January 16, 2015 - Attention Stryker Rejuvenate and Stryker ABGII Hip Implant Recipients - In addition to reading about the specific issues associated with these hip implants, please take time to inform yourself about the recent Stryker Hip Settlement Agreement. You may be entitled to compensation.
Dual Modularity
Prior to the Rejuvenate and ABGII hip implant devices, most hip implants were constructed from a design known as single modularity, meaning there was one junction between the ball and the neck. One piece was inserted into the natural socket of the hip, while the other piece was inserted into the femur bone. Survival rates and long-term results were good with such a single modularity device. Stryker, realizing that one size really did not fit all, developed the dual modularity hip device. Because surgeons were hindered by the fact that when the socket was cemented it, serious problems could result if the positioning was not just right. With the Rejuvenate and ABGII, the surgeon could insert the metal shell into the natural hip socket while retaining the ability to adjust the inside diameter of the bearing and the alignment.
The ball length could be changed to create adjustments to the tension in the soft tissue as well as the leg length once the component was placed in the femur. For those patients with hip regularities, the dual modularity of the Rejuvenate and ABGII offered real advantages. While patients liked the idea of a custom fitted implant, as well as the fact that the Stryker ball was not made out of metal, the manufacturer had not counted on the possibility of neck-body disassociation as well as the level of metal ions released by the dual-modularity design. Dual modularity designs are also responsible for an increase in crevice corrosion as well as galvanic corrosion.
Coating on the Stryker Rejuvenate and ABGII
There are two types of coatings associated with the Stryker Rejuvenate and ABGII. The first is a hydroxyapatite porous coating, which is meant to encourage bone ingrowth. The coating on the Stryker ABGII is much shallower and smoother than that of the Rejuvenate, which is rougher and deeper. This coating may release into the bone tissue, putting Stryker Rejuvenate and ABGII patients at risk for osteolysis, or weakening of the bone. Osteolysis can lead to pain, loss of range-of-motion, and other health risks. Some patients notice a popping or cracking noise coming from their Stryker hip implant. Later they found that the bone surrounding the implant had suffered significant deterioration, possibly as a result of the HA coating. This coating also makes it much more difficult to remove the implant during revision surgery, particularly the Rejuvenate.
The second coating is known as TMZF and is a proprietary blend of titanium, molybdenum, zirconium and iron, which was sprayed on the implant, ostensibly to avoid the problem of mismatched metals. Studies done over a decade ago, as well as some more recent ones caution against pairing dissimilar metals as excess corrosion and the release of cobalt and chromium ions can result. The taper junction of the Rejuvenate and ABGII stem and the cobalt and chromium neck of the implants make them particularly prone to corrosion. The TMZF spray was actually a selling point for the Rejuvenate and ABGII, as it was meant to prevent this corrosion. Unfortunately, the coating failed to deliver on those promises as it did not prevent the problem of the mismatched metals. Although the TMZF spray probably did not contribute to the problems associated with the Stryker recall, neither did it avert those problems.
Are Dissimilar Metals a Problem With the Recalled Stryker Hip Implants?
The high levels of corrosion seen in the Stryker ABGII and the Stryker Rejuvenate may be attributed to the use of mismatched components. One study showed that 28% of hip implants which used similar alloys exhibited moderate to severe corrosion while 42% showed the same levels of corrosion when mixed metals were used. The theory is that the much harder cobalt and chromium metals paired with the softer titanium stem result in extra corrosion. Among hip implants removed during revision surgery, those with mixed alloys showed a much higher level of corrosion than those which used similar metals. It is unclear why Stryker chose to use mismatched metals in the Rejuvenate and ABGII, particularly in light of the research and studies warning against using dissimilar metals.
Frictional Torque on the Stryker Rejuvenate and ABGII
The stress point on most hip implants is on the ball and neck joint; a considerable amount of frictional torque is placed on that joint. The design of the Rejuvenate and ABGII allow surgeons to more closely fit the implant to the patient’s size and activity level, therefore frictional torque was not really considered a serious problem in these implants. At the time the hip device is implanted, the taper junction is hammered into the stem. It could be that this action releases the cobalt and chromium ions into the body. Researchers were very surprised to find that the frictional torque of the Rejuvenate and ABGII was occurring at the neck-stem junction, possible due to the “lever arm effect” of the taper, which is eccentrically loaded. In other words, the modular neck fits into the stem of the hip device, allowing movement and excessive frictional torque, which, in turn, causes wear on the hip joint, particularly among those who are very active.