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Electrohydrodynamic spraying is a well established process used to deposit, coat, analyse and synthesise materials within the biomedical remit. Recently, electrohydrodynamic printing has been developed to afford structures for potential applications in the biomedical and medical engineering fields. Both of these processes rely on the formation of an electrically-induced jet, however the resulting products can be made strikingly different and offer potential in broader applications. Here we show how spraying and printing are linked by elucidating the ease of transition between the processes. Changes in the deposition distance can result in either spray (>10 mm) or print formation (<3 mm), with an overlap of the two in between this range. For the optimal printing distance of 0.5 mm, gradual changes in the applied voltage (0–4.5 kV) encounters transitional printing modes (dripping, micro-dripping, rapid micro-dripping, unstable and stable jetting) which can be utilised for patterning. The results indicate the robustness of the electrohydrodynamic route in the nano-materials processing arena, with emphasis on biomedical materials. [ABSTRACT FROM AUTHOR]

Hydroxyapatite [Ca10(PO4)6(OH)2], (HA) is similar in composition to bone mineral and has been found to promote new bone formation when implanted in a skeletal defect. However, its use in biomedical applications is limited by its relatively slow rate of biological interaction, and there is also a requirement to improve the success rate of HA implants in younger active patients, particularly where implants will be in place long-term. The addition of silicon (Si) into HA has been demonstrated to enhance the speed, and quality of the bone repair process. This paper describes the synthesis and detailed characterisation of nanocrystalline silicon-substituted hydroxyapatite (SiHA) thin coatings applied to a titanium substrate via a magnetron co-sputtering process. Amorphous SiHA coatings (∼1 μm thick) with varying Si content up to 4.9 wt% were produced before being transformed into crystalline films by heat-treatment. The crystalline coating was characterised by X-ray diffraction (XRD) and infrared (IR) analysis, and confirmed to be a single-phase apatite. The substitution of Si into HA resulted in an increase in both the a- and c-axes of the unit cell parameters, but a decrease in the crystallite size, with increasing Si substitution. This substitution also caused a decrease in the intensities of both the O–H and P–O bands in the IR spectra. Hence, these findings confirmed that the crystal structure of HA was altered with Si substitution. In vitro cell culture work showed that these SiHA thin coatings exhibited enhanced bioactivity and biofunctionality. An increase in the attachment and growth of human osteoblast-like (HOB) cells on these coatings was observed throughout the culture period, with the formation of extracellular matrix. In addition, confocal microscopy revealed that HOBs developed mature cytoskeletons with clear evidence of actin stress fibres, along with defined cell nuclei. [ABSTRACT FROM AUTHOR]

Reports on the production of biocompatible submicrometer sized relics containing nanosized hydroxyapatite (HA) by electrospraying. Procedure's offer of the potential to create both micro- and nano- scale surface topography and coverage for a favorable cell response; Exploration of the processing of nHA suspensions using different modes of electrospraying and a variety of substrates having different surface porosities.

Phase pure hydroxyapatite (HA) and a 0.8 wt % silicon substituted hydroxyapatite (SiHA) were prepared by aqueous precipitation methods. Both HA and SiHA were processed into granules 0.5–1.0 mm in diameter and sintered at 1200 °C for 2 h. The sintered granules underwent full structural characterization, prior to implantation into the femoral condyle of New Zealand White rabbits for a period of 23 days. The results show that both the HA and SiHA granules were well accepted by the host tissue, with no presence of any inflammatory cells. New bone formation was observed directly on the surfaces and in the spaces between both HA and SiHA granular implants. The quantitative histomorphometry results indicate that the percentage of bone ingrowth for SiHA (37.5%±5.9) was significantly greater than that for phase pure HA (22.0%±6.5), in addition the percentage of bone/implant coverage was significantly greater for SiHA (59.8%±7.3) compared to HA (47.1%±3.6). These findings indicate that the early in vivo bioactivity of hydroxyapatite was significantly improved with the incorporation of silicate ions into the HA structure, making SiHA an attractive alternative to conventional HA materials for use as bone substitute ceramics. [ABSTRACT FROM AUTHOR]

A new synthesis/processing method has been devised to produce magnesium/carbonate co-substituted hydroxyapatite ceramics that do not decompose to tricalcium phosphate (TCP) on sintering. Using this method, a series of magnesium/carbonate co-substituted hydroxyapatite (Mg/CO3–HA) compositions, containing between 0 and 0.35 wt % Mg and approximately 0.9 wt % CO3 were prepared. Sintering the Mg/CO3–HA compositions in a CO2/H2O atmosphere yields a single crystalline phase that appears to be identical to stoichiometric HA. In contrast, when the compositions were prepared in the absence of carbonate and were sintered in air, the phase composition was a biphasic mixture of HA and TCP e.g. for 0.25 wt % Mg substitution the phase composition was approximately 60%HA/40% TCP. Clearly, both the synthesis route and the processing (i.e. sintering) route are of importance in the production of a single-phase Mg/CO3–HA ceramic. Fourier transform infrared (FTIR) spectroscopy has indicated that the Mg/CO3–HA ceramics still contained carbonate groups after sintering at 1200 °C. Chemical analysis by X-ray fluorescence spectroscopy (XRF) and C–H–N analysis has shown that the cation/anion molar ratio (i.e. [Ca+Mg]/[P+C/2]) of the different compositions were 1.68(±0.01), which is equivalent to the Ca/P molar ratio of stoichiometric HA. Although the magnesium/carbonate co-substitution had a positive effect in preventing phase decomposition during sintering, it appeared to have a negative effect on the densification of the MgCO3–HA ceramics, compared to stoichiometric HA. [ABSTRACT FROM AUTHOR]

Heat treatments are used when sintering hydroxyapatite to make porous blocks and granules and during plasma spraying of coatings. Calcium : phosphorus ratio is known to affect the thermal decomposition behavior of hydroxyapatite. Hydroxyapatite with carbonate ions substituted for phosphate ions is more similar in composition to bone mineral. While it has been shown that carbonate apatite may be sintered, relatively little is known about its high temperature stability. Various atmospheres have been used in investigations into the thermal stability of hydroxyapatites and carbonate hydroxyapatites, including nitrogen, wet carbon dioxide air, water vapor and wet oxygen, but few of these studies were directly comparable. Previous work has shown that loss of carbonate from CHA at high temperature is time dependent, which suggests that rapid high temperature treatment may prevent carbonate loss during processing. This study investigated the effect of dry carbon dioxide, carbon dioxide containing 3% water, nitrogen and nitrogen containing 3% water on the phase composition of hydroxyapatite containing between 1.0 and 11.5 wt % carbonate rapidly heated to temperatures of between 700 and 1400 °C. Carbonate ion substitution was observed to decrease the temperature at which crystallisation occurred to a minimum of 700 °C for 11.8 wt % carbonate apatite heated in wet atmospheres. Atmosphere was found to appreciably affect the crystallization temperature and phase transformations of carbonate apatite containing 7.8 wt % carbonate. In wet and dry carbon dioxide atmospheres, crystallisation began in this material at 1100 and 900 °C, βTCP was formed at 1500 and 1300 °C respectively. The high temperature decomposition of carbonate hydroxyapatite would appear to depend on the composition of the apatite and the atmosphere in which it is heated. [ABSTRACT FROM AUTHOR]

Hydroxyapatite (HA) reinforced high density polyethylene (HDPE) was invented as a biomaterial for skeletal applications. In this investigation, tribological properties (e.g. wear rate and coefficient of friction) of unfilled HDPE and HA/HDPE composites were evaluated against the duplex stainless steel in dry and lubricated conditions, with distilled water or aqueous solutions of proteins (egg albumen or glucose) being lubricants. Wear tests were conducted in a custom-built test rig for HDPE and HA/HDPE containing up to 40 vol % of HA. It was found that HA/HDPE composites had lower coefficients of friction than unfilled HDPE under certain conditions. HA/HDPE also exhibited less severe fatigue failure marks than HDPE. The degradation and fatigue failure of HDPE due to the presence of proteins were severe for low speed wear testing (100 rpm) as compared to high speed wear testing (200 rpm). This was due possibly to the high shear rate at the contact which could remove any degraded film instantaneously at high sliding speed, while with a low sliding speed the build-up of a degraded layer of protein could occur. The degraded protein layer would stay at the contact for a longer time and mechanical activation would induce adverse reactions, weakening the surface layer of HDPE. Both egg albumen and glucose were found to be corrosive to steel and adversely reactive for HDPE and HA/HDPE composites. The wear modes observed were similar to that of ultra-high molecular weight polyethylene. Specimens tested with egg albumen also displayed higher wear rates, which was again attributed to corrosion accelerated wear. [ABSTRACT FROM AUTHOR]

A bone cement, poly(ethylmethacrylate)/n-butylmethacrylate (PEMA/nBMA) has been developed with lower exotherm and monomer leaching compared to the traditional poly(methylmethacrylate)/methylmethacrylate (PMMA/MMA) cement. This study compares the in vitro biological response to the cements using primary human osteoblast-like cells (HOB). Cell attachment was qualified by immunolocalization of vinculin and actin cytoskeleton, showing more organization on PEMA/nBMA compared to PMMA/MMA. Proliferation was assessed using tritiated thymidine incorporation, and phenotype expression determined by measuring alkaline phosphatase (ALP) activity. An increase in proliferation and ALP activity was observed on PEMA/nBMA compared to PMMA/MMA. The results confirm the biocompatability of PEMA/nBMA, and an enhanced cell attachment and expression of differentiated cell phenotype. [ABSTRACT FROM AUTHOR]

A detailed study has been undertaken of the anisotropic mechanical behaviour of oriented hydroxyapatite reinforced polymer composite (HAPEXTM). The oriented material was produced by hydrostatic extrusion of isotropic HAPEXTM. It was found that the tensile and shear moduli and strengths of the oriented HAPEXTM are substantially increased compared with isotropic HAPEXTM. In particular, the properties along the extrusion direction exceed the lower range of the properties of cortical bone, showing that the oriented HAPEXTM is suitable for use in skeletal major load bearing applications. [ABSTRACT FROM AUTHOR]

The effect of different calcination temperatures on the powder characteristics and the sintered density of synthetic hydroxyapatite (HA) powders, produced using two different processing routes, was examined. Powders were produced by either drying, milling and sieving an as-precipitated HA or by spray-drying a slurry of precipitated HA. Calcining the two powders at temperatures between 400 and 1000 °C did not significantly affect the powder particle size. The specific surface areas of the two powders, however, were reduced from 70–80 m2/g for a calcination temperature of 400 °C to approximately 5–7 m2/g for 1000 °C. Analysis of the surfaces of the HA powders using scanning electron microscopy (SEM) illustrated the coarsening and subsequent sintering of the sub-micron crystallites that constitute a powder particle as the calcination temperature increased, corresponding to the decrease in surface area of the powders. The sintered densities of the final ceramics were not significantly affected by calcining the powders. Microhardness measurements of ceramics prepared from powders calcined at different temperatures showed no significant variations with calcination temperature or powder processing method. The results of this study have illustrated that for applications where HA may be used in powder form, for example in plasma-spraying and for the production of HA-polymer composites, calcining the HA will significantly affect the powder properties, namely the surface area and morphology of the powders. For applications requiring HA in a dense ceramic form, for example as granules or blocks, calcining the powders does not significantly affect the properties of the final ceramic. [ABSTRACT FROM AUTHOR]

The effect of different sintering conditions on the sintered density and microstructure of two different hydroxyapatite (HA) powders was examined. The powder characteristics of a laboratory synthesized HA powder (Lab HA) were low crystallinity, a bimodal particle size distribution, a median particle size of 22 μm and a high specific surface area (SSA) of 63 m2/g. By contrast, a commercial calcined HA (commercial HA) was crystalline and had a median particle size of 5 μm and a low SSA of 16 m2/g. The different powder characteristics affected the compactability and the sinterability of the two HA powders. Lab HA did not compact as efficiently as commercial HA, resulting in a lower green density, but the onset of sintering of powder compacts of the former was approximately 150 °C lower than the later. The effect of compaction pressure, sintering temperature, time and heating rate on the sintered densities of the two materials was studied. Varying all these sintering conditions significantly affected the sintered density of commercial HA, whereas the sintered density of Lab HA was only affected significantly by increasing the sintering temperature. The Vickers hardness, Hv, of Lab HA was greater than commercial HA for low sintering temperatures, below 1200 °C, whereas for higher sintering temperatures the commercial HA produced ceramics with greater values of hardness. These trends can be related to the sinterability of the two materials. [ABSTRACT FROM AUTHOR]

HAPEXTM, a bone analog material, with similar properties to cortical bone, has been studied in vitro with particular reference to the effect of surface topography. The stimulation of a favorable bone response by this composite depends on optimization of the hydroxyapatite (HA) content in relation to the material bioactivity without compromising the mechanical characteristics. In this study we have started to investigate the effects of surface topography on cell attachment and subsequent cellular behavior in relation to proliferation. Different volumes of HA (20% and 40%) were added to a high density polyethylene (HDPE) matrix to produce the test materials. Scanning electron microscopy (SEM) and confocal laser scanning microscopy (CLSM) were used to examine cell morphology on HAPEXTM, and the surface characteristics produced by different machining protocols. The measurement of cellular DNA and tritiated thymidine ([3H]-TdR) incorporation has been used to asses cell proliferation upon the materials. The results show that the material surface topography has a large influence on cell proliferation and attachment, and with a controled material topography the 40% vol HA/HDPE composite gives the greater biological response compared to the 20% vol HA/HDPE composite. [ABSTRACT FROM AUTHOR]

The structural changes that occur during the transformation of a Ca-deficient apatite, prepared by a wet chemical method, to β-TCp were investigated. X-ray diffraction (XRD) analysis of as-prepared samples and samples calcined at temperatures between 500 and 1100 °C showed that the transformation occurs over the temperature range 710–740 °C, under non-equilibrium conditions. The change in crystallite size with increasing calcination/sintering temperature was studied by XRD using the Scherrer formula. Fourier transform infra-red (FTIR) analysis indicated considerable structural change in samples above and below this temperature range. Changes were observed in the hydroxyl, carbonate and phosphate bands as the calcination temperature was increased from 500 to 1100 °C. Even once a single β-TCP phase is obtained at 740 °C there remains a considerable amount of structural change at temperatures between 740 and 1100 °C. This effect was illustrated by an unusual change in the lattice parameters of the β-TCP structure and significant changes in the phosphate bands of FTIR spectra as the calcination temperature was increased. The results obtained in this study show that the combined experimental techniques of XRD and FTIR are excellent complimentary methods for characterizing structural changes that occur during phase transformations. [ABSTRACT FROM AUTHOR]

This work documents an investigation into the effect of water on the density and microstructure of carbonate hydroxyapatite in carbon dioxide sintering atmospheres. Carbonate apatites with carbonate contents of between 3.2 and 7.8 wt % were precipitated and the precipitates were formed into dry gels. Isothermal and isochronal sintering experiments were performed under dry carbon dioxide and wet carbon dioxide (containing 3 wt % water) atmospheres. The effect of carbonate content was studied by using two gels both with a green density of 37% and with carbonate contents of 5.8 and 7.8 wt %. Both isothermal and isochronal experiments demonstrated that bloating of the apatite occurred and this behavior was associated with the loss of carbonate from the apatite. It was found that only in wet carbon dioxide atmospheres fully dense translucent carbonate apatite could be formed. 93% dense carbonate apatite was formed after 4 h sintering at temperatures as low as 700 °C. © 2000 Kluwer Academic Publishers [ABSTRACT FROM AUTHOR]

Dynamic mechanical analysis (DMA) was used to characterize biomedical composites consisting of synthetic hydroxyapatite (HA) particulate reinforced polyethylene (PE). The effects of the HA volume fraction, temperature and HA particle on the storage modulus ( EI) and damping (tan δ) were investigated. Increasing HA volume fractions increased EI and decreased tan δ. EI was found to be linearly related to the Young's modulus values obtained from quasi-static tensile tests. Relative modulus and damping studies showed that the viscoelastic behavior of unfilled PE was different to that of the filled matrix due to the presence of thermally induced tensile stresses in the matrix at the filler-matrix interface. [ABSTRACT FROM AUTHOR]

Mechanical fatigue of bone leads to micro-cracking which is associated with remodeling, establishing a balance in the microcrack population of the living tissue, thus, in the steady-state, the microstructure of bone provides sites of discontinuity acting as stress raisers. Hence fracture toughness plays a decisive role in bone functionality by determining the level to which the material can be stressed in the presence of cracks, or, equivalently, the magnitude of cracking which can be tolerated at a given stress level. Cortical bone, which behaves as a quasi-brittle solid when fractured, was tested as short-rod chevron-notched tension specimens (CNT). The main features of the CNT specimen are its geometry and the V shaped notch. The notch leads to steady-state crack propagation whilst the requested geometry allows a diameter 40% smaller than the thickness of a standard compact tension specimens (CT). These features are essential to distinguish the inhomogeneties in the fracture properties of materials like bone. Bone structure and crack propagation of the CNT specimens were analyzed using X-ray computed micro-tomography (XMT), which is a non-invasive imaging technique. The unique feature of the micro-CT is the high resolution three-dimensional image which consists of multi-sliced tomographs taken in a fine pitch along the rotational axis. Fracture toughness ( KIC) computed according to the peak load was 4.8 MNm-3/2 while that derived from experimental calibration tests using XMT was 4.9 MNm-3/2. [ABSTRACT FROM AUTHOR]

PEMA–based bone cement has previously been shown to possess many advantages over traditional PMMA cements. One of these is the option of adding up to 40 wt % HA without a decrease in static mechanical strength, thus providing the potential for enhanced bioactivity. Bone cement, in vivo, is subjected to an aqueous environment and therefore, it is important to understand the influence of this upon the mechanical integrity of experimental cements. In this current investigation the static and dynamic properties of PEMA cement, with and without 30 wt % untreated and silanated HA, were examined after periods of immersion in Ringer’s solution. A commercial PMMA cement was also tested in a similar manner. Relatively small changes in static mechanical properties were observed after 12 weeks storage for the PEMA cements, the largest change being for the PEMA cement reinforced with silanated HA. The PMMA cement exhibited the largest change in static strength with a decrease of 16.6%. In contrast to these results, the fatigue properties of the PEMA cements were found to decrease significantly after storage in Ringer’s solution, again with the largest changes to the PEMA cement reinforced with silanated HA. This effect was attributed to the reduction in efficiency of the silane coupling agent in the presence of water. The fatigue resistance of the PMMA cement was not reduced after immersion in a saline environment. [ABSTRACT FROM AUTHOR]

Impact stress transmission of Ti-Ni alloy was evaluated for biomedical stress shielding. Transformation temperatures of the alloy were investigated by means of DSC. An impact compression test was carried out with use of split-Hopkinson pressure-bar technique with cylindrical specimens of Ti-Ni alloy, titanium and stainless steel. As a result, the transmitted pulse through Ti-Ni alloy was considerably depressed as compared with those through titanium and stainless steel. The initial stress reduction was large through Ti-Ni alloy and titanium, but the stress reduction through Ti-Ni alloy was more continuous than titanium. The maximum value in the stress difference between incident and transmitted pulses through Ti-Ni alloy or titanium was higher than that through stainless steel, while the stress reduction in the maximum stress through Ti-Ni alloy was statistically larger than that through titanium or stainless steel. Ti-Ni alloy transmitted less impact stress than titanium or stainless steel, which suggested that the loading stress to adjacent tissues could be decreased with use of Ti-Ni alloy as a component material in an implant system. ©2000 Kluwer Academic Publishers [ABSTRACT FROM AUTHOR]

Hydroxyapatite reinforced high density polyethylene composite (HAPEX™) has been developed for bone substitution. To improve its mechanical properties, HAPEX™ was hydrostatically extruded at different extrusion ratios after compression moulding. Substantial increases in the tensile and flexural properties of both unfilled polyethylene and HAPEX™ were achieved. It was evident that the higher the extrusion ratio, the stiffer and the stronger the extruded rods. The ductility of HAPEX™ was also significantly enhanced by hydrostatic extrusion. Hydrostatically extruded HAPEX™ possesses mechanical properties that are within the bounds for human cortical bone, which indicates its potential for load-bearing skeletal implant applications. [ABSTRACT FROM AUTHOR]

The nature of the orthopedic implant surface affects the interaction between cells and subsequent bone formation. The bone/cement interface in cement-held prostheses is considered to be the main cause of fracture leading to implant revision. It is thought that the introduction of a bioactive phase, such as hydroxyapatite (HA), to cement may permit a stronger implant by encouraging direct bone apposition rather than encapsulation of the implant by fibrous tissue. Thus, a poly(methylmethacrylate) (PMMA) cement incorporating 17.5% HA by weight has been investigated. In this study, in order to analyze the interaction at the cellular level, the in vitro biological response of the HA/PMMA to a similar PMMA without HA incorporation has been studied. Primary human osteoblast-like cells (HOB) were used as they are a model of the cell type the cements might encounter in vivo. Cell proliferation and growth were assessed by measurement of total cellular DNA and tritiated thymidine ([3H]-TdR) incorporation. Alkaline phosphatase (ALP) production was measured as an indicator of HOB phenotype upon the cements. The results showed that HA/PMMA was a better substrate for HOB cells, resulting in increased proliferation and ALP activity. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) showed that HOB cells cultured on the HA-filled PMMA preferentially anchored to HA particles exposed at the cement surface, with a close intimacy observed between HA and HOB cells. [ABSTRACT FROM AUTHOR]

Hydroxyapatite has been investigated for use in the osseous environment for over 20 years and the biocompatibility of the ceramic and its osseoconductive behavior is well established. Therefore, the use of porous hydroxyapatite for the repair of osseous defects seems promising with potential for complete penetration of osseous tissue and restoration of vascularity throughout the repair site. However, there have been few systematic studies of the effects of physical properties such as macropore size and pore connectivity on the rate and quality of bone integration within porous hydroxyapatite implants. This paper quantifies the early biological response to a well-characterized series of implants with uniform microstructure and phase composition, but differing macrostructures and demonstrates the dependence of the rate of osseointegration on the apparent density of porous hydroxyapatite as a function of pore connectivity. Furthermore, compression testing established that bony ingrowth has a strong reinforcing effect on porous hydroxyapatite implants, which is more pronounced in the lower density implants, as a result of a greater relative volume of bone ingrowth. © 1999 Kluwer Academic Publishers [ABSTRACT FROM AUTHOR]

A drug delivery system (DDS) which provides a sustained release of antibiotics at the focal site either singly, or in combination with a bone stimulating factor could both eliminate infection and increase the number of potentially healthy osteogenic cells. In this study, we address the use of a degradable gelatin DDS, for the combined release of therapeutic levels of both gentamicin and growth hormone (GH). An initial bolus release was observed during the first 24 h followed by a reduced, but sustained, release for both drugs up to 14 days. Bioactivity of gentamicin was demonstrated by growth inhibition of Staphylococcus aureus for over 96 h with a mean zone of inhibition of 29.4 mm (±0.19) for the time period studied. Furthermore, GH was shown to have a direct effect on primary human osteoblast-like (HOB) cells, stimulating proliferation and enhancing their differentiation. Site-specific drug delivery offers the advantage of localizing a drug directly at the target site, thus minimizing systemic effects. The results of this study suggest that gelatin is a good DDS for the combined release of drugs. In addition, gelatin is both biocompatible and biodegradable, thus making it a promising DDS for the management of acute and chronic bone and tissue infection such as osteomyelitis. © 1999 Kluwer Academic Publishers [ABSTRACT FROM AUTHOR]

Hydroxyapatite has been considered for use in the repair of osseous defects for the last 20 years. Recent developments have led to interest in the potential of porous hydroxyapatite as a synthetic bone graft. However, despite considerable activity in this field, regarding assessment of the biological response to such materials, the basic materials characterization is often inadequate. This paper documents the characterization of the chemical composition, mechanical integrity, macro- and microstructure of a porous hydroxyapatite, Endobon ® (E. Merck GmbH), intended for the bone-graft market. Specimens possesed a range of apparent densities from 0.35 to 1.44 g cm-3. Chemical analysis demonstrated that the natural apatite precursor of Endobon® was not converted to pure hydroxyapatite, but retained many of the ionic substituents found in bone mineral, notably carbonate, sodium and magnesium ions. Investigation of the microstructure illustrated that the struts of the material were not fully dense, but had retained some traces of the network of osteocyte lacunae. Macrostructural analysis demonstrated the complex inter-relationship between the structural features of an open pore structure. Both pore size and connectivity were found to be inversely dependent on apparent density. Furthermore, measurement of pore aspect ratio and orientation demonstrated a relationship between apparent density and the degree of macrostructural anisotropy within the specimens, while, it was also noted that pore connectivity was sensitive to anisotropy. Compression testing demonstrated the effect of apparent density and macrostructural anisotropy on the mechanical properties. An increase in apparent density from 0.38 to 1.25 g cm-3 resulted in increases in ultimate compressive stress and compressive modulus of 1 to 11 MPa and 0.2 to 3.1 GPa, respectively. Furthermore, anisotropic high density (> 0.9 g cm-3) specimens were found to possess lower compressive moduli than isotropic specimens with equivalent apparent densities. These results underline the importance of full structural and mechanical characterization of porous ceramic implant materials. ©1999 Kluwer Academic Publishers [ABSTRACT FROM AUTHOR]

A primary human cell culture model was used to investigate a range of hydroxyapaptite (HA)-reinforced high-density polyethylene (HDPE) composites (HAPEXTM). These materials are being developed as potential bone-substitute materials. When designing and optimizing a second-generation biomaterial, it is important to achieve a balance between mechanical and biological properties without compromising either. Biochemical and histological parameters have been used to compare the biological response of 20% and 40% volume HA in HDPE. Cellular DNA and incorporation of tritiated thymidine was measured to assess cell proliferation. Alkaline phosphatase (ALP) production was used as a marker of osteoblast phenotype expression. In this preliminary study, osteoblasts cultured on the 20% HAPEXTM showed a greater increase in the rate of proliferation and osteoblast expression as indicated by an increase in ALP activity compared to the 40% HAPEXTM over the time period studied. Osteoblast-like cells showed a flattened morphology on both composites and in some cases a greater covering was observed on the 20% HAPEXTM. These results indicate that the composites may not be identical in terms of bioactivity and that further research on surface topography and physico-chemical properties is required to assess fully the biological response of these composites. © 1998 Kluwer Academic Publishers [ABSTRACT FROM AUTHOR]

The polymerization behavior of a new bone cement based on poly(ethylmethacrylate), hydroxyapatite powder and n-butylmethacrylate monomer and a commercial cement have been studied. Polymerization kinetics were analyzed by means of differential scanning calorimetry (DSC). DSC data have been used to evaluate a phenomenological model describing the cure kinetics of this new bone cement. The kinetic model coupled with the energy balance was then used to obtain temperature and degree of conversion profiles in the bone–cement–prosthesis system, under non-isothermal conditions, as function of initial temperature and thickness of the cement. Material properties, boundary and initial conditions and the kinetic behavior were the input data for the numerically solved heat-transfer model. The modeling results have been compared with in vitro results. © 1998 Kluwer Academic Publishers [ABSTRACT FROM AUTHOR]

Substituted apatite ceramics are of clinical interest as they offer the potential to improve the bioactive properties of implants. Carbonate hydroxyapatite (CHA) has been synthesized by an aqueous precipitation method and precipitates with two different levels of carbonate, processed as powders. Sintering experiments were performed to establish the influence of carbonate in significantly reducing the temperature required to prepare high-density ceramics when compared with stoichiometric hydroxyapatite (HA). High-temperature X-ray diffraction was used to characterize the phase stability of the apatites on sintering. Increasing carbonate content was shown to reduce the temperature at which decomposition occurred, to phases of CaO and β-TCP. Mechanical testing, performed using biaxial flexure, showed that the CHA specimens had strengths similar to stoichiometric HA. © 1998 Kluwer Academic Publishers [ABSTRACT FROM AUTHOR]

Dahllite is a synthetic carbonated hydroxyapatite with a carbonate ion content similar to bone mineral. The first objective of this study was to investigate the stoichiometric conditions under which dahllite formation occurs in a powder mixture of α-Ca3(PO4)2, Ca(H2PO4)2 and CaCO3. The second objective was to identify how these conditions apply to commercially available cement, Skeletal Repair System (SRSTM), and other α-Ca3(PO4)2-based cements currently under investigation. The stoichiometric coefficients were found to be a function of both the percentage of carbonate ions incorporated into the hydroxyapatite structure, and the amount of CO2 released during the reaction. As a consequence, a stability field has been obtained where different initial proportions of the reactants in the powder mixture should give the same reaction product if sufficient CO2 is released into the solution. However, increasing amounts of CaCO3 in the initial mixture have been shown to affect the solution pH in such a way that only partial reaction of the reactants takes place. SRSTM and other α-Ca3(PO4)2-based cements have been located inside the stability field and a comparison between their reported setting and hardening properties has been performed. © 1998 Kluwer Academic Publishers [ABSTRACT FROM AUTHOR]

Titanium–zirconium based alloys containing a small amount of niobium were investigated in order to evaluate their possible use as biomedical materials. Zirconium, which belongs to the IVa group, is known to have good corrosion resistance and biocompatibility similar to titanium. As the titanium–zirconium system shows a complete solid solution, a wide variation of alloy design is available and large quantities of solid-solution hardening must be possible. Niobium, having a β-phase stabilizing effect, was chosen as a ternary element in order to control desirably the microstructure. There have been no reports which suggest its harm to a living body. The alloys containing 2% or 3% niobium showed the highest hardness value after aging heat treatment at 773 K. In contrast to this, no alteration of hardness was seen in specimens aged at 1073 K. Through conventional X-ray diffractometry and in situ X-ray analysis using a hot stage, β-phase precipitation in the A matrix was identified. From the above results, it is concluded that alloys containing 2%–3% niobium are hopeful candidates for new kinds of biomedical alloys, when they are heat treated under suitable conditions. © 1998 Kluwer Academic Publishers [ABSTRACT FROM AUTHOR]

Tensile and shear properties were determined for hydroxyapatite reinforced poly-ethylene composites (HAPEXTM) for medical applications. Properties of talc or alumina filled polyethylene were also obtained. Hydroxyapatite particles of different median sizes and morphologies were used to reinforce the polyethylene. Furthermore, chemical coupling of hydroxyapatite to polyethylene was investigated. The obtained results are discussed in terms of possible use as orthodontic materials. © 1998 Kluwer Academic Publishers [ABSTRACT FROM AUTHOR]

The development of a wide range of hydroxyapatite polyethylene composites for medical applications is increasing the need for accurate predictive modelling. The objective of this work was to elucidate the observed mechanical processes and failure processes in this material using the finite element analysis method. The need for full three-dimensional modelling of this material has been shown. The results from this predictive model lead to accurate predictions of measured mechanical properties, and allow deduction of possible routes to improved ductility at high volume fractions. [ABSTRACT FROM AUTHOR]

A new bone cement based on poly(ethylmethacrylate) (PEMA), hydroxyapatite powder (HA) and n-butylmethacrylate monomer (n-BMA) has been studied using isothermal and non-isothermal polymerization. Methacrylate monomers are highly reactive and release a considerable amount of heat during polymerization. A quantitative understanding of the methacrylate polymerization is necessary because the thermal history of the polymerization has considerable influence on the final properties of a bone cement. In the first part, polymerization kinetics are analysed by means of differential scanning calorimetry (DSC). DSC data are used to evaluate a phenomenological model describing the cure kinetics of this new bone cement. In the second part, a kinetic model coupled with the energy balance is used to obtain temperature and degree of conversion profiles in the bone–cement–prosthesis system, under non-isothermal conditions, as function of initial temperature and thickness of the cement. Material properties, boundary and initial conditions and the kinetic behaviour are the input data for the numerically solved heat-transfer model. The temperature at the bone/cement interface, can be considered as a weak point, often responsible for total joint replacement failure. For this particular bone cement exhibiting a low exotherm and low glass transition temperature, the interfacial temperature is lower than the threshold level for thermal tissue damage (50 °C). The conversion occurs almost completely, avoiding problems with unreacted monomers that can be released by the cement, giving rise to tissue damage. © 1998 Chapman & Hall [ABSTRACT FROM AUTHOR]

Bone cement, used to fix prostheses into the bone, must be sterilized prior to implantation. Two sterilization techniques, γ and β radiation, were investigated, examining the influence upon molecular weight, static and dynamic mechanical characteristics and rheological properties. A number of experimental cements were studied prepared from methylmethacrylate (MMA) co-polymers, either single powders or powder blends, mixed with MMA monomer. It was found that with both γ and β radiation, there was a decrease in molecular weight of all powders, including a MMA/styrene co-polymer, in relation to the radiation dose. This fall in molecular weight resulted in a drop in tensile strength, Young’s modulus and strain to failure of all cements tested. However, the deterioration in mechanical strength was highlighted by the dynamic testing. Fatigue lives of cements after testing in tension–tension, at 2 Hz under load control and irradiated with 25 kGy γ radiation, displayed significant decreases. This result indicated the utmost importance of conducting such tests upon experimental bone cements prior to in vivo use. The rheological time profiles of curing cements were also found to be influenced by 25 kGy γ radiation, with a reduction of complex viscosity after sterilization. [ABSTRACT FROM AUTHOR]

Asceptic loosening of cemented joint prostheses in many cases is related to the mechanical failure of the acrylic bone cement. Poly(methylmethacrylate) bone cements are widely used in orthopaedic surgery although there are well-known disadvantages. A lower modulus bone cement based on poly(ethylmethacrylate)–n-butylmethacrylate with a lower polymerization exotherm, and a low monomer extractibility, is a promising alternative. The effect of incorporating crosslinking agents in order to improve the mechanical performance of the PEMA bone cement is reported. Three different bifunctional dimethacrylate crosslinking agents with different chain lengths and degrees of flexibility were incorporated in the monomer phase, and cements formulated. The setting time was found to decrease in the presence of the cross-linking agents and the polymerization exotherm decreased in the presence of triethylene glycol dimethacrylate and polyethylene glycol dimethacrylate, n=400. Incorporation of triethylene glycol dimethacrylate showed an increase in the tensile strength and modulus with a decrease in the strain at maximum stress. However, polyethylene glycol dimethacrylate, n=400, did not improve the mechanical properties appreciably which may be attributed to the low crosslinking density and higher flexibility of the spacer group in the crosslinking agent. [ABSTRACT FROM AUTHOR]

The bioactivity and biocompatibility of Bioglass®-reinforced high-density polyethylene composite (Bioglass®/HDPE) have been evaluated in simulated body fluid (SBF) and by in vitro cell culture, respectively. The formation of a biologically active hydroxy-carbonate apatite (HCA) layer on the composite surface after immersion in SBF was demonstrated by thin-film X-ray diffraction, infrared spectroscopy and scanning electron microscopy, indicating the in vitro bioactivity of Bioglass®/HDPE composites. The HCA layer was formed on the 40 vol% composite surface within 3 days immersion in SBF at a formation rate comparable to those on bioactive glass-ceramics, showing that in vitro bioactivity could be obtained in a composite. Furthermore, the composite was biocompatible to primary human osteoblast-like cells. In comparison with unfilled HDPE and tissue culture plastic control, a significant increase in cellular metabolic activity was found on the composite. Therefore, Bioglass®/HDPE composites have a promising biological response as a potential implant material. [ABSTRACT FROM AUTHOR]

Porous hydroxyapatite (Endobon®) specimens were implanted into the femoral condyle of New Zealand White rabbits for up to 6 months. After sacrifice, specimens were sectioned for histology and mechanical testing, where the extent of reinforcement by bony ingrowth was assessed by compression testing and fixation was assessed by push-out testing. From histological observations, it was established that the majority of bone ingrowth occurred between 10 day and 5 weeks after implantation and proceeded predominantly from the deep end of the trephined defect, with some integration from the circumferential sides. At 3 months, the implants were fully integrated, exhibiting bony ingrowth, vascularization and bone marrow stroma within the internal macropores. After 5 weeks, the mean ultimate compressive strength of retrieved implants (6.9 MPa) was found to be greater than that of the original implant (2.2 MPa), and by 3 months the fully integrated implants attained a compressive strength of approximately 20 MPa. Push-out testing demonstrated that after 5 weeks in vivo, the interfacial shear strength reached 3.2 MPa, increasing to 7.3 MPa at 3 and 6 months. [ABSTRACT FROM AUTHOR]

The effect of the concentration of benzoyl peroxide in poly(methyl methacrylate) bone cement formulations on their setting characteristics, particularly peak temperature and setting time, were studied. An optimization of the concentration of benzoyl peroxide was made with respect to curing parameters and compared with the residual monomer content. The mechanical properties of the different formulations were also determined and the results indicated that a composition of 1.5% wt/wt and 0.82% wt/wt of benzoyl peroxide and N,N-dimethyl-p-toluidine concentrations, respectively, gave the highest yield strength. Studies on the preparation of bone cement formulations containing different amounts of barium sulphate were also performed to assess the effect on the polymerization process and mechanical properties of the cements. [ABSTRACT FROM AUTHOR]

A composite of polyhydroxybutyrate (PHB) polymer, reinforced with synthetic hydroxyapatite (HA) particles, with potential as a bone-analogue material, was examined microscopically using scanning electron microscopy and transmission electron microscopy. These imaging techniques provide the means of understanding and monitoring the morphological and structural behaviour of retrieved implants. Scanning electron microscopy was used to assess the overall mechanism of new bone formation at the implant interface after up to 6 months implantation. This procedure was followed by a detailed ultrastructural examination at lattice plane resolution level, using high resolution electron microscopy and selected area diffraction of the regions showing bone apposition. Fine hydroxyapatite crystallites were found to form at the interface after in vivo implantation into cortical bone. [ABSTRACT FROM AUTHOR]

Apatites were prepared with three different fluoride concentrations: 0.0 mM (pure hydroxyapatite) 2.5 mM and 5 mM. Reactions were performed in aqueous medium using a reaction between diammonium orthophosphate and calcium nitrate 4-hydrate and ammonium fluoride at temperatures of 3°, 25°, 60° and 90°C. The effects of reaction temperature and fluoride concentration on the crystal morphology, phase purity and crystallinity of the precipitates were observed, using transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy and ion chromatography. Transmission electron micrographs revealed that the crystallites precipitated at 3°C were spheroidal, but became increasingly acicular with increasing precipitation temperature. X-ray diffraction results indicated that all the materials produced were phase pure and that the crystallinity of apatites prepared at higher precipitation temperatures was higher than those prepared at lower precipitation temperatures. A significant difference in the a-axis dimension of fluoride-substituted apatites was observed, as compared to hydroxyapatite. FTIR spectroscopy revealed a hydroxyl band at 3568 cm-1, along with a broad peak of adsorbed water in the region of 3568 cm-1 to 2670 cm-1 in the hydroxyapatite and fluoride-substituted apatite powders. Hence by careful selection of the precipitation conditions and fluoride contents, the composition and morphology of fluoride-substituted apatite may be controlled and this has interesting implications for the development of these materials for biomedical implantation. [ABSTRACT FROM AUTHOR]

An understanding of the interfacial relationship between a bioceramic implant and the adjacent bonet issue is facilitated by precise characterization of the associated structures. The structure of different commercial synthetic hydroxyapatite powders and a novel carbonated apatite have been studied with photo-acoustic (PAS) Fourier transform infrared (FTIR) spectroscopy. The PAS technique is an ideal method for analysing biomaterials, as materials can be analysed without the need to reduce the particle size or to dilute with KBr. Spectra from carbonated apatite appear to be different from those of commercial hydroxyapatite powders, with the main difference lying in the carbonate and phosphate ratio. Commercial hydroxyapatite powders from different sources have also been analysed and compared. [ABSTRACT FROM AUTHOR]