Hip devices containing cobalt-chromium parts have been used successfully in orthopedics for years and have been known to produce slightly elevated metal levels in patients who receive them1. Over the past decade, however, the profit-driven flurry of new metal-on-metal hip devices has left thousands of patients with much higher levels than previously observed.
Specifically, the inherent design of metal-on-metal devices often results in metal wear, which releases metal ions into the body and causes elevated metal levels. An unacceptable proportion of metal-on-metal hip recipients have needed revision surgery due to implant loosening, tissue necrosis, and pseudotumors – all of which have been attributed to elevated metal levels.
Since the DePuy ASR recall in August 2010, surgeons have been moving away from metal-on-metal implants. Many device makers have taken their metal-on-metal implants off of the market, either by issuing a recall or by “phasing them out.” It is apparent that the metal-on-metal design is causing high failure rates.
However, in relation to how widespread a problem this is, experts have come up with few definitive answers as to how interpret elevated metal levels found in patients.
Why Don’t We Know More by Now?
Most metal-on-metal devices gained FDA approval through a regulatory shortcut known as the 510(k) approval process. This process requires the manufacturer to prove that the device is “substantially equivalent” to another that is already on the market. They are not required to conduct pre-market studies to prove safety and efficacy.
Since the manufacturers of MoM devices did not study the effects of metal wear, people have likened the advent of metal-on-metal hips to a large, uncontrolled experiment. Thus, the studies that are now underway are reactive in nature and have minimal foundation to build from.
As Nick Freemantle, professor of clinical epidemiology and biostatistics at University College London, pointed out, the studies should have been done before the product is used – not after.
“We shouldn’t be in this position where we don’t know and there’s so much uncertainty. The stability of a compound should have been ascertained before it was used widely in people. As yet, we don’t know the consequences of this.”
What do we know about elevated metal levels?
According to the Medicines and Healthcare products Regulatory Agency (MHRA), patients whose levels are above a threshold level of 7 μg/L should be further monitored and tested. However, it is unclear where they got this number, or if it is reliable.
Earlier this year, BMJ published a study that examined the association between elevated metal ion concentrations and early failure of hip devices. They did so by documenting the clinical course of patients who had elevated cobalt and chromium concentrations but were not experiencing symptoms of a failed device.
The results of the study provided the first real evidence that blood metal ion tests can be used to measure the risk of early joint failure. They also suggested that elevated metal levels, regardless of symptoms, warrant concern.
The study also noted differences in failure rates between men and women. Of men and women who had the same metal ion levels, women appeared to be at greater risk of joint failure than their male counterparts. This is not the first time that higher failure rates have been observed in female patients. Previously, however, the theory was that women were more prone to elevated metal levels due to heavier wear of smaller implants.
However, since the failure rates were still higher in women than men who had the same metal levels, the differences must also have a biological explanation. That is, females may be more likely to have an adverse immune response to the ions. Indeed, the prevalence of a number of immune conditions is higher in women, which, according to the study, further suggests that woman may be more likely to have problems than men.
An important conclusion to the BMJ study is that metal ion concentration levels are a useful clinical tool in predicting device failure. The data presented in the study will help doctors to develop standards and guidelines for future treatment and evaluation of metal-on-metal patients. The data also strengthens the link between the metal-on-metal design of a device and device failure, which manufacturers have contested by pointing to other potential reasons for failure.
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