In the early 1990s, Harris and his team began to focus their attention on finding a way to decrease the wear and tear of the polyethylene cushion in the joint, with an ultimate goal of eliminating osteolysis. The team's initial work involved designing a hip simulator that could accurately replicate the motions and forces of the human hip and measure the wear performance of the implant. The MGH group then turned to a team of polymer chemists from Massachusetts Institute of Technology (MIT) for help in figuring out just how to make a polyethylene cushion that would resist wear and erosion through years of constant motion and weight. They eventually found the solution by "crosslinking" the polyethylene, which involves using a high dose of irradiation to bond molecules more tightly together, producing a much stronger and more durable material. Out of this MGH-MIT collaboration emerged a highly crosslinked, ultra-high-molecular-weight polyethylene.
The research team improved and refined the material by putting it through a melting process to eliminate any free radicals that could cause oxidation and lead to the degradation of the implant material. The material continued to prove strong and reliable in several studies, showing virtually no wear even after being subjected to excessive use and intense abuse. In 1999, the FDA approved highly crosslinked polyethylene for use in implants, the manufacture of which has been licensed to Zimmer, Inc. In subsequent years, the compound has continued to hold up exceptionally well, improving the long-term outlook for patients and expanding the field of total joint replacement.
"The availability of implants with crosslinked polyetheylene has made a great deal of difference for patient care," Harris says. "For example, we used to be reluctant to do total hip replacements in young people because of th
Contact: Sue McGreevey
Massachusetts General Hospital