The July, 2012 Stryker ABGII hip implant recall affected thousands of patients—potentially as many as 10,000. Many of these patients had experienced adverse health symptoms related to their ABGII implant, soon after their surgery. For others, the symptoms had not manifested at the time of the recall, but the potential for problems remained. Many recipients of the ABGII—as well as their surgeons—were shocked to find that the hip implant they believed to be safe, came with many serious health risks. Although the Stryker ABGII hip implant is not considered a “true” metal-on-metal hip implant, the health issues associated with the ABGII are much the same.
One might wonder whether the human body would ever be a hospitable environment for metal implants since metals are inherently susceptible to corrosion. Metal hip implant components undergo a process known as pre-passivation prior to final packaging which involves using an acid bath or other electrochemical anodizing process. The particular metals used for hip implants are chosen based on whether the component will be load bearing as well as their biocompatibility with the human body. Metal devices introduced into the body are continually exposed to body fluids, which in turn lead to varying levels of corrosion.
Stryker ABGII Design Problems
The ABGII has a ceramic ball, rather than a metal ball, and the design implements a variety of neck and stem components which allow surgeons to construct a hip implant device which is personalized to the individual patient. The ABGII hip implant device was believed to not only last for 15-20 years, but to increase anatomical stability due to the titanium femoral stem which follows the contours of the natural femur. This conformity with the femur bone was believed to ensure an even load transfer pattern which closely imitated the natural stress distribution in the human body. The ABGII hip implant offered eight left stems, eight right stem and ten modular necks in varying lengths and angles; once the ideal implant was constructed, based on the patient’s body size and activity level, bone remodeling would be encouraged. While the theory of the ABGII design appears sound, the reality turned out to be vastly different.
Health Issues Associated With the Stryker ABGII
At the time of the recall, Stryker noted the ABGII hip implant had a much higher risk of fretting and corrosion, leading to excess generation of ion debris. These issues can cause the ABGII hip device to fail relatively soon after implantation, and certainly much sooner than the 15-20 year mark. When metal ions enter the bloodstream, a variety of health issues can occur, including gastrointestinal disorders, renal and thyroid disorders, neurological and cardiovascular problems, the potential for certain types of cancer, DNA disruption, the development of pseudo-tumors, irritability, depression, anxiety, vertigo, chronic headaches and the loss of vision and hearing. Metal ions which lodge into hip tissue can lead to inflammation, infection, chronic pain, bone and tissue loss and the failure of the implant.
Once the implant fails, revision surgery becomes necessary. Revision surgery, particularly for the ABGII device, is both risky and expensive. The surgical procedure can take as long as six hours, exposing the patient to infection and other medical issues, and the recovery time can be as long as six weeks. The stem of the ABGII is longer than in other hip implants, and is placed deeply within the femur during the original hip implantation surgery. Removing the stem can require extreme measures—some have likened the process to removing rebar from hardened cement.
The Stryker ABGII Hip Implant and the FDA
Many wonder how a hip implant with so many risks received FDA approval. The ABGII received FDA approval based on a process known as the 510(k) in which the manufacturer of a medical device must only show the device is substantially similar to a device already on the market. In many cases, the device the new implant gains approval under has experienced problems of its own. No clinical trials are required under the 510(k) process, only that the manufacturer of the implant will perform follow up post-market data. Unfortunately, by the time post-market data is amassed, it is much too late for many recipients of the hip implant.
Stryker ABGII Coating Defects
Although Stryker has remained largely silent regarding the reasons why the ABGII failed, there are a variety of likely theories. The mismatched components may be an issue in the ABGII, as studies done over a decade ago showed that pairing dissimilar metals can lead to excess corrosion. The taper junction of the titanium stem and the cobalt and chromium neck of the ABGII make it especially prone to corrosion and the release of metal ions. It was not expected these issues would be a problem since at the time the ABGII was marketed, one of its “selling points” was the fact that Stryker sprayed the implant with a proprietary blend of titanium, molybdenum, zirconium and iron (TMZF).
This coating was believed to stop the problems associated with mismatched components, covering the titanium stem and cobalt and chromium neck with a mixture of TMZF and plasma spray. In fact, Stryker claimed the TMZF alloy further improved stress transfer to the proximal femur over more conventional alloys. Interestingly, in most metal implants, the cobalt and chromium ions release in roughly the same amounts. Studies show the Stryker ABGII hip implant releases greater amounts of cobalt than chromium. Scientists are uncertain whether this relates to the TMZF coating or another issue.
Whether the TMZF coating actually contributed to the corrosion associated with the ABGII is not clear, however it is clear that the TMZF coating failed to deliver as promised. There is also another coating on the stem of the ABGII known as hydroxyapatite porous coating, or HA. The goal of the HA coating is to encourage bone ingrowth, however the coating may release into bone tissue, putting ABGII patients at risk for osteolysis, or weakening of the bone, leading to pain, loss of range-of-motion and other health risks. Many patients experienced popping and cracking noises from their ABGII implant, and later found that the bone surrounding the implant had deteriorated, possibly as a result of the HA coating. The HA coating also makes it much more difficult to remove the stem during revision surgery, and although the coating on the ABGII is smoother and shallower than on the Rejuvenate, it remains a problem.