloys was assumed to be induced by a hydrated titania which was formed by an ion exchange of the alkali ion in the alkali titanate layer and the hydronium ion in SBF. The resultant surface structure changed gradually from the outermost apatite layer to the inner Ti and its alloys through a hydrated titania and titanium oxide layers. This provides not only the strong bonding of the apatite layer to the substrates but also a uniform gradient of stress transfer from bone to the implants. The present chemical surface modification is therefore expected to allow the use the bioactive Ti and its alloys as artificial bones even under load-bearing conditions.

Interfacial shear strength and histology of plasma sprayed and sintered hydroxyapatite implants in vivo

The interfaces of bone with sintered hydroxyapatite (SHA) and plasma sprayed hydroxyapatite-coated (HAC) implants in the femora of six dogs were examined by light microscopy, scanning electron microscopy, energy dispersive X-ray analysis, and push-out tests. The results demonstrated that there was no significant difference at 12 and 24 weeks after insertion between the interfacial shear strengths with bone for the two types of implants, however, the histological characteristics of the bone around the plasma sprayed HA could be distinguished from that of the sintered HA. The HAC implants showed an early surface biodegradation as compared with the SHA implants. The observed differences in the interfacial zones may be attributed to different bone cell activities and variations in the dynamics of bone formation, possibly resulting from a higher level of dissolution/reprecipitation along the plasma sprayed HA surface.

Water vapour-treated hydroxyapatite coatings after plasma spraying and their characteristics

A novel way to enhance the ability of hydroxyapatite (HA) coatings in resisting degradation was revealed. The as-received plasma sprayed HA coatings were kept in water vapour at 125 degrees C, with a pressure of 0.15 MPa for 6 h; most of the amorphous phase in the coating was converted into crystalline HA and enhanced the crystallinity significantly. Meanwhile, the alpha-tricalcium phosphate, tetracalcium phosphate and CaO which decomposed from HA during plasma spraying were also transformed into crystalline HA. The dissolution experiment in distilled water at room temperature showed that the post-water vapour-treated coatings were more stable than post-heat-treated ones. The average interfacial tensile bond strength between HA and substrate before and after water vapour treatment was 45.0 and 39.1 MPa, respectively.

Effect of temperature on electrochemical deposition of calcium phosphate coatings in a simulated body fluid

Calcium phosphate coatings were deposited on titanium plate by an electrochemical method in simulated body fluid at 5-62 degrees C. X-ray diffractometry and FTIR studies demonstrated that the deposits at 5, 22 and 37 degrees C were amorphous and those at 52 and 62 degrees C contained Mg(OH)2, CaCO3 and carbonate apatite of low crystallinity. The calcium, magnesium and phosphorus contents of deposit increased in direct proportion to the square root of loading time of cathodic potential. Induction periods, which might be thought to be the time required to decrease the pH of the electrolyte around the cathode by the formation of H2 gas and to start deposition of calcium phosphate, were observed on all the regression lines. It is concluded that in the electrochemical synthesis of calcium phosphate in this temperature range the diffusion process is a rate-determining step.

Sintering effects on the strength of hydroxyapatite

Mechanisms underlying temperature-strength interrelations for dense (> 95% dense, pores closed) hydroxyapatite (HAp) were investigated by comparative assessment of temperature effects on tensile strength, Weibull modulus, apparent density, decomposition (HAp:tricalcium phosphate ratio), dehydroxylation and microstructure. Significant dehydroxylation occurred above approximately 800 degrees C. Strength peaked at approximately 80 MPa just before the attainment of closed porosity (approximately 95% dense). For higher temperatures (closed porosity), the strength dropped sharply to approximately 60 MPa due to the closure of dehydroxylation pathways, and then stabilized at approximately 60 MPa. At very high temperatures (> 1350 degrees C), the strength dropped catastrophically to approximately 10 MPa corresponding to the decomposition of HAp to tricalcium phosphate and the associated sudden release of the remaining bonded water.

Formation and characterization of anodic titanium oxide films containing Ca and P

Commercially pure titanium was anodized in an electrolytic solution that was dissolved calcium and phosphorus compounds in water, and an AOFCP (anodic titanium oxide film containing Ca and P) was formed. It was found that sodium beta-glycerophosphate (beta-GP) and calcium acetate (CA) were suitable for the electrolytes to form the AOFCP having an equivalent Ca/P ratio to hydroxyapatite (HA). The AOFCP was characterized by scanning electron microscopy (SEM), an energy-dispersive X-ray microanalysis (EDX), and X-ray diffraction (XRD). Numerous micropores and microprojections were observed on the AOFCP by SEM. The composition of the AOFCP, which was measured by EDX, changed according to beta-GP and CA concentration, and the electrolytic voltage. Ca and P in the AOFCP seem to be incorporated into the TiO2 matrix from CA and beta-GP in the electrolyte during the anodic oxidation. Despite the existence of Ca and P in the AOFCP, no calcium phosphate peak was detected by XRD, and the AOFCP consisted of anatase and only a little rutile. The AOFCP, whose contents of Ca and P were low, had a high adhesive strength after soaking in a simulated body fluid for 300 days. When the AOFCP having an equivalent Ca/P ratio to HA was hydrothermally heated at 300 degrees C, HA crystals were precipitated on the AOFCP and completely covered the surface.

Effect of Ca/P coating resorption and surgical fit on the bone/implant interface

The effect of coating resorption on bone apposition and attachment strength to resorbable hydroxyapatite (HA), nonresorbable HA-coated, and uncoated rough titanium implants was evaluated in interference- and noninterference-fit (gap of 2-3 mm) surgical models 2, 4, and 12 weeks postoperatively. Interference and noninterference fits showed differences in bone bridging. Bone apposition was circumferential around the implants in noninterference fit. Significantly greater bone apposition was seen to nonresorbable HA-coated implants than uncoated and resorbable HA-coated implants at 4 and 12 weeks. Only resorbable HA coatings showed significantly lower bone apposition for noninterference versus interference fit and from 4-12 weeks. At 2 weeks, strengths of bone attachment to resorbable HA-coated implants were greater than the other implants, and decreased to lower values (not significant) than the nonresorbable HA-coated implants at 4 and 12 weeks. Differences in push-out shear strengths between interference- and noninterference-fit surgical models were significant for uncoated implants at 4 weeks, but not for HA-coated implants at any time period. Significant differences were seen between the three implant types only for the noninterference-fit model, where the HA-coated implants showed greater strengths than the uncoated implants (significant at 2 and 4 weeks). This study showed that presence of resorbable or nonresorbable HA coatings is beneficial when a gap of 2-3 mm is present between the implant and the bone. The resorbable HA-coated implants showed greatest strengths at the early time period. At later time periods, resorbable HA-coated implants showed lower bone apposition and attachment strengths than nonresorbable HA coatings.(ABSTRACT TRUNCATED AT 250 WORDS)

Dissolution/reprecipitation and protein adsorption studies of calcium phosphate coatings by FT-IR/ATR techniques

The surfaces of bioactive Ca-P ceramics immediately change when exposed to proteinaceous solutions. The dissolution behavior and protein interactions of these bioactive materials at the bone/implant interface need to be investigated to understand their material-cellular interactions fully. In this study, FT-IR/ATR techniques were used to study the in situ phosphate release kinetics of Ca-P coatings. The net loss of phosphate molecules from coatings was slower in saline solutions compared with alpha-MEM solutions. Coatings exposed to alpha-MEM solutions containing fibronectin released phosphate molecules slower than coatings exposed to alpha-MEM solutions containing albumin. Conformational changes in fibronectin and albumin adsorbed onto Ca-P and uncoated germanium surfaces were also investigated using FT-IR/ATR spectroscopy. Analysis of changes in the amide I bands indicated that there was a greater loss of beta-sheet structure in adsorbed fibronectin on Ca-P coatings when compared with bare germanium surfaces. Although albumin did change its structure upon adsorption on both Ca-P and germanium, unlike fibronectin, adsorbed albumin structure was similar on Ca-P coatings and germanium. Furthermore, with time the conformation of adsorbed fibronectin and albumin appeared to be very stable on Ca-P coatings, whereas albumin adsorbed to germanium exhibited an increase in ratio of alpha-helix to beta-turn.

Underlying mechanisms at the bone-biomaterial interface

In order to understand how biomaterials influence bone formation in vivo, it is necessary to examine cellular response to materials in the context of wound healing. Four interrelated properties of biomaterials (chemical composition, surface energy, surface roughness, and surface topography) affect mesenchymal cells in vitro. Attachment, proliferation, metabolism, matrix synthesis, and differentiation of osteoblast-like cell lines and primary chondrocytes are sensitive to one or more of these properties. The nature of the response depends on cell maturation state. Rarely do differentiated osteoblasts or chondrocytes see a material prior to its modification by biological fluids, immune cells and less differentiated mesenchymal cells in vivo. Studies using the rat marrow ablation model of endosteal wound healing indicate that ability of osteoblasts to synthesize and calcify their extracellular matrix is affected by the local presence of the material. Changes in the morphology and biochemistry of matrix vesicles, extracellular organelles associated with matrix maturation and calcification, seen in normal endosteal healing, are altered by implants. Moreover, the material exerts a systemic effect on endosteal healing as well. This may be due to local effects on growth factor production and secretion into the circulation, as well as to the fact that the implant may serve as a bioreactor.

Post-deposition heat treatments for ion beam sputter deposited calcium phosphate coatings

Calcium phosphate coatings produced using the ion beam sputter deposition process are amorphous. To produce crystalline coatings, a series of different post-deposition heat treatments were conducted. Heat treatments conducted at temperatures less than 500 degrees C did not produce crystalline phases in the coatings. A 500 degrees C post-deposition heat treatment provided the energy required to produce a hydroxyapatite (HA)-type coating as determined using X-ray diffraction and Fourier transform IR spectroscopy (FTIR). Although post-deposition heat treatments can reduce the adhesion of a coating to its substrate, the 500 degrees C heat treatment did not decrease the coating bond strength. Heat treatments conducted at 600 degrees C were found to produce crystalline HA-type coatings but these heat treatments significantly reduced the coating bond strength. Cracks were observed on the surface of the 600 degrees C heat treated coatings. FTIR analysis revealed a new absorption band at 820 cm-1 for the 600 degrees C heat treated coatings, suggesting the formation of a new phase.

Characterization of calcium phosphate powders by ESCA and EDXA

Calcium phosphate (CaP) materials can be well characterized by traditional methods such as wet chemistry and X-ray diffraction (XRD). These methods, however, offer limitations when non-destructive evaluation of CaP coatings on curved surfaces is required. Since the source powders for these coatings are generally commercially available CaP powders, careful characterization of the source powders may allow inferences to be made regarding the effects of plasma spraying on coating composition. Nine commercially available CaP powders were characterized by scanning electron microscopy, wet chemistry and XRD. These techniques showed that major differences exist between individual powders claiming to be hydroxyapatite. Analysis of these nine powders by electron spectroscopy for chemical analysis (ESCA) and energy dispersive X-ray analysis (EDXA) suggest that these techniques can provide the chemical composition of CaP in a non-destructive manner and thus may be of use in determining the composition of CaP in configurations (such as coatings on metal surfaces) not readily amenable to traditional methods. A calibration curve is required, however, to relate this surface chemical composition result to the material's bulk composition as determined by wet chemistry analysis. Errors of less than 10% can be obtained using ESCA and EDXA. These studies suggest that non-destructive chemical composition evaluation by EDXA and ESCA may also be applicable to CaP coatings.

Structure and integrity of a plasma sprayed hydroxylapatite coating on titanium

Plasma sprayed hydroxylapatite (HA) coatings on titanium substrates were analyzed for process-induced compositional and structural changes. The HA starting powder and the resulting HA coatings were characterized using x-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and Raman spectroscopy. The integrity of the ceramic-to-substrate bond strength was also determined, by subjecting plasma sprayed HA coatings to shear/cantilever bond testing. The ceramic coatings retained the basic apatitic crystal structure of the starting powder; however, a considerable amount of amorphous material was created during the plasma spray process. FTIR and Raman spectroscopy revealed that the resulting coatings were partially dehydroxylated. Both XRD and FTIR spectroscopy results also suggested that amorphous material, as well as additional calcium phosphate phases such as alpha-tricalcium phosphate (TCP) not in the starting powder, were present in the HA coating. Average bond strengths of the HA coatings to Ti were determined to be 14.8 MPa +/- 3.5, with fracture occurring at the interface and within the coating itself.

Development of a glass reinforced hydroxyapatite with enhanced mechanical properties. The effect of glass composition on mechanical properties and its relationship to phase changes

Utilizing glasses of the types xNa2O-(1-x)P2O5 and xCaO-(1-x)P2O5 (where x = 0.2, 0.3, and 0.5), a systematic study of the effect of increasing network modifying oxides in glasses was made on the mechanical properties of a glass reinforced hydroxyapatite, at glass additions of 2.5 and 5 wt%. For the soda type glass, phase changes were promoted much more readily compared to the lime type glass. This was true for both 2.5 and 5 wt% additions of glass. For the lime type glass, considerable differences between the effects of 2.5 and 5 wt% additions were seen. At 5 wt%, the increased amount of liquid phase present promotes an increased level of phase inversion to alpha and beta tricalcium phosphate. At 2.5 wt%, a larger percentage of the HA remains stable at higher temperatures. Furthermore, the effect of the composition may be seen. As the mol% of network forming oxide increases, (i.e., the Ca/P ratio moves towards 1.67, (the ratio for HA) the HA remains more stable. This is seen in the maintenance of both the HA phase and also the enhanced mechanical properties.

Analysis of apatite deposits on substrates

Calcium phosphate coatings, important for medical implant applications can occur in several different phases. Because of differences in properties of these phases it is imperative to be able to identify their presence in a substrate's coating. In this study we characterized hydroxy apatite (HA) surfaces applied by four different coating procedures. Included were plasma sprayed, sputtered, a composite, and a composite overlayed with plasma sprayed. It was found that a combination of reflectance infrared and Raman spectroscopy could be utilized to make an in situ characterization of the calcium phosphate phases. It was shown that plasma sprayed and sputtered coatings of HA lost OH and thus decreased in crystallinity, while composite coatings remained unchanged from the original HA powder.

The effect of calcium phosphate ceramic composition and structure on in vitro behavior. II. Precipitation

The formation of a biologically equivalent carbonate-containing apatite on the surface of synthetic calcium phosphate ceramics (CPC) may be an important step leading to bonding with bone. Reactions of several single phases CPCs upon immersion into a simulated physiologic solution (SPS) with an electrolyte composition of human plasma were determined. The CPCs covered a wide range of solution stabilities from low-soluble hydroxyapatites (HA) to metastable tricalcium phosphates (TCP) and tetracalcium phosphate (TTCP). Changes in chemical compositions of SPS and infrared spectral features after CPC immersion were analyzed. New phase formation was observed on all the CPCs. However, kinetics, compositions, and structures of the new phases were significantly different. The studied CPCs can be characterized by the time to new phase formation in vitro; the minimum time for measurable precipitate formation was found to increase in the order: not-well-crystallized HAs < well-crystallized HAs < alpha-TCP, TTCP < beta-TCP. Among the CPCs only not-well-crystallized HAs led to immediate new phase formation. The metastable CPCs, beta-TCP, alpha-TCP, and TTCP required an induction time during which dissolution occurred. beta-TCP showed the longest induction time and the lowest lattice ion uptake rate of all the CPCs tested. Only the not-well-crystallized HAs elicited immediate formation of carbonated HA. The well-crystallized HAs and beta-TCP did not elicit carbonated apatite formation within the time frame of the experiment. Instead, intermediate phases were formed. On alpha-TCP amorphous calcium phosphate (ACP) with a relatively low carbonate content was formed. TTCP was found to transform extensively to poorly crystallized carbonated apatite after 2 days of immersion.

In vitro evaluation of amorphous calcium phosphate and poorly crystallized hydroxyapatite coatings on titanium implants

Studies of various apatite coatings on metal orthopaedic prostheses suggest that coating dissolution may promote enhanced bone bonding. Little is known concerning the effects of crystallinity and the underlying roughness on calcium phosphate (Ca/P) coating dissolution rate. To address these issues, the surface chemistry of amorphous Ca/P and poorly crystallized hydroxyapatite (HA) coatings on "smooth" and "rough" titanium (Ti) alloy (Ti-6A1-4V) implants was studied following immersion in Hank's physiologic solution at pH 7.2 and 5.2 for 0-, 4-, and 12-week periods. Changes in Calcium (Ca) ion concentrations in the solutions, coating chemistry, and surface morphology were studied by ion selective electrode, x-ray diffraction (XRD), and scanning electron microscopy (SEM) respectively. The amount of Ca dissolved from Ca/P-coated implants was strongly dependent on the chemistry of the coating and less dependent on pH or time of incubation. The effect of the underlying surface (smooth vs. rough) was not significant. The poorly crystallized HA coating underwent the most degradation, greatest crystallographic alteration, and greatest surface film formation. The amorphous coating was more stable in the saline environment, and may be more suitable in vivo if coating longevity is desired. These results suggest that this in vitro method is an effective way of determining differences in HA coating integrity.

The effect of calcium phosphate ceramic composition and structure on in vitro behavior. I. Dissolution

Synthetic calcium phosphate ceramic (CPC) surfaces can be transformed to a biological apatite through a sequence of reactions which include dissolution, precipitation, and ion exchange. By virtue of the reactions being material-dependent, it is important to determine parametric rate effects. In this study we focused on the effect of stoichiometry and crystal structure of CPCs on the dissolution kinetics. Monophase, biphase, and multiphase CPCs with a Ca/P ratio equal to or greater than 1.5 were studied. The experiments were performed in a calcium- and phosphate-free Tris buffer solution at pH 7.3. The dissolution behavior of the CPCs studied was found to vary over a wide range. The dissolution rate of the monophase CPCs increased in the order of stoichiometric hydroxyapatite, calcium deficient hydroxyapatite, oxyhydroxyapatite, beta-tricalcium phosphate, alpha-tricalcium phosphate, and tetracalcium phosphate. Dissolution of biphase and multiphase CPCs increased prorated the concentration of more soluble component.

The effect of phase differences on the time-dependent variation of the zeta potential of hydroxyapatite

The osteoconductive nature of calcium phosphate ceramics (CPC) follows from several proven effects, such as a direct bone attachment and enhanced bone tissue formation. Mechanisms leading to these phenomena are still largely undiscovered. Specifically, little is known about the CPC surface and cell-driven reactions. These atomic and molecular level events are involved in tissue attachment and enhanced tissue formation. It is hypothesized that the zeta potential of these ceramics is directly related to the surface reactivity governing osteoconductivity. As a first step in our analysis, the zeta potential of stoichiometric and Ca-deficient hydroxyapatite was determined as a function of immersion time. It is concluded that, under the conditions of the experiment, the observations support the hypothesis in a dual way. First, the absolute values of the zeta potential which were measured are related to electrokinetic potentials known to cause substantial effect on the cellular activities and bone tissue formation when applied exogenously. Second, the magnitude and duration of the changes in zeta potential are related to an ion exchange between the hydrated layer around the ceramic and the ceramic surface, and a net precipitation of new material. If these findings could be confirmed in other solutions, i.e., solutions with a substantially equivalent composition as the fluids in developing bone tissue, a basis would be provided to explain the bridging of the ceramic surface with the surrounding developing tissue.

Bone tissue growth enhancement by calcium phosphate coatings on porous titanium alloys: the effect of shielding metal dissolution product

The possible mechanism of minimization of prosthesis-derived bone growth inhibitors by shielding of the metal and the reduction, if not elimination, of the associated metal dissolution was investigated. Titanium, aluminium and vanadium release rates were determined in vitro for Ti alloy specimens both with and without a calcium phosphate coating. Ti orderly oriented wire mesh (OOWM) porous coatings on Ti-6Al-4V substrates were used as the metal specimens. Half of the specimens were coated with a 75 microns calcium phosphate ceramic (CPC coating). Seven reference (OOWM) and seven coated (OOWM-CPC) specimens were immersed and placed along with seven control solutions for various periods in an incubator maintained at 37 degrees C and 5% CO2 - air atmosphere. Whereas the reference solutions showed a Ti release increasing as a function of time, the solutions that had the CPC-coated specimens contained no measurable amounts of titanium. The Al in solution around the CPC-coated specimens was significantly greater than the concentration around non-coated specimens. The Al, however, did not increase significantly with time, at least up to 4 wk immersion. The ceramic coating had a small beneficial effect on V concentration. In the absence of a significant adverse effect of Ti on local bone tissue formation, we focus on the Al data of our study. The possible adverse effect of this element is well documented. The calcium phosphate coating produced a significant increase of biological fixation, yet at the same time a greater Al release into solution, calling into question the significance of CPC coating in shielding adverse metal passive dissolution to explain enhanced bone growth [corrected].

Physico-chemical considerations of titanium as a biomaterial

Physico-chemical properties of titanium are discussed. Special attention is paid to those of amorphous TiO 2 that contact tissues in vivo. In aqueous environments TiO 2. (aq) has low ion-formation tendency and low reactivity with macromolecules. This is accompanied by low toxicity. Titanium does not facilitate reactive oxygen radical generation during inflammatory conditions as observed in in-vitro experiments. The outermost layers of the oxide are in the Ti(IV) oxidation state, although using electron spin resonance (ESR) techniques, formation of Ti(III) is observed at atmospheric conditions. The impact of similarities between water and TiO 2 is speculated upon, and the physico-chemical properties of titanium are tentatively linked to some in-vivo consequences.

Compositional variations in the surface and interface of calcium phosphate ceramic coatings on Ti and Ti-6Al-4V due to sintering and immersion

The compositions of the surface and the interface of calcium phosphate ceramic (CPC) coatings electrophoretically deposited and sintered on titanium or its alloy, were determined by scanning Auger electron spectroscopy before and after 4 wk of immersion in a simulated physiological solution. In the CPC coating-metal interfaces, the phosphorus diffused beyond the titanium oxide layer. The phosphorus concentration in the interface followed a Gaussian distribution for both unalloyed and alloyed titanium. The diffusion depleted P in the ceramic adjacent to the metal. The surface of the ceramic, however, was substantially unchanged. A major change in the compositional depth profiles was induced by immersion: thick and uniform titanium phosphide layers of constant composition were observed on the Ti-based metal substrates.

Effect of calcium phosphate coating characteristics on early post-operative bone tissue ingrowth

The synthesis of model porous metal-CPC materials, and their use in one-parametric studies of bone tissue ingrowth enhancement were considered. By using the same starting calcium-deficient hydroxyapatite powder, three different coatings, CAP1, CAP2 and CAP3, were obtained of thicknesses 50 +/- 5, 75 +/- 5 and 75 +/- 5 microns, respectively. CAP1 and 2 were either the starting powder mixed in a 3:1 ratio CPC: poly(lactic acid) or the powder by itself. The CAP3 coating was the result of a thermal treatment producing a mixture of oxyhydroxyapatite, alpha- and beta-tricalcium phosphate. Orderly oriented wire mesh porous coated specimens were implanted, along with the same specimens lined with CAP 1, 2 or 3. Subsequently, the total of 156 specimens was retrieved at 2, 4 or 6 wk, and tested mechanically and processed for histomorphometry. The data produced considerable evidence for the CPC-dependent enhancement of bone tissue ingrowth in porous metals immediately after implantation. They prove that the materials processing of CPC coatings influences the resulting biological behaviour substantially. Furthermore, they support the hypothesis that ceramic dissolution is a causative factor on the bone tissue growth enhancement mechanisms.