Stryker ABGII Design Problems
The Stryker ABGII hip implant device was recalled in July, 2012, after being on the market since only 2009, when it received FDA approval. At the time of the recall, Stryker noted the risk of fretting and corrosion, leading to excess metal ion debris generation. These microscopic cobalt and chromium ions travel to the bloodstream or become lodged in the hip tissues, leading to serious health problems in some people. Stryker has been relatively quiet about why the ABGII hip implant has had so many problems, particularly since the design was believed to be a particularly novel approach which would correct many of the problems associated with all-metal hip implants.
The ABGII implemented a ceramic ball rather than the cobalt and chromium ball used in most metal-on-metal hip implants. Since the head-neck junction was considered to be the primary load-bearing junction, designers were surprised to find that in the case of the ABGII, the neck-stem junction was bearing the primary load. The design of the ABGII was also considered groundbreaking, in that it offered a variety of stem and neck component pieces of differing sizes and angles. In fact, the ABGII hip implant offered eight right stems, eight left stems and ten modular necks. This allowed the surgeon to precisely fit the hip device to the individual patient, taking into account the patient’s body size and type as well as their overall level of activity.
The Stryker ABGII was believed to more closely follow the natural contours of the femur, adding stability and maintaining bone stress levels. Trunnions, on both ends of the neck component allow that portion of the implant to “pop” into the stem on one end and to the ball on the other. The stem was implanted into the femur, and the ball was fitted inside the cup. Although it was first believed that the corrosion was contained to the neck-stem taper junction, it is now known that body fluids can collect beneath the metal trunnions, leading to corrosion and further elevated levels of cobalt and chromium among recipients. Despite the innovative design of the Stryker ABGII and the claims that the implant would last from 15-20 years and was ideal for the younger, more active patient, adverse reports began coming in to the FDA soon after the device gained approval.
Does the FDA Know what is wrong with the Stryker ABGII?
The FDA approved the Stryker ABGII under the 510(k) process which allows medical devices to gain approval based on their substantial similarity to a device already on the market. No clinical trials are mandated, and the only requirements are that the manufacturer must collect data on the implants after the fact. Unfortunately, for the many people who were harmed by an ABGII implant, it is much too late once the device is implanted in their body. During the years following FDA approval of the ABGII, several surgeons contacted the FDA as well, with reports of patients who were experiencing signs of metallosis and metal toxicity following their ABGII implant. One surgeon removed an ABGII device and noted that it was covered with what appeared to be “black rust.”
Stryker ABGII Fretting and Corrosion and the Risks to Patients
Once the Stryker ABGII has undergone corrosion and fretting, the metal shards may lodge into the hip tissues, causing inflammation, infection, chronic pain, and the destruction of bone and tissue. When the metal ions enter the bloodstream a variety of adverse health effects are possible, including: gastrointestinal disorders, neurological and cardiovascular disorders, renal and thyroid problems, loss of vision and hearing, depression, anxiety, irritability, vertigo, chronic headaches, reproductive disorders, the formation of pseudo-tumors and even disruption of the patient’s DNA. All of these health risks fall under the category of metal toxicity or metal poisoning. Some people are more sensitive to metals, and even relatively low levels of cobalt and chromium can render them extremely ill.
Others are less sensitive to the metals and may test high, yet have few overt symptoms. Because the failure rate of the ABGII is possibly as high as 40%, even those patients who have not yet experienced symptoms of metallosis or metal toxicity, have a fairly good chance of developing those symptoms in the future. Should the metal levels be extremely high, or the inflammation reach a point where the implant begins to fail, revision surgery will likely be necessary.
Stryker ABGII Component Friction
Although Stryker has made few statements regarding design defect theories, researchers have come up with several issues which may have rendered the ABGII unsafe, and contributed to the recall. One of those issues is mismatched components, meaning the titanium stem and the cobalt and chromium neck are well-known to create corrosion at the taper junction, possibly due to the much harder cobalt and chromium neck. The coating used on the neck and stem may also have caused some issues in the ABGII. Finally, frictional torque could be a contributor to the downfall of the Stryker ABGII. Frictional torque was previously rarely seen in the neck-stem taper portion which locks the two components securely together.
When implanting the ABGII, the Morse taper of the neck is pounded into the stem, which may also be responsible for the release of metal ions. Scientists believe the lever-arm effect of the ABGII neck taper causes the increased amount of movement at the neck-stem junction, eccentrically loading this taper junction. Other hip implants are a one-piece construction, meaning there is little to no movement in this area, however with a goal of providing a hip implant which more fully acts as a normal hip joint, the ABGII does allow for additional movement. This movement creates high levels of frictional torque when the patient engages in physical activity. It is believed that the formation of pseudo-tumors is also related to the excessive wear seen at the neck-stem taper junction. Although pseudo-tumors are generally benign, they must still be surgically removed to avoid further problems.
Up until the April, 2012 Urgent Field Safety Notification sent to hospitals and surgeons which warned of potential Stryker ABGII risks, Stryker maintained that adverse reactions to the hip device were “rare,” affecting less than 1% of recipients. This number is a far cry from the actual percentage of ABGII patients who have experienced very real, very serious health issues and must now determine how to deal with their recalled ABGII hip implant.