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More second-generation highly crosslinked polyethylene options coming
Biomet and Stryker bring new polyethylene products into a competative marketplace.
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A few orthopedic manufacturers recently went back to basics and developed what they are calling a new generation of polyethylenes for total hip and knee replacements.
Developers at Stryker and Biomet say their second-generation, highly crosslinked polyethylene materials improve on the wear resistance of the first generation of highly crosslinked ultra-high-molecular-weight (UHMW) polyethylenes introduced a decade ago, many of which were irradiated and melted.
In the clinical setting, the new materials may offer other needed benefits, including hip liners that pair with larger, more anatomical femoral heads.
Stryker introduced its solution — X3 bearing technology — in late 2005. Some other companies are reportedly developing new polyethylenes, but they did not supply information on their efforts following requests from Orthopedics Today for this article.
“Most people will be looking at some kind of second-generation [product] to improve upon issues with first-generation material, whether it was remelted or annealed,” said John M. Susaraba, Biomet vice president of product management and Orthopedics Today industry council member.
Vitamin E doping
Biomet, which introduced its Arcom XL highly crosslinked polyethylene in 2005, recently announced another new polyethylene offering: the UHMW E-Poly material, which is stabilized with vitamin E, a natural antioxidant. The material is expected to reduce oxidation in vivo, according to Dave Schroeder, Biomet biomaterials research, bearing technologies manager.
“The E-Poly was designed to give us the maximum amount of wear resistance in a polyethylene bearing that would be able to be used for both hips and knees,” Schroeder told Orthopedics Today.
Ringloc-brand acetabular cup liners made of E-Poly are expected to launch early this summer. The 28-, 32- and 36-mm sizes are expected first, followed shortly by the 38- and 40-mm components, Susaraba said.
Biomet’s goals in developing the E-Poly: to achieve maximum wear resistance with a polyethylene bearing to complete its offering of bearing surfaces, according to Schroeder.
“We have the metal-on-metal, ceramic-on-ceramic [articulations] and we wanted a highly crosslinked polyethylene that could get us that maximum reduction in the wear,” he said.
Collaborative effort
E-Poly was developed in conjunction with Massachusetts General Hospital (MGH) and Cambridge Polymer Group in Boston. The technology is also co-licensed to Zimmer. At the time Zimmer signed the development agreement, Zimmer officials said in a press release that they expected to produce a next-generation product to succeed their current brands of polyethylene like Longevity, Durasul and Prolong, which are crosslinked through a melting process.
In laboratory tests, Biomet E-Poly hip liners demonstrated 95% to 99% less wear compared to the already low wear rates of the firm’s Arcom polyethylene liners, officials said in a press release and confirmed by Schroeder.
Orhun K. Muratoglu, PhD, co-director of the Harris Orthopaedic Biomechanics and Biomaterials Laboratory at MGH, told Orthopedics Today that lab tests showed that no oxidation occurs with the E-Poly. “It has much better mechanical properties and better fatigue strength than the first generation of crosslinked polyethylenes,” he said.
Henrik Malchau, MD, PhD, co-director of the lab, is managing E-Poly research, including a Biomet-sponsored global, multicenter, nonrandomized, prospective clinical study on using it in hip arthroplasty. The study is expected to involve physicians in seven countries, including the United States, and will include nearly 500 patients, according to a press release.
“We are particularly interested in the study for the E-Poly material, as our scientists conducted the research that led to the development of this revolutionary technology,” Malchau said.
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Image: Stryker |
Image: Biomet |
Sequential and crosslinked
Stryker first rolled out its highly crosslinked X3 UHMW polyethylene for hips, followed soon thereafter by X3 tibial inserts, according to Patrick Treacy, vice president of reconstructive marketing. “The X3 bearing technology has the potential to provide for longer lasting implants, thereby reducing costs to the health care system,” he told Orthopedics Today.
The firm’s goals in developing the X3 bearing were to preserve material strength, reduce wear and resist oxidation. Products like Stryker’s highly crosslinked UHMW Crossfire polyethylene do a reasonable job of meeting those challenges, but developers saw a need for a higher performing material, particularly for knee arthroplasty, according to Treacy.
Crossfire has been a clinically successful product in hip applications shown in the lab to reduce wear up to 90% over conventional polyethylene. Developers saw a need for a material that would stand up to the high stress demands of knee applications.
The X3 sterilization process involves sequential crosslinking, where it is irradiated and then annealed three times for a 9 Mrad total dose of irradiation. Because the molecular chains stay closer to each other with sequential crosslinking, they are more easily re-crosslinked following heat treatment.
“The density of the crosslinking … in X3 is far higher if it’s done with the proprietary technology three times in a row, rather than all at once,” Treacy explained.
The Triathlon knee/X3 insert construct demonstrated 96% wear reduction in tests Stryker conducted vs. Smith & Nephew’s Genesis II knee with Oxinium, according to Treacy. In hips, “X3 reduces wear by about 70% further over Crossfire. … What we’re now seeing is that wear performance with the X3 bearing technology is really in the order of magnitude of what some people are reporting in metal-on-metal type articulations.”
Stryker’s surgeon customers have rapidly adopted the X3 bearings, which now constitute about two-thirds of all Triathlon knee inserts sold.
X3 technology is available for acetabular revisions with the Tritanium porous titanium revision acetabular implant and will soon be introduced for the revision Triathlon knee prosthesis. It remains a knee polyethylene option for cruciate retaining, posterior stabilized and patellar designs.
“It is the highest crosslinked product on the market today for knees,” Treacy said, but he noted that X3 bearings generally cost more than other products because the technology is more expensive.
For more information:
- Henrik Malchau, MD, PhD, can be reached at Orthopaedics Biomechanics and Biomaterials Laboratory, MGH, 55 Fruit St., GRJ-12-1206, Boston, MA 02114; 617-726-3886; e-mail: hmalchau@partners.org.
- Orhun K. Muratoglu, PhD, can also be reached at MGH; 617-726-3869; e-mail: omuratoglu@partners.org.
- Dave Schroeder and John Susaraba can be reached at Biomet, P.O. Box 587, Warsaw, IN 46581; 574-267-6639.
- Patrick Treacy can be reached at Stryker Orthopaedics, 325 Corporate Drive, Mahwah, NJ 07430; 201-831-5000.
Reference:
- Oral E, Christensen SD, Malhi AS, et al. Wear resistance and mechanical properties of highly cross-linked, ultra-high-molecular-weight polyethylene doped with vitamin E. J Arthroplasty. 2006;21:580-591.