What Type of Stryker Rejuvenate Design Problems Were Present?

Stryker Orthopedics, a Michigan-based corporation, recalled the Rejuvenate, a metal hip implant device, in July, 2012. The recall came on the heels of an Urgent Field Safety Notification sent to hospitals and surgeons in April. Stryker stated a risk of fretting and corrosion in the Rejuvenate which could potentially lead to excess metal debris being released into the body. With over 9,000 Rejuvenate hip devices implanted in consumers, those risks may have serious, far-reaching repercussions.  The Rejuvenate garnered FDA approval in 2008, only a few short years prior to the recall.

Was the FDA Aware of Stryker Rejuvenate Design Problems?

The process under which the Rejuvenate was approved has come under fire over the past few years: the 510(k) process allows medical devices to be approved based on their similarity to a device already on the market. In the case of the Rejuvenate, approval was gained based on the Wright Profemur Total Hip System which was, itself, based on yet another hip implant device. The fact that these hip implants may also have had problems of their own apparently has little bearing on new approvals. The Rejuvenate and the Profemur do share one design similarity: they both use interchangeable titanium neck components. The 510(k) process also allows manufacturers to circumvent the normal requirements of clinical trials as well as skipping important safety testing.

Stryker Rejuvenate Defects

While the majority of the focus has been on all-metal hip implants, and the Rejuvenate implements a ceramic ball, it appears the same problems exist in the Rejuvenate as with the all-metal implants. In fact, the Stryker Rejuvenate may have the same level of risk of metallosis and metal toxicity as the recalled metal-on-metal hip implants. While Stryker has declined to put a number on the failure rate of the recalled Rejuvenate, independent studies place that number as high as 40%, despite the use of a ceramic ball.

Metal toxicity can occur when the metal components of the hip device rub against one another during normal physical activity. In the case of the Rejuvenate, the titanium stem rubs against the cobalt and chromium neck piece at the junction between the two, causing small metal ions to shear away and enter the body. Further, the Rejuvenate implements tiny metal trunnions located on either end of the neck piece which can also cause corrosion when body fluids become trapped underneath.

What is Wrong with the Stryker Rejuvenate?

Symptoms of metal toxicity include gastrointestinal disorders, cardiovascular and neurological disorders, renal and thyroid problems, loss of hearing and vision, disruption of DNA, the development of pseudo-tumors, the increase in certain types of cancer, vertigo, memory loss, depression, anxiety and irritability. When the tiny metal ions enter the surrounding hip tissue, infection, inflammation, chronic pain and the deterioration of bone and tissue can occur, leading to total failure of the hip implant. Because the Rejuvenate stem component is placed so deeply within the femur bone, should hip revision become necessary, removing the stem can be difficult, dangerous and costly.

What About Stryker Rejuvenate Mismatched Components?

Among the many problems associated with the Rejuvenate, it appears that mismatched components may be a major issue. Despite studies done years before, stating the dangers of mismatched components, Stryker chose to pair a titanium stem with a cobalt and chromium neck.

Hoping to alleviate some of the known risks, Stryker used a hydroxyapatite porous coating—which they expected to minimize the negative interaction between the dissimilar metals—however the coating failed to deliver. One study in particular noted that while moderate to severe corrosion was seen in approximately 28% of similar alloy hip implant components, that number rose to over 42% when mixed alloys were used. Scientists are not certain what causes the metal ion shear in mismatched components, but believe it is related to the fact that cobalt and chromium are considerably harder than titanium.

In looking at hip components removed during revision surgery, those implants which did not mix metals showed no evidence of corrosion, while the implants which used mismatched metal components showed significant levels of corrosion. Galvanic corrosion is the term used to describe the electrochemical differences between two dissimilar metals; in scientific terms the anode, or active metal is attacked by the cathode, or more resistant metal. Some metals will form a stable passivation field once they are implanted in the body, however research has shown that this field is only present and protective when there is no motion and no wear of the implant. The cobalt chromium alloy has a hardness factor of 4.5 GPa (Gigapascal—1 GPa = 140,000 psi) while the titanium alloy has a hardness factor of 3.0 GPa, meaning the cobalt and chromium component is less susceptible to galling, and, overall, more resistant to fretting and corrosion than the titanium component. 

Were Stryker Rejuvenate Defects Known Once the Devices Began Being Implanted?

The question many consumers have is whether Stryker was aware of the problems associated with the Rejuvenate long before the recall? Why did the manufacturer choose to use mismatched components in the Rejuvenate despite scientific evidence proving there were problems associated with mixed alloys? Perhaps one reason Stryker used a titanium stem component due to the higher modulus of elasticity as compared to human bone in the cobalt and chromium components.

The titanium stem was also found to be more fatigue-resistant and easier for the manufacturer to shape into the proper design. Finally, the titanium components allowed for more bone ingrowth—thought to be a positive aspect which would allow the hip implant to gain stability. Unfortunately, should the Rejuvenate require removal, it can be extremely difficult due to this bone ingrowth. As the number of lawsuits against Stryker continues to grow, the question of whether Stryker Rejuvenate defects were known prior to marketing is likely to surface.

Did Frictional Torque Cause Stryker Rejuvenate Fretting?

The Stryker Rejuvenate hipimplant was approved by the FDA in 2008, yet only a few short years later, the manufacturer recalled the device, citing a higher-than-normal rate of failure and fretting and corrosion leading to metal ion debris. Those who were implanted with a recalled Rejuvenate are likely wondering just what is wrong with the Stryker Rejuvenate. These implant recipients are victims of a system which did not require clinical trials or extensive safety testing prior to marketing the hip device. Further, when the recall was issued in July, 2012, many recipients may not have even been notified of the recall, or, if they were notified, may not have fully understood what fretting and corrosion meant for their health.

Stryker Rejuvenate Defects and Design

The Rejuvenate was believed to be a truly innovative design following several dismal results from all-metal hip implants implementing a metal ball. The Rejuvenate used a ceramic ball, and offered a variety of lengths and angles in the stem and neck component pieces. This allowed surgeons to fit the implant device more precisely to the individual patient, according to the patient’s body type and size and level of activity. Because frictional torque had been a problem in the all-metal implants, primarily at head-neck junction, it was believed that the ceramic ball used in the Rejuvenate solved that problem. Unfortunately, this belief turned out not to be true.

The Rejuvenate began showing considerable wear at the neck-stem taper junction, causing metal ions to shear away and flood the body. Additionally, four small metal trunnions, located on either end of the neck piece were also sites where corrosion was present, due to body fluids being trapped beneath the trundles. Many of those implanted with a Rejuvenate began having serious health issues soon after, and the FDA received a number of adverse reports from victims of the device. Surgeons also sent information to the FDA regarding implants they had removed from their patients which appeared to have some sort of “black rust” on them. As more and more patients relayed health problems to their doctors and surgeons, and more surgeons noted health issues among their Rejuvenate patients, questions began being asked regarding the safety of the implant.

The FDA and the Rejuvenate

Although the Rejuvenate was approved by the FDA, that approval process fell under the 510(k) program, which has increasingly come under scrutiny. This particular program allows medical devices to gain approval based on their similarity to a device already on the market. In the case of the Rejuvenate, the titanium stem is really the only similarity between the implant the Rejuvenate claimed to be substantially similar to—the Wright Profemur.  As with many other devices approved under this process, the Wright Profemur has had problems of its own, and the device the Profemur was considered to be substantially similar to, also experienced problems.

Was Stryker Aware of Safety Issues Related to the Rejuvenate?

Recipients of the Rejuvenate may wonder whether the company knew there were Stryker Rejuvenate design problems prior to its release, and whether consumer safety was sacrificed in order to put more profit into Stryker’s coffers.  Although those questions may never be fully answered, many believe Stryker definitely downplayed the potential risks of the Rejuvenate up until they had little choice but to issue a recall. The specific health issues seen in those with a Rejuvenate hip implant were metallosis and metal toxicity. When the metal ions which shear away from the implant lodge in surrounding tissues the following can occur:

·         Infection

·         Severe inflammation

·         Chronic and severe pain

·         Bone deterioration and loss

·         Tissue deterioration and death

When the metal ions find their way into the patient’s bloodstream, metal toxicity, including one or more of the following symptoms can occur:

·         Gastrointestinal disorders

·         Alterations in DNA

·         The development of pseudo-tumors

·         Neurological and cardiovascular disorders

·         Irritability, depression and anxiety

·         Vertigo

·         Memory loss

·         Chronic and severe headaches

·         Reproductive disorders

·         Loss of hearing and vision

·         The development of certain types of cancer

Although Stryker stated fretting and corrosion at the time of the recall, there was little information of the causes of that fretting and corrosion. Among the design defect theories of the Stryker Rejuvenate, mismatched components, the powder coating and frictional torque are considered likely suspects. Mismatched components relates to the titanium stem Stryker paired with the much-harder cobalt and chromium stem. Scientists believe the harder cobalt and chromium reacts negatively with the titanium, causing an excess amount of metal ions.

Stryker Rejuvenate Component Friction

Because frictional torque had previously been seen primarily in the head-neck taper portion of metal implants, it was not expected in the neck-stem taper of the Rejuvenate. The taper portion of the components allows them to lock together securely, and when implanting a Rejuvenate, the Morse taper of the neck is pounded into the stem (also responsible for the release of metal ions). Scientists believe the lever-arm effect of the Stryker neck taper is responsible for increased movement at the neck-stem junction and this particular junction is considered to be eccentrically loaded. 

In other hip implants which are constructed of one tight-cast piece, there is little to no movement at the neck-stem area, however the Stryker Rejuvenate allows for additional movement, creating frictional torque where the two components come together. Among Rejuvenate recipients who are particularly active, the frictional torque is much higher, leading to excess wear on the joint. The formation of pseudo-tumors may also be related to the excessive wear at the neck-stem taper junction. Once pseudo-tumors form, they must be surgically removed, even though they are not usually cancerous. It is likely that as time goes on and more lawsuits are filed, more facts will come out regarding the safety issues surrounding the recalled Stryker Rejuvenate.