Biomedical subjects

S M Kurtz

Publications and source records attributed to S M Kurtz.

At least 19 recordsLinked to original sources

PubMed2006

Digital photogrammetry for quantitative wear analysis of retrieved TKA components.

The use of new materials in knee arthroplasty demands a way in which to accurately quantify wear in retrieved components. Methods such as damage scoring, coordinate measurement, and in vivo wear analysis have been used in the past. The limitations in these methods illustrate a need for a different methodology that can accurately quantify wear, which is relatively easy to perform and uses a minimal amount of expensive equipment. Off-the-shelf digital photogrammetry represents a potentially quick and easy alternative to what is readily available. Eighty tibial inserts were visually examined for front and backside wear and digitally photographed in the presence of two calibrated reference fields. All images were segmented (via manual and automated algorithms) using Adobe Photoshop and National Institute of Health ImageJ. Finally, wear was determined using ImageJ and Rhinoceros software. The absolute accuracy of the method and repeatability/reproducibility by different observers were measured in order to determine the uncertainty of wear measurements. To determine if variation in wear measurements was due to implant design, 35 implants of the three most prevalent designs were subjected to retrieval analysis. The overall accuracy of area measurements was 97.8%. The error in automated segmentation was found to be significantly lower than that of manual segmentation. The photogrammetry method was found to be reasonably accurate and repeatable in measuring 2-D areas and applicable to determining wear. There was no significant variation in uncertainty detected among different implant designs. Photogrammetry has a broad range of applicability since it is size- and design-independent. A minimal amount of off-the-shelf equipment is needed for the procedure and no proprietary knowledge of the implant is needed.

Biocompatible Materials↗

PubMed2006

Economic burden of revision hip and knee arthroplasty in Medicare enrollees.

UNLABELLED: The economic burden to Medicare due to revision arthroplasty procedures has not yet been studied systematically. The economic burden of revisions was calculated as annual reimbursements for revision arthroplasties relative to the sum total reimbursements of primary and revision arthroplasties. We evaluated this revision burden for total hip and knee arthroplasties through investigation of trends in charges and reimbursements in the Medicare population (Parts A and B claims from 1997-2003), while taking into account age and gender effects. Mean annual economic revision burdens were 18.8% (range, 17.4-20.2%) and 8.2% (range, 7.5-9.2%) for total hip arthroplasties and total knee arthroplasties, respectively. Procedural charges increased while reimbursements decreased over the study period, with higher charges observed for revisions than primary arthroplasties. Reimbursements per procedure were 62% to 68% less than associated charges for primary and revision total hip and knee arthroplasties. The effect of age and gender on reimbursements varied by procedure type. Unless some limiting mechanism is implemented to reduce the incidence of revision surgeries, the diverging trends in reimbursements and charges for total hip and knee arthroplasties indicate that the economic impact to the Medicare population and healthcare system will continue to increase. LEVEL OF EVIDENCE: Prognostic study, level II-1 (retrospective study). See Guidelines for Authors for a complete description of levels of evidence.

Aged↗

PubMed2006

Biomechanics of the Birmingham hip resurfacing arthroplasty.

The effects of the method of fixation and interface conditions on the biomechanics of the femoral component of the Birmingham hip resurfacing arthroplasty were examined using a highly detailed three-dimensional computer model of the hip. Stresses and strains in the proximal femur were compared for the natural femur and for the femur resurfaced with the Birmingham hip resurfacing. A comparison of cemented versus uncemented fixation showed no advantage of either with regard to bone loading. When the Birmingham hip resurfacing femoral component was fixed to bone, proximal femoral stresses and strains were non-physiological. Bone resorption was predicted in the inferomedial and superolateral bone within the Birmingham hip resurfacing shell. Resorption was limited to the superolateral region when the stem was not fixed. The increased bone strain observed adjacent to the distal stem should stimulate an increase in bone density at that location. The remodelling of bone seen during revision of failed Birmingham hip resurfacing implants appears to be consistent with the predictions of our finite element analysis.

Arthroplasty, Replacement, Hip↗

PubMed2005

Failure property distributions for conventional and highly crosslinked ultrahigh molecular weight polyethylenes.

To make stochastic (probabilistic) failure predictions of a conventional or highly crosslinked ultrahigh molecular weight polyethylene (UHMWPE) material, not only must a failure criterion be defined, but it is also necessary to specify a probability distribution of the failure strength. This study sought to evaluate both parametric and nonparametric statistical approaches to describing the failure properties of UHMWPE, based on the Normal and Weibull model distributions, respectively. Because fatigue and fracture properties of materials have historically been well described with the use of Weibull statistics, it was expected that a nonparametric approach would provide a better fit of the failure distributions than the parametric approach. The ultimate true stress, true strain, and ultimate chain stretch data at failure were analyzed from 60 tensile tests conducted previously. The ultimate load and ultimate displacement from 121 small punch tests conducted previously were also analyzed. It was found that both Normal and Weibull models provide a reasonable description of the central tendency of the failure distribution. The principal difference between the Normal and Weibull models can be appreciated in the predicted lower-bound response at the tail end of the distribution. The data support the use of both parametric and nonparametric methods to bracket the lower-bound failure prediction in order to simulate the failure threshold for UHMWPE.

Gamma Rays↗

PubMed2005

Validation of a micro-CT technique for measuring volumetric wear in retrieved acetabular liners.

In this study, a novel micro-CT-based technique for evaluating wear in retrieved acetabular liners was introduced and validated. Six UHMWPE acetabular components ranging in implantation time from 2.7 to 14.4 years were collected and evaluated with the use of a high-resolution micro-CT scanner. The components were scanned with a uniform volumetric resolution of 74 microns (16-bit precision) with the use of a 1,024 x 1,024 in-plane image matrix. Manual rigid 3D image registration of the interior hemispherical portion of the acetabular cup with geometric primitives by trained observers allowed for isolation, visualization, and measurement of the wear volume. Results for these six components indicated an average wear rate of 65 mm(3)/year. Overall scanner error was quantified gravimetrically and associated with a maximum uncertainty of 0.6%. Intra-- and interobserver uncertainty analysis showed the method to be both accurate and repeatable.

Acetabulum↗

PubMed2005

Notch strengthening and hardening behavior of conventional and highly crosslinked UHMWPE under applied tensile loading.

This study examined the engineering and true axial stress-strain behavior of smooth cylindrical and shallow and deep notched cylindrical test specimens, under applied axial tensile loading using non-contacting methods, of both conventional and highly crosslinked ultra-high molecular weight polyethylenes (UHMWPEs). The smooth specimens experienced a uniaxial stress state, while the notched specimens experienced a triaxial stress state in the vicinity of the notch. Materials were all prepared from a single batch of medical grade GUR 1050 resin (Ticona, Bayport, TX). The two conventional UHMWPEs were as-received (virgin) and gamma radiation sterilized at 30 kGy in a nitrogen atmosphere (radiation sterilized). The two highly crosslinked UHMWPEs were each irradiated at 100 kGy and then post-processed with one of either of the two thermal treatments: annealing, which was done below the melt transition temperature (T(m)), at 110 degrees C for two hours (110 degrees C-annealed), and remelting, which was done above T(m), at 150 degrees C (150 degrees C-remelted). All of the materials showed notch strengthening; that is, a significant elevation of axial yield properties (both engineering and true) for the shallow and deep notched conditions. Axial ultimate properties (engineering and true) were significantly decreased for the notched conditions compared with the smooth condition. Hardening ratios (both true and engineering), which are defined in this work as the ratio of ultimate stress or strain to yield stress or strain, were also found to significantly decrease with notching. The extent of change was dependent on the UHMWPE material. The micromechanism of fracture differed between the smooth and notched conditions. This study suggests that notches inherent in the design of UHMWPE joint replacement components (posts, undercuts, grooves) will have different notch sensitivities depending on the UHMWPE formulation.

Biocompatible Materials↗

PubMed2005

Static and fatigue mechanical behavior of bone cement with elevated barium sulfate content for treatment of vertebral compression fractures.

The use of bone cement to treat vertebral compression fractures in a percutaneous manner requires placement of the cement under fluoroscopic image guidance. To enhance visualization of the flow during injection and to monitor and prevent leakage beyond the confines of the vertebral body, the orthopedic community has described increasing the amount of radiopacifier in the bone cement. In this study, static tensile and compressive testing, as well as fully reversed fatigue testing, was performed on three PMMA-based bone cements. Cements tested were SimplexP with 10% barium sulfate (Stryker Orthopedics, Mahwah, NJ) which served as a control; SimplexP with 36% barium sulfate prepared according to the clinical recommendation of Theodorou et al.; and KyphX HV-R with 30% barium sulfate (Kyphon Inc., Sunnyvale, CA). Static tensile and compressive testing was performed in accordance with ASTM F451-99a. Fatigue testing was conducted in accordance with ASTM F2118-01a under fully reversed, +/-10-, +/-15-, and +/-20-MPa stress ranges. Survival analysis was performed using three-parameter Weibull modeling techniques. KyphX HV-R was found to have comparable static mechanical properties and significantly greater fatigue life than either of the two control materials evaluated in the present study. The static tensile and compressive strengths for all three PMMA-based bone cements were found to be an order of magnitude greater than the expected stress levels within a treated vertebral body. The static and fatigue testing data collected in this study indicate that bone cement can be designed with barium sulfate levels sufficiently high to permit fluoroscopic visualization while retaining the overall mechanical profile of a conventional bone cement under typical in vivo loading conditions.

Barium Sulfate↗

PubMed2005

Molecular chain stretch is a multiaxial failure criterion for conventional and highly crosslinked UHMWPE.

The development of accurate theoretical failure, fatigue, and wear models for ultra-high molecular weight polyethylene (UHMWPE) is an important step towards better understanding the micromechanisms of the surface damage that occur in load bearing orthopaedic components and improving the lifetime of joint arthoplasties. Previous attempts to analytically predict the clinically observed damage, wear, and fatigue failure modes have met with limited success due to the complicated interaction between microstructural deformations and continuum level stresses. In this work, we examined monotonic uniaxial and multiaxial loading to failure of UHMWPE using eight failure criteria (maximum principal stress, Mises stress, Tresca stress, hydrostatic stress, Coulomb stress, maximum principal strain, Mises strain, and chain stretch). The quality of the predictions of the different models was assessed by comparing uniaxial tension and small punch test data at different rates with the failure model predictions. The experimental data were obtained for two conventional (unirradiated and gamma radiation sterilized in nitrogen) and two highly crosslinked (150kGy, remelted and annealed) UHMWPE materials. Of the different failures models examined, the chain stretch failure model was found to capture uniaxial and multiaxial failure data most accurately for all of the UHMWPE materials. In addition, the chain stretch failure criterion can readily be calculated for contemporary UHMWPE materials based on available uniaxial tension data. These results lay the foundation for future developments of damage and wear models capable of predicting multiaxial failure under cyclic loading conditions.

Arthroplasty, Replacement↗

PubMed2004

[Stress levels in bones and bone cement in the thoracolumbar spine afer kyphoplasty. Finite element study].

The study quantified the stress levels in treated and untreated vertebral bodies following kyphoplasty. Three-dimensional FE models of treated and untreated T11, T12, L1, and L2 vertebral bodies were evaluated to examine the stress levels within the bone and bone cement. A validated T12-L1 model was used to investigate the effect of kyphoplasty treatment on adjacent vertebral stresses and strains. Using the single vertebral models, bone cement modulus changes had minimal effect on the stresses in the bone or the cement. The presence of bone cement had a minimal effect on the stress magnitudes or distribution in the adjacent T12 vertebra. This study provides quantification of the stress levels in bone cement and bone in vertebral bodies treated with kyphoplasty under in vivo-like loading conditions. The presence of bone cement immediately following kyphoplasty has only a slight effect on the stress magnitudes or distributions in adjacent vertebrae.

Biomechanical Phenomena↗

PubMed2004

An augmented hybrid constitutive model for simulation of unloading and cyclic loading behavior of conventional and highly crosslinked UHMWPE.

Ultra-high molecular weight polyethylene (UHMWPE) is extensively used in total joint replacements. Wear, fatigue, and fracture have limited the longevity of UHMWPE components. For this reason, significant effort has been directed towards understanding the failure and wear mechanisms of UHMWPE, both at a micro-scale and a macro-scale, within the context of joint replacements. We have previously developed, calibrated, and validated a constitutive model for predicting the loading response of conventional and highly crosslinked UHMWPE under multiaxial loading conditions (Biomaterials 24 (2003) 1365). However, to simulate in vivo changes to orthopedic components, accurate simulation of unloading behavior is of equal importance to the loading phase of the duty cycle. Consequently, in this study we have focused on understanding and predicting the mechanical response of UHMWPE during uniaxial unloading. Specifically, we have augmented our previously developed constitutive model to also allow for accurate predictions of the unloading behavior of conventional and highly crosslinked UHMWPE during cyclic loading. It is shown that our augmented hybrid model accurately captures the experimentally observed characteristics, including uniaxial cyclic loading, large strain tension, rate-effects, and multiaxial deformation histories. The augmented hybrid constitutive model will be used as a critical building block in future studies of fatigue, failure, and wear of UHMWPE.

Biocompatible Materials↗

PubMed2003

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.

Materials Testing↗

PubMed2003

Prediction of multiaxial mechanical behavior for conventional and highly crosslinked UHMWPE using a hybrid constitutive model.

The development of theoretical failure, fatigue, and wear models for ultra-high molecular weight polyethylene (UHMWPE) used in joint replacements has been hindered by the lack of a validated constitutive model that can accurately predict large deformation mechanical behavior under clinically relevant, multiaxial loading conditions. Recently, a new Hybrid constitutive model for unirradiated UHMWPE was developed Bergström et al., (Biomaterials 23 (2002) 2329) based on a physics-motivated framework which incorporates the governing micro-mechanisms of polymers into an effective and accurate continuum representation. The goal of the present study was to compare the predictive capability of the new Hybrid model with the J(2)-plasticity model for four conventional and highly crosslinked UHMWPE materials during multiaxial loading. After calibration under uniaxial loading, the predictive capabilities of the J(2)-plasticity and Hybrid model were tested by comparing the load-displacement curves from experimental multiaxial (small punch) tests with simulated load-displacement curves calculated using a finite element model of the experimental apparatus. The quality of the model predictions was quantified using the coefficient of determination (r(2)). The results of the study demonstrate that the Hybrid model outperforms the J(2)-plasticity model both for combined uniaxial tension and compression predictions and for simulating multiaxial large deformation mechanical behavior produced by the small punch test. The results further suggest that the parameters of the HM may be generalizable for a wide range of conventional, highly crosslinked, and thermally treated UHMWPE materials, based on the characterization of four material properties related to the elastic modulus, yield stress, rate of strain hardening, and locking stretch of the polymer chains. Most importantly, from a practical perspective, these four key material properties for the Hybrid constitutive model can be measured by relatively simple uniaxial tension or compression tests.

Biocompatible Materials↗

PubMed2003

Failure micromechanisms during uniaxial tensile fracture of conventional and highly crosslinked ultra-high molecular weight polyethylenes used in total joint replacements.

Highly crosslinked UHMWPEs have demonstrated improved in vitro wear properties; however, there is concern regarding loss of fracture resistance and ductility. The goals of this study were to evaluate the micromechanisms of failure under uniaxial tension and to determine the effect of gamma radiation-induced crosslinking and post-irradiation thermal processing on the estimated fracture toughness (Kc) of UHMWPE. Kc was estimated for two conventional and two highly crosslinked UHMWPE materials from tensile tests. A 32% decrease in Kc was found following crosslinking at 100kGy. The highly crosslinked materials also exhibited less ductile fracture behavior. Kc was slightly dependent on displacement rate but was insensitive to changes in crystallinity (and thus, to thermal processing). The same basic failure mechanism, microvoid nucleation and slow coalescence followed by comparatively rapid fracture after the defect reached a critical size, was observed for all of the conventional and highly crosslinked UHMWPE specimens. These observations will be used in the development of a theoretical failure model for highly crosslinked UHMWPE, which, in conjunction with a validated constitutive model, will provide the tools for predicting the risk of failure in orthopaedic components, fabricated from these new orthopaedic bearing materials.

Biocompatible Materials↗

PubMed2002

Interlaboratory validation of oxidation-index measurement methods for UHMWPE after long-term shelf aging.

An international oxidation index standard would greatly benefit the orthopedic community by providing a universal scale for reporting oxidation data of ultra-high molecular weight polyethylene (UHMWPE). We investigated whether severe oxidation associated with long-term shelf aging affects the repeatability and reproducibility of area-based oxidation index measurement techniques based on normalization with the use of 1370- or 2022-cm(-1) infrared (IR) absorption reference peaks. Because an oxidation index is expected to be independent of sample thickness, subsurface oxidation was examined with the use of both 100- and 200-microm-thick sections from tibial components (compression-molded GUR 1120, gamma irradiated in air) that were shelf aged for up to 11.5 years. Eight institutions in the United States and Europe participated in the present study, which was administered in accordance with ASTM E691. On average, the 100-microm-thick samples were associated with significantly greater interlaboratory relative standard uncertainty (40.3%) when compared with the 200-microm samples (21.8%, p = 0.002). In contrast, the intralaboratory relative standard uncertainty was not significantly affected by the sample thickness (p = 0.21). The oxidation index method did not significantly influence either the interlaboratory or intralaboratory relative standard uncertainty (p = 0.32 or 0.75, respectively). Our interlaboratory data suggest that with the suitable choice of specimen thickness (e.g., 200 microm) and either of the two optimal oxidation index methods, interlaboratory reproducibility of the most heavily oxidized regions in long-term shelf-aged components can be quantified with a relative standard uncertainty of 21% or less. Therefore, both the 1370-cm(-1) and the 2022-cm(-1) reference peaks appear equally suitable for use in defining a standard method for calculating an oxidation index for UHMWPE.

Artifacts↗

PubMed2002

Accelerated aging studies of UHMWPE. I. Effect of resin, processing, and radiation environment on resistance to mechanical degradation.

The resin and processing route have been identified as potential variables influencing the mechanical behavior, and hence the clinical performance, of ultra-high molecular weight polyethylene (UHMWPE) orthopedic components. Researchers have reported that components fabricated from 1900 resin may oxidize to a lesser extent than components fabricated from GUR resin during shelf aging after gamma sterilization in air. Conflicting reports on the oxidation resistance for 1900 raise the question of whether resin or manufacturing method, or an interaction between resin and manufacturing method, influences the mechanical behavior of UHMWPE. We conducted a series of accelerated aging studies (no aging, aging in oxygen or in nitrogen) to systematically examine the influence of resin (GUR or 1900), manufacturing method (bulk compression molding or extrusion), and sterilization method (none, in air, or in nitrogen) on the mechanical behavior of UHMWPE. The small punch testing technique was used to evaluate the mechanical behavior of the materials, and Fourier transform infrared spectroscopy was used to characterize the oxidation in selected samples. Our study showed that the sterilization environment, aging condition, and specimen location (surface or subsurface) significantly affected the mechanical behavior of UHMWPE. Each of the three polyethylenes evaluated seem to degrade according to a similar pathway after artificial aging in oxygen and gamma irradiation in air. The initial ability of the materials to exhibit post-yield strain hardening was significantly compromised by degradation. In general, there were only minor differences in the aging behavior of molded and extruded GUR 1050, whereas the molded 1900 material seemed to degrade slightly faster than either of the 1050 materials.

Biocompatible Materials↗

PubMed2002

Accelerated aging studies of UHMWPE. II. Virgin UHMWPE is not immune to oxidative degradation.

In Part I of this series, we showed that aging at elevated oxygen pressure is more successful at increasing the depth to which degradation occurs although it, too, generally causes greater degradation at the surface than at the subsurface. Therefore we hypothesized that thermal degradation alone, in the absence of free radicals, could be sufficient to artificially age UHMWPE in a manner analogous to natural aging. In the present study, virgin and air-irradiated UHMWPE (extruded GUR 1050 and compression-molded 1900) were aged up to 4 weeks at elevated oxygen pressure, and the mechanical behavior at the surface and subsurface was examined. All the materials were substantially degraded following 4 weeks of aging, but the spatial variations in the nonirradiated materials more closely mimicked the previously observed subsurface peak of degradation seen in naturally aged UHMWPE following irradiation in air. This aged material could provide a more realistic model for subsurface mechanical degradation, making it suitable for further mechanical testing in venues such as wear simulation.

Biocompatible Materials↗

PubMed2002

Deconvolution of surface topology for quantification of initial wear in highly cross-linked acetabular components for THA.

Evaluation of the surface morphology of short-term retrieved cross-linked acetabular components requires differentiation between the features generated during machining and the smaller-scale morphologies generated during the in vivo wear process. Previously, the distinction between the waviness of machining and the roughness of wear has been related to the grain size of the UHMWPE. Here a low-frequency cutoff is proposed, based on the maximum spectral frequency of machining marks, rather than on the grain size of the bulk UHMWPE material, as a reliable method for deconvolving machining marks from in vivo wear following short-term implantation. To this end, as-machined articulating surfaces of conventional (GUR 1050) and two groups of highly cross-linked UHMWPE acetabular components were examined to determine whether they exhibited a periodic surface morphology with a well-defined spatial frequency. The surface frequency spectra revealed low-frequency peaks associated with the machining marks, which were unique to each type of implant. Furthermore, the surface frequency spectra appeared uniform within a single group of implants. Statistically significant differences in the surface roughness and waviness were observed between the three groups of new implants. Our research suggests that machining marks can be effectively deconvolved from the articulating surface with the use of a Fourier transform algorithm with a single cutoff frequency of 0.08 1/microm, corresponding to a wavelength of 12.5 microm. The results of this study provide a unified conceptual framework for discriminating between waviness and roughness of the articulating surface for machined orthopedic components. The distinction between waviness and roughness is expected to be crucial for the comprehensive evaluation of wear surfaces after short-term implantation, when machining marks may be partially worn away or plastically deformed in vivo.

Acetabulum↗

PubMed2002

Constitutive modeling of ultra-high molecular weight polyethylene under large-deformation and cyclic loading conditions.

When subjected to a monotonically increasing deformation state, the mechanical behavior of UHMWPE is characterized by a linear elastic response followed by distributed yielding and strain hardening at large deformations. During the unloading phases of an applied cyclic deformation process, the response is characterized by nonlinear recovery driven by the release of stored internal energy. A number of different constitutive theories can be used to model these experimentally observed events. We compare the ability of the J2-plasticity theory, the "Arruda-Boyce" model, the "Hasan-Boyce" model, and the "Bergström-Boyce" model to reproduce the observed mechanical behavior of ultra-high molecular weight polyethylene (UHMWPE). In addition a new hybrid model is proposed, which incorporates many features of the previous theories. This hybrid model is shown to most effectively predict the experimentally observed mechanical behavior of UHMWPE.

Biocompatible Materials↗