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

Transcription

1 SPECIAL ISSUE PAPER 119 Metal-on-metal bearings surfaces: materials, manufacture, design, optimization, and alternatives G H Isaac1*, J Thompson1, S Williams2, and J Fisher2 1 DePuy International Ltd, Leeds, UK 2 Institute of Medical and Biological Engineering, University of Leeds, Leeds, UK The manuscript was received on 26 May 2005 and was accepted after revision for publication on 13 October DOI: / X68953 Abstract: 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. Keywords: total hip replacement, wear, hard-on-hard bearings, metal-on-metal bearings, ceramics, polyethylene 1 INTRODUCTION inherent risk of introducing modified materials and new designs of bearing surface into clinical use Total hip replacement is a very successful procedure has been counterbalanced by patient demand and that provides pain relief and restoration of mobility. changes in patient demographics. Replacement-hip There are no hard data; however, it is estimated that patients are becoming younger, have higher expecapproximately one million such procedures are tations, are generally more active, may specifically carried out per year worldwide. A critical component wish to participate in impact sports, and finally have of a replacement hip joint is the bearing surface, and a greater longevity. All these factors are driving the good wear performance is now seen to be necessary search for new hip joint designs. Specifically, new for the long-term survivorship and overall perform- bearing surfaces are being sought which give significant ance of the joint. reductions in both the volume of wear debris Currently, bearing surfaces for hip joints fall into generated and its effect on the body. two broad categories: firstly, the so-called hard-onsoft Renewed interest in metal-on-metal bearings has combinations in which hard materials such been fuelled by encouraging medium-term results as metal (sometimes coated) or ceramics articulate when used with contemporary conventional hip against polyethylene; secondly, the so-called hard- replacement and the re-introduction of surfacereplacement-type on-hard bearing surfaces in which the same hard components for which a metal-onon-hard components articulate against themselves. The metal system is currently the only practical solution. That both of these arthroplasty solutions are targeted * Corresponding author: DePuy International Ltd, Number One, at younger, more active patients makes it even more White Rose Office Park, Millshaw Park Lane, Leeds LS11 0EA, imperative to optimize performance because of the UK. gisaac@dpygb.jnj.com increased likelihood that these components are in

2 120 G H Isaac, J Thompson, S Williams, and J Fisher situ for extremely long periods of time, potentially of high-density polyethylene articulating against years. stainless steel in This combination, initially The aim of this paper is to review the development using 22 mm stainless steel heads, became the of metal-on-metal bearings, to examine the ways by benchmark for bearing surfaces producing very low which their performance may be optimized, to con- volumes of wear debris. From the late 1960s this sider the advantages and disadvantages when compared bearing surface entered general widespread use with other currently available solutions, and with low wear rates being reported, for example by finally to highlight the potential future solutions for Griffith et al. [2]. Refinements on this basic principle wear reduction. included heads of slightly larger diameter (28 2 BEARING SURFACE HISTORY 32 mm) made from cobalt chromium alloys, the introduction of ceramic heads, and a more advanced polyethylene called ultra-high molecular weight polyethylene. However, this design remained essentially Replacement hip joints were first used in significant unaltered until the early 1990s when longer-term quantities in the late 1940s. However, the first designs problems became apparent with this bearing surface which have the appearance of modern devices are combination. These include an osteolytic reaction typified by the components developed by McKee in caused by polyethylene wear debris [3], susceptibility the UK in the mid-1950s. These consisted of a of gamma-irradiated polyethylene to oxidation [4], femoral stem with a large-diameter (35 40 mm) and the effect of roughened femoral heads [5, 6]. This head, made from a cobalt chromium alloy, articulating led to the development of harder (ceramic) femoral against a similar-sized acetabular cup made from heads and new highly cross-linked polyethylenes. the same material (Fig. 1). Similar devices were subsequently By 2000 there was an increasing belief that these developed by Ring (in about 1962) and advanced polyethylenes would minimize the adverse Muller (in about 1965). Such components were introduced effects associated with wear debris but would not with little or no testing and certainly nothing solve the problem entirely and alternative solutions that simulated their performance as replacement were sought. joints. These devices tended to suffer from a high As discussed above, after a high early failure rate initial failure rate and their use declined. However, with first generation metal-on-metal devices, the sur- reviews of long-term results indicated a very low viving components had a relatively high level of longterm wear rate of surviving components (less than 8 mm/ success. This led to renewed interest from the year) after years [1]. This prompted renewed 1980s onwards and the introduction of re-engineered interest from the late 1980s onwards. metal-on-metal bearing surfaces manufactured from At the same time as the early work on hard-on- cobalt chromium alloys in conjunction with more hard bearing surfaces, John Charnley was developing conventional acetabular and femoral components. replacement hip joints based on the low-frictional These have produced some encouraging results. For torque principle, fixed in place with acrylic bone example, Wagner and Wagner [7] reviewed a series cement. A combination of science, determination, of 78 patients with a 5 year follow-up. There were and serendipity led to the clinical testing by Charnley three revisions all unrelated to wear. There was no evidence of metallosis, and no osteolysis apparent radiographically. Similar short- to medium-term Fig. 1 McKee Farrar prosthesis. (Image supplied by results were recently reported by Jacobs et al. [8] and Lombardi et al. [9]. Sieber et al. [10] reported on the analysis of 118 retrieved second-generation metalon-metal implants. Linear annual wear was found to be 25 mm in the first year, dropping to 5 mm after the third year, and was equally divided between head and cup. The self-polishing capacity of metal-onmetal joints was noted whereby scratches are worn away by further joint movement. Ceramic-on-ceramic components had a similar history. They were first introduced by Boutin et al. [11] in the 1970s who in 1988 reported disappointing clinical results. However, a more encouraging out- D. Dowson, Leeds University, UK) come was reported by Mittelmeier and Heisel [12],

3 Metal-on-metal bearings surfaces 121 which helped to re-establish the concept. More may be an increase by a factor of 1000 in the number recently Bizot et al. [13] reported on a group of of particles [16]. 234 patients with ceramic-on-ceramic components The potential harmful effects of this debris, or at an average follow-up of 7.8 years. There were 11 more specifically the effect of the breakdown of revisions none of which was related to wear. Radio- the debris into metal ions, were first highlighted graphic analysis showed no signs of osteolysis, a conclusion by Black [17] who indicated that isolated clinical also reached by Pignatti et al. [14] in their observations [of adverse carcinogenic, metabolic, review of 123 ceramic-on-ceramic components. immunological, and bacteriological effects] support These results have led to the more widespread the presence of systemic effects, especially associated use of these so-called alternative or hard-on-hard with immune responses and metal overload and bearings. accumulation conditions. Despite extensive searches Semlitsch and Willert [15] reviewed available clinical for evidence of this relationship, most notably by data. They noted the reduction in wear when Paavolainen et al. [18], no link has been found after polyethylene articulated against a ceramic compared 40 years of use. Willert et al. [19] recently reported with a metal. They also identified that a much greater on a group of patients who had early post-operative reduction in clinical wear rate could be achieved by pain. The histological findings indicated few using both metal-on-metal and ceramic-on-ceramic metal particles but were consistent with a possible bearings, as shown in Fig. 2. lymphocyte-dominated immunological response. 3 CONTEMPORARY ISSUES WITH METAL-ON- METAL BEARINGS Because of these concerns it would still be prudent to try to minimize the volume of wear debris generated in metal-on-metal bearings. 3.1 Biological issues 3.2 Theory The key to the performance of metal-on-metal bear- Despite encouraging results there remain concerns ings, is lubrication theory which identifies three with metal-on-metal bearings. These are related to lubrication regimes. the long-term effect of wear debris. The osteolytic effect of polyethylene debris is well known but the 1. Boundary lubrication. This occurs by slippery long-term effect of the debris from hard-on-hard molecules chemically adhering to moving sur- bearings is less clear. The wear particles are much faces, in which there is direct asperity contact. smaller (10 60 nm) than those from polyethylene Examples of boundary lubricants in the human (usually in the range of less than mm) and it body are phospholipids and glycoproteins. has been estimated that as a consequence, whilst 2. Fluid-film lubrication. Conditions are such that there is significant reduction in wear volume, there an interposed fluid separates the moving surfaces. Fig. 2 Review of clinical wear rates with various bearing surfaces. (After Semlitsch and Willert [15])

4 122 G H Isaac, J Thompson, S Williams, and J Fisher An example of a fluid lubricant in the body is All metal-on-metal joints are cobalt chromium synovial fluid. molybdenum alloys. It was reported as early as Mixed lubrication. The load is partially supported by Streicher et al. [27] that those alloys with a higher by a combination of contact with boundary lubricarbon carbon content outperform those classified as low- cants (at the asperity tips) and by pressure develis alloys. It is now generally accepted that this oped in a fluid film that separates some, but not the case. It has also been reported by Cawley et al. all, of the asperities of the interacting surfaces. [28] that a superior wear performance was achieved with high-carbon alloys that have a microstructure Ideally, for hard-on-hard bearings to work, full consisting of coarse blocky carbides compared with fluid-film lubrication is required but the service cycle other conditions. This was attributed to better abraof hip joints, the engineering materials available for sion resistance. It was further proposed by McMinn use, and the variable properties of lubricants render [29] that the use of heat-treated high-carbon alloys this very difficult. However, evidence of satisfactory was responsible for early failures in some surface clinical performance of a hip joint bearing surface replacement components. Conversely Chan et al. was reported by Clarke et al. [20] which, from the [30] found no difference between cast and wrought lubrication theory adapted for hip joint replacement high-carbon material. In addition, Bowsher et al. [31] proposed by Jin et al. [21], was calculated by Dowson found no difference between as-cast and cast and et al. [22] to be operating in the mixed regime rather heat-treated material. Dowson et al. [32] reported a than in a regime of continuous fluid-film lubrication. hip simulator study which compared the wear per- The full benefits of fluid film can nevertheless be formance of several high-carbon cobalt chromium encouraged by the optimization of key manufacturmolybdenum alloys using 36 mm components ing parameters. with various manufacturing routes which produced The adapted lubrication theory proposed by Jin extremes of microstructure. The microstructures et al. [21] for hip joints identified the factors affecting varied from the as-cast condition with the aforementheir wear. This work indicated that wear would be tioned blocky carbides to the wrought condition with reduced by using large-diameter components, which a fine dispersion of small carbides. No significant have a small diametral clearance between the difference was found between any of these confemoral head and acetabular cups. The components ditions, as illustrated by Fig. 3. This conclusion from should be as smooth as possible, which suggests that simulator studies has important implications for the components should be as hard as possible since manufacture of metal-on-metal bearings, specifically harder surfaces are more likely to achieve and mainthat material choice is not dictated by wear performtain a very smooth surface in the human body. At ance but by other factors such as ease of manufacfirst sight the inverse relationship between femoral ture and non-wear-related benefits. Therefore, for head diameter and wear is at odds with the lowexample, it makes sense for metal femoral heads of friction arthroplasty principle proposed by Charnley intermediate diameter (28 36 mm) to be made from [23] and observed clinically by Livermore et al. [24] wrought bar, this material condition lending itself to and Kabo et al. [25], which indicated that volumetric high-volume precision manufacture using modern wear is directly proportional to femoral head diam- computer numerically controlled technology. Coneter. The explanation for this apparent dichotomy is versely the intricate internal geometry required for that all polyethylene bearings operate in the bound- the femoral portion of surface-replacement-type ary lubrication regime in which wear is related to components lends itself to a cast route. The use of sliding distance, which increases with increasing sintering to attach beads or a mesh for fixation femoral head diameter. Conversely, hard-on-hard naturally leads to a cast and heat-treated material for bearings, if correctly engineered, have the potential either femoral or acetabular single-piece compoto operate in the mixed and fluid-film lubrication nents. On the other hand for modular acetabular regime where the increased head diameter leads to components the fixation portion is separate from increased sliding speeds, which aids lubrication and the bearing surface. In this configuration the former hence reduces wear. Interestingly this state of affairs is usually forged (and possibly heat-treated was correctly predicted by Charnley et al. [26] in depending on fixation methods). The latter would favour wrought bar for ease of manufacture. 3.3 Materials 3.4 Design optimization The effect of materials is unclear and, as discussed above, there is some dispute regarding their effect. The theories associated with metal-on-metal bearings have been tested in hip joint simulators and

5 Metal-on-metal bearings surfaces 123 Fig. 3 Bar chart showing the effect of materials on wear (36 mm diameter components; highcarbon ( per cent) alloy; clearance, mm). (After Dowson et al. [32]) were reported by Chan et al. [30], Scholes et al. [33], diametral clearance of mm, noted low ion and Dowson et al. [22]; they indicated how the performance levels when compared with larger-diameter (48 mm) of metal-on-metal designs may be optim- components which were known to be in the range ized. It was found that reducing the clearance of mm. This result is entirely consistent with between the femoral and acetabular components lubrication theory. However, Farrar and Schmidt [35] reduced the amount of wear debris generated, as reported an increase in wear when clearances shown by Fig. 4. A significant reduction (p=0.05) was dropped to 30 mm. This was thought to be caused by noted for large diameters (54 mm) when clearances geometrical errors in manufacturing rather than by a were reduced from 287 to 107 mm. A modest re- breakdown in lubrication theory. They also reported duction in clearances (from 143 to 105 mm) with seizure of components which had negative clear- medium-diameter (36 mm) components also pro- ances. A factor that does need to be taken into consideration duced a significant reduction (p=0.05) in wear rates. when designing clearances is that of cup A previous study by Smith et al. [34] determined the flexibility. This has been discussed in detail by Jin percentage of the loading cycle when the components et al. [36] elsewhere in this volume who reported were separated by a fluid film by measuring that, in very thin cups, diametrical squeezing of up to the voltage drop between the femoral and acetabular 100 mm may be observed when cups are implanted. components. This showed that relatively small This is obviously incompatible with the low clearances differences in clearance with 36 mm components necessary to minimize wear and a compromise resulted in no separation (170 mm) or separation for needs to be reached whereby the minimum clearance the entire loading cycle (130 mm). Fears have been between the femoral head and acetabular cup expressed that reducing clearance may limit fluid is always significantly more than the maximum entrainment and ultimately lead to lubricant starvation. flexibility of the cup. Whilst component seizure was reported in The effect of diameter was examined by Dowson early designs [1], these were short-term failures attributed et al. [22] and their results are summarized in Fig. 5. to negative clearance which caused equatorial At small diameters (16 and 22 mm), wear increased surface contact associated with high frictional with increasing bearing surface diameter. This indi- torques and high wear. Late failures attributed to cated that these components were operating in the long-term wear processes have not been reported. boundary lubrication regime in which wear increases There is no evidence to support these concerns in with increasing sliding distance. At larger diameters modern, appropriately engineered components. On (28, 36, and 54.5 mm) the wear rate decreased with the contrary, Clarke et al. [20], when reporting on the increasing diameter, clearly indicating that these performance of conventional hip replacements used components were operating in the mixed and fluid- in conjunction with 28 mm bearings which had a film lubrication regimes. It also shows that this

6 124 G H Isaac, J Thompson, S Williams, and J Fisher Fig. 4 Bar charts showing the effect of clearance on running-in wear (2 106 cycles) with (a) 36 mm and (b) 54 mm diameter heads. (After Dowson et al. [22]) Fig. 5 Graph showing the effect of diameter on wear: curve a, 16 mm, n=5, and C d #53 70 mm; curve b, mm, n=5, and C d #46 66 mm; curve c, 28 mm, n=4, and C d #55 70 mm; curve d, 36 mm, n=3, and C d #76 78 mm; curve e, 54.5 mm, n=4, and C d # mm. (After Dowson et al. [22])

7 Metal-on-metal bearings surfaces 125 improvement in performance continued up to diameters which are significantly greater than those which are generally used for total hip replacement. However, such diameters are commonplace with surface replacement implants, a design philosophy which is being increasingly used for younger patients. It is encouraging that such components not only have the clinical advantages of conserving bone, a large range of motion, and reduced likelihood of dislocation [37] but also, if correctly engineered, have the potential for the lowest wear rates. An example of a surface replacement device is shown in Fig. 6. Wear in metal-on-metal components is known to consist of two distinct phases. A relatively high-wear running-in phase which lasts for (0.5 2) 106 cycles, followed by a steady state phase when the wear rate is relatively constant and much lower [10, 28, 38]. Figure 7, derived from data reported earlier [19], demonstrates that the effects of optimizing wear characteristics in metal-on-metal components are most marked in the bedding-in phase. Figure 7(a) shows that wear rates are reduced when bearing surface diameters are increased both in the running-in phase [(0 2) 106 cycles] and in the steady state phase [(2 5) 106 cycles]. Figure 7(b) shows that reduced clearances have the most marked effect in the running-in phase. This was attributed by Hu et al. [39] to greater penetration and hence to greater Fig. 7 Bar charts showing volumetric wear rates in the running-in phase [(0 2) 106 cycles] and the volumetric wear necessary to develop a given contact steady state phase [(2 5) 106 cycles]: (a) effect area with components which have a larger clear- of diameter [28 mm, with clearance 62 mm ance. A comprehensive review of steady state wear (n=4); 36 mm, with clearance 77 mm (n=3); reported in various hip simulators was undertaken 54.5 mm, with clearance 108 mm (n=4)]; by Dowson [40] which showed that theoretical film (b) effect of clearance [54 mm, with clearance thickness alone, independent of small variations in 276 mm (n=3); 54.5 mm, with clearance 108 mm surface roughness, was a good predictor of the longterm performance of metal-on-metal bearings. A film (n=4)]. (After Dowson et al. [22]) thickness of less than 12 nm produced relatively high in the range nm, with increasing film thickness wear rates. A dramatic drop in wear rates occurred having little further effect on wear rates. The effects of surface roughness are well known and have been reported by Chan et al. [30] who concluded that, in line with lubrication theory, increased surface roughness was associated with higher wear rates. However, recently researchers have tended to ignore the effects of surface roughness because of the self-polishing capacity of metal-on-metal components [10]. There is evidence to suggest that, below certain limits, roughness does not affect wear rates [40], and the limitations placed on surface finish are due to the nature of the metal itself. Metal-on-metal components have given good clinical results but there are still concerns over longterm exposure to elevated levels of metal ions. Hip Fig. 6 Typical surface replacement component. (Image supplied by DePuy International Ltd) simulator studies have demonstrated that significant reductions in wear and thus metal ion levels can be

8 126 G H Isaac, J Thompson, S Williams, and J Fisher made if designs are optimized by utilizing larger- gamma irradiated with 4 MRad in a vacuum and foil diameter components with reduced clearances packed, GVF GUR1020 polyethylene, has been shown between the femoral head and acetabular cup but to have a wear rate of 35 mm3 per 106 cycles against clinical studies measuring metal ion levels are still metallic femoral heads and 25 mm3 per 106 cycles required to validate these laboratory studies. against alumina ceramic femoral heads in a hip joint It can be seen from the above discussion that the simulator [49, 50] compared with 50 mm3 per 106 metal-on-metal combination provides an attractive cycles for material gamma irradiated and stored in option as a bearing surface. However, continued concerns air. Intentionally cross-linked polyethylene (based regarding their use warrants a review of other on a higher-molecular-weight resin GUR1050) was solutions for low-wear bearing couples. reported by McKellop et al. [51] to have produced an 4 CURRENT ALTERNATIVE SOLUTIONS 80 per cent reduction in wear with a medium level of cross-linking 5MRad. Over 95 per cent reduction in wear, essentially zero wear, in highly cross-linked polyethylene (10 MRad) was reported by Muratoglu et al. [52]. The virtually zero wear and the retention 4.1 Polyethylene-on-metal and ceramic femoral of surface machining marks found in these early heads simulator studies with highly cross-linked polyethylene Polyethylene-on-metal or ceramic femoral heads [52] has not been found to represent clinical have been the most widely used bearing combina- performance where finite wear rates and loss of surface tion over the last 40 years [2, 23 25]. Higher demand machining marks have been found in retrievals and more active patients with longer life expect- by Bradford et al. [53]. A more recent hip simulator ancies have led to increased levels of failure due to study of highly cross-linked polyethylene, with lower, polyethylene wear debris-induced osteolysis [41]. physiologically relevant, serum protein levels, and Acceleration of wear and wear debris-induced osteolysis higher loads consistent with the ISO standards reirradiated in conventional polyethylene bearings gamma ported on the effects of various levels of radiation in air have been associated with oxidative [54]. Wear decreased with increased exposure to radi- ageing of the polyethylene and roughening and ation but even at the 10MRad level exhibited finite damage to the femoral heads. A greater than twofold wear rates up to 9 mm3 per 106 cycles, loss of machin- increase in wear rate was found with oxidative ageing ing marks and linear penetrations consistent with following a shelf life of 5 years [42], while a longer clinical retrievals in bearings of size 28 mm, as shown shelf life was found to cause further increase in wear in Fig. 8. These clinical and realistic laboratory wear [43]. Oxidation was also found to produce smaller rates should be compared with the value of 1.6 mm3 and more reactive wear particles [43, 44]. Damage to per 106 cycles reported for the same-diameter components femoral heads [5] was found to work synergistically in metal-on-metal bearings [22]. with oxidation to increase wear further [43]. Tipper However, the biological response to wear debris et al. [45] quantified a series of retrieved Charnley is quite different for polyethylene and metal. Poly- hip prostheses with a lifetime of years and ethylene debris produces an inflammatory response found a doubling of the polyethylene wear rate from [41], and indeed it has been shown recently that 40 to 80 mm3 per year cycles for prostheses with higher-molecular-weight and cross-linked polyethylene damaged femoral heads. This was supported by wear debris is smaller and more reactive than simulator tests on polyethylene gamma sterilized in the conventional lower-molecular-weight GUR1020 air which showed a threefold increase in wear with polyethylene debris [55]. In contrast, metal wear scratched metal heads [46]. In contrast, alumina particles produce a very low level of inflammatory ceramic femoral heads have been found to be resistant response [56]. However, concerns remain about cyto- to scratching and damage in vitro and in vivo toxicity of metal particles if they accumulate in high [47, 48]. concentrations [57] and the potential for a hypersen- In the last 10 years, there has been a focus on sitivity reaction in a small number of patients [19]. improving both the polyethylene material, by making It is important to note that the small nanometre- it oxidation resistant and through intentional cross- sized metallic particles are readily transported away linking, and also by more widespread use of ceramic from the prosthesis and disseminated throughout femoral heads. Polyethylene sterilized in an inert the body. atmosphere and stored in an oxygen-free environment As there is a move towards larger femoral heads, has been shown to be more resistant to oxi- 36 mm or greater, there is concern that this will dation and wear. One example of such a material further increase wear in polyethylene bearings, in

9 Metal-on-metal bearings surfaces 127 Fig. 8 Graph showing the effect of various levels of gamma irradiation of the wear and creep of GUR 1050 ultra-high molecular weight polyethylene. (After Galvin et al. [54]) contrast with an improved lubrication and reduction design and materials, and issues with fixation [11], in wear in metal-on-metal bearings. A further the overall clinical experience with the bearing concern is that the reduced polyethylene thickness surface has been good [58, 59]. associated with larger-diameter components may Laboratory hip simulator studies which have been lead to a weakened overall construct. carried out by Nevelos and co-workers [60 64] under 4.2 Ceramic-on-ceramic bearings standard conditions have shown extremely low wear rates for alumina ceramic-on-ceramic bearings, as Alumina ceramic-on-ceramic bearings have an low as mm3 per 106 cycles. These wear rates extensive clinical history. Both the Mittelmeier Auto- can be tenfold lower than metal-on-metal hips, as phor prosthesis specifically (Ceramtec AG, Plochingen, shown by Fig. 9, and over a hundredfold lower than Germany) and other ceramic-on-ceramic polyethylene. However, these standard simulator bearings produced by the same manufacturer have tests did not replicate the wear rate, wear mechan- over 20 years clinical experience. Although during isms, and patterns of wear debris found clinically or that period there have been substantial changes in on retrievals. Explant studies on two different types Fig. 9 Average volumetric wear of metal-on-metal, ceramic-on-ceramic, and ceramic-on-metal bearing couples after cycles in a hip joint simulator. (After Nevelos et al. [60] and Firkins et al. [70])

10 128 G H Isaac, J Thompson, S Williams, and J Fisher of ceramic-on-ceramic hip have shown higher wear surgery, through incorrect insertion into the metal rates, of the order of mm3 per year [62], characteristic acetabular shell, or alternatively post-surgery stripe wear scars on the head [62, 63], and a through a high-impact trauma which overloads the bimodal debris distribution with both nanometreand component. A recent review of this issue in ceramic micrometre-size wear particles [64]. bearings by Hannouche et al. [69] reported on a large A new simulator methodology has been introduced series of total hip arthroplasties with a liner fracture which included microseparation of the head rate of five out of 3300 implanted. They concluded and cup, and this resulted in contact of the head on that Ceramic fracture, although of great concern, has the rim of the cup at heel strike and stripe wear for- to be considered with the perspective of the gain mation on the head [62]. Microseparation simulator obtained in using a highly performing material testing produced similar wear rates, wear mechan- regarding wear. isms, and wear debris to that found on retrieved Currently in orthopaedics, softer polyethylenes prostheses [62, 65, 66]. The alumina ceramic wear are always used with a dissimilar harder, metal, or debris has been found to be highly biocompatible ceramic femoral head. This is consistent with [67], producing a lower level of osteolytic cytokines common engineering practice which tends to avoid than cross-linked polyethylene, and being less cytotoxic wherever possible the use of like materials as bearing than metallic debris [57]. This makes alumina- surface combinations and favours the use of dis- on-alumina ceramic bearing couples an attractive similar materials with a differential hardness. How- option with sizes in the range mm. Larger- ever, hard-on-hard bearings such as metal-on-metal diameter components, which in theory should or ceramic-on-ceramic are used as like-on-like further reduce wear, are not used clinically because bearings. Ceramic-on-metal articulation offers a of limitations dictated by the commensurate reduction combination with an order-of-magnitude hardness in cup thickness. differential, while the use of materials that can be However, concerns remain about brittle fracture of finished to very tight manufacturing tolerances also ceramic components and chipping of acetabular maintains fluid-film lubrication during articulation. inserts during implantation. This, together with Firkins et al. [70] investigated the wear properties general design constraints, limits their current use in and debris morphology of a novel differential many countries. More recently a new alumina matrix hardness prosthesis combining an alumina ceramic composite, namely zirconia-toughened alumina, has femoral head and high-carbon cobalt chromium been developed. The increased toughness gives lower molybdenum alloy acetabular liner, in comparison fracture risk, more design flexibility, and most with metal-on-metal articulations in a physiological importantly reduction in wear under microseparation anatomical hip joint simulator. The results revealed conditions [68]. that the ceramic-on-metal pairings were found to have wear rates approximately a hundredfold lower than the metal-on-metal pairings. The wear rate of 5 FUTURE ALTERNATIVE SOLUTIONS the ceramic-on-metal pairings, as shown in Fig. 9, was found to be approximately 0.01 mm3 per 106 Despite the significant improvements offered by the cycles for the duration of the test with very little technologies discussed above, there nevertheless wear detected on the surface of the components. remain concerns related to polyethylene wear (crossof The initial bedding-in period which is characteristic linked polyethylenes), metallic debris, and subseabsent conventional metal-on-metal components was quent potential metal ion release (metal-on-metal), in the ceramic-on-metal tests. The higher and ceramic liner fracture (ceramic-on-ceramic). wear rate of metal-on-metal articulation continued Therefore, in addition to these current alternative into the steady state wear rate of 1.23±0.5 mm3 per solutions, it would seem appropriate to review some 106 cycles. potential future alternatives to metal-on-metal The size and shape of the particles from both bearings. articulations were similar, metallic, round to oval in shape, and in the nanometre size range, with a diameter 5.1 Ceramic-on-metal bearings of 30±2.25 nm for metal-on-metal bearings and 17.57±1.37 nm for ceramic-on-metal bearings after The brittle nature of ceramics is such that liners can 106 cycles. be subject to occasional fractures, despite the best The same factors that optimize hard-on-hard bearings efforts of the component designers to minimize the in general apply to a ceramic-on-metal coupling. risks. Typically, ceramic liners may fracture either in Therefore fluid-film thickness should be maximized

11 Metal-on-metal bearings surfaces 129 and design issues such as diametrical clearance, simulator. CrN heads were articulated with CrN sphericity, and surface roughness remain important inserts and CrCN heads with CrCN inserts in a to achieving successful articulation. It should further cycle hip joint simulator study (Leeds Mark II). As be emphasized that the testing and analysis reported illustrated by Fig. 10, the overall wear of the CrN-onhere employed the combination of a ceramic femoral CrN and CrCN-on-CrCN bearing couples was at least head and an acetabular metal liner. The use of the 22-fold lower, with overall wear being less than combination in reverse (metallic head and ceramic 0.05 mm3 per 106 cycles compared with 1.2 mm3 per liner) has not been tested. The known historical 106 cycles for metal-on-metal couples [72, 73]. failures of polyethylene heads articulating against Wear particles produced in all hip simulator metal liners, as well as the potential for scratching tests were characterized by transmission electron of the head if rim contact occurred during articu- microscopy using a method described by Tipper et al. lation make this unlikely to be a viable bearing [74]. All material combinations produced particles combination. generally less than 30 nm in size. The metal ion levels In summary, ceramic-on-metal bearings demon- in the serum lubricant were measured; cobalt and strated the advantages of exceptionally low wear, chromium ion levels were determined using graphite reducing the osteolytic potential and ion release furnace atomic absorption spectroscopy, and molybassociated with polymeric and metallic wear debris denum ion levels were determined by inductively respectively. Furthermore it offers reduced potential coupled plasma mass spectrometry. Compared with for acetabular component liner fracture, relative metal-on-metal uncoated bearings; the cobalt, chroto ceramic-on-ceramic articulation, and reduced mium, and molybdenum ion release was less in all damage to femoral heads under the microseparation serum samples from the coated bearing couples. and potential rim contact conditions reported by The cytotoxicity of the metallic, CrN, and CrCN Lombardi et al. [71]. wear particles was assessed by co-culture with U macrophages and L929 fibroblast cells by measuring Surface engineered coatings on metal-onthe effect on cell viability [75]. A significant reduction metal hip replacements in the viability of macrophage cells was observed Surface engineered coatings have been investigated with metallic wear particles at concentrations of 50, by Fisher et al. [72, 73] to examine the potential in 5, and 0.5 mm3 of wear debris per cell compared with reducing the volume of wear, the concentration of the cell only control. The fibroblast cells only showed metal debris, and the levels of cobalt, chromium, and a significant reduction in viability with 50 mm3 of molybdenum ions released. wear debris per cell. The CrN wear debris at concen- Thick (8 12 mm) surface engineered coatings trations at 50 and 5 mm3 of wear debris per cell [chromium nitride (CrN) and chromium carbonviability, caused a significant reduction in macrophage cell itride (CrCN)] were deposited by arc evaporative whereas only the highest concentration physical vapour deposition on cobalt chromium (50 mm3 of wear debris per cell) caused a significant molybdenum heads and cups and tested in a hip reduction in the viability of fibroblast cells. The CrCN Fig. 10 Wear rates of CrN- and CrCN-coated hip replacements compared with metal-on-metal bearings throughout testing. (After Fisher et al. [72, 73])

12 130 G H Isaac, J Thompson, S Williams, and J Fisher wear debris caused no significant reduction in cell nents are effective in reducing the volume of wear viability at any concentration tested with either cell debris, thereby validating the test methods, the data type. generated, and the designs which resulted. These initial findings support further development and clinical trials of surface engineered metal-onmetal bearings. In particular, surface engineered REFERENCES coatings may offer an alternative to metal-on-metal 1 Semlitsch, M., Streicher, R.M., and Weber, H. The surface replacement designs because of the reduced wear behavior of capsules and heads of CoCrMo wear and ion release compared with metal-on-metal casts in long-term implanted all-metal hip prosbearings and increased design flexibility compared theses. Othopade, 1989, 18, with ceramics. 2Griffith, M. J., Seidenstein, M., Williams, D., and 6 CONCLUSIONS Charnley, J. Socket wear in Charnley low friction arthroplasty of the hip. Clin. Orthop., 1978, 137, Willert, H.-G. and Semlitsch, M. Reactions of the articular capsule to wear products of artificial joint 1. Improvements in polyethylene and the introduc- prostheses. J. Biomed. Mater. Res., 1977, 11, Sutula, L. C., Collier, J. P., Saum, K. A., Currier, tion of ceramic femoral heads have reduced wear B. H., Currier, J. H., Sanford, W. M., Mayor, M. B., rates but not eliminated the risk of osteolysis. Wooding, R. E., Sperling, D. K., Williams, I. R., 2. Clinical evidence has indicated that hard-on-hard Kasparak, D. J., and Surprenant, B. A. The Otto bearing surfaces are a viable alternative to poly- Aufranc Award. Impact of gamma sterilization on ethylene. clinical performance of polyethylene in the hip. 3. Laboratory and theoretical evidence indicates that Clin. Orthop., 1995, 319, metal-on-metal bearings can produce very low 5 Isaac, G. H., Atkinson, J. R., Dowson, D., Kennedy, P. D., and Smith, M. R. The causes of femoral head wear volumes when optimized by using comporoughening in explanted Charnley hip prostheses. nents which have large diameters, smooth sur- Engng Medicine, 1987, 16, faces, and small diametral clearance between 6 Fisher, J., Firkins, P., Reeves, E. A., Hailey, J. L., and femoral and acetabular components. Isaac, G. H. The influence of scratches to metallic 4. Ceramic-on-ceramic bearings produce signifiweight polyethylene. Proc. Instn Mech. Engrs, Part counterfaces on the wear of ultra-high molecular cantly reduced amounts of less reactive wear H: J. Engineering in Medicine, 1995, 209, debris than metal-on-metal components but their 7 Wagner, M. and Wagner, H. Medium-term results use is limited by concerns of component fracture of a modern metal-on-metal system in total hip and consequent limitations of applicability in replacement. Clin. Orthop., 2000, 379, large-diameter thin designs of hip replacement. 8 Jacobs, M., Gorab, R., Mattingley, D., Trick, L., and 5. New technologies such as ceramic-on-metal and Southworth, C. Three- to six-year results with the surface engineered coatings have been tested and Ultima metal-on-metal hip articulation for primary total hip arthroplasty. J. Arthroplasty, 2004, 19(7), may produce further improvements in terms of Suppl. 2, performance and applicability of hard-on-hard 9 Lombardi, A. V., Mallory, T. H., Cuckler, J. M., bearings. Williams, J., Bernend, K. R., and Smith, T. M. Midterm results of a polyethylene-free metal-on-metal Encouraging short- to medium-term clinical articulation. J. Arthroplasty, 2004, 19(7), Suppl. 2, results with second-generation components, and increasing confidence in the quality of preclinical 10 Sieber, H.-P., Rieker, C.B., and Kottig, P. Analysis of testing in hip joint simulators have led to a signifihip 118 second-generation metal-on-metal retrieved implants. J. Bone Jt Surg., 1998, 80B, cant increase in the use of hard-on-hard bearings. 11 Boutin, P., Christel, P., Dorlot, J. M., Meunier, A., The in vivo performance of bearings using polyethylde Roquancourt, A., Blanquaert, D., Herman, S., ene can be measured using X-rays. Similar measure- Sedel, L., and Witvoet, J. The use of dense alumina ments cannot be made with hard-on-hard bearings alumina ceramic combination in total hip replacement. because the materials are radio-opaque and the J. Biomed. Mater. Res., 1988, 22, volume of wear, as measured by change of dimenence 12 Mittelmeier, H. and Heisel, J. Sixteen-years experi- with ceramic hip prostheses. Clin. Orthop., sion, is too small. However, wear can be deduced 1992, 282, indirectly for metal-on-metal bearings by measuring 13 Bizot, P., Larrouy, M., Witvoet, J., Sedel, L., and the level of metal ions in whole blood, serum, or Nizard, R. Press-fit metal-backed alumina sockets: urine. Only when these studies are complete will it a minimum 5-year followup study. Clin. Orthop., be known whether the latest generation of compo- 2000, 379,

13 Metal-on-metal bearings surfaces Pignatti, G., Stagni, C., Fravisini, M., and chrome molybdenum alloys used in metal-on-metal Girunti, A. Ceramic ceramic coupling. Total hip resurfacing hip arthroplasty. Wear, 2003, 255, arthroplasty in young patients. Chir. Organi Movimento, 2003, 88, McMinn, D. J. W., Development of metal/metal hip 15 Semlitsch, M. and Willert, H.-G. Clinical wear resurfacing. Hip Int., 2003, 13, Suppl. 2, S41 S53. behaviour of ultra-high molecular weight polyethylene 30 Chan, F. W., Bobyn, J. D., Medley, J. B., Krygier, J. J., cups paired with metal and ceramic ball and Tanzer, M. Wear and lubrication of metal-on- heads in comparison to metal-metal pairings of hip metal hip implants. Clin. Orthop., 1999, 369, joint replacements. Proc. Instn Mech. Engrs, Part H: 31 Bowsher, J. G., Nevelos, J., Pickard, J., and Shelton, J. Engineering in Medicine, 1997, 211, J. C. Do heat treatments influence the wear of large 16 Goldsmith, A. A. J., Dowson, D., Isaac, G. H., and diameter metal-on-metal hip joints? An in vitro Lancaster, J. G. A comparative joint simulator study study under normal and adverse conditions. of the wear of metal-on-metal and alternative mate- Transactions of 49th Annual Meeting, Orthopaedic rial combinations in hip replacements. Proc. Instn Research Society, New Orleans, Louisiana, USA, Mech. Engrs, Part H: J. Engineering in Medicine, 2 5 February 2003, Poster , 214, Dowson, D., Hardaker, C., Flett, M., and Isaac, G. H. 17 Black, J. Systemic effects of biomaterials. Bio- A hip joint simulator study of the performance of materials, 1984, 5, metal-on-metal joints. Part I: the role of materials. 18 Paavolainen, P., Pukkala, E., Pulkkinen, P., and J. Arthroplasty, 2004, 19(8), Suppl. 3, Visuri, T. Cancer incidence in Finnish hip replacement 33 Scholes, S. C., Green, S. M., and Unsworth, A. The patients from : a nationwide cohort wear of metal-on-metal total hip prostheses meas- study involving 31,651 patients. J. Arthroplasty, 2000, ured in a hip simulator. Proc. Instn Mech. Engrs, Part 15, H: J. Engineering in Medicine, 2001, 215, Willert, H.-G., Buchhorn, G. H., Fayyazi, A., 34 Smith, S. L., Dowson, D., and Goldsmith, A. A. J. Flury, R., Windler, M., Koster, G., and Lohmann, The effect of femoral head diameter upon C. H. Metal-on-metal bearings and hypersensitivity lubrication and wear of metal-on-metal total hip in patients with artificial hip joints. J. Bone Jt Surg., replacements. Proc. Instn Mech. Engrs, Part H: J. 2005, 87, Engineering in Medicine, 2001, 215, Clarke, M. T., Lee, P. T., Arora, A., and Villar, R. N. 35 Farrar, R. and Schmidt, M. B. The effect of diametral Levels of metal ions after small- and large-diameter clearance on wear between head and cup for metal-on-metal hip arthroplasty. J. Bone Jt Surg., metal on metal articulations. Transactions of 43rd 2003, 85B, Annual Meeting, Orthopaedic Research Society, San 21 Jin, Z. M., Dowson, D., and Fisher, J. Analysis of Francisco, California, USA, 9 13 February fluid film lubrication in artificial hip joint replacements 36 Jin, Z. M., Meakins, S., Morlock, M. M., Parsons, P., with surfaces of high elastic modulus. Proc. Hardaker, C., Flett, M., and Isaac, G. H. Defor- Instn Mech. Engrs, Part H: J. Engineering in Medicine, mation of press-fitted metallic resurfacing cups. Part 1996, 211, : experimental simulation. Proc. Instn Mech. Engrs, 22 Dowson, D., Hardaker, C., Flett, M., and Isaac, G. H. Part H: J. Engineering in Medicine (in press). A hip joint simulator study of the performance of 37 Cuckler, J. M., Moore, K. D., Lombardi, A. V., metal-on-metal joints. Part II: design. J. Arthroplasty, McPhereson, E., and Emerson, R. Large versus 2004, 19(8), Suppl. 3, small femoral heads in metal-on-metal total hip 23 Charnley, J. Low Friction Arthroplasty of the Hip, arthroplasty. J. Arthroplasty, 2004, 19(8), Suppl. 3, 1979 (Springer-Verlag, Berlin) Livermore, J., Ilstrup, D., and Morrey, B. Effect of 38 Clarke, I. C., Good, V., Williams, P., Schroeder, D., femoral head size on wear of the polyethylene Annissian, L., Stark, A., Oonishi, H., Schuldies, J., acetabular component. J. Bone Jt Surg., 1990, 72A, and Gustafson, G. Ultra-low wear rates for rigid-on rigid bearings in total hip replacements. Proc. Instn 25 Kabo, J. M., Gebhard, J. S., Loren, G., and Amstutz, Mech. Engrs, Part H: J. Engineering in Medicine, H. C. In vivo wear of polyethylene acetabular com- 2000, 214, ponents. J. Bone Jt Surg., 1993, 75B, Hu, X. Q., Isaac, G. H., and Fisher, J. Changes in the 26 Charnley, J., Kamangar, A., and Longfield, M. D. contact area during the bedding-in wear of different The optimum size of prosthetic heads in relation to sizes of metal on metal hip prostheses. Biomed. the wear of plastic sockets in total replacement of Engng Mater., 2004, 14, the hip. Med. Biol. Engng, 1969, 7, Dowson, D. The relationship between steady-state 27 Streicher, R. M., Semlitsch, M., Schon, R., wear rate and theoretical film thickness in metalon-metal Weber, H., and Rieker, C. Metal-on-metal articulation total replacement hip joints. In Tribol- for artificial hip joints: laboratory study and ogical Research and Design for Engineering Systems, clinical results. Proc. Instn Mech. Engrs, Part H: J. Proceedings of the 29th Leeds Lyon Symposium on Engineering in Medicine, 1996, 210, Tribology, Elsevier Tribology Series, Vol. 41 (Eds D. 28 Cawley, J., Metcalf, J. E. P., Jones, A. H., Band, T. J., Dowson, M. Priest, G. Dalmaz, and A. A. Lubrecht), and Skupien, D. S. A tribological study of cobalt 2003, pp

14 132 G H Isaac, J Thompson, S Williams, and J Fisher 41 Ingham, E. and Fisher, J. Biological reactions to 53 Bradford, L., Barker, D., Graham, J., Chawan, A., wear debris in total joint replacement. Proc. Instn Ries, M., and Pruitt, L. Wear and surface cracking Mech. Engrs, Part H: J. Engineering in Medicine, in early retrieved highly cross-linked polyethylene 2000, 214, acetabular liners. J. Bone Jt Surg., 2004, 86A, 42 Fisher, J., Chan, K. L., Hailey, J. L., Shaw, D., and Stone, M. Preliminary study of the effect of aging 54 Galvin, A., Endo, M., Tipper, J., Ingham, E., and following irradiation on the wear of ultra-high Fisher, J. Non cross linked and cross linked polymolecular weight polyethylene. J. Arthroplasty, 1995, ethylene hip prostheses. In Proceedings of the 10, Australian Society for Biomaterials, Sydney, 43 Besong, A. A., Tipper, J. L., Ingham, E., Stone, Australia, 2004, p M. H., Wroblewski, B. M., and Fisher J. Quantitative 55 Ingram, J. H., Stone, M., Fisher, J., and Ingham, E. comparison of wear debris from UHMWPE that has The influence of molecular weight, cross linking and and has not been sterilised by gamma irradiation. counterface roughness on TNF-alpha production by J. Bone Jt Surg., 1998, 80B, macrophages in response to ultra high molecular 44 Green, T. R., Fisher, J., Stone, M. H., Wroblewski, weight polyethylene particles. Biomaterials, 2004, B. M., and Ingham, E. Polyethylene particles of a 25, critical size are necessary for the induction of cyto- 56 Ingham, E. and Fisher, J. The role of macrophages kines by macrophages in vitro. Biomaterials, 1998, in osteolysis in total joint replacement. Biomaterials. 19, , 26, Tipper, J. L., Ingham, E., Hailey, J. L., Besong, A. 57 Germain, M. A., Hatton, A., Williams, S., Matthews, A., Fisher, J., Wroblewski, B. M., and Stone, M. H. J. B., Stone, M. H., Fisher, J., and Ingham, E. Quantitative analysis of polyethylene wear debris, Comparison of the cytotoxicity of clinically relevant wear rate and head damage in retrieved Charnley cobalt chromium and alumina ceramic wear parhip prostheses. Mater. Sci., Mater. Medicine, 2000, ticles in vitro. Biomaterials, 2003, 24, , Prudhommeaux, F., Hamadouche, M., Nevelos, J., 46 Barbour, P. S. M., Stone, M. H., and Fisher, J. A hip Doyle, C., Meunier, A., and Sedel, L. Wear of joint simulator study using new and physiologically alumina-on-alumina total hip arthroplasties of a scratched femoral heads with ultra-high molecular mean 11-year follow-up. Clin. Orthop., 2000, 379, weight polyethylene acetabular cups. Proc. Instn Mech. Engrs, Part H: J. Engineering in Medicine, 59 Roy, B. R., Nevelos, A. B., Ingham, E., Shaw, D. L., 2000, 214, and Fisher, J. Comparison of ceramic on ceramic to 47 Cooper, J. R., Dowson, D., Fisher, J., and Jobbins, B. ceramic on polyethylene total hip replacement. In Ceramic bearing surfaces in artificial joints. Proceedings of the 13th International Society for Resistance to third body damage. J. Med. Engng Ceramic in Medicine, Bioceramics, Bologne, Italy, Technol., 1991, 15, , Vol. 13, pp Minakawa, H., Stone, M. H., Wroblewski, B. M., 60 Nevelos, J. E., Ingham, E., Doyle, C., Nevelos, A. B., Lancaster, J. G., Ingham, E., and Fisher, J. and Fisher, J. Wear of HIPed and non-hiped Quantification of third-body damage and its effect alumina alumina hip joints under standard and on UHMWPE wear with different types of femoral severe simulator testing conditions. Biomaterials, head. J. Bone Jt Surg., 1998, 80B, , 22, Endo, M., Tipper, J. L., Barton, D. C., Stone, M. H., 61 Nevelos, J. E., Ingham, E., Doyle, C., Nevelos, A. B., Ingham, E., and Fisher, J. Comparison of wear, wear debris and functional biological activity of moderon the wear of BIOLOX Forte alumina ceramic and Fisher, J. The influence of acetabular cup angle ately crosslinked and non-crosslinked polyethylenes in hip prostheses. Proc. Instn Mech. Engrs, Part H: J. bearing couples in a hip joint simulator. J. Mater. Engineering in Medicine, 2002, 216, Sci.: Mater. Medicine, 2001, 12, Williams, S., Butterfield, M., Stewart, T., Ingham, 62 Nevelos, J. E., Ingham, E., Doyle, C., Nevelos, A. B., E., Stone, M. H., and Fisher, J. Wear and deforcomponents for Mittelmeier total hip prostheses. and Fisher, J. Analysis of retrieved alumina ceramic mation of ceramic-on-polyethylene total hip replacements with joint laxity and swing phase Biomaterials, 1999, 20, microseparation. Proc. Instn Mech. Engrs, Part H: J. 63 Nevelos, J. E., Prudhommeaux, F., Hamadouche, Engineering in Medicine, 2003, 217, M., Doyle, C., Ingham, E., Meunier, A., Nevelos, A. 51 McKellop, H., Shen, F. W., Lu, B., Campbell, P., and B., Sedel, L., and Fisher, J. Comparative analysis of Salovey, R. Development of an extremely wearthesis retrieved for aseptic loosening. J. Bone Jt two different types of alumina alumina hip prosresistant ultra high molecular weight polyethylene for total hip replacement. J. Orthop. Res., 1999, 17, Surg., 2001, 83B, Hatton, A., Nevelos, J. E., Nevelos, A. B., Banks, 52 Muratoglu, O. K., Bragdon, C. R., O Connor, D. O., R. E., Fisher, J., and Ingham, E. Alumina alumina Jasty, M., and Harris, W. H. A novel method of crosscharacterisation artificial hip joints. Part I: a histological analysis and linking ultra-high-molecular-weight polyethylene to of wear debris by laser capture mic- improve wear, reduce oxidation, and retain mechanical rodissection of tissues retrieved at revision. properties. J. Arthroplasty, 2001, 16, Biomaterials, 2002, 23,

15 Metal-on-metal bearings surfaces Stewart, T., Tipper, J. L., Streicher, R., Ingham, E., 71 Lombardi, A. V., Mallory, T. H., Dennis, D. A., and Fisher. J. Long-term wear or HIPed alumina on Komistek, R. D., Fada, R. A., and Northcut, E. J. An alumina bearings for THR under microseparation in vivo determination of total hip arthroplasty pisconditions. J. Mater. Sci.: Mater. Medicine, 2001, 12, toning during activity. J. Arthroplasty, 2000, 15, Tipper, J. L., Hatton, A., Nevelos, J. E., Ingham, E., 72 Fisher, J., Hu, X., Williams, S., Tipper, J., Doyle, C., Streicher, R., Nevelos, A. B., and Fisher, J. Ingham, E., Stone, M., Hatt, P., Berry, G., Alumina alumina artificial hip joints. Part II: Hardaker, C., and Isaac, G. H. New bearing surfaces: characterisation of the wear debris from in vitro hip What does the future hold? Semin. Arthroplasty, joints simulations. Biomaterials, 2002, 23, , 14, Fisher, J., Hu, X., Stewart, T., Williams, S., Tipper, J., 67 Hatton, A., Nevelo, J. E., Matthew, J. B., Fisher, J., Ingham, E., Stone, M., Davies, C., Hatto, P., and Ingham, E. Effects of clinically relevant alumina Bolton, J., Riley, M., Hardaker, C., Isaac, G. H., and ceramic wear particles on TNF-a production by Berry, G. Wear of surface engineered metal on metal human peripheral blood mononuclear phagocytes. hip prostheses. J. Mater. Sci.: Mater. Medicine, 2004, Biomaterials, 2003, 24, , Stewart, T. D., Tipper, J. L., Insley, G., Streicher, R. 74 Tipper, J., Firkins, P., Ingham, E., Fisher, J., M., Ingham, E., and Fisher, J. Long-term wear of Stone, M., and Farrar, R. Quantitative analysis of ceramic matrix composite materials for hip proswear and wear debris from low and high carbon theses under severe swing phase microseparation. content cobalt chrome alloys used in metal on metal J. Biomed. Mater. Res., Appl. Biomater., Part B, 2003, total hip replacements. J. Mater. Sci.: Mater. 66, Medicine, 1999, 10, Hannouche, D., Nich, C., Bizot, P., Meunier, A., 75 Williams, S., Tipper, J., Ingham, E., Stone, M., Nizard, R., and Sedel, L. Fractures of ceramic bearand Fisher, J. In vitro analysis of the wear, wear ings history and present status. Clin. Orthop., debris and biological activity of surface engineered 2003, 417, coatings for use in metal-on-metal total hip replace- 70 Firkins, P. J., Tipper, J. L., Ingham, E., Stone, M. H., Farrar, R., and Fisher, J. A novel low wearing ments. Proc. Instn Mech. Engrs, Part H: J. Engindifferential hardness, ceramic-on-metal hip joint eering in Medicine, 2003, 217, prostheses. J. Biomechanics, 2001, 34,

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