The Stryker hip recall
lawsuits began soon after the company’s Rejuvenate and ABGII were pulled from
the market in July, 2012. The company cited fretting and corrosion at the neck
juncture which could result in metal ion debris being released into the body
and the necessity of early revision surgery as the reason for the recall. In
addition to the reasons stated by Stryker at the time of the recall, it was
later discovered that the small metal trunnions located on each end of the
modular neck piece were subject to corrosion as body fluids become trapped
underneath.
In addition to the corrosion
at the neck juncture and under the metal trunnions, higher levels of cobalt
were found to be coming from the Morse taper—a slightly angular junction
between the stem and the neck. Research shows that the area where the Morse
taper connects to the stem releases cobalt ions causing cross-ionization
between the titanium stem and the cobalt and chromium neck. When these
microscopic ions of cobalt and chromium are released into the body they can
become lodged in the surrounding hip tissues or travel into the bloodstream,
wreaking havoc in the body.
How the Morse Taper and Mixed Metals Contribute to Ion Release
The cup and ball joint is the
one which is expected to receive the majority of the wear and the Morse taper
joint should have had little to no wear, therefore no ion release. Instead, the
taper joint has been found to have significant wear and is responsible for
causing additional stress and fraying sending cobalt ions into the body.
Corrosion has also been found to occur more often among hip implant components
which are made from dissimilar metals. The Stryker Rejuvenate and ABGII are
constructed with a titanium stem and a cobalt and chromium neck. Titanium has a
different set of mechanical properties from cobalt and chromium plus the fact
that the cobalt and chromium is a much harder metal which can result in higher
levels of wear.
Over twenty years ago research
was conducted to study the taper interface of 139 modular femoral components
which had been removed from patients. Some of these implants were constructed
of a cobalt and chromium neck and head while others were constructed of a
cobalt and chromium head and a titanium neck. While the components constructed
of the same metal alloys showed little evidence of corrosion, the mixed metal
components showed substantial levels of corrosion. When metals are mixed, taper
wear is compounded, releasing ions into the body and leading to serious medical
issues. Bearing diameter may also contribute to a greater number of cobalt ions
released into the body.
Stryker Rejuvenate and ABGII Hip Implants Show Wear at the Neck and
Stem Juncture
A more recent analysis was done regarding how and where hip implants
show the most wear. The results of this 2012 study showed
the majority of excessive wear occurred at the juncture of the modular neck and
stem, at the top of the notch area. Neck corrosion can occur in titanium alloy
stems or full-up front tapers and can be responsible for the formation of
pseudo-tumors. In some cases surgeons were removing only the neck portion
during a revision surgery and replacing the neck with one made of a different
substance. Unfortunately, this would not correct the issue of corrosion and
metal ion debris. It is expected that Stryker hip recall lawsuits will continue
to escalate as more Rejuvenate or ABGII hip implant recipients begin to suffer
health problems directly related to their recalled Stryker device.
