Stryker ABGII Design Problems
With over 9,000 Stryker ABGII hip implant devices sold prior to the Stryker recall in July, 2012, there are thousands of patients wondering just what is wrong with the Stryker ABGII. The ABGII modular-neck hip implant submitted premarket notification to the FDA in August, 2009, and received approval notice under the controversial 510(k) FDA process in October, 2009. Marketing for the Stryker ABGII began in earnest in early 2010, yet soon thereafter adverse reports began filtering in to the FDA from recipients of the device. There were complaints of inflammation, chronic pain, hip failure and symptoms of metal toxicity from ABGII implant recipients across the United States.
Deficiencies in the 510(k) Process
The 510(k) application for the ABGII submitted by Stryker claimed the ABGII was substantially similar in nature to “other commercially available hip stems in regard to intended use, design materials and operational principles…such as the Stryker Modular Hip System and Rejuvenate Monolithic Hip Size 4 Stem.” Of course the Stryker Rejuvenate gained 510(k) approval in 2008, based on its substantial similarity to the Wright Profemur which has received hundreds of adverse event reports submitted to the FDA. In fact, a 2008 report issued by the Australian National Joint Replacement Registry showed the Wright ProFemur had a revision rate of three times that of comparable hips. The fact is, the 510(k) process requires no clinical trials, only post-market surveillance. This means Stryker was required to keep tabs on how their ABGII implant affected patients after implantation, however problems could only be spotted once they occurred in the implant recipients.
Stryker ABGII Defects
After a significant number of adverse reports regarding the ABGII were received by the FDA, Stryker issued an Urgent Field Safety Notification to surgeons and hospitals in April, 2012 which noted the ABGII had a higher-than-normal failure rate. The notification also stated the ABGII was subject to fretting and corrosion, leading to excess metal ion debris generation. When metal ions shear away from the implant during periods of activity, those ions may either lodge in surrounding hip tissues or travel to the bloodstream. The microscopic metal shards which enter the hip tissue are responsible for inflammation, chronic pain, potential infection and the loss of bone and tissue. When the issue becomes severe enough, total failure of the implant is likely, resulting in the necessity of hip revision surgery to remove the ABGII.
Because the ABGII design implemented a ceramic ball rather than a metal one, and offered an innovative modular design, Stryker claimed the implant would give patients many more years of service than other metal hip implants—from 15-20. Surgeons were allowed to choose from a separate stem, neck, and ball and cup component based on the activity level of the patient as well as the patient’s body type and size. While the highest levels of wear are generally seen at the head-neck junction, in the ABGII, the wear occurs primarily at the neck-stem taper junction. Wear and corrosion are also seen under the metal trundles located on either end of the neck piece. When corrosion is present and the tiny metal ions travel to the bloodstream symptoms of metal toxicity can occur such as:
· Gastrointestinal disorders
· Cardiovascular and renal problems
· Thyroid issues
· Loss of hearing and vision
· DNA disruption
· The formation of pseudo-tumors
· Anxiety, depression and irritability
· Vertigo
· Memory Loss
· Reproductive disorders
· Certain types of cancer
Removal of a Stryker ABGII
Should it become necessary to remove the ABGII due to metallosis or metal toxicity, the neck implant piece must be removed with an ABGII Modular Neck Extractor and may need tapping with a mallet to complete the removal. The ABGII stem will be removed next, using a locking arm which engages the taper, pushing the stem from its position in the femur. Unfortunately, in some cases the femur can shatter during extraction, resulting in even more time added to the recovery time of the revision surgery. While the ABGII design is substantially similar to the Rejuvenate—which was recalled at the same time—the ABGII implements a shallower hydroxyapatite porous coating, meaning the bone does not grow as deeply into the implant. Both implants use a titanium alloy blend for the stem and a cobalt and chromium neck—a design issue which may be responsible for some of the corrosion taking place in the ABGII.
Stryker ABGII Mismatched Components
Mismatched components may be a primary issue in the development of corrosion on the ABGII. Studies done years earlier stated the dangers of mismatched components—in the case of the ABGII, a titanium stem and a cobalt and chromium neck. Despite the use of a hydroxyapatite porous coating which was believed to minimize the negative reactions between the two metals, the problem persisted. In one research study, some 28% of hip implants using similar alloys exhibited moderate to severe corrosion while 42% showed such levels of corrosion among those with mixed metals. Scientist hypothesize that the extra corrosion which emanates from the neck-stem junction are a result of the much harder metal of the cobalt and chromium.
Yet another study looked at hip components removed during revision surgery. While the implants which used mixed alloys showed considerable evidence of corrosion, those using similar metals did not. The electrochemical difference among two dissimilar metals is known as galvanic corrosion; the anode (active metal) is attacked by the cathode (resistant metal). In the case of the ABGII, the resistant metal is the cobalt and chromium while the active metal is the titanium blend. Cobalt and chromium has a hardness factor of 4.5 GPa as compared to 3.0 GPa in titanium. GPa is the abbreviation for a gigapascal; one gigapascal equals 140,000 psi. The cobalt and chromium neck piece is less susceptible to galling and, as compared to the titanium component, somewhat more resistant to fretting and corrosion.
Why Stryker Chose to Use Mismatched Components
The question naturally arises as to why Stryker chose to use mismatched metal components in the ABGII when research had proven it to be potentially risky. Titanium is a more fatigue-resistant metal, and considerably easier—from a manufacturing perspective—to shape into the proper stem design. Further, the cobalt and chromium metals have a higher modulus of elasticity when compared to human bone, even though cobalt and chromium does decrease wear on bearing surfaces. It is likely that as time passes and more Stryker ABGII lawsuits are filed, more information will come to light regarding the design defects of the ABGII.