Increased Metal Levels from Stryker Hip Implants: Cobalt Metal Poisoning from Recalled Stryker Rejuvenate & ABGII Hips
Wednesday, May 29, 2013
How Stryker’s Rejuvenate and ABGII are Different from the DePuy ASR
A Stryker hip recall for the ABGII and Rejuvenate implants occurred in 2012 while the DePuy ASR was recalled in 2010. Although there are similarities between the hip devices, there are very important differences as well. Patients who received a recalled DePuy ASR hip implant and have developed metal toxicity could find their cobalt and chromium levels in the 20’s, 30’s, 40’s or even 50’s parts per billion. The acceptable “danger” level is generally agreed to be anything over 7 parts per billion although some doctors put that number much lower, between 2 and 3 parts per billion. DePuy ASR patients with elevated levels of cobalt and chromium will generally find those numbers very comparable. This means if the cobalt levels in the hip replacement are 20 ppb, then it is likely the chromium levels will be very much the same.
Patients with Stryker ABGII or Rejuvenate hip implants could see a wide variance between the cobalt and chromium levels with the cobalt levels being considerably more elevated. The probable reason for the difference in these numbers is that the wear in the Rejuvenate and ABGII hip implants does not come from the cup and ball portion of the joint rather it comes from the stem and neck. The design of these Stryker devices implements a Morse taper between the stem and the neck. The Morse taper is a slightly angular junction which the physician hammers into the stem until it is snug, then hammers again so it will lock tightly.
It is believed that where the Morse taper meets the stem, the cobalt and chromium ions are being released, causing cross ionization between the titanium stem and the cobalt and chromium neck—more specifically the cobalt in the neck. For reasons not clearly understood, it appears the cobalt ions are pulling out from the neck more than the chromium ions. When these ions enter the surrounding tissues or bloodstream, necrosis, tissue damage and death to bone and muscle can occur. The Morse taper, which was meant to provide more “give” and more readily adapt to different patient sizes has instead created an added stress which causes micro-motion and fraying, sending ion debris into the body. The Morse taper is not considered a wear joint like the cup and ball therefore there should be no wear and no cobalt and chromium release of ions.
Losing Bone vs. Losing Muscle Due to High Levels of Cobalt
When cobalt ions from the Stryker Rejuvenate or ABGII burrow into the surrounding hip tissue, muscle and bone, the consequences can be very serious. When metal cobalt ions kill bone, it is definitely a concern however bone can be re-built using bone stock and cement. In other words, bone can be artificially recreated but the muscles in the body are a different story. Although the human body has roughly 640 muscles, when cobalt exposure kills a muscle it cannot be regenerated or artificially recreated therefore is gone for good. When muscles die, the strength of the surrounding muscles can be increased in order to continue to carry the loads placed on them but this could lead to a burning sensation, a limp, a serious gait or even the necessity of a walker or wheelchair for life.
How the Stryker Rejuvenate Gained FDA Approval
When Stryker launched the ABGII and Rejuvenate in 2009 there were two ways to gain FDA approval. The company chose the path of least resistance known as the 510k process which requires minimal testing and virtually no in-depth research regarding the potential cobalt ion shear. Stryker did not anticipate that the Morse taper would wear in the specific way it has or that it would release the high levels of cobalt and chromium into the body of the implant recipient. Little was known about the short or long-term effects of the Rejuvenate or ABGII therefore many doctors counseled their patients to have the device removed once it was found that cobalt levels in the body were elevated. The 510k process has come under fire due to the number of medical devices which received this type of approval and were later found to be dangerous to consumers.
Cobalt Poisoning—How Much is Too Much
Because every person has a different tolerance level for metals in the body, one person with levels only at 2 parts per billion could be extremely sick while another person testing above 7 parts per billion might not be experiencing significant side effects. However, it is believed the tolerance ranges for cobalt in those with a Stryker ABGII or Rejuvenate hip stem are considerably lower as they are emanating from a joint which was not meant to sustain serious amounts of wear.
Therefore a patient with a Rejuvenate or ABGII hip implant device could require revision surgery when cobalt levels reached a 3, while a patient with another type of hip implant could test at a level 10 and still not be a candidate for revision surgery. Physicians will generally recommend revision surgery even when cobalt levels are not as high in a patient with a Stryker Rejuvenate or ABGII implant. It is important for those with a Stryker Rejuvenate or ABGII hip implant to have a cobalt blood level test done to determine the levels of toxic metals in the bloodstream then to discuss cobalt poisoning levels with their physician.