CWRU IMPROVING DESIGN OF ARTIFICIAL JOINT REPLACEMENTS
30 October 2003 - Case Western Reserve University

Clare Rimnac, director of the Musculoskeletal Mechanics and Materials Laboratories, talks with Jay Bensusan, a senior research engineer in mechanical and aerospace engineering, about research that seeks to improve the wear and performance of artificial joints made of ultra-high molecular weight polyethylene. The team is studying how plastic in the joints is damaged, in order to help design better-performing implants.

"Total hip replacement and total knee replacement are orthopedic success stories which enable hundreds of thousands of people around the globe to live fuller, more active lives," said Clare Rimnac, director of the Musculoskeletal Mechanics and Materials Laboratories at CWRU and associate professor of mechanical and aerospace engineering at the Case School of Engineering. "Unfortunately, in some cases these joints wear out more rapidly than we would like," she added.

"Instead of implanting the joint and waiting to see what activities cause the material to fail, we're developing computer simulated models to give us that information," Rimnac said. "The ultimate goal of our research is to make it possible to predict the performance of new implant designs before they are implanted into patients."

This retrieved hip replacement component shows severe damage in the form of fracture and loss of material from the plastic portion of the component. A normally active person will load and unload a hip or knee joint by flexing and extending it between one million and three million times per year, Rimnac said. The force across the joint may vary from three times a person's body weight when simply standing to more than six times body weight when climbing and descending stairs and jogging. "Imagine how much stress is put on the knee or hip of a 200-pound man," she said.

This x-ray image shows a patient's hip after a total hip replacement. When the plastic is damaged, particles are released into the body, which may cause bone loss. Then, instead of having an implant that is well-fixed into the bone, the reaction to the debris leads to gaps in the bone which compromise the mechanical and structural integrity of the implant.

Using a mechanical testing machine, Rimnac's lab subjects sample pieces of the polyethylene to different loading conditions until the sample fails by fracture. The research team then studies how the samples deform and how cracks form and grow, developing a model of the plastic's behavior. They are conducting this research on several new formulations of the polyethylene that recently have been introduced in total hip replacements and that are being implanted into patients.

"Although these new polyethylene materials are already in clinical use, our goal is to provide better computational simulations of these joint replacements to identify potential design concerns associated with these new materials and to, in general, develop design strategies for improving the long-term performance of total joint replacements," said Rimnac.

The hip is much like a ball-and-socket joint, and total hip replacements mimic this with a metal ball that articulates in a plastic cup.

Rimnac notes that these new formulations of polyethylene are of great clinical interest because research suggests that they are more resistant to wear than previous polyethylene formulations. With these new polyethylene materials, the long-term performance of implants may be extended because fewer wear particles will be released and there should be less bone loss.

Rimnac is working with Steven M. Kurtz, a principal engineer at Exponent, who serves as the co-principal investigator, and Jorgen Bergstrom, a co-investigator from Exponent's Natick, Massachusetts, office.

The CWRU laboratory is responsible for physical and mechanical testing of the polyethylene materials, while Exponent is supervising the computer simulations of the joint replacements and development of the mathematical relationships of the materials. The goal is to develop new analytical models for predicting the response of the plastic components of total joint replacements to a variety of clinically relevant loading conditions.

Rimnac's research is funded by a three-year, $750,000 grant from the National Institutes of Health's National Institute of Arthritis and Musculoskeletal and Skin Diseases. Exponent is a second performance site on this grant.

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