Biological reactions to wear debris in total joint replacement

Three- to six-year results with the Ultima metal-on-metal hip articulation for primary total hip arthroplasty

One hundred seventy-one primary total hip arthroplasties were evaluated in a prospective, randomized study. Ninety-five involved a metal-backed cup with an all-metal liner and 76 involved a metal-backed polyethylene cup that was used as the control. All were implanted with an S-ROM cementless femoral component with a 28 mm head. The mean follow-up period was 3.7 years (range, 3.0-5.7). The average postoperative Harris hip score was 95.4 (range, 65-100) for the metal-on-metal group and 96.1 (range, 65-100) for the metal-on-polyethylene group. Radiographic results were not statistically different between the 2 groups. Early results show the metal-on-metal articulation has been successful to date and justify continued clinical use.

Biological response to wear debris generated in carbon based composites as potential bearing surfaces for artificial hip joints

UHMWPE wear particles have been implicated in osteolysis, implant loosening, and long-term failure of total hip arthroplasties in vivo. This study examined four carbon-based composite materials as alternatives for UHMWPE in joint bearings. These materials were HMU-CVD, SMS-CVD, P25-CVD, and CFR-PEEK. New bearing materials should satisfy certain criteria: they should have good wear properties that at least match UHMWPE, and produce wear particles with low levels of biological activity. Of the four materials tested in multidirectional pin-on-plate tribological tests, SMS-CVD, P25-CVD, and CFR-PEEK showed lower volumetric wear factors than UHMWPE. P25-CVD had the lowest wear factor of 0.54 +/- 0.34 x 10(-7) mm(3)/Nm. Analysis of P25-CVD wear particles by transmission electron microscopy showed that the debris was very small, with the vast majority of particles being under 100 nm in size, which was similar in size to metal wear particles. The P25-CVD particles were isolated and cultured with L929 fibroblasts and U937 monocytic cells to assess their effect on cell viability. P25-CVD particles were significantly less cytotoxic (p < 0.01, ANOVA) to both cell types than CoCr metal wear particles. This work suggests that carbon-carbon composite materials may have potential for use in total hip replacement bearings. Of the materials tested P25-CVD had the lowest wear factor, and produced very small wear debris that had minimal cytotoxic effect on L929 and U937 cells in vitro. Therefore carbon-carbon composites, such as P25-CVD, may be important in the development of next-generation implants with lower wear rates and reduced cytotoxic potential.

Comparison of the response of primary human blood monocytes and the U937 human monocytic cell line to two different sizes of alumina ceramic particles

It is well recognized that wear particles derived from orthopaedic implants have the potential to induce inflammation, which may eventually lead to aseptic loosening of the artificial joint. We hypothesized that alumina ceramic particles of different sizes cause a differential cytokine response by human monocytes. To test this hypothesis a human monocytic cell line (U937) and primary human blood monocytes obtained from healthy volunteers were exposed to ceramic particles within the range known to be generated in vivo. Cellular responses were measured by quantifying the relative gene expression of 12 different cytokines using TAQman Real-Time Polymerase Chain Reaction (RT-PCR). Our results demonstrate that at a particle to cell ratio of 100:1, 0.5 microm ceramic particles consistently provoked higher amounts of Interleukin-1alpha (IL-1alpha), IL-1beta, IL-8, IL-10 and Tumor necrosis factor-alpha (TNF-alpha) steady state mRNA by U937 cells. As expected, the variability of cytokine expression in primary blood monocytes was much higher compared to the cell line however, a similar trend was observed. These results show a differential response to ceramic particle size, which may imply that 0.5 microm particles are less biocompatible. New ceramic implants can be designed to generate a known particle size range in vivo. Implant materials of this type may induce relatively lower levels of production of inflammatory cytokines resulting in a reduced incidence of failure due to aseptic loosening.

An easy technique to digest and isolate UHMWPE wear particles from a hip joint simulator

The size and morphology of polyethylene wear particles isolated from in vitro tests were analysed in this study. There are some major controversies emerging in the literature that will only be resolved by careful particles analysis. Since it is difficult to identify the factors that affect size and morphology of the wear particles, the goal of this study was to compare four different polymer wear debris isolation techniques (base method and acid treatment) under identical conditions in a hip joint simulator to obtain polyethylene wear particles. We investigated achieving particulate isolation by using a different approach to the one reported in the literature that involved an easier and streamline method of particle debris isolation. The new method, compared to the previous one, used a strong base, normal centrifugation, and filtration to digest the serum constituents thus isolating the polyethylene particle debris from lubricant. In all four methods the isolation involved some or all of the following steps: density gradient separation, centrifugation or ultracentrifugation, and washing. However, a requirement for all these techniques was that the lubricant had to be devoid of organic compounds in order to effectively image and count the particle debris. The results from these studies clearly show that this new method of particle isolation is easier and more streamlined than the three methods analysed and reported in literature because it did not involve ultracentrifugation and is quicker.

Damage to F-actin and cell death induced by chromium VI and nickel in primary monolayer cultures of rat hepatocytes

The toxicity of hexavalent chromium and nickel was investigated using primary cultures of hepatocytes as an in vitro system. Cr VI and Ni are widely used in the steel and orthopaedic implant industry. Although their toxicity has been extensively investigated, the mechanism(s) of action is/are not fully understood. Monolayer cultures of hepatocytes (10(5) cells/cm2) were exposed to various concentrations of Cr VI and Ni for 24 h. Cells were stained with phalloidin-FITC for the detection of the cytoskeletal component, F-actin, and Annexin V-FITC and propidium iodide for the detection of the mode of cell death. Exposure of cells to Cr VI (1, 5, 10 and 50 microM) resulted in the loss of the cell cytoskeleton, and this was accompanied by membrane blebbing and shrinking of the cell. Ni, on the other hand, induced detectable damage to the cytoskeleton only at 500 microM. Staining of the cells with Annexin V and propidium iodide showed that Cr VI induces apoptosis at low concentrations (5 microM), and necrosis at higher concentrations (25 and 50 microM). Ni almost exclusively induced necrosis at 500 microM with very few cells undergoing apoptosis. Below this concentration it had no discernable effect on hepatocytes. Damage to the cell cytoskeleton caused by Cr VI may be an early indication of apoptosis in hepatocytes.

Prediction of lubrication regimes in wrist implants with spherical bearing surfaces

The wrist joint is frequently affected by rheumatoid arthritis, resulting in wrist pain, deformity and ultimately loss of function. Artificial wrist implants have been introduced to treat the rheumatoid wrist, to attempt to alleviate pain and restore some function to the joint. The aim of this study was to predict the likely lubrication regimes that occur in wrist implants with spherical bearing surfaces. The implant was modelled as an equivalent ball-on-plane. Elastohydrodynamic lubrication theory was used to determine the minimum film thickness for the implant under different load, entraining velocity, lubricant viscosity, size of implant and material combinations. The results show that the highest film thickness is found in large implants, with high viscosity, high entraining velocity and low load. Hard-on-soft material combinations will operate with a boundary lubrication regime. Material combinations involving ceramic bearing surfaces have the potential to operate with a mixed lubrication regime.

Effect of slide track shape on the wear of ultra-high molecular weight polyethylene in a pin-on-disk wear simulation of total hip prosthesis

Prosthetic joints appear to show a strong relationship between the type of relative motion and wear, requiring careful consideration in the design of wear simulators. This relationship was studied with a 12-station pin-on-disk device, specifically adapted for the wear simulation of prosthetic hip joints. Each station had a unique motion, characterized by the so-called slide track, the track of the pin on the disk. The slide track shapes included 10 ellipses, their aspect ratio (AR) varying from 1.1 to 11.0, and a circle and a straight line as extreme cases. Hence for the first time in hip wear simulation, the motion was systematically varied over a wide range. Conventional UHMWPE pins were tested against polished CoCr disks in diluted calf serum three times for 3 million cycles. Below the AR value of 5.5, the polyethylene wear factor and wear mechanisms agreed with clinical observations. Above this value, the wear factor decreased to unrealistically low values, and the wear surface topography differed from that of retrieved acetabular cups. The wear particles, however, were similar to those isolated from periprosthetic tissues, irrespective of the AR value. In conclusion, it is recommended that the AR value be kept well below the critical point of 5.5.

Biologic and tribologic considerations of alternative bearing surfaces

Patients who are young or active or both who require total joint replacement pose a unique challenge; their high activity demands wear-resistant bearings that will perform for decades, without suffering from the adverse effects of accumulated wear products. We discuss the tribologic and biologic properties of newly introduced bearing materials for hip prostheses. The new PEs are intended to address the aseptic loosening problem by reducing the volume of submicron PE particles to a level well below that historically associated with osteolysis. However, choosing among the several variations of the cross-linked thermally-stabilized PEs is confounded by conflicting opinions regarding the optimum balance between long-term wear resistance and mechanical strength, and regarding potential effects of differences in morphologic features of the submicron-sized wear particles on their relative osteolytic potential. Metal-on-metal bearings have clinically proven wear resistance and the advantage of self-polishing, but the long-term biologic effects of metallic ions remain unknown. Ceramic-on-ceramic bearings have the advantage of high biocompatibility and usually very low wear, but fracture remains a rare but catastrophic complication. The choice of an appropriate bearing couple should be made after a thorough consideration of the relative risks and potential benefits of each of these materials.

Influence of orthopedic particulate biomaterials on inflammation and synovial microcirculation in the murine knee joint

The purpose of the present study was to examine changes in the synovial microcirculation as well as synovial tissue responses to exposure to titanium, polymethylmethacrylate (PMMA), ceramic (Al(2)O(3)), cobalt-chromium alloy (Co-Cr), and polyethylene (PE) particles in an in vivo model. The particulate biomaterials were injected into the left knee joint of female Balb/c mice and assessment of the synovial microcirculation using intravital fluorescence microscopy as well as histological evaluation of the synovial tissue response were performed on day 7 after particle administration. Intravital microscopic measurements revealed that all tested biomaterials caused significantly (p < 0.05) enhanced leukocyte-endothelial cell interactions and an increase of functional capillary density compared to controls. In the histological examination PMMA, Al(2)O(3), PE, and Co-Cr particles provoked significantly (p < 0.05) enhanced inflammatory tissue responses in comparison to tissue from control animals. Titanium particles showed significantly (p < 0.05) less leukocyte-endothelial cell interactions than the other particulate biomaterials and caused significantly (p < 0.05) minor membrane thickening compared to PE and PMMA particles. In conclusion, all tested particulate biomaterials were capable of inducing inflammatory responses in the present study. Our data suggest that titanium particles may cause less leukocyte activation and inflammatory tissue responses than other particulate biomaterials used in total joint arthroplasty.

In vivo surface wear mechanisms of femoral components of cemented total hip arthroplasties: the influence of wear mechanism on clinical outcome

The appearance and mechanism of femoral stem wear was studied in 172 retrieved femoral components, of which 74 stems had been stable in vivo. Macroscopic, microscopic, and nano-level scales of examination were used. Loss of stem surface in response to micromotion (wear) was found to affect 93% of stems. However, changes were frequently difficult to see with the naked eye, and in 19% of cases they would have been missed completely without the use of light microscopy. The surface finish of the prosthesis determined the mechanism of stem wear. Matte surfaces showed typical abrasive processes that also damage the cement, releasing particulate debris from the cement and metal surfaces. This may destabilize the stem within the cement. Polished stems showed a typical fretting appearance with retention of debris on the stem surface and without significant damage to the cement. These differences in wear mechanism between matte and polished stems have significant effects on stem function.

Metal-on-metal bearing wear with different swing phase loads

There is currently much discussion about the most clinically relevant testing methods for evaluating total hip replacements. This study examined the effect of different swing phase loads, including microseparation, on the wear, friction, and wear particles of metal-on-metal (MOM) hip replacements. MOM hip replacements were tested for 5 million cycles with the use of a hip simulator; prostheses were tested with a low (100-N) and ISO (280-N) swing phase load, and under microseparation conditions. Increasing the swing phase load from 100 to 280 N in the same hip simulator increased the wear of the MOM hip replacements by over tenfold. Introducing microseparation into the gait cycle increased wear further, and stripe wear was observed on the femoral heads, accompanied by corresponding rim damage on the acetabular cups. No significant difference in wear particle size was observed between wear particles produced by low load and microseparation hip simulator conditions. Introducing microseparation into the hip simulator gait cycle increased the wear of MOM prostheses. Joint laxity and separation may lead to increased wear rates of MOM prostheses in vivo. Additionally elevated positive swing phase loads may also increase wear. Variable swing phase load conditions in vivo may contribute to variations in clinical wear rates.

The potential environmental impact of engineered nanomaterials

With the increased presence of nanomaterials in commercial products, a growing public debate is emerging on whether the environmental and social costs of nanotechnology outweigh its many benefits. To date, few studies have investigated the toxicological and environmental effects of direct and indirect exposure to nanomaterials and no clear guidelines exist to quantify these effects.

Ion release in stable hip arthroplasties using metal-on-metal articulating surfaces: a comparison between short- and medium-term results

The use of metallic heads articulating with metallic cups could solve the problem of polyethylene (PE) wear in total hip replacement (THR) with metal-on-PE bearings. A conspicuous release of metal ions from new models of metal-on-metal bearings has been found in the short-term, but it is yet unclear whether the medium-term corrosion rate is high or, on the contrary, it becomes negligible, because of the continuous surface finishing. Our purpose was to compare the serum ion values (nanograms per milliliter) in 15 patients with metal-on-metal stable prosthesis (Group A), in the short-term (subgroup A(1); mean follow-up: 24 mo) and medium-term (subgroup A(2); mean follow-up: 52 mo), in order to determine whether the ion release decreased with time of implant. Chromium (Cr), cobalt (Co), molybdenum (Mo) and aluminum (Al) were analyzed. Twenty-two presurgical patients were used for comparison (Group B). The reference range was obtained from a population of 27 healthy subjects (Group C). Co and Cr levels in the medium-term (subgroup A(2)) were not decreased in comparison with the short-term values (subgroup A(1)) and were significantly higher (p < 0.001) than presurgical and reference values. Otherwise, Mo and Al concentrations were not significantly increased in comparison with reference values. In conclusion, despite the apparent advantage of metal-on-metal coupling, especially in younger patient populations, there is a major concern about the extent and duration of the relevant "internal" exposure to Cr and Co ions. This exposure should be carefully monitored, in order to clarify the biologic effects of ion dissemination and, consequently, to identify risks concerning long-term toxicity of metals.

Polyethylene cross-linking by two different methods reduces acetabular liner wear in a hip joint wear simulator

Advances in cross-linking have led to the development of wear resistant ultrahigh molecular weight polyethylene for total joint replacement. This study compared wear reduction by two different cross-linking methods as measured in a hip wear simulator. One highly cross-linked polyethylene was treated with 7.5 Mrad gamma irradiation with post-irradiation annealing and a sterilization dose of 2.5 Mrad (10 Gamma), while the other used 9.5 Mrad warm irradiation with 10 MeV electron-beam (9.5 EB). Liners of the same design, made from nominally cross-linked (gamma sterilized) polyethylene were also tested. Gravimetric wear analysis was performed every 500,000 cycles for 5,000,000 cycles. After correcting for weight gain due to water absorption, the nominally cross-linked liners demonstrated mean wear rates of 15.7 (+/-1.7) and 12.5 (+/-1.0) mg/million cycles. Both highly cross-linked polyethylene liners demonstrated significantly less wear than their respective controls (with mean wear rates of 1.5 (+/-1.2) and -1.4 (+/-1.5) mg/million cycles). The 9.5 EB liners gained weight presumably due to increased fluid absorption, in addition to that measured in loaded-soaked control implants. Any wear occurring was therefore assumed to have been more than offset by weight gain. Highly cross-linked polyethylene was significantly more wear resistant than non- or nominally cross-linked polyethylene. The differences in wear rates between the two highly cross-linked polyethylene designs (9.5 EB or 10 Gamma) are probably too small to be clinically significant.

Elastohydrodynamic lubrication analysis of a functionally graded layered bearing surface, with particular reference to 'cushion form bearings' for artificial knee joints

Elastohydrodynamic lubrication of a functionally graded layered (FGL) bearing surface, whose elastic modulus increases with depth from the bearing surface, was investigated in this study. The finite difference method was employed to solve the Reynolds equation, simultaneously with the elasticity equation of the bearing surface, under circular point contacts. The finite element method was adopted to solve the elasticity equation for the FGL bearing surface. The displacement coefficients thus obtained were used to calculate the elastic deformation of the bearing surface, required for the elastohydrodynamic lubrication analysis. Good agreement of the predicted film thickness and pressure distribution was obtained, between the present method and a previous study for a single layered bearing surface with a uniform elastic modulus. The general numerical methodology was then applied to an FGL bearing surface with both linear and exponential variations in elastic modulus, with particular reference to the 'cushion form bearing' for artificial knee joints. The predicted film thickness and pressure distribution were shown to be quite close to those obtained for a single layer under typical operating conditions representative of artificial knee joints, provided that the elastic modulus of the single layer was chosen to be the average elastic modulus of the graded layer.

Contact mechanics of metal-on-metal hip implants employing a metallic cup with a UHMWPE backing

The contact mechanics in metal-on-metal hip implants employing a cobalt chromium acetabular cup with an ultra-high molecular weight polyethylene (UHMWPE) backing were analysed in the present study using the finite element method. A general modelling methodology was developed to examine the effects of the interfacial boundary conditions between the UHMWPE backing and a titanium shell for cementless fixation, the coefficient of friction and the loading angle on the predicted contact pressure distribution at the articulating surfaces. It was found that the contact mechanics at the bearing surfaces were significantly affected by the UHMWPE backing. Consequently, a relatively constant pressure distribution was predicted within the contact conjunction, and the maximum contact pressure occurred towards the edge of the contact. On the other hand, the interfacial boundary condition between the UHMWPE backing and the titanium shell, the coefficient of friction and the loading angle were found to have a negligible effect on the contact mechanics at the bearing surfaces. Overall, the magnitude of the contact pressure was significantly reduced, compared with a similar cup without the UHMWPE backing. The importance of the UHMWPE backing on the tribological performance of metal-on-metal hip implants is discussed.

In vitro analysis of the wear, wear debris and biological activity of surface-engineered coatings for use in metal-on-metal total hip replacements

Extremely low wear rates have been reported for metal-on-metal total hip replacements, but concerns remain about the effects of metal ion release, dissolution rates and toxicity. Surface-engineered coatings have the potential to improve wear resistance and reduce the biological activity of the wear debris produced. The aim of this study was to examine the wear and wear debris generation from surface-engineered coatings: titanium nitride (TiN), chromium nitride (CrN) and chromium carbon nitride (CrCN) applied to a cobalt-chrome alloy (CoCr) substrate. The coatings were articulated against themselves in a simple geometry model. The wear particles generated were characterized and the cytotoxic effect on U937 macrophages and L929 fibroblasts assessed. The CrN and CrCN coatings showed a decrease in wear compared to the CoCr bearings and produced small (less than 40 nm in length) wear particles. The wear particles released from the surface engineered bearings also showed a decreased cytotoxic effect on cells compared to the CoCr alloy debris. The reduced wear volumes coupled with the reduced cytotoxicity per unit volume of wear indicate the potential for the clinical application of this technology.

Normal and prosthetic synovial joints are lubricated by surface-active phospholipid: a hypothesis

Much evidence supports the hypothesis that surface-active phospholipid (SAPL), which imparts the thin hydrophobic outermost lining to the normal articular surface, is the boundary lubricant reducing friction to remarkably low levels. We review this evidence and further hypothesize that SAPL produced in type B synoviocytes will also lubricate prostheses after implantation. This could explain why implanted hips display far less wear than hips in simulated wear trials do, even using protein as the lubricant whereas rougher surfaces can be tolerated in vivo. We introduce the concept that a deficiency of SAPL might explain the selective failure of prostheses just as osteoarthritic articular surfaces are deficient. This, in turn, leads to the replenishment of SAPL, as tested in OA, and the concept of prelubricating prostheses before implantation.

Wear and deformation of ceramic-on-polyethylene total hip replacements with joint laxity and swing phase microseparation

Wear of polyethylene and the resulting wear debris-induced osteolysis remains a major cause of long-term failure in artificial hip joints. There is interest in understanding engineering and clinical conditions that influence wear rates. Fluoroscopic studies have shown separation of the head and the cup during the swing phase of walking due to joint laxity. In ceramic-on-ceramic hips, joint laxity and microseparation, which leads to contact of the head on the superior rim of the cup, has led to localized damage and increased wear in vivo and in vitro. The aim of this study was to investigate the influence of joint laxity and microseparation on the wear of ceramic on polyethylene artificial hip joints in an in vitro simulator. Microseparation during the swing phase of the walking cycle produced contact of the ceramic head on the rim of the polyethylene acetabular cup that deformed the softer polyethylene cup. No damage to the alumina ceramic femoral head was found. Under standard simulator conditions the volume change of the moderately crosslinked polyethylene cups was 25.6 +/- 5.3 mm3/million cycles and this reduced to 5.6 +/- 4.2 mm3/million cycles under microseparation conditions. Testing under microseparation conditions caused the rim of the polyethylene cup to deform locally, possibly due to creep, and the volume change of the polyethylene cup when the head relocated was substantially reduced, possibly due to improved lubrication. Joint laxity may be caused by poor soft tissue tension or migration and subsidence of components. In ceramic-on-polyethylene acetabular cups wear was decreased with a small degree of joint laxity, while in contrast in hard-on-hard alumina bearings, microseparation accelerated wear. These findings may have significant implications for the choice of fixation systems to be used for different types of bearing couples.

Ion release in patients with metal-on-metal hip bearings in total joint replacement: a comparison with metal-on-polyethylene bearings

Polyethylene (PE) wear has been shown to be a problem in long-term joint replacement using metal-on-PE bearing. The use of metallic heads articulating with metallic cups could solve this problem: success will be enhanced if wear and corrosion of the articulating surfaces are maintained at a low level. New models with metal-on-metal bearing have been proposed, to be used mainly for young subjects: such coupling seems to have a reduced release, but it is unclear yet if the medium-term corrosion rate is really negligible or, on the contrary, it is significantly higher than in the metal-on-PE bearing. Aim of our study was the comparison of ion release in the serum of two groups of patients who had the same type of stable cementless prosthesis, but different bearing: twenty-six patients with metal-on-metal (Group A) and fifteen patients with metal-on-PE bearing (Group B) were examined. The follow-up was 14-38 months for group A and 18-34 months for group B. The serum concentration of chromium (Cr), cobalt (Co) and molybdenum (Mo) was measured. Twenty-two patients before surgery were used for comparison (Group C). The reference values were obtained from a population of twenty-two healthy subjects (Group D). Our findings indicate that metal-on-metal bearings produce a significantly higher systemic release of cobalt and chromium (ng/ml) when compared with levels found in metal-on-PE, pre-surgery and reference groups. Such a high release should induce to improve the bearing materials or, at least, to study the biologic fate of metal ions and consequently their long-term effects. In such a way a risk-to-benefit ratio for the patient could be established.

Wear of historical polyethylenes in hip prostheses. Biomechanical success and a biological failure

;Polyethylene wear debris induced osteolysis is a major cause of failure in artificial hip joints. Sub micrometre size particles are taken up by macrophages which are stimulated to release osteolytic cytokines such as TNFα. This leads to bone resorption, loosening and failure. In vitro cell culture studies have shown particles in the size range 0.1 to 1 micrometre to be at least six times more reactive than larger particles. Studies of historically used gamma irradiated in air polyethylene show increased wear rate with damaged femoral heads and with aged and oxidised polyethylene. The aged and oxidised polyethylene also produced a greater percentage of smaller particles leading to increased osteolytic potential. Combined tribological and biological simulation models have been developed for pre-clinical assessment of osteolytic potential of artificial hip joints.

Alternative bearing couples in total hip replacements: Solutions for young patients

;There is now considerable clinical concern about the effect of polyethylene wear debris induced osteolysis in long term failure of hip replacements. This paper compares the wear of stabilised and crosslinked polyethylene to alternative hard on hard bearings. The volumetric wear rates of stabilised and moderately crosslinked polyethylene 50 to 35 mm³/million cycles were less than previously reported for historical gamma irradiated in air polyethylene, but still of a level that in the long term could cause osteolysis. The moderately crosslinked polyethylene produced less wear than non-crosslinked polyethylene, but particles were smaller and more reactive resulting in little change in the osteolytic potential. Alumina ceramic on ceramic produced substantially less wear and osteolytic potential. Metal on metal also produced less wear than polyethylene but the particles adversely influence cell viability.

Morphological characteristics of total joint arthroplasty-derived ultra-high molecular weight polyethylene (UHMWPE) wear debris that provoke inflammation in a murine model of inflammation

It is recognized that the chronic inflammation in peri-prosthetic tissue that contributes to implant failure frequently is provoked by the presence of wear debris. Some wear debris is inevitable because of the nature of the prosthesis, but not all patients develop severe inflammatory responses. The precise factors that mediate the severity of tissue inflammation to wear debris has yet to be fully defined. Because wear debris retrieved from peri-prosthetic tissue consists of a heterogeneous mixture of materials with various sizes and shapes, this study evaluated the influence of two major physical aspects of ultra-high molecular weight polyethylene (UHMWPE) wear debris (shape and surface texture) using a model of tissue inflammation. UHMWPE debris particulates recovered from 50 peri-prosthetic tissue samples were examined by scanning electron microscopy and categorized into four groups based upon aspect ratio and surface texture of the material. The four groups were defined as: 1) smooth and globular, 2) smooth and fibular, 3) rough and globular, and 4) rough and fibular. Histological analysis and ELISA assays were conducted to evaluate variations in cellular responses and cytokine production between the groups. The strongest expression of tumor necrosis factor alpha and interleukin-1 beta was found in tissues exposed to UHMWPE debris with both a rough surface texture and fibular shape, and this response was significantly elevated over debris particles with a smooth surface texture and globular shape. The data suggest that both shape and texture influence the severity of specific inflammatory responses and that rough debris surface texture exerts a marked effect on adverse tissue responses when combined with particles that have a sharp, elongated shape.

Retrieval study of uncemented metal-metal hip prostheses revised for early loosening

A tribologic assessment was performed on 22 metal-metal hip prostheses from a single manufacturer, following removal for early aseptic loosening after a mean service life of 32 months (range, 12-59 months). The mean linear wear rate was 7.6 microm/year (range, 2.9-12.8 microm/year). This was below the rates previously observed in other modern metal-metal combinations. A novel contour analysis technique using a coordinate measuring machine showed the mean volumetric wear rate to be 2.02 mm(3)/year (range, 0.55-3.74 mm(3)/year), which corresponds to a mean gravimetric wear rate of 16.9 mg/year (range, 4.6-31.4 mg/year). The mean clearance of 39.8 microm (range, 30-50 microm) was within the optimal range for hard-hard bearing combinations. Evidence of abrasive, adhesive, and third-body wear was found on all bearing surfaces. The tribologic assessment did not indicate manufacturing defects as a cause of early loosening. Equally, third-body wear was too low to be considered a causative factor for early loosening.

Effects of clinically relevant alumina ceramic wear particles on TNF-alpha production by human peripheral blood mononuclear phagocytes

The recent introduction of microseparation of the components of ceramic-on-ceramic hip prostheses during hip simulations has produced clinically relevant wear rates, wear patterns and wear particles. This provided an opportunity to determine the response of primary human peripheral blood mononuclear cells to clinically relevant alumina ceramic wear particles in vitro. Alumina ceramic wear particles were generated in a hip joint simulator under microseparation conditions. The particles showed a bi-modal size distribution with nanometer sized (5-20nm) and larger particles (0.2->10 micrometer). The particles were cultured with human peripheral blood mononuclear cells obtained from six different donors at particle volume to cell number ratios of 1, 10, 100 and 500 micrometer(3). After 24h incubation the viability of the cells and the levels of TNF-alpha were determined. The response to the microseparation wear particles was compared to that of commercially available alumina powder with a uniform morphology and mean size of 0.5 micrometer. All six Donors PBMNC produced significantly elevated levels of TNF-alpha when stimulated with 100 micrometer(3) of the alumina powder per cell. Volumetric concentrations of 10 and 1.0 micrometer(3) per cell failed to stimulate a significant response by the cells from any of the six donors. Three of the six Donors PBMNC secreted significantly elevated levels of TNF-alpha when stimulated with 100 micrometer(3) of the microseparation wear particles, whereas the other three failed to respond to the wear debris at this concentration. All of the Donors PBMNC produced significantly elevated levels of TNF-alpha when stimulated with 500 micrometer(3) of the microseparation wear particles per cell. Thus, a greater volume of the microseparation wear particles was required to activate the PBMNC than the alumina powder. This was probably due to the microseparation wear particles having fewer particles in the critical size range (0.1-1 micrometer) for macrophage activation compared to the alumina powder. It can be concluded that alumina ceramic wear particles generated under microseparation conditions are capable of inducing osteolytic cytokine production by human mononuclear phagocytes. However, the volumetric concentration of the particles needed to generate this response is extremely high and given the low wear rates (<4mm(3) per million cycles) of ceramic-on-ceramic bearings, even under severe microseparation conditions, it is unlikely that this concentration threshold will be achieved in vivo.

Comparison of the cytotoxicity of clinically relevant cobalt-chromium and alumina ceramic wear particles in vitro

Concern over polyethylene wear particle induced aseptic loosening of metal-on-polyethylene hip prostheses has led to renewed interest in alternative materials such as metal-on-metal and alumina ceramic-on-alumina ceramic for total hip replacement. This study compared the effects of clinically relevant cobalt-chromium and alumina ceramic wear particles on the viability of U937 histiocytes and L929 fibroblasts in vitro. Clinically relevant cobalt-chromium wear particles were generated using a flat pin-on-plate tribometer. The mean size of the clinically relevant metal particles was 29.5+/-6.3 nm (range 5-200 nm). Clinically relevant alumina ceramic particles were generated in the Leeds MkII anatomical hip simulator from a Mittelmieier prosthesis using micro-separation motion. This produced particles with a bimodal size distribution. The majority (98%) of the clinically relevant alumina ceramic wear debris was 5-20 nm in size. The cytotoxicity of the clinically relevant wear particles was compared to commercially available cobalt-chromium (9.87 microm+/-5.67) and alumina ceramic (0.503+/-0.19 microm) particles. The effects of the particles on the cells over a 5 day period at different particle volume (microm(3)) to cell number ratios were tested and viability determined using ATP-Lite(TM). Clinically relevant cobalt-chromium particles 50 and 5 microm(3) per cell reduced the viability of U937 cells by 97% and 42% and reduced the viability of L929 cells by 95% and 73%, respectively. At 50 microm(3) per cell, the clinically relevant ceramic particles reduced U937 cell viability by 18%. None of the other concentrations of the clinically relevant particles were toxic. The commercial cobalt-chromium and alumina particles did not affect the viability of either the U937 histiocytes or the L929 fibroblasts.Thus at equivalent particle volumes the clinically relevant cobalt-chromium particles were more toxic then the alumina ceramic particles. This study has emphasised the fact that the nature, size and volume of particles are important in assessing biological effects of wear debris on cells in vitro.

The influence of shape and sliding distance of femoral head movement loci on the wear of acetabular cups in total hip arthroplasty

Wear of the polyethylene acetabular component is the most serious threat to the long-term success of total hip replacements (THRs). Greatly reduced wear rates have been reported for unidirectional, compared to multidirectional, articulation in vitro. This study considers the multidirectional motions experienced at the hip joint as described by movement loci of points on the femoral head for individual THR patients. A three-dimensional computer program determined the movement loci of selected points on the femoral head for THR patients and normal subjects using kinematic data obtained from gait analysis. The sizes and shapes of these loci were quantified by their sliding distances and aspect ratios with substantial differences exhibited between individual THR patients. The average sliding distances ranged from 10.0 to 18.1 mm and the average aspect ratios of the loci ranged from 2.5 to 9.2 for the THR patients. Positive correlations were found between wear rate and average sliding distance, the inverse of the average aspect ratio of the loci and the product of the average sliding distance and the inverse of the average aspect ratio of the loci. Patients with a normal hip joint range of motion produce multidirectional motion loci and tend to experience more wear than patients with more unidirectional motion loci. Differing patterns of multidirectional motion at the hip joint for individual THR patients may explain widely differing wear rates in vivo.

[Transmission electron microscopy image of wear particles of joint endoprostheses and ultrastructural cell changes]

Wear particles from joint endoprostheses vary considerably in size, and may be detectable in tissue only by electron microscopy. Wear debris plays a central role in the non-infectious late loosening of prostheses, and it has been estimated that the submicron particles induce increased liberation of mediators of osteolysis by activated macrophages. From the types of prostheses currently in use, bone cement and polyethylene particles greatly predominate over metallic and ceramic particles. Since it had formerly not been possible to reliably identify wear particles in the transmission electron microscopy, and descriptions of them in the literature varied considerably, we analysed ultrathin sections obtained from periprosthetic tissue containing wear particles previously identified by laser microprobe mass analysis. Using this method, it proved possible to classify almost all the wear particles detected in the electron microscope, to determine their size range and to represent the cellular alterations caused by them.

Particle disease. A comprehensive theory of periprosthetic osteolysis: a review

Aseptic loosening and osteolysis are considered the main long-term problems of hip arthroplasty. Pathogenesis of periprosthetic osteolysis is multifactorial, and both the biological and mechanical factors seem to play an important role. Bearing surfaces continuously generate excessive amounts of micron and submicron particles provoking an adverse inflammatory response of periprosthetic connective tissues. In general, a key role has been attributed to macrophages. Cytokines, growth factors, PGE2, and enzymes are secreted with activated periprosthetic cells resulting in formation of osteolytic granulomas. The final osteolytic step is taken predominantly by osteoclasts which are getting ready for action mainly by an osteoprotegerin ligand (RANKL) and TNFalpha. Rankl is expressed by activated macrophages, osteoblasts, and lymphocytes. In parallel, a repetitive hydraulic effect of the joint fluid is manifested on the susceptible bone.

Effects of short-term alendronate treatment on the three-dimensional microstructural, physical, and mechanical properties of dog trabecular bone

The bisphosphonate, alendronate, is well known for its potent inhibition of osteoclast-mediated bone resorption. It has been used clinically for the treatment of osteoporosis and has also recently been used to reduce osteolysis around prostheses in a canine revision model of implant loosening (femoral condyle). In this study, the effects of alendronate on trabecular bone properties were assessed in dogs at an oral dose of 0.5 mg/kg per day over a 12 week period, and compared with control dogs. Cubic cancellous bone specimens were produced from lumbar vertebrae (L-1 and L-2) and bilateral proximal humeri. These specimens were scanned using a high-resolution microcomputed tomography (micro-CT) system. From accurate data sets, three-dimensional microstructural properties were calculated and physical and mechanical properties were determined. Treatment with alendronate increased bone volume fraction by 9.5%, 7.7%, 7.4%, and 18.4%, respectively, in L-1, L-2, humeral greater tuberosity, and humeral head trabecular bone. In the lumbar vertebrae, the alendronate-treated trabeculae were thicker and lower in bone surface-to-volume ratio. In the greater tuberosity, the alendronate-treated trabeculae were thicker, lower in bone surface-to-volume ratio, and less anisotropic. In the humeral head, the alendronate-treated trabeculae were thicker, less anisotropic, lower in surface density, and showed decreased trabecular separation. Alendronate significantly increased apparent density and collagen density in the lumbar vertebrae and humeral heads, and significantly decreased collagen concentration in the vertebrae. In the lumbar vertebrae, Young's modulus in the cephalocaudal direction, ultimate stress, and failure energy were significantly increased in the alendronate-treated group. The changes in mechanical properties in the humeral head trabecular bone were similar to those seen in the lumbar vertebrae. Our results demonstrate that alendronate increases the mechanical properties of healthy canine trabecular bone after short-term treatment. The physical and microstructural changes of trabecular bone are consistent with the significantly increased mechanical properties.

An in vitro study of the reduction in wear of metal-on-metal hip prostheses using surface-engineered femoral heads

Although the wear of existing metal-on-metal (MOM) hip prostheses (1 mm3/10(6) cycles) is much lower than the more widely used polyethylene-on-metal bearings, there are concerns about the toxicity of metal wear particles and elevated metal ion levels, both locally and systemically, in the human body. The aim of this study was to investigate the possibility of reducing the volume of wear, the concentration of metal debris and the level of metal ion release through using surface-engineered femoral heads. Three thick (8-12 microm) coatings (TiN, CrN and CrCN) and one thin (2 microm) coating (diamond-like carbon, DLC), were evaluated on the femoral heads when articulating against high carbon content cobalt-chromium alloy acetabular inserts (HC CoCrMo) and compared with a clinically used MOM cobalt-chromium alloy bearing couple using a physiological anatomical hip joint simulator (Leeds Mark II). This study showed that CrN, CrCN and DLC coatings produced substantially lower wear volumes for both the coated femoral heads and the HC CoCrMo inserts. The TiN coating itself had little wear, but it caused relatively high wear of the HC CoCrMo inserts compared with the other coatings. The majority of the wear debris for all half-coated couples comprised small, 30 nm or less, CoCrMo metal particles. The Co, Cr and Mo ion concentrations released from the bearing couples of CrN-, CrCN- and DLC-coated heads articulating against HC CoCrMo inserts were at least 7 times lower than those released from the clinical MOM prostheses. These surface-engineered femoral heads articulating on HC CoCrMo acetabular inserts produced significantly lower wear volumes and rates, and hence lower volumetric concentrations of wear particles, compared with the clinical MOM prosthesis. The substantially lower ion concentration released by these surface-engineered components provides important evidence to support the clinical application of this technology.

Analysis of contact mechanics in ceramic-on-ceramic hip joint replacements

The contact mechanics in ceramic-on-ceramic hip implants has been analysed in this study using the finite element method. Only the ideal conditions where the contact occurs within the acetabular cup were considered. It has been shown that the contact pressure distribution and the contact area at the main articulating bearing surfaces depend largely on design parameters such as the radial clearance between the femoral head and the acetabular cup, as well as the thickness of the ceramic insert. For the ceramic-on-ceramic hip implants used in clinics today, with a minimum 5-mm-thick ceramic insert, it has been shown that the radius of the contact area between the femoral head and the acetabular cup is relatively small compared with that of the femoral head and the ceramic insert thickness. Consequently, Hertz contact theory can be used to estimate the contact parameters such as the maximum contact pressure and the contact area.

Risk analysis for a radio-carpal joint replacement

Risk analysis is required by the medical device directives to provide evidence that manufacturers have eliminated or reduced risks as far as possible so that a medical device does not compromise the safety of patients or health workers. This paper presents a risk analysis for the Swanson wrist implant, which is made from an implantable-grade silicone elastomer and used to replace the radiocarpal joint in the rheumatoid wrist. The main hazards identified were that the implant fractures and that silicone synovitis occurs in patients. The results of this risk analysis will be used to aid the design of a new wrist implant.

Surface roughness of the proximal and distal bearing surface of mobile bearing total knee prostheses

Proximal and distal articulations surface roughness measurements were done on 8 mobile bearing knee designs immediately after removal from sterile packaging. Roughness parameters Ra and Rp based on ISO 97, representing mean deviation from the smooth surface line and mean peak to smooth surface line, were recorded using a contact surface profilometer at 10 random sites in the anteroposterior and mediolateral direction on the femoral and tibial metal components and the proximal and distal surface of the ultra-high molecular weight polyethylene (PE) inserts. No differences were found for surface roughness values for the metal components. Surface roughness values were greater for the distal PE bearing surfaces compared with the proximal PE bearing surfaces for each design tested. The roughness values for the PE inserts showed a directional dependence. Complex kinematics of mobile bearing knees coupled with this rougher distal interface could influence the rate of generation of wear particles and total volume of particles produced especially in the early postoperative period.

Comparison of wear, wear debris and functional biological activity of moderately crosslinked and non-crosslinked polyethylenes in hip prostheses

The wear, wear debris and functional biological activity of non-crosslinked and moderately crosslinked ultrahigh molecular weight polyethylene (UHMWPE) acetabular cups have been com pared when articulating against smooth and intentionally scratched femoral heads. Volumetric wear rates were determined in a hip joint simulator and the debris was isolated from the lubricant and characterized by the percentage number and volumetric concentration as a function of particle size. The volumetric concentration was integrated with the biological activity function determined from in vitro cell culture studies to predict an index of specific biological activity (SBA). The product of specific biological activity and volumetric wear rate was used to determine the index of functional biological activity (FBA). On smooth femoral heads the crosslinked UHMWPE had a 30 per cent lower wear rate, but it had a greater percentage volume of smaller, more biologically active particles, which resulted in a similar index of FBA compared with the non-crosslinked material. On the scratched femoral heads the volumetric wear rate was three times higher for the moderately crosslinked UHMWPE and two times higher for the non-crosslinked UHMWPE compared with the smooth femoral heads. This resulted in a higher wear rate for the moderately crosslinked material on the scratched femoral heads. All the differences in wear rate were statistically significant. There were only small differences in particle volume concentration distributions, and this resulted in similar indices of FBA which were approximately twice the values of those found on the smooth femoral heads. Both materials showed lower wear and FBA than for previously studied aged and oxidized UHMWPE gamma irradiated in air. However, this study did not reveal any advantage in terms of predicted FBA for moderately crosslinked UHMWPE compared with non-crosslinked UHMWPE.

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.

Apoptosis and cytokine release in human monocytes cultured on polystyrene and fibrinogen-coated polystyrene surfaces

The effects of polystyrene (PS) material surface preadsorption with fibrinogen (3 mg/ml) and a low concentration of lipopolysaccharide (LPS; 10 ng/ml) and polystyrene particles (PS; 10(5)/ml) on human monocyte adhesion, viability and cytokine release were studied during 24h culture in vitro. LPS caused an upregulation of CD14 in adherent cells. In comparison with unstimulated cells on uncoated polystyrene surfaces, LPS did not alter the number of adherent cells but caused a markedly increased release of the proinflammatory cytokines (IL-1alpha and TNF-alpha) and the down-regulating IL-10. The expression of indicators of various stages of cell death, TdT, annexin-V, propidium iodide (PI) and lactate dehydrogenase (LDH), were unaltered, decreased, decreased and increased, respectively, after LPS stimulation. PS particles (3 microm psi) caused an increased DNA fragmentation but had a reduced proportion of annexin-V and PI positive cells in comparison with unstimulated cells on uncoated PS. In contrast, 1microm psi particles had a similar proportion of TdT, annexin-V and PI expressing cells as unstimulated controls. Cultures stimulated with particles (irrespective of size), had a similar concentration of proinflammatory cytokines as unstimulated controls, whereas a higher level of IL-10 was detected. Precoating of PS with fibrinogen revealed an enhanced cell adhesion and a concomitant reduction of CD14 expression. irrespective of stimulation with various agonists. The proportions of TdT, annexin-V and PI positive cells were unaltered or reduced on fibrinogen-coated PS in both unstimulated and agonist-challenged cultures. However, depending on the presence and type of agonist, fibrinogen mediated either a markedly increased (LPS) or equivalent (particles and unstimulated) IL-1alpha and TNFalpha release. Further, in comparison with uncoated substrates, fibrinogen was associated with a reduction of IL-10 release, irrespective of the type of stimuli. These observations, using low concentrations of bacterial and material products, indicate that fibrinogen modulates cell material interactions and up- and down-regulates specific events depending on the nature/ type of immediate stimuli.

The wear of metal-on-metal total hip prostheses measured in a hip simulator

New generation metal-on-metal prostheses have been introduced to try and overcome the problem of osteolysis often attributed to the wear particles of the polyethylene component of conventional metal-on-ultra-high molecular weight polyethylene (UHMWPE) joints. The wear rates of four metal-on-metal joints (two different clearances) were assessed along with that of a conventional metal-on-UHMWPE joint. Friction measurements of the metal-on-metal joints were taken before and after the wear test and compared. Two distinct wear phases were discernible for all the metal-on-metal joints: an initial wear phase up to 0.5 x 10(6) cycles and then a lower steady state wear phase. The steady state wear rate of the 22 microm radial clearance metal-on-metal joint was lower than that for the 40 microm radial clearance joint, although this difference was not found to be significant (p > 0.15). The wear rates for all the joints tested were consistent with other simulator studies. The friction factors produced by each joint were found to decrease significantly after wear testing (p < 0.05).

Quantitative evaluation of the prosthetic head damage induced by microscopic third-body particles in total hip replacement

The increase of the femoral head roughness in artificial hip joints is strongly influenced by the presence of abrasive particulate entrapped between the articulating surfaces. The aim of the present study is to evaluate the dependence of such damage on the geometry of the particles entrapped in the joint, with reference to the UHMWPE/chrome-cobalt coupling. Five chrome-cobalt femoral heads and their coupled UHMWPE acetabular cups, retrieved at revision surgery after a short period of in situ functioning, have been investigated for the occurrence of third-body damage. This was found on all the prosthetic heads, where the peak-to-valley height of the scratches, as derived from profilometry evaluations, ranged from 0.3-1.3 microm. The observed damage has been divided into four classes, related to the particle motion while being embedded into the polymer. Two kinds of particle morphology have been studied, spherical and prismatic, with size ranging from 5-50 microm. In order to provide an estimation of the damage induced by such particles, a finite element model of the third-body interaction was set up. The peak-to-valley height of the impression due to the particle indentation on the chrome-cobalt surface is assumed as an index of the induced damage. The calculated values range from 0.1-0.5 microm for spherical particles of size ranging from 10-40 microm. In the case of prismatic particles, the peak-to-valley height can reach 1.3 microm and depends both on the size and width of the particle's free corner, indenting the chrome-cobalt. As an example, a sharp-edged particle of size 30 microm can induce on the chrome-cobalt an impression with peak-to-valley height of 0.75 microm, when embedded into the polyethylene with a free edge of 5 microm facing the metallic surface. Negligible damage is induced, if a free edge of 7.5 microm is indenting the counterface. Our findings offer new support to the hypothesis that microscopic third-body particles are capable of causing increased roughening of the femoral head and provide a quantitative evaluation of the phenomenon.

Quantitative characterization of polyethylene debris isolated from periprosthetic tissue in early failure knee implants and early and late failure Charnley hip implants

This study isolated and characterized UHMWPE particles from 3 explant groups: early Charnley hip failures (ECE; < 10 years), late Charnley hip failures (LCE; > 10 years) and early knee failures (EKE; < 10 years). Debris isolated from the 3 groups had percentage particle number and percentage volumetric concentration distributions that were not significantly different. The greatest number of particles were found in the 0.1-0.5 microm size range and 19-20.6% of the volumetric concentration was below 1 microm in size in all groups. However, there were significant differences in the total volumetric concentration of debris isolated per g of tissue. LCE had significantly higher volumes of debris than ECE and EKE, there was no significant difference in the volume of debris from the EKE and ECE. The mean aspect ratio and mean irregularity ratio of the LCE group were also significantly higher than the ECE and EKE, suggesting that different wear mechanisms were occurring in the late Charnley group compared to the early Charnley and knee groups. These results also suggest that early knees, with normal surface wear, may have similar wear mechanisms to early Charnley hips and indicate that similar volumes of biologically active micrometer and sub-micrometer UHMWPE particles were produced. This may have important implications in the longer-term outcome of total knee arthroplasties, because it indicates a similar potential for osteolysis induced by wear debris.

Design revision of a partially cemented hip stem

In a previous preclinical study the prototype version of a partially cemented hip stem, cement-locked uncemented (CLU) prosthesis, showed optimal primary stability and moderate stress shielding. However, numerical analysis suggested that the prototype design would induce relatively high stresses in the cement and a significant relative motion between cement and metal. The present study aimed to verify if these problems could be eliminated once the CLU design is improved. The revised design was analysed using a complete finite element model of an implanted human femur. To further strengthen the predictions of the finite element analysis, the cement damage induced by a severe load history was assessed experimentally in synthetic femurs implanted with the improved CLU stem or with a clinically successful fully cemented stem. The modifications made to the CLU stem design did not reduce its good primary stability but decreased the metal-cement relative micromotion. The same load induced stresses in the cement mantle of the improved CLU stem that were significantly lower than those predicted for the prototype design. Although the presence of modelling artefacts produced a highly localized stress peak of 13 MPa. 99 per cent of the cement volume was subjected to a principal tensile stress lower then 4 MPa. These levels of stress compare favourably with the tensile fatigue limit of the acrylic cement used in this study (9.7 MPa). The experimental results further supported these findings. The cemented stem showed a number of cracks per volume unit approximately ten times higher than the partially cemented stem under investigation.

Effect of counterface roughness on the wear of conventional and crosslinked ultrahigh molecular weight polyethylene studied with a multi-directional motion pin-on-disk device

The effect of counterface roughness on the wear of conventional gamma-sterilized, and electron-beam-crosslinked ultrahigh molecular weight polyethylene was studied with a circularly translating pin-on-disk device. The counterfaces, CoCr disks, were either polished, or roughened so that they represented the type of roughening and the range of surface roughness values (R(a) = 0.014-0.24 microm) observed in explanted femoral heads of total hip prostheses. The lubricant was diluted calf serum, and the test length 3 million cycles. A total of 24 tests were done. With both types of polyethylene, there was a strong correlation between R(a) and wear factor k. The power equations were k = 5.87 x 10(-5)(R(a))(0.91) for conventional polyethylene (R(2) = 0.94), and k = 7.87 x 10(-5)(R(a))(2.49) for crosslinked polyethylene (R(2) = 0.82). Crosslinking improved wear resistance significantly. The wear of crosslinked polyethylene against the roughest counterfaces was lower than the wear of conventional polyethylene against the polished counterfaces. Against rough counterfaces, the wear of crosslinked polyethylene was an order of magnitude lower than that of conventional polyethylene. On the crosslinked polyethylene pins that were tested against polished counterfaces, remains of original machining marks were still visible after the test. The average size of wear particles produced by both types of polyethylene against rough counterfaces was similar, 0.4 microm, whereas that produced by conventional and crosslinked polyethylene against polished counterfaces was significantly smaller, 0.2 and 0.1 microm, respectively.

The influence of phospholipid concentration in protein-containing lubricants on the wear of ultra-high molecular weight polyethylene in artificial hip joints

There is considerable interest in the wear of polyethylene and the resulting wear-debris-induced osteolysis in artificial hip joints. Proteins play an important role as boundary lubricants in vivo in the pseudosynovial fluid, and these are reproduced in in vitro tests through the use of bovine serum. Little is known, however, about the effect of phospholipid concentrations within proteinaceous solutions on the wear of ultra-high molecular weight polyethylene (UHMWPE). The effects of protein-containing lubricants with 0.05, 0.5 and 5 per cent (w/v) phosphatidyl choline concentrations on the wear of ultra-high molecular weight polyethylene (UHMWPE) were compared with 25 per cent (v/v) bovine serum which had 0.01 per cent (w/v) lipid; the effects were compared in a hip joint simulator with smooth (n = 4) and scratched (n = 3) femoral heads. The control bovine serum lubricant produced UHWMPE wear of 55 and 115 mm3/10(6) cycles on the smooth and rough heads respectively. The increased phospholipid concentration significantly reduced the wear rate. At the higher concentration (5% w/v phosphatidyl choline) the average wear was reduced to less than 2 mm3/10(6) cycles. Even with the relatively low concentrations of 0.05% w/v phosphatidyl choline the wear was reduced by at least threefold compared with the bovine serum tests for both the smooth and rough femoral heads. There may be considerable differences in the phospholipid concentrations in patients' synovial fluid and this is highly likely to produce considerable variation in wear rates. In vitro, differences in the phospholipid concentration of lubricants may also cause variation in wear rates between different simulator tests.

A novel method for the prediction of functional biological activity of polyethylene wear debris

The comparative performance of artificial hip joints has been extensively investigated in vitro through measurements of wear volumes. In vivo a major cause of long-term failure is wear-debris-induced osteolysis. These adverse biological reactions are not simply dependent on wear volume, but are also controlled by the size and volumetric concentration of the debris. A novel model is presented which predicts functional biological activity; this is determined by integrating the product of the biological activity function and the volumetric concentration function with the wear volume over the whole particle size range. This model combines conventional wear volume measurements with particle analysis and the output from in vitro cell culture studies to provide a new indicator of osteolytic potential. The application of the model is demonstrated through comparison of the functional biological activity of wear debris from polyethylene acetabular cups articulating under three different conditions in a hip joint simulator.

Elastohydrodynamic lubrication analysis of ultra-high molecular weight polyethylene hip joint replacements under squeeze-film motion

Elastohydrodynamic lubrication was analysed under squeeze-film or normal approach motion for artificial hip joint replacements consisting of an ultra-high molecular weight polyethylene (UHMWPE) acetabular cup and a metallic or ceramic femoral head. A simple ball-in-socket configuration was adopted to represent the hip prosthesis for the lubrication analysis. Both the Reynolds equation and the elasticity equations were solved simultaneously for the lubricant film thickness and hydrodynamic pressure distribution as a function of the squeeze-film time was solved using the Newton-Raphson method. The elastic deformation of the UHMWPE cup was calculated by both the finite element method and a simple equation based upon the constrained column model. Good agreement of the predicted film thickness and pressure distribution was found between these two methods. A simple analytical method based upon the Grubin-Ertel-type approximation developed by Higginson in 1978 [1] was also applied to the present squeeze-film lubrication problem. The predicted squeeze-film thickness from this simple method was found to be remarkably close to that from the full numerical solution. The main design parameters were the femoral head radius, the radial clearance between the femoral head and the acetabular cup, and the thickness and elastic modulus for the UHMWPE cup; the effects of these parameters on the squeeze-film thickness generated in current hip prostheses were investigated.

Chemical eluates from ultra-high molecular weight polyethylene and fibroblast proliferation

Although polyethylene wear particles have been implicated in osteolysis and implant loosening, this study is the first to test whether chemical eluates extracted from ultra-high molecular weight polyethylene (UHMWPE) could also be involved in this process. Eluates were prepared from UHMWPE bar stock and examined for their effects on (3)H-thymidine incorporation by human foreskin fibroblasts grown in 96-well culture plates. Low concentrations of eluates stimulated (3)H-thymidine uptake; whereas, high concentrations inhibited uptake in a dose-dependent manner. Maximum inhibition of proliferation for eluates (87+/-0.03% inhibition, n = 45 paired wells) was greater than that observed for particles (54+/-0.07% inhibition, n = 45 paired wells). Ethylene oxide sterilization of UHMWPE reduced (3)H-thymidine uptake at low eluate concentrations relative to sterilization by gamma-irradiation. It was concluded that leachable eluates from UHMWPE implantse contribute to the osteolytic process at the bone-implant interface.