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- MATERIALS AND METHODS
Metal-on-metal (MoM) bearings are at the forefront in hip resurfacing arthroplasty. Because of their good wear characteristics and design flexibility, MoM bearings are gaining wider acceptance with market share reaching nearly 10% worldwide. However, concerns remain regarding potential detrimental effects of metal particulates and ion release. Growing evidence is emerging that the local cell response is related to the amount of debris generated by these bearing couples. Thus, an urgent clinical need exists to delineate the mechanisms of debris generation to further reduce wear and its adverse effects. In this study, we investigated the microstructural and chemical composition of the tribochemical reaction layers forming at the contacting surfaces of metallic bearings during sliding motion. Using X-ray photoelectron spectroscopy and transmission electron microscopy with coupled energy dispersive X-ray and electron energy loss spectroscopy, we found that the tribolayers are nanocrystalline in structure, and that they incorporate organic material stemming from the synovial fluid. This process, which has been termed “mechanical mixing,” changes the bearing surface of the uppermost 50 to 200 nm from pure metallic to an organic composite material. It hinders direct metal contact (thus preventing adhesion) and limits wear. This novel finding of a mechanically mixed zone of nanocrystalline metal and organic constituents provides the basis for understanding particle release and may help in identifying new strategies to reduce MoM wear. © 2009 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 28:436–443, 2010
Replacement of the arthritic or traumatized hip joint is a routinely performed procedure. Because of an aging population and the extension of the procedure to younger patients, technological and surgical aspects of joint replacement strategies are continually reviewed and advanced. Hip resurfacing arthroplasty has regained significant popularity in recent years, combining the preservation of bone stock and a reduced risk of dislocation with a contemporary, low-wearing metal-on-metal (MoM) joint articulation.1 Hip resurfacing has increased the popularity of the MoM articulation. For example, hip resurfacing accounts already for 7.5% of all hip replacements in Australia.2 Thus, the MoM market share of 9.1% worldwide will be growing3 (and 2008 internal estimation of Zimmer GmbH, Winterthur). A cause for concern with MoM joints, however, has been systemic metal ion release. Despite today's very low-wear rates ranging from 0.5 to 2.5 µm/year,4–6 increased ion levels in serum compared with other established bearing combinations are observed.7 Metal ion release, which can form metal/protein complexes,8 and the generation of nanoscopic wear debris,9–11 raise concerns regarding particle induced osteolysis, perivascular lymphocytic tissue responses, and metal hypersensitivity.12–15
Considerable progress has been made in understanding and controlling manufacturing variables such as alloy composition, bearing diameter, design and clearance tolerances, and surface finish. Further wear reductions will only be possible if underlying wear mechanisms are better understood. In vitro16 and retrieval17 studies found that the governing wear mechanisms are not adhesion and abrasion as in other bearings, but predominantly tribochemical reactions (TCR) and surface fatigue. TCRs occur when the surfaces of two contacting metal bearings react with the interfacial medium (e.g., synovial fluid), resulting in the alternating formation and removal of chemical reaction products at the surfaces.18 The observed nanometer-sized wear debris must stem from the uppermost tribochemically transformed zone; otherwise, small wear rates would be impossible. Indeed, using transmission electron microscopy (TEM) the top surface layer can be seen to recrystallize to nanometer grain sizes.19 The interplay between lubricant and the nanocrystalline surface layer is not well defined. An investigation into this interaction is critical because TCRs affect the composition of the layer and determine its mechanical and chemical properties (and thus stability). Our purpose was to provide a better understanding of TCRs in MoM joints by virtue of chemical and microstructural analyses of retrieved MoM bearing couples.
- Top of page
- MATERIALS AND METHODS
Considering the environmental conditions of the specific tribosystem of the artificial hip joint, all four major wear mechanisms (abrasion, adhesion, surface fatigue, and tribochemical reactions) can apply.18 Typically, MoM hip joints operate in boundary or mixed lubrication mode,24 depending on the head diameter and clearance tolerances. Hence, TCR layers are expected for MoM joints and have been described.25–29 TCR layers were recognized as “deposits” and/or “precipitates,” which belies their importance in the tribosystem. In this study we demonstrated that TCR layers do not simply adsorb onto the bearing surface; TCRs also modify the cobalt-alloy substrate, transforming subsurface layers from purely metallic to composite-like.
The TCR layer consists of organic, ceramic, and metallic constituents that are well mixed. At first glance these findings are unexpected, but the specific composition of the TCR layers explains the success of self-mating cobalt–chromium alloy joints in the human body: direct metal–metal contact never occurs in the presence of a TCR layer—even without fluid film separation. Thus, adhesion, which could lead to catastrophic seizure of the contacting surfaces, is prevented. Indeed, no signs of adhesion were identified on 84 articulating surfaces of this retrieval collection.17 Obviously, TCR layers are essential to keeping wear rates low.
Tribochemical reactions depend on the mechanical and chemical interaction between body and counterbody, the interfacial medium, and the environment. According to classical theory, reaction layers are generated within or adjacent to the contacting areas and require mechanical action. Friction between the contacting bodies causes an increase in temperature and a rise of the inner energy of the uppermost layers of the deformed materials in contact. Both features enhance the surface reactivity, and oxidized islands are generated.30–33 These oxide layers flake off the surface after reaching a critical thickness. Now, freshly activated, bare metal is presented to the interfacial medium causing metal–ion release. The interfacial medium is likely involved in the generation (reformation) of TCR layers. For example, proteins can stick to the activated surfaces, forming deposits. This may slow the repassivation process, yet a chromium–oxide layer is still generated.34 The specific bonding mechanism is not well understood, but can be attributed to the high number of free Co and Cr ions close to the surface, which easily form metal/protein complexes.8 In turn, these complexes are adsorbed onto the metallic surfaces.35 These protein layers adhere rigidly to the surfaces23 and are typically found on passive metal films.17
The subsurface carbon must stem from these or other environmental carbon sources. At Point 2 in Figure 3, within the first 100 nm of the TCR layer, the nonmetallic elements were 89% C, 7% N, and 4% O. This is similar (though not equal) to albumin, a major protein constituent of synovial fluid. Human albumin contains 63% C, 17% N, 19% O, and <1% S (neglecting hydrogen).36 XPS and EELS readings suggested the presence of carbon clusters, not dissolved carbon (furthermore, the measured C content is far too high to be attributed to carbides). However, it is still unclear how carbon clusters can extend up to 200 nm into the bulk, given the thermodynamic conditions of the hip joint. Although locally elevated temperatures between 60 and 80°C are conceivable,16 no thermally driven diffusion process can be postulated that would account for driving organic matter into a metallic solid solution within a time frame of years. Similarly, a mechanically driven diffusion process37 is implausible under mild sliding wear conditions: the essential impact energy for this process is not present in total hip joints. Therefore, another mechanism must be operant, capable of blending organic material with a metal substrate. Based on recent molecular dynamics (MD) simulations, such a mechanism was investigated by Rigney et al. and termed “mechanical mixing.”38–42
Putting two different metallic materials A and B into contact, MD simulations revealed the formation of vortices in the vicinity of the interface during sliding conditions. The convective material transport is most pronounced in regions with high vorticity. Interestingly, the material transport is not restricted by the interface A/B, but material exchange between both bodies can take place. Such a mechanism is capable of mixing materials over a number of atomic distances and has been experimentally validated for several tribosystems.43–45 In the case of MoM hip replacements, the tribosystem is very complex, and the computer simulation of all aspects (e.g., organic constituents of the synovial fluid; materials with strain induced phase transformation) is currently impossible. However, the same principles apply, suggesting that areas with oxide layers and/or adsorbed proteins are cluster-wise incorporated into the convective material transport. This, in concert with the external shear stresses due to friction, facilitates the transformation of the uppermost subsurface layers into a nanocrystalline microstructure of cobalt–chromium alloy. The nanocrystals are known to rotate under mechanical shear stresses,46 which would then support the mixing process even further.
All retrievals were first generation McKee-Farrar type MoM components from various manufacturers. They were made of cast cobalt–chromium alloy according to ASTM F75/ISO 5832-4. Today, MoM bearings are typically manufactured from wrought (forged) low or high carbon CoCrMo alloys (acc. to ASTM F1537 and ISO 5832/12). This is a limitation of the study; however, similar microstructural surface changes were observed for wrought low19 and high carbon16 cobalt–chromium alloys after in vitro testing. Furthermore, microstructural surface changes were found in other tribosystems with austenitic stainless steels sliding against each other in boundary or mixed lubrication mode.22 These reports suggest that our findings likely apply to current MoM bearings, and thus provide a clear direction for investigating these bearings. Recently, a mechanically mixed zone of nanocrystalline metal and organic constituents was documented for a modern, retrieved hip resurfacing implant.47
The mechanism is similar to the action of antiwear additives in high-performance engine lubricants. These additives form surface films that protect the underlying material.48 Further work is required to determine if current MoM devices exhibit the protective nanocrystalline TCR layers and could benefit from strategies to stabilize them. To make MoM bearings more durable and further reduce their wear, the generation of nanocrystalline TCR layers might be enhanced. Strategies should be employed to stabilize these layers.
In conclusion, TCR layers are found frequently on MoM bearing surfaces. These layers are generated through mechanical mixing with organic carbon stemming from the synovial fluid and are a nanocrystalline composite of metallic, ceramic, and organic material. One strategy to lower wear rates of these bearings is to promote the formation and stability of TCR layers.