Bone Cement Specimens Research Articles

Statistical distribution of micro and macro pores in acrylic bone cement- effect of amount of antibiotic content

Aseptic loosening due to mechanical failure of bone cement is considered to be a leading cause of revision of joint replacement systems. Detailed quantified information on the number, size and distribution pattern of pores can help to obtain a deeper understanding of the bone cement's fatigue behavior. The objective of this study was to provide statistical descriptions for the pore distribution characteristics of laboratory bone cement specimens with different amounts of antibiotic contents. For four groups of bone cement (Palacos) specimens, containing 0.3, 0.6, 1.2 and 2.4 wt/wt% of telavancin antibiotic, seven samples per group were micro computed tomography scanned (38.97 μm voxel size). The images were first preprocessed in Mimics and then analyzed in Dragonfly, with the level of threshold being set such that single-pixel pores become visible. The normalized pore volume data of the specimens were then used to extract the logarithmic histograms of the pore densities for antibiotic groups, as well as their three-parameter Weibull probability density functions. Statistical comparison of the pore distribution data of the antibiotic groups using the Mann–Whitney non-parametric test revealed a significantly larger porosity (p < 0.05) in groups with larger added antibiotic contents (2.4 and 0.6 wt/wt% vs 0.3 wt/wt%). Further analysis revealed that this effect was associated with the significantly larger frequency of micropores of 0.1–0.5 mm diameter (p < 0.05) in groups with larger antibiotic content (2.4 wt/wt% vs and 0.6 and 0.3 wt/wt%), implying that the elution of the added antibiotic produces micropores in this diameter range mainly. Based on this observation and the fatigue test results in the literature, it was suggested that micropore clusters have a detrimental effect on the mechanical properties of bone cement and play a major role in initiating fatigue cracks in highly antibiotic added specimens.

Comparison of antibacterial activity and biocompatibility of non-leaching nitrofuran bone cement loaded with vancomycin, gentamicin, and tigecycline

BackgroundNon-leaching antibacterial bone cement can generate long-term antibacterial activity, it cannot treat serious infections that have occurred like antibiotic-loaded bone cement. Currently, the antibacterial activity and biocompatibility of non-leaching cement when loaded with antibiotics have yet to be determined.MethodsNon-leaching antibacterial nitrofuran bone cement (NFBC) specimens were prepared with low-dose and high-dose antibiotics. The antibacterial activity and biocompatibility of NFBC loaded with vancomycin, gentamicin, and tigecycline were compared. The agar diffusion method was employed to observe the inhibition zone of the samples against two bacterial strains from day one to day seven. The CCK-8 assay and acute liver and kidney toxicity test were conducted to assess the effects of the samples on mouse embryo osteoblast precursor cells and C57 mice, respectively.ResultsGentamicin-loaded cement exhibited the most potent antibacterial activity, effectively inhibiting both bacterial strains at a low dose. Tigecycline-loaded cement demonstrated superior biocompatibility, showing no acute liver and kidney toxicity in mice and minimal cytotoxicity to osteoblasts.ConclusionsNFBC loaded with gentamicin, vancomycin, and tigecycline not only maintains sustained antibacterial activity but also exhibits excellent biocompatibility.

Influence of chitosan and chitosan oligosaccharide on dual antibiotic-loaded bone cement: In vitro evaluations.

The purpose of this study was to investigate the effect of incorporating chitosan (Ch) and chitosan oligosaccharides (ChO) into the commercially premixed antibiotic-loaded bone cement (ALBC). We compare antibiotic release profiles, antibacterial activity, and mechanical properties among different ALBC formulations. The hypothesis was that increasing the amount of Ch and ChO in the cement mixture would increase the antibiotics released and bacterial control. ALBC mixed with Ch or ChO may create a greater effect due to its superior dissolving property. The bone cement samples used in this project were made from Copal® G+V composed of vancomycin and gentamicin. To prepare the Ch and the ChO mixed bone cement samples, different amounts of Ch and ChO were added to the polymethylmethacrylate matrix with three concentrations (1%, 5%, and 10%). Drug elution assay, antimicrobial assay, in vitro cytotoxicity, and mechanical properties were conducted. Bone cement samples made from Copal® G+V alone or combined with Ch or ChO can release vancomycin and gentamicin into the phosphate-buffered saline. Mixing ChO into the bone cements can increase the amount of drug released more than Ch. ChO 10% gave the highest amount of antibiotics released. All samples showed good antibacterial properties with good biocompatibility in vitro. The microhardness values of the Ch and ChO groups increased significantly compared to the control group. In all groups tested, the microhardness of bone cements was reduced after the drug eluted out. However, this reduction of the Ch and ChO groups was in line with the control. Various attempts have been made to improve the ALBC efficacy. In our study, the best bone cement formulation was bone cement mixed with ChO (10%), which had the highest drug release profiles, was biocompatible, and contained antibacterial properties with acceptable mechanical properties. This phenomenon could result from the superior water solubility of the ChO. When ChO leaves the bone cement specimens, it generates pores that could act as a path that exposes the bone cement matrix to the surrounding medium, increasing antibiotic elution. From all above, ChO is a promising substance that could be added to ALBC in order to increase the drug elution rate. However, more in vitro and in vivo experiments are needed before being used in the clinic.

Mechanical strength of antibiotic-loaded PMMA spacers in two-stage revision surgery

BackgroundAntibiotic-loaded polymethylmethacrylate (PMMA) bone cement spacers provide high local antibiotic concentrations and patient mobility during the interim period of two-stage revision for periprosthetic joint infection (PJI). This study compares mechanical characteristics of six dual antibiotic-loaded bone cement (dALBC) preparations made from three different PMMA bone cements. The study`s main objective was to determine the effect of time and antibiotic concentration on mechanical strength of dALBCs frequently used for spacer fabrication in the setting of two-stage revision for PJI.MethodsA total of 84 dual antibiotic-loaded bone cement specimens made of either Copal spacem, Copal G + V or Palacos R + G were fabricated. Each specimen contained 0.5 g of gentamicin and either 2 g (low concentration) or 4 g (high concentration) of vancomycin powder per 40 g bone cement. The bending strength was determined at two different timepoints, 24 h and six weeks after spacer fabrication, using the four-point bending test.ResultsPreparations made from Copal G + V showed the highest bending strength after incubation for 24 h with a mean of 57.6 ± 1.2 MPa (low concentration) and 50.4 ± 4.4 MPa (high concentration). After incubation for six weeks the bending strength had decreased in all six preparations and Palacos R + G showed the highest bending strength in the high concentration group (39.4 ± 1.6 MPa). All low concentration preparations showed superior mechanical strength compared to their high concentration (4 g of vancomycin) counterpart. This difference was statistically significant for Copal spacem and Copal G + V (both p < 0.001), but not for Palacos R + G (p = 0.09).ConclusionsThis study suggests that mechanical strength of antibiotic-loaded PMMA bone cement critically decreases even over the short time period of six weeks, which is the recommended interim period in the setting of two-stage revision. This potentially results in an increased risk for PMMA spacer fracture at the end of the interim period and especially in patients with prolonged interim periods. Finally, we conclude that intraoperative addition of 4 g of vancomycin powder per 40 g of gentamicin-premixed Palacos R + G (Group D) is mechanically the preparation of choice if a dual antibiotic-loaded bone cement spacer with high antibiotic concentrations and good stability is warranted. In any case the written and signed informed consent including the off-label use of custom-made antibiotic-loaded PMMA bone cement spacers must be obtained before surgery.

In vitro and in vivo effectiveness of a novel injectable calcitonin-loaded collagen/ceramic bone substitute.

This study aimed to evaluate the effectiveness of a novel calcitonin-loaded calcium phosphate composite bone cement in vitro and in vivo. The novel composite bone cements were composed of NuROs injectable bone graft substitute, type I collagen, and/or salmon calcitonin. The setting time, porosity, wettability, compressive strength, compressive modulus, and crystallographic structures of cement specimens were determined. Degradation rate, calcitonin release rate, and osteoinductivity were assessed in vitro. In addition, osteogenic effect was examined in a rabbit model of femoral defect. The results revealed that addition of collagen/calcitonin did not substantially alter physical properties and degradation rate of bone cement specimens. Calcitonin was released into culture medium in a two-phase manner. Osteogenic effect of conditioned medium derived from calcitonin containing bone cement was observed. Finally, de novo bone growth and bone mineralization across the bone defect area were observed in rabbits after implantation of composite bone cement specimens. In conclusion, this novel calcitonin-loaded composite calcium phosphate bone cement exhibits biocompatibility, bioresorbability, osteoinductivity, and osteoconductivity, which may be suitable for clinical use.

Microencapsulation of rifampicin: A technique to preserve the mechanical properties of bone cement.

Two-stage exchange with antibiotic-loaded bone cement spacers remains the gold standard for chronic periprosthetic joint infection (PJI). Rifampicin is highly efficient on stationary-phase staphylococci in biofilm; however, its addition to PMMA to manufacture spacers prevents polymerization and reduces mechanical properties. Isolation of rifampicin during polymerization by microencapsulation could allow manufacturing rifampicin-loaded bone cement maintaining elution and mechanical properties. Microcapsules of rifampicin with alginate, polyhydroxybutyratehydroxyvalerate (PHBV), ethylcellulose and stearic acid (SA) were synthesized. Alginate and PHBV microcapsules were added to bone cement and elution, compression, bending, hardness, setting time and microbiological tests were performed. Repeated measures ANOVA and Bonferroni post-hoc test were performed, considering a p < 0.05 as statistical significance. Bone cement specimens containing alginate microcapsules eluted more rifampicin than PHBV microcapsules or non-encapsulated rifampicin over time (p < 0.012). Microencapsulation of rifampicin allowed PMMA to preserve mechanical properties in compression and bending tests. Cement with alginate microcapsules showed similar behavior in hardness tests to control cement over the study period (73 ± 1.68HD ). PMMA with alginate microcapsules exhibited the largest zones of inhibition in microbiological tests. Statistically significant differences in mean diameters of zones of inhibition between PMMA loaded with alginate-rifampicin (p = 0.0001) and alginate-PHBV microcapsules (p = 0.0001) were detected. Rifampicin microencapsulation with alginate is the best choice to introduce rifampicin in PMMA preserving mechanical properties, setting time, elution, and antimicrobial properties. The main applicability of this study is the opportunity for obtaining rifampicin-loaded PMMA by microencapsulation of rifampicin in alginate microparticles, achieving high doses of rifampicin in infected tissues, increasing the successful of PJI treatment. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:459-466, 2018.

Extended fatigue life of a catalyst free self-healing acrylic bone cement using microencapsulated 2-octyl cyanoacrylate.

The tissue adhesive 2-octyl cyanoacrylate (OCA) was encapsulated in polyurethane microshells and incorporated into bone cement to form a catalyst free, self-healing bone cement comprised of all clinically approved components. The bending strength, modulus, and fatigue lifetime were investigated in accordance with ASTM and ISO standards for the testing of PMMA bone cement. The bending strength of bone cement specimens decreased with increasing wt % capsules content for capsules without or with OCA, with specimens of <5 wt % capsule content showing minimal effect. In contrast, bone cement bending modulus was insensitive to capsule content. Load controlled fatigue testing was performed in air at room temperature on capsule free bone cement (0 wt %), bone cement with 5 wt % OCA-free capsules (5 wt % No OCA), and 5 wt % OCA-containing capsules (5 wt % OCA). Specimens were tested at a frequency of 5 Hz at maximum stresses of 90%, 80%, 70%, and 50% of each specimen's bending strength until failure. The 5 wt % OCA exhibited significant self-healing at 70% and 50% of its reference strength (p < 0.05). Fatigue testing of all three specimen types in air at 22 MPa (50% of reference strength of the 5 wt % OCA specimens) showed that the cycles to failure of OCA-containing specimens was increased by two-fold compared with the OCA-free and capsule-free specimens. This study represents the first demonstration of dynamic, catalyst free self-healing in a biomaterial formulation.

Material Mismatch Effect on the Fracture of a Bone-Composite Cement Interface.

The interfacial mechanics at the bone-implant interface is a critical issue for implant fixation and the filling of bone defects created by tumors and/or their excision. Our previous study found that micron and nano sizes MgO particles improved the fracture toughness of bone-cement interfaces under tension loading. The strength of bonding of different types of bone with different types of implants may not be the same. The aims of this research were to determine the influences of material mismatch due to bone orientation and a magnesium oxide (MgO) filler material for PMMA bone cement on the mechanical strength between bone and bone cement specimens. This research studied the longitudinal and transverse directions bovine cortical bone as different bone materials and poly Methyl MethAcrylate (PMMA) bone cement with and without MgO additives as different implant materials. The scope of work for this study was: (1) to determine the bending strength and modulus of different bone and bone cement specimens, (2) to determine whether inclusion of MgO particles on PMMA has any influence on these mechanical properties of PMMA, and (3) to determine whether bone orientation and inclusion of MgO particles with PMMA has any influence on the interface strength between bone and PMMA. This study showed that bone orientation has statistically significant effect on the bonding strength between bone and bone cement specimens (P value<0.05). This study also found that while MgO additive decreased the bending strength and modulus of PMMA bone cement, but the inclusion of MgO additives with PMMA bone cement has no statistically significant effect on the bonding strength between bone and bone cement specimens (P value>0.05).

METHOD TO ANALYZE THE FATIGUE CRACKS IN ACRYLIC BONE CEMENT

Acrylic bone cement is a poly(methyl methacrylate)-based material that ensures short-term stability of orthopedic implants after surgery. Its long-term performance can be affected by many factors (e.g., composition, cement mixing and delivery method, temperature, humidity). Furthermore, patient activities produce a spectrum of cyclic loads that generate microdamage within the acrylic bone cement mantle. Therefore, pre-clinical studies on fatigue damage of acrylic bone cements are essential for predicting the long-term stability of cemented implants. There are several methods for analyzing damage of acrylic bone cement. However, they present a number of limitations. The aim of this study was to validate the use of a high-resolution scanner to analyze the presence of microcracks in acrylic bone cement. The proposed method met predetermined criteria to overcome limitations of previous methods, ensuring approximate spatial resolution of 5 microns, reduction of image acquisition time, and reduction of artifacts due to operator and/or environment during image acquisition. Additionally, the described method was applied to three types of acrylic bone cement specimens that previously were subjected to a fatigue test. The presented method enables the accurate assessment of fatigue damage induced during cycling loading, including quantification of the number, length, type and position of cement cracks.

Effect of vacuum mixer brand on tensile properties of acrylic bone cement

The effect of vacuum mixer brand is studied on six commercial bone cements, three with prophylactic doses of antibiotics and three without. The modulus of elasticity, ultimate tensile strength, maximum strain, and 0.2% yield strength was analysed. Bone cement specimens were manufactured and tested in accordance with ASTM D638 specifications using three different types of vacuum mixers in an operating room. The specimens were tested in a universal testing machine. Statistical analysis revealed that a proprietary mixer did not generally outperform the other two mixers for its own cement. It was found that the mixer had a significant effect on the results and that the Stryker/Simplex mixer generally outperformed Biomet and Zimmer. However, for the Simplex bone cement, the mixer type did not have a significant effect for most of the data analysed. The overall conclusion is that the proprietary mixer does not perform the best with its own cement.

Incorporation of tocopherol acetate-containing particles in acrylic bone cement

Acrylic bone cement (BC) is used in orthopaedic surgery to anchor cemented prostheses to bone. Association of antioxidant molecules to BC may suppress reactive species injury which contributes to implant failure. Tocopherol acetate (ATA)-loaded polymethylmethacrylate (PMMA) particles (ATA(PMMA)) were prepared by single emulsion solvent evaporation technique and were incorporated into BC. An encapsulation efficiency of 84% (w/w) was obtained and drug release studies showed distinct ATA release profiles and mechanisms before and after particle incorporation into BC. Experimental data, analysed using first-order, Higuchi and Korsmeyer-Peppas models revealed that ATA was released from particles by a Fickian diffusion mechanism while a non-Fickian transport was observed upon particle incorporation in BC. There were no changes in the mechanical properties of BC specimens containing ATA(PMMA) particles, in contrast to what was observed when ATA was loaded directly into BC. Overall, ATA(PMMA) particles are potential carriers for the incorporation of an antioxidant drug into BC.

Effect of stem preheating on the fatigue behaviour of bone cement around hip prostheses

Tensile fatigue behaviour of bone cement specimens obtained from cement mantles moulded in vitro, simulating the surgical scenario, was investigated. The effect of stem preheating, before its insertion into the cement dough, on specimen fatigue life was studied. A commercial bone cement was selected for this study. Bone cement mixing was conducted in air, following the manufacturer's instructions, and injected simulating the clinical practice. Two conditions were considered: stem maintained at the surgical room temperature (23 degrees C), and stem preheated to 45 degrees C. Four repetitions of the whole procedure were performed for each condition obtaining a total of 32 specimens. All specimens underwent fatigue testing (stress ratio, 0; maximum tensile stress, 15 MPa) until failure. Both two-parameter and three-parameter Weibull distributions were initially used to analyse the fatigue life data set. However, the two-parameter distribution was chosen for both groups on the basis of the coefficient of determination used to test the goodness of fit. Stem preheating seems to have a negligible effect on fatigue behaviour of the studied bone cement in the low range of fatigue lives (up to 10(4)). However, above this number of cycles, stem preheating seems to reduce the probability of failure. These findings are discussed in the text.

Incorporation of Methotrexate in Calcium Phosphate Cement: Behavior and Release in Vitro and in Vivo

The application of antibiotic-loaded calcium phosphate bone cement in the treatment and prevention of osteomyelitis suggests that calcium phosphate cement could also be used as an anticancer drug carrier to reduce the local recurrence of bone tumors and systemic toxicities of chemotherapy. We added 0 to 400 mg of methotrexate to 40 g cement, with final methotrexate concentrations of 0% to 1% (weight/weight). The setting times, mechanical properties, microstructures, and in vitro methotrexate release kinetics of these methotrexate-calcium phosphate bone cement specimens were evaluated, along with in vivo methotrexate release kinetics in 24 rabbits. Methotrexate did not significantly alter the cement setting time. The compressive and tensile strengths of the methotrexate-calcium phosphate bone cement specimens were significantly less when 400 mg methotrexate was used, compared to control samples without methotrexate addition. Nevertheless, the cement remained compliant with the minimum requirements for clinical application. The scanning electron microscopy micrographs showed that the basic crystal structure did not alter. The methotrexate release kinetics in vitro and in vivo confirmed that methotrexate-calcium phosphate bone cement was a monolithic matrix system, with a burst effect in the initial stage and a sudden drop thereafter. Drug delivery in vivo was faster than in vitro. We estimated that the incorporated methotrexate could be continuously released over 2 to 4 months at a higher than minimum concentration. The methotrexate had no apparent toxicity on the host rabbits, even at the highest dose of methotrexate tested. The methotrexate-calcium phosphate bone cement system may be a potentially effective therapy for bone tumors in humans.

Evaluation of an accelerated aging medium for acrylic bone cement based on analysis of nanoindentation measurements on laboratory‐prepared and retrieved specimens

The thrust of the study was a critical evaluation of the efficacy of a medium (30% v/v H(2)O(2), at 60 degrees C) that has been suggested in a literature report as being suitable for simulating the oxidative aging process, seen in vivo, in the acrylic bone cement mantles of total hip and knee joint replacements. For this purpose, quasi-static and dynamic nanoindentation measurements were used to obtain material properties--elastic modulus, E; hardness, H; and the variation of the storage and loss moduli with the frequency of the applied indenting force--of PalacosR acrylic bone cement specimens after various periods of immersion (7, 14, 21, and 28 days) in the aging solution, and of specimens prepared from cement mantles retrieved from cemented total hip joint replacements after various times in vivo (0.92-21 years). Also, best-fit relationships were obtained between E and time in the H(2)O(2) solution (t), H and t, E and in vivo time (T), and H and T. This body of results points to the possibility that the aging solution is effective, although the evidence is not conclusive.

Assessment of a three-dimensional measurement technique for the porosity evaluation of PMMA bone cement

In vitro testing of bone cement has historically resulted in the belief that porosity should be minimised to help reduce the risk of prosthesis failure through aseptic loosening. Traditional porosity measurement techniques rely on the analysis of a two dimensional representation of a three dimensional structure. However, with an increasing interest in the number, size and distribution of pores in bone cement, the reliability of a two dimensional approach is questionable. The purpose of this study was to investigate the use of micro computed tomography (micro-CT) for the three dimensional measurement of bone cement porosity by comparison with two traditional techniques. Eighteen bone cement specimens were analysed for porosity using each technique. Levels of agreement between techniques were evaluated, and technique precision was assessed in terms of repeatability and sensitivity to changes in threshold. Micro-CT data was used to illustrate the effectiveness of predicting the porosity of a whole structure from a sample region; an approach often used with traditional techniques. In summary, poor agreement was found between all techniques. However, micro-CT was found to be significantly more repeatable and less sensitive to changes in threshold. The results demonstrated that porosity cannot be reliably determined using traditional techniques and that a large proportion of a specimen is required to provide an accurate porosity measurement.

Why would cement porosity reduction be clinically irrelevant, while experimental data show the contrary

Laboratory bench tests have shown that porosity reduction increases the fatigue life of bone cement specimens. Clinically, however, the effect porosity reduction is subject to debate. We hypothesized that the discrepancy between clinical and experimental findings is related to differences in the stress distribution, which is typically uniform in experimental test specimens, while stress concentrations exist in cement around hip implants. We simulated fatigue failure of cement in a finite element model of an experimental test specimen and of a transverse slice of a total hip arthroplasty with a sharp-cornered stem. Four levels of porosity were introduced. In the fatigue test specimen model, the fatigue life clearly was dependent on the level of porosity, while in the transverse slice model, the level of porosity had virtually no effect on failure of the cement mantle. The results of the simulations confirmed our hypothesis. In simulations of laboratory tests, pores clearly acted as crack initiators, while in the simulation of a real total hip reconstruction, crack formation was governed by local stress singularities. This explains why the beneficial effect of cement porosity reduction on the lifetime of total hip reconstructions may be hard to detect clinically.

Mechanical properties of oligomer-modified acrylic bone cement

The aim of this study was to determine the mechanical properties of acrylic bone cement modified with an experimental oligomer filler, based on an amino acid of trans-4-hydroxy- L-proline synthesized in the laboratory. The test specimens were tested either dry, or after being stored in distilled water or in simulated body fluid (SBF) for 1 week and then tested in distilled water. The three-point bending test was used to measure the flexural strength and flexural modulus of the cement, and the compression tests were used to measure the compression strength and modulus. One test specimen from each group was examined under a scanning electron microscope (SEM) to determine the nature of the oligomer filler in the polymethylmethacrylate–polymethylacrylate copolymer-based (PMMA–PMA/PMMA) polymer blend. In dry conditions, the flexural strength of the test specimens tested in air was 66 MPa , and the compression strength was 93 GPa ( p<0.001) for the plain bone cement. For the test specimens including 20 wt% of oligomer filler, the flexural strength was 37 MPa , and the compression strength was 102 MPa (p<0.001) in dry conditions. The storage in wet conditions (in distilled water and the SBF) decreased the flexural strength of the test specimens with 20 wt% of oligomer filler ( p<0.001) by 60% and the flexural modulus by 44% compared to the plain bone cement specimens stored in the same conditions. The reduction in compression strength in wet conditions was 32%, and that of the compression modulus was 30% ( p<0.001). No significant differences were found between test specimens stored in distilled water or SBF (ANOVA, p<0.001). In the SEM examinations, random voids were observed in the oligomer-PMMA–PMA/PMMA polymer blend after water or SBF storage. The results suggest that both water and SBF storage decrease the mechanical properties of the PMMA–PMA/PMMA bone cement modified with oligomer, while at the same time, there was porous formation in the bone cement structure.

Key issues involved with the use of miniature specimens in the characterization of the mechanical behavior of polymeric biomaterials--a review.

This article is arranged in three parts. In the first, a brief history of the use of miniature specimens for characterizing the mechanical behavior of materials in general is given. In the second part, several trends in literature reports of small punch and small shear punch tests of ultra-high-molecular-weight polyethylene and acrylic bone cement specimens are examined critically. In this exercise, special attention is paid to one test metric; namely, the work to failure [which is the area under the punch load (P) versus punch displacement (Delta) plot up to the failure point]. In the third part of the article, seven key issues in this field are discussed. In each case, the gaps in the current knowledge base are pointed out. Among those issues are determination of sensitivity of test results to test variables, development of methods for converting P-Delta results to bulk material properties, and constitutive modeling of the mechanical behavior of a polymeric biomaterial, under both small punch and small shear punch loading.

The static and cyclic strength of a bone-cement bond.

Four-point bending static and fatigue tests were carried out on bone-cement bonds. The effects of the pressurization and the washing of the bone joint face on the bond strength were investigated. The results are summarized as follows. When the bond surface of cancellous bone is washed prior to the application of the bone cement, both the static and fatigue strengths of the bond are increased relative to the corresponding properties of unwashed bone-cement bonds. From observations of bone-cement interfaces as well as the fracture surfaces of bone-cement specimens, it has been determined that bone cement was able to infiltrate into fine holes present in washed cancellous bone. However, such infiltration occurred to a much lesser degree in the case of unwashed cancellous bone. Increasing the molding pressure during the time of cement application to the bone from 39200 to 117600 Pa had a beneficial effect on the bending strength and fatigue properties, particularly in the case of washed bone cement specimens. An increase in molding pressure also resulted in a reduction in the amount of scatter in test results.

Viscoelastic behaviour of acrylic bone cements

Local contact stresses at the bone–cement interface are thought to play an important role in the initiation of component loosening. A reduced-modulus bone cement can lower these local contact stresses. The viscoelastic properties of such a cement raised the question of long term subsidence of the implant system. In this study, the viscoelastic properties of a reduced-modulus bone cement were compared with standard polymethylmethacrylate, PMMA, bone cement using stress relaxation tests. Unconstrained stress relaxation tests were performed at 37′C in an aqueous environment by applying 1%, 2.5%, and 5% strains on bone cement specimens and monitoring the diminishing load for 100 h. The initial rapid stress relaxation occurring over the first hour and the steady state stress relaxation occurring between 15 and 100 h were analyzed. A fast stress diminution occurred in PBMMA specimens indicating that, in a total hip arthroplasty application, PBMMA bone cement would transfer the stress quickly and distribute it over a larger area of endosteal bone surface. Steady state stress relaxation experiments showed a significant difference in 2.5% and 5% stress relaxation values ( P<0.05) between PMMA and PBMMA specimens, but not at the 1% stress values. Length measurements indicated that the viscoelastic PBMMA specimens demonstrated little recovery after 100 h of imposed strain whereas the elastic PMMA specimens showed substantial recovery. This seems to indicate relatively larger subsidence rates in unconstrained PBMMA specimens compared to PMMA specimens. In vivo, the cement is surrounded by endosteal bone at the outer side and by an implant on the inner side. Therefore, constrained creep tests are necessary to obtain the data required for an assessment of in vivo subsidence.