The relationship between porosity and fatigue characteristics of bone cements

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.

Influence of Porosity on Fracture Toughness and Fracture Behavior of Antibiotic-Loaded PMMA Bone Cement

Aseptic loosening is the most common reason for the long-term revision of cemented arthroplasties with fracture of the cement being a postulated cause or contributing factor. In our previous studies we showed that adding an antibiotic to a polymethylmethacrylate (PMMA) bone cement led to detrimental effects on various mechanical properties of the cement such as bending strength, compressive strength and fracture toughness (KIC). This finding implied that the mechanical failure of antibiotic-loaded PMMA bone cement was influenced by its pore volume fraction. Up to now this aspect has not been studied. Hence the purposes of this study were to determine (1) the influence of antibiotic (telavancin) loading on the KIC of a widely used PMMA bone cement brand (Palacos®R) and (2) the influence of pore size and pore distribution on the fracture behavior of the KIC specimens. For (2) both experimental and numerical methods (extended finite element method [XFEM]) were used allowing a comparison between the two sets of results. We found that: (1) KIC decreased with increased porosity with the drop (relative to the value for the control cement) being significant when the telavancin loading was 4.8 wt/wt % (2 g of telavancin added to 40 g of control cement powder); (2) there was a critical pore size above which there was a significant decrease in KIC and is 1 mm; (3) crack propagation was strongly influenced by pore size and pore locations (pore-pore interactions); and, (4) there was good agreement between the experimental and XFEM results. The implications of these findings for the use of a telavancin-loaded PMMA bone cement in cemented total joint arthroplasties are commented upon.

Advanced G-MPS-PMMA Bone Cements: Influence of Graphene Silanisation on Fatigue Performance, Thermal Properties and Biocompatibility

The incorporation of well-dispersed graphene (G) powder to polymethyl methacrylate (PMMA) bone cement has been demonstrated as a promising solution to improving its mechanical performance. However, two crucial aspects limit the effectiveness of G as a reinforcing agent: (1) the poor dispersion and (2) the lack of strong interfacial bonds between G and the matrix of the bone cement. This work reports a successful functionalisation route to promote the homogenous dispersion of G via silanisation using 3-methacryloxypropyltrimethoxy silane (MPS). Furthermore, the effects of the silanisation on the mechanical, thermal and biocompatibility properties of bone cements are presented. In comparison with unsilanised G, the incorporation of silanised G (G_MPS1 and G_MPS2) increased the bending strength by 17%, bending modulus by 15% and deflection at failure by 17%. The most impressive results were obtained for the mechanical properties under fatigue loading, where the incorporation of G_MPS doubled the Fatigue Performance Index (I) value of unsilanised G-bone cement-meaning a 900% increase over the I value of the cement without G. Additionally, to ensure that the silanisation did not have a negative influence on other fundamental properties of bone cement, it was demonstrated that the thermal properties and biocompatibility were not negatively impacted-allowing its potential clinical progression.

Polymethyl Methacrylate-Based Bone Cements Containing Carbon Nanotubes and Graphene Oxide: An Overview of Physical, Mechanical, and Biological Properties

Every year, millions of people in the world get bone diseases and need orthopedic surgery as one of the most important treatments. Owing to their superior properties, such as acceptable biocompatibility and providing great primary bone fixation with the implant, polymethyl methacrylate (PMMA)-based bone cements (BCs) are among the essential materials as fixation implants in different orthopedic and trauma surgeries. On the other hand, these BCs have some disadvantages, including Lack of bone formation and bioactivity, and low mechanical properties, which can lead to bone cement (BC) failure. Hence, plenty of studies have been concentrating on eliminating BC failures by using different kinds of ceramics and polymers for reinforcement and also by producing composite materials. This review article aims to evaluate mechanical properties, self-setting characteristics, biocompatibility, and bioactivity of the PMMA-based BCs composites containing carbon nanotubes (CNTs), graphene oxide (GO), and carbon-based compounds. In the present study, we compared the effects of CNTs and GO as reinforcement agents in the PMMA-based BCs. Upcoming study on the PMMA-based BCs should concentrate on trialing combinations of these carbon-based reinforcing agents as this might improve beneficial characteristics.

Self-healing biomaterials: The next generation is nano

The U.S. Agency for Healthcare Research and Quality estimates that there are over 1 million total hip and total knee replacements each year in the U.S. alone. Twenty five percent of those implants will experience aseptic loosening, and bone cement failure is an important part of this. Bone cements are based on poly(methyl methacrylate) (PMMA) systems that are strong but brittle polymers. PMMA-based materials are also essential to modern dental fillings, and likewise, the failure rates are high with lifetimes of 3-10 years. These brittle polymers are an obvious target for self-healing systems which could reduce revision surgeries and visits to dentist. Self-healing polymers have been described in the literature since 1996 and examples from Roman times are known, but their application in medicine has been challenging. This review looks at the development of self-healing biomaterials for these applications and the challenges that lie between development and the clinic. Many of the most promising formulations involve introducing nanoscale components which offer substantial potential benefits over their microscale counterparts especially in composite systems. There is substantial promise for translation, but issues with toxicity, robustness, and reproducibility of these materials in the complex environment of the body must be addressed. This article is categorized under: Therapeutic Approaches and Drug Discovery > Emerging Technologies Nanotechnology Approaches to Biology > Nanoscale Systems in Biology Implantable Materials and Surgical Technologies > Nanomaterials and Implants.

Evaluation of the Effect of Ciprofloxacin and Vancomycin on Mechanical Properties of PMMA Cement; a Preliminary Study on Molecular Weight

Antibiotic-loaded bone cement (ALBC) is commonly used in joint replacement therapy for prevention and treatment of bone infection and mechanical properties of the cement is still an important issue. The effects of ciprofloxacin and vancomycin was investigated on mechanical characterization of PMMA bone cement. Different properties of cement containing (0, 2.5, 5 and 10% W/W) antibiotics, including compressive and bending properties, microstructural, porosity and density were evaluated. Both antibiotics significantly reduced the density values and mechanical properties (compressive and flexural strength and modulus) in all groups in comparison to control over first two weeks (p < 0.05). This reduction was due to increased porosity upon antibiotic addition (3.05 and 3.67% for ciprofloxacin and vancomycin, respectively) in comparison to control (2.08%) (p < 0.001) and exposure to aqueous medium. Vancomycin as antibiotic with higher molecular weight (MW = 1485) had significant effect on compressive strength reduction of the cement at high amount compared to ciprofloxacin (MW = 367) (P < 0.01), there was no difference between two antibiotics at lower concentrations (P > 0.05). The effect of antibiotic loading is both molecular weight and drug content dependent. The time is also an important parameter and the second week is the probably optimum time to study mechanical behavior of ALBC.

Compressive fatigue properties of commercially available standard and low-modulus acrylic bone cements intended for vertebroplasty

Vertebroplasty (VP) is a minimally invasive surgical procedure commonly used to relieve severe back pain associated with vertebral compression fractures. The poly(methyl methacrylate) bone cement used during this procedure is however presumed to facilitate the occurrence of additional fractures next to the treated vertebrae. A reason for this is believed to be the difference in stiffness between the bone cement and the surrounding trabecular bone. The use of bone cements with lower mechanical properties could therefore reduce the risk of complications post-surgery. While intensive research has been performed on the quasi-static mechanical properties of these cements, there is no data on their long-term mechanical properties. In the present study, the in vitro compressive fatigue performance as well as quasi-static mechanical properties of two commercially available acrylic bone cements - a low-modulus cement (Resilience^®) and a standard cement (F20) from the same manufacturer - were determined. The quasi-static mechanical properties of the low-modulus and standard cements after 24 h of setting were in the range of other vertebroplastic cements (σ = 70-75 MPa; E= 1600-1900 MPa). F20 displayed similar mechanical properties over time in 37 °C phosphate buffered saline solution, while the mechanical properties of the Resilience^® cement decreased gradually due to an increased porosity in the polymeric matrix. The standard cement exhibited a fatigue limit of approx. 47 MPa, whereas the low-modulus cement showed a fatigue limit of approx. 31 MPa. In summary, the low-modulus bone cement had a lower fatigue limit than the standard cement, as expected. However, this fatigue limit is still substantially higher than the stresses experienced by vertebral trabecular bone.

Mechanical Properties and Porosity of Acrylic Cement Bone Loaded with Alendronate Powder

Aseptic loosening is the most common complication of joint replacement. Previous studies showed that acrylic bone cement loaded with a commercially-available alendronate powder (APAC) had good promise against wear debris-mediated osteolysis for prevention of aseptic loosening. The purpose of the present study was to investigate the effect of adding alendronate powder to an acrylic bone cement on quasi-static mechanical properties (namely, compressive strength, compressive modulus, tensile strength, and flexural strength), fatigue life, porosity, and microstructure of the cement. The results showed that adding up to 1 wt./wt.% alendronate powder exerted no detrimental effect on any of the quasi-static mechanical properties. However, the fatigue life of APAC decreased by between ~17% and ~27 % and its porosity increased by between ~ 5-7 times compared with corresponding values for the control cement (no alendronate powder added). Fatigue life was negatively and significantly correlated with porosity. Considering that fatigue life of the cement plays a significant role in joint replacement survival, clinical use of APAC cannot be recommended.

Towards the optimization of the preparation procedures of PMMA bone cement

The mechanical properties and thermal history of polymethyl-methacrylate bone cement vary significantly with the preparation procedure used. Because the polymerization reaction is exothermic, many researchers have attempted to minimize thermal osteonecrosis due to heat generation by altering procedures in the preparation of the cement. In most previous studies, only one or two aspects of the preparation procedure were controlled, and there has been little research that comprehensively examines the effects of preparation on the cure kinetics and resulting properties of bone cement. In this study, cement viscosity, cement layer thickness, initial cement temperature, initial metal component temperature, and mixing method were varied to assess the effects on the cement. Maximum temperature, polymerization time, necrosis index, bending strength, and porosity were chosen to evaluate the different preparation procedures, where an optimal procedure would minimize necrosis, reduce cement cure time, and maximize bending strength. Design of Experiments (DOE) was used to examine the main effects and interactions of preparation techniques. Among the most prominent results, it was found that the cure kinetics and the related quantities are primarily controlled by the initial metal component temperature and that the bending strength is most dependent on the mixing method. For the two formulations studied, the optimum preparation procedures should keep cement and metal components at room temperature prior to mixing with a vacuum mixing system. Reducing cement mantle thickness may also be advantageous, as it reduces the maximum temperature and the risk of tissue damage. © 2015 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:915-923, 2016.

Mechanical and thermal behaviour of an acrylic bone cement modified with a triblock copolymer

The basic formulation of an acrylic bone cement has been modified by the addition of a block copolymer, Nanostrength(®) (NS), in order to augment the mechanical properties and particularly the fracture toughness of the bone cement. Two grades of NS at different levels of loading, between 1 and 10 wt.%, have been used. Mechanical tests were conducted to study the behaviour of the modified cements; specific tests measured the bend, compression and fracture toughness properties. The failure mode of the fracture test specimens was analysed using scanning electron microscopy (SEM). The effect of NS addition on the thermal properties was also determined, and the polymerisation reaction using differential scanning calorimetry. It was observed that the addition of NS produced an improvement in the fracture toughness and ductility of the cement, which could have a positive contribution by reducing the premature fracture of the cement mantle. The residual monomer content was reduced when the NS was added. However this also produced an increase in the maximum temperature and the heat delivered during the polymerisation of the cement.

Fatigue failure of dentin-composite disks subjected to cyclic diametral compression

OBJECTIVE

Our aim was to establish the relationship between cyclic loading and fatigue life of the dentin-composite interface using the newly developed disk in diametral compression tests. The results were then used to estimate the fatigue life of restored teeth under occlusal loading.

METHODS

Disk specimens (5mm dia.×2mm thick) were prepared using bovine incisors and restored with either a methacrylate-based composite Z100™ with Adper Single Bond Plus (Z100) or silorane-based composite Filtek™ LS with LS System adhesive (LS). The dentin-composite disks were tested under cyclic diametral compression to determine the number of cycles to failure (Nf) at three load levels (n=3 per group). Finite element analysis (FEA) was used to calculate the interfacial stresses (σ) within the specimen, to establish the σ vs. Nf curves, and those within a restored tooth under normal chewing forces (15N maximum). These were then used to estimate the lifetime of the restored tooth for the two restorative systems.

RESULTS

The disks restored with LS had a higher fatigue resistance than those restored with Z100. The maximum interfacial stress in the restored tooth determined by FEA was ∼0.5MPa. Based on the estimate of 300,000 cycles of chewing per year, the predicted lifetime under occlusal loading for teeth restored with LS and Z100 was 33 and 10 years, respectively.

SIGNIFICANCE

The disk in cyclic diametral compression has been used successfully to provide fatigue data which allows the lifetime of composite-restored teeth under occlusal loading to be predicted using numerical simulation.

Optimisation of a two-liquid component pre-filled acrylic bone cement system: a design of experiments approach to optimise cement final properties

The initial composition of acrylic bone cement along with the mixing and delivery technique used can influence its final properties and therefore its clinical success in vivo. The polymerisation of acrylic bone cement is complex with a number of processes happening simultaneously. Acrylic bone cement mixing and delivery systems have undergone several design changes in their advancement, although the cement constituents themselves have remained unchanged since they were first used. This study was conducted to determine the factors that had the greatest effect on the final properties of acrylic bone cement using a pre-filled bone cement mixing and delivery system. A design of experiments (DoE) approach was used to determine the impact of the factors associated with this mixing and delivery method on the final properties of the cement produced. The DoE illustrated that all factors present within this study had a significant impact on the final properties of the cement. An optimum cement composition was hypothesised and tested. This optimum recipe produced cement with final mechanical and thermal properties within the clinical guidelines and stated by ISO 5833 (International Standard Organisation (ISO), International standard 5833: implants for surgery-acrylic resin cements, 2002), however the low setting times observed would not be clinically viable and could result in complications during the surgical technique. As a result further development would be required to improve the setting time of the cement in order for it to be deemed suitable for use in total joint replacement surgery.

Nonparametric pore size distribution using d-PFG: comparison to s-PFG and migration to MRI

Here we present the successful translation of a pore size distribution (PSD) estimation method from NMR to MRI. This approach is validated using a well-characterized MRI phantom consisting of stacked glass capillary arrays (GCA) having different diameters. By employing a double pulsed-field gradient (d-PFG) MRI sequence, this method overcomes several important theoretical and experimental limitations of previous single-PFG (s-PFG) based MRI methods by allowing the relative diffusion gradients' direction to vary. This feature adds an essential second dimension in the parameters space, which can potentially improve the reliability and stability of the PSD estimation. To infer PSDs from the MRI data in each voxel an inverse linear problem is solved in conjunction with the multiple correlation function (MCF) framework, which can account for arbitrary experimental parameters (e.g., long diffusion pulses). This scheme makes no a priori assumptions about the functional form of the underlying PSD. Creative use of region of interest (ROI) analysis allows us to create different underlying PSDs using the same GCA MRI phantom. We show that an s-PFG experiment on the GCA phantom fails to accurately reconstruct the size distribution, thus demonstrating the superiority of the d-PFG experiment. In addition, signal simulations corrupted by different noise levels were used to generate continuous and complex PSDs, which were then successfully reconstructed. Finally, owing to the reduced q- or b- values required to measure microscopic PSDs via d-PFG MRI, this method will be better suited to biomedical and clinical applications, in which gradient strength of scanners is limited.

Pore size distribution of bioresorbable films using a 3-D diffusion NMR method

Pore size distribution (PSD) within porous biomaterials is an important microstructural feature for assessing their biocompatibility, longevity and drug release kinetics. Scanning electron microscopy (SEM) is the most common method used to obtain the PSD of soft biomaterials. The method is highly invasive and user dependent, since it requires fracturing of the sample and then considers only the small portion that the user had acquired in the image. In the current study we present a novel nuclear magnetic resonance (NMR) method as an alternative method for estimation of PSD in soft porous materials. This noninvasive 3-D diffusion NMR method considers the entire volume of the specimen and eliminates the user's need to choose a specific field of view. Moreover, NMR does not involve exposure to ionizing radiation and can potentially have preclinical and clinical uses. The method was applied on four porous 50/50 poly(dl-lactic-co-glycolic acid) bioresorbable films with different porosities, which were created using the freeze-drying of inverted emulsions technique. We show that the proposed NMR method is able to address the main limitations associated with SEM-based PSD estimations by being non-destructive, depicting the full volume of the specimens and not being dependent on the magnification factor. Upon comparison, both methods yielded a similar PSD in the smaller pore size range (1-25μm), while the NMR-based method provided additional information on the larger pores (25-50μm).

Carboxyl functionalised MWCNT/polymethyl methacrylate bone cement for orthopaedic applications

The incorporation of carboxyl functionalised multi-walled carbon nanotube (MWCNT-COOH) into a leading proprietary grade orthopaedic bone cement (Simplex P™) at 0.1 wt% has been investigated. Resultant static and fatigue mechanical properties, in addition to thermal and polymerisation properties, have been determined. Significant improvements ( p ≤ 0.001) in bending strength (42%), bending modulus (55%) and fracture toughness (22%) were demonstrated. Fatigue properties were improved ( p ≤ 0.001), with mean number of cycles to failure and fatigue performance index being increased by 64% and 52%, respectively. Thermal necrosis index values at ≥44℃ and ≥55℃ were significantly reduced ( p ≤ 0.001) (28% and 27%) versus the control. Furthermore, the onset of polymerisation increased by 58% ( p < 0.001), as did the duration of the polymerisation reaction (52%). Peak energy during polymerisation increased by 672% ( p < 0.001). Peak area of polymerisation increased by 116% ( p < 0.001) indicating that the incorporation of MWCNT-COOH reduced the rate of polymerisation significantly. A non-significant reduction (8%) in percentage monomer conversion was also recorded. Raman spectroscopy clearly showed that the addition of MWCNT-COOH increased the ratio between normalised intensities of the G-Band and D-Band ( IG/ID), and also increased the theoretical compressive strain (−1.72%) exerted on the MWCNT-COOH by the Simplex P™ cement matrix. Therefore, demonstrating a level of chemical interactivity between the MWCNT-COOH and the Simplex P™ bone cement exists and consequently a more effective mechanism for successful transfer of mechanical load. The extent of homogenous dispersion of the MWCNT-COOH throughout the bone cement was determined using Raman mapping.

Creep and fatigue behavior of a novel 2-component paste-like formulation of acrylic bone cements

The fatigue and creep performance of two novel acrylic bone cement formulations (one bone cement without antibiotics, one with antibiotics) was compared to the performance of clinically used bone cements (Osteopal V, Palacos R, Simplex P, SmartSet GHV, Palacos R+G and CMW1 with Gentamicin). The preparation of the novel bone cement formulations involves the mixing of two paste-like substances in a static mixer integrated into the cartridge which is used to apply the bone cement. The fatigue performance of the two novel bone cement formulations is comparable to the performance of the reference bone cements. The creep compliance of the bone cements is significantly influenced by the effects of physical ageing. The model parameters of Struik's creep law are used to compare the creep behavior of different bone cements. The novel 2-component paste-like bone cement formulations are in the group of bone cements which exhibit a higher creep resistance.

Is the Stryker Revolution mixing system fit for purpose? Reliability and a micro-CT assessment of cement porosity

The Stryker Revolution(TM) is a new mixing system that employs a high vacuum and a motorised mixing spatula in an effort to reduce cement porosity. We have compared Revolution(TM) with Depuy Cemvac(®), in terms of system reliability and cement porosity. Standardised Simplex P(®) and SmartSet(®) HV cement samples were produced using both mixing systems and analysed using a micro-CT scanner. The overall porosity, number and volume of voids were measured. Void analysis was subdivided into macro-pores (>0.5 mm3) and micro-pores (0.0005-0.5 mm3). Both systems were easy to use and no breakages were encountered. There was no significant difference in overall porosity between Revolution(TM) and Cemvac(®). Revolution(TM) produced over a five-fold decrease in average macro-pore size with medium viscosity cement (p=0.02), but produced a greater number of micro-pores (p<0.01). SmartSet(®) HV specimens had a higher porosity compared to Simplex P(®). This study demonstrated that the Revolution(TM) system was reliable and reduced porosity at least as effectively as the established Cemvac(®) system. The Revolution(TM) produced a greater number of smaller pores and further testing is required to establish if this results in a significant mechanical benefit.

Fatigue and biocompatibility properties of a poly(methyl methacrylate) bone cement with multi-walled carbon nanotubes

Composites of multi-walled carbon nanotubes (MWCNT) of varied functionality (unfunctionalised and carboxyl and amine functionalised) with polymethyl methacrylate (PMMA) were prepared for use as a bone cement. The MWCNT loadings ranged from 0.1 to 1.0 wt.%. The fatigue properties of these MWCNT-PMMA bone cements were characterised at MWCNT loading levels of 0.1 and 0.25 wt.% with the type and wt.% loading of MWCNT used having a strong influence on the number of cycles to failure. The morphology and degree of dispersion of the MWCNT in the PMMA matrix at different length scales were examined using field emission scanning electron microscopy. Improvements in the fatigue properties were attributed to the MWCNT arresting/retarding crack propagation through the cement through a bridging effect and hindering crack propagation. MWCNT agglomerates were evident within the cement microstructure and the degree of agglomeration was dependent on the level of loading and functionality of the MWCNT. The biocompatibility of the MWCNT-PMMA cements at MWCNT loading levels upto 1.0 wt.% was determined by means of established biological cell culture assays using MG-63 cells. Cell attachment after 4h was determined using the crystal violet staining assay. Cell viability was determined over 7 days in vitro using the standard colorimetric MTT assay. Confocal scanning laser microscopy and SEM analysis was also used to assess cell morphology on the various substrates.

Setting properties and biocompatibility of dicalcium silicate with varying additions of tricalcium aluminate

This study sought to prepare biphasic mixtures by adding tricalcium aluminate (Ca3Al2O6) into dicalcium silicate (Ca2SiO4) and to evaluate the effect of Ca3Al2O6 on setting properties and biocompatibility of the Ca2SiO4/Ca3Al2O6 mixtures as compared to pure Ca2SiO4. The results indicated that the addition of Ca3Al2O6 into Ca2SiO4 reduced the setting time and improved the compressive strength. Furthermore, Ca2SiO4/Ca3Al2O6 mixtures were bioactive and biocompatible, and had a stimulatory effect on the L929 cell growth when the content of Ca3Al2O6 was below 10%. Therefore, the mixtures with 10% Ca3Al2O6 produced the best compromise between setting and in vitro biological properties.

Porosity and color of maxillofacial silicone elastomer

PURPOSE

Prosthesis color production and stability as a result of pore entrapment during mixing has not been investigated for maxillofacial silicone prostheses. The purpose of this study was to investigate pore numbers and percentages of a maxillofacial silicone elastomer mixed by two different techniques, using X-ray microfocus computerized tomography (Micro-CT), and to investigate the effect of porosity on color reproducibility and stability after two different aging conditions.

MATERIALS AND METHODS

Sixty-four disk-shaped specimens were prepared (8-mm diameter, 3-mm thick) by mixing TechSil S25 silicone elastomer (Technovent, Leeds, UK) following two techniques: manual mixing (n = 32) and mechanical mixing under vacuum (n = 32). Half the specimens in each group were intrinsically pigmented, and the other half remained unpigmented. Pore numbers, volumes, and percentages were calculated using the Micro-CT, and then specimens of each subgroup were stored in simulated sebum for 6 months (n = 8), and exposed to accelerated daylight aging for 360 hours (n = 8). Color change (ΔE) was measured at the start and end of conditioning. Pore numbers and percentages were analyzed using one-way Analysis of Variance (ANOVA) and Dunnett's-T3 post-hoc tests (p < 0.05). Independent t-test was used to detect differences (p < 0.05) in ΔE between manually and mechanically mixed specimens, in both unpigmented and pigmented states and to detect differences (p < 0.05) in ΔE before and after conditioning within each mixing method.

RESULTS

Mechanical mixing under vacuum reduced the number and percentage of pores in comparison to manual mixing, within pigmented and unpigmented silicone specimens (p < 0.05). Perceptible ΔE between manual and mechanical mixing techniques were 5.93 and 5.18 for both unpigmented and pigmented specimens, respectively. Under sebum storage, manually mixed unpigmented specimens showed lower ΔE (p < 0.05) than those that were mechanically mixed; however, pigmented silicone specimens showed the same ΔE (p > 0.05). After light aging, mixing method had no effect on ΔE of unpigmented specimens (p > 0.05). Furthermore, mechanically mixed pigmented specimens showed lower ΔE (p < 0.05).

CONCLUSIONS

Within silicone elastomers (whether pigmented or unpigmented), mechanical mixing under vacuum reduced pore numbers and percentages in comparison to manual mixing. For selected skin shade, pores affected the resultant color of prosthesis (color reproducibility). Additionally, silicone pores affected silicone color stability upon service.

CLINICAL SIGNIFICANCE

In fabricating maxillofacial prostheses, mechanically mixing silicone under vacuum produces pore-free prostheses, tending to enhance their color production and stability.

Effect of tricalcium aluminate on the properties of tricalcium silicate-tricalcium aluminate mixtures: setting time, mechanical strength and biocompatibility

AIM

To prepare biphasic mixtures by adding Ca(3) Al(2) O(6) into Ca(3) SiO(5) and to evaluate the effect of Ca(3) Al(2) O(6) on physical and ex vivo biological properties of the Ca(3) SiO(5) /Ca(3) Al(2) O(6) mixtures derived from mineral trioxide aggregate (MTA).

METHODOLOGY

Combinations of Ca(3) SiO(5) and Ca(3) Al(2) O(6) (0, 5%, 10% and 15%) powders were mixed with deionized water. After hydration, setting time, compressive strength, ex vivo bioactivity and biocompatibility of each mixture were investigated and compared to pure Ca(3) SiO(5) .

RESULTS

With the addition of Ca(3) Al(2) O(6) from 0% to 15%, the initial setting time and final setting time of the Ca(3) SiO(5) /Ca(3) Al(2) O(6) mixtures decreased from 110 to 43min and from 220 to 97min, respectively (P≤0.05). However, the compressive strength increased from 6.75 to 16.20MPa after one day (P≤0.05) and from 17.73 to 29.13 Mpa after 28 days. Furthermore, the mixtures with 10% Ca(3) Al(2) O(6) or less had similar bioactivity and biocompatibility when compared to the pure Ca(3) SiO(5).

CONCLUSIONS

The addition of Ca(3) Al(2) O(6) into Ca(3) SiO(5) accelerated the hydration process, reduced the setting time and improved the compressive strength. Furthermore, these mixtures were bioactive and biocompatible and had a stimulatory effect on the L929 cell growth when the content of Ca(3) Al(2) O(6) was below 10%. Therefore, the mixtures with 10% Ca(3) Al(2) O(6) produced the best compromise between hydration and ex vivo biological properties.

Computational assessment of the effect of polyethylene wear rate, mantle thickness, and porosity on the mechanical failure of the acetabular cement mantle

Clinical studies have revealed that aseptic loosening is the dominant cause of failure in total hip arthroplasty, particularly for the acetabular component. For a cemented polyethylene cup, failure is generally accompanied by the formation of fibrous tissue at the cement-bone interface. A variety of reasons for the formation of this tissue have been suggested, including osteolysis and mechanical overload at the cement-bone interface. In this study, a computational cement damage accumulation method was used to investigate the effect of polyethylene cup penetration, cement mantle thickness, and cement porosity on the number of cycles required to achieve mechanical fatigue failure of the cement mantle. Cup penetration was found to increase cement mantle stresses, resulting in a reduction in cement mantle fatigue life of 9% to 11% for a high cup penetration rate. The effect of using a thin (2 mm) over a thick (4 mm) cement mantle also reduced cement mantle fatigue life between 9% and 11%, and greatly raised cancellous bone stresses. Cement porosity was found to have very little effect on cement mantle fatigue life. Failure modes and cement stresses involved suggest that only extreme combinations of a thin cement mantle and high cup penetration may lead to mechanical failure of the cement mantle, thereby allowing wear debris access to the cement-bone interface. A thin cement mantle may also lead to the mechanical overload of the cement-bone interface. In this manner, the authors suggest that the mechanical factors may contribute to the failure mode of cemented polyethylene cups.

Comparison of two methods of fatigue testing bone cement

Two different methods have been used to fatigue test four bone cements. Each method has been used previously, but the results have not been compared. The ISO 527-based method tests a minimum of 10 samples over a single stress range in tension only and uses Weibull analysis to calculate the median number of cycles to failure and the Weibull modulus. The ASTM F2118 test regime uses fewer specimens at various stress levels tested in fully reversed tension-compression, and generates a stress vs. number of cycles to failure (S-N) or Wöhler curve. Data from specimens with pores greater than 1mm across is rejected. The ISO 527-based test while quicker to perform, provides only tensile fatigue data, but the material tested includes pores, thus the cement is closer to cement in clinical application. The ASTM regime uses tension and compression loading and multiple stress levels, thus is closer to physiological loading, but excludes specimens with defects obviously greater than 1mm, so is less representative of cement in vivo. The fatigue lives between the cements were up to a factor 15 different for the single stress level tension only tests, while they were only a factor of 2 different in the fully reversed tension-compression testing. The ISO 527-based results are more sensitive to surface flaws, thus the differences found using ASTM F2118 are more indicative of differences in the fatigue lives. However, ISO 527-based tests are quicker, so are useful for initial screening.

Physical aging and the creep behavior of acrylic bone cements

The creep deformation of two acrylic bone cements, Palacos R+G and SmartSet GHV, was investigated for different physical aging times ranging from 45 min to 2 (1/2) years. The experiments were carried out in a three-point-bending set-up in 37 degrees C Ringer's solution applying 10 MPa or 25 MPa creep loads. Both bone cements exhibit a significant decrease of their creep compliance with increasing physical aging time. The experimental data were analyzed with a creep law discussed in the context of physical aging by Struik, and a modified Burgers' model which can be used to separate the strain response of the bone cements into an elastic, a visco-elastic and a creep component. The creep behavior of the bone cements could be described essentially with only one parameter of Struik's creep law. The analysis with the modified Burgers' model showed that physical aging influences all model parameters which are directly related to the mobility of the polymer chains. The effect of physical aging should be taken into account particularly if the mechanical performance of bone cements shortly after curing is investigated.

Statistical distribution of the fatigue strength of porous bone cement

This paper reports on the damaging effects of different percentage porosities on the fatigue life of acrylic bone cement as used in the fixation of orthopaedic implants. Both hand-mixed (HM) and vacuum-mixed (VM) specimens containing different levels of porosity were fatigue tested to failure. A negative correlation between porosity level and fatigue life was demonstrated for both techniques. Considerable scatter was present in the data. Using the pore size distributions for HM and VM cement virtual HM and VM specimens were created containing various levels of porosity. Incorporating the effect of pore size and pore clustering quantified previously using the theory of critical distances a fatigue life prediction could be obtained for the virtual specimens. The virtual data agreed strongly with the experimental findings, predicting the correlation and more significantly the scatter in the experimental results. Using the virtual porosity failure model, it was demonstrated that given a constant porosity the fatigue life can vary by over an order of magnitude in both HM and VM cement. This suggests that not only porosity level but pore size distribution is extremely important in controlling the fatigue life of bone cement. It was verified that pore clustering and pore size are the major contributors to failure in HM and VM cement respectively. Furthermore, given the beneficial effects of porosity it has been proposed that an even distribution of small pores would provide an optimal bone cement mantle. Using the virtual model, it was determined that neither technique was capable of achieving such a distribution indicating a need for a new more reliable technique. The TCD based virtual porosity failure model should prove to be a powerful tool in the design of such a technique.

Accounting for inclusions and voids allows the prediction of tensile fatigue life of bone cement

Previous attempts by researchers to predict the fatigue behavior of bone cement have been capable of predicting the location of final failure in complex geometries but incapable of predicting cement fatigue life to the right order of magnitude of loading cycles. This has been attributed to a failure to model the internal defects present in bone cement and their associated stress singularities. In this study, dog-bone-shaped specimens of bone cement were micro-computed-tomography (microCT) scanned to generate computational finite element (FE) models before uniaxial tensile fatigue testing. Acoustic emission (AE) monitoring was used to locate damage events in real time during tensile fatigue tests and to facilitate a comparison with the damage predicted in FE simulations of the same tests. By tracking both acoustic emissions and predicted damage back to microCT scans, barium sulfate (BaSO(4)) agglomerates were found not to be significant in determining fatigue life (p=0.0604) of specimens. Both the experimental and numerical studies showed that diffuse damage occurred throughout the gauge length. A good linear correlation (R(2)=0.70, p=0.0252) was found between the experimental and the predicted tensile fatigue life. Although the FE models were not always able to predict the correct failure location, damage was predicted in simulations at areas identified as experiencing damage using AE monitoring.

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.

Crack initiation processes in acrylic bone cement

A major constraint in improving the understanding of the micromechanics of the fatigue failure process and, hence, in optimizing bone cement performance is found in the uncertainties associated with monitoring the evolution of the internal defects that are believed to dominate in vivo failure. The present study aimed to synthesize high resolution imaging with complementary damage monitoring/detection techniques. As a result, evidence of the chronology of failure has been obtained. The earliest stages of crack initiation have been captured and it is proposed that, in the presence of a pore, crack initiation may occur away from the pore due to the combined influence of pore morphology and the presence of defects within regions of stress concentration. Furthermore, experimental evidence shows that large agglomerations of BaSO(4) are subject to microcracking during fatigue, although in the majority of cases, these are not the primary cause of failure. It is proposed that cracks may then remain contained within the agglomerations because of the clamping effect of the matrix during volumetric shrinkage upon curing.

Quantitative analysis of the effect of porosity on the fatigue strength of bone cement

This paper reports on the effects of porosity and its distribution on the fatigue strength of bone cement. Hand-mixed (HM) and vacuum-mixed (VM) bone cement samples were fatigue tested to failure. The point of failure commonly coincided with large single pores (in the VM materials) and multiple pores in clusters (in the HM material). The effect of pores was analysed using the Theory of Critical Distances (TCD), a theory previously developed to explain the effect of notches and other stress concentrations on fatigue and fracture. Clusters of pores were analysed by developing a criterion to decide whether local cracking would act to link pores together, forming a single stress concentration of more complex shape. This approach enabled us to predict the high-cycle fatigue strength of samples containing clusters of pores, with good accuracy (errors less than 13%). We then used the analysis to develop general rules for the effect of pore size and proximity on fatigue strength. For example, we showed that a single pore of 2mm diameter or more would cause a significant decrease in the fatigue strength (compared to that of pore-free material); however, two pores of only 1mm diameter in close proximity would be equally damaging. This demonstrates the importance not only of pore size but also of pore density and distribution. However, pores do have beneficial effects such as improved drug dispersion, bone ingrowth and crack tip blunting. Therefore, given the findings from this study, a possible step forward in the development of surgical bone cements may involve a compromise in which relatively small pores are evenly distributed throughout the material.

Incorporation of large amounts of gentamicin sulphate into acrylic bone cement: Effect on handling and mechanical properties, antibiotic release, and biofilm formation

Bacterial infection remains a significant complication following total joint replacement. If infection is suspected when revision surgery is being performed, a large dose of antibiotic, usually gentamicin sulphate, is often blended with the acrylic bone cement powder in an attempt to reduce the risk of recurrent infection. In this in-vitro study the effect of small and large doses of gentamicin sulphate on the handling and mechanical properties of the cement, gentamicin release from the cement, and in-vitro biofilm formation by clinical Staphylococcus spp. isolates on the cement was determined. An increase in gentamicin loading of 1, 2, 3, or 4 g, in a cement powder mass of 40 g, resulted in a significant decrease in the compressive and four-point bending strength, but a significant increase in the amount of gentamicin released over a 72 h period. When overt infection was modelled, using Staphylococcus spp. clinical isolates at an inoculum of 1×107 colony-forming units/ml, an increase in the amount of gentamicin (1, 2, 3, or 4 g) added to 40 g of poly(methyl methacrylate) cement resulted in an initial decrease in bacterial colonization but this beneficial effect was no longer apparent by 72 h, with the bacterial strains forming biofilms on the cements despite the release of high levels of gentamicin. The findings suggest that orthopaedic surgeons should carefully consider the clinical consequences of blending large doses (1 g or more per 40 g of poly(methyl methacrylate)) of gentamicin into Palacos® R bone cement for use in revision surgery as the increased gentamicin loading does not prevent bacterial biofilm formation and the effect on the mechanical properties could be important to the longevity of the prosthetic joint.

In vitro study of the efficacy of acrylic bone cement loaded with supplementary amounts of gentamicin: effect on mechanical properties, antibiotic release, and biofilm formation

BACKGROUND

Infection remains a severe complication following a total hip replacement. If infection is suspected when revision surgery is being performed, additional gentamicin is often added to the cement on an ad hoc basis in an attempt to reduce the risk of recurrent infection.

METHODS AND RESULTS

In this in vitro study, we determined the effect of incorporating additional gentamicin on the mechanical properties of cement. We also determined the degree of gentamicin release from cement, and also the extent to which biofilms of clinical Staphylococcus spp. isolates form on cement in vitro. When gentamicin was added to unloaded cement (1-4 g), there was a significant reduction in the mechanical performance of the loaded cements compared to unloaded cement. A significant increase in gentamicin release from the cement over 72 h was apparent, with the amount of gentamicin released increasing significantly with each additional 1 g of gentamicin added. When overt infection was modeled, the incorporation of additional gentamicin did result in an initial reduction in bacterial colonization, but this beneficial effect was no longer apparent by 72 h, with the clinical strains forming biofilms on the cements despite the release of high levels of gentamicin.

INTERPRETATION

Our findings indicate that the addition of large amounts of gentamicin to cement is unlikely to eradicate bacteria present as a result of an overt infection of an existing implant, and could result in failure of the prosthetic joint because of a reduction in mechanical performance of the bone cement.

Fatigue crack propagation under variable amplitude loading in PMMA and bone cement

Fatigue failure of PMMA bone cement is an important factor in the failure of cemented joint replacements. Although these devices experience widely varying loads within the body, there has been little or no study of the effects of variable amplitude loading (VAL) on fatigue damage development. Fatigue crack propagation tests were undertaken using CT specimens made from pure PMMA and Palacos R bone cement. In PMMA, constant amplitude loading tests were carried out at R- ratios ranging from 0.1 to 0.9, and VAL tests at R = 0.1 with 30% overloads every 100 cycles. Palacos R specimens were tested with and without overloads every 100 cycles and with a simplified load spectrum representing daily activities. The R- ratio had a pronounced effect on crack propagation in PMMA consistent with the effects of slow crack growth under constant load. Single overloads caused pronounced crack retardation, especially at low da/dN. In Palacos R, similar overloads had little effect, whilst individual overloads at low da/dN caused pronounced acceleration and spectrum loading retarded crack growth relative to Paris Law predictions. These results demonstrate that VAL can have dramatic effects on crack growth, which should be considered when testing bone cements.

Analysis of rheological properties of bone cements

The rheological properties of three commercially available bone cements, CMW 1, Palacos R and Cemex ISOPLASTIC, were investigated. Testing was undertaken at both 25 and 37 degrees C using an oscillating parallel plate rheometer. Results showed that the three high viscosity cements exhibited distinct differences in curing rate, with CMW 1 curing in 8.7 min, Palacos R and Cemex ISOPLASTIC in 13 min at 25 degrees C. Furthermore it was found that these curing rates were strongly temperature dependent, with curing rates being halved at 37 degrees C. By monitoring the change of viscosity with time over the entire curing process, the results showed that these cements had differing viscosity profiles and hence exhibit very different handling characteristics. However, all the cements reached the same maximum viscosity of 75 x 10(3) Pa s. Also, the change in elastic/viscous moduli and tan delta with time, show the cements changing from a viscous material to an elastic solid with a clear peak in the viscous modulus during the latter stages of curing. These results give valuable information about the changes in rheological properties for each commercial bone cement, especially during the final curing process.

Ultrasonic characterization of the mechanical properties and polymerization reaction of acrylic-based bone cements

In this investigation the pulse-echo technique was validated as a method that could be used to monitor the complete polymerization of acrylic bone cement in a surgical theatre. Currently, orthopaedic surgeons have no objective method to quantify the state of cure of bone cement as it progresses through its polymerization cycle. Clear benefits of the pulse-echo technique are that it is easy to use, non-invasive, and non-destructive. Furthermore, the test results were found to be highly reproducible with minor deviations. Three proprietary cements were used to confirm the validity of the technique; CMW Endurance, Palacos R and Simplex P. The results showed that the acoustic properties of bone cement clearly demonstrated a relationship with the different stages of polymerization, and in particular with the transitions between the waiting, dough, and setting phases. Additionally, the cure time of the poly(methyl methacrylate) cements consistently correlated with the attainment of 75 per cent of the average maximum velocity of sound value. The measured cure times concurred with the ISO and ASTM standards. Moreover, measurements of the final sound velocity and broadband ultrasonic attenuation correlated strongly with the density and mechanical properties of the cured bone cement samples.

Effect of surface modification of polymer beads on the mechanical properties of acrylic bone cement

The effect of surface modification of polymer filler on the static mechanical properties of acrylic bone cement was studied. The surface of polymer beads was modified with carboxylic and amino groups by photochemical reaction with azide compounds. Monomer modifiers (maleic anhydride, methacrylic acid and p-aminostyrene) are attached to the functionalized surface of polymer beads. Functional allyl groups, which are capable of the graft polymerisation reaction, are attached to the surface via photochemical reaction with N-(2-nitro-4-azidophenyl)-N-(-propen) amine. This approach to bone cement provides the additional covalent bonds between the polymer beads and the inter-bead matrix. The static mechanical properties of bone cements containing modified polymer beads were investigated and compared with the static mechanical properties of unmodified cements. The absolute values of compressive strength for the modified and unmodified cements were found to be similar. An increase in flexural strength for the modified cements (dry and after water storage) was observed. The structure of the surface functional groups affects the methyl methacrylate grafting resulting in a higher value of flexural strength for the maleic anhydride- and p-aminostyrene-modified cements. The scanning electron microscopy examination of the fracture surface of the cement samples showed an improvement of the adhesion between the beads and the matrix after modification.

Augmentation of acrylic bone cement with multiwall carbon nanotubes

Acrylic bone cement, based on polymethylmethacrylate (PMMA), is a proven polymer having important applications in medicine and dentistry, but this polymer continues to have less than ideal resistance to mechanical fatigue and impact. A variety of materials have been added to bone cement to augment its mechanical strength, but none of these augmentative materials has proven successful. Carbon nanotubes, a new hollow multiwalled tubular material 10-40 nm in diameter, 10-100 microm long, and 50-100 times the strength of steel at 1/6 the weight, have emerged as a viable augmentation candidate because of their large surface area to volume ratio. The objective of this study was to determine if the addition of multiwall carbon nanotubes to bone cement can alter its static or dynamic mechanical properties. Bar-shaped specimens made from six different (0-10% by weight) concentrations of multiwall carbon nanotubes were tested to failure in quasi-static 3-point bending and in 4-point bending fatigue (5 Hz). Analyses of variance and the 3-Parameter Weibull model were used to analyze the material performance data. The 2 wt % MWNT concentration enhanced flexural strength by 12.8% (p=0.003) and produced a 13.1% enhancement in yield stress (p=0.002). Bending modulus increased slightly with the smaller (<5 wt % MWNT) concentrations, but increased 24.1% (p<0.001) in response to the 10 wt % loading. While the 2 wt % loading produced slightly improved quasi-static test results, it was associated with clearly superior fatigue performance (3.3x increase in the Weibull mean fatigue life). Weibull minimum fatigue life (No), Weibull modulus (alpha), and characteristic fatigue life (beta) for bone cement augmented with carbon nanotubes were enhanced versus that observed in the control group. These data unambiguously showed that the bone cement-MWNT polymer system has an enhanced fatigue life compared to "control" bone cement (no added nanotubes). It is concluded that specific multiwall carbon nanotube loadings can favorably improve the mechanical performance of bone cement.

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.

Effects of porosity on the fatigue performance of polymethyl methacrylate bone cement: an analytical investigation

Porosity has been shown to affect the fatigue life of bone cements, but, although vacuum mixing is widely used to reduce porosity in the clinical setting, results have been mixed and the effects of porosity are not well understood. The aim of this study was to investigate the effects of porosity using stress analysis and fracture mechanics techniques. The stress concentrations arising at voids in test specimens were found using analytical solutions and boundary element methods. The fatigue life of specimens containing voids of various sizes was predicted using fracture mechanics techniques. For spherical voids that do not occupy a significant proportion of the cross-section, the resulting stress concentration is independent of void size and too small to account for the observed crack initiation. Cracks must therefore initiate at additional stress raisers such as radiopacifier particles or additional voids. For large voids, the stress increases as the remaining cross-section of the specimen decreases, and this may account for much of the observed reduction in fatigue strength in hand-mixed cement. Although crack initiation may be largely independent of void size, there is an effect on crack growth rate. Cracks are predicted to grow faster around larger voids, since they remain in the stress concentration around the void for longer. This effect may account for the relationship between porosity and fatigue life that has been observed in samples without large voids. Since porosity appears to affect crack growth more than initiation, it may be less damaging in high-cycle clinical fatigue, which may be predominantly initiation controlled, than in short laboratory tests.

Validation of the small-punch test as a technique for characterizing the mechanical properties of acrylic bone cement

This paper examines the validity of using the small-punch test technique as a means of quantifying the mechanical properties of acrylic bone cement under different test conditions. The elastic moduli calculated using the small-punch test method were compared with data measured using the international standard for acrylic bone resin, ISO 5833. Conclusions from the study indicate that the small-punch test is a reproducible miniature specimen test method that can be used to characterize the mechanical properties of retrieved acrylic bone cement as used in total joint replacement surgery. Moreover, the test conditions were found to influence the elastic modulus of acrylic bone cement. The test temperature had a greater effect on the elastic behaviour of the bone cement than the test medium.

The effects of fluid penetration and interfacial porosity on the fixation of cemented femoral components

We have investigated the role of the penetration of saline on the shear strength of the cement-stem interface for stems inserted at room temperature and those preheated to 37°C using a variety of commercial bone cements. Immersion in saline for two weeks at 37°C reduced interfacial strength by 56% to 88% after insertion at room temperature and by 28% to 49% after preheating of the stem. The reduction in porosity as a result of preheating ranged from 71% to 100%. Increased porosity correlated with a reduction in shear strength after immersion in saline (r = 0.839, p < 0.01) indicating that interfacial porosity may act as a fluid conduit.

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.