The effect of ultra-high molecular weight polyethylene wear debris on MG63 osteosarcoma cells in vitro

BACKGROUND: Focal osteolysis due to ultra-high molecular weight polyethylene wear debris involves effects on both bone resorption and bone formation. METHODS: The response of MG63 osteoblast-like osteosarcoma cells to ultra-high molecular weight polyethylene wear debris isolated by enzymatic digestion of granulomatous tissue obtained from the sites of failed total hip arthroplasties was examined. Scanning electron microscopy, particle-size analysis, and Fourier transform infrared spectroscopy were used to characterize the number, morphology, size distribution, and chemical composition of the particles. Cell response was assessed by adding particles at varying dilutions to confluent cultures and measuring changes in cell proliferation (number of cells and [3H]-thymidine incorporation), osteoblast function (alkaline-phosphatase-specific activity and osteocalcin production), matrix production (collagen production and proteoglycan sulfation), and local cytokine production (prostaglandin-E2 production). RESULTS: The mean size of the particles was 0.60 micrometer, and 95 percent of the particles had a size of less than 1.5 micrometers. The number of particles per gram of tissue ranged from 1.39 to 3.38x10(9). Three of the four batches of particles were endotoxin-free. Exposure of the cells to particles of wear debris significantly increased the number of cells (p<0.05) and the [3H]-thymidine incorporation (p<0.05) in a dose-dependent manner. In contrast, the addition of particles decreased alkaline-phosphatase-specific activity and osteocalcin production. Collagen production and proteoglycan sulfation were also decreased, while prostaglandin-E2 synthesis was increased by the addition of particles. CONCLUSIONS: Ultra-high molecular weight polyethylene particles isolated from human tissue stimulated osteoblast proliferation and prostaglandin-E2 production and inhibited cell differentiation and matrix production. These results indicate that particles of wear debris inhibit cell functions associated with bone formation and that osteoblasts may produce factors in response to wear debris that influence neighboring cells, such as osteoclasts and macrophages. CLINICAL RELEVANCE: Particles of wear debris, especially ultra-high molecular weight polyethylene, have been implicated in the loosening of implants and the development of osteolysis. The present study shows that particles of ultra-high molecular weight polyethylene isolated from human tissue inhibit osteoblast functions associated with bone formation. In addition, particles of wear debris induced osteoblasts to secrete factors capable of influencing neighboring cells, such as osteoclasts and macrophages. These results suggest that osteoblasts may play a role in the cascade of events leading to granuloma formation, osteolysis, and failure of orthopaedic implants.     

In vitro activation of human fibroblasts by retrieved titanium alloy wear debris

Titanium-aluminum-vanadium wear particles isolated from the soft-tissue membrane of a failed total hip arthroplasty were added to human fibroblasts in cell culture. The cellular response to particle challenge was determined by assaying for levels of interleukin-1β, interleukin-6, tumor necrosis factor-α, prostaglandin E₂, basic fibroblast growth factor, platelet-derived growth factor-AB, and transforming growth factor-β. Collagenase and gelatinase activities were analyzed by zymography and [³H]collagen degradation. Cell viability was assessed by measuring the uptake of [³H]thymidine. Over the range of particle concentrations tested, cell viability, as demonstrated by [³H]thymidine uptake, remained unaffected. Fibroblasts exhibited a dose-dependent release of interleukin-6 in response to exposure to titanium-aluminum-vanadium particles. At 6 and 48 hours, the highest concentration of titanium alloy particles (0.189% [vol/vol]) resulted in 7-fold and 16-fold increases in interleukin-6 release, respectively, when compared with negative controls. Neither interleukin-1β nor tumor necrosis factor-α was detected in the culture medium at any particle concentration tested for both dermal and foreskin fibroblasts. The pattern of prostaglandin E₂ release by fibroblasts mirrored the pattern of interleukin-6 release. Fibroblasts exposed to the highest concentration of titanium alloy particles showed an increase in collagenase activity, starting at 12 hours. When medium samples were treated with amino phenylmercuric acetate to activate latent enzymes, a statistically significant increase in collagenase activity was observed as early as 6 hours (p < 0.001). Substrate gel analysis of medium from fibroblasts stimulated by high particle concentrations also showed an increase in gelatinolytic activity when compared with unstimulated controls. Analysis of medium samples for growth factors showed an increase in basic fibroblast growth factor at low particle concentrations, beginning at 12 hours. Levels of platelet-derived growth factor-AB and transforming growth factor-β were not detectable in the controls or at any particle concentration tested. The results of this study showed that fibroblasts exposed to titanium alloy wear particles become activated and release proinflammatory mediators that influence bone metabolism. These data support the hypothesis that direct activation of fibroblasts by particulate wear may play a role in particle-mediated osteolysis. Fibroblast activation coupled with the biologic response of macrophages to wear debris in the loosening membrane may have a synergistic effect on pathologic bone resorption.     

Nitric oxide and prostaglandin E2 production in response to ultra-high molecular weight polyethylene particles depends on osteoblast maturation state

BACKGROUND: Recent studies have shown that osteoblast-like cells respond directly to ultra-high molecular weight polyethylene particles in culture, suggesting that they may be involved in aseptic loosening of endoprostheses. We tested the hypothesis that the state of cell maturation plays a role in the response of osteogenic cells to ultra-high molecular weight polyethylene particles. METHODS: MG63 cells (immature osteoblast-like cells), OCT-1 cells (mature secretory osteoblast-like cells), and MLO-Y4 cells (osteocyte-like cells) were treated for twenty-four hours with commercial ultra-high molecular weight polyethylene particles with an average diameter of 1 mm. The effect of particle treatment on cell proliferation was assessed by measuring the number of cells, whereas the effects on differentiation and local factor production were assessed by measuring the production of osteocalcin, prostaglandin E2, and nitric oxide. The effect of particles on apoptosis was also evaluated. RESULTS: The addition of ultra-high molecular weight polyethylene particles increased the number of MG63 cells, did not affect the number of OCT-1 cells, and led to a decrease in the number of MLO-Y4 cells. The observed changes in cell number were not due to programmed cell death, as no more than 3% of the cells in cultures treated with the highest concentration of particles were undergoing apoptosis. Osteocalcin production was not affected by the addition of particles. Prostaglandin E2 production was increased in all three types of cultures, but the effect was greatest in OCT-1 cell cultures, as was the absolute amount of prostaglandin E2 produced. Nitric oxide production was unaffected in MG63 cell cultures, but it was stimulated in OCT-1 and MLO-Y4 cell cultures. CONCLUSIONS: The results of the present study support the hypothesis that osteoblast cell maturation state plays an important role in the response to ultra-high molecular weight polyethylene particles and that the terminally differentiated osteocyte may be involved in the bone response to wear debris in vivo.     

Effect of polymer molecular weight and addition of calcium stearate on response of MG63 osteoblast-like cells to UHMWPE particles.

Periprosthetic osteolysis and implant loosening is associated with the presence of ultrahigh molecular weight polyethylene (UHMWPE) wear debris particles. Osteoblast phenotypic expression in vitro is affected by UHMWPE particles, suggesting that bone formation may also be affected by wear debris. Here we tested the hypothesis that the response of osteoblasts to UHMWPE can be modified by changes in UHMWPE particle chemistry. We used four different commercially available preparations of GUR UHMWPE particles to determine if chemical composition (+/- Ca-stearate) or polymer molecular weight (3.1-4.2 million or 5.4-6.5 million g/mol) modulates osteoblast response. Particles were characterized by size distribution, morphology, and number of particles added to the culture medium. They had an average equivalent circle diameter ranging from 0.46-1.26 microm. MG63 cell response was assessed by measuring cell number, cellular and cell layer alkaline phosphatase, and prostaglandin E2 (PGE2) production. There were dose-dependent effects of the particles on cell response. Cell number and PGE, production were increased, while alkaline phosphatase specific activity was decreased. In addition, there was a marked difference between cultures treated with particles containing Ca-stearate and as a function of polymer molecular weight. Particles of higher molecular weight caused a greater stimulation of proliferation and inhibition of alkaline phosphatase than particles of lower molecular weight. The presence of Castearate exerted a more pronounced depression of osteoblast phenotype as well as a significantly greater increase in PGE2 release by the cells. The present study shows that chemical composition and polymer molecular weight of UHMWPE are capable of modulating osteoblast response to particles. The results suggest that osteoblast differentiation is inhibited by UHMWPE particles, whereas cell proliferation and PGE2 production are stimulated. This may have direct effects on osteoblasts and bone formation, but also paracrine effects on cells of the monocytic lineage inducing bone resorption and promoting inflammation which may lead to aseptic loosening. The present results suggest that the cellular events in aseptic loosening may be modulated or even accelerated by changes in the composition of the UHMWPE used to fabricate implants.     

Human monocyte response to particulate biomaterials generated in vivo and in vitro

We studied the ability of four clinically relevant particle species to stimulate human peripheral blood monocytes to release bone-resorbing agents, including interleukin-1 (both interleukin-1α and interleukin-1β), interleukin-6, and prostaglandin E₂. The species studied were titanium-6%aluminum-4%vanadium (TiAlV), commercially pure titanium, fabricated ultrahigh molecular weight polyethylene, and polyethylene retrieved from interfacial membranes of failed uncemented total hip arthroplasties. For all species, the mean size was less than 1 μm. Human peripheral blood monocytes were challenged with these particles in a uniform manner on the basis of surface area. Phorbol 12-myristate acetate, zymosan, and nonphagocytosable titanium particles served as controls. Stimulation of human monocytes is a function of the composition and concentration of particles. In this study. TiAlV particles appeared to be the most competent to elicit the synthesis and release of inflammatory mediators. Particles of commercially pure titanium and of fabricated ultrahigh molecular weight polyethylene also could induce the release of various cellular mediators, albeit at a lower level, whereas the particles of polyethylene retrieved from interfacial membranes were less stimulatory in these short-term in vitro experiments.     

Analysis of polyethylene particles produced in different wear conditions in vitro

The production of ultrahigh molecular weight polyethylene wear particles is a major factor limiting the life of prosthetic joints. The aim of the current study was to determine whether the morphologic features and size distribution of polyethylene particles produced in wear tests were in agreement with clinical findings. Particles from two hip simulators, a pin-on-disk hip wear device and a knee wear simulator, were studied and compared with particles found from synovial fluid of a prosthetic hip, and with published findings on clinical wear particles. Scanning electron microscopy and digital image analysis were used for characterization and sizing. The average equivalent circle diameter ranged from 0.27 microm to 0.69 microm, which corresponded well with published clinical findings. Common to all wear tests was that the lubricant contained albumin or globulin, and that the relative motion was multidirectional. In the hip wear simulation, the particle size distribution was not sensitive to the type of loading, counterface material, protein content of the lubricant, and whether the polyethylene was irradiated. In the knee wear simulation, the debris on average was larger than in the hip wear simulation. The simulators produced wear particles similar to those seen clinically, which indicates that the current test methods are relevant for assessing wear of prosthetic joints.     

Antioxidant impregnated ultra‐high molecular weight polyethylene wear debris particles display increased bone remodeling and a superior osteogenic:osteolytic profile vs. conventional UHMWPE particles in a murine calvaria model

Periprosthetic osteolysis remains a major limitation of long‐term successful total hip replacements with ultra‐high molecular weight polyethylene (UHMWPE) bearings. As intra and extracellular reactive oxygen species are know to contribute to wear debris‐induced osteoclastic bone resorption and decreased osteoblastic bone formation, antioxidant doped UHMWPE has emerged as an approach to reduce the osteolytic potential of wear debris and maintain coupled bone remodeling. To test this hypothesis in vivo, we evaluated the effects of crosslinked UHMWPE wear debris particles (AltrX^?), versus similar wear particles made from COVERNOX^? containing UHMWPE (AOX^?), in an established murine calvaria model. Eight‐week‐old female C57B/6 mice (n = 10/Group) received a pre‐op micro‐CT scan prior to surgical implantation of the UHMWPE particles (2mg), or surgery without particles (sham). Dynamic labeling was performed by intraperitoneal injection of calcein on day 7 and alizarin on day 9, and the calvaria were harvested for micro‐CT and histology on day 10. Surprisingly, we found that AOX particles induced significantly more bone resorption (1.72‐fold) and osteoclast numbers (1.99‐fold) vs. AltrX (p < 0.001). However, AOX also significantly induced 1.64‐fold more new bone formation vs. AltrX (p < 0.01). Moreover, while the osteolytic:osteogenic ratio of both particles was very close to 1.0, which is indicative of coupled remodeling, AOX was more osteogenic (Slope = 1.13 ± 0.10 vs. 0.97 ± 0.10). Histomorphometry of the metabolically labeled undecalcified calvaria revealed a consistent trend of greater MAR in AOX vs. AltrX. Collectively, these results demonstrate that anti‐oxidant impregnated UHMWPE particles have decreased osteolytic potential due to their increased osteogenic properties that support coupled bone remodeling. © 2015 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:845–851, 2016.

     

Response of three murine macrophage populations to particulate debris: Bone resorption in organ cultures

Particulate wear debris from bone cement or prosthetic components can stimulate macrophages to cause bone resorption. We compared the effect of particle composition (titanium and polymethylmethacrylate as inherent components of prosthetic materials or bone cement and polystyrene as a reference material) on the secretion of interleukin-1 and prostaglandin E₂ by periotoneal macrophages and monocyte/macrophage cell lines (P388D₁ and IC-21) and on the bone-resorbing activity of conditioned medium harvested from these particle-challenged macrophages. Titanium particles (1–3 μm) in peritoneal macrophage cultures exhibited significantly enhanced bone-resorbing activity measured as ⁴⁵Ca release, whereas polymethylmethacrylate and polystyrene exhibited this effect to a greater extent in the P388D₁ and IC-21 monocyte/macrophage cultures. Although exogenous prostaglandin E₂ and recombinant human interleukin-1 could significantly increase the ⁴⁵Ca release and indomethacin significantly reduced both the spontaneous calcium efflux and active ⁴⁵Ca release from calvarial bones labeled in vivo, the levels of interleukin-1 and prostaglandin E₂, alone or together, did not always correlate with the bone-resorbing activity of conditioned media. Thus, the actual levels of potent bone-resorbing agents (prostaglandin E₂ and interleukin-1) measured in conditioned tissue culture media did not necessarily reflect the bone-resorbing capability. An important result of this study is that different macrophage populations may respond differently to the same microenvironmental signal, which in our investigation was particulate wear debris of differing composition and size.     

Technique for generating submicrometer ultra high molecular weight polyethylene particles

Ultra high molecular weight polyethylene wear debris is believed to have a major role in aseptic loosening of prosthetic joints. In order to study the cellular and host response to this and other such particulate debris, a source of fine ultra high molecular weight polyethylene debris is needed. We have described a technique to fracture the GUR 4150 primary ultra high molecular weight polyethylene grain, which reproducibly generated particles less than 1 μm in size. Furthermore, the particle morphology was similar to that of ultra high molecular weight polyethylene particles generated in vivo and retrieved from interfacial tissues. The fabricatd polyethylene particls ranged from 0.1 to 33 μm in diameter, with a mean of 2.3 ± 0.2 μm. Sixty percent of the particles were smaller than 1 μm and 90% were smaller than 7 μm. Using filtration and sedimentation, it is possible to acquire finer particle fractions. These particles are currently being used for biological response studies.     

Third-body wear testing of a highly cross-linked acetabular liner: the effect of large femoral head size in the presence of particulate poly(methyl-methacrylate) debris

The hip simulator wear performance of an electron beam cross-linked and subsequently melted ultrahigh molecular weight polyethylene against femoral heads of 28-, 38-, and 46-mm diameter in the presence of poly(methyl-methacrylate) particulate debris was contrasted with that of conventional polyethylene against a 46-mm diameter head. Over 5 million cycles of testing, the average wear rate of the conventional polyethylene liners was 29.3 +/- 3.0 mg per million cycles. All highly cross-linked components exhibited marked reduction in wear, with the highest wear measuring 0.74 +/- 0.85 mg per million cycles. This study, using a clinically relevant third-body material, showed the electron beam cross-linked material to be far more resistant to this third-body wear than conventional ultrahigh molecular weight polyethylene, even when very large diameter femoral heads were used.     

The effect of entrapped bone particles on the surface morphology and wear of polyethylene

Clinically retrieved highly cross-linked ultrahigh molecular weight polyethylene (HXPE) acetabular liners have demonstrated scratching, whereas conventional ultrahigh-molecular-weight polyethylene (UHMWPE) implants show a smoother surface early after implantation. In the present study, the potential of bone particles and soft tissues, rather than cement, to scratch the articular surface of HXPE and UHMWPE (gamma radiated) acetabular components was evaluated; multiple bone particles located at the articular surface for 3600 simulated walking cycles replicated the scratches observed on retrieved implants. By remelting, these scratches were confirmed to be due to plastic deformation of the polyethylene, not wear. Furthermore, it was shown using wear testing that these scratches did not affect the subsequent wear rate of HXPE or conventional UHMWPE. Wear rates of scratched conventional and cross-linked polyethylene were not significantly different from unscratched conventional and cross-linked polyethylene, respectively.     

Comparison of cross-linked polyethylene materials for orthopaedic applications

Cross-linked polyethylenes are being marketed by orthopaedic manufacturers to address the problem of osteolysis caused by polyethylene particulate wear debris. Wear testing of these cross-linked polyethylenes in hip simulators has shown dramatic reduction in wear rate compared with standard ultrahigh molecular weight polyethylene, either gamma irradiated in air or nitrogen - or ethylene oxide-sterilized. However, this reduction in wear rate is not without cost. The cross-linking processes can result in materials with lower mechanical properties than standard ultrahigh molecular weight polyethylene. To evaluate the effect of the various cross-linking processes on physical and mechanical properties of ultrahigh molecular weight polyethylene, commercially available cross-linked polyethylenes from six orthopaedic manufacturers were tested. This study was the culmination of collaboration with these manufacturers, who provided cross-linked polyethylene for this study, wear characteristics of the material they provided, and review of the physical and mechanical properties measure for their polyethylene. Cross-linked materials were evaluated as received and after an accelerated aging protocol. Free radical identity and concentration, oxidation, crystallinity, melt temperature, ultimate tensile strength, elongation at break, tensile stress at yield, and toughness are reported for each material. By comparing these physical and mechanical properties, surgeons can evaluate the trade-off that results from developing materials with substantially lower wear rates.     

Effects of polyethylene on macrophages

A system was developed to expose macrophages to polyethylene in vitro. Exposure of macrophages to these particles in isolation led to the release of tumor necrosis factor alpha and prostaglandin E₂. Exposure of macrophages in co-culture with osteoblasts to polyethylene particles increased the release of prostaglandin E₂ and also led to the release of interleukin-6. Incubation of radiolabelled calvariae with conditioned medium from macrophages exposed to polyethylene particles alone or to particles in co-culture with osteoblasts led to bone resorption reflected by release of ⁴⁵Ca. Incubation with pamidronate was effective in inhibiting resorption stimulated by conditioned medium from macrophages exposed to these particles alone or in co-culture with osteoblasts. This demonstrates that pamidronate, or other bisphosphonates, may be effective in inhibiting bone resorption at the implant/bone interface in association with the macrophage ressponse to particulate polyethylene. Further investigation into the possible use of pamidronate or other bisphosphonates in the treatment of aseptic loosening is warranted.     

Polyethylene particles from a hip simulator cause (45)Ca release from cultured bone

Periprosthetic osteolysis is a dominant factor in the success or failure of total hip prostheses. Polyethylene wear debris has been implicated in the process of bone resorption and subsequent implant loosening. The present study is the first to examine the effect of ultra high molecular weight polyethylene (UHMWPE) wear debris produced by a hip simulator on calvarial bone resorption in vitro. (45)Ca release was measured in cultured mouse calvarial bone samples. Although short-term exposure to UHMWPE particles (2 h) decreased (45)Ca release, longer-term exposure for 1-2 days increased release in a dose-dependent manner. After one-day exposure to 7.5 x 10(6) particles per mL, 18% more (45)Ca was released from cultured calvarial bone than from control samples. It was concluded that UHMWPE wear particles either directly or indirectly stimulated osteoclasts to activate bone resorption. Polyethylene wear debris contributes to the osteolytic process at the bone-implant interface.     

On the origins of high in vivo wear rates in polyethylene components of total joint prostheses

Scanning electron microscope examination of the polyethylene components of 8 total hip and 16 total knee prostheses which had been implanted 4--96 months revealed that in many cases severe wear may not necessarily be caused by the presence of acrylic cement debris or other abrasives. The craters and cracks observed on severely worn surfaces are associated with fusion defects in the plastic itself. The fusion defects were seen to occur as a result of the low temperature necessary to mold ultrahigh molecular weight polyethylene and are (at least at present) difficult to avoid. As a consequence of the above and variability of molecular weight in this material, relatively wide variations in wear rate should be expected even in the absence of acrylic debris.     

Differentiated cellular function in fetal chondrocytes cultured with insulin-like growth factor-I and transforming growth factor-β

This study examined fetal chondrocyte proliferation and function following exposure to transforming growth factor-β and insulin-like growth factor-I. Fetal equine articular chondrocytes of the early third-trimester were isolated and cultured in monolayer conditions, then exposed to 0,1,5, or 10 ng/ml transforming growth factor-β or 0,10,50, or 100 ng/ml insulin-like growth factor-I for 48 hours. Proliferative responses were assessed by cell counts and [³H]thymidine uptake into precipitable DNA. Differentiated chondrocyte metabolic activity was determined by sulfated glycosaminoglycan quantitation, ³⁵[SO₄] incorporation into precipitable glycosaminoglycan, and proteoglycan molecular sizing by CL-2B column chromatography. Morphological changes seen on phase-contrast microscopy included a larger proportion of rounded cells in monolayer cultures supplemented with insulin-like growth factor-I and cytotoxic changes in cells treated with transforming growth factor-β. Both insulin-like growth factor-I and transforming growth factor-β resulted in significant elevations of [³H]thymidine uptake; however, cell numbers did not rise sufficiently over the 48-hour culture period to reach significant levels. Maximum mitogenic responses were evident at 50 and 100 ng/ml insulin-like growth factor-I and 5 ng/ml transforming growth factor-β. The production of proteoglycan was also enhanced (435%) by exposure to 50 ng/ml insulin-like growth factor-I, and an increased proportion of larger proteoglycan monomer species was evident in cultures treated with 50 and 100 ng/ml insulin-like growth factor-I. A similar dose-response was also evident in cultures treated with transforming growth factor-β (maximal 164%' increase with 5 ng/ml), although the presence of serum in the culture medium altered the pattern of enhanced proteoglycan synthesis to favor the lower concentration of 1 ng/ml (191%). Additionally larger proteoglycan molecules were synthesized in response to high concentrations of transforming growth factor-β in serum-free cultures. Significant biochemical changes resulted from the addition of transforming growth factor-β to fetal chondrocyte cultures; however, monolayer cultures that were treated with transforming growth factor-β and supplemented with serum began to develop cellular toxicity, including nuclear pyknosis and cytoplasmic fragmentation. Degenerative cellular changes were not evident in cultures treated with insulin-like growth factor-I, and significant differentiated metabolic activity resulted from the presence of insulin-like growth factor-I in the culture medium. These data suggest that the responses of fetal chondrocytes to insulin-like growth factor-I and transforming growth factor-β were enhanced compared with the responses of chondrocytes derived from postnatal animals and that these metabolically active cells can be primed by endogenous or exogenous growth factors to provide enhanced articular function and repair.     

The effectiveness of polyethylene versus titanium particles in inducing osteolysis in vivo.

Bearing surface wear and periprosthetic osteolysis due to wear particles are among the most common reasons for joint replacement failure. A murine calvarial model of wear particle-induced osteolysis has been used to identify different biologic factors associated with this problem and to test nonsurgical methods of modulating the host response to particulate debris. This model has utilized titanium particles, however, in clinical practice the most common source of particulate debris is polyethylene particles from bearing surface wear. We now report a calvarial model of wear particle-induced osteolysis based on commercially available polyethylene particles. We found that compared to sham surgery osteoclast recruitment and bone resorption can be induced by introduction of the titanium particles or polyethylene particles. However, bone resorption was significantly higher with polyethylene particles compared to titanium particles (p=0.02). We consider the polyethylene based murine calvarial model of wear particle-induced osteolysis a reliable and clinically relevant tool to understand the host factors and potential pharmacologic interventions that can influence wear debris generated osteolysis. This model might serve as an extension of the well-established titanium based bone resorption model.     

Characterization of a highly cross-linked ultrahigh molecular-weight polyethylene in clinical use in total hip arthroplasty

This article reports on a commercially available extensively cross-linked ultrahigh molecular-weight polyethylene (HXPE) produced by subjecting molded GUR 1050 ultrahigh molecular-weight polyethylene (UHMWPE) to 100 ± 10 kGy of electron beam radiation followed by melt annealing and sterilization by gas plasma. When compared to contemporary conventional molded GUR 1050 UHMWPE sterilized by 37 kGy of gamma radiation, the HXPE material has enhanced wear properties, has no detectable free radicals, and is resistant to oxidation and oxidative-related material property changes. The relative wear improvement of the HXPE is maintained in the presence of bone cement or alumina particles. The HXPE produced greater than 90% fewer wear particles in all size ranges and statistically significantly (P < .0001) smaller average-size particles than did the conventional UHMWPE.     

Particle size and morphology of UHMWPE wear debris in failed total knee arthroplasties--a comparison between mobile bearing and fixed bearing knees.

Osteolysis induced by ultrahigh molecular weight polyethylene wear debris has been recognized as the major cause of long-term failure in total joint arthroplasties. In a previous study, the prevalence of intraoperatively identified osteolysis during primary revision surgery was much higher in mobile bearing knee replacements (47%) than in fixed bearing knee replacements (13%). We postulated that mobile bearing knee implants tend to produce smaller sized particles. In our current study, we compared the particle size and morphology of polyethylene wear debris between failed mobile bearing and fixed bearing knees. Tissue specimens from interfacial and lytic regions were extracted during revision surgery of 10 mobile bearing knees (all of the low contact stress (LCS) design) and 17 fixed bearing knees (10 of the porous-coated anatomic (PCA) and 7 of the Miller/Galante design). Polyethylene particles were isolated from the tissue specimens and examined using both scanning electron microscopy and light-scattering analyses. The LCS mobile bearing knees produced smaller particulate debris (mean equivalent spherical diameter: 0.58 microm in LCS, 1.17 microm in PCA and 5.23 microm in M/G) and more granular debris (mean value: 93% in LCS, 77% in PCA and 15% in M/G).