Ultra-high Molecular Weight Polyethylene Insert Research Articles

A multiscale computational framework for wear prediction in knee replacement implants

Wear damage represents a long-term material failure that affects the service life of joint implants. Prediction of wear in bearing inserts under arbitrary joint kinematics and kinetics holds an important key to improving the performance of entire joint implant device through new material development and customized geometry design. In this work, a hierarchical multiscale computational framework that predicts the wear rate of ultra-high molecular weight polyethylene (UHMWPE) insert by resolving the microstructure details and microscale deformation mechanisms is developed. A fully dynamic finite element simulation is carried out at the macroscale level to reproduce the interactions between femoral component and UHMWPE insert according to the knee simulator input profiles. The stress evolution history from the initial wear site is extracted and applied back to the representative volume element (RVE) to further study the microscale deformation mechanisms that are related to wear damage. Activation of different slip systems in the crystalline phase as well as the interplay between the crystalline phase and amorphous phase are captured through a semi-crystal plasticity model. Results shown that improving wear resistance requires minimizing the plastic deformation upon the same external loading while maintaining a load-sharing balance between the crystalline phase and the amorphous phase. A combination of high crystallinity, well-dispersed crystallite with small size and texture alignment along the axial load direction can promote activation of chain pull slip that leads to improved wear resistance.

Investigation of wear of ultra high molecular weight polyethylene in a soft tissue behaviour knee joint prosthesis wear test simulator

Knee joint prostheses are commonly employed for therapy in over-use disorder, and as a result of traffic accident and sports injuries. However, errors that can occur due to their use need to be defined. In order to determine the types and rates of wear that might occur in prosthetic components, wear tests were performed in knee joint simulators that mimiced knee joint motion. In this study, the wear was specifically examined in ultra high molecular weight polyethylene (UHMWPE) insert material in the four-axis knee joint prosthesis wear test simulator, and the causes for it were examined. Wear tests were performed for 3 different cycles, 1 × 106, 2 × 106 and 3 × 106. Following wear tests, microscopic images, and mass loss and surface roughness measurements were taken from the UHMWPE insert condyle surface. With an increase in the number of cycles, pitting wear, scratches, agglomerated particles and delamination were seen more clearly on the UHMWPE material surface, due to repetitive forces on different axes. In addition, surface roughness measurements taken from worn surfaces increased the with the number of cycles, and flexion/extension (F/E) motion range increased with surface roughness. Wear rates in the medial region of the UHMWPE insert were higher than in the lateral region.

Effects of knee simulator control method and radiation dose on UHMWPE wear rate, and relationship between wear rate and clinical revision rate in National Joint Registry

The wear rate of five types of cruciate-retaining artificial knee joint ultrahigh-molecular-weight polyethylene (UHMWPE) inserts was examined using two custom-made knee joint simulators satisfying ISO 14243–1 (load control) and ISO 14243–3 (displacement control). The wear rate of knee joints composed of a UHMWPE insert and a Co-Cr-Mo alloy or oxidized zirconium femoral component linearly increased with increasing number of wear cycles, and the volumetric wear rate per million cycles was approximately 6–18 mm3/Mc. The wear rate was the lowest in the highly crosslinked knee joint irradiated at 90 kGy (Scorpio) among the five UHMWPE inserts. The extent of oxidation in UHMWPE after a knee simulator test of up to 5 × 106 cycles was small. The load-controlled wear rates measured in this work were close to the displacement-controlled wear rates reported in the literature. The effect of the control method on the wear rate was small for Nexgen and Scorpio knee joints. However, it was larger for the PFC Sigma knee joint having a high curvature of the surface. The wear rate of various knee joints made of highly crosslinked UHMWPE (XLPE) markedly decreased when they were subjected to a radiation dose of 40 kGy or more. The 10-year cumulative percentage revision rate since the primary operation slightly decreased with decreasing volumetric knee simulator wear rate for conventional UHMWPE (CPE) and XLPE knee joint inserts. The XLPE knee joint was shown to exhibit reduced in vivo wear and lower rates of revision for total knee replacement. On the other hand, Nexgen and PFC Sigma (both CPE) knee joints showed the lowest revision rate in the AOA and NJR national joint replacement registries. The volumetric wear rates of 3 mm3/Mc for XLPE and 15 mm3/Mc for CPE knee joint inserts are recommended as a goal for the development of new knee joints.

Effect of head size on wear properties of metal-on-metal bearings of hip prostheses, and comparison with wear properties of metal-on-polyethylene bearings using hip simulator

The effects of articular head size on the wear losses of the metal insert and articular head for a metal-on-metal bearing were examined using a hip simulator manufactured to satisfy ISO 14242-1. The wear properties of metal-on-metal and metal-on-polyethylene bearings were also compared under the same conditions. The total wear losses of the metal insert and articular head decreased with increasing diameter of the metal insert in the range from 28 to 44mm. The total wear loss was greater for a diameter of 48mm than for a diameter of 44mm. When the articular metal insert diameter was smaller than 44mm, the wear loss was reduced because the contact surface pressure increased with increasing metal insert diameter. However, the increase in wear loss observed for the 48-mm-diameter insert might have been due to the considerable increase in the rotation moment with increasing insert diameter. The tendency of decreasing contact pressure calculated using the Hertzian contact stress equation nearly conformed to the change in wear loss. On the other hand, the wear loss of an artificial hip joint consisting of a cross-linked ultrahigh-molecular-weight polyethylene insert (UHMWPE) and a Co–Cr–Mo articular head was small.

Material and surface factors influencing backside fretting wear in total knee replacement tibial components

Retrieval studies have shown that the interface between the ultra-high molecular weight polyethylene insert and metal tibial tray of fixed-bearing total knee replacement components can be a source of substantial amounts of wear debris due to fretting micromotion. We assessed fretting wear of polyethylene against metal as a function of metal surface finish, alloy, and micromotion amplitude, using a three-station pin-on-disc fretting wear simulator. Overall, the greatest reduction in polyethylene wear was achieved by highly polishing the metal surface. For example, highly polished titanium alloy surfaces produced nearly 20 times less polyethylene wear compared with blasted titanium alloy, whereas, decreasing the micromotion amplitude from 200 to 50 μm produced approximately four times less polyethylene wear for the same blasted titanium alloy surface. Although the effect of the metal alloy was much smaller than the effect of metal surface roughness or the micromotion amplitude, CoCr discs produced slightly greater polyethylene fretting wear than titanium alloy discs under each condition. The results are essential in design and manufacturing decisions related to fixed-bearing total knee replacements.

Mechanical Studies on Biomaterials for Reconstruction of Lower Limb Functions

In this paper, mechanical studies on biomaterials for reconstruction and regeneration of bone and joints of lower limb are summarized. Finite element analyses of knee and hip prostheses were performed to characterize the mechanical conditions of the artificial components and the mechanical interaction between bone and prosthesis. The computational results showed that for the knee prosthesis, the transitional behavior of stress concentration on ultra-high molecular weight polyethylene insert corresponded to actual damage formations, and for the hip prosthesis, the disturbed stress on the injured acetabulum was effectively reduced by the joint replacement, however, stress shielding effect in the femur was also induced by stem insertion. Strength enhancement of bioabsorbable porous poly(L-lactide) scaffold was also investigated using layered structurization. It was found that those properties were effectively improved by introducing layered structures.

The influence of tibial tray design on the wear of fixed-bearing total knee replacements

Debris-induced osteolysis due to surface wear is a potential long-term problem in total knee replacements (TKRs). Wear between the tibial tray and ultra-high molecular weight polyethylene insert is thought to contribute to the wear. This study investigated the influence of tibial tray design on the wear of fixed-bearing TKRs. Specifically, this study investigated the influence of the material's surface finish and design characteristics of the locking mechanism of the tibial tray on the wear in fixed-bearing knees for both cruciate-retaining (CR) and posterior-stabilized designs. A new fixed-bearing tibial tray design using Co-Cr and with an improved locking mechanism significantly reduced polyethylene wear from 22.8 +/- 6.0 mm3 per 10(6) cycles to 15.9 +/- 2.9 mm3 per 10(6) cycles compared with a previous titanium alloy tray with a CR design. The wear rates were similar to those of a fixed-bearing insert clamped into a tibial tray, suggesting that the decrease in wear was due to a reduction in backside wear. There was no significant difference between the wear rates of a cruciate-retaining design and a posterior-stabilized design under the two kinematic conditions tested.

The effect of oxidative aging on the wear performance of highly crosslinked polyethylene knee inserts under conditions of severe malalignment

First-generation crosslinked polyethylene developed for total hip arthroplasty has not gained wide acceptance for knee arthroplasty because of the increased potential for failure under high stresses and the increased risk for oxidative damage caused by free radicals. Sequentially crosslinked polyethylene (SQXL) is a second-generation crosslinked polyethylene that is reported to reduce the level of free radicals and preserve mechanical properties. Three groups of ultrahigh molecular weight polyethylene inserts were wear tested after artificial aging as per ASTM F2003: gamma-irradiated in air (GA-aged), sequentially crosslinked (SQXL-aged), and electron-beam irradiated and remelted (EBeam-aged). Inserts were tested in an AMTI knee wear simulator under malalignment conditions that were two standard deviations from the mean reported for computer-navigated and conventionally aligned techniques. GA-aged inserts delaminated by 500,000 cycles, and were severely damaged after 1 million cycles. None of the highly crosslinked inserts (SQXL-aged or EBeam-aged) delaminated or showed any signs of severe wear. Mean wear rate for GA-aged inserts was 124.6 +/- 49.4 mg/million cycles. Mean wear rates for SQXL-aged and EBeam-aged inserts were significantly lower (1.74 +/- 0.3 and 4.72 +/- 0.7, respectively). These results support the low levels of free radicals and preservation of mechanical properties reported in second-generation crosslinked ultrahigh molecular weight polyethylene.

Ceramic Total Knee Prosthesis with Smooth Surface Keeps High Wear Resistance in Long Clinical Use

In the late 1970s, based on good clinical results in total hip prostheses, the use of a combination of alumina ceramics and ultra-high molecular weight polyethylene (UHMWPE) was begun for total knee prostheses (TKPs) in order to reduce UHMWPE wear. In this study, to examine the in vivo efficacy of the alumina ceramic bearing surfaces of TKPs, we compared retrieved alumina ceramic TKPs with cobalt-chrome (Co-Cr) alloy TKPs by surface observations and linear wear measurements. In scanning electron microscopic observations, many scratches due to clinical use were observed only on the retrieved Co-Cr alloy femoral components. The damage in the form of scratches on the articulating surface was linear and was produced by rubbing against the Co-Cr alloy surface. The linear wear rate of the retrieved Co-Cr alloy TKPs was 0.027–0.358 mm/year. In contrast, the wear of the retrieved alumina ceramic TKPs was stably low and linear; the linear wear rate was estimated to be 0.026 mm/year. The lower wear rate and milder nature of wear observed in the TKPs with the combination of UHMWPE inserts and alumina ceramic femoral components suggest the possibility of the retention of high performance even during prolonged clinical use.

Osteolysis Caused by Tibial Component Debonding in Total Knee Arthroplasty

Late failure of total knee arthroplasties usually results from ultrahigh molecular weight polyethylene wear or implant loosening. Early failure from osteolysis is uncommon. However, we treated a patient with a failed total knee arthroplasty from osteolysis that developed 2 years postoperatively. The failure was associated with tibial component debonding from the cement mantle with abundant cement and metal debris. Although there was some third-body debris in the ultrahigh molecular weight polyethylene insert surface, the insert wear was not extensive. Although abundant cement and metal debris were found in the periarticular soft tissues, no ultrahigh molecular weight polyethylene was seen in histologic specimens under polarized light. The osteolysis seems to have been caused primarily by debris generated from debonding and torsional motion at the tibial baseplate-cement interface rather than the bearing surface. Although this failure mechanism has been well recognized in cemented total hip arthroplasties, it has not been reported to be a substantial cause of failure in total knee arthroplasties.

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.

Nanotribology of CoCr–UHMWPE TJR prosthesis using atomic force microscopy

Ultra high molecular weight polyethylene (UHMWPE) submicron wear debris particles in total joint replacement (TJR) prosthesis have been observed clinically to cause osteolysis. In this study, the formation of UHMWPE submicron wear debris was investigated by modeling a nanoscale asperity on the surface of a CoCr femoral component of a total knee replacement (TKR) prosthesis using a Si 3N 4 AFM commercial tip. Contact mode atomic force microscope (AFM) was used as a tribosystem to represent CoCr–UHMWPE asperity contact in TKR prostheses in order to evaluate nanoscale coefficient of friction ‘ μ’. Nanoindentation was used to understand the mechanical response of the TJR prosthesis components. Moreover, the threshold for UHMWPE plastic deformation and subsequently wear as a function of frictional and normal load at a nanoscale between CoCr alloy and UHMWPE asperity-to-asperity contact was investigated. Results from energy dispersive X-ray (EDX) analysis and back scattering energy (BSE) technique illustrated the presence of silicon carbide (SiC) particles in the polished surface of a femoral component of total knee replacement (TKR) prosthesis. Sliding of the model asperity on direct compression moulded UHMWPE for a total normal load range of 1–11 nN caused contact pressures ranging from 14 to 80 MPa which resulted in the dynamic ‘ μ’ at the interface of the nanotribocontact equal to 0.22±0.01. Transients in frictional response of the UHMWPE as a function of normal load range (17–340 nN) during sliding against the modeled CoCr asperity were observed. The first transient in frictional force was observed at a normal load of 197±9 nN, corresponding to a contact pressure of 462±9 MPa. The second transient in the frictional force was observed at a normal load of 288±13 nN corresponding to a contact pressure of 518±13 MPa. Following the second transition in frictional response for the normal load range selected, plowing of the UHMWPE surface by the hard asperity was observed. This indicated that the deep scratches noticed in retrieved TJR prosthesis could be due to the abrasion from protruding hard carbide asperities that exist within the cast CoCr alloy of TJR prosthesis. From this study it can be stated that the plastic deformation of UHMWPE under multiaxial loading conditions (normal and shear) during sliding can be used to explain the initiation of failure of the UHMWPE insert in TJR prosthesis eventually leading to submicron wear debris generation.

Tibial Insert Undersurface as a Contributing Source of Polyethylene Wear Debris

Sixty-seven ultrahigh molecular weight polyethylene tibial inserts from cementless total knee arthroplasties were retrieved at autopsy and revision surgery and analyzed for evidence of articular and nonarticular surface wear after a mean implantation time of 62.8 months (range, 4-131 months). Polyethylene cold flow and abrasive wear on the nonarticular insert surface (undersurface) were assigned a wear severity score (Grade 0-4). The severity of articular wear was assessed quantitatively and graded. Corresponding prerevision radiographs were evaluated for evidence of tibial metaphyseal osteolysis and osteolysis around tibial fixation screws. Exact nonparametric conditional inference methods were used to establish correlations between different variables and the occurrence of tibial metaphyseal osteolysis. Severe Grade 4 wear of the tibial insert undersurface was associated with tibial metaphyseal osteolysis or osteolysis around fixation screws. Time in situ statistically was related to Grade 4 undersurface wear and tibial metaphyseal osteolysis. The occurrence of tibial osteolysis was not related statistically to articular wear severity, insert thickness, or implant type. The main articulation between the femoral implant and ultrahigh molecular weight polyethylene insert has been assumed to be the primary source of polyethylene debris contributing to osteolysis and total knee arthroplasty implant failure. The undersurface of the insert is an additional source of polyethylene debris contributing to tibial metaphyseal osteolysis. To lessen polyethylene debris produced at this modular interface, the tibial implant locking mechanism should fix the insert firmly to the metal backing to decrease relative micromotion. Because motion between the insert and metal backing may be inevitable, the wear characteristics of the inner tray surface should be optimized to minimize wear debris production at this other articulation.