On the Importance of Considering Porosity When Simulating the Fatigue of Bone Cement

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.

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.

Four decades of finite element analysis of orthopaedic devices: where are we now and what are the opportunities?

Finite element has been used for more than four decades to study and evaluate the mechanical behaviour total joint replacements. In Huiskes seminal paper "Failed innovation in total hip replacement: diagnosis and proposals for a cure", finite element modelling was one of the potential cures to avoid poorly performing designs reaching the market place. The size and sophistication of models has increased significantly since that paper and a range of techniques are available from predicting the initial mechanical environment through to advanced adaptive simulations including bone adaptation, tissue differentiation, damage accumulation and wear. However, are we any closer to FE becoming an effective screening tool for new devices? This review contains a critical analysis of currently available finite element modelling techniques including (i) development of the basic model, the application of appropriate material properties, loading and boundary conditions, (ii) describing the initial mechanical environment of the bone-implant system, (iii) capturing the time dependent behaviour in adaptive simulations, (iv) the design and implementation of computer based experiments and (v) determining suitable performance metrics. The development of the underlying tools and techniques appears to have plateaued and further advances appear to be limited either by a lack of data to populate the models or the need to better understand the fundamentals of the mechanical and biological processes. There has been progress in the design of computer based experiments. Historically, FE has been used in a similar way to in vitro tests, by running only a limited set of analyses, typically of a single bone segment or joint under idealised conditions. The power of finite element is the ability to run multiple simulations and explore the performance of a device under a variety of conditions. There has been increasing usage of design of experiments, probabilistic techniques and more recently population based modelling to account for patient and surgical variability. In order to have effective screening methods, we need to continue to develop these approaches to examine the behaviour and performance of total joint replacements and benchmark them for devices with known clinical performance. Finite element will increasingly be used in the design, development and pre-clinical testing of total joint replacements. However, simulations must include holistic, closely corroborated, multi-domain analyses which account for real world variability.

A review of probabilistic analysis in orthopaedic biomechanics

Probabilistic analysis methods are being increasingly applied in the orthopaedics and biomechanics literature to account for uncertainty and variability in subject geometries, properties of various structures, kinematics and joint loading, as well as uncertainty in implant alignment. As a complement to experiments, finite element modelling, and statistical analysis, probabilistic analysis provides a method of characterizing the potential impact of variability in parameters on performance. This paper presents an overview of probabilistic analysis and a review of biomechanics literature utilizing probabilistic methods in structural reliability, kinematics, joint mechanics, musculoskeletal modelling, and patient-specific representations. The aim of this review paper is to demonstrate the wide range of applications of probabilistic methods and to aid researchers and clinicians in better understanding probabilistic analyses.

Nondestructive evaluation of bone cement and bone cement/metal interface failure

To quantify the failure mechanisms related to the loosening of cemented hip joint replacements, novel techniques, capable of monitoring, nondestructively, the initiation and progression of failure during in vitro fatigue tests, were employed. Fatigue testing of model cement and cement-stem test pieces was monitored using acoustic emission (AE) sensors. Once damage was detected, an ultrasonic imaging system was used to obtain an image of the damage site and to measure the stiffness of the affected region. This method of examination provided a detailed insight into the internal crack propagation and delamination patterns. Initial work was conducted on bulk cement specimens subjected to bending and tension. The second stage of the work examined a model stem-cement interface under tensile opening loading conditions. A novel ultrasonic technique was used to measure the bond quality at the cement-metal interface. Progressive delamination was identified over time, and the AE technique was able to identify critical areas of delamination before they could be identified conclusively by ultrasonic imaging. The work has demonstrated the potential of the AE technique as a tool for the preclinical assessment of total hip replacements.

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.

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.

Cement mantle fatigue failure in total hip replacement: Experimental and computational testing

One possible loosening mechanism of the femoral component in total hip replacement is fatigue cracking of the cement mantle. A computational method capable of simulating this process may therefore be a useful tool in the preclinical evaluation of prospective implants. In this study, we investigated the ability of a computational method to predict fatigue cracking in experimental models of the implanted femur construct. Experimental specimens were fabricated such that cement mantle visualisation was possible throughout the test. Two different implant surface finishes were considered: grit blasted and polished. Loading was applied to represent level gait for two million cycles. Computational (finite element) models were generated to the same geometry as the experimental specimens, with residual stress and porosity simulated in the cement mantle. Cement fatigue and creep were modelled over a simulated two million cycles. For the polished stem surface finish, the predicted fracture locations in the finite element models closely matched those on the experimental specimens, and the recorded stem displacements were also comparable. For the grit blasted stem surface finish, no cement mantle fractures were predicted by the computational method, which was again in agreement with the experimental results. It was concluded that the computational method was capable of predicting cement mantle fracture and subsequent stem displacement for the structure considered.