Hip Metallosis and Corrosion—A Million Harmed Due to FDA Inaction

On February 18, 2016, FDA rule 78FR4094 became “effective,” which requires the arthroprosthetic industry to file a premarket approval (PMA) application for all metal-on-metal hip replacements (MoMHR), which are currently on the market. These implants had bypassed comprehensive testing required by PMA and were expedited to market a decade ago using the FDA’S premarket notification process (510(k)), a process which only requires an implant to have “substantial equivalence” to a current or previously approved medical device, regardless of whether the antecedent device underwent premarket trials. The new rule is too little too late. Industry has already removed the MoMHR from the market through formal and silent recalls by 2014. It has no incentive to fund the now required clinical trials, which were avoided by the 510(k) approval process because it is known, from unfortunate clinical experience, to be largely a failed technology. Rule 78FR4094 is emblematic of dysfunctional FDA medical device approval processes. This rule does not obligate industry tomonitor patients already implanted with nonrecalledMoMHR for adverse reactions to metallic (chrome-cobalt) debris (ARMD). Periprosthetic ARMD results in inflammation, with severity ranging from minor pain to irreparable tissue loss and hip tissue necrosis. Systemic ARMD consists of multiple organ cobalt poisoning (cobaltism) most commonly manifesting as encephalopathy or cardiomyopathy, with morbidity ranging from moodiness and cognitive decline to death from heart failure. The MoMHR (Fig. 1) uses a chrome-cobalt femoral ball articulating on a chrome-cobalt acetabular socket. Its predicate device used a metal-on-metal articulation that was voluntarily withdrawn from the market in the 1980s. A million of the reintroducedMoMHRwere implanted in Americans between 2003 and 2013. By 2010, it became apparent that some of these newer MoMHR had rates of failure that were 10 to 20 times greater than that of metal-on-plastic hip replacement (MoPHR) models from the 1970s, like the Charnley low-friction arthroplasty. One particularly poorly performing MoMHR, the DePuy ASR, was formally recalled in 2010. The Australian Total Joint Registry (ATJR) rather than the FDA or the manufacturer, Johnson & Johnson, was the first to sound the alarm regarding the high failure rate of the ASR. The ATJR later found that the high failure rate generalized to all MoMHR. Unfortunately, problems with 510(k) extend well beyond the approval of the MoMHR. The original MoPHR has been modified and now uses a modular chrome-cobalt head that articulates on a plastic socket. The Stryker Rejuvenate MoPHR was designed with a second junction, allowing the chromecobalt neck to be a separate component. This was thought to be advantageous, allowing the surgeon additional intraoperative flexibility to maximize the stability of the arthroplasty. The taper junctions between chrome-cobalt components, such as those involved in the Rejuvenate MoPHR, are prone to mechanically assisted crevice corrosion (MACC). Chrome-cobalt metallosis produced by corrosion seems to be an order of magnitude more toxic than the metal debris generated by the wear of chrome-cobalt articular surfaces that plagued MoMHR. The Rejuvenate MoPHR became the “canary in the mine” for MACC. Some Rejuvenate implanted patients developed ARMD within months because the supernumerary distal neck taper is particularly prone to corrosion. The design change that birthed the Rejuvenate hip was deemed substantially equivalent by the FDA and was approved without testing. Subsequently, MACC has been identified at the proximal headneck taper used in most of the 4 million MoPHR, which have been implanted in the United States over the past 20 years (Fig. 2). The more rigorous PMA is not a guarantee of safe medical devices, despite favorable premarket clinical studies. Trials may have inadequate follow-up periods or be insufficiently controlled or powered to ensure that a new device is as safe as a market standard. Several brands of metal-on-metal hip surface replacements (MoMHSR) have PMA status and yet are failing at 10 to 20 times the annual rate of the Charnley MoPHR.