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.