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.
