Influence of gelatin and bovine serum lubricants on ultra-high molecular weight polyethylene wear debris generated in in vitro simulations

Ultra-high molecular weight polyethylene (UHMWPE) wear debris induced osteolysis has a major role in the late aseptic loosening and ultimate failure of total hip replacements (THR). Clinically relevant in vitro simulations of wear are essential to predict the osteolytic potential of bearing surfaces in artificial hip joints. Newborn calf or bovine serum has been accepted as a boundary lubricant for such in vitro tests, but its biological stability has been questioned. This study compared the wear factors, number of wear particles and levels of microbial contamination produced in bovine serum and a gelatin-based lubricant. The wear factors produced by the two lubricants were not significantly different, however the wear debris morphology produced was substantially different. The bovine serum became contaminated with micro-organisms within 28 h, whereas the protein-based lubricant remained uncontaminated. The results showed that bovine serum was not a stable boundary lubricant. They also showed that although the wear factors for the two solutions were not significantly different, the protein-based lubricant was not a suitable alternative to bovine serum because the wear debris produced was not clinically relevant.     

The influence of lubricant on the morphology of ultra-high molecular weight polyethylene wear debris generated in laboratory tests.

Since the implication of polyethylene wear debris as a major cause of osteolysis in total joint replacements, there has been much interest in polyethylene wear studies and in cell culture studies using ultra-high molecular weight polyethylene (UHMWPE) wear debris. Studies have shown that particles in the 0.1-10 microns size range are particularly important in causing adverse cellular reactions resulting in osteolysis. The morphology, the mass and size distributions, and the number of wear particles produced at the joint surfaces are influenced by the tribological conditions at the joint. Laboratory wear tests are used to investigate the wear properties of prosthetic joint materials and different research groups have used different lubricants in these tests. This paper shows that the volumetric wear and morphology of UHMWPE particles generated in vitro are influenced by the type of lubricant used. This study compared, quantitatively, UHMWPE wear debris generated in deionized water to debris that was generated in a system lubricated by bovine serum which was diluted to 25 per cent. The wear factors of UHMWPE in water and serum lubricants were significantly different (p < 0.05). UHMWPE wore 14 times more in water than in serum. Quantitative analysis of the wear particles showed that the debris that was generated in serum was morphologically different from debris that was produced in a water-lubricated system. Furthermore, the particles produced in serum showed a closer similarity to those found in retrieved acetabular tissues.     

Influence of protein and lipid concentration of the test lubricant on the wear of ultra high molecular weight polyethylene

To gain a better understanding of the ultra-high molecular weight polyethylene (UHMWPE) wear mechanism in the physiological environment, the effects of protein and lipid constituents of synovial fluid on the specific wear rate of UHMWPE were examined experimentally. The multidirectional sliding pin-on-plate wear tester was employed to simulate the simplified sliding condition of hip joint prostheses. Bovine serum γ-globulin and synthetic l--DPPC were used as model protein and lipid constituents of synovia, respectively. Results of the wear test indicated that the UHMWPE wear rate primarily depended on the protein concentration of the test lubricant. Lipids acted as a boundary lubricant and reduced polyethylene wear in the low protein lubricants. However, the polyethylene wear rate increased with increasing lipid concentrations if the protein concentration was within the physiological level. Increased interactions between protein and lipid molecules and lipid diffusion to polyethylene surface might be responsible for the increased wear.     

The friction and wear behavior of irradiated very high molecular weight polyethylene

The friction and wear behavior of gamma irradiated very high molecular weight polyethylene has been studied for radiation doses of 20 to 1000 MRad. For tests run at low loads the effect of irradiation is to increase the friction coefficient and the wear factor. As the load is increased the friction coefficient decreases and the wear factor remains constant. At a critical load there is an abrupt increase in the wear factor. However, further increase in the load leads to a decrease in the wear factor to about the original value. This wear behavior is quite different from that shown by unirradiated polyethylene and is connected to changes in structure resulting from the temperatures generated during sliding.     

The wear behavior of ultrahigh molecular weight polyethylene

The wear behaviour of ultrahigh molecular weight polyethylene has been studied using a thrust-washer testing machine. The data are examined in the light of findings of other workers and of clinical results. The wear factor exhibits a maximum at about 500 lb in^?2 surface pressure for polyethylene samples tested in water. The use of plasma in place of water does not change the wear factor and this indicates that the wear of polyethylene is not sensitive to the type of liquid environment likely to be encountered in a total joint prosthesis.     

Sliding wear behaviour of ion implanted ultra high molecular weight polyethylene against a surface modified titanium alloy Ti-6Al-4V

A study has been made of the sliding wear behaviour of untreated and ion implanted ultra high molecular weight polyethylene (UHMWPE) against a surface modified titanium alloy (Ti-6Al-4V) using a pin on disc apparatus. It was found that the presence of water lubrication and a very smooth counterface was necessary to maintain low wear rates of the UHMWPE. A ‘zero wear’ effect was observed when nitrogen implanted UHMWPE was tested against very smooth counterfaces (R_a ≈ 0.03 μm) of either surface oxidized or nitrogen implanted Ti-6Al-4V under water lubrication. The enhanced mechanical and physical properties of the surface treated materials are believed to be responsible for the improved wear performance.     

Mechanical and tribological properties of complex-modified material based on ultra high molecular weight polyethylene and CuO

Results of studying nanocomposite materials based on ultra-high-molecular-weight polyethylene filled with copper oxide nanopowder produced in plasma of low-pressure arc discharge are presented. The process of the vacuum-plasma synthesis of powdered composite mixtures, which involves the treatment of powders of the polymer matrix material in a device for synthesizing nanopowders at low temperatures, is described. Results of the experimental study of the specimens are presented.     

On the pressure dependence of the wear of ultrahigh molecular weight polyethylene

The wear of polyethylene in bovine serum was evaluated as a function of load and molecular weight. The range and distribution of contact loadings simulated those which exist in currently available total hip and total knee prostheses. The wear increased exponentially with load at constant molecular weight. An increasing molecular weight parametrically displaced the exponential curve to higher loads, lowering the overall rate. It is proposed that the behavior of these materials be described in terms of a critical pressure-velocity product although the specific mechanisms for wear acceleration are not known in this case.     

The influence of bone and bone cement debris on counterface roughness in sliding wear tests of ultra-high molecular weight polyethylene on stainless steel

Studies of explanted hip prostheses have shown high wear rates of ultra-high molecular weight polyethylene (UHMWPE) acetabular cups and roughening of the surface of the metallic femoral head. Bone and bone cement particles have also been found in the articulating surfaces of some joints. It has been proposed that bone or bone cement particles may cause scratching and deterioration in the surface finish of metallic femoral heads, thus producing increased wear rates and excessive amounts of wear debris. Sliding wear tests of UHMWPE pins on stainless steel have been performed with particles of different types of bone and bone cement added. Damage to the stainless steel counterface and the motion of particles through the interface have been studied. Particles of bone cement with zirconium and barium sulphate additives and particles of cortical bone scratched the stainless steel counterface. The cement particles with zirconium additive produced significantly greater surface damage. The number of particles entering the contact and embedding in the UHMWPE pin was dependent on particle size and geometry, surface roughness and contact stress. Particles are likely to cause surface roughening and increased wear rates in artificial joints.     

Ferrographic analysis of wear debris generated in locomotive diesel engines

A preliminary trial in which ferrography was applied to the condition monitoring of locomotive diesel engines is described in this paper. Typical particles generated by locomotive diesel engines are illustrated. Results of observations of those particles are discussed.     

Post failure analysis of 0°/90° ultra high molecular weight polyethylene composite after ballistic testing

The ballistic performance of a 0°/90° ultra high molecular weight polyethylene (UHMWPE) composite was investigated. Two areal densities were considered: one for fragmentation protection and one for high velocity bullet protection.
The role of microscopy in the post-failure analysis of the UHMWPE was explored. Stereo, optical and environmental scanning electron microscopy were utilized and attempts were made to correlate the results obtained with other characterization techniques available. These included high speed photography, visual inspection and ultrasonic inspection.
Visual inspection revealed the gross effects of ballistic testing. These primarily included deformation and delamination of the composite plate. The result obtained corresponded with the predicted impact event and, for high velocity bullet impact, corresponded well to the images obtained using high speed photography. Optical, stereo and electron microscopy allowed for fibre and matrix characterization. Fibre displacement and pull-out were also observed on impact and exit faces.     

Fluid composition impacts standardized testing protocols in ultrahigh molecular weight polyethylene knee wear testing

Wear of total knee replacements is determined gravimetrically in simulator studies. A mix of bovine serum, distilled water, and additives is intended to replicate the lubrication conditions in vivo. Weight gain due to fluid absorption during testing is corrected using a load soak station. In this study, three sets of ultrahigh molecular weight polyethylene tibial plateau were tested against highly polished titanium condyles. Test 1 was performed in two different institutions on the same simulator according to the standard ISO 14243-1, using two testing lubricants. Test 2 and test 3 repeated both previous test sections. The wear and load soak rates changed significantly with the lubricant. The wear rate decreased from 16.9 to 7.9 mg weight loss per million cycles when switching from fluid A to fluid B. The weight gain of the load soak specimen submersed in fluid A was 6.1 mg after 5 x 10(6) cycles, compared with 31.6 mg for the implant in fluid B after the same time period. Both lubricants were mixed in accordance with ISO 14243 (Implants for surgery - wear of total knee-joint prostheses), suggesting that calf serum should be diluted to 25 +/- 2 per cent with deionized water and a protein mass concentration of not less than 17 g/l. The main differences were the type and amount of additives that chemically stabilize the lubricant throughout the test. The results suggest that wear rates can only be compared if exactly the same testing conditions are applied. An agreement on detailed lubricant specifications is desirable.     

Improved wear resistance of ultra-high molecular weight polyethylene by plasma immersion ion implantation

Surface modification of ultra-high molecular weight polyethylene (UHMWPE) has been explored using the novel non-line-of-slight plasma immersion ion implantation (PIII) with nitrogen. The modified surfaces were characterised by SEM and a Nano Test 600 testing machine. The tribological behaviour of PIII treated UHMWPE sliding against AISI 316L stainless steel counterfaces was evaluated using a pin-on-disc tribometer under water lubricated conditions. The experimental results show that PIII is a very promising surface engineering technique to improve such surface mechanical properties as surface hardness and elastic modulus of UHMWPE. As a result, the wear resistance of UHMWPE was significantly enhanced by a factor of three following PIII treatment, as compared with untreated material. It was found that the significantly improved wear resistance of PIII treated UHMWPE can be mainly attributed to ion bombardment induced cross-linking, and thus surface hardening.     

Isolation and analysis of articular joints wear debris generated in vitro

The total replacement of damaged or diseased synovial joints represents one of the greatest advances in orthopaedic surgery of the 20th century. Whereas replacements are available for the shoulder, ankle, elbow, and knee, hip accounts particularly for the most surgical interventions. In France, 100,000 hip joints per year are replaced and all the implants consist of a sliding pair represented by a hard counter face, either metal or ceramic, and commonly a softer polymer. Ultra high molecular weight polyethylene (UHMWPE) was first used in joint replacement in the early 1960s. Since that time, it had been the dominant polymer for bearing surfaces in orthopaedic surgery. However, generation of UHMPWE wear debris from bearing surfaces in patients is the major problem for long term implants. Both volume and morphology of the wear particles determine the response of the body to debris, and subsequent effects on secure fixing.This paper presents a review of the type of particles most frequently found in biopsies of tissues from explanted prostheses. Indeed, the size and the amount of these debris are very important factors for a better understanding of wear and corrosion wear processes in artificial joints. Real wear particles are also described in this paper.     

Biological activity of polyethylene wear debris produced in the patellofemoral joint

Polyethylene wear is considered a threat to the long-term survival of total knee replacements. The aim of this study was to investigate the contribution that resurfacing the patella makes to wear debris-induced osteolysis following total knee replacement. Ultra-high molecular-weight polyethylene wear particles were isolated from simulator lubricant. Particle shape, size, and volume distributions were recorded allowing the osteolytic potential of the wear debris produced in the patellofemoral joint to be estimated using the concept of specific biological activity and functional biological activity. Values were compared with those reported for the tibiofemoral joint. Specific biological activity for the patellofemoral joint was not significantly different from the values for the tibiofemoral joint of total knee replacement devices, and therefore, has a similar potential to stimulate osteolytic cytokine release from macrophages. Functional biological activity was significantly lower for the patellofemoral joint compared with the tibiofemoral joint. Functional biological activity was significantly lower for the patellofemoral joint compared with the fixed bearing and rotating platform total knee replacement devices. However, as patellar resurfacing is commonly fitted as part of a total knee replacement system, this results in a 20% increase in overall functional biological activity for the system. Therefore, implanting a patellar resurfacing will increase the potential for osteolysis in the knee.     

A simple fully integrated contact-coupled wear prediction for ultra-high molecular weight polyethylene hip implants

A fully coupled contact and wear model was developed in the present study for hip implants employing an ultra-high molecular weight polyethylene (UHMWPE) cup in combination with a metallic or ceramic femoral head. A simple elasticity equation based on the concept of constrained column model was employed to solve the contact mechanics between the acetabular cup and the femoral head under the three-dimensional physiological loading condition. The wear model was based on the classical Archard-Lancaster equation in common with all other studies reported in the literature. The fully coupled contact and wear model was applied to both conventional and cross-linked UHMWPE cups under a wide range of design parameters such as the clearance and the femoral head radius. The predicted linear and volumetric wear as well as their rates for conventional UHMWPE cups were found to be in good agreement with those obtained from a similar analysis by Maxian but using the finite element method for the contact mechanics analysis. The predicted maximum contact pressure was found to decrease rapidly within the first 10(6) cycles, and below the limit to cause plastic deformation within the UHMWPE cup with a nominal radial clearance of 0.2 mm. The effect of the clearance between the head and the cup on the predicted wear was found to be negligible. For the cross-linked UHMWPE cup with relatively large diameters up to 48 mm and a fixed outside diameter of 50 mm, the predicted wear, which was found to increase with increasing femoral head radius, remained small owing to the small wear factor associated with these materials. Furthermore, if the head diameter increases beyond 42 mm, a rapid increase in the contact pressure was predicted, owing to the decrease in the wall thickness of the cross-linked UHMWPE cup.     

An axisymmetric contact model of ultra high molecular weight polyethylene cups against metallic femoral heads for artificial hip joint replacements.

Contact mechanics of ultra high molecular weight polyethylene (UHMWPE) cups against metallic femoral heads for artificial hip joints is considered in this study. Both the experimental measurement of the contact area and the finite element prediction of the contact radius, maximum contact pressure and maximum Von Mises stress have been carried out for a wide range of contemporary artificial hip joints. Good agreement of the contact radius has been found between the experimental measurements and the finite element predictions based upon an elastic modulus of 1000 MPa and a Poisson's ratio of 0.4 for UHMWPE material under various loads up to 2.5 kN. It has been shown that the half contact angle for all the cup/head combinations considered in this study is between 40 degrees and 50 degrees under a load of 2.5 kN. The importance of this result has been discussed with respect to the anatomical position of the cup when placed in the body and the selection of a simple wear-screening test for artificial hip joints. The predicted contact radius and maximum contact pressure from the finite element model have also been compared with a simple elasticity analysis. It has been shown that the difference in the predicted contact radius between the two methods is reduced for more conforming contacts between the femoral head and the acetabular cup and smaller UHMWPE cup thickness. However, good agreement of the predicted maximum contact pressure has been found for all the combinations of the femoral head and the acetabular cup considered in this study. The importance of contact mechanics on the clinical performance of artificial hip joint replacements has also been discussed.     

Effect of stair descent loading on ultra-high molecular weight polyethylene wear in a force-controlled knee simulator

A loading protocol approximating forces, torques and motions at the knee during stair descent was developed from previously published data for input into a force-controlled knee simulator. A set of total knee replacements (TKRs) was subjected to standard walking cycles and stair descent cycles at a ratio of 70: 1 for 5 million cycles. Another set of implants with similar articular geometry and the same ultra-high molecular weight polyethylene (UHMWPE) resin (GUR 415), sterilization and packaging was tested with standard walking cycles only. Implant kinematics, gravimetric wear and surface roughness of the UHMWPE inserts were analysed for both sets of implants. Contact stresses were calculated for both loading protocols using a Hertzian line contact model. Significantly greater weight loss (p < 0.05) and more severe surface damage of UHMWPE inserts resulted with the walking + stair descent loading protocol compared to walking cycles only. Anterior-posterior (AP) tibiofemoral contact point displacements were lower during stair descent than walking, but not significantly different (p = 0.05). Contact stresses were significantly higher during stair descent than walking, owing to higher axial loads and the smaller radius of curvature of the femoral components at higher flexion angles. High contact stresses on UHMWPE components are likely to accelerate the fatigue of the material, resulting in more severe wear, similar to what is observed in retrieved implants. Thus the inclusion of loading protocols for activities of daily living in addition to walking is warranted for more realistic in vitro testing of TKRs.     

Effect of contact pressure on wear and friction of ultra-high molecular weight polyethylene in multidirectional sliding

Computational wear models need input data from valid tribological tests. For the wear model of a total hip prosthesis, the contact pressure dependence of wear and friction of ultra-high molecular weight polyethylene (UHMWPE) against polished CoCr in diluted calf serum lubricant was studied, and useful input data produced. Two test devices were designed and built: a heavy load circularly translating pin-on-disc (HL-CTPOD) wear test device and an HL-CTPOD friction measurement device. Both can be used with a wide range of loads. The wear surface diameter of the test pin was kept constant at 9 mm, whereas the load was varied so that the nominal contact pressure ranged from 0.1 to 20 MPa. The wear factor decreased with increasing contact pressure, whereas the coefficient of friction first increased with increasing contact pressure with low pressure values and then decreased. Up to the pressure of 2.0 MPa, the wear mechanisms and wear factors were in good agreement with clinical findings. In the critical range of 2.0-3.5 MPa, the wear mechanisms and wear factors started to differ from clinical ones, and the decrease of the wear factor steepened. The discrepancy became more and more evident as the pressure was gradually increased beyond 3.5 MPa. It appears that the pressure value of 2.0 MPa should not be exceeded in pin-on-disc wear tests that are to reproduce the clinical wear of UHMWPE acetabular cups.     

Effect of contact stress on friction and wear of ultra-high molecular weight polyethylene in total hip replacement

This paper studies the effect of contact stress on friction and wear of ultra-high molecular weight polyethylene (UHMWPE) acetabular cups by means of friction and wear joint simulator testing under serum lubrication. For a given applied load, increasing the contact stress by increasing the ball/socket radial clearance decreased both the coefficient of friction and the wear rate. Friction and wear were highly correlated. The dependence of friction on contact stress for the UHMWPE socket under serum lubrication was similar to that of semi-crystalline polymers under dry sliding. This finding indicates the occurrence of partial dry contact at asperity levels for the metal-polyethylene ball-in-socket joint under serum lubrication.