First Clinical Use: New Form Of Polyethylene Will Allow Stronger, More Versatile Joint Replacement

"We think this material could be used for any joint in the body and in any implant design, even those demanding higher flexion and more mobility," says Orhun Muratoglu, PhD, co-director of the Harris Orthopædics Biomechanics and Biomaterials Laboratory (OBBL) at MGH, who developed the new material in collaboration with scientists at the Cambridge Polymer Group.

Total replacements for hips and other joints were developed in the late 1960s, but it soon became apparent that hip implants could start loosening about 5 years after surgery and would eventually fail completely. A team led by William Harris, MD, DSc, now director emeritus of the MGH OBBL, investigated this complication and found that long-term friction of the implant's head against the polyethylene-lined joint socket would break off small particles of polyethylene. The body's immune system reacted against these foreign particles, eventually destroying adjacent bone tissue and causing the implant to loosen -- a condition called periprosthetic osteolysis.

Harris and his colleagues, working with polymer chemists from MIT, found that high doses of radiation would "crosslink" the polyethylene, bonding molecules together to produce a much more durable material. The procedure also generates free radicals that could lead to oxidization and degradation of the implant, but the research team found that melting the material would eliminate free radicals. The first-generation highly crosslinked polyethylene was approved by the FDA for use in implants in 1999 and has been licensed to Zimmer, Inc.

However, the MGH researchers knew that the first-generation material had limitations in strength that made it unsuitable for some types of joint replacement implants. Subsequently, Muratoglu found that oxidation could be blocked by diffusing the antioxidant vitamin E throughout the polyethylene material. Both mechanical testing and animal studies have shown that the new material resists wear as well as the first generation and is much stronger. Vitamin-E-stabilized, highly crosslinked polyethylene has also received FDA approval for use in joint implants and has been licensed to both Zimmer and to Biomet, Inc., which made the implant used in the first surgical procedure on July 16.

"This material will allow us to offer our patients very long-term, high-performance joint replacements," says Andrew A. Freiberg, MD, chief of the Arthoplasty Service in the MGH Department of Orthopædics, who performed the first implant with the new material. "It should be suitable for higher-stress applications in younger patients, those who are more active and those who are heavier."