Corrosion of a metal is observed as a breakdown of the material over time due to a chemical or electrochemical reaction. Virtually all metal orthopaedic implants used in patients will corrode to some extent. Often the extent of this corrosion is minimal, having no impact on the patient or implant itself. Occasionally however an implant (or part of an implant) will corrode to such an extent that its function is impaired, or the debris generated has an adverse effect in the patient.
The phenomenon of trunnionosis is well documented in the use of large-diameter metal-on-metal hip (LD-MOM) implants, in which metal released from the junction of the femoral stem trunnion and head taper leads to the patient experiencing an adverse reaction to metal debris (ARMD), Figure 1.
We know the predominant mechanism of this metal release to be due to corrosion at this interface . Specifically, it is the process of mechanically assisted crevice corrosion that occurs; micromotion of the trunnion in the taper damages the protective oxide layer on the surface and the ingress of fluid within crevices between the two surfaces leads to local chemical reactions. This process may be further exacerbated if mixed alloys are used, leading to galvanic corrosion; this explains why cobalt-chromium (CoCr) femoral heads often appear to corrode more than the titanium alloy stems they are paired with. Taper junction corrosion in LD-MOM has particularly been an issue due to the larger diameter heads generating greater rotational and lever arm forces, leading to more micromotion at this interface .
In cases where patients with LD-MOM hips have experienced ARMD, metrology analysis has measured volumes of CoCr released from these corroded taper surfaces to be as high as 25mm3 . This contrasts with volumes of 1.45mm3 released from tapers of hips in patients with no evidence of ARMD . The bearing surfaces of MOM hips however have been shown to release significantly more material, as high as 300mm3, due to a process of mechanical wear . Importantly however, we find that patients with corroded hip tapers often experience similar adverse reactions to those with worn bearing surfaces, even though the amount of metal released from the tapers is so much lower. Our synchrotron work conducted at the Diamond Light Source in Oxford helped explain these clinical findings; we found that the speciation of chromium metal present in tissue taken from around the taper junction of patients was different (and potentially more toxic) than that found in tissue from around the bearing surfaces . These differences were attributed to the different degradation mechanisms: corrosion versus mechanical wear.
Beyond the taper junctions, we’ve also seen evidence of considerable corrosion at the stem-cement interface of some cobalt-chromium cemented femoral stems, Figure 2 . In some circumstances, these have been the predominant source of metal ion release, even in patients with LD-MOM hips.
Corrosion of CoCr liners has also been observed at the liner-shell interface of modular cups (the shells are composed of titanium alloy) and this was a contributing factor in the high failure rates of one design that was ultimately recalled .
Blood metal ion tests have been demonstrated as being a useful biomarker for detecting when a CoCr implant is corroding in the patient. Previous studies have shown that when these components corrode, Cr ions tend to be preferentially retained within the black corrosion deposits that are often observed on their surfaces. Consequently, more Co ions are found in the blood stream in the presence of a corroding implant, and this is reflected by a greater ratio of Co to Cr ions measured in blood samples. Our retrieval analysis of approximately 400 corroded hip tapers found a trend of greater corrosion, Figure 3, being associated with higher Co/Cr ratios . In the patient, sequential blood tests showing an increasing Co/Cr ratio are likely to be indicative of an increasingly corroding implant.
To conclude, we know that all implants are likely to corrode to some degree in vivo. Clinical-engineering analysis of retrieved implants has helped us better understand (1) when this corrosion is clinically significant, (2) which factors influence the extent of corrosion and (3) how we can better predict and detect its occurrence in patients.
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- Hothi, H. S., Berber, R., Whittaker, R. K., Blunn, G. W., Skinner, J. A., & Hart, A. J. (2016). The Relationship Between Cobalt/Chromium Ratios and the High Prevalence of Head-Stem Junction Corrosion in Metal-on-Metal Total Hip Arthroplasty. The Journal of arthroplasty, 31(5), 1123–1127.
Harry Hothi, Sean Bergiers, Johann Henckel, Anna Di Laura, John Skinner, Alister Hart
The Implant Science Centre at The Royal National Orthopaedic Hospital