As more and more recipients of
metal-on-metal hip implants experience serious adverse symptoms from the
devices there will be more information on the design defects in these devices
which have caused the problems. Stryker Orthopedics recalled their Rejuvenate
and ABGII in 2012, only a few short years after the implants received FDA
approval and began being marketed. Like the recalled DePuy ASR, as time passes
more and more defects are being found in the design of the hip implants. When
Stryker recalled the Rejuvenate and ABGII they stated the potential of fretting
and corrosion at the neck juncture. Later it was found that corrosion could
occur under the small metal trunnions located at both ends of the neck portion
as body fluids became trapped underneath.
More recently it has come to
light that there may be a serious mismatch between the titanium stem portion—which
is coated with a proprietary mix of titanium, molybdenum, zirconium and iron
mixed with plasma spray—and the cobalt and chromium neck. Studies as far back
as 2002 found that corrosion is significantly higher in hip implants with mixed
alloy parts versus parts which were all made of the same metal. In fact, mixed
alloy hip implants were responsible for moderate to severe corrosion in 42% of
hip implants as compared to 28% in implants constructed of only one metal. A
study done as recently as 2012 concluded that when a titanium stem and cobalt
and chromium adapter sleeve were mixed, a galvanic reaction which produced
corrosion was much more likely. While it is theorized that the problem may
occur because the cobalt and chromium components are much harder, scientists
have not yet found a definitive reason for this phenomenon.
Why Frictional Torque May Be a Problem in the Stryker Rejuvenate and
ABGII
Frictional torque could also
be an issue in Stryker’s recalled hip implant devices. Stryker’s design was
considered highly innovative as it implemented a variety of parts which allowed
the surgeon to choose the best sizes for the individual patient. The modular
neck of the Rejuvenate fits into the stem which leads to frictional torque
where the two parts meet. This torque leads to significantly more wear on the
joint and, in fact, a wear analysis concluded the majority of the wear occurred
at the stem and neck juncture. As the bearing diameter increases, there is also
an increase in mechanical stress which has been found to cause excessive cobalt
ion release. This excessive wear is also responsible for pseudo-tumor
formation. Retrieved head-neck tapers from a 1992 study showed that over
one-third of the head-neck tapers showed moderate to severe corrosion due to
the frictional torque at the juncture.
Cobalt Toxicity Due to TMZF Coating Issues
The TMCF coating used to coat
the Stryker components was meant to circumvent the issue of mixing alloys as
well as offer resistance to corrosion and fretting, however data disproves that
theory. Fretting and corrosion do still occur, sending microscopic metal ions
into the tissue surrounding the implant and into the bloodstream. When the ions
have built up in the body, metal toxicity can occur leading to such symptoms as
deterioration of vision and hearing, DNA disruption, the formation of
pseudo-tumors, pain, cardiovascular, renal, thyroid and neurological disorders,
depression, anxiety, irritability, vertigo, skin disorders and gastrointestinal
issues. The ions which are shearing away from the neck/stem juncture appear to
be comprised of more cobalt than chromium. This is somewhat puzzling as in
other metal-on-metal hip implants measured levels of cobalt and chromium and
generally much the same, while in patients of the Stryker Rejuvenate and ABGII
the cobalt levels outpace the chromium levels. It is expected that Stryker
Rejuvenate hip recall lawsuits will continue to mount as those who were
implanted with the recalled devices will continue to experience adverse
symptoms.
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