Wednesday, June 13, 2012

Is it Chromium or Cobalt that is the biggest culpret in metalosis?

Acta Biomater. 2012 Jun 8. [Epub ahead of print]

Cobalt from metal-on-metal hip replacements maybe the clinically relevant active agent responsible for periprosthetic tissue reactions.


Department of Orthopaedic Surgery, Imperial College and Imperial College Healthcare NHS Trust, Charing Cross Hospital Campus, Fulham Palace Rd, London W6 8RF, UK.


Some types of metal-on-metal (MOM) hip replacements have unacceptably high rates of failure, such as the Ultima TPS MOM hip with 13.8% failure at 5 years. This has been attributed to an inflammatory reaction following the release of cobalt (Co) and chromium (Cr) from the bearing surfaces and modular junctions. There is in vitro evidence that Co is more important than Cr in the inflammatory process, but there are no reported human tissue studies of the analysis of the implant-derived metal. To better understand the mechanism of failure of the "Ultima TPS" hip implant, we studied the distribution, relative amounts and chemical form of cobalt (Co) and chromium (Cr) in periprosthetic tissue taken at revision surgery from 6 patients. We made a comparison with tissue from 6 patients with current generation, large diameter MOM hips (the Non-Ultima group). Both types of implants had Co-Cr bearing surfaces but the Ultima type had a titanium (Ti) acetabular cup and a cemented CoCr stem that had macroscopic corrosion of the femoral stem at revision surgery. Both types of hip can cause an adverse tissue response but its severity is likely to be affected by the amount and type of implant-derived metal released from the hip. All patients were revised due to severe pain in the absence of infection. Synchrotron X-ray spectroscopy was used to map the tissue samples for Co and Cr. 63,173 tissue areas (4 x 4 microns) were analysed for their relative amounts of Co and Cr and some of these underwent detailed analysis to determine the oxidation state of the Co (4 patients) and Cr (6 patients). The findings differed according to the hip type: Co was much more abundant in the Ultima hips and had a ratio of Co to Cr that was ten times greater than the Non-Ultima hips (p=0.011). This was independent of the concentration of the metals at those locations. Interestingly, the oxidation states of the most abundant chemical forms of Co and Cr were similar for both hip types: Co was a mixture of metallic (zero oxidation state) and a Co(II) species; Cr was Cr(III) in both types of hips. We attribute the greater amount of Co in the periprosthetic tissues of the Ultima TPS to the accelerated corrosion of the femoral stem, which occurs when used together with a MOM bearing. Interestingly, the oxidation state of the Co in the tissues did not differ between the groups so that the dose rather than type of Co appears to be the biologically relevant issue. This may be generalizable to other MOM hip designs that use a cemented CoCr femoral stem and titanium acetabular component and implant designs that can minimise the Co release may have increased longevity. Furthermore, biocompatibility tests that rely on Co induced readouts maybe the most realistic.

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