The mechanical properties of cement and loosening of the femoral component of hip replacements

Quasi-static and dynamic mechanical properties of a linoleic acid-modified, low-modulus bone cement for spinal applications

Background

Polymethylmethacrylate (PMMA) bone cement is extensively used in spinal procedures such as vertebroplasty and kyphoplasty, while its use in percutaneous cement discoplasty (PCD) is not yet widely spread. A main issue for both application sites, vertebra and disc, is the mismatch in stiffness between cement and bone, potentially resulting in adjacent vertebral fractures and adjacent segment disease. Tailoring the cement modulus using additives is hence an interesting strategy. However, there is a lack of data on the tensile and tension-compression fatigue properties of these cements, relevant to the newly researched indication of PCD.

Method

A commercial PMMA cement (VS) was modified with 12%vol of linoleic acid (VSLA) and tested for quasi-static tensile properties. Additionally, tension-compression fatigue testing with amplitudes ranging from +/-5MPa to +/-7MPa and +/-9MPa was performed, and a Weibull three-parameter curve fit was used to calculate the fatigue parameters.

Results

Quasi-static testing revealed a significant reduction in VSLA's Young's Modulus (E=581.1±126.4MPa) compared to the original cement (E=1478.1±202.9MPa). Similarly, the ultimate tensile stress decreased from 36.6±1.5MPa to 11.6±0.8MPa. Thus, VSLA offers improved compatibility with trabecular bone properties. Fatigue testing of VSLA revealed that as the stress amplitude increased the Weibull mean number decreased from 3591 to 272 and 91 cycles, respectively. In contrast, the base VS cement reached run-out at the highest stress amplitude. However, the lowest stress amplitude used exceeds the pressures recorded in the disc in vivo, and VSLA displayed a similar fatigue life range to that of the annulus fibrosis tissue.

Conclusions

While the relevance of fully reversed tension-compression fatigue testing can be debated for predicting cement performance in certain spinal applications, the results of this study can serve as a benchmark for comparison of low-modulus cements for the spine. Further investigations are necessary to assess the clinical feasibility and effectiveness of these cements.

Functional Properties of Low-Modulus PMMA Bone Cements Containing Linoleic Acid

Acrylic bone cements modified with linoleic acid are a promising low-modulus alternative to traditional high-modulus bone cements. However, several key properties remain unexplored, including the effect of autoclave sterilization and the potential use of low-modulus cements in other applications than vertebral augmentation. In this work, we evaluate the effect of sterilization on the structure and stability of linoleic acid, as well as in the handling properties, glass transition temperature, mechanical properties, and screw augmentation potential of low-modulus cement containing the fatty acid. Neither 1H NMR nor SFC-MS/MS analysis showed any detectable differences in autoclaved linoleic acid compared to fresh one. The peak polymerization temperature of the low-modulus cement was much lower (28-30 °C) than that of the high-modulus cement (67 °C), whereas the setting time remained comparable (20-25 min). The Tg of the low-modulus cement was lower (75-78 °C) than that of the high-stiffness cement (103 °C). It was shown that sterilization of linoleic acid by autoclaving did not significantly affect the functional properties of low-modulus PMMA bone cement, making the component suitable for sterile production. Ultimately, the low-modulus cement exhibited handling and mechanical properties that more closely match those of osteoporotic vertebral bone with a screw holding capacity of under 2000 N, making it a promising alternative for use in combination with orthopedic hardware in applications where high-stiffness augmentation materials can result in undesired effects.

In vivo response to a low-modulus PMMA bone cement in an ovine model

Poly(methyl methacrylate) (PMMA) is the most commonly used material for the treatment of osteoporosis-induced vertebral compression fractures. However, its high stiffness may introduce an increased risk of adjacent vertebral fractures post-surgery. One alternative in overcoming this concern is the use of additives. This presents its own challenge in maintaining an adequate biocompatibility when modifying the base cement. The aim of this study was to evaluate the in vivo biocompatibility of linoleic acid (LA)-modified acrylic bone cement using a large animal model for the first time, in order to further advance towards clinical use. A worst-case approach was used, choosing a slow-setting base cement. The in vitro monomer release from the cements was also assessed. Additional material characterization, including mechanical tests, are summarized in Appendix A. Unmodified and LA-modified cements were injected into a total of 56 bone defects created in the femur and humerus of sheep. Histopathologic and histomorphometric analysis indicated that LA-modified cement showed a harmless tissue response similar to that of the unmodified cement. Adjacent bone remodeling was observed microscopically 4 weeks after implantation, suggesting a normal healing process of the bone tissues surrounding the implant. LA-modified cement exhibited lower mechanical properties, with a reduction in the elastic modulus of up to 65%. The handling properties were slightly modified without negatively affecting the injectability of the base cement. LA-modified bone cement showed good biocompatibility as well as bone compliant mechanical properties and may therefore be a promising material for the treatment of osteoporotic vertebral fractures.

STATEMENT OF SIGNIFICANCE

The benefits of using linoleic acid to reduce the stiffness of poly(methyl methacrylate) bone cement has been demonstrated previously, with the in vitro and in vivo response of the modified cement in small animals reported as comparable to the base cement. However, biocompatibility evaluation of modified cement in large animal models for future clinical use has yet to be performed. In this study, modified and unmodified cements were injected into bone defects created in sheep. We showed that the inflammatory response of the modified cement was similar to the base cement, allowing remodelling of the bone surrounding the implant. This demonstrates the potential of low-modulus PMMA cement in the field of bone augmentation.

Acrylic bone cement: current concept review

Acrylic bone cement has had for years an important role in orthopedic surgery. Polymethylmethacrylate (PMMA) has been extended from the ophthalmological and dental fields to orthopedics, as acrylic cement used for fixation of prosthetic implants, for remodeling osteoporotic, neoplastic and vertebral fractures repair. The PMMA bone cement is a good carrier for sustained antibiotic release in the site of infection. Joint prostheses chronic infection requires surgical removal of the implant, in order to eradicate the infection process. This can be performed in the same surgical time (one-stage procedure) or in two separate steps (two-stage procedure, which involves the use of an antibiotic-loaded cement spacer). The mechanical and functional characteristics of the spacers allow a good joint range of motion, weight-bearing in selected cases and a sustained release of antibiotic at the site of infection. The improvement of fixation devices in recent years was not accompanied by the improvement of elderly bone quality. Some studies have tested the use of PMMA bone cement or calcium phosphate as augmentation support of internal fixation of these fractures. Over the past 20 years, experimental study of acrylic biomaterials (bone cement, bioglass ceramic, cement additives, absorbable cement, antibiotic spacers) has been of particular importance, offering numerous models and projects.

Eugenol derivatives immobilized in auto-polymerizing formulations as an approach to avoid inhibition interferences and improve biofunctionality in dental and orthopedic cements

Auto-polymerizing formulations based on poly(ethyl methacrylate) (PEMA) and eugenol derivatives are reported for dental and orthopedic applications. Spherical beads of PEMA were used as the pre-polymer powder and mixed with combinations of ethyl methacrylate and eugenyl methacrylate (EgMA) or ethoxyeugenyl methacrylate (EEgMA). A range of concentrations from 10 to 30 wt.% of EgMA or EEgMA were used to impart bioactivity properties to the cements. Increasing concentrations of the eugenol derivatives decreased the maximum polymerization temperatures from 69 to 37 degrees C without altering the working or setting time. At concentrations of 10 and 15 wt.% of EgMA or EEgMA a noticeable increase in the compressive (8%), flexural (40%) and tensile (24%) strengths were recorded in comparison to the control cements containing PEMA/ethyl methacrylate only. In addition to the improvement in mechanical properties the cements yielded a slightly crosslinked network due to the participation of the allylic group present in the eugenol derivatives, the presence of which has an intrinsically bactericidal effect against Escherichia coli and Streptococcus mutans strains, as reported in a previous study, thus enhancing the properties of the cements.

Alternative acrylic bone cement formulations for cemented arthroplasties: present status, key issues, and future prospects

All the commercially available plain acrylic bone cement brands that are used in cemented arthroplasties are based on poly (methyl methacrylate) and, with a few exceptions, have the same constituents. It is well known that these brands are beset with many drawbacks, such as high maximum exotherm temperature, lack of bioactivity, and volumetric shrinkage upon curing. Furthermore, concerns have been raised about a number of the constituents, such as toxicity of the activator (N,N,dimethyl-p-toluidine) and possible involvement of the radiopacifier (BaSO(4) or ZrO(2) particles) in third-body wear. Thus, over the years, many research efforts have been expended to address these drawbacks, culminating in a large number of alternative formulations, which may be grouped into 16 categories. Although there are a number of reviews of the large literature that now exists on these formulations, each covers only some of the categories and none contains a detailed discussion of the germane issues. The objective of the present work, therefore, was to present a comprehensive and critical review of the whole field. In addition to succinct descriptions of the cements in each category, there are explicative summaries of literature reports, a detailed discussion of several key issues surrounding the potential for use of these cements in cemented arthroplasties, and a presentation of numerous ideas for future studies.

The role of polymethylmethacrylate bone cement in modern orthopaedic surgery

Polymethylmethacrylate remains one of the most enduring materials in orthopaedic surgery. It has a central role in the success of total joint replacement and is also used in newer techniques such as percutaneous vertebroplasty and kyphoplasty. This article describes the current uses and limitations of polymethylmethacrylate in orthopaedic surgery. It focuses on its mechanical and chemical properties and links these to its clinical performance. The behaviour of antibiotic-loaded bone cement are discussed, together with areas of research that are now shedding light upon the behaviour of this unique biomaterial.

Prediction of the long-term creep behaviour of hydroxyapatite-filled polyethylmethacrylate bone cements

The creep behaviour of bone cements based on polyethylmethacrylate, with and without addition of hydroxyapatite filler has been investigated, in order to determine the effect of hydroxyapatite filling and to investigate methods of predicting the long-term creep behaviour from short-term tests. The materials were produced under laboratory conditions and tested in tension in Ringer's solution, as the study was intended to investigate the inherent materials behaviour rather than to simulate realistic conditions. The effects of adding hydroxyapatite were to increase the short-term stiffness and more significantly to decrease the creep rate. Short-term creep tests of up to 10(6) s were conducted at various temperatures, stresses and ageing states. These were then used to investigate various methods of extrapolation to long-term behaviour. The use of time-temperature superposition was found to be useful, though it takes no account of ongoing physical ageing and so gives a significant overestimate of long-term creep strains. Stress-time superposition was less useful and also excludes ageing effects. The use of 'effective time' theory was more successful, but requires a large number of short-term tests. The most effective method was that of the 'integrated time' approach, which required fewer tests yet still gave good correlations with longer-term data.

Histologic and mechanical evaluation of impaction grafting for femoral component revision in a goat model

A goat model of revision hip arthroplasty was used to examine the histology and mechanical performance of impaction grafting using two femoral stems varying in stem surface finish. There were no significant mechanical or qualitative histologic differences between smooth, tapered, polished stems and step-cut, grit-blasted stems. Allograft distribution, bone incorporation, and cement mantle thickness were not uniform within the femoral canal. Efforts to improve the impaction grafting technique may be more important than stem design.

Fatigue testing and performance of acrylic bone-cement materials: state-of-the-art review

Over the past three decades or so, a very large volume of literature has been generated on the impact of an assortment of variables on the fatigue lifetimes of a large number of acrylic bone-cement formulations. In the present article, this literature is examined critically to reveal areas of agreement, areas of disagreement, as well as a welter of underexplored and unexplored topics. For example, there is unanimity of support for the notion that an increase in the molecular weight of the powder constituents or the fully cured cement leads to an increase in the cement's fatigue life, whereas there is disagreement as to whether vacuum mixing the cement constituents leads to an increase in the fatigue life of the fully cured cement (relative to the hand-mixed counterpart). Among the underexplored topics is systematic study of the effect of test frequency on the fatigue results, whereas determination of the optimal concentration of the antibiotic in an antibiotic-loaded cement is an example of the unexplored topics. It is pointed out that resolving the controversies, addressing the underexplored topics, and filling the lacunae will allow comprehensive evaluations of acrylic bone-cement materials to be made. This enhanced body of knowledge will prove invaluable in the continued use of acrylic bone cement as the anchoring agent in cemented arthroplasties.

Structural degradation of acrylic bone cements due to in vivo and simulated aging

Acrylic bone cement is the primary load-bearing material used for the attachment of orthopedic devices to adjoining bone. Degradation of acrylic-based cements in vivo results in a loss of structural integrity of the bone-cement-prosthesis interface and limits the longevity of cemented orthopedic implants. The purpose of this study is to investigate the effect of in vivo aging on the structure of the acrylic bone cement and to develop an in vitro artificial aging protocol that mimics the observed degradation. Three sets of retrievals are examined in this study: Palacos brand cement retrieved from hip replacements, and Simplex brand cement retrieved from both hip and knee replacement surgeries. In vitro aging is performed using oxidative and acidic environments on three acrylic-based cements: Palacos, Simplex, and CORE. Gel permeation chromatography (GPC) and Fourier transform infrared spectroscopy (FTIR) are used to examine the evolution of molecular weight and chemical species within the acrylic cements due to both in vivo and simulated aging. GPC analysis indicates that molecular weight is degraded in the hip retrievals but not in the knee retrievals. Artificial aging in an oxidative environment best reproduces this degradation mechanism. FTIR analysis indicates that there exists a chemical evolution within the cement due to in vivo and in vitro aging. These findings are consistent with scission-based degradation schemes in the cement. Based on the results of this study, a pathway for structural degradation of acrylic bone cement is proposed. The findings from this investigation have broad applicability to acrylic-based cements and may provide guidance for the development of new bone cements that resist degradation in the body.

Lysineurethanedimethacrylate--a novel generation of amino acid based monomers for bone cements and tissue repair

A novel amino acid based dimethacrylate monomer (lysineurethanedimethacrylate, LUDM) was prepared by the addition of hydroxyethylmethacrylate (HEMA) to lysinediisocyanate (LDI). The structure was confirmed by FT-IR and 1H and 13C NMR spectroscopy as well as FAB-MS. Photopolymerized LUDM exhibited low volume shrinkage upon polymerization, good mechanical properties (Young's modulus: 3740 MPa) and high thermal stability. Osteoblast adhesion and growth on polymerized LUDM samples evidenced the biocompatibility. Further improvement of the mechanical properties was obtained by using Ca-hydroxyapatite as inorganic filler varying between 10 and 30 wt%. The Young's and flexural moduli increased with increasing filler content ranging from 3740 to 5250 MPa and from 2020 to 3690 MPa, respectively. The mechanical properties and the good biocompatibility of the lysine-based methacrylate networks make them interesting materials for medical applications, e.g. bone cements, and tissue engineering.

Gentamicin release from hydroxyapatite/poly(ethyl methacrylate)/poly(methyl methacrylate)composites

In this work the release kinetics of gentamicin sulfate (GEN) in samples composed by hydroxyapatite, poly(methyl methacrylate), and poly(ethyl methacrylate) has been studied. The release study was performed by soaking three samples in simulated body fluid at 37 degrees C; the medium was periodically replaced during 70 days. The concentration of GEN was determined by the o-phtaldialdehyde method. The release profile shows three stages: the first stage, occurring during the first 10 h, corresponds to a fast release (nearly 30% of the drug is released in this period). The second stage is slower and includes from the first 10 h to 16 days, releasing 60% of the total amount of GEN. The final stage is the slowest and it takes from 16 to 70 days (10% of GEN is released). The fraction of released GEN versus square root of time can be fitted to a third order polynomial, corresponding with the model proposed by Cobby et al. (J Pharm Sci 1974;63:725-732). The characterization of the samples after the release study shows that a carbonate hydroxyapatite layer has grown on the whole surface of the composites.

Mechanical evaluation of a bio-active bone cement for total hip arthroplasty

A new bio-active bone cement, known as CAP, has been developed as an alternative to acrylic bone cement. CAP has improved mechanical properties, with a high modulus that is over five times that of PMMA. The effects of this high modulus are examined by finite element analysis, when the CAP is used in place of PMMA to fix the femoral component in total hip prostheses. The results show a higher tensile stress of 8.76 MPa in the CAP cement, compared with 1.99 MPa in the PMMA cement. However, it is also shown that CAP has a superior fatigue strength of approximately 40 MPa, obtained from a cyclic loading test.

Properties of acrylic bone cement: state of the art review

Acrylic bone cement occupies a distinctive place in the hierarchy of synthetic biomaterials, because it is the only material currently used for anchoring the prosthesis to the contiguous bone in a cemented arthroplasty. However, the cement is not without its drawbacks. The main one is the role that it has been postulated to play in the aseptic loosening and, hence, clinical life of the arthroplasty. In turn, this role is directly related to the mechanical properties of the cement, especially the resistance to fracture of the cement in the mantle at the cement-prosthesis interface or the cement-bone interface. The present work is a detailed critical review of the recent literature on the properties of bone cement that are considered germane to its use in the stated application. The relevant properties are identified and a case is made for including each of them. Compilations of the values of these properties, obtained under clearly identified conditions, are presented for the six commercial formulations of bone cement in current popular orthopedic use. The gaps and unresolved questions in the current data base, efforts that should be made to address these issues, and research directions are covered.

Polymethylmethacrylate-based bone cement modified with hydroxyapatite

A commercial acrylic bone cement was modified by the incorporation of different weight fractions of polycrystalline hydroxyapatite (HA), and the modified formulation was investigated. The influence of the filler proportion on the flow characteristics and the mechanical behavior of the resultant composite was evaluated. The residual monomer present in the cured materials was measured by gas chromatography. The comparison of the residual monomer present in the cements with and without reinforcement demonstrated that the degree of polymerization was not affected by the addition of HA. Porosity morphology was analyzed by optical and scanning electron microscopy. Image examination revealed that the porosity and the pore size of the hardened cement increased with an increasing amount of particulate filler. Flexural, compressive, and fracture properties of the cement with varying amounts of HA reinforcement were measured. It was found that up to 15 wt% HA could be added for increases in flexural modulus and fracture toughness. HA acts as a rigid filler that enhances fracture resistance and flexural modulus. Our results show that the workability of the modified formulation limited the incorporation of the ceramic filler to a maximum value of 15 wt%.

Total hip arthroplasty using bone cement containing tri-n-butylborane as the initiator

We performed total hip arthroplasty (THA) using a special acrylic self-curing bone cement (Bonemite), which contains tri-n-butylborane as the initiator. Its maximum temperature at curing is lower than that of a conventional bone cement (CMW). Fifty-eight THAs using Bonemite and 35 THAs using CMW were followed up for more than 8 years (12.5 years on average). At the 10-year follow-up, the survival rates, using revision surgery or aseptic loosening impending revision as the endpoint for failure, were 92.2% for the patients in the Bonemite group and 91.0% for those in the CMW group. No statistical differences were observed between the patients in these two groups with regard to survival rate (p = 0.39). Bonemite showed no clear superiority compared with CMW, although the results suggest that Bonemite is safe and reliable for clinical use and stable in situ for long time.

Characterization of new acrylic bone cement based on methyl methacrylate/1-hydroxypropyl methacrylate monomer

New formulations of acrylic bone cement based on methyl methacrylate/1-hydroxypropyl methacrylate (MMA/HPMA) monomers were developed with the purpose of obtaining more ductile materials with reduced polymerization shrinkage. In this way, the ductility of such materials increased, but the introduction of high percentages of the hydrophilic component produced an important decrease in Young's modulus and strength. To ascertain the reason for the deterioration of the tensile parameters, an analysis by scanning electron microscopy of these formulations was carried out; it revealed poor adhesion between the matrix and poly(MMA) beads. We also observed that the polymerization shrinkage increased as the amount of hydrophilic monomer in the formulation decreased, and the 50% (v/v) HPMA modified bone cement compensated for this volume reduction with its water uptake swelling. Measurements taken on the setting time and polymerization exotherm showed a decrease in the former and an increase in the latter, because of the introduction of a more reactive monomer in the bone cement formulation.

Bioactive bone cements

Poly(methylmethacrylate) (PMMA) bone cement, used to fix implants into the bone, produces good surgical results if used correctly. However, prostheses do eventually become loose and the breakdown of the cement mantle is a factor in this failure. Limitations of PMMA cement, which lead to problems with the fixation of the implant, include its mechanical characteristics and its influence upon surrounding bone, associated with the polymerization reaction. A bioactive bone cement is particularly designed to produce a better interface between the cement and bone. However, an improvement in mechanical properties, especially fatigue, creep and fracture toughness, are an added necessary requirement to increase the lifetime of a cemented implant. The development of a bioactive cement has been conducted mainly in two ways; firstly, to improve existing PMMA cement by the addition of various bioactive agents and secondly, to design an alternative matrix for the bioactive material to be combined with. The most promising investigations which have been conducted, along with their relative benefits and drawbacks, are discussed.

Dye incorporation to enhance the laser ablation of standard and reduced-modulus bone cements

Laser ablation of acrylic bone cement is an alternative method of cement removal that can be used during revision arthroplasty of cemented implants. This study investigated the feasibility of using a continuous-wave Argon ion laser (wavelength = 514 nm) with the addition of methylene blue or red dye no. 13 to enhance the ablation of two types of bone cements: polymethylmethacrylate and polybutylmethylmethacrylate. Six cement/dye combinations were studied while power (0.5, 0.75, and 1.0 W) and exposure times (30, 45, 60, and 90 seconds) were varied. The Argon laser was unable to ablate undyed polymethylmethacrylate or polybutylmethylmethacrylate. However, ablation was shown for both cements with either dye. The red dye had a stronger absorption peak at 514 nm than did the blue dye. Statistically larger ablation areas were seen for red polymethylmethacrylate than for blue polymethylmethacrylate (p < 0.013) at all levels tested. Ablation areas were larger in red than in blue polybutylmethylmethacrylate cement. Blue polybutylmethylmethacrylate cement produced larger ablation areas than did blue polymethylmethacrylate cements at all energy levels tested, with smaller surrounding damage areas. Red polybutylmethylmethacrylate cement also produced larger ablation areas than did red polymethylmethacrylate cement (at 0.75 and 1.0 W), again with smaller damage areas. Damage zones were smallest in red polybutylmethylmethacrylate cements at all test levels. These results suggest that, by using dyes to selectively alter the absorption characteristics of bone cement, laser ablation can be an effective method for cement removal. Changes in the chemical structure of the cement can also influence the response to laser treatment. Furthermore, the absorption spectra of the bone cement can be altered to maximize energy absorption at a wavelength that is not absorbed by bone tissue; this potentially minimizes damage to bone during revision surgery.

Effect of crosslinking agents on acrylic bone cements based on poly(methylmethacrylate)

Three different crosslinking agents were added to the monomer content of the bone cement formulation based on poly(methylmethacrylate), PMMA, in various concentrations, and their effects on the curing parameters and mechanical properties were determined. The different crosslinking agents were dimethacrylates containing a range of chain lengths and degrees of flexibility. For the parent formulation of the bone cement, PMMA powder was used, and the properties and kinetics of curing were compared for the same system with and without crosslinking agents. It was found that at low concentrations of crosslinking agents, the mechanical properties were superior but steadily decreased with increasing concentrations. Poly(ethyleneglycoldimethacrylate), EGDMA(400), even when used at very low concentrations, produced a steady improvement in the mechanical properties and could be used in cement formulations with a view to reducing creep and improving mechanical properties.

The theory of early loosening of hip prostheses

The issue of prosthetic loosening is currently a matter of debate, particularly with regard to the timing and nature of the precipitating events. The theory presented here postulates that loosening begins at an early stage due to either insufficient initial fixation or early loss of fixation. The loosened prosthetic component is then affected by varying degrees of mechanical stress associated with normal daily activity, which differs according to patient characteristics (body weight and level of physical activity) and the components used (prosthetic design, positioning, friction, and wear). This theory of early loosening can explain without supplementary ad hoc assumptions the rapid early prosthetic migration detected by roentgen stereophotogrammetry, the development of focal osteolysis and wear granulomas, the phenomenon known as stress-shielding, and, to a great extent, the epidemiology of clinical failure.

Assessment of bioactivity for orthopedic coatings in a gap-healing model

In order to study bone growth conducting capacities of new biomaterials under standardized conditions, a goat model was developed based on a canine model by Soballe. Titanium alloy implants with and without a hydroxyapatite coating were used as positive and negative controls, and these were implanted with a circumferential gap of one millimeter in the spongious bone of the knee condyles of two groups of four goats. These goats were sacrificed at 6 and 25 weeks. A second experiment was done on two groups of four goats with the same type of titanium alloy and hydroxyapatite-coated implants as controls and with Polyactive 55-45 coated titanium alloy implants for testing. These goats were sacrificed at 9 and 25 weeks, respectively. Qualitative and quantitative differences in gap healing were evaluated through light microscopy, and initiation and direction of bone apposition were determined with fluorescence microscopy. Apposition of bone was seen directly on all hydroxyapatite surfaces and on some of the noncoated titanium alloy surfaces. The difference between the percentage of bone growth on the titanium alloy implants and the hydroxyapatite-coated implants appeared to be divergent in time: the bone growth on the noncoated implants declined after 9 weeks in contrast to the steady increase of bone growth on the hydroxyapatite-coated implants towards the 25 week follow-up time (p = 0.02). No significant difference was found between the first and the second experiment: apposition of bone on the implants differed only 6.6% on a scale of 0% to 100%. Only scarce bone growth was seen on the polyactive-coated implants in this model. The newly tested Polyactive 55-45 coating apparently needs initial bone contact for bone-bonding and therefore showed hardly any direct bone formation on its surface. The clear differences in the reaction of bone to the coated and noncoated implants in this goat study and the reproducibility of these reactions of bone to the different controls indicate the sensitivity of the currently used animal model and its suitability for use as a bioactivity assay.

Effects of cement creep on stem subsidence and stresses in the cement mantle of a total hip replacement

In cemented total hip prostheses, the role of creep of the acrylic cement (polymethyl methacrylate, [PMMA]) in increasing or decreasing the chance of failure of the cement mantle is a subject of ongoing controversy. In the present study we used a three-dimensional finite-element model of a cemented stem to assess the influence of cement creep on subsidence of the stem, and on the stress and strain in the cement under cyclic load, both in the short and long term. The cement layer was assigned the shear and bulk creep moduli of Zimmer regular PMMA cement, which were obtained experimentally. The stem-cement interface was modeled either as (1) completely bonded, (2) completely debonded with friction, or (3) completely debonded and frictionless. Under the cyclic load some cement creep occurred with all three bonding conditions, allowing additional subsidence of the stem and a decrease in the stress components within the cement. During the unloaded period the full recovery of the preload conditions could be reached with the completely bonded and with the frictionless interfaces. With the frictional interface there was residual cement creep, residual stresses within the cement, and residual subsidence of the stem during the unloaded period; however, the reduction of the stress was at most 13% and the subsidence was about 0.46 mm. The much larger subsidence of debonded stems that is often observed clinically might be attributed to the factors which were not included in the present model, such as circumferential bone remodeling.

Effect of co-monomer composition on the integrity of bioactive growth hormone released from novel PEMA based polymers

The release of human growth hormone (hGH) from hormone loaded bone cement was previously shown to enhance osteoid formation. hGH is a complex protein and its incorporation into such cements may compromise its bioactivity. We therefore characterized the release of hGH from a series of methacrylate systems based upon poly(ethylmethacrylate) (PEMA). Different mixtures of two monomers, hydroxyethylmethacrylate (HEMA) and n-butylmethacrylate (n-BM) were used to provide polymers with graded water uptakes. Exclusive use of only one of the monomers resulted in enhanced cytotoxicity and also reduced release of the bioactive hormone. Combinations of the monomers improved the recovery of bioactivity from the polymers and reduced their cytotoxicity. hGH released from the polymer with the lowest water uptake (100% n-BM, 0% HEMA) had an exceptionally low bioactivity: immunoactivity ratio, suggesting that the bioactive site of the hormone is particularly susceptible to disruption when it is incorporated into this matrix.

Effects of distal femoral centralizers on bone-cement in total hip arthroplasty. An experimental analysis of cement-centralizer bonding, cement void formation, and crack propagation

Distal femoral centralizers of five different designs were inserted into model femoral stems and cemented into closed-ended tubes simulating a proximal femoral canal. Specimens underwent cyclic loading from 50 to 500 lb. for 0, 1, 2, 5, and 10 million cycles. Each specimen was then sectioned transversely at multiple levels to obtain serial cross-sections, beginning at the femoral stem tip and proceeding distally so as to include the full extent of the centralizer. The area of each section occupied by a centralizer and the total amount of porosity present in the cement surrounding the centralizers were measured using an image analyzer. A dye penetrant was then applied to each section to visualize cement cracks and areas of incomplete bonding between cement and centralizers. The number, length, and location of cement cracks were catalogued for each section. No cement cracks or lack of bonding was observed at the interface between cement and centralizers. There was greater porosity in the specimens containing centralizers than in controls without centralizers (P < .05). The cement surrounding two of the centralizer designs had a significantly smaller amount of porosity than the cement surrounding the other three designs (P < .05). The number of cracks did not depend on whether a centralizer was used, the type of centralizer, or the cycling duration. In the control specimens, failure to adequately plug the centralizer receptacle hole in the stem tip resulted in very large cement voids.

In situ analysis of the degree of polymerization of bone cement by using FT-Raman spectroscopy

A method has been developed which enables an in situ analysis of the degree of polymerization of bone cement. The concentration of monomeric double bond is monitored continuously during the entire curing process and hence the method can be used for quantitative studies of polymerization kinetics. In this study, Fourier Transform Raman (FTR) spectroscopy was utilized to investigate the degree of polymerization of a novel bone cement in situ, and the results are compared with the thermal profile obtained for polymerization.

Impaction grafting: preliminary report of a new method for exchange femoral arthroplasty

ABSTRACTGie et al described a method for femoral component revision using compressed morselized cancellous allograft and a cemented collarless polished taper stem. We have reviewed our first 37 exchanges performed in this manner. Results after 2 to 5 years have reflected few complications, a majority of satisfactory results, evidence of graft incorporation, and no mechanical loosening. Further study is necessary; however, these preliminary findings give rise to cautious optimism.

Polymerization kinetics, glass transition temperature and creep of acrylic bone cements

Sulfix-6 and Zimmer LVC 60/30 bone cements were selected and the polymerization kinetics and resulting glass transition temperature Tg; creep behaviour in the dry or water-saturated state; and sorption and diffusion of water were studied. The calculation of conversion was based on a comparison of the residual polymerization heat measured by differential scanning calorimetry and the corresponding theoretical value. The conversion reached 99% after 90 min of quasi-adiabatic polymerization starting at 23 degrees C or after 10 min of isothermal polymerization at 37 degrees C. The Tgs of the cements prepared in the former way were about 82 and 100 degrees C, respectively. Creep rate of the bone cements at 37 degrees C decreased with the time of creeping. Sorbed water enhanced the compliance, but reduced the creep rate for long times so that water sorption during the service time may not have detrimental effects on the creep resistance of the cements. Both types of cements contained about 1% of low molar mass substances extractable by water. Measurements of the sorption kinetics of water showed that the diffusion coefficient is 0.14 x 10(-11) and 0.22 x 10(-11) m2/s and 1 yr sorption achieves 2.11% and 2.89% for Sulfix and Zimmer, respectively.

The fracture toughness of titanium-fiber-reinforced bone cement

Fracture of the poly(methyl methacrylate) bone cement mantle can lead to the loosening and ultimate failure of cemented total joint prostheses. The addition of fibers to the bone cement increases fracture resistance and may reduce, if not eliminate, in vivo fracturing. This study discusses the effect of incorporating titanium (Ti) fibers on fracture toughness. Essential characteristics of the composite bone cement included a homogeneous and uniform fiber distribution, and a minimal increase in apparent viscosity of the polymerizing cement. Ti fiber contents of 1%, 2%, and 5% by volume increased the fracture toughness over non-reinforced bone cement by up to 56%. Bone cements of two different viscosities were used as matrix material, but when reinforced with the same fiber type and content, they showed no difference in fracture toughness. Four different fiber aspect ratios (68, 125, 227, 417) were tested. At 5% fiber content, there was no statistically significant dependence of fracture toughness on fiber aspect ratio. Scanning electron microscopy revealed important toughening mechanisms such as fiber/matrix debonding, local fracture path alteration, and ductile fiber deformation and fracture. Fiber fracture was evidence that the critical fiber length was exceeded. The surfaces of the Ti fibers were rough and irregular, indicating that a high degree of mechanical interlock between matrix and fiber was likely. The energy absorption contribution of plastic deformation and ductile fracture is absent in brittle fibers, like carbon, but is a distinction of the Ti fibers used in this study.

Experimental studies of the biological response to a new bone cement: II Soft tissue reactions in the rat

The biological response to a new bone cement (London Hospital cement), which contains polyethylmethacrylate/n-butylmethacrylate, has been examined experimentally and compared with polymethylmethacrylate. The cellular reaction was identical when the polymer powders of the cements were inserted subcutaneously in rats, there being a macrophage response to the polymer beads of both materials. Experimental implantation of the two cements into the paraspinal musculature of rats showed that there was less tissue damage adjacent to the new material than to polyethylmethacrylate. There was a significantly smaller macrophage and giant cell reaction in relation to the polyethylmethacrylate/n-butylmethacrylate cement. These differences may be related to the lower heat of polymerisation of the new cement and to its smoother surface contour, as demonstrated by scanning electron microscopy.

A quantitative assessment of cross-sectional cortical bone remodeling in the femoral diaphysis following hip arthroplasty in elderly females

A quantitative assessment of cross-sectional cortical bone remodeling in the femoral diaphysis following hip arthroplasty was made by direct in vitro measurements of cross-sectional geometric properties. We obtained eight femora from four female cadavers ranging in age from 77 to 96 years. In three cases unilateral uncemented Austin Moore implants were used, and in one case a unilateral cemented Thompson prosthesis had been implanted. The time of implantation in the two specimens where this information could be obtained was greater than 40 months. Sections were made at 12 diaphyseal locations from the superior aspect of the lesser trochanter through the distal diaphysis. Section properties (areas and second moments of area, or area moments of inertia) were determined by tracing photographs of the cross-sections with a digitizer. In this sample of prosthetic femora, we found reductions in both total subperiosteal area (TA) and endosteal area (ENDA) relative to the contralateral unoperated side in most sections distal to the lesser trochanter. The average pairwise reduction in ENDA for this region was 21.1 mm2, reaching statistical significance in one distal diaphyseal section. The average decline in TA in this region was 10.2 mm2. Because the reduction in endosteal dimensions was generally greater than the reduction in subperiosteal dimensions, cortical area (CA) was maintained or increased throughout the distal 80% of this region in prosthetic femora with an average increase in CA of 9.3 mm2, reaching statistical significance in one mid-diaphyseal section. A completely different pattern of remodeling occurred in the two most proximal sections through the lesser trochanter and base of the femoral neck.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of retention of the femoral neck and of cement upon the stability of a proximal femoral prosthesis

Radiological measurements of downward femoral component migration are reported for 203 hips in the first 2 years following total hip arthroplasty. The femora differed with respect to the retention of the femoral neck--in 167 it was retained and in 36 it was damaged--and to the use of cement--142 hips were press-fits and 61 were cemented. Multivariate analysis demonstrated that both retention of the neck and the use of cement retarded migration (ie, increased stability) of the femoral component (P = .0003 and .0037, respectively).