The Stryker hip recall in 2012 resulted in numerous lawsuits alleging Stryker knew—or should have known—of many of the design defects which have caused harm to those implanted with the Rejuvenate or ABGII. The official reason for the recall was the higher-than-normal risk of corrosion and fretting at the neck juncture of the implant, however since that time many more issues have come to light. The four small metal trunnions located at either end of the neck portion of the Rejuvenate were found to suffer corrosion when body fluids become trapped underneath. Further, higher levels of cobalt were found to be coming from the Morse taper which is a slightly angular junction between the stem and the neck.
Problems with the Morse Taper
A Morse taper is implanted through a physician hammering the taper into the stem until snug, then hammering again to lock it tightly in place. It is believed that the area where the Morse taper meets the stem is responsible for cobalt ions being released, causing cross ionization between the titanium stem and the cobalt/chromium neck. These ions can cause damage and even death to the bone, tissue and muscle surrounding the hip joint. Although the Morse taper was meant to provide more “give,” adapting to various patient sizes, it has instead caused added stress and fraying, sending more cobalt debris into the body. The Morse taper should have had no wear and no ion release as it is not considered a “wear” joint like the cup and ball however it has been found to have significant wear.
How Dissimilar Metals May Cause Corrosion
Trunnion corrosion which occurs between dissimilar metals is another issue associated with the recalled Stryker Rejuvenate and ABGII. A 2002 report found significantly higher levels of corrosion among implants which were constructed of mixed metals as compared to those constructed of the same metal. The titanium alloy vs. the cobalt and chromium alloy have different mechanical properties. The cobalt and chromium alloy is considerably harder therefore it causes higher levels of wear when paired with titanium components. Research done in 1992 studied the taper interface between the head and neck of 139 modular femoral components removed from patients.
Those implants which implemented cobalt/chromium in both the head and neck were found to show little evidence of corrosion while those implants which were made up of a titanium neck and a cobalt/chromium head showed significant levels of corrosion. Keep in mind, this study was done over 20 years ago, yet despite the results Stryker still chose to pair a titanium stem with a cobalt/chromium neck. Mixing alloys leads to excessive taper wear and the release of metal ions into the body. Those ions can cause a number of serious medical issues including cardiovascular, gastrointestinal, renal, thyroid and neurological disorders, loss of hearing and vision, vertigo, depression, anxiety, irritability, the formation of pseudo-tumors, DNA disruption and skin disorders. Experts in the area of the Stryker recalled implants also note significantly higher cobalt levels when the diameter of the bearing is increased.
Excessive Wear between the Stryker Rejuvenate Modular Neck and Stem
A 2012 wear analysis done on a modular neck showed excessive wear where the modular neck meets the stem, right at the top of the notch area. This neck corrosion is responsible for metal ion release as well as the formation of pseudo-tumors and the stress additionally created fracture and abrasion of the oxide-film which covers the cobalt and chromium surface. While this specific type of corrosion is more common in full-up front tapers, it has also been reported in titanium alloy stems.
Because the corrosion is occurring at the neck-stem taper of the Rejuvenate and ABGII, should a surgeon choose to only remove and replace the modular neck, the problem will likely continue to escalate. Stryker hip recall lawsuits have been escalating since the recall last July and it is expected they will continue to increase as more recipients of the implants begin experiencing metal toxicity or the destruction of bone, tissue and muscle, forcing them to undergo revision surgery.