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Hip implant materials swing sharply to polyethylene | PlasticsToday.com.

Hip implant materials swing sharply to polyethylene

By Doug Smock
Published: July 13th, 2012

A two-day hearing held at the end of June in Washington, D.C. put an unusual, very public focus on the types of materials used in medical implants, specifically hip replacement and resurfacing systems.

On the docket were a Who’s Who of medical device manufacturers and international medical professional associations. Precipitating the hearings by the Food and Drug Administration’s Orthopaedic and Rehabilitation Devices panel was a rising tide of failures of metal-on-metal hip implants.

There have been two metal-on-metal (MoM) total hip replacement device recalls in the United States: the Zimmer Durom Total Hip System (2008) and the DePuy ASR Total Hip System (2010).

“Wear of articulating components may lead to the production and accumulation of metal ions and debris,” said Elizabeth Frank, biomedical engineer with the FDA. She said the purpose of the hearings was to:

  • Review currently available data regarding MoM hip systems,
  • Create open and transparent dialogue of the issues,
  • Characterize any potential and real safety risks, and
  • Generate scientifically based recommendations for the clinical and patient communities on how to best communicate and mitigate risks

Dr. Paul Manner, associate professor in the Department of Orthopaedics and Sports Medicine at the University of Washington (Seattle, WA) said there was a push for new materials after a report in 2001 identified ultra-high molecular weight polyethylene (UHMWPE) wear debris generated at the articulating surface as a principal culprit in hip failures. Engineers took two approaches to solve the problem: decrease opportunities for wear and change materials. “Hard-on-hard” bearings looked like an attractive alternative. New MoM Systems were approved for use in the United States in 2005 and 2006.

Other materials’ technologies including improved polyethylene were also developed in an effort to improve outcomes for total hip arthroplasty (THA) and hip resurfacing arthroplasty (HRA). These included:

  • Metal on Polyethylene (MoP),
  • Ceramic on Polyethylene (CoP),
  • Ceramic on Metal (CoM), and
  • Ceramic on Ceramic (CoC).

Dr. Manner noted, however, that: “Data comparing metal metal-on on-metal THA/HRA to other bearing surfaces are sparse and drawing conclusions is difficult.”

Data from a survey conducted this year by the American Association of Hip and Knee Surgeons, however, shows a massive move to bearing combinations using polyethylene. Metal on polyethylene systems were used for 85% of operations; ceramic on polyethylene for 14%, and metal on metal for 1%.

Meanwhile, improvements are being made in metal systems, and in some cases their use may be preferred, according to testimony in the hearing.

“Metal-on-metal implants allow for larger femoral heads to be used, which provide greater stability and lower risk of dislocation,” said Paul Voorhorst, director of biostatistics & data management at DePuy Orthopaedics.

Device manufacturers also pointed out there are differences in various metal-on-metal systems, ranging from metals used to processing methods that affect metallurgy.

One questions that still lingered at the end of the hearing:

What steps can be taken to improve predictability of device effectiveness prior to approval? And this comes amidst growing pressure to speed approvals.

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