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Wednesday, June 13, 2012
Is it Chromium or Cobalt that is the biggest culpret in metalosis?
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.