Wear and creep of highly crosslinked polyethylene against cobalt chrome and ceramic femoral heads

The wear and creep characteristics of highly crosslinked ultrahigh-molecular-weight polyethylene (UHMWPE) articulating against large-diameter (36mm) ceramic and cobalt chrome femoral heads have been investigated in a physiological anatomical hip joint simulator for 10 million cycles. The crosslinked UHMWPE/ceramic combination showed higher volume deformation due to creep plus wear during the first 2 million cycles, and a steady-state wear rate 40 per cent lower than that of the crosslinked UHMWPE/cobalt chrome combination. Wear particles were isolated and characterized from the hip simulator lubricants. The wear particles were similar in size and morphology for both head materials. The particle isolation methodology used could not detect a statistically significant difference between the particles produced by the cobalt chrome and alumina ceramic femoral heads.     

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.     

Tribological behaviour of DLC coatings compared to different materials used in hip joint prostheses

The goal of the study carried out in the laboratory was to quantify the wear and the friction of two materials used for the manufacturing of hip prostheses. Tests used had to obtain in a short time the tribological behaviour laws of the materials. Tests on a hip simulator have been excluded because their cost and their duration were too high for a program of preliminary development of new materials.

To amplify wear phenomena, dry friction tests were carried out for two configurations: ball-on-disc and pin-on-disc. The influence of the contact pressure at constant sliding velocity on the wear of materials has been clearly shown.

Results obtained with several different tested materials (stainless steel/UHMWPE, stainless steel+DLC coating/UHMWPE, stainless steel+DLC coating/stainless steel+DLC coating, titanium alloy+DLC coating/UHMWPE, titanium alloy+DLC coating/titanium alloy+DLC coating, zirconium dioxide/UHMWPE, alumina/UHMWPE, alumina/alumina) have shown the superiority of DLC coatings. Promising results obtained during this study are in the validation stage on a hip simulator.     

Wear and biological activity of highly crosslinked polyethylene in the hip under low serum protein concentrations

Crosslinked ultra-high molecular weight polyethylene (UHMWPE) has been developed and introduced into clinical practice in order to reduce wear in the hip. Zero wear of highly crosslinked UHMWPE in vitro has been reported by some groups using lubricants with high concentrations of serum proteins in hip simulators. In contrast, some clinical studies have reported finite wear rates. The aim of this study was to compare the wear rates, wear surfaces, and wear debris produced by UHMWPE with different levels of crosslinking in a hip joint simulator, with lower, more physiologically relevant concentrations of protein in the lubricant. The UHMWPEs were tested in the Leeds ProSim hip joint simulator against cobalt-chromium (CoCr) femoral heads. The wear particles were isolated and imaged using a field emission gun scanning electron microscope (FEGSEM) at high resolution. The highly crosslinked UHMWPEs had significantly lower wear volumes than the non-crosslinked UHMWPEs. No significant difference was found in the percentage number and percentage volume of the particles in different size ranges from any of the materials. They had similar values of specific biological activity. The functional biological activity (FBA), which takes into account the wear volume and specific biological activity, showed that the highly crosslinked UHMWPEs had lower FBAs due to their lower wear volume.     

Effect of motion inputs on the wear prediction of artificial hip joints

Hip joint simulators have been largely used to assess the wear performance of joint implants. Due to the complexity of joint movement, the motion mechanism adopted in simulators varies. The motion condition is particularly important for ultra-high molecular weight polyethylene (UHMWPE) since polyethylene wear can be substantially increased by the bearing cross-shear motion. Computational wear modelling has been improved recently for the conventional UHMWPE used in total hip joint replacements. A new polyethylene wear law is an explicit function of the contact area of the bearing and the sliding distance, and the effect of multidirectional motion on wear has been quantified by a factor, cross-shear ratio. In this study, the full simulated walking cycle condition based on a walking measurement and two simplified motions, including the ISO standard motion and a simplified ProSim hip simulator motion, were considered as the inputs for wear modelling based on the improved wear model. Both the full simulation and simplified motions generated the comparable multidirectional motion required to reproduce the physiological wear of the bearing in vivo. The predicted volumetric wear of the ProSim simulator motion and the ISO motion conditions for the walking cycle were 13% and 4% lower, respectively, than that of the measured walking condition. The maximum linear wear depths were almost the same, and the areas of the wear depth distribution were 13% and 7% lower for the ProSim simulator and the ISO condition, respectively, compared with that of the measured walking cycle motion condition.     

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.     

Experimental analysis of tribological behavior of UHMWPE against AISI420C and against TiAl6V4 alloy under dry and lubricated conditions

Friction and wear behavior of ultra-high molecular weight polyethylene (UHMWPE) sliding against AISI420C austenitic stainless steel and against TiAl6V4 alloy under dry and lubricated conditions were investigated with a reciprocating pin-on-flat tribometer for comparative purposes. The tests were conducted by varying frequency of the pin alternative motion and the applied normal load. For the tests in lubricated conditions a fluid containing a large amount of sodium hyaluronate has been chosen. By using an electronic precision balance the wear mass loss of the UHMWPE samples was evaluated accordingly. Friction is greatly reduced by the presence of UHMWPE and this is believed to be due to the formation of a lubricating film of UHMWPE in the contact zone. Furthermore, the experimental investigation shows that the AISI420C/UHMWPE gives, in dry conditions, better values in the wear rate and in the friction coefficient than the TiAl6V4/UHMWPE.     

The effect of minimum load on the fluid uptake and wear of highly crosslinked UHMWPE total hip acetabular components

Thirteen total hip replacement acetabular components of the same design (32 mm diameter, cup liners made of highly crosslinked ultra-high molecular weight polyethylene (UHMWPE)) were tested in an AMTI hip simulator (six in wear stations and seven in load-soak stations). Two additional liners were immersed in undiluted bovine calf serum but not loaded. The test conditions were otherwise similar for all wear and load-soak samples, with the following exceptions: (a) no motion was applied onto load-soak liners; (b) the minimum load during the swing phase was set at 40 N for three of the wear liners and four of the load-soak liners (group A: low minimum load), and 230 N for the remaining three wear liners and three load-soak liners (group B: high minimum load). It was found that: (a) load-soak liners gained about 10 times more weight than no-load soak liners; (b) load-soak liners in the low minimum load group gained twice as much weight as those in the high minimum load group, and (c) the observed wear rates were low for both groups; surprisingly, however, the wear rates of the low minimum load liners were also higher than those of the high minimum load liners. This study showed for the first time that the fluid uptake and wear rates of highly crosslinked UHMWPE are sensitive to small differences in the loading pattern.     

A hip simulator study of the influence of patient activity level on the wear of crosslinked polyethylene under smooth and roughened femoral conditions

Over the last three decades, tribological studies of polyethylene total hip replacements have been undertaken using a simplified model of normal walking. As hip prostheses are being implanted in younger and more active patients, coupled with the increased wear resistance of crosslinked polyethylene, such in vitro approximations in activity are very limiting. Using a hip joint simulator, the influence of a significant increase in patient activity was studied by applying a series of simulated walking, stumbling and jogging sequences, at varying cycle speeds, using crosslinked UHMWPE/CoCrMo components, with both smooth and roughened femoral heads. All tests were performed using 25% bovine calf serum, and all components were positioned physiologically. The effects on wear, frictional torque, counterface roughness, lubricant temperature, material deformation and particle morphology were measured and analysed. It was found that with smooth heads and non-constraining socket fixtures, the occurrence of excessive stumbling at 1 Hz (5 kN max) had a negligible effect on the wear rate of polyethylene, whilst simulated jogging at 1.75 Hz (4.5 kN max) only showed a median increase in wear volume of 40% compared to normal walking. Fast walking showed the largest wear rate, and was consistently greater than for simulated jogging, thus, suggesting that short periods of increased load and speed have a relatively small effect on polyethylene wear. However, increasing the femoral roughness R_a to 0.38 μm under simulated jogging, 1.75 Hz, led to a massive increase in wear and frictional torque, generating wear rates >3000 mm³/10⁶ cycles for crosslinked polyethylene. Surface topography, sliding speed and the type of socket fixturing were shown to be the most influential factors when simulating increased patient activity.     

Wear properties of UHMWPE/aromatic thermosetting copolyester blends in unlubricated sliding

To improve the wear resistance of ultrahigh molecular weight polyethylene (UHMWPE), blends of UHMWPE, and an aromatic thermosetting copolyester (ATSP) (50/50, v/v) were developed, taking advantage of the crosslinked structure and good wear resistance of ATSP. As a compatibilizer, poly(ethylene-co-acrylic acid) (PEA) was added into the blends with its contents changing from 0 to 20% (w/w). Dynamometer wear tests (sliding against stainless steel surface with contact pressure ranging from 600 to 2500 kPa) showed that the UHMWPE/ATSP blend with 10% PEA had lower wear rate than the UHMWPE sample. The improved wear resistance resulted from the change of the wear mechanism. Scanning electron microscopy (SEM) images of the worn surfaces revealed that the presence of ATSP and PEA would prevent the lamellar alignment in the UHMWPE phase and adding PEA effectively enhanced the interaction between UHMWPE and ATSP.     

Type of motion and lubricant in wear simulation of polyethylene acetabular cup.

The wear of ultra-high molecular weight polyethylene, the most commonly used bearing material in prosthetic joints, is often substantial, posing a significant clinical problem. For a long time, there has been a need for simple but still realistic wear test devices for prosthetic joint materials. The wear factors produced by earlier reciprocating and unidirectionally rotating wear test devices for polyethylene are typically two orders of magnitude too low, both in water and in serum lubrication. Wear is negligible even under multidirectional motion in water. A twelve-station, circularly translating pin-on-disc (CTPOD) device and a modification of the established biaxial rocking motion hip joint simulator were built. With these simple and inexpensive devices, and with the established three-axis hip joint simulator, realistic wear simulation was achieved. This was due to serum lubrication and to the fact that the direction of sliding constantly changed relative to the polyethylene specimen. The type and magnitude of load was found to be less important. The CTPOD tests showed that the subsurface brittle region, which results from gamma irradiation sterilization of polyethylene in air, has poor wear resistance. Phospholipid and soy protein lubrication resulted in unrealistic wear. The introduction of devices like CTPOD may boost wear studies, rendering them feasible without heavy investment.     

Evaluation of a hip joint simulator

To evaluate the functioning of the Durham hip joint wear simulator, the wear rates of ultra high molecular weight polyethylene (UHMWPE) and polytetrafluoroethylene (PTFE) acetabular cups articulating against 22 mm diameter cobalt-chromium-molybdenum (CoCrMo) femoral heads were studied. A wear test was conducted in a lubricant of distilled water at 37 degrees C for a duration of 4.8 million cycles. The average penetration rate for the CoCrMo femoral heads against UHMWPE acetabular cups was 0.03 mm/10(6) cycles, while penetration rate for PTFE cups was some twenty times greater. These results are of a similar order of magnitude to other simulator studies in distilled water and are in a similar ratio to clinical data.     

Investigation of an Approach to Balance Wear Resistance and Mechanical Properties of Crosslinked UHMWPE

In order to produce ultra-high molecular weight polyethylene (UHMWPE) with increased wear resistance for bearing applications, yet minimize degradation in stiffness and oxidation resistance, the authors have proposed irradiation crosslinking of the polymer at moderate radiation doses. It was theorized that this would minimize or eliminate the need for a subsequent stabilization of free radicals, and thus retain a significant proportion of the original stiffness of the crystalline polymer. In this study, four groups of UHMWPE were investigated: (a) non-crosslinked, (b) crosslinked with 50 kGy electron-beam irradiation, thermally stabilized, (c) crosslinked, non-stabilized, and (d) crosslinked, non-stabilized, and exposed to accelerated oxidation to simulate prolonged exposure. Crosslinking and subsequent thermal stabilization significantly reduced the elastic modulus of UHMWPE, while omission of the stabilization step yielded a more moderate reduction in elastic modulus. It was shown that thermal stabilization reduced crystallinity more so than did omission of stabilization. Accelerated aging did not further decrease the storage modulus of UHMWPE over that of non-stabilized, non-aged polymer. Crosslinking showed a significant increase in wear resistance, while aging of the non-stabilized polymer showed no adverse effects on wear. These results suggest the potential for the use of moderate UHMWPE crosslinking without stabilization for industrial and some biomedical applications.     

A new formulation for the prediction of polyethylene wear in artificial hip joints

Artificial joints employing ultra-high molecular weight polyethylene (UHMWPE) are widely used to treat joint diseases and trauma. Wear of the polymer bearing surface largely limits the use of these joints in younger and more active patients. Previous studies have shown the wear factor used in Archard's law for the conventional polyethylene to be highly dependent on contact pressure and this has produced variability in experimental data and has constrained the reliability and applicability of previous computational predictions. A new wear law is proposed, based on wear volume being dependent on, and proportional to, the product of the sliding distance and contact area. The dimensionless proportional constant, wear coefficient, which was independent of contact pressure, was determined from a multi-directional pin on plate study. This was used in computational predictions of the wear of the conventional UHMWPE hip joints. The wear of the polyethylene cup was independently experimentally determined in physiological full hip joint simulator studies. The predicted wear rate from the new computational model was generally increased, with an improved agreement with the experimental measurement compared with the previous computational model. It was shown that wear in the UHMWPE hip joints increased as head size and contact area increased. This resulted in a much larger increase in the wear rate as the head size increased, compared with the previous computational model, and is consistent with clinical observations. This new understanding of the wear mechanism in artificial joints using the UHMWPE bearing surfaces, and the improved ability to predict wear independently and to address previously described discrepancies offer new opportunities to optimize design parameters.     

Development and testing of a novel joint wear simulator and investigation of the viability of an elastomeric polyurethane for total-joint arthroplasty devices

The use of artificial joints for the treatment of degenerative diseases of the hip and knee is becoming more widespread as life expectancy increases. Because of the latter, there is also the need for joints of higher durability than the commonly used artificial joints with ultra-high molecular weight polyethylene (UHMWPE) articulating against a metallic counterface. This requires the use and testing of novel materials. Relatively inexpensive and effective screening devices that would allow investigators to rapidly characterize the wear behavior of such materials are thus needed. This paper reports the design and development of a Dual Axis Wear Simulator (DAWS) to screen materials for wear behavior in a simulated in vivo environment. The machine allows for direct control of the applied normal load and the two-dimensional wear path shape of the pin against a cylindrical counterface. With this new machine, the effects of wear path shape and applied load were investigated. A 39 N load coupled with a 6.4-mm square wear path was shown to produce wear amounts comparable to those from other screening devices and also from artificial hips retrieved after in vivo use. The results further showed the importance of multidirectional sliding motion and a trend for higher wear rates as the aspect ratio of the wear path was increased. The wear rate of polytetrafluoroethylene was found to be orders of magnitude higher than that of UHMWPE, while that of polyacetal was somewhat lower. The development and use of compliant materials that simulate the mechanical properties of natural articular cartilage will likely lengthen the pain-free lifetimes of artificial joints. In view of this, the elastomeric polyurethane Pellethane™ 2363-80A, which is currently used in non-orthopedic biomedical applications, was tested for wear and its wear rate was found to be much lower than that of UHMWPE, likely due to its ability to conform to the counterface and thus reduce the contact pressure. This investigation showed the DAWS to be an effective wear simulator for the screening of new biomaterials for use in artificial joints and will be useful in the development of such joints.     

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.     

Carbon fiber reinforced polyether ether ketone composite as a bearing surface for total hip replacement

This paper investigates the tribological performance of a carbon fiber reinforced PEEK composite as a bearing surface for total hip replacement. Extensive hip joint simulator tests were conducted to optimize the microstructure of the composite and the counterface material. A softer and more graphitic carbon fiber is preferred to a harder and more abrasive fiber. A ceramic counterface is preferred to a metal counterface. An excellent wear couple was identified to be a 30 wt% pitch-based carbon fiber reinforced PEEK composite acetabular insert articulating against a zirconia ceramic head. When tested on a hip simulator run for 10 million cycles, a reduction in the wear rate of almost two orders of magnitude was achieved with this wear couple in comparison to a conventional UHMWPE/metal or UHMWPE/ceramic couple.     

交联超高分子量聚乙烯人工髋关节磨损仿真模拟

人工髋关节超高分子量聚乙烯（UHMWPE）关节面磨损仍是影响置换关节远期寿命的主要因素,其仿真建模是对关节模拟机磨损测试手段的重要补充,也是实现置换关节临床前性能评估的有效方法。由多向运动产生的交叉剪切效应是影响UHMWPE磨损的主要原因之一,也是仿真建模的关键。现有理论方法将磨损深度确定为滑动距离的函数,并将90°交叉剪切运动条件下的磨损作为度量基准计算不同角度下的交叉剪切效应,但尚未考虑接触应力变量对磨损深度的影响。针对以上问题,提出了在垂直交叉剪切运动条件下将磨损深度表示为摩擦功函数的方法。该方法利用UHMWPE摩擦因数与接触压力的定量关系计算摩擦因数并确定摩擦功,解决了UHMWPE磨损交叉剪切效应中滑动距离与接触应力的耦合问题。基于磨损仿真新模型研究了36 mm直径的交联UHMWPE髋关节,并与已有ProSim模拟机试验结果进行了验证。结果显示该仿真模型可准确计算体积磨损和线性磨损等磨损量以及髋关节载荷方向改变对磨损的影响。磨损新模型为进一步仿真模拟奠定了有效基础。     

An in vitro wear study of alumina-alumina total hip prostheses.

Four 28 mm diameter alumina-alumina hip prostheses were tested in the Mkll Durham hip simulator for 5 x 10(6) cycles using 25 per cent bovine serum as lubricant. Wear of the heads and cups was measured gravimetrically. The mean and standard deviation of the wear rate for the alumina cups was 0.097 +/- 0.039 mm3/10(6) cycles. The femoral heads produced such low wear that it could not be measured by weighing but could be detected byincreased surface roughness measurements. Such low wear rates represent about one-five-hundredthof the wear of ultra-high molecular weight polyethylene (UHMWPE) against ceramic in a similar test and supports work which indicates that fluid film lubrication exists in these joints.     

Improved mathematical model of the wear of the cup articular surface in hip joint prostheses and comparison with retrieved components

This paper presents an analytical model of the cobalt-based alloy-ultra-high molecular weight polyethylene (UHMWPE) wear coupling. Based on a previous model in which the cup wear volume over a gait cycle (WG) was calculated under the simplifying assumption of an ideal rigid coupling, the current version proposes a more realistic wear simulation. All three components of the hip loading force were considered for the contact pressure calculation and all three components of the hip motion were taken into account for the sliding distance calculation. The contact pressure distribution was calculated on the basis of the Hertzian theory for the elastic contact of two bodies with non-conforming geometrical shapes. The wear factor was taken from hip simulator wear tests. The calculated WG is 67 x 10(-6) mm3 for a standard reference patient. The parametric model simulations show that WG increases linearly with the patient weight, femoral head diameter and surface roughness. It increases non-linearly to a maximum and decreases to an asymptotic value with increasing cup/head clearance and with cup isotropic elastic modulus. The cup orientation in the pelvis affects only slightly the total amount of WG whereas it is the dominant factor affecting the shape of the wear distribution. The iso-wear maps show paracentral patterns at low cup inclination angles and marginal patterns at higher inclination angles. The maximum wear depth is supero-posterior when the cup is in neutral alignment and supero-anterior at increasing anteversion angles. Complex patterns with a combination of paracentral and marginal wear were obtained at specific clearance values and cup orientations. The results of the simulations are discussed in relation to the wear distribution measured on the articular surface of 12 UHMWPE components retrieved from failed hip joint prostheses, after a period of in situ functioning.