Just
this past July Stryker Orthopedics issued a voluntary recall of their
Rejuvenate and ABGII hip implant systems citing a potential for fretting and
corrosion at the metal neck junction. The FDA convened a panel shortly before
this recall to discuss the merits of continuing to use metal-on-metal hip
implants due to the high number of complications suffered from recipients. The
design of the Rejuvenate was believed to overcome most issues associated with
the all-metal hip implants as it allowed surgeons to custom-fit the stems and
necks of the implants to the size and relative activity level of the patient.
The Rejuvenate Design
The
Stryker Rejuvenate consists of a ceramic ball which fits into a plastic-lined
socket. Because there was no metal-on-metal ball and socket it was believed
there were no metal surfaces which could potentially rub against one another,
causing metal ion debris to enter the surrounding tissue or bloodstream. The
Rejuvenate is essentially a two-piece system in that the properly sized metal
stem and neck connect to one another then connect to the ceramic ball. The neck
is made of chromium and cobalt, just like the all-metal implants and the stem
is coated with titanium. Even so, by using a ceramic ball and a plastic-lined
socket, it was believed there would be no issues regarding metal ion debris.
The Problem with Trundles
What
the company may not have accounted for are the trundles on either side of the
neck piece of the Stryker Rejuvenate implant. Trundles are essentially “divets”
which allow the neck piece to slip into the stem on one side and into the ball
on the other side. In other words, the stem portion of the implant goes into
the femur bone, then the stem connects to the neck by popping together via the
trundles, the neck connects to the ball with trundles, then the ball fits into
the acetabular cup. While there is no metal ball rubbing on a metal cup,
corrosion can occur where the trundles are located. Body fluids can be trapped
inside the trundles, causing corrosion which leads to chromium and cobalt
spilling out of that implant joint.
The Potential for Metal Toxicity
While
there would not be the same high levels of metal ions resulting from this
trundle corrosion as from an all-metal implant, any level of metal in the body
is too much and can lead to health risks for recipients. The trundles in the
implants of those patients who are typically more active can be jostled more,
leading to greater levels of cobalt and chromium ion shedding. These ions can
enter the surrounding tissues or the bloodstream causing necrosis, pain,
infection and metal toxicity. As the trundles have not been specifically
mentioned as a potential problem in the recall by Stryker, it is unlikely
physicians would even think to look at the trundle area of removed implants.
