Hip Simulator Wear Research Articles

In silico total hip replacement wear testing in the framework of ISO 14242-3 accounting for mixed elasto-hydrodynamic lubrication effects

Currently, preclinical wear tests for total hip replacements is carried out by using mechanical simulators under a simplified normal gait cycle as recommended by the international guidelines ISO-14242. Nevertheless, the wear tests have a long duration and are expensive so that the aim of this paper was to propose a novel in silico approach for wear calculation in lubricated total hip replacements.The proposed in silico simulation is based on the accurate modelling of the tribological phenomena acting in the joint during certain kinematical and dynamical conditions under non-Newtonian unsteady mixed elasto-hydrodynamic lubrication conditions (MEHL). The analyzed hip joint is modeled as soft polymeric acetabular cup, PE GUR1020 (EtO), against hard metallic CoCrMo femoral head. The calculation of the fluid film pressure field within the joint gap is achieved during the in silico simulations by imposing the ISO 14242-3 kinematics and dynamics, to evaluate the cumulated wear volume over a single gait cycle by using a modified Archard’s model.The main purpose of this study is to propose an in-silico wear calculation approach reproducing the classical in vitro wear testing of total hip replacements accounting for unsteady synovial lubrication effects between the femoral head and the acetabular cup. The predicted wear, extrapolated from a novel proposed approach based on the wear slopes, was compared with the value from in vitro simulations showing a satisfactory agreement, allowing the authors to conclude that the proposed model is suitable to be used as in silico hip joint wear simulator.

Influence of Metallic Deposition on Ceramic Femoral Heads on the Wear Behavior of Artificial Hip Joints: A Simulator Study.

Several retrieval studies have reported on metallic depositions on ceramic femoral heads, but the effect on the wear behavior of artificial hip joints has not been investigated in wear simulator studies. In the present study, retrieved ceramic heads with metallic depositions as third particles were tested against cross-linked ultra-high-molecular-weight polyethylene (UHMWPE) liners in a hip wear simulator. The amount of liner wear and expansion of metallic depositions on the heads were determined before and after wear testing with digital microscopy. The surface roughness of the heads was investigated in areas with and without metallic depositions by laser scanning microscopy. After five million load cycles, a non-significant reduction in the metallic formation on the retrieved heads was found. The metallic areas showed a higher surface roughness compared to unconcerned areas. The liners showed a higher wear rate of 1.57 ± 1.36 mg/million cycles for 28 mm heads and 2.42 ± 0.82 mg/million cycles for 36 mm heads with metallic depositions, in comparison with new ceramic heads with a 28 mm size ((−0.06 ± 0.89) mg/million cycles) and 36 mm size ((2.04 ± 0.46) mg/million cycles). Metallic transfer on ceramic heads can lead to an increased surface roughness and higher wear rates at the UHMWPE liners. Therefore, metallic contact of the ceramic femoral head should be avoided.

Influence of Different Damage Patterns of the Stem Taper on Fixation and Fracture Strength of Ceramic Ball Heads for Total Hip Replacement.

Background Modularity finds frequent application in total hip replacement, allowing a preferable individual configuration and a simplified revision by retaining the femoral stem and replacing the prosthetic head. However, micromotions within the interface between the head and the stem taper can arise, resulting in the release of wear debris and corrosion products. The aim of our experimental study was to evaluate the influence of different taper damages on the fixation and fracture stability of ceramic femoral heads, after static and dynamic implant loading. Methods Ceramic ball heads (36 mm diameter) and 12/14 stem tapers made of titanium with various mild damage patterns (intact, scratched, and truncated) were tested. The heads were assembled on the taper with a quasistatic load of 2 kN and separated into a static and a dynamic group afterwards. The dynamic group (n = 18) was loaded over 1.5 million gait cycles in a hip wear simulator (ISO 14242-1). In contrast, the static group (n = 18) was not mechanically loaded after assembly. To determine the taper stability, all heads of the dynamic and static groups were either pulled off (ASTM 2009) or turned off (ISO 7206-16). A head fracture test (ISO 7206-10) was also performed. Subsequent to the fixation stability tests, the taper surface was visually evaluated in terms of any signs of wear or corrosion after the dynamic loading. Results In 10 of the 18 cases, discoloration of the taper was determined after the dynamic loading and subsequent cleaning, indicating the first signs of corrosion. Pull-off forces as well as turn-off moments were increased between 23% and 54% after the dynamic loading compared to the unloaded tapers. No significant influence of taper damage was determined in terms of taper fixation strength. However, the taper damage led to a decrease in fracture strength by approximately 20% (scratched) and 40% (truncated), respectively. Conclusion The results suggest that careful handling and accurate manufacturing of the stem taper are crucial for the ceramic head fracture strength, even though a mild damage showed no significant influence on taper stability. Moreover, our data indicate that a further seating of the prosthetic head may occur during daily activities, when the resulting hip force increases the assembly load.

Effect of inward-outward rotation on hip wear simulation

The ISO 14242-1 standard specifies a three-axis motion for the wear testing of prosthetic hips. Multidirectionality of the relative motion and serum-based lubrication are known to be necessary for the reproduction of clinical wear mechanisms. For multidirectionality however, biaxial motion has been shown to be sufficient. To a biaxial hip joint simulator that incorporated flexion-extension (FE, range 46°) and abduction-adduction (AA, range 12°), a third motion component, inward-outward rotation (IOR, range 12°) was added according to the ISO 14242-1 standard. Due to the addition of the IOR, the wear rate of vitamin E stabilized, extensively cross-linked polyethylene (VEXLPE) liners decreased by 50 per cent. This was probably attributable to the increased linearity of the relative motion in the stance phase, caused by the simplified motion waveforms and their relative phases specified in the standard. In order not to underestimate the wear rate, the established biaxial motion is preferred.

Effect of wear, acetabular cup inclination angle, load and serum degradation on the friction of a large diameter metal-on-metal hip prosthesis

BackgroundThe large-scale clinical problem caused by unacceptable tribological behaviour of certain large diameter metal-on-metal prosthetic hips has directed attention to adverse condition testing. High metal-on-metal wear is connected with adverse reaction to metal debris. Friction is important because high friction may be associated with high wear, risk the fixation of the cup, and cause detrimental heating of periprosthetic tissues. MethodsA friction measurement system was added to a multidirectional, established hip joint wear simulator, and its functionality was evaluated. In preliminary tests, a 50 mm diameter metal-on-metal prosthesis was tested in an optimal acetabular cup inclination angle (48°) and in a steep angle (70°) using a normal peak load (2 kN) and an increased peak load (3 kN). The test length was 100 h. Long-term adverse condition tests of 3 million cycles were run for three 52 mm metal-on-metal prostheses. The lubricant was diluted calf serum at 37 °C. FindingsIn the 100 h tests, metal-on-metal frictional torque was not highly sensitive to the angle, load and serum degradation, and it was close to that of a conventional 28 mm prosthesis with a polyethylene cup, mostly below 5 Nm. However, a manyfold higher frictional torque (10 to 20 Nm) was observed in long-term metal-on-metal tests with substantial wear. InterpretationTo obtain a realistic prediction of the frictional behaviour of a hip design, long-term, multidirectional wear tests are necessary. The friction should preferably be measured during the wear test. In addition to normal conditions, adverse condition testing is strongly recommended.

An antioxidant stabilized, chemically cross-linked UHMWPE with superior toughness.

Chemical cross-linking of ultrahigh molecular weight polyethylene (UHMWPE) using an organic peroxide followed by high temperature melting results in a large increase in toughness accompanied by a decrease in cross-link density, which, surprisingly does not compromise the wear resistance. We compared the mechanical properties and wear behavior of a vitamin E blended, chemically cross-linked and high temperature melted UHMWPE produced by ram extrusion (PRX HTM) to those measured with the clinically available 100-kGy irradiated and melted UHMWPE (CISM 100). We also assessed the local biocompatibility of PRX-HTM in rabbit subcutaneous pouch and osteochondral defect models. The ultimate tensile strength and pin-on-disc wear rate were similar to CISM 100; whereas the elongation-at-break and impact toughness were much higher with PRX-HTM. The stress intensity factor range at crack inception was also higher with PRX-HTM. Accelerated aging did not result in any measurable oxidation or changes in mechanical properties. Hip simulator wear rate of acetabular liners made with PRX-HTM was 0.3 ± 0.4 mg/million-cycle, similar to that reported for CISM 100 liners. The wear particles were largely spherical with a number-averaged particle size of 0.95 μm with ~75% of particles below 1 μm. The subcutaneous and osteochondral rabbit implantations showed no histological differences between PRX-HTM and the control CISM 100. Pre-clinical wear, mechanical, and biocompatibility testing of PRX HTM showed feasibility for the use of this material as a total joint arthroplasty implant bearing surface. This process has the potential of eliminating the additional step of radiation cross-linking by combining consolidation and cross-linking while improving toughness. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater 107B: 1945-1952, 2019.

Bioceramics are Not Bioinert: The Role of Oxide and Non-Oxide Bioceramics on the Oxidation of UHMWPE Components in Artificial Joints

The following research is aimed at understanding the influence of Zirconia-Toughened Alumina (ZTA) and Silicon Nitride (Si3N4) on Ultra-High Molecular Weight Polyethylene (UHMWPE) acetabular liners. Bioceramic femoral heads were systematically tested against UHMWPE in controlled environment according to static/load-free coupling in hydrothermal environment, pin-on-ball wear testing, and hip-simulator wear testing. In addition, a retrieved ZTA femoral head has been analyzed and results have been compared to the simulations. Experimental results from X-ray photoelectron (XPS), cathodoluminescence (CL), Raman and Fourier-Transformed Infrared spectroscopy suggest that, despite conventional notions imply that bioceramics are inert, the surface chemistry of bioceramics was relevant to the oxidation rate of polyethylene liners. Non-biointertness could either be advantageous or disadvantageous toward polyethylene oxidation. The main reason resides in the peculiar chemical interactions between polyethylene and different ceramics, and, more specifically, depends on the direction of oxygen flow at the interface between the ceramic and the polymer. ZTA femoral heads were found to release a non-negligible amount of oxygen moieties from their surfaces, thus accelerating oxidative degradation of polyethylene. Conversely, Si3N4 ceramics exerted a protective role towards the polyethylene liner by scavenging oxygen from the tribolayer. The results of this work provide new insights into the interaction between bioceramics and polymers, which should also be considered when designing the next generation artificial hip joints with significantly elongated lifetimes.

Towards wear testing of high demanding daily activities on total hip replacement: preliminary results

A hip joint wear simulator is any device used to provide highly accurate predictions of the in vitro wear of a broad spectrum of bearing biomaterials. In recent years, more severe conditions have been successfully modeled and mechanical in vitro wear testing on total hip replacement is moving towards the simulation of the typical loads of high demanding daily activities including jogging, stair climbing, and other movements characteristic of daily life. In this work, we developed a flexible setup on a hip wear-testing machine capable of simulating various loading conditions simultaneously, with electrohydraulic valves capable to adapt rapidly to the input file changes. A high demanding activity protocol was developed to simulate the most important demanding activities that a prothesized patient could perform, analyzing also the amount of the various tasks accomplished in a day. The newly developed simulator showed to be highly flexible for each analysis purpose, allowing to simulate contemporaneously different loading conditions on different stations to analyze loads acting on the articular component and to precisely replicate the flexion extension condition thanks to the encoder feedback system installed.

Technical note: Comparison of metal-on-metal hip simulator wear measured by gravimetric, CMM and optical profiling methods

Simulation of wear in artificial joint implants is critical for evaluating implant designs and materials. Traditional protocols employ the gravimetric method to determine the loss of material by measuring the weight of the implant components before and after various test intervals and after the completed test. However, the gravimetric method cannot identify the location, area coverage or maximum depth of the wear and it has difficulties with proportionally small weight changes in relatively heavy implants. In this study, we compare the gravimetric method with two geometric surface methods; an optical light method (RedLux) and a coordinate measuring method (CMM). We tested ten Adept hips in a simulator for 2 million cycles (MC). Gravimetric and optical methods were performed at 0.33, 0.66, 1.00, 1.33 and 2 MC. CMM measurements were done before and after the test. A high correlation was found between the gravimetric and optical methods for both heads (R2 = 0.997) and for cups (R2 = 0.96). Both geometric methods (optical and CMM) measured more volume loss than the gravimetric method (for the heads, p = 0.004 (optical) and p = 0.08 (CMM); for the cups p = 0.01 (optical) and p = 0.003 (CMM)). Two cups recorded negative wear at 2 MC by the gravimetric method but none did by either the optical method or by CMM. The geometric methods were prone to confounding factors such as surface deformation and the gravimetric method could be confounded by protein absorption and backside wear. Both of the geometric methods were able to show the location, area covered and depth of the wear on the bearing surfaces, and track their changes during the test run; providing significant advantages to solely using the gravimetric method.

Scanning Electron Microscopy and Energy-Dispersive X-Ray Spectroscopy as a Valuable Tool to Investigate the Ultra-High-Molecular-Weight Polyethylene Wear Mechanisms and Debris in Hip Implants

BackgroundThe use of scanning electron microscopy (SEM) and energy-dispersive spectrometry (EDS) was investigated to understand the wear mechanisms from a metal-on-polyethylene bearing couple. Morphological features of femoral head acetabular liner, and isolated particles resulting from hip wear testing were evaluated. EDS was proposed to investigate the polymeric nature of the particles isolated from the wear testing. MethodsIn this work, 28-mm conventional ultra-high-molecular-weight polyethylene acetabular liners paired with metallic heads were tested in a hip wear simulator over 2 million cycles. SEM-EDS was employed to investigate wear mechanisms on hip implant components and associated wear debris. ResultsSEM showed worn surfaces for both hip components, and a significant volume of ultra-high-molecular-weight polyethylene wear particles resulting from hip wear testing. Particles were classified into 3 groups, which were then correlated to wear mechanisms. Group I had particles with smooth surfaces, group II consisted of particles with rough surfaces, and group III comprised aggregate-like particles.Group I EDS revealed that particles from groups I and II had a high C/O ratio raising a concern about the particle source. On the other hand, particles from group III had a low C/O ratio, supporting the hypothesis that they resulted from the wear of acetabular liner. Most of particles identified in group III were in the biologically active size range (0.3 to 20 μm). ConclusionThe use of optical and electron microscopy enabled the morphological characterization of worn surfaces and wear debris, while EDS was essential to elucidate the chemical composition of isolated debris.

Why Coating Technologies for Hip Replacement Systems, and the Importance of Testing Them In Vitro

The rationale behind novel surface engineering and coating technologies for implant articular surfaces is described especially for the significant number of metal-sensitive patients with total hip replacement. We list the important coating characteristics, and emphasize adhesion as the most challenging. We report on 2 large scale experimental hip wear simulator studies we conducted on samples of 2 such coating technologies implemented by external experts in their respective fields. One was a titanium nitride ceramic coating on a metal-on-metal total hip replacement, which aimed at wear reduction and a barrier against metal-ion diffusion. The other was a nanocrystalline homometallic surface treatment where same element metals were vapor-deposited in a novel ion-beam-assisted deposition process onto CoCr alloy femoral heads in CoCr metal on ultra high molecular weight polyethylene (UHMWPE) hips. The gradual reduction in crystal size toward the surface into nano size aimed at increasing hardness and reducing wear. Both test series were conducted on an AMTI hip simulator for 5 million cycles under the 14242-1 ISO standard, simulating walking gait, without any deliberate edge-loading or harsh conditions, and using uncoated specimens of identical design and materials as controls in each test. Both tests resulted in some of the coating eventually rubbing away (separating) to various levels on all specimens. The wear rate of the coated specimens was generally higher not lower than the uncoated controls, but not multiply so. Our discussion compares and contrasts our results with others, and we speculate whether a metal-sensitive arthritic patient who badly needs a hip replacement may still benefit from a coated implant despite the risk.

Wear resistance of poly(2-methacryloyloxyethyl phosphorylcholine)-grafted carbon fiber reinforced poly(ether ether ketone) liners against metal and ceramic femoral heads.

Younger, active patients who undergo total hip arthroplasty (THA) have increasing needs for wider range of motion and improved stability of the joint. Therefore, bearing materials having not only higher wear resistance but also mechanical strength are required. Carbon fiber-reinforced poly(ether ether ketone) (CFR-PEEK) is known as a super engineering plastic that has great mechanical strength. In this study, we focused on poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC)-grafted CFR-PEEK and investigated the effects of PMPC grafting and the femoral heads materials on the wear properties of CFR-PEEK liners. Compared with untreated CFR-PEEK, the PMPC-grafted CFR-PEEK surface revealed higher wettability and lower friction properties under aqueous circumstances. In the hip simulator wear test, wear particles generated from the PMPC-grafted CFR-PEEK liners were fewer than those of the untreated CFR-PEEK liners. There were no significant differences in the size and the morphology of the wear particles between the differences of PMPC-grafting and the counter femoral heads. Zirconia-toughened alumina (ZTA) femoral heads had significantly smoother surfaces compared to cobalt-chromium-molybdenum alloy femoral heads after the hip simulator test. Thus, we conclude that the bearing combination of the PMPC-grafted CFR-PEEK liner and ZTA head is expected to be a lifelong bearing interface in THA. © 2017 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 106B: 1028-1037, 2018.

CORR Insights®: Prediction of Polyethylene Wear Rates from Gait Biomechanics and Implant Positioning in Total Hip Replacement.

CORR Insights®: Prediction of Polyethylene Wear Rates from Gait Biomechanics and Implant Positioning in Total Hip Replacement.

In-vitro wear simulation of sequentially crosslinked and annealed polyethylene acetabular liners: 14 years of results

Sequentially crosslinked and annealed polyethylene (SXL) is a second generation highly crosslinked polyethylene that has been available for joint replacement with clinical success. The purpose of this study is to characterize SXL cups through an analysis of almost 14 years of historical in vitro wear simulation data. Three characteristics were evaluated in this study (1) Wear rate, (2) linear penetration and (3) shelf-life effect on wear.Specimens were SXL acetabular cups with internal diameters ranging from 22mm to 44mm, and thicknesses ranging from 3.9mm to 19.6mm. All cups were tested for 1–5 million cycles. Wear rates were tested for normality and suspected outliers were tested using the generalized extreme studentized deviate (ESD).A total of 290 samples were evaluated, which included suspected outliers. After ESD statistical outlier removal, distribution was non-normal and the range of wear rates was −4.3 to 8.2mm3/mc, with a mean of 1.8±1.6mm3/mc and median of 1.6mm3/mc. Individual bearing sizes and thicknesses reveal non-normal distributions for the 22mm, 32mm, and 40mm groups and for the 10mm thickness group. The sample with the highest calculated linear penetration rate, 22.2μm/mc, was for a 22mm bearing size with 19.6mm thickness. Before and after outlier removal, regression results of bearing size, thickness, and shelf age produce linear coefficients of correlation, R2<0.1, for all comparisons.Published clinical data performance at 5 years documented with 32mm bearings show clinical values ranging from 0.008mm/yr to 0.015mm/yr. The 32mm bearing size represents 127 samples in this data set, and the mean calculated linear penetration is 0.008mm/mc. This value compares favorably to the reported penetration values, assuming that 1 million hip wear simulation cycles approximates closely to one year of in-vivo cycles. In addition, wear analysis showed no effect of shelf-life on wear and maintained its low wear characteristics regardless of polyethylene thickness or head size.

Wear and Corrosion Resistance of Low Temperature Degradation Free ZTA for Artificial Joint

Low-temperature-degradation-free (LTD-free) zirconia-toughened alumina (ZTA) has been developed as an alternative to alumina and zirconia. It has been reported that ZTA has more beneficial mechanical characteristics than alumina, because of the stress-induced transformation and the spontaneous transformation of the zirconia phase that occurs for some ZTA in a hydrothermal environment. To achieve high-reliability artificial-joint prostheses, it is necessary to improve both the mechanical characteristics and phase stability of the joint material simultaneously. Thus, in this study, we evaluate the wear characteristics and corrosion resistance of LTD-free ZTA using a hip wear simulator, so as to confirm the long-term stability and reliability of this material in clinical use. The ZTA with advantageous mechanical characteristics used in this study has extremely high wear and corrosion resistance. Therefore, this ZTA could be quite useful as a bearing material in artificial joints for longer-term clinical use. It is also confirmed that the newly developed LTD-free ZTA is extremely stable under sliding conditions. Thus, it can be expected that the initial superior surface characteristics will be maintained through long-term clinical use.

Transfer of metallic debris after in vitro ceramic-on-metal simulation: Wear and degradation in Biolox ® Delta composite femoral heads

Abstract This study was aimed at investigating the effects of the transfer of metallic debris during an in vitro wear test on ceramic femoral heads articulating against metallic acetabular cups. In particular, Biolox® Delta ceramic femoral heads of two different diameters (32 and 36 mm, three samples of each set) were run for five million cycles onto a hip wear simulator using bovine calf serum as lubricant. Wear and degradation of Biolox® Delta Composite femoral heads were evaluated by a gravimetric method and by micro-Raman spectroscopy, which was used to investigate possible phase changes upon the particles deposit, by assessing the monoclinic zirconia content and quantifying the tetragonal → monoclinic zirconia transformation. Our results showed that after five million cycles, the 32 mm-acetabular cups lost a mass higher than the 36 mm-ones. Metal transfer was observed on all the Biolox® Delta ceramic femoral heads and determined a worsening of all roughness parameters. The micro-Raman analyses of the in vitro tested femoral heads confirmed the results previously obtained on retrievals, i.e. the stress related to metal transfer appeared to be a particularly severe condition able to induce the tetragonal→monoclinic zirconia phase transformation. The extent of the transformation appeared higher than that observed in vivo for Biolox® Delta-on-Biolox® Delta couplings. On average, the 36 mm-femoral heads seemed more detrimentally affected by metal transfer than the 32 mm-ones; one femoral head belonging to the first set of samples underwent the highest extent of phase transformation and the highest depth involved, which in the worn centre area appeared on average higher for the 36 mm-components than for the 32 mm-ones.

Friction measurement in a hip wear simulator.

A torque measurement system was added to a widely used hip wear simulator, the biaxial rocking motion device. With the rotary transducer, the frictional torque about the drive axis of the biaxial rocking motion mechanism was measured. The principle of measuring the torque about the vertical axis above the prosthetic joint, used earlier in commercial biaxial rocking motion simulators, was shown to sense only a minor part of the total frictional torque. With the present method, the total frictional torque of the prosthetic hip was measured. This was shown to consist of the torques about the vertical axis above the joint and about the leaning axis. Femoral heads made from different materials were run against conventional and crosslinked polyethylene acetabular cups in serum lubrication. Regarding the femoral head material and the type of polyethylene, there were no categorical differences in frictional torque with the exception of zirconia heads, with which the lowest values were obtained. Diamond-like carbon coating of the CoCr femoral head did not reduce friction. The friction factor was found to always decrease with increasing load. High wear could increase the frictional torque by 75%. With the present system, friction can be continuously recorded during long wear tests, so the effect of wear on friction with different prosthetic hips can be evaluated.

Can physical joint simulators be used to anticipate clinical wear problems of new joint replacement implants prior to market release?

One of the most important mandates of physical joint simulators is to provide test results that allow the implant manufacturer to anticipate and perhaps avoid clinical wear problems with their new products. This is best done before market release. This study gives four steps to follow in conducting such wear simulator testing. Two major examples involving hip wear simulators are discussed in which attempts had been made to predict clinical wear performance prior to market release. The second one, involving the DePuy ASR implant systems, is chosen for more extensive treatment by making it an illustrative example to explore whether wear simulator testing can anticipate clinical wear problems. It is concluded that hip wear simulator testing did provide data in the academic literature that indicated some risk of clinical wear problems prior to market release of the ASR implant systems. This supports the idea that physical joint simulators have an important role in the pre-market testing of new joint replacement implants.

Electrochemistry measurements during hip wear simulation for high and low femoral head assembly

Electrochemistry measurements during hip wear simulation for high and low femoral head assembly

Evaluation of Two Total Hip Bearing Materials for Resistance to Wear Using a Hip Simulator

Electron beam crosslinked ultra high molecular weight polyethylene (UHMWPE) 32 mm cups with cobalt alloy femoral heads were compared with gamma-irradiation sterilized 26 mm cups and zirconia ceramic heads in a hip wear simulator. The testing was performed for a total of ten million cycles with frequent stops for cleaning and measurement of mass losses due to wear. The results showed that the ceramic on UHMWPE bearing design exhibited higher early wear than the metal on highly crosslinked samples. Once a steady state wear rate was reached, the wear rates of the two types of hip bearing systems were similar with the ceramic on UHMPWE samples continuing to show a slightly higher rate of wear than the highly crosslinked samples. The wear rates of each of the tested systems appear to be consistent with the expectations for low rates of wear in improved hip replacement systems.