A comparative joint simulator study of the wear of metal-on-metal and alternative material combinations in hip replacements

While total hip replacement represents the major success story in orthopaedic surgery in the twentieth century, there is much interest in extending even further, early in the twenty first century, the life of implants. Osteolysis has been identified as a major factor limiting the life of prostheses, with indications that fine polyethylene wear debris, generated primarily at the interface between the femoral head and the acetabular cup, promotes the process. There is therefore considerable interest in the introduction of alternative wear resistant systems to limit the deleterious effects of wear. These alternatives include ceramic-on-ceramic and metal-on-metal configurations and the present paper is primarily concerned with the latter. Some six pairs of new metal-on-metal implants of 36 mm diameter and four pairs of existing metal-on-metal implants of 28 mm diameter were tested in a ten-station hip joint simulator in the presence of a 25 per cent bovine serum solution. The implants were tested in the anatomical position to 5 x 10(6) cycles. The new heads and cups were manufactured from CoCrMo alloy with careful attention being paid to sphericity and surface finish of both components. The wear performance of the new and existing metal-on-metal total hip replacements have been evaluated and compared. The overall wear rates have then been compared with previously reported wear rates for a zirconia-on-polyethylene prosthesis of 22 mm diameter tested on the same simulator. The comparison is taken further by recalling published penetration data for metal-on-polyethylene implants of 22 and 28 mm diameter and converting these to volumetric wear rates. It was found that the heads and cups in metal-on-metal joints wore by almost equal amounts and that the opposing surfaces converged to similar surface roughness as the testing time increased. Steady state wear rates were generally achieved after 1-2 x 10(6) cycles. The mean long-term wear rates for the metal-on-metal prostheses were very low, being 0.36 mm3/10(6) cycles and 0.45 mm3/10(6) cycles for the new implants of 36 mm diameter and established implants of 28 mm diameter respectively. These wear rates compare with 6.3 mm3/10(6) cycles for zirconia-on-ultra-high molecular weight polyethylene tested on the same simulator and representative clinical values for metal-on-polyethylene of 36 mm3/year for heads of 22 mm diameter and a reported range of 60-180 mm3/year for 28 mm heads. These values do not translate directly into numbers of particles, since the metallic debris from metal-on-metal joints is very fine. The number of metallic particles may exceed the number of polyethylene wear particles from an otherwise similar metal-on-polyethylene joint by a factor of 10(3). A detailed discussion of the size and morphology of wear debris and tissue reaction to various forms of debris is beyond the scope of this paper, but the biological response to polymeric, metallic and ceramic wear debris forms a major subject for further study. The present investigation nevertheless confirms the potential of carefully designed and manufactured metal-on-metal total replacement joints for the treatment of diseased and damaged hips.     

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.     

Tribological assessment of a flexible carbon-fibre-reinforced poly(ether-ether-ketone) acetabular cup articulating against an alumina femoral head

New material combinations have been introduced as the bearing surfaces of hip prostheses in an attempt to prolong their life by overcoming the problems of failure due to wear-particle-induced osteolysis. This will hopefully reduce the need for revision surgery. The study detailed here used a hip simulator to assess the volumetric wear rates of large-diameter carbon-fibre-reinforced pitch-based poly(ether-ether-ketone) (CFR-PEEK) acetabular cups articulating against alumina femoral heads. The joints were tested for 25 x 10(6) cycles. Friction tests were also performed on these joints to determine the lubrication regime under which they operate. The average volumetric wear rate of the CFR-PEEK acetabular component of 54 mm diameter was 1.16 mm(3)/10(6) cycles, compared with 38.6 mm(3)/10(6) cycles for an ultra-high-molecular-weight polyethylene acetabular component of 28 mm diameter worn against a ceramic head. This extremely low wear rate was sustained over 25 x 10(6) cycles (the equivalent of up to approximately 25 years in vivo). The frictional studies showed that the joints worked under the mixed-boundary lubrication regime. The low wear produced by these joints showed that this novel joint couple offers low wear rates and therefore may be an alternative material choice for the reduction of osteolysis.     

Metal-on-metal hip simulator study of increased wear particle surface area due to 'severe' patient activity

This study investigated changes in metal-on-metal (MOM) hip wear and wear particle characteristics arising from a more aggressive patient activity level compared with normal walking. The test hypothesis was that 'severe'-gait conditions will change wear, wear particle sizes, and morphology owing to a decline in joint lubrication. Four carbon MOM hip bearings 40 mm high were subjected to normal-walking and fast-jogging simulations in an orbital hip joint simulator with 25 per cent alpha-calf serum as a lubricant. Co-Cr-Mo wear particles were extracted using an enzymatic method, and prolate ellipsoid equations were used to estimate particle volume and surface area. Fast-jogging simulations generated a sevenfold increase in volumetric wear, a 33 per cent increase in mean wear particle size, and a threefold increase in the number of larger (needle) particles compared with walking. This resulted in a twentyfold increase in total wear particle surface area per 10(6) cycles compared with walking, thereby confirming our hypothesis. The clinical significance of this result suggests that highly active MOM patients may exhibit greater ion release than less active patients.     

In vitro comparison of the two hard-hard articulations for total hip replacements

Polyethylene particle disease is one of the major causes of late aseptic loosening of total hip replacement. Two hard-hard articulations (alumina-on-alumina and metal-on-metal) have been developed in Europe as an alternative to the ultra-high molecular weight polyethylene (UHMWPE) articulations. Even though these hard-hard articulations are on the market and numerous reports have been published about them, only a very limited number of studies allowing a direct in vitro comparison of the two articulations have been published so far. This paper compares in vitro these two types of articulation (alumina-on-alumina and metal-on-metal), which have been tested with a hip simulator for their tribological behaviour using exactly the same experimental methodology. This comparison shows that these two types of hard-hard articulation have very similar abrasive wear behaviour with four main features: 1. A running-in wear period (1 x 10(6) cycles) gives a cumulative wear of about 20 microns with head diameters of 28 mm. 2. After the running-in wear, there is a stabilization of the linear wear behaviour with a low linear wear rate/10(6) cycles for both types of articulation. 3. The volumetric wear rate of both articulations (< 2.0 mm3/year for head diameters of 28 mm) is significantly lower than that observed for metal-on-polyethylene or ceramic-on-polyethylene articulations having the same head diameter. 4. Abrasive wear is readily apparent (indicating a mixed lubrication regime) with both types of articulation. The extremely low wear performance of these articulations is confirmed and they constitute a low-wear alternative to the UHMWPE articulations currently used.     

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).     

Metal-on-metal hip joint tribology

The basic tribological features of metal-on-metal total hip replacements have been reviewed to facilitate an understanding of the engineering science underpinning the renaissance of these hard-on-hard joints. Metal-on-polymer hip replacements operate in the boundary lubrication regime, thus leading to the design guidance to reduce the femoral head diameter as much as is feasible to minimize frictional torque and volumetric wear. This explains why the gold-standard implant of this form from the past half-century had a diameter of only 22.225 mm (7/8 in). Metal-on-metal implants can operate in the mild mixed lubrication regime in which much of the applied load is supported by elastohydrodynamic films. Correct tribological design leads to remarkably low steady state wear rates. Promotion of the most effective elastohydrodynamic films calls for the largest possible head diameters and the smallest clearances that can reasonably be adopted, consistent with fine surface finishes, good sphericity and minimal structural elastic deformation of the cup on its foundations. This guidance, which is opposite in form to that developed for metal-on-polymer joints, is equally valid for solid (monolithic) metallic heads on metallic femoral stems and surface replacement femoral shells. Laboratory measurements of friction and wear in metal-on-metal joints have confirmed their potential to achieve a very mild form of mixed lubrication. The key lies in the generation of effective elastohydrodynamic lubricating films of adequate thickness compared with the composite roughness of the head and cup. The calculation of the film thickness is by no means easy, but the full procedure is outlined and the use of an empirical formula that displays good agreement with calculations based upon the full numerical solutions is explained. The representation of the lambda ratio, lambda, embracing both film thickness and composite roughness, is described.     

A 12-station anatomic hip joint simulator

A novel 12-station hip joint simulator with an anatomic position of the prosthesis was designed and built. The motion of the simulator consists of flexion-extension and abduction-adduction. The load is of the double-peak type. The validation test was done with three similar 28 mm CoCr-polyethylene joints in diluted calf serum lubricant for 3.3 x 10(6) cycles. The bearing surfaces of the polyethylene cups were burnished, the CoCr heads were undamaged, the wear particles were in the 0.1-1 microm size range, and the mean wear factor of the polyethylene cups was 5.7 x 10(-7) mm(3)/N m. These essential observations were in good agreement with clinical findings. In addition, three similar 50 mm CoCr/CoCr joints, representing the contemporary large-diameter metal-on-metal articulation were tested. The wear of the CoCr/CoCr joints was calculated from the Co and Cr concentrations of the used lubricant quantified with atomic absorption spectroscopy. The bearing surfaces of the CoCr/CoCr joints showed mild criss-cross scratching only. The average wear factor of polyethylene cups was 275 times that of the CoCr/CoCr joints. The tribological behaviour of the large-dia. CoCr/CoCr appeared to be dominated by fluid film lubrication, as indicated by very low frictional heating and wear, making it tribologically superior to the conventional CoCr/polyethylene, and therefore very interesting clinically. In conclusion, the simulator proved to be a valid, reliable, practical, economical, and easy-to-operate tool for wear studies of various hip replacement designs.     

Predictive role of the lambda ratio in the evaluation of metal-on-metal total hip replacement

The wear of metal-on-metal bearings is affected by various design parameters, such as the clearance or surface roughness. It would be very useful to have a significant indicator of wear according to these design parameters, such as the lambda ratio. Three different batches of cast high- and low-carbon cobalt-chromium hip implants (28 mm, 32 mm, and 36 mm diameters) were tested in a hip joint simulator for 2 x 10(6) cycles. Bovine calf serum was used as lubricant, and the samples were weighed at regular intervals during the test. The predictive role of the lambda ratio on the wear behaviour was investigated. Three different configurations were tested to explore the wear rate for a broad range of lambda ratios. The results of these studies clearly showed that the femoral heads of 36 mm diameter had the best wear behaviour with respect to the other two smaller configurations tested. From a predictive point of view, the lambda ratios associated with the configurations tested could clearly indicate that the femoral heads of 36 mm diameter worked in the mixed-lubrication regime (lambda > 1); all the smallest configurations (28mm size) had lambda < 1, thus showing their aptitude to work in the boundary lubrication regime, with substantially higher volume depletion due to wear. The lambda values associated with the 32 mm size varied in a range around 1 (0.95 < lambda < 1.16), suggesting the possibility of operating in the mixed-lubrication regime.     

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.     

The effect of 'running-in' on the tribology and surface morphology of metal-on-metal Birmingham hip resurfacing device in simulator studies

It is well documented that hard bearing combinations show a running-in phenomenon in vitro and there is also some evidence of this from retrieval studies. In order to investigate this phenomenon, five Birmingham hip resurfacing devices were tested in a hip wear simulator. One of these (joint 1) was also tested in a friction simulator before, during, and after the wear test and surface analysis was conducted throughout portions of the testing. The wear showed the classical running in with the wear rate falling from 1.84 mm3 per 10(6) cycles for the first 10(6) cycles of testing to 0.24 mm3 per 10(6) cycles over the final 2 x 10(6) cycles of testing. The friction tests suggested boundary lubrication initially, but at 1 x 10(6) cycles a mixed lubrication regime was evident. By 2 x 10(6) cycles the classical Stribeck curve had formed, indicating a considerable contribution from the fluid film at higher viscosities. This continued to be evident at both 3 x 10(6) and 5 x 10(6) cycles. The surface study complements these findings.     

The wear of high-carbon metal-on-metal bearings after different heat treatments

To study the tribological performance of metal-on-metal hip joint resurfacings, the wear performance of three pairs of Co-Cr-Mo alloy samples (pins and plates) were tested in a multidirectional pin-on-plate wear machine. An 'as-cast', a single-heat-treated, and a double-heat-treated set of specimens were tested to 3 x 10(6) cycles. The two heat treatments resulted in partial and full solution of the carbides into the matrix. An increasing trend in wear rate was found from 'as-cast' to the double-heat-treated specimens. The as-cast specimens showed the lowest wear rate (1.69 x 10(-6) mm3/N m), the reduced carbide samples had the next lowest wear rate (2.1 x 10(-6) mm3/N m), while the specimens without carbides wore the most (2.41 x 10(-6) mm3/N m).     

'Severe' wear challenge to 'as-cast' and 'double heat-treated' large-diameter metal-on-metal hip bearings

The wear generation of double-heat-treated and as-cast large-diameter metal-on-metal (MOM) hip bearings was investigated using standard- and 'severe'-gait simulations. The test hypothesis was that double heat treatment would change MOM hip wear compared with the as-cast condition. Two groups of high-carbon MOM bearings of 40 mm diameter were manufactured and subjected to either hot isostatic pressing (HIP) and solution annealing (SA) or no heat treatment (as cast). The results showed no statistical difference between the two groups under both running-in and steady state conditions. Even under the most 'severe'-gait simulation published to date, the mean volumetric wear rates were 2.9 and 3.9 mm3 per 10(6) cycles for the HIP-SA and as-cast bearings respectively, showing a ten-fold increase in wear compared with walking. These differences were not statistically different; therefore our hypothesis was negated. Changes in alloy microstructure do not appear to influence the wear behaviour of high-carbon cast MOM articulations with similar chemical compositions. This is in sharp contrast with the published significance of bearing diameter and radial clearance on the wear of MOM hip bearings.     

A multi-station hip joint simulator study of the performance of 22 mm diameter zirconia-ultra-high molecular weight polyethylene total replacement hip joints.

The commissioning of a new form of 10-station hip joint simulator is described and the results of a study of the performance of zirconia-ultra-high molecular weight polyethylene (UHMWPE) total replacement hip joints in the familiar Charnley head size of 7/8 inch (22.225 mm) diameter are presented. The head size is referred to as 22 mm for brevity and consistency throughout the paper. The simulator provided very consistent and repeatable results and the new machine, together with the methods of investigation adopted, offer an excellent facility for the further evaluation of existing and new prostheses. The findings are compared with the outcome of previous laboratory simulator and clinical studies of ceramic-polyethylene implants of similar diameter. It was found that a relatively rapid penetration of the head into the cup was followed by a very low, steady, long-term penetration rate after about two million loading cycles. The mean long-term volumetric penetration rate was 6.28 mm3/10(6) loading cycles. When the linear penetration rates were assessed by direct measurement on a coordinate measuring machine, or deduced from the tunnelling expression, the resulting values were very similar and small at 0.019 and 0.016 mm/10(6) loading cycles respectively. It is generally assumed that one million loading cycles is equivalent to about one year of service in the body and if this equivalence is accepted, these penetration rates compare very favourably with a clinical evaluation of alumina heads of the same diameter, which yielded a mean long-term penetration rate of 0.022 mm/year.     

A five-station hip joint simulator

A five-station hip joint wear simulator was designed and built which featured simplified motion and loading. An elliptical wear path was produced using approximately sinusoidal motion in the flexion/extension and internal/external rotation axes and the dynamic loading approximated to a square wave. Five 28 mm diameter zirconia femoral heads articulated against ultra-high molecular weight polyethylene acetabular cups in 25 per cent bovine serum for 5 x 10(6) cycles. Gravimetric wear measurement was used with moisture absorption compensation using a dynamically loaded soak control. With motion of physiological magnitude, the mean acetabular cup wear rate was 52.2 mm3/10(6) cycles which is comparable with a number of clinical studies.     

Influence of simulator kinematics on the wear of metal-on-metal hip prostheses

There is now considerable interest in metal-on-metal bearings for hip prostheses. Extremely low wear rates (0.1 mm3/10(6) cycles) have been reported in some simulator studies, while in vivo studies, although still very low, have shown wear rates of the order of 1 mm3/10(6) cycles. The aim of this study was to compare wear rates of metal-on-metal bearings in two hip simulators with different kinematic inputs. In the simulator with three independent input motions which produced an open elliptical wear path with a low level of eccentricity, the wear rates were very low as recorded previously in other simulators. In the simulator with two input motions which produced an open elliptical wear path with greater eccentricity the wear rate was at least ten times higher and closer to clinical values. The motion and kinematic conditions in the contact are critical determinants of wear in metal-on-metal bearings.     

Simplified motion and loading compared to physiological motion and loading in a hip joint simulator

Two wear tests were conducted using the Durham Hip Joint Wear Simulator to investigate the effects of simplified motion and loading on ultra-high molecular weight polyethylene (UHMWPE) acetabular cup wear rates. Bovine serum was used as a lubricant and a gravimetric technique was used to measure wear. The first wear test duration was 7.1 x 10(6) cycles and investigated the effect of simplified loading. This was achieved by using full physiological motion and loading for the first 5 x 10(6) cycles of the test, then physiological motion with simplified loading for the final 2.1 x 10(6) cycles of the wear test. The UHMWPE acetabular cup wear rates using full physiological motion and loading were 32.2 and 51.7 mm3/10(6) cycles against zirconia and CoCrMo femoral heads respectively. Using simplified loading the cup wear rates were 30.1 and 49.2 mm3/10(6) cycles against zirconia and CoCrMo respectively which was not significantly different from wear rates with physiological loading. The effect of simplified motion was investigated in a second wear test of 5.0 x 10(6) cycles duration. Physiological loading was applied across the prosthesis with physiological motion in the flexion/extension plane only. Mean wear of the acetabular component dropped to 0.197 mm3/10(6) cycles. The surfaces of all the acetabular cups were subject to gross examination, optical microscopy and scanning electron microscopy. No notable difference was observed between the cups subjected to physiological motion and loading and those subjected to simplified loading. The cups worn with a single plane of motion had a much smaller worn area and a notable difference in surface features to the other cups. Simplifed loading is therefore an acceptable simplification in simulator testing but simplifying motion to the flexion/extension plane axis only is unacceptable.     

The tribological behaviour of different clearance MOM hip joints with lubricants of physiological viscosities

Clearance is one of the most influential parameters on the tribological performance of metal-on-metal (MOM) hip joints and its selection is a subject of considerable debate. The objective of this paper is to study the lubrication behaviour of different clearances for MOM hip joints within the range of human physiological and pathological fluid viscosities. The frictional torques developed by MOM hip joints with a 50 mm diameter were measured for both virgin surfaces and during a wear simulator test. Joints were manufactured with three different diametral clearances: 20, 100, and 200 microm. The fluid used for the friction measurements which contained different ratios of 25 percent newborn calf serum and carboxymethyl cellulose (CMC) with the obtained viscosities values ranging from 0.001 to 0.71 Pa s. The obtained results indicate that the frictional torque for the 20 microm clearance joint remains high over the whole range of the viscosity values. The frictional torque of the 100 microm clearance joint was low for the very low viscosity (0.001 Pa s) lubricant, but increased with increasing viscosity value. The frictional torque of the 200 microm clearance joint was high at very low viscosity levels, however, it reduced with increasing viscosity. It is concluded that a smaller clearance level can enhance the formation of an elastohydrodynamic lubrication (EHL) film, but this is at the cost of preventing fluid recovery between the bearing surfaces during the unloaded phase of walking. Larger clearance bearings allow a better recovery of lubricant during the unloaded phase, which is necessary for higher viscosity lubricants. The selection of the clearance value should therefore consider both the formation of the EHL film and the fluid recovery as a function of the physiological viscosity in order to get an optimal tribological performance for MOM hip joints. The application of either 25 per cent bovine serum or water in existing in vitro tribological study should also be revised to consider the relevance of clinic synovial fluid viscosities and to avoid possible misleading results.     

Comparison of friction and lubrication of different hip prostheses

It is well documented that an important cause of osteolysis and subsequent loosening of replacement hip joints is polyethylene wear debris. To avoid this, interest has been renewed in metal-on-metal and ceramic-on-ceramic prostheses. Various workers have assessed the lubrication modes of different joints by measuring the friction at the bearing surfaces, using different lubricants. Measurements of friction factors of a series of hip prostheses were undertaken using carboxymethyl cellulose (CMC) fluids, silicone fluids, synovial fluid and different concentrations of bovine serum as the lubricant. The experimental results were compared with theoretical predictions of film thicknesses and lubrication modes. A strong correlation was observed between experiment and theory when employing CMC fluids or silicone fluids as the lubricant. Mixed lubrication was found to occur in the metal-on-metal (CoCrMo/CoCrMo) joints with all lubricants at a viscosity within the physiological range. This was also the case for the metal-on-plastic (CoCrMo/ultra-high molecular weight polyethylene) joints. The ceramic-on-ceramic (Al2O3/Al2O3) joints, however, exhibited full fluid film lubrication with the synthetic lubricants but mixed lubrication with the biological lubricants. Employing a biological fluid as the lubricant affected the friction to varying degrees when compared with the synthetic lubricants. In the case of the ceramic-on-ceramic joints it acted to increase the friction factor tenfold; however, for the metal-on-metal joints, biological fluids gave slightly lower friction than the synthetic lubricants did. This suggests that, when measuring friction and wear of artificial joints, a standard lubricant should be used.     

Biological effects of clinically relevant wear particles from metal-on-metal hip prostheses

The problems of osteolysis and late aseptic loosening associated with ultra-high molecular weight polyethylene (UHMWPE) particles has lead to a renewed interest in metal-on-metal prostheses. Wear particles generated by modern Co-Cr-on-Co-Cr prostheses are nanometre in size (range: 10-120 nm; mean: about 40 nm), an order of magnitude smaller than the size of UHMWPE known to be critical for activation of osteolytic cytokines by macrophages. Co-Cr wear particles will induce osteolytic cytokine production by human macrophages, but only at high volumetric concentrations. Unlike UHMWPE, Co-Cr particles are not inert. Co-Cr particles have the potential to release metal ions; they may be toxic to cells, induce deoxyribonucleic acid damage or cause host hypersensitivity. The nanometre size range of Co-Cr wear particles means that they may be disseminated widely in the body. The potential for metal-on-metal bearings to induce adverse effects clinically will be dependent upon the rate of wear. What constitutes a safe wear rate for modern metal-on-metal bearings is unknown. However, the wear of metal-on-metal prostheses is critically dependent upon the design and, in particular, the carbon content of the alloy, the radial clearance, and the head diameter. Thus, the potential for adverse biological reactions associated with metal-on-metal bearings can be reduced by selection of appropriately designed implants.