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How Ions May Cause Metal-on-Metal Hip Implants to Go Bad

Home › MPMN › Medtech Pulse Blog › How Ions May Cause Metal-on-Metal Hip Implants to Go Bad

From the recalls last of the DePuy Orthopaedics ASR XL acetabular system and the ASR hip resurfacing system to FDA‘s recent order that all metal-on-metal hip implant manufacturers conduct postmarket surveillance studies, the flood of news stories detailing problems with metal-on-metal hip implants seems unstoppable. And it’s affecting Not just DePuy, but Stryker Corp., Zimmer Holdings Inc., and Biomet Inc. as well. Just last week, the Qmed Daily—echoing an earlier report in the New York Times—reported that the National Joint Registry for England and Wales has observed that hip failure rates are skyrocketing,

Now, the New York Times is at it again. In the October 1 issue, Barry Meier reports that the numbers of complaints associated with metal-on-metal implants is increasing. In one case, for example, the area around a failed artificial hip in a 53-year-old patient resembled a ‘biological dead zone’ consisting of matted strands of tissue stained gray and black, while a large segment of muscle near the hip no long contracted. Similar scenes, the reporter notes, are occurring at hospital around the country as increasing numbers of patients are attempting to have their metal-on-metal hip implants removed and replaced.

What’s noteworthy about this article is that it highlights a biological process that results from the interaction between cells in the body and metal debris cast off by hip implants. While all orthopedic implants create particles as they wear, researchers believe that metal-on-metal hip replacements are more dangerous because scavenger cells, in response to the presence of foreign bodies, devour the metal debris and convert it into biologically active metallic ions. In some patients, this process can result in a heightened immune-system response that can lead to early device failure and tissue damage.

While only a small proportion of the approximately 500,000 patients that have received metal-on-metal hip implants over the last decade have suffered from injuries, studies indicate that those rates will increase over time. For example, take the case of Young-Min Kwon, an orthopedic surgeon at Massachusetts General Hospital. In the last year, Kwon’s caseload of patients experiencing problems with their metal-on-metal hip implants has tripled. And in the first six months of this year, FDA received more than 5000 reports about problems associated with metal-on-metal hip replacements.

The situation is complicated by the fact that different patients react differently to the presence of metallic debris, according to the New York Times article. While some patients with high levels of metallic debris are suffering from tissue damage, other patients seem to be healthy. On the other hand, some patients with normal or low metallic blood levels and even some symptom-free patients are experiencing tissue damage.

The growing spate of injuries suffered by hip-replacement patients should be seen as a call to arms by materials suppliers, working in conjunction with the OEMs, to invest time and money to find suitable replacements for metal-on-metal implants. For example, while PEEK fell into disfavor in the 1990s for use in total joint arthroplasty procedures, it seems to be experiencing a resurgence as a potentially compatible material for use in hip-replacement surgeries. But perhaps other materials, material combinations, or surface-preparation techniques may create better implants in the future, including novel metals, new polymer formulations, ceramics, or hybrid products.

For more articles and blogs on hip implants, see “What’s Hip in Orthopedic Implant Design” and “Lesson Learned from the DePuy Recall: It’s All Material.” —Bob Michaels

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