METAL PARTICLES AND METAL IONSMetal wear particles are produced by the in vivo removal of surface asperities, a.k.a. surface protrusions, when bearing surfaces move and act against each other. Metal particles are also created by fretting between components and by aseptic loosening .
Metal ParticlesCorrosion of metal particles creates metal ions through electrochemical processes . All ions carry either a positive or a negative electrical charge. Because of their electrical charge metal ions easily bond with naturally existing proteins creating ion-protein complexes. Worries and concerns about metal ions are actually about ion-protein complexes and the potential to activate the immune system. With current generation metal-on-metal bearing couples, the levels of cobalt and chromium ions are the main concern for immune system activation.
- larger than atoms
- smaller than polyethylene wear particles
- Volume produced
- lower than polyethylene wear particles
Metal IonsProper control of material selection, design and manufacturing processes reduces corrosion, fretting, and wear, therefore reducing metal particle and metal ion production.
- slightly larger than atoms OR
- slightly smaller than atoms
- extremely small
- Produces through
- corrosion of metal in the body
- corrosion rate related to volume & surface area of exposed metal
BIOLOGIC CONCERNSBecause of the low rate of production, the low volume produced, and the size of metal particles, they are not typically a large concern. Since the critical biologic load is not easily or quickly reached, the human body’s own defenses are usually able to eliminate metal particles and in turn, metal ions. It could be argued that the increased metal ion levels found in the blood and urine of many metal-on-metal bearing recipients is simply a sign of the effective biologic handling of these wear products. The liver and the spleen are the major waste management systems of the human body. It is reasonable to expect that any toxic effects from metal particles would be seen here first , most likely in the form of small localized clumps of particles. An autopsy study of a patient 30 years post-metal-on-metal total hip replacement showed no evidence of metal particle accumulation or organ damage . Repeated searches of the orthopaedic literature have not found any reports of organ damage or toxicity in otherwise healthy patients.
Biologic IssuesCobalt and chromium ions are of the greatest concern in metal-on-metal THR patients. In animal models, elevated levels of these metal ions are associated with increased carcinoma rates [4,5]. However, these cancer rates have not been seen in humans with metal-on-metal THR nor is it clear how to best translate cancer rates from animal studies to human populations. There are published exposure levels from the EPA for chromium, but these refer to inhaled and ingested levels not implanted sources of metal ions. Only one article has specifically examined cancer risk after metal-on-metal THR . Visuri et al found that the metal-on-metal THR group did not have any significantly increased risk of cancer when compared to the metal-on-polyethylene THR group. Hypersensitivity to metal ions appears to have the potential to affect a larger portion of the orthopaedic population. Indeed, metal hypersensitivity is higher in the total joint population, regardless of bearing type, than in the general population [4,7,8,9]. Although there are some concernst that hypersensitivity may affect aseptic loosening and therefore failure, it remains unclear if hypersensitivity plays any role at all [1,4,9,10,11,12]
Relationships between Metal Particles, Metal Ions, & Biologic Issues - UNPROVEN
- no reliable reports of organ toxicity due to metal-on-metal THR
- metal ions from THR have not been shown to cause cancer in humans
- hypersensitivity is a minimal risk
- unclear if hypersensitivity contributes to implant failure
Design IssuesAs already discussed, metal ions are primarily produced from metal particles and metal particles are primarily products of wear. Therefore, when wear is reduced, ion production is also reduced. Current metal-on-metal, second-generation, bearing designs are optimized to address wear issues from first-generation designs. Advances in metallurgy and stricter tolerances in surface finish, clearance, and sphericity reduce wear particle production .
- surface finish
MetallurgyThe cobalt-chrome alloys currently in use may be wrought or cast alloys, and they may be high or low carbon content; both of which affect wear characteristics. The magnitude and distribution of surface carbides, a factor in wear resistance, is markedly affected by carbon content [14,15,16,17]. Various studies have shown differing results, clearly systematic evaluation of alloys and carbon content are needed . Results range from higher carbon alloys are associated with less wear [15,18], to carbon content does not affect wear , and from cast alloys are more resistant than high carbon wrought alloys  to wrought alloys are superior to cast .
Design VariablesSmooth, consistent surface finish on both bearing surfaces reduces the production of metal particles and metal ions through decreased wear. Smoother surface finishes, less rough surface finishes, have fewer surface asperities and therefore have reduced wear rates; there is simply less to wear off. Sphericity and clearance are closely related since both are dependent on femoral head geometry. Sphericity is controlled by the manufacturing process and refers directly to the smooth roundness of the femoral head. Poor sphericity causes poor clearance and increased wear. Clearance is the difference between the three-dimensional radius of the femoral head and the acetabular cup. Proper clearance results in increased polar contact at the apex of the femoral head, but not in the equatorial region. Low clearance results in decreased wear, but if clearance is too low, synovial fluid cannot enter the interface to act as lubricant. Additionally, wear particles will not be ejected from between the bearing surfaces, causing more wear to be produced.
SUMMARYUnfortunately, there are no hard and fast conclusions. It seems clear that excessive metal particle production, and in turn metal ion generation, is not a good thing. But what is “excessive”? If “excessive” metal particle and metal ion levels can be defined as the point where adverse effects occur for the general metal-on-metal THR population, then it is also clear that current metal-on-metal bearing surfaces do not generate “excessive” levels. It may be that in the very long-term, some patients are not as well served by metal-on-metal THR as might be hoped. But given the drive for further knowledge and development in the orthopaedic industry, science, and practice, innovative solutions will be discovered. References
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Copyright (c) 2005 Susan G Capps, PhD