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 1 mm 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 1 mm, 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.     

Modelling evaluation of the testing condition influence on the maximum stress induced in a hip prosthesis during ISO 7206 fatigue testing

In vivo fatigue failure of hip prosthesis stems has been extensively reported in literature. The ISO 7206 international standard has been developed to assess the fatigue reliability of hip prostheses. It describes the fatigue testing apparatus and procedure and it is currently adopted by several testing laboratories throughout the world. In this work we evaluate the maximum stress in a titanium alloy commercial stem in different testing conditions, ranging within the standard specification, using the finite element method applied to a 3D model of the stem. The calculated maximum von Mises stress ranges from +4.5 to -1.5% (for different cement constraint levels) and from +6.7 to -6.8% (for different stem angular orientations) with respect to that calculated at the nominal testing conditions. The results suggest that the ISO 7206 testing specification will give experimental data of reasonable accuracy, with probably no more scatter than that found in typical specimen test results. This is particularly important in the case of components manufactured from materials showing a fatigue resistance highly sensitive to stress variations, such as the Ti6A14V alloy, for which a small increase of the maximum applied stress corresponds to a significant decrease of the statistical fatigue life.     

Estimation of the minimum number of test specimens for fatigue testing of acrylic bone cement

In the literature on fatigue testing of acrylic bone cements, data sets of various sizes have been used in different test series for the same cement formulation. There are two important consequences of this situation. First, it means that some test series last much longer than others, with all the implications for the cost of testing. Second, it makes drawing conclusions about the fatigue performance of a cement, based on the results of different literature series, a problematic issue. Clearly then, a recommendation as to what should be the minimum number of test specimens to use that would allow for confidence in the results of the statistical treatment of the test results (Gmin) would be desirable. In the present work, a method that could be used to culminate in such a recommendation is described. This method involves (i) obtaining experimental fatigue test results and (ii) analyzing those results using the Weibull probability distribution function and other statistical methods. This methodology is illustrated using fatigue life results obtained from uniaxial tension-compression fatigue tests on specimens fabricated from the polymerizing dough of one commercially available acrylic bone cement. For a tolerable error of 5%, we estimated Gmin to be either 7 (if the fatigue life results are treated using the two-parameter Weibull distribution function) or 11 (if the fatigue life results are treated using the three-parameter Weibull distribution function). To be on the conservative side, we therefore recommend that Gmin be 11. Three key limitations of the methodology presented here are discussed.     

The influence of the viscosity classification of an acrylic bone cement on its in vitro fatigue performance

The goal of the present work was to investigate the influence of the viscosity classification of an acrylic bone cement on its in vitro fatigue performance, as determined in fully-reversed tension-compression (+/-15 MPa) fatigue tests. The test matrix comprised six commercially available bone cements [Orthoset1, (OS1), Orthoset(R)3 (OS3), CemexRX (CRX), Cemex XL (CXL), Palacos R (PR) and Osteopal (OP)], two methods of mixing the cement constituents (hand-mixing and vacuum-mixing), two methods of fabricating the test specimens (direct molding and molding followed by machining), two specimen cross-sectional shapes (rectangular or "flat" and circular or "round"), and four test frequencies (1, 2, 5, and 10 Hz). In total, 185 specimens, distributed among 20 sets, were tested. The test results (number of fatigue stress cycles, N_f) were processed using the linearized transformation of the three-parameter Weibull distribution, whence estimates of the Weibull mean, N_[WM], were obtained. Statistical analysis of the ln N_f results (Mann-Whitney test; alpha<0.05) and a comparison of the N_[WM] estimates for specimen sets in which the formulations have essentially the same composition but different viscosity classification (namely, OS1 versus OS3, CRX versus CXL, and PR versus OP) showed that, in the majority of the comparisons carried out, the viscosity classification of a bone cement does not exert a significant influence on its in vitro fatigue performance.     

The relationship between stress, porosity, and nonlinear damage accumulation in acrylic bone cement.

The long-term survival of cemented hip replacements depends on the ability of the cemented fixation to resist fatigue damage. Damage has been assumed to accumulate linearly (Miner's law) even though it is unlikely to be the case in such a porous brittle material. This study addresses the nonlinear stress-dependent nature of fatigue damage accumulation in acrylic bone cement. Specimens were subjected to a zero-to-tension fatigue load in water at 37 degrees C. A total of 15 specimens were tested, i.e., five specimens at each of three stress levels. The specimens were cyclically loaded to a certain fraction of their fatigue lives and the amount of microcracking present at that time was quantified by counting each crack and measuring its length. This procedure was repeated until the specimen failed. A total of 801 cracks formed in the 15 specimens. All cracks were found to initiate at pores. Crack propagation directions were distributed normally about the direction perpendicular to the applied load at the lower stress levels, but at higher stress, the distribution tended to be broader. At higher stresses, more cracks were produced per pore. The damage accumulation process in acrylic bone cement was found to be nonlinear with the degree of nonlinearity increasing with stress. Furthermore, great variability was found which was attributed to the differences in porosity between specimens. A power law equation is given which describes the predicted relationship between damage accumulation and number of loading cycles as a function of the stress level.     

Biaxial flex-fatigue and viral penetration of natural rubber latex gloves before and after artificial aging

Barrier integrity of unaged and oven-aged (at 70 degrees C) natural rubber latex examination gloves was assessed with a biaxial flex-fatigue method where failure was detected electronically, and by live viral penetration testing performed according to a modified version of ASTM F1671-97a. When no change in barrier properties was detected during flex testing, no virus passage was found after viral challenge. Conversely, when a change in the barrier properties was indicated by the electrical signal, virus passage was found in 74% of the specimens. Flex-fatigue results indicated that unaged test specimens from powdered (PD) and powder-free (PF) nonchlorinated gloves had significantly longer fatigue lives than powder-free chlorinated (CL) gloves from the same manufacturer. Biaxial flexing of oven-aged glove specimens showed a marginal increase in fatigue life for the PF gloves, but no increase for the PD gloves. The fatigue life of the CL gloves was observed to increase significantly after oven aging. However, this appears to be due to a design feature of the test apparatus, wherein peak volume displacement of the worked specimen is held constant. An aging-induced change in the viscoelastic properties of the CL gloves-permanent deformation of the specimens early in the fatigue test-relieves the stress magnitude applied as the test progresses. Thus, permanent deformation acts as a confounding factor in measuring durability of latex gloves by fixed displacement flex-fatigue.     

Effect of test specimen cross-sectional shape on the in vitro fatigue life of acrylic bone cement

Constant-amplitude uniaxial tension-compression fatigue tests were conducted on specimens fabricated from 12 sets of acrylic bone cements, covering cement formulations with three different viscosities (so-called "high-", "medium-" and "low-viscosity" varieties), two different methods of mixing the cement constituents (so-called "hand-" and "vacuum-mixed" methods) and two test specimen shapes (rectangular-cross-sectioned or "flat" and circular-cross-sectioned or "round"). The test results-namely, the number of fatigue stress cycles, N(f)-were analyzed using the linearized transformation of the three-parameter Weibull relationship, allowing the values of the Weibull mean, N(WM), to be determined for each set. Values ranged from 14,300 to 1,284,331 for the round specimen sets and from 2898 to 72,960 for the flat specimen sets. Statistical analysis of the ln N(f) data, together with an examination of the N(WM) values, showed that, for any combination of cement formulation and mixing method, round specimens had significantly longer fatigue lives compared to flat ones. These results are explained in terms of two factors. The first is the smaller surface area of the waisted zone in the round specimens compared to that in the flat specimens (nominal value of 157mm(2) versus nominal value of 185mm(2)), leading to the possibility of fewer crack initiation sites on the round specimens compared to the flat ones. Secondly, it is postulated that the crystallinity of the round specimens was higher than that of the flat ones, a consequence of the significantly lower measured residual liquid monomer contents of the former compared to the latter (3.40+/-1.28wt%/wt compared to 3.81+/-1.48wt%/wt). The significance of the present finding is that it indicates that, for a set of bone cement formulation and experimental conditions, discriminating fatigue test results are more likely to be obtained if flat, rather than round, test specimens are used.     

皮质骨试样制备及其材料参数识别方法研究

有限元方法已经成为损伤生物力学研究的重要手段,准确的皮质骨材料本构参数对研究人体冲击损伤和人工骨骼等医学问题具有重要意义。基于长骨皮质骨的生理结构,提出批量制备高精度规则试样的加工方法,并开展准静态三点弯曲试验;采用基于梁理论和基于特定样本有限元模型优化反求两种方法,研究皮质骨材料的各向异性和区域差异,并通过线性回归分析建立两种识别方法所得材料参数的回归模型。制备横向和轴向试样各16个,所提出的试样制备方法能够保证试样间的尺寸偏差不超过4%,基于梁理论方法求得的新鲜牛股骨皮质骨轴向和横向试样的杨氏模量分别为(21.70±3.33) GPa和(14.47±2.10) GPa,极限应力分别为(249.21±38.83) MPa和(106.81±21.05) MPa,极限应变分别为1.95%±0.35%和1.06%±0.20%,与文献结果吻合。对利用优化方法和梁理论方法求得的杨氏模量、屈服应力和有效塑性应变进行线性回归分析,回归模型拟合精度分别为96.4%、98.7%和89.3%(P<0.05)。研究结果表明,所提出的试样制备方法能够高效地加工尺寸精度较高的规则试样,并良好地保持试样的生物力学特性。在皮质骨准静态三点弯曲试验中,基于特定样本有限元模型的材料参数反求方法能够识别更准确的材料本构参数,而基于梁理论识别参数的回归模型则能够快速准确地预测皮质骨的弹性模量、屈服应力和有效塑性应变。     

The use of dynamic mechanical analysis to assess the viscoelastic properties of human cortical bone

The purpose of this study was to examine the use of a dynamic mechanical analyzer (DMA) system to study the viscoelastic nature of bone. Cortical bone specimens from human femora were tested isothermally for 150 min at 37 degrees C and the loss factor (tan delta) and storage modulus (E') were measured. To explore the effects of test conditions on tan delta and E', different levels of applied stress, two specimen sizes, and two hydration conditions (wet and vacuum-dried) were evaluated. Finally, nonisothermal tests were performed, wherein specimens were heated up to 70 degrees C at different heating rates: 1 degrees C/min, 3 degrees C/min, and 5 degrees C/min. The results indicated that a threshold level of minimum applied stress was required to obtain repeatable and relatively constant values of tan delta. Specimen size did not significantly affect tan delta although it influenced E'. Moisture content had a significant effect on tan delta; vacuum-dried specimens exhibited a lower tan delta compared to wet specimens. Lastly, heating rates influenced tan delta values with lower rates producing more consistent results. The study demonstrated that DMA can be used as an effective tool to test bone.     

Screening of bone marrow and kidney transplant donors and recipients for cytomegalovirus antibody by commercial latex agglutination or competitive enzyme-linked immunosorbent assays

A commercial competitive enzyme-linked immunosorbent assay and a sensitive in-house radioimmunosorbent test detected cytomegalovirus (CMV) antibody in similar numbers of serum specimens (66 and 67 of 152, respectively) obtained from bone marrow and renal transplant donors and recipients. In contrast a commercial latex agglutination assay for CMV antibody apparently gave false positive results when testing serum specimens obtained from organ donors. The implications for CMV screening of organ transplant donors and recipients are discussed.     

Work of fracture of a composite resin: fracture-toughening mechanisms

The aim of this work was to investigate those mechanical parameters able to describe the fracture behavior of dental composite resins. A commercially available fine-particle micro-hybrid resin composite was used. Classical parameters as Young's modulus, strength distribution, and critical stress intensity factor were considered. Strength values were determined using the diametrical compression of discs test and for the critical stress intensity factor both unstable and controlled fracture tests were used. Controlled fracture tests allowed determining the work of fracture. Microstructure was studied by optical and field emission scanning electron microscopy. The obtained properties have been Young's modulus, 17.7 +/- 0.6 GPa; Weibull modulus, m = 14 (upper and lower limits for 90% confidence: 17 and 10); characteristic strength 51 MPa (upper and lower limits for 90% confidence: 53 and 49 MPa); critical stress intensity factor in mode I, K(IC) = 1.3 +/- 0.1 and work of fracture, gamma(wof) = 8-9 J/m(2). Pores and bubbles formed during the packing of the composite were identified as critical defects in the tested specimens. Crack deflection and branching have been identified as toughening mechanisms. Classical mechanical parameters (Young's modulus, hardness...) are not able to efficiently predict the major clinical failure mode of composite resins by fatigue. Work of fracture analysis, which is dependant on microstructural parameters such as particle size and shape, have to be included when testing mechanical properties of dental composite resins in future research studies.     

Occult blood testing using tetramethylbenzidine in an extraction procedure for patients on unrestricted diets

An extraction test for stool occult blood was developed using 3,3', 5,5' tetramethylbenzidine (TMB), which gave reliable results in the presence of interfering substances, such as vitamin C, meat fibers, and vegetable enzymes. The new test was applied to 1,320 fecal specimens from hospitalized patients not on dietary restrictions. Of 189 specimens unequivocally positive by slide tests, 48 were negative by the extraction test, 42 were trace reactions, and 99 were 2+ to 4+. Extraction testing was performed on 351 of the 1,082 specimens negative by slide tests. Four of these were from patients taking supplemental vitamin C. Two of these specimens were extraction test positive ( ) and two were extraction test negative. One other specimen, from a patient not on supplemental vitamin C, was extraction test positive. Forty-nine specimens gave equivocal results, with two commercial slide tests for occult blood. By the extraction test, 26 of these were negative, 13 had trace amounts of blood, and 10 were 2+ or 4+.     

Multiaxial fatigue behavior of conventional and highly crosslinked UHMWPE during cyclic small punch testing

Previous observations of reduced uniaxial elongation, fracture resistance, and crack propagation resistance of highly crosslinked ultrahigh molecular weight polyethylene (UHMWPE) have contributed to concern that the technology may not be appropriate for systems undergoing cyclic fatigue loading. Using a "total life" approach, we examined the influence of radiation crosslinking on the fatigue response of UHMWPE under cyclic loading via the small punch test. Our goal in this study was to evaluate the suitability of the small punch test for conducting miniature-specimen, cyclic loading, and fatigue experiments of conventional and highly crosslinked UHMWPE. We subjected four types of conventional and highly crosslinked UHMWPE to cyclic loading at 200 N/s and at body temperature in a small punch test apparatus. After failure, the fracture surfaces were characterized with the use of field emission scanning electron microscopy to evaluate the fatigue mechanisms. Cyclic small punch testing under load control was found to be an effective and repeatable method for relative assessment of the fatigue resistance of conventional and highly crosslinked UHMWPE specimens under multiaxial loading conditions. For each of the four conventional and highly crosslinked UHMWPE materials evaluated in this study, fatigue failures were consistently produced according to a power law relationship in the low cycle regimen, corresponding to failures below 10000 cycles. The fatigue failures were all found to be consistent with a single source of initiation and propagation to failure. Our long-term goal in this research is to develop miniature-specimen fatigue testing techniques for characterization of retrieved UHMWPE inserts.     

Lifetime predictions for resin-based composites using cyclic and dynamic fatigue

Because dental restorative materials undergo fatigue in use, testing is often performed in the laboratory to evaluate material responses to cyclic loading. The purpose of this study was to compare the lifetime predictions resulting from two methods of fatigue testing: dynamic and cyclic fatigue. Model composites were made in which one variable was the presence of a silanizing agent, and specimens tested in 4-point flexure. Cyclic fatigue was carried out at a frequency of 5 Hz, while dynamic fatigue testing spanned seven decades of stress rate application. Data were reduced and the crack propagation parameters for each material were calculated from both sets of fatigue data. These parameters were then used to calculate an equivalent static tensile stress for a 5-year survival time. The 5-year survival stresses predicted by dynamic fatigue data were approximately twice those predicted by cyclic fatigue data. In the absence of filler particle silanization, the survival stress was reduced by half. Aging in a water-ethanol solution reduced the survival stresses by a factor of four to five. Cyclic fatigue is a more conservative means of predicting lifetimes of resin-based composites.     

Mechanical characterization in shear of human femoral cancellous bone: torsion and shear tests

In order to investigate and compare the mechanical behaviour of human cancellous bone during different shear loading modes, two tests were performed to characterise human femoral cancellous bone in shear: a torsion test until failure and a shear test using a sharpened stainless steel tube. Paired cylindrical samples were core drilled from 12 human femoral heads, symmetrically with respect to the coronal plane and along the primary trabecular direction. The distal part of the sample was assigned to a torsion test and the shear test was performed on the proximal part along two perpendicular anatomical directions. Apparent densities and tissue densities were measured on both torsion and shear specimens. The mean torsion properties were shear modulus G, 289 (183) MPa, ultimate stress tau(torsion), 6.1 (2.7) MPa, ultimate strain gamma(ultimate), 4.6 (1.3)%, yield stress tau(yield), 4.3 (1.9) MPa and yield strain gamma(yield), 1.8 (0.3)%. Strong correlation was obtained between G and tau(torsion) (r'=0.853, p<0.001). These torsion properties were correlated with apparent density of torsion specimens showing, respectively: r'=0.713, p=0.005 and r'=0.671, p=0. 008. Properties from the shear test were invariable with regard to the two tested directions then isotropic ultimate shear stress and isotropic elementary shear stress, which represent the mean values of the two tested directions were, respectively, tau(shear), 10.0 (4. 5) MPa and tau(elem), 18.8 (6.1) MPa. Both shear stresses were correlated with apparent density of shear specimens: tau(shear), r'=0.564, p=0.045 and tau(elem), r'=0.636, p=0.024. Apparent densities for shear specimens were superior than for torsion specimens (p=0.06) and the comparison was the opposite for tissue densities (p=0.028), showing strong density gradients of cancellous bone in the femoral head. These torsion and shear tests which permit the evaluation of cancellous bone behavior under two different types of shear loading, may be performed on different human sites and the measured shear properties may be compared to structural properties of cancellous bone.     

Tensile characteristics of ten commercial acrylic bone cements

The mechanical properties of acrylic bone cement, used in orthopedic surgery, are very influential in determining successful long-term stability of a prosthesis. A large number of commercial formulations are available, differing in chemical composition and physical properties of both powder and monomer constituents. In this study, the static and dynamic tensile characteristics of a number of the most commonly used bone cements (Palacos R, Simplex P, CMW 1 & 3, Sulfix-60, Zimmer Dough), along with some newer formulations (Endurance, Duracem 3, Osteobondtrade mark and Boneloc), have been investigated under the same testing regimes. Testing was performed in air at room temperature. Significant differences in both static and fatigue properties were found between the various bone cements. Tensile tests revealed that Palacos R, Sulfix-60, and Simplex P had the highest values of ultimate tensile strength, closely followed by CMW 3, while Zimmer Dough cement had the lowest strength. Fatigue testing was performed under stress control, using sinusoidal loading in tension-tension, with an upper stress level of 22MPa. The two outstanding cements when tested in these cyclic conditions were Simplex P and Palacos R, with the highest values of Weibull median cycles to failure. Boneloc bone cement demonstrated the lowest cycles to failure. While the testing regimes were not designed to replicate exact conditions experienced by the bone cement mantle in vivo, there was a correlation between these results and clinical outcome.     

Compliance calibration for fracture testing of anisotropic biological materials

The compliance technique has been used to monitor crack length during fracture and fatigue testing of materials. Difficulties arise when this technique is applied to anisotropic biological materials such as bone. In this tutorial, two different methods of analyzing compliance calibration data are described: the standard ASTM method and a new approach developed by the authors specifically for anisotropic materials. An example is given showing how data from equine cortical bone can be analyzed. In this example, calibration tests were conducted on thirty-six three point bend specimens machined from the mid-diaphysis of six pairs of equine third metacarpal bones. Cracks were propagated in three orientations with respect to the long axis of the bone: transverse, longitudinal, and radial. Specimen compliance was determined for a crack range of 0.30 to 0.65 times the specimen width from load vs. crack opening displacement data. The results demonstrate that the ASTM method is not applicable to anisotropic biomaterials such as bone. Rather, it is necessary to develop separate compliance calibration equations for each crack propagation orientation investigated.     

Interfacial fracture toughness between bovine cortical bone and cements

To evaluate the bonding strength of the interfaces within the cemented arthroplasty system, various mechanical tests have been used. Conventional push-out and pull-out tests cannot reveal the actual bonding property of the interface because of the significant influence of surface roughness on the measured adhesion and the failure to account for the mismatch of elastic modulus across the interface. An alternative fracture mechanics approach, which considers the mix of opening and shear modes of the crack tip loading associated with the testing system and the elastic mismatch of materials across the interface, was used to evaluate the bonding ability of various cements. The four-point bend interfacial delamination test by Charalambides et al. (J. Appl. Mech. 56 (1989) 77; Mech. Mater. 8 (1990) 269) was used to quantify the bonding ability of cements. This method is arguably more suitable since the applied loading mode is comparable to the nature of loading within the prosthetic system, which is primarily bending. The bovine bone specimens were polished to mirror finish to eliminate bonding by mechanical interlocking. The results revealed minimal bonding for the conventional bone cement (PMMA) whereas substantial bonding was evident for the glass-ionomer cements tested. However, only the conventional glass-ionomer cements showed evidence of bonding on testing, while the resin-modified glass-ionomer cement (poly-HEMA) did not. The latter appeared to debond before testing because of excessive expansion stresses associated with swelling in water.     

Enhancing the mechanical integrity of the implant-bone interface with BoneWelding technology: determination of quasi-static interfacial strength and fatigue resistance

The BoneWelding technology is an innovative bonding method, which offers new alternatives in the treatment of fractures and other degenerative disorders of the musculoskeletal system. The BoneWelding process employs ultrasonic energy to liquefy a polymeric interface between orthopaedic implants and the host bone. Polymer penetrates the pores of the surrounding bone and, following a rapid solidification, forms a strong and uniform bond between implant and bone. Biomechanical testing was performed to determine the quasi-static push-out strength and fatigue performance of 3.5-mm-diameter polymeric dowels bonded to a bone surrogate material (Sawbones solid and cellular polyurethane foam) using the BoneWelding process. Fatigue tests were conducted over 100,000 cycles of 20-100 N loading. Mechanical test results were compared with those obtained with a comparably-sized, commercial metallic fracture fixation screw. Tests in surrogate bone material of varying density demonstrated significantly superior mechanical performance of the bonded dowels in comparison to conventional bone screws (p < 0.01), with holding strengths approaching 700 N. Even in extremely porous host material, the performance of the bonded dowels was equivalent to that of the bone screws. For both cellular and solid bone analog materials, failure always occurred within the bone analog material surrounding and distant to the implant; the infiltrated interface was stronger than the surrounding bone analog material. No significant decrease in interfacial strength was observed following conditioning in a physiological saline solution for a period of 1 month prior to testing. Ultrasonically inserted implants migrated, on average, less than 20 microm over, and interfacial stiffness remained constant the full duration of fatigue testing. With further refinement, the BoneWelding technology may offer a quicker, simpler, and more effective method for achieving strong fixation and primary stability for fracture fixation or other orthopaedic and dental implant applications.