Biomedical subjects

S M Kurtz

Publications and source records attributed to S M Kurtz.

At least 37 records · Page 2Linked to original sources

PubMed2002

Thermomechanical behavior of virgin and highly crosslinked ultra-high molecular weight polyethylene used in total joint replacements.

Three series of uniaxial tension and compression tests were conducted on two conventional and two highly crosslinked ultra-high molecular weight polyethylenes (UHMWPEs) all prepared from the same lot of medical grade GUR 1050. The conventional materials were unirradiated (control) and gamma irradiated in nitrogen with a dose of 30 kGy. The highly crosslinked UHMWPEs were gamma irradiated at room temperature with 100 kGy and then thermally processed by either annealing below the melt transition at 100 degrees C or by remelting above the melt transition at 150 degrees C. The true stress-strain behavior of the four UHMWPE materials was characterized as a function of strain rate (between 0.02 and 0.10 s(-1)) and test temperature (20-60 degrees C). Although annealing and remelting of UHMWPE are primarily considered as methods of improving oxidation resistance, thermal processing was found to significantly impact the crystallinity, and hence the mechanical behavior, of the highly crosslinked UHMWPE. The crystallinity and radiation dose were key predictors of the uniaxial yielding, plastic flow, and failure properties of conventional and highly crosslinked UHMWPEs. The thermomechanical behavior of UHMWPE was accurately predicted using an Arrhenius model, and the associated activation energies for thermal softening were related to the crystallinity of the polymers. The conventional and highly crosslinked UHMWPEs exhibited low strain rate dependence in power law relationships, comparable to metals. In light of the unifying trends observed in the true stress-strain curves of the four materials investigated in this study, both crosslinking (governed by the gamma radiation dose) and crystallinity (governed by the thermal processing) were found to be useful predictors of the mechanical behavior of UHMWPE for a wide range of test temperatures and rates. The data collected in this study will be used to develop constitutive models based on the physics of polymer systems for predicting the thermomechanical behavior of conventional and crosslinked UHMWPE used in total joint replacements.

Biocompatible Materials↗

PubMed2001

Interlaboratory reproducibility of standard accelerated aging methods for oxidation of UHMWPE.

During accelerating aging, experimental uncertainty may arise due to variability in the oxidation process, or due to limitations in the technique that is ultimately used to measure oxidation. The purpose of the present interlaboratory study was to quantify the repeatability and reproducibility of standard accelerated aging methods for ultra-high molecular weight polyethylene (UHMWPE). Sections (200 microm thick) were microtomed from the center of an extruded rod of GUR 4150 HP, gamma irradiated in air or nitrogen, and circulated to 12 institutions in the United States and Europe for characterization of oxidation before and after accelerated aging. Specimens were aged for 3 weeks at 80 degrees C in an air circulating oven or for 2 weeks at 70 degrees C in an oxygen bomb (maintained at 503 kPa (5 atm.) of O2) in accordance with the two standard protocols described in ASTM F 2003-00. FTIR spectra were collected from each specimen within 24 h of the start and finish of accelerated aging, and oxidation indices were calculated by normalizing the peak area of the carbonyl region by the reference peak areas at 1370 or 2022 cm(-1). The mean relative interlaboratory uncertainty of the oxidation data was 78.5% after oven aging and 129.1% after bomb aging. The oxidation index measurement technique was not found to be a significant factor in the reproducibility. Comparable relative intrainstitutional uncertainty was observed after oven aging and bomb aging. For both aging methods, institutions successfully discriminated between air-irradiated and control specimens. However, the large interinstitutional variation suggests that absolute performance standards for the oxidation index of UHMWPE after accelerated aging may not be practical at the present time.

Biocompatible Materials↗

PubMed2001

A small punch test technique for characterizing the elastic modulus and fracture behavior of PMMA bone cement used in total joint replacement.

Polymethylmethacrylate (PMMA) bone cement is used in total joint replacements to anchor implants to the underlying bone. Establishing and maintaining the integrity of bone cement is thus of critical importance to the long-term outcome of joint replacement surgery. The goal of the present study was to evaluate the suitability of a novel testing technique, the small punch or miniaturized disk bend test, to characterize the elastic modulus and fracture behavior of PMMA. We investigated the hypothesis that the crack initiation behavior of PMMA during the small punch test was sensitive to the test temperature. Miniature disk-shaped specimens, 0.5 mm thick and 6.4 mm in diameter, were prepared from PMMA and Simplex-P bone cement according to manufacturers' instructions. Testing was conducted at ambient and body temperatures, and the effect of test temperature on the elastic modulus and fracture behavior was statistically evaluated using analysis of variance. For both PMMA materials, the test temperature had a significant effect on elastic modulus and crack initiation behavior. At body temperature, the specimens exhibited "ductile" crack initiation, whereas at room temperature "brittle" crack initiation was observed. The small punch test was found to be a sensitive and repeatable test method for evaluating the mechanical behavior of PMMA. In light of the results of this study, future small punch testing should be conducted at body temperature.

Arthroplasty, Replacement↗

PubMed2000

Surface morphology and wear mechanisms of four clinically relevant biomaterials after hip simulator testing.

The surfaces of worn components hold clues to the underlying wear mechanisms. Previous evidence suggested that the absolute wear rates of acetabular components in a hip simulator were related to mechanical behavior; we hypothesized that the surface morphology of the liners might also be sensitive to mechanical properties. A noncontact, three-dimensional surface topography measurement system based on white light interferometry was used to quantify the surface morphology of ultra-high molecular weight polyethylene, polytetrafluoroethylene, high-density polyethylene, and polyacetal liners, and their corresponding femoral heads, after 3 million cycles in a multi-directional hip simulator. Comparisons were made with the fatigue soaked and control (as machined) components. Statistically significant power law relationships were observed between the arithmetic mean surface roughness (R(a)) of the worn acetabular liners and the volumetric wear rate in the hip simulator (p < 0.01, r(2) = 0.52). Significant relationships were also observed between R(a) and the elastic and large deformation mechanical behavior of the liner materials, measured directly from the wear-tested liners using the small punch test (p < 0.01, r(2) = 0.54-0.81). The results support the hypothesis that wear mechanisms of acetabular liners during hip simulator testing are related to surface morphology in conjunction with the mechanical behavior of the polymeric materials.

Acetals↗

PubMed2000

Degradation of mechanical behavior in UHMWPE after natural and accelerated aging.

Ultra-high molecular weight polyethylene (UHMWPE) is known to degrade during natural (shelf) aging following gamma irradiation in air, but the mechanical signature of degradation remains poorly understood. Accelerated aging methods have been developed to reproduce the natural aging process as well as to precondition total joint replacement components prior to joint simulator wear testing. In this study, we compared the mechanical behavior of naturally (shelf) aged and accelerated aged tibial inserts using a previously validated miniature specimen testing technique known as the small punch test. Tibial inserts made-of GUR 1120 and sterilized with 25 to 40 kGy of gamma radiation (in air) in 1988, 1993, and 1997 were obtained; a subset of the 1997 implants were subjected to 4 weeks of accelerated aging in air at 80 degrees C. To determine the spatial variation of mechanical properties within each insert, miniature disk shaped specimens were machined from the surface and subsurface regions of the inserts. Analysis of variance of the test data showed that aging significantly affected the small punch test measures of elastic modulus, initial load, ultimate load, ultimate displacement, and work to failure. The accelerated aging protocol was unable to reproduce the spatial mechanical profile seen in shelf aged components, but it did mechanically degrade the surface of GUR 1120 tibial components to an extent comparable to that seen after 10 years of natural aging. Test specimens showed a fracture morphology consistent with the decreased ductility and toughness which was corroborated by the small punch test metrics of this study. Our data support the hypothesis that UHMWPE undergoes a spatially nonuniform change towards a less ductile (more brittle) mechanical behavior after gamma irradiation in air and shelf aging.

Bone Substitutes↗

PubMed2000

The relationship between the clinical performance and large deformation mechanical behavior of retrieved UHMWPE tibial inserts.

Many aspects of the proposed relationship between material properties and clinical performance of UHMWPE components remain unclear. In this study, we explored the hypothesis that the clinical performance of tibial inserts is directly related to its large-deformation mechanical behavior measured near the articulating surface. Retrieval analysis was performed on three conventional UHMWPE and three Hylamer-M tibial components of the same design and manufacturer. Samples of material were then obtained from the worn regions of each implant and subjected to mechanical characterization using the small punch test. Statistically significant relationships were observed between the metrics of the small punch test and the total damage score and the burnishing damage score of the implants. We also examined the near-surface morphology of the retrievals using transmission electron microscopy. TEM analysis revealed lamellar alignment at and below the wear surfaces of the conventional UHMWPE retrievals up to a maximum depth of approximately 8 microm, consistent with large-deformation crystalline plasticity. The depth of the plasticity-induced damage layer varied not only between the retrievals, but also between the conventional UHMWPE and Hylamer-M components. Thus, the results of this study support the hypothesis that the clinical performance of UHMWPE tibial inserts is related to the large-deformation mechanical behavior measured near the articulating surface.

Biocompatible Materials↗

PubMed2000

Influence of mechanical behavior on the wear of 4 clinically relevant polymeric biomaterials in a hip simulator.

The elastic and large-deformation mechanical behavior of 4 materials with known clinical performance was examined and correlated with the wear behavior in a hip simulator. Acetabular liners of a commercially available design were machined from ultra-high molecular weight polyethylene (UHMWPE), high-density polyethylene (HDPE), polytetrafluoroethylene (PTFE), and polyacetal and wear tested in a multidirectional hip joint simulator. Elastic and large-deformation mechanical behavior was directly measured from the wear-tested liners using the small punch test. The finite element method was used to compute elastic modulus from the measured small punch test initial stiffness, and the contact stress for the liners was calculated using the theory of elasticity solution. Positive, statistically significant correlations were observed between the hip simulator wear rate and the initial peak load, ultimate load, and work to failure from the small punch test. Negative correlations were observed between the wear rate and the elastic modulus and contact stress. The results of this study support the hypothesis that the large-deformation mechanical behavior of a polymer plays a greater role in the wear mechanisms prevalent in total hip replacements than the elastic behavior.

Biocompatible Materials↗

PubMed1999

A miniature specimen mechanical testing technique scaled to articulating surface of polyethylene components for total joint arthroplasty.

The small punch test was developed to investigate the mechanical behavior of polyethylene using miniature specimens (< 14 mg) measuring 0.5 mm in thickness and 6.4 mm in diameter. The objective of this study was to demonstrate the feasibility and reproducibility of the small punch test when applied to clinically relevant polyethylenes. Mechanical behavior was characterized during 66 tests performed on GUR4150HP and GUR4120 specimens following alternate sterilization methods and 4 weeks of accelerated aging at 80 degrees C. The small punch test was found to be highly reproducible with regard to characterizing the ductility, ultimate strength, and fracture resistance of sterilized and aged polyethylene. In the future, the small punch test can be used to directly measure mechanical properties near the articulating surface of retrieved components.

Air↗

PubMed1999

Evolution of morphology in UHMWPE following accelerated aging: the effect of heating rates.

Accelerated aging methods are used to evaluate the oxidative stability of UHMWPE components for total joint replacements. In this study, we traced the evolution of the crystalline morphology during accelerated thermal aging of UHMWPE in air with the intent of explaining previous, counterintuitive heating rate effects. GUR4150HP extruded rod stock material was machined into miniature (0.5 mm thick) specimens that were either gamma irradiated in air or in nitrogen (27 +/- 3 kGy) or left unirradiated (control). Accelerated aging in an air furnace (at 80 degrees C, atmospheric pressure) was performed on half of the test samples at a heating rate of 0.1 degrees C/min and at 5 degrees C/min for the remaining half. Although the initial heating rate, as measured by changes in density, did influence the absolute degradation rate by up to 214%, the heating rate effect did not appear to influence the relative ranking of UHMWPE in terms of its oxidative stability. The heating rate effect is more consistent with a kinetic mechanism of the oxidation process than it is with a previously hypothesized diffusion mechanism. UHMWPE morphology, as characterized using a transmission electron microscope (TEM), demonstrated considerable rearrangement of the crystalline regions as a result of the accelerated aging. The stacking of the lamellae observed after accelerated aging was not consistent with the morphology of naturally aged UHMWPE components. The observed differences in crystalline morphology likely result from the enhanced mobility of the polymer chains due to thermal aging and may be analogous to an annealing process.

Algorithms↗

PubMed1999

Simulation of initial frontside and backside wear rates in a modular acetabular component with multiple screw holes.

A sliding distance-based finite element formulation was implemented to predict initial wear rates at the front and back surfaces of a commercially available modular polyethylene component during in vitro loading conditions. We found that contact area, contact stress, and wear at the back surface were more sensitive to the liner/shell conformity than the presence of multiple screw holes. Furthermore, backside linear and volumetric wear rates were at least three orders of magnitude less than respective wear estimates at the articulating surface. This discrepancy was primarily attributed to the difference in maximum sliding distances at the articulating surfaces (measured in mm) versus the back surface (measured in microm). This is the first study in which backside wear has been quantified and explicitly compared with frontside wear using clinically relevant metrics established for the articulating surface. The results of this study suggest that with a polished metal shell, the presence of screw holes does not substantially increase abrasive backside wear when compared with the effects of backside nonconformity.

Acetabulum↗

PubMed1999

Radiation and chemical crosslinking promote strain hardening behavior and molecular alignment in ultra high molecular weight polyethylene during multi-axial loading conditions.

The mechanical behavior and evolution of crystalline morphology during large deformation of eight types of virgin and crosslinked ultra high molecular weight polyethylene (UHMWPE) were studied using the small punch test and transmission electron microscopy (TEM). We investigated the hypothesis that both radiation and chemical crosslinking hinder molecular mobility at large deformations, and hence promote strain hardening and molecular alignment during the multiaxial loading of the small punch test. Chemical crosslinking of UHMWPE was performed using 0.25% dicumyl peroxide (GHR 8110, GUR 1020 and 1050), and radiation crosslinking was performed using 150 kGy of electron beam radiation (GUR 1150). Crosslinking increased the ultimate load at failure and decreased the ultimate displacement of the polyethylenes during the small punch test. Crosslinking also increased the near-ultimate hardening behavior of the polyethylenes. Transmission electron microscopy was used to characterize the crystalline morphology of the bulk material, undeformed regions of the small punch test specimens, and deformed regions of the specimens oriented perpendicular and parallel to the punch direction. In contrast with the virgin polyethylenes, which showed only subtle evidence of lamellar alignment, the crosslinked polyethylenes exhibited enhanced crystalline lamellae orientation after the small punch test, predominantly in the direction parallel to the punch direction or deformation axis. Thus, the results of this study support the hypothesis that crosslinking promotes strain hardening during multiaxial loading because of increased resistance to molecular mobility at large deformations effected by molecular alignment. The data also illustrate the sensitivity of large deformation mechanical behavior and crystalline morphology to the method of crosslinking and resin of polyethylene.

Biocompatible Materials↗

PubMed1999

Advances in the processing, sterilization, and crosslinking of ultra-high molecular weight polyethylene for total joint arthroplasty.

Despite the recognized success and worldwide acceptance of total joint arthroplasty, wear is a major obstacle limiting the longevity of implanted UHMWPE components. Efforts to solve the wear problem in UHMWPE have spurred numerous detailed studies into the structure, morphology, and mechanical properties of the polymer at every stage of its production from original resin into stock material and final fabricated form. Scientific developments in this field are occurring at an accelerating rate, and periodic review of UHMWPE technology is therefore increasingly necessary. The present article provides a four-part comprehensive review of technological advancements in the processing, manufacture, sterilization, and crosslinking of UHMWPE for total joint replacements. The first part of this article describes the recently updated nomenclature of UHMWPE, including the process of resin production and conversion to stock material. The second part outlines the methods of manufacturing UHMWPE into joint replacement components and provides overviews of alternate forms of UHMWPE, namely carbon-fiber reinforced UHMWPE (Poly II) and UHMWPE recrystallized under high temperature and pressure (Hylamer). The third part summarizes the sterilization and degradation of UHMWPE. Newly developed methods for accelerating the oxidation of UHMWPE after sterilization (for preconditioning of test specimens), as well as methods for quantifying the oxidation of UHMWPE, are also discussed. Finally, the fourth part reviews the development and properties of crosslinked UHMWPE, a promising alternate biomaterial for total joint replacements.

Biocompatible Materials↗

PubMed1999

Plasticity-induced damage layer is a precursor to wear in radiation-cross-linked UHMWPE acetabular components for total hip replacement. Ultra-high-molecular-weight polyethylene.

The mechanism for the improved wear resistance of cross-linked ultra-high-molecular-weight polyethylene (UHMWPE) remains unclear. This study investigated the effect of cross-linking achieved by gamma irradiation in nitrogen on the tribologic, mechanical, and morphologic properties of UHMWPE. The goal of this study was to relate UHMWPE properties to the wear mechanism in acetabular-bearing inserts. Wear simulation of acetabular liners was followed by detailed characterization of the mechanical behavior and crystalline morphology at the articulating surface. The wear rate was determined to be directly related to the ductility, toughness, and strain-hardening behavior of the UHMWPE. The concept of a plasticity-induced damage layer is introduced to explain the near-surface orientation of the crystalline lamellae observed in the wear-tested acetabular liners. Cross-linking reduces abrasive wear of acetabular components by substantially reducing--but not eliminating--the plasticity-induced damage layer that precedes abrasive wear.

Acetabulum↗

PubMed1999

Medical education initiatives in communication skills.

Medical educators at undergraduate, postgraduate and continuing medical education levels acknowledge that communication is a fundamental medical skill. Responding to patient, professional and governmental advocates, as well as to advances in research on patient-physician communication and its teaching, some medical educators are in the process of starting new communication curricula, while others are working at expanding, integrating and further developing already well-established programs. For most people working in this area, the question is no longer whether to teach and assess communication skills and attitudes but, rather, how to do so most efficiently and effectively. In order to enhance the development of communication curricula at all levels, we first provide a brief look at how communication education has become widely encouraged in many parts of the globe, and we set out the underlying assumptions that frame the teaching and learning of communication in medicine. We then summarize critical components common to many established communication curricula and identify a series of specific strategies for teaching communication skills. We include a chart that describes a sample of the wide variety of resources available to assist in the development and teaching of communication curricula in medicine. Finally, we consider gaps in current communication curricula and suggest the next steps and ideas for moving forwards.

Attitude of Health Personnel↗

PubMed1998

Predictive model for tensile true stress-strain behavior of chemically and mechanically degraded ultrahigh molecular weight polyethylene.

The gamma radiation sterilization of ultrahigh molecular weight polyethylene (UHMWPE) components in air generates long-lived free radicals that oxidize slowly over time during shelf storage and after implantation. To investigate the combined effects of chemical and mechanical degradation on the mechanical behavior of UHMWPE, sterilized tensile specimens were immersed in 0.5% hydrogen peroxide solution at 37 degrees C for up to 9 months and concurrently subjected to cyclic stress levels of 0 (control), 0 to 5, and 0 to 10 MPa. After chemical and mechanical preconditioning, specimen density was measured using the density gradient column technique. The true stress-strain behavior was measured up to 0.12 true strain and characterized using a multilinear material model, the parameters of which were found to vary linearly with density and cyclic stress history. The mechanical behavior of as-irradiated and degraded UHMWPE was accurately predicted by an analytical composite beam model of the tensile specimens. The results of this study support the hypothesis that chemical and mechanical degradation affect the true stress-strain behavior of UHMWPE. In the future, the material model data presented in this study will enable more accurate prediction of the stresses and strains in UHMWPE components following gamma sterilization in air and subsequent in vivo degradation.

Biocompatible Materials↗

PubMed1998

Backside nonconformity and locking restraints affect liner/shell load transfer mechanisms and relative motion in modular acetabular components for total hip replacement.

Nonconformity between the polyethylene liner and the metal shell may exist in modular acetabular components by design, due to manufacturing tolerances, or from locking mechanisms that attach the polyethylene liner to the metal shell. Relative motion at the liner/shell interface has been associated with backside wear, which may contribute to osteolysis which has been clinically observed near screw holes. The purpose of this study was to investigate the effect of nonconformity and locking restraints on the liner/shell relative motion and load transfer mechanisms in a commercially available, metal-backed acetabular component with a polar fenestration. The finite element method was used to explore the hypothesis that backside nonconformity and locking restraints play important roles in long-term surface damage mechanisms that are unique to modular components, such as backside wear and liner extrusion through screw holes. The three-body quasi-static contact problem was solved using a commercially available explicit finite element code, which modeled contact between the femoral head, polyethylene liner, and the metal shell. Four sets of liner boundary conditions were investigated: no restraints, rim restraints, equatorial restraints, and both rim and equatorial restraints. The finite element model with a conforming shell predicted between 8.5 and 12.8 microm of incremental extrusion of the polyethylene through the polar fenestration, consistent with in vitro experiments of the same design under identical loading conditions. Furthermore, idealized rim and/or equatorial liner restraints were found to share up to 71% of the load across the liner/shell interface. Consequently, the results of this study demonstrate that backside nonconformity and locking restraints substantially influence backside relative motion as well as load transfer at the liner/shell interface.

Acetabulum↗

PubMed1998

The yielding, plastic flow, and fracture behavior of ultra-high molecular weight polyethylene used in total joint replacements.

The yielding, plastic flow, and fracture behavior of UHMWPE plays an important role in wear and failure mechanisms of total joint replacement components. The primary objective of this study was to compare the yielding, plastic flow, and fracture behavior of two implantable grades of UHMWPE (GUR 1120 vs 4150 HP). The first part of this work explored the hypothesis that up to the polymer yield point, the monotonic loading behavior of UHMWPE displays similar true stress strain behavior in tension and compression. Uniaxial tension and compression tests were conducted to compare the equivalent true stress vs strain response of UHMWPE up to 0.12 true strain. During monotonic loading, the equivalent true stress strain behavior was similar in tension and compression up to the yield point. However, investigation of the unloading behavior and permanent plastic deformations showed that classical deviatoric rate independent plasticity theory may dramatically overpredict the permanent strains in UHMWPE. A secondary goal of this study was to determine the ultimate true stress and strain for UHMWPE and to characterize the fracture surfaces after failure. Using a fracture mechanics approach, the critical flaw sizes were used in combination with the true ultimate stresses to predict the fracture toughness of the two resins. A custom video-based strain measurement system was developed and validated to characterize the true stress-strain behavior up to failure and to verify the accuracy of the incompressibility assumption in calculating the true stress-strains up to failure. In a detailed uncertainty analysis, theoretical expressions were derived for the relative uncertainty in digital video-based estimates of nominal strain, true strain, homogeneous stress, and true stress. Although the yielding behavior of the two UHMWPE resins was similar, the hardening and plastic flow behavior clearly discriminated between the GUR 1120 and 4150 HP. A statistically significant difference between the fracture toughness of the two resins was also evident. The long-term goal of this research is to provide detailed true stress strain data for UHMWPE under uniaxial tension and compression for future numerical simulations and comparison with more complex multiaxial loading conditions.

Biocompatible Materials↗

PubMed1998

Postirradiation aging affects stress and strain in polyethylene components.

Ultrahigh molecular weight polyethylene components oxidatively degrade because of gamma radiation sterilization and subsequent shelf aging in air. The effects of shelf aging on the stresses and strains associated with surface damage in tibial and acetabular components were examined. A material model was developed to predict the stress and strain relationship of oxidatively degraded polyethylene as a function of density using samples of polyethylene that were gamma radiation sterilized and evaluated immediately after irradiation and after 42 months of shelf aging. The finite element method was used to determine the stresses and strains before and after shelf aging for two tibial components with different conformities between the articulating surfaces and for an acetabular component. The stresses increased by 10% to 14% in the conforming tibial model after 42 months of aging, whereas the stresses in the nonconforming tibial model and in the acetabular model increased by only 4% to 8%. Aging decreased the principal strains by 5% to 10% in both tibial models and by 15% to 17% in the acetabular model. Postirradiation aging during shelf storage of polyethylene joint components is likely to worsen long term wear, based on the increased stresses and decreased strains predicted to occur as a result of aging.

Gamma Rays↗