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All-Metal Hip Implants: The End of an Era or Primed for a Comeback?

Oh, how the mighty have fallen. Horror stories about osteolysis, a widely publicized recall, a litany of lawsuits, and a full-scale media assault during the past few years have all contributed to causing what some might classify as irreparable damage to the reputation and public image of metal-on-metal (MoM) hip implants. But do several new research projects stand a chance at helping MoM implants overcome their safety stigma and restoring these all-metal hip implants to their former glory? Or are they futile efforts to save a sinking ship?

hip implantResearchers at the University of Alabama at Birmingham (UAB), for example, found that applications of nanodiamond coatings to metal implants could enhance wear characteristics, potentially resulting in safer implants. Although the grinding motion of the ball-and-socket joint tends to produce some wear particles regardless of the bearing materials, metal-on-metal hip implants have drawn fire for their increased association with osteolysis, increased inflammation in the tissue surround the artificial joint, and increased need for early revision surgery as a result of implant loosening and failure compared with other material combinations. Problems such as these typically arise when the wear debris triggers activation of macrophages, which are cells that absorb the wear particles and then secrete chemicals that lead to inflammation. In turn, this series of events prompts a reaction that results in resorption of bone tissue and subsequent bone loss.

The UAB researchers, however, believe that the use of nanodiamond coatings on MoM implants could prevent the production of these pesky metal particulates. But, of course, there's a catch: Nanodiamond coatings would likely be subjected to the brunt of the grinding and, thus, shed wear debris instead of the metal alloys. The good news is that the researchers believe that nanodiamonds  shed less debris and yield smaller particles than is common with other materials; they also appear to be nontoxic in living cells.

During the study, the researchers observed the interaction between nanodiamonds and macrophages in a dish, deducing that the macrophages released fewer inflammatory chemicals when they absorbed the nanodiamonds than has been seen when they engulf metal or polyethylene particles. "Past studies on diamond-joint surfaces have shown a marked reduction in wear-debris volume compared to first-generation alloy and polyethylene joint parts, but the work continues to ensure they are safe," says Yogesh Vohra, director of the UAB Center for Nanoscale Materials and Biointegration. "We hope the reduced wear volume and particle size expected for diamond articulation will represent a major advance over conventional orthopedic bearings."

Also evaluating metal-on-metal hip implants is a multiuniversity research team that has discovered that a carbonaceous, graphite-containing film forms on cobalt-chromium (CoCr) implants over time. These so-called tribofilms, according to the researchers, appear to originate in the synovial fluid surrounding the hip implant and act as industrial lubricants. The success of the CoCr implant, they say, is tied to this film. By gaining a better understanding of this film, the researchers believe that better alloys prone to less particle shedding could be developed.

"Today, cobalt-chromium with molybdenum is used in metal-on-metal implants," Markus Wimmer, associate professor of orthopedics at Rush University Medical Center, told MPMN in a recent interview. "But other metals such as newly developed high-nitrogen stainless steels may have even better properties."

While these research studies are intriguing and promising, are they enough to help metal-on-metal hip implants eventually regain favor? It's tough to tell. MoM hip implants took a brutal, public beating during the past few years. And there are too many critics, lawsuits, and unfavorable research studies to simply ignore. Plus, when even patients are informed and wary of a medical device, well, that's really difficult to bounce back from.

But it might not be a completely lost cause. After all, I made a prediction late last year that the final days of MoM hip implants were drawing nigh. However, I am going to amend that prediction: MoM implants may fall out of favor during the next few years; but, once the dust (and lawsuits) settle from the controversy, they could be primed for a comeback. That said, they only really have a chance at a comeback if new materials and designs, along with some seriously solid marketing and research studies, are employed and strongly emphasized. Trust isn't always easy to earn once it has been violated. But the media, researchers, physicians, and patients may be forgiving--or forgetful of past transgressions--if MoM implants can come back several years down the line and truly show that they are new and improved. Let us know what you think about the future of MoM hip implants in our poll below. --Shana Leonard

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