The radiation induces adiabatic heating, which melts the material, causing crosslinking and recombining of the residual free radicals. The result, he said, is a material that shows two melting peaks, suggesting that it has two phases. In tests on a mechanical hip simulator with a peak load of 750 lbs., the material has shown no measurable wear after 20 million cycles, Muratoglu said. The hospital has licensed its WIAM technology to Sulzer Orthopedics, headquartered in Baar, Switzerland, and Zimmer in Warsaw, Ind. Sulzer has incorporated the technology into its Durasul hip liners, which has recently received FDA approval. Zimmer is using the technology in its Longevity hip liners.
Some experts believe that the overall effects of gamma radiation in air have been misrepresented.
This is the opinion of Aiguo Wang, head of the tribology department at Howmedica Osteonics Corp. in Rutherford, N.J., who said that the overall effect of gamma radiation is to increase the crosslinking rather than to cause chain scission. He said the company has developed techniques to maximize the benefits of gamma radiation while minimizing the effects of oxidation. The crosslinking is achieved with an elevated dose of gamma radiation, followed by heating the material close to its melting point. The poly- ethylene is then machined and packaged in a nitrogen atmosphere, and sterilized with gamma radiation.