The modular components of the Stryker Rejuvenate consist of various head dimensions and modular neck orientations. These attach through a Morse taper, allowing for variability in leg length and offset. The Morse taper is a slightly angular junction on either end of the hip implant components which the surgeon taps into the stem until it is locked tightly. The benefit of using a Morse taper is the surgeon’s ability to couple various sizes and materials, creating a customized fit for each patient. Stryker apparently did not want to pay Wright Medical to use the widely accepted Morse taper, so they designed their own version of a modified Morse taper. This modified Morse taper has much more surface contact area between this taper and the stem, and is simply insufficient to prevent micro erosion at a junction which must support seven times the patient’s body weight. Therefore, at the point the modified Morse taper meets the stem, cobalt and chromium ions are released. In turn cross ionization between the cobalt and chromium neck and the titanium stem can occur.
For reasons not yet clearly understood, it appears the neck portion of the Stryker Rejuvenate releases more cobalt and chromium ions than the stem. Modified Morse taper use has led to concern over fretting and corrosion between components which can lead to complications such as necrosis, inflammation, aseptic loosening and osteolysis from this added stress location. Several studies have concluded that these modified Morse tapers can lead to weakening of the entire construction of the implant—variables such as a totally pristine taper, a taper covered with blood and fat or a taper with the blood and fat wiped away determine the final outcome. Other potential variables include impaction force, the number of assembly-disassembly cycles and wet/dry conditions.
Why Stryker’s Innovative Design Likely Failed
Stryker’s Rejuvenate design was considered to be extremely innovative as well as a safe alternative to the all-metal implants such as the DePuy ASR. The variety of components was believed to offer a perfect fit for each recipient, and replacing the metal ball with a ceramic ball was believed to put an end to the corrosion and subsequent release of metal ions. Unfortunately, the high hopes for the Rejuvenate and ABGII fell flat. The metal neck juncture of the Rejuvenate and ABGII suffered corrosion and fretting, leading to metal debris being released into the body and eventual failure of the implant in some patients. Small metal trunnions located on either end of the neck portion were also found to experience corrosion and cobalt and chromium ion release as well. Further, the majority of the wear of the Rejuvenate and ABGII comes from the stem and neck rather than the cup and ball. Unfortunately, these are surfaces which were not meant to undergo this level of wear.
Some twenty years ago researchers studied taper interface in hip components removed from patients. The components constructed of the same metal alloys showed much less evidence of corrosion, while the mixed metal components showed much greater levels of corrosion. Mixing metals as well as the diameter of the bearing appears to compound taper wear, contributing to even more cobalt and chromium ions being released into the body. While it is unclear why Stryker modified the Morse taper of the Rejuvenate, what is clear are the negative health effects for recipients of the recalled device. The recall for the ABGII and Rejuvenate was issued in July, 2012; those who have experienced adverse health effects from these recalled hip implants should speak to their physician and have regular blood tests done to monitor levels of cobalt and chromium.