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

Does diametrical clearance influence the wear of Pinnacle hip implants?

Aims

The optimum clearance between the bearing surfaces of hip arthroplasties is unknown. Theoretically, to minimize wear, it is understood that clearances must be low enough to maintain optimal contact pressure and fluid film lubrication, while being large enough to allow lubricant recovery and reduce contact patch size. This study aimed to identify the relationship between diametrical clearance and volumetric wear, through the analysis of retrieved components.

Methods

A total of 81 metal-on-metal Pinnacle hips paired with 12/14 stems were included in this study. Geometrical analysis was performed on each component, using coordinate and roundness measuring machines. The relationship between their as-manufactured diametrical clearance and volumetric wear was investigated. The Mann-Whitney U test and unpaired t-test were used, in addition to calculating the non-parametric Spearman's correlation coefficient, to statistically evaluate the acquired data.

Results

The hips in this study were found to have had a median unworn diametrical clearance of 90.31 μm (interquartile range (IQR) 77.59 to 97.40); 32% (n = 26) were found to have been below the manufacturing tolerance. There was no correlation found between clearance and bearing (r_s = -0.0004, p = 0.997) or taper (r_s = 0.0048, p = 0.966) wear rates. The wear performance of hips manufactured within and below these specifications was not significantly different (bearing: p = 0.395; taper: p = 0.653). Pinnacles manufactured from 2007 onwards had a greater prevalence of bearing clearance below tolerance (p = 0.004).

Conclusion

The diametrical clearance of Pinnacle hips did not influence their wear performance, even when below the manufacturing tolerance. The optimum clearance for minimizing hip implant wear remains unclear.Cite this article: Bone Joint Res 2020;9(8):515-523.

Effect of Biomet Recap Diameter and Clearance on Friction and Lubrication Behaviour Using Serum-Based Lubricants

The friction and lubrication behaviour of six Biomet ReCap components with nominal diameters of 46, 48, 50, 52, 54 and 56 mm and diametral clearances in the range 66-314μm have been investigated using a friction hip simulator. Friction testing was carried out using pure bovine serum (BS) and aqueous solutions of BS with and without carboxymethyl cellulose (CMC), adjusted to a range of viscosities (0.001-0.24 Pas). The Stribeck analyses suggested mixed lubrication as the dominant mode for all the ReCaps, but depending on the diameter and mainly clearance, the ReCaps articulated at different range of friction factors within the as mentioned viscosity range. The 56 and 54mm ReCaps with 233μm clearances had lower friction factors in the range 0.0311-0.079 and 0.053-0.11, respectively, as compared to the 46mm ReCap with 66μm clearance having friction factors in the range 0.0915-0.1558. The Stribeck curves showed the lowest achievable friction factors for clearances of greater than 150μm and less than 240μm. The 50mm ReCap with the highest clearance of 314μm had the highest friction factors in the range 0.1277-0.1515 suggesting that this clearance cannot be considered as the optimum gap between the bearings.

Can physical joint simulators be used to anticipate clinical wear problems of new joint replacement implants prior to market release?

One of the most important mandates of physical joint simulators is to provide test results that allow the implant manufacturer to anticipate and perhaps avoid clinical wear problems with their new products. This is best done before market release. This study gives four steps to follow in conducting such wear simulator testing. Two major examples involving hip wear simulators are discussed in which attempts had been made to predict clinical wear performance prior to market release. The second one, involving the DePuy ASR implant systems, is chosen for more extensive treatment by making it an illustrative example to explore whether wear simulator testing can anticipate clinical wear problems. It is concluded that hip wear simulator testing did provide data in the academic literature that indicated some risk of clinical wear problems prior to market release of the ASR implant systems. This supports the idea that physical joint simulators have an important role in the pre-market testing of new joint replacement implants.

Simultaneous measurement of friction and wear in hip simulators

We propose and have evaluated a method to measure hip friction during wear testing on a popular multi-station hip simulator. A 6-degree-of-freedom load cell underneath the specimen sensed forces and torques during implant wear testing of simulated walking. This included internal–external and adduction–abduction rotations which are often neglected during friction testing on pendulum-type machines. Robust mathematical analysis and data processing provided friction estimates in three simultaneous orthogonal rotations, over extended multi-million cycle wear tests. We tested various bearing couples including metal-on-plastic, ceramic-on-plastic, and metal-on-metal material couples. In one test series, new and intentionally scratched CoCrMo 40-mm-diameter femoral heads were tested against conventional ultrahigh-molecular-weight polyethylene, highly cross-linked, and highly cross-linked with vitamin E versions. The scratching significantly increased friction and doubled the wear of all groups. Before scratching, friction levels for the aforementioned plastic groups were 0.056 ± 0.0060, 0.062 ± 0.0080, and 0.070 ± 0.0045, respectively, but after scratching increased to 0.088 ± 0.018, 0.076 ± 0.0066, and 0.082 ± 0.0049, respectively, all statistically significant increases (p = 0.00059, 0.00005, 0.0115, respectively). In another test series of 44-mm femoral head diameter hips, metal-on-plastic hips with conventional ultrahigh-molecular-weight polyethylene showed the lowest friction at 0.045 ± 0.0085, followed by highly cross-linked with 0.046 ± 0.0035 (not significantly different). In a ceramic-on-plastic design with conventional ultrahigh-molecular-weight polyethylene, higher friction 0.079 ± 0.0070 was measured likely due to that ceramic surface being rougher than usual. Metal-on-metal hips were compared without and with a TiN coating, resulting in 0.049 ± 0.014 and 0.097 ± 0.020 friction factors, respectively (statistically significant, p < 0.001), and the coating wore away on all coated hips eventually. Higher friction mostly correlated with higher wear or damage to femoral heads or implant coatings, except for the highly cross-linked wear resistant ultrahigh-molecular-weight polyethylene which had slightly higher friction, confirming the same finding in other independent studies. This type of friction measurements can help screen for clamping and elevated wear of metal-on-metal and resurfacing total hip replacements, surgical malpositioning, and abraded and otherwise damaged surfaces.

Wear of the Charité® lumbar intervertebral disc replacement investigated using an electro-mechanical spine simulator

The Charité^® lumbar intervertebral disc replacement was subjected to wear testing in an electro-mechanical spine simulator. Sinusoidally varying compression (0.6–2 kN, frequency 2 Hz), rotation (±2°, frequency 1 Hz), flexion–extension (6° to −3°, frequency 1 Hz) and lateral bending (±2°, frequency 1 Hz) were applied out of phase to specimens immersed in diluted calf serum at 37 °C. The mass of the ultra-high-molecular weight polyethylene component of the device was measured at intervals of 0.5, 1, 2, 3, 4 and 5 million cycles; its volume was also measured by micro-computed tomography. Total mass and volume losses were 60.3 ± 4.6 mg (mean ± standard deviation) and 64.6 ± 6.0 mm³. Corresponding wear rates were 12.0 ± 1.4 mg per million cycles and 12.8 ± 1.2 mm³ per million cycles; the rate of loss of volume corresponds to a mass loss of 11.9 ± 1.1 mg per million cycles, that is, the two sets of measurements of wear agree closely. Wear rates also agree closely with measurements made in another laboratory using the same protocol but using a conventional mechanical spine simulator.

Wear assessments of a new cervical spinal disk prosthesis: Influence of loading and kinematic patterns during in vitro wear simulation

Surgical treatment is one of the effective methods of treatment in cervical spondylosis. The traditional method of operation is decompression fusion; however, this surgery results in restricted movement of cervical vertebra and adjacent segment degeneration. Due to the deficiency of traditional surgery, scholars have widely carried out artificial cervical disk replacement surgery and have achieved good clinical effects. Comparing to the characteristics of the common artificial cervical disk which is used frequently, we developed a new artificial cervical intervertebral disk prosthesis. The purpose of this study was to determine the wear behavior in a cervical total disk replacement system. The total disk replacement system tested consists of a ultra-high-molecular-weight polyethylene inlay articulating between a Ti6Al4V alloy superior plate and an inferior plate, using a spine wear simulator, per the ISO 18192-1:2011 standard test methods. Three rotations and axial force were applied on each station. The specimens were removed at 5 × 10(5) and 10(6) cycles and at intervals of 10(6) cycles thereafter to determine the actual mass loss. The serum was replaced every 5 × 10(5) cycles. The specimens were changed periodically among the different stations. A mean ultrahigh molecular weight polyethylene inlay wear rate of 0.53 mg per million cycles (standard = 0.13 mg per 10(6) cycles) was found after 10(7) cycles. All inferior plates showed slight scratching after 10(7) cycles. The impingement wear simulation introduced here proved to be suitable to predict in vivo impingement behavior in regard to the contact pattern seen on retrieved devices of the Pretic-I disk arthroplasty design in a preclinical test.

Effect of head size on wear properties of metal-on-metal bearings of hip prostheses, and comparison with wear properties of metal-on-polyethylene bearings using hip simulator

The effects of articular head size on the wear losses of the metal insert and articular head for a metal-on-metal bearing were examined using a hip simulator manufactured to satisfy ISO 14242-1. The wear properties of metal-on-metal and metal-on-polyethylene bearings were also compared under the same conditions. The total wear losses of the metal insert and articular head decreased with increasing diameter of the metal insert in the range from 28 to 44mm. The total wear loss was greater for a diameter of 48mm than for a diameter of 44mm. When the articular metal insert diameter was smaller than 44mm, the wear loss was reduced because the contact surface pressure increased with increasing metal insert diameter. However, the increase in wear loss observed for the 48-mm-diameter insert might have been due to the considerable increase in the rotation moment with increasing insert diameter. The tendency of decreasing contact pressure calculated using the Hertzian contact stress equation nearly conformed to the change in wear loss. On the other hand, the wear loss of an artificial hip joint consisting of a cross-linked ultrahigh-molecular-weight polyethylene insert (UHMWPE) and a Co-Cr-Mo articular head was small.

Wear in metal-on-metal total disc arthroplasty

The wear of a model metal-on-metal ball-and-socket total disc arthroplasty was measured in a simulator. The ball had a radius of 10 mm, and there was a radial clearance between ball and socket of 0.015 mm. The model was subjected to simultaneous flexion–extension, lateral bending, axial rotation (frequency: 1 Hz) and compression (frequency: 2 Hz, maximum load: 2 kN). Throughout the tests, the models were immersed in calf serum diluted to a concentration of 15 g protein per litre, at a controlled temperature of 37 °C. Tests were performed on three models. At regular intervals (0, 0.5, 1, 2, 3, 4 and 5 million cycles), mass and surface roughness were determined; mass measurements were converted into the volume lost as a result of wear. All measurements were repeated six times. Wear occurred in two stages. In the first stage (duration about 1 million cycles), there was a linear wear rate of 2.01 ± 0.04 mm3 per million cycles; in the second stage, there was a linear wear rate of 0.76 ± 0.02 mm3 per million cycles. Surface roughness increased linearly in the first million cycles and then continued to increase linearly but more slowly.

What are the risks accompanying the reduced wear benefit of low-clearance hip resurfacing?

BACKGROUND

Clearance is an important determinant of metal-metal bearing function. Tribologic theory and laboratory evidence suggest low clearance (LC) reduces wear but with a potential to increase friction and clinical reports show LC resurfacings have high implant failure rates. Thus, the role of LC is unclear.

QUESTIONS/PURPOSES

We asked: is in vivo wear as reflected by cobalt (Co) and chromium (Cr) levels reduced in LC bearings, and if so, is this benefit offset by increased friction as assessed by implant-bone interface changes?

METHODS

We retrospectively reviewed 26 patients with LC resurfacings. We assessed Co and Cr levels in blood and urine, hip function, and radiographic adverse features. These data were compared with those from 26 patients with a similar resurfacing but with conventional clearance (CC) from a previous study. Minimum followup was 4.0 years (mean, 4.1 years; range, 4.0-4.7 years).

RESULTS

Co and Cr ion comparisons showed three phases: in the first 2 months, there was no difference between the cohorts; at 2 to 24 months, the CC group showed higher levels; and subsequently, levels in the two groups converged. A mean Oxford hip score of 13 and step activity of 1.9 million cycles per year in the LC group were similar to those of the CC group. Cup radiolucencies were seen in three patients in the LC group and none in the CC group.

CONCLUSIONS

Lower Co and Cr levels suggest lower wear in the LC resurfacings in the intermediate term, but the presence of radiolucencies raises the concern that higher bearing friction is affecting implant fixation. A larger clearance than the theoretically predicted ideal may be required to allow for minor manufacturing imperfections, component deformation, and progressive changes in the in vivo lubricant.

LEVEL OF EVIDENCE

Level III, therapeutic study. See the Guidelines for Authors for a complete description of levels of evidence.

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.

Wear of metal-on-metal hip bearings: metallurgical considerations after hip simulator studies

Metal-on-metal hip-bearing components with different percentages of carbon content (low and high carbon) were tested in 6 different configurations using a hip joint simulator. The aim of this study was to characterize metallurgical and tribological events occurring at the articulating surfaces of these articulations. Also, ion release was evaluated and correlated with wear. After the test, for the high-carbon components, carbides were observed below the matrix surface. In the low-carbon content components, most carbides were "carbide-free", while a minority were worn below the matrix surface with increased test cycles. In the cast alloy components, some carbides were pulled out resulting in micropits. Scanning microscope electron characterization of the tested specimens showed scratches and holes. The surface showed a dominant severe wear mechanism due to third-body particles. A greater amount of ions was released in the lubricant used during the wear test for the smaller diameter compared with the bigger one. This study showed that the metallurgical and tribological events taking place at the articulating surfaces of metal-metal hip implants are numerous and complex. The surface morphology after the test showed the effect of more critical working conditions with smaller diameters.

In vitro investigation of friction under edge-loading conditions for ceramic-on-ceramic total hip prosthesis

Edge-loading generates higher wear rates in ceramic-on-ceramic total hip prosthesis (THP). To investigate the friction coefficient (FC) in these conditions, three alumina ceramic (Biolox Forte) 32 mm-diameter components were tested using a hip friction simulator. The cup was positioned with a 75 degrees abduction angle to achieve edge-loading conditions. The motion was first applied along the edge and then across the edge of the cup. First, tests were conducted under lubricated conditions with 25% bovine serum. Next, to simulate an extremely high contact pressure, the tests were run with the addition of a third body alumina ceramic chip inserted between the edge of the cup and the head. Engineering blue was used to analyze the contact area. Reference values were determined using a 0 degrees cup abduction angle. Edge loading was achieved. The FC increased by three- to sixfold when the motion was applied along the edge, and by 70% when the motion was applied across the edge. However, the FC value was still low (about 0.1), which is similar to metal-on-metal THP. With the third body alumina ceramic particle inserted, the FC was 26 times higher than in the ideal conditions and intermittent squeaking occurred. High cup abduction angles may generate edge-loading and an increase in the friction coefficient for ceramic THP.

Changes in whole blood metal ion levels following resurfacing: serial measurements in a multi-centre study

Seventy-seven patients implanted with unilateral resurfacing prosthesis were recruited from four centres. Serial whole blood samples were collected and ion levels were analysed. In most cases, the ion levels stabilized by 3 months. The 24 month median ion levels were 1.49ug/l for chromium and cobalt. In approximately 50% of patients the increase in chromium and cobalt level was less than 1ug/l. There were 6 patients with abnormally high metal ion levels. Of these 4 were significant outliers, had high ion levels that became apparent between 12 and 24 months after implantation, and had a high cup abduction angle. Not all patients with high cup abduction angles demonstrated high levels. There were differences in ion levels between the four centres that correlated with variation in acetabular component placement. Variability in ion levels was seen with the same prosthesis, underscoring the importance of surgical technique, longitudinal analysis, and multi-centre trials.

A meta-analysis of design- and manufacturing-related parameters influencing the wear behavior of metal-on-metal hip joint replacements

This article aims to clarify the influence of design- and manufacturing-related parameters on wear of metal-on-metal (MoM) joint bearings. A database search for publications on wear simulator studies of MoM bearings was performed. The results of published studies were normalized; groups with individual parameters were defined and analyzed statistically. Fifty-six investigations studying a total of 200 implants were included in the analysis. Clearance, head size, carbon content, and manufacturing method were analyzed as parameters influencing MoM wear. This meta-analysis revealed a strong influence of clearance on running-in wear for implants of 36-mm diameter and an increase in steady-state wear of heat treated components.

Comparing ceramic-metal to metal-metal total hip replacements--a simulator study of metal wear and ion release in 32- and 38-mm bearings

Our 32 and 38 mm alumina ceramic-on-metal (COM) bearings were run in a hip simulator study for comparison with 32 mm metal-on-metal (MOM) controls. The 32 mm MOM bearings demonstrated an overall wear rate of 1.58 mm(3)/million cycles (Mc) that was comparable to previous simulator studies. The peak run-in MOM wear-rates (10, 15.7 mm(3)/Mc) were higher than in previous simulator studies. There was a noticeable graying in color of serum lubricants with MOM wear rates of 2-3 mm(3)/Mc and with wear rates of 10-15 mm(3)/Mc the serum became much darker. The COM lubricants darkened during two "break-away" wear events with wear-rates 5.8-6.7 mm(3)/Mc. The 32 and 38 mm COM bearings demonstrated overall wear-rates of 0.38 and 0.29 mm(3)/Mc, approximately four-fold reduced compared to MOM controls. The COM wear-rates were also much higher than in the one previous COM study. There may be methodological reasons that could explain this discrepancy. Our ion concentrations assessed from serum lubricants had Cobalt (Co) 68% and Chromium (Cr) 32% for average ratio of metal ion composition (i.e. averaging Co/Cr ratios: 2.26) in the parent alloy. Comparing Co ion concentrations during run-in, the COM bearings represented a 35-fold reduction compared to MOM. At 3.0 Mc, the COM represented a 33-fold reduction compared to MOM. Overall, our simulator study confirmed previously published advantages of low wear and reduced metal ions with the ceramic-metal coupling compared to standard metal-metal bearings.

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 3×106 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 (1.69×10−6 mm3/N m), the reduced carbide samples had the next lowest wear rate (2.1×10−6 mm3/N m), while the specimens without carbides wore the most (2.41×10−6 mm3/N m).

The exercise-related rise in plasma cobalt levels after metal-on-metal hip resurfacing arthroplasty

Wear of metal-on-metal bearings causes elevated levels of cobalt and chromium in blood and body fluids. Metal-on-metal bearings have two distinct wear phases. In the early phase, the wear rate is high. Later, it decreases and the bearing enters a steady-state phase. It is expected that as the wear rates decline, the level of cobalt detected in plasma will also decrease. We studied the baseline and exercise-related cobalt rise in 21 patients (13 men and eight women) with a mean age of 54 years (38 to 80) who had undergone successful hip resurfacing at a mean of 44 months (10 to 96) earlier. Our results showed that circulating baseline cobalt levels were not significantly correlated with the time since implantation (r = 0.08, p = 0.650). By contrast, the exercise-related cobalt rise was directly correlated with the inclination angle of the acetabular component (r = 0.47, p = 0.032) and inversely correlated with the time since implantation (r = −0.5, p = 0.020).

Inclination of the acetabular component should be kept less than 40° to decrease the production of wear debris.

Design considerations and life prediction of metal-on-metal bearings: the effect of clearance

BACKGROUND

Clinical observations suggest that metal-on-metal arthroplasties that have been implanted for more than twenty years do fail. It is proposed that there are not two, but three distinct phases of wear life for any metal-on-metal implant system: bedding-in, steady state, and end point. In this study, we asked two questions: can we explain late failure due to wear, and will there be a late failure mechanism due to a change in the frictional torque?

METHODS

In order to characterize wear failure, an analysis was made of five retrieved metal-on-metal couples that were mapped with use of a roundness machine. A geometrical model was developed on the basis of these observations, and wear at the end point was calculated. The literature on first-generation metal-on-metal implants retrieved for aseptic loosening was reviewed to assess the agreement with the retrieval findings as well as the wear model.

RESULTS

A wear patch of an appreciable and constant size could be measured in all five retrieved couples. The end point of revision was observed to occur when the wear progression reached a contact area corresponding to approximately 75% of the projected diameter of the ball. The wear volume was calculated from the geometry. The available literature describing the wear characteristics of retrieved bearings after successful clinical use showed good agreement with the calculated wear model.

CONCLUSIONS

During the implant life of long-term successful metal-on-metal devices, a wear patch develops, as evident from retrieved failed devices. Failure often occurs through loosening, and the observed wear patch is similar in size for devices measured by us and for those described in the literature. We hypothesized that failure by loosening occurs through the accumulation of wear, which eventually leads to high friction within the bearing and increased torsional forces across the joint and its fixation.

Wear of an experimental metal-on-metal artificial disc for the lumbar spine

STUDY DESIGN

In vitro wear simulation.

OBJECTIVE

To determine the type and amount of wear produced by experimental metal-on-metal artificial discs for the lumbar spine. To minimize the amount of wear by changing the carbon content, clearance, and presence of a keel and notch.

SUMMARY OF BACKGROUND DATA

In contrast to the extensive number of hip joint replacement simulator studies examining the effects of individual design variables on wear, existing artificial lumbar disc wear publications have measured wear using only the final version of each product. That is, the effects of individual variables such as material, diameter, or clearance on wear of artificial discs are not known, even though the importance of such variables has been established in artificial hip wear studies.

METHODS

Experimental metal-on-metal artificial discs for the lumbar spine were tested in a 3-station, biaxial spine wear simulator designed and constructed by the investigators. Two versions of the implants were manufactured with differences in carbon content, clearance, and the presence of a keel. Additionally, implants were tested with or without a surgical notch.

RESULTS

The wear rates of the experimental metal-on-metal lumbar discs in the current study ranged from 6.2 to 15.8 mm3/million cycles. However, changing the carbon content of the ball from low to high, decreasing the initial clearance, and eliminating the anteroposterior keel reduced the wear rate from 12.4 to 7.6 mm3/million cycles. Furthermore, removing the surgical notch reduced the wear rate from 7.6 to 6.2 mm3/million cycles. The surface damage was generally consistent with low lubrication and varying degrees of abrasive and fatigue wear, with impingement of nonbearing surfaces observed at 1.5 million cycles for the longer-term test.

CONCLUSION

Although the implants tested in the current study were experimental, the results suggest that metal-on-metal lumbar discs have the potential to produce wear of this magnitude and mechanism in vivo. Therefore, careful consideration of individual design variables, including those considered in the current study, is necessary to avoid production of excessive wear in artificial lumbar discs.

Can cobalt levels estimate in-vivo wear of metal-on-metal bearings used in hip arthroplasty?

High levels of cobalt and chromium ions are detected in the blood and urine of patients with metal-on-metal (MoM) hip replacement. These elements are released as a result of wear at the bearing surfaces. Wear rates depend on a multitude of factors, which include the bearing geometry, carbon content, manufacturing processes, lubrication, speed and direction of sliding of the surfaces, pattern of loading, and orientation of the components. In-vivo wear of MoM bearings cannot be reliably measured on X-rays because no distinction can be made between the bearing surfaces. Hip simulator studies have shown that wear rates are higher during the initial bedding-in phase and subsequently drop to very low levels. Accordingly, metal ion levels would be expected to decrease with the use of the bearing, measured as implantation time following surgery. However, several clinical studies have found that metal ion levels either gradually rise or fluctuate instead of decreasing to lower levels. Moreover, hip simulator studies predict that large-diameter bearings have lower wear rates than small-diameter bearings. In clinical studies, however, metal levels in patients with large-diameter bearings are unexpectedly higher than those in patients with small-diameter bearings.

As a consequence, high cobalt ion levels in patients do not necessarily imply that their MoM bearings produce much wear debris at the time that their levels were measured; it may simply be due to accumulation of wear debris from the preceding time. Exercise-related cobalt rise may overcome this limitation and give a better assessment of the current wear status of a MoM bearing surface than a measure of cobalt levels only.

Advanced nanocomposite materials for orthopaedic applications. I. A long-term in vitro wear study of zirconia-toughened alumina

The use of ceramic-on-ceramic (alumina- and zirconia-based) couplings in hip joint prostheses has been reported to produce lower wear rates than other combinations (i.e., metal-on-polyethylene and ceramic-on-polyethylene). The addition of zirconia into an alumina matrix (zirconia-toughened alumina, ZTA) has been reported to result in an enhancement of flexural strength, fracture toughness, and fatigue resistance. The development of new processing routes in nonaqueous media has allowed to obtain high-density ZTA nanocomposites with a very homogeneous microstructure and a significantly smaller and narrower particle-size distribution of zirconia than conventional powder mixing methods. The aim of the present study was to set up and validate a new ZTA nanocomposite by testing its biocompatibility and wear behavior in a hip-joint simulator in comparison with commercial alumina and experimental alumina specimens. The primary osteoblast proliferation onto ZTA nanocomposite samples was found to be not significantly different from that onto commercial alumina samples. After 7 million cycles, no significant differences were observed between the wear behaviors of the three sets of cups. In this light, it can be affirmed that ZTA nanocomposite materials can offer the option of improving the lifetime and reliability of ceramic joint prostheses.

Friction of total hip replacements with different bearings and loading conditions

Metal-on-ultra-high molecular weight polyethylene (UHMWPE) total hip replacements have been the most popular and clinically successful implants to date. However, it is well documented that the wear debris from these prostheses contributes to osteolysis and ultimate failure of the prosthesis, hence alternative materials have been sought. A range of 28 mm diameter bearings were investigated using a hip friction simulator, including conventional material combinations such as metal-on-UHWMPE, ceramic-on-ceramic (CoC), and metal-on-metal (MoM), as well as novel ceramic-on-metal (CoM) pairings. Studies were performed under different swing-phase load and lubricant conditions. The friction factors were lowest in the ceramic bearings, with the CoC bearing having the lowest friction factor in all conditions. CoM bearings also had low friction factors compared with MoM, and the trends were similar to CoC bearings for all test conditions. Increasing swing phase load was shown to cause an increase in friction factor in all tests. Increased serum concentration resulted in increased friction factor in all material combinations, except MoM, where increased serum concentration produced a significant reduction in friction factor.

Laboratory studies on the tribology of hard bearing hip prostheses: Ceramic on ceramic and metal on metal

Total hip replacements offer relief to a great many patients every year around the world. With an expected service life of around 25 years on most devices, and with younger and younger patients undergoing this surgery, it is of great importance to understand the mechanisms of their function. Tribological testing of both conventional and hard bearing joint combinations have been conducted in many centres throughout the world, and, after being initially abandoned owing to premature failures, hard bearing combinations have been revisited as viable options for joint replacements. Improved design, manufacturing procedures, and material compositions have led to improved performance over first-generation designs in both metal-on-metal and ceramic-on-ceramic hip prostheses. This paper offers a review of the work conducted in an attempt to highlight the most important factors affecting joint performance and tribology of hard bearing combinations. The tribological performance of these joints is superior to that of conventional metal- or ceramic-on-polymer designs.

Prediction of lubrication regimes in two-piece metacarpophalangeal prostheses

Various designs of two-piece finger prosthesis with conforming spherical surfaces have been proposed. These were compared by calculating the lubrication regime for the material combinations and operating conditions expected at the metacarpophalangeal joints of the fingers. Consideration was given to a range of loads from 2 to 50 N, a range of entraining velocities from 0 to 30 mm/s, and a range of prosthesis radii from 3 to 10 mm. This theoretical lubrication analysis indicated that the optimum material combination of those available for two-piece metacarpophalangeal prostheses is in the order: ceramic-on-ceramic; metal-on-metal; pyrocarbon-on-pyrocarbon; and metal-on-polymer. However, it should be recognised that other factors may take precedence when choosing a material combination for a design of finger prosthesis.

Metal-on-metal hip resurfacing compares favorably with THA at 2 years followup

Metal-on-metal total hip resurfacing is a bone-conserving reconstructive option for patients with advanced articular damage. While intended to address several problems with conventional THA, the safety and efficacy is not well established. We therefore retrospectively compared the outcomes of 52 patients (57 hips) with resurfacing arthroplasty to 84 patients (93 hips) with cementless primary THAs. The patients had a minimum 2-year followup (mean 3 years). The patients with resurfacing arthroplasty had a mean age of 47 years (range, 22-64) while those with cementless primary THA had a mean age of 57 years (range, 17-92). After controlling for age, gender, and preoperative differences, the total Harris Hip Scores (HHS), function scores, and pain scores were similar between the two groups. However, the resurfacing group had higher activity scores (14 versus 13, p < 0.001) and range of motion (ROM) scores (5.0 versus 4.8, p < 0.001). The complication rates (5.3% for resurfacing versus 14.0% for THA) and reoperation rates (3.5% for resurfacing versus 4.3% for THA) were similar. The total hip arthroplasty and metal-on-metal resurfacing groups both showed improvement in HHS, pain, activity, and ROM and had similar early complication and reoperation rates.

The effects of proteins on the friction and lubrication of artificial joints

The tribological testing of artificial hip and knee joints in the laboratory has been ongoing for several decades. This work has been carried out in an attempt to simulate the loading and motion conditions applied in vivo and, therefore, the potential for the success of the joint. However, several different lubricants have been used in these tests. The work documented in this paper compares results obtained using different lubricants and makes suggestions for future work. Hip joints and knee joints of different material combinations were tested in a friction simulator to determine their friction and lubrication properties. Both carboxymethyl cellulose (CMC) fluids and bovine serum (with CMC fluids added) were used as the lubricants. These were prepared to various viscosities to produce the Stribeck plots. Human synovial fluid, of just one viscosity, was used as the lubricant with some of the joints to give a true comparison with physiological lubricants. The results showed that, in most cases, the lubricant had a significant effect on the friction developed between the joint surfaces. This is thought to be due to the proteins that are present within the bovine serum adsorbing to the bearing surfaces, creating 'solid-like' films which rub together, protecting the surfaces from solid-to-solid contact. This would be beneficial in terms of wear but can either increase or decrease the friction between the contacting surfaces. It is important to simulate the conditions in vivo as closely as possible when testing these joints to try to obtain a better comparison between the joints and to simulate more accurately the way that these joints will operate in the body. In an attempt to simulate synovial fluid, bovine serum seems to be the most popular lubricant used at present. It would be beneficial, however, to develop a new synthetic lubricant that more closely matches synovial fluid. This would allow us to predict more accurately how these joints would operate long-term in vivo.

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.

Development rationale for an articular surface replacement: a science-based evolution

Hip resurfacing has an enduring appeal because of the advantages of bone conservation and maximal joint stability. However, a far from satisfactory experience with earlier resurfacing designs led to its virtual disappearance in the 1980s. The concept was reintroduced in the late 1990s. The current generation of resurfacing devices generally consisted of a large-diameter metal-on-metal articulation, the femoral components being cemented and the acetabular components utilizing various forms of cementless fixation. The encouraging medium-term results, with a follow-up of up to 8 years using the current generation of surface replacement joints, combined with favourable reports related to long-term performance of some metal bearings have led to a rapid increase in the use of such components with these devices. This trend is most marked in younger, more active patients who have expectations of restoration of lifestyle in addition to improved mobility and pain relief and in whom failure with conventional total hip replacement is much higher than previously reported with more sedentary patients. The aim of this paper is, firstly, to highlight a number of areas of improvement and, secondly, to explain how these may be addressed by making modifications to the design of both implants and instrumentation and to the surgical technique. The areas identified for improvement were tissue preservation (thinner components, and reduced steps between sizes), acetabular cup issues (fixation, insertion, and positioning), femoral component issues (design, loading, and cementation), improved bearing surface characteristics, and simplified precise instrumentation with a low-trauma surgical technique.

The tribology of metal-on-metal total hip replacements

Total hip surgery is an effective way of alleviating the pain and discomfort caused by diseased or damaged joints. However, in the majority of cases, these joints have a finite life. The main reason for failure is osteolysis (bone resorption). It is well documented that an important cause of osteolysis, and therefore the subsequent loosening and failure of conventional metal- or ceramic-on-ultra-high molecular weight polyethylene joints, is the body's immunological response to the polyethylene wear particles. To avoid this, interest has been renewed in metal-on-metal joints.

The intention of this paper is to review the studies that have taken place within different laboratories to determine the tribological performance of new-generation metal-on-metal total hip replacements. These types of joint offer a potential solution to enhance the longevity of prosthetic hip systems; however, problems may arise owing to the effects of metal ion release, which are, as yet, not fully understood.

Investigation on stick phenomena in metal-on-metal hip joints after resting periods

Insufficient understanding of tribological behaviour in total joint arthroplasty is considered as one of the reasons for prosthesis failure. Contrary to the continuous motion input profiles of hip simulators, human locomotion contains motion interruptions. These occurring resting periods can cause stick phenomena in metal-on-metal hip joints. The aim of the present study was to investigate the tribological sensitivity of all-metal bearings to motion interruptions on in vitro test specimens and retrieved implants. Friction and wear with and without resting periods were quantified. Unlike the metal-on-polyethylene joints, the static friction of metal-on-metal joints increased up to μs = 0.3 with rest, while wear appeared to be unaffected. This effect is caused by the interlocking of firmly adhered carbon layers, which were generated from the protein-containing lubricant through tribochemical reactions. Since more than 80 per cent of the retrieved implants exhibited macroscopically visible carbon layers, the increase in friction presumably also occurs under physiological conditions, which is then transferred to the bone-implant interface. These recurrent tangential stress peaks should be considered for the design features of the cup-bone interface, in particular when larger-sized implant heads are used.

Metal-on-metal bearings surfaces: Materials, manufacture, design, optimization, and alternatives

When first introduced, total hip replacements offered pain relief and improved mobility in elderly patients. The success of this procedure in terms of long-term durability and restoration of function has led to its use in younger, more active patients. This has resulted in a commensurate increase in patient expectation regarding longevity and the degree to which function and lifestyle is restored.

The bearing surface is a key feature of the performance of replacement joints. It is generally accepted that excessive amounts of wear debris preclude their long-term survivorship and hence there is an ongoing requirement for bearing surfaces which minimize debris generation. The purpose of this paper is to review the factors which affect the performance of so-called metal-on-metal bearings, to compare their performance with that of the other commonly used contemporary alternatives, metal and ceramic articulating against highly cross-linked polyethylene, and ceramic-on-ceramic, and finally to consider the potential solutions offered by new developments such as ceramic-on-metal and coatings applied to metal-on-metal bearings.

Effect of swing phase load on metal-on-metal hip lubrication, friction and wear

There is renewed interest in metal-on-metal (MOM) total hip replacements (THRs), however, variable wear rates have been observed clinically. It is hypothesised that changes in soft tissue tensioning during surgery may alter loading of THRs during the swing phase of gait leading to changes in fluid film lubrication, friction and wear. This study aimed to assess the effect of swing phase load on the lubrication, friction and wear of MOM hip replacements. Theoretical lubrication modelling was carried out using elastohydrodynamic theory. All the governing equations were solved numerically for the lubricant film thickness between the articulating surfaces under the transient dynamic conditions with low and high swing phase loads. Friction testing was completed using a single axis pendulum simulator, simplified loading cycles were applied with low and high swing phase loads. MOM hip replacements were tested in a hip simulator, modified to provide different swing phase loading regimes; a low (100 N) and a high load (as per ISO 14242-1; 280 N). Results demonstrated that the performance of MOM bearings is highly dependent on swing phase load. Hence, changes in the tension of the tissues at surgery and variations in muscle forces may increase swing phase load, reduce lubrication, increase friction and accelerate wear. This may explain some of the variations that have been observed with clinical wear rates.

Effect of Wear of Bearing Surfaces on Elastohydrodynamic Lubrication of Metal-on-Metal Hip Implants

The effect of geometry change of the bearing surfaces owing to wear on the elastohydrodynamic lubrication (EHL) of metal-on-metal (MOM) hip bearings has been investigated theoretically in the present study. A particular MOM Metasul™ bearing (Zimmer GmbH) was considered, and was tested in a hip simulator using diluted bovine serum. The geometry of the worn bearing surface was measured using a coordinate measuring machine (CMM) and was modelled theoretically on the assumption of spherical geometries determined from the maximum linear wear depth and the angle of the worn region. Both the CMM measurement and the theoretical calculation were directly incorporated into the elastohydrodynamic lubrication analysis. It was found that the geometry of the original machined bearing surfaces, particularly of the femoral head with its out-of-roundness, could lead to a large reduction in the predicted lubricant film thickness and an increase in pressure. However, these non-spherical deviations can be expected to be smoothed out quickly during the initial running-in period. For a given worn bearing surface, the predicted lubricant film thickness and pressure distribution, based on CMM measurement, were found to be in good overall agreement with those obtained with the theoretical model based on the maximum linear wear depth and the angle of the worn region. The gradual increase in linear wear during the running-in period resulted in an improvement in the conformity and consequently an increase in the predicted lubricant film thickness and a decrease in the pressure. For the Metasul™ bearing tested in an AMTI hip simulator, a maximum total linear wear depth of approximately 13 μm was measured after 1 million cycles and remained unchanged up to 5 million cycles. This resulted in a threefold increase in the predicted average lubricant film thickness. Consequently, it was possible for the Metasul™ bearing to achieve a fluid film lubrication regime during this period, and this was consistent with the minimal wear observed between 1 and 5 million cycles. However, under adverse in vivo conditions associated with start-up and stopping and depleted lubrication, wear of the bearing surfaces can still occur. An increase in the wear depth beyond a certain limit was shown to lead to the constriction of the lubricant film around the edge of the contact conjunction and consequently to a decrease in the lubricant film thickness. Continuous cycles of a running-in wear period followed by a steady state wear period may be inevitable in MOM hip implants. This highlights the importance of minimizing the wear in these devices during the initial running-in period, particularly from design and manufacturing points of view.

Current concepts of metal-on-metal hip resurfacing

The second-generation, metal-on-metal (MOM) bearing for total hip replacements was launched in the 1980s, and resurfacing followed in the mid-1990s. Remaining challenges include long-term bone remodeling of the femoral resurfacing and consideration of adverse MOM wear conditions. Precise understanding of manufacturing variables such as alloy types, bearing diameters, design tolerances, and surface finish is imperative in obtaining clinical consistency and safety in the patient. This review examines femoral fixation, bone remodeling, and wear studies of MOM implants and provides a brief overview of the latest outcome and retrieval data and how these data integrate with the in vitro wear studies.

Effect of ion implantation on the tribology of metal-on-metal hip prostheses

Nitrogen ion implantation (which considerably hardens the surface of the bearing) may represent one possible method of reducing the wear of metal-on-metal (MOM) hip bearings. Currently there are no ion-implanted MOM bearings used clinically. Therefore a physiological hip simulator test was undertaken using standard test conditions, and the results compared to previous studies using the same methods. N2-ion implantation of high carbon cast Co-Cr-Mo-on-Co-Cr-Mo hip prostheses increased wear by 2-fold during the aggressive running-in phase compared to untreated bearing surfaces, plus showing no wear reductions during steady-state conditions. Although 2 specimens were considered in the current study, it would appear that ion implantation has no clinical benefit for MOM.

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.

Wear evaluation of cobalt-chromium alloy for use in a metal-on-metal hip prosthesis

Wear of the polyethylene in total joint prostheses has been a source of morbidity and early device failure, which has been extensively reported in the last 20 years. Although research continues to attempt to reduce the wear of polyethylene joint-bearing surfaces by modifications in polymer processing, there is a renewed interest in the use of metal-on-metal bearing couples for hip prostheses. Wear testing of total hip replacement systems involving the couple of metal or ceramic heads on polymeric acetabular components has been performed and reported, but, until recently, there has been little data published for pin-on-disk or hip-simulator wear studies involving the combination of a metallic femoral head component with an acetabular cup composed of the same or a dissimilar metal. This study investigated the in vitro wear resistance of two cobalt/chromium/molybdenum alloys, which differed primarily in the carbon content, as potential alloys for use in a metal-on-metal hip-bearing couple. The results of pin-on-disk testing showed that the alloy with the higher (0.25%) carbon content was more wear resistant, and this alloy was therefore chosen for testing in a hip-simulator system, which modeled the loads and motions that might be exerted clinically. Comparison of the results of metal-on-polyethylene samples to metal-on-metal samples showed that the volumetric wear of the metal-on-polyethylene bearing couple after 5,000,000 cycles was 110-180 times that for the metal-bearing couple. Polyethylene and metal particles retrieved from either the lubricant for pin-on-disk testing or hip simulator testing were characterized and compared with particles retrieved from periprosthetic tissues by other researchers, and found to be similar. Based upon the results of this study, metal-on-metal hip prostheses manufactured from the high carbon cobalt/chromium alloy that was investigated hold sufficient promise to justify human clinical trials.