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The effect of micro-structure on the wear of cobalt-based alloys used in metal-on-metal hip implants

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My View:

The scientists, surgeons, manufacturers have all known about the impact of heat treatment on the wearing of cobalt chrome metal-on-metal hips for nearly 10 years. But there have been a series of trade-offs:

  • cast cobalt chrome is more brittle and can lead to fracture of the femoral neck of the replacement hip.
  • heat-treated cobalt chrome is more flexible and so a replacement hip failing due to fracture is less likely.
  • heat-treated (tempered) cobalt chrome hip implants wear faster.
  • every one has known for years that cobalt chrome wears and releases metal ions and these destroy tissue and bone and can cause systemic health issues.
  • all parties involved in this problem seem to have a view that up to about 10% of recipients having problems is OK  but above this, it is not!
  • trouble is 10% in the USA is 30,000 or more people – individuals, whose lives are screwed up by this – some much more than others.
  • in Australia we are talking about around 3,000 people, and in the UK about 5,000 – so globally this is a huge medical problem, but the medical device companies, the surgeons and regulators all keep treating us as statistics – ” a low percentage is OK” – but we are individuals, not just numbers!

In this study, the researchers found that the wear was strongly affected by the dissolved carbon content of the alloys and mostly independent of grain size or the carbide characteristics. The increased carbon in solid solution caused reductions in volumetric wear because carbon helped to stabilize a face-centred cubic crystal structure, thus limiting the amount of strain-induced transformation to a hexagonal close-packed crystal structure.

Introduction:

The dominant clinical problem that reduces the longevity of metal-on-polyethylene (MPE) total hip replacement is periprosthetic osteolysis [1]. This ‘disease’ is caused by macrophage-induced bone resorption [2, 3] at the bone-implant interface owing to the repeated ingress and egress of small wear par¬ticles in this area [1]. Macrophages are cells which react to the foreign wear particles by enveloping them and they eventually fuse with other macro- phages to form cysts. For MPE hip implants, the macrophages found in the vicinity of bone resorption usually contain a large number of polyethylene wear particles [4, 5]. Additional bone resorption is caused by cytokines signalling osteoblasts to reduce their activity [6].

Interestingly, retrievals of some cast cobalt- chromium-molybdenum (CoCrMo) metal-on-metal (MM) hips [7], implanted in the early 1970s, revealed very low wear of the articulating surfaces compared with MPE implants. As a result, a renewed interest developed regarding the optimization of the wear performance of CoCrMo MM implants used in total hip replacement. Most modern MM hip implants are made from investment castings (that are later subjected to a solution-annealing procedure) or machined from wrought alloys. The wrought state of these alloys is achieved through thermomechanical processing of the cast material by hot rolling or forg¬ing into bar stock. Thermomechanical processing of the material refines the grain structure and removes any macrodefects that are present, thus improving the overall mechanical properties of the alloy [8]. In addition, the precision surface grinding that is used in manufacturing the head and cup components has allowed implants to have precise clearance ranges that effectively eliminated any problems associated with equatorial contact that was detrimental to the performance of the older-generation MM hip im¬plants.

Although MM implants have been shown to exhibit much less wear than their conventional MPE counterparts, research is required to reduce further the volume of wear particles that are generated at the joint interface. Since implants made from differ¬ent CoCrMo alloy grades exhibit different amounts of wear [9], it is necessary to investigate the role of alloy microstructure in the wear of these implants in order to take full advantage of the use of the MM configuration. As a prerequisite, a better understand¬ing of the parameters controlling wear is required.

Discussion (excerpts):

Carbides had been considered to be very important second-phase materials with respect to wear owing to their relatively hard nature. In terms of wear resist¬ance, carbides might influence the wear of materials not only because of their high hardness character¬istics but also as a protective barrier against matrix delamination [29]. In the present study, identical sur¬faces were in contact with one another (with essen¬tially identical microstructures). Thus, hard carbides opposed equally hard carbides and they all sustained cracking, fracture, and pull-out, thus causing third- body abrasive damage. Furthermore, the high hard¬ness of the carbides caused abrasive damage to the respective counterfaces because, between the car¬bides on both surfaces, a softer matrix material was vulnerable. The argument that larger carbides might be better for resisting wear in that they would provide a larger region of hard surface and would be more difficult to gouge out compared with smaller carbides was not considered to be an accurate statement for the present scenario of self-mated materials [30]. In fact, in the present study, alloys that contained the largest carbides (F75 in particular) had carbides that were the most damaged and even pulled out. Thus, carbides, regardless of size, might not be effective in protecting against abrasive wear in self-mating appli¬cations such as this.

However, there was another aspect to consider. The as-cast specimens had the lowest wear in the present study and, in particular, they had lower wear than the F75 specimens that had very similar micro- structure but had been solution annealed. As men¬tioned previously, the solution annealing caused coarsening of the blocky M23C6 and precipitation of fine satellite M23C6 (Fig. 3(c)). Thus, it was considered possible that, under the present conditions of direct surface contact, the F75 carbides were more prone to fragmentation with partial pull-out, giving more third-body abrasive wear and thus a higher overall wear at 1 Mc. While this might explain the higher overall wear, it was noted that the steady state wear rates (based on data from 0.5-1 Mc) were very similar.

Somewhat different results were obtained by Bowsher et al. [27] in their simulator studies. They showed no significant differences in either the run- in or the steady state wear of implants made from an alloy equivalent to the F75 compared with those made from as-cast alloy. Then, they subjected both types of implant to adverse conditions and both experienced a statistically equivalent increase in the wear rate. One explanation for the difference between the present results and those of Bowsher et al. clearly lay in the different test devices used. The pin-on-plate apparatus of the present study had quite different, initially perhaps more extreme, con¬tact conditions. Eventually, the pin-on-plate appar¬atus gave wear rates that approached zero (Fig. 7) while much higher wear rates were found by Bowsher et al. Thus, it was tempting simply to dismiss this aspect of the present results as not being of clinical significance and to concentrate instead on the more fundamental issues of microstructure influences on wear. However, during the run-in wear of the present specimens, direct contact occurred between the sur¬faces whereas this might have occurred to a lesser extent in the simulator studies of Bowsher et al. that did not permit the frequent direct contact that would occur during stop-dwell-start motion in vivo. Thus, it was considered possible that hip implants might show somewhat lower wear in vivo, when made from as-cast material.

Also, it was mentioned that carbides might act as a protective barrier against matrix delamination. In general, carbides were expected to have good coher¬ency with the surrounding matrix. Even though there were crystal structure differences between the car¬bides and matrix, good coherency was probably still attained [31]. Thus, the carbides might have acted as a barrier to matrix delamination at the surface and this would provide one advantage for the HC alloys compared with the LC alloys. There might also be an influence of carbon in solid solution on matrix delamination, as considered in the next section.

CONCLUSIONS

In the present study, a clear distinction in the wear of LC and HC alloys was observed. Among these, the as-cast alloy exhibited the lowest wear. In addition, solution annealing of the wrought F1537 HC alloy reduced the amount of wear. These observations were explained by the correlation between dissolved carbon (i.e. in solid solution) and wear. It was found that the wear decreased with increased dissolved carbon owing to stabilization of the FCC phase (inhibiting SIT to the detriment of the HCP phase). These findings, coupled with the observed surface twinning and fracture in the wear zone, supports the theory of a deformation-controlled mechanism for wear in self-mated MM contacts. To reduce wear, further heat treatments should be sought to increase the dissolved carbon in cobalt-based alloy for MM hip implants and implants should be designed with large diameters and low clearances to reduce contact stress.

Source:

Proc. IMechE Vol. 220 Part H: J. Engineering in Medicine

R Varano1, J D Bobyn2, J B Medley3*, and S Yue1 1Department of Mining, Metals and Materials, McGill University, Montreal, Quebec, Canada 2 Jo Miller Orthopaedic Research Laboratory, Montreal General Hospital, Montreal, Quebec, Canada 3Department of Mechanical Engineering, University of Waterloo, Ontario, Canada The manuscript was received on 12 October 2005 and was accepted after revision for publication on 10 November 2005. DOI: 10.1243/09544119JEIM110

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APPENDIX

Notation
EDS    energy-dispersive spectroscopy
EDTA    ethylenediaminetetraacetic acid
FCC    face-centred cubic
GSAS    general structure analysis system
HC    high carbon
HCP    hexagonal close-packed
LC    low carbon
LC    low carbon
Mc    million cycles
MPE    metal-on-polyethylene
r.m.s.    root mean square
SEM    scanning electron microscopy
SIT    strain-induced transformation

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