Today's News

Saturday, February 15, 1997

New polyethylene form highly wear-resistant

A new highly cross-linked form of polyethylene with no free radicals has been found to be highly wear-resistant as a bearing surface in a total hip replacement.

When acetabular components manufactured from the material were evaluated in a hip simulator, there was no measurable wear at 2.6 million cycles, Murali Jasty, MD, clinical associate professor in orthopaedic surgery, Massachusetts General Hospital, Boston, reported in scientific paper 302 Friday. However, the wear in the conventional ultra high molecular weight polyethylene (UHMWPE) measured 30 cubic millimeters per million cycles. "Even the machining marks remained in the cross-linked UHMWPE while the conventional UHMWPE was worn to a glossy finish," Dr. Jasty said.

UHMWPE is a semicrystalline polymer with a crystalline and amorphous phase. The crystalline region is ordered while the amorphous is not. The amorphous region is susceptible to large-strain plastic deformation while the crystalline region imparts strength and toughness to the material.

Cross-linking the polyethylene effectively increases the intermolecular connections in the amorphous region and makes it resistant to large-strain plastic deformation.

Dr. Jasty said that recent studies from retrieved acetabular components have shown that the UHMWPE at the articulating surface undergoes large strain plastic deformation and subsequently breaks up due to multidirectional motions of the hip.

"Cross-linking of UHMWPE using ionizing irradiation in the solid state leads to the production of free radicals due to the dissociation of hydrogen atoms from the carbon atoms," Dr. Jasty said. "Some of these recombine with each to produce cross-links. Others, however, remain for a long time and react with oxygen diffusing from the outside. In time, this leads to chain scission recrystallization and embrittlement of the material. This leads to degradation of the wear resistance.

"Therefore, we elected to perform the irradiation in the molten state. The enhanced molecular mobility allows the free radicals to recombine with each other, thus enhancing the cross-linking efficiency and quenching of all the free radicals. The melt-irradiated UHMWPE is expected to have long-term resistance to oxidative degradation."

The researchers used electron beam irradiation because it can be delivered faster than gamma irradiation and can be carried out while the material is hot. However, electron beam irradiation has limited penetration. The three million volts delivered by the irradiator used in the study can obtain penetration depth of only 1 centimeter. The researchers had to preform the material into a rough shape of an acetabular component, irradiate it in the molten state and remachine the final shape of the acetabular component.

Other co-authors of the study from Massachusetts General Hospital are Charles R. Bragdon, BS, senior research associate; Daniel O. O'Connor, senior research associate, orthopaedic biomechanics laboratory; Orhun K. Muratoglu, PhD, orthopaedic Biomechanics Laboratory; and William H. Harris, MD, chief, hip and implant service, orthopaedic department. Other co-authors, both from Massachusetts Institute of Technology, are Edward W. Merrill, PhD, professor, department of chemical engineering, and Premnath Venugopalen, graduate student in chemical engineering.