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A distinguished pursuit: More durable replacement joints



Clare Rimnac '83 Ph.D. and her mentor, Richard Hertzberg, professor emeritus of materials science and engineering.

When you stand on one leg, says Clare Rimnac, you triple the impact of your body’s weight on the working knee.

When you run or jump, you impose eight to 12 times more force on your knees than you do while standing still.

Rimnac, who earned a Ph.D. in metallurgy and materials engineering from Lehigh in 1983, has spent her career studying artificial knee and hip replacements and the myriad factors—material, biological, chemical and mechanical—that determine their success or failure.

As the Wilbert J. Austin Professor of Engineering and chair of the department of mechanical and aerospace engineering at Case Western Reserve University in Cleveland, Rimnac leads a research team that has examined more than 1,000 failed knee and hip implants. She is investigating a type of ultra-high-molecular-weight polyethylene whose polymeric chains, when crosslinked, show promise in preventing the loosening that can cause artificial joints to fail.

Rimnac recently returned to Lehigh to give a presentation on her work and to receive the Distinguished Alumni Award from the university’s department of materials science and engineering.

“So many fond memories”

Already the first woman to earn a Ph.D. from the department, Rimnac also became the first to win the alumni award. She told her audience in Whitaker Auditorium that she had benefited as a graduate from her department’s collegial environment.

“I have so many fond memories of that time,” said Rimnac, who started her presentation by showing photos of herself and her colleagues from a quarter-century ago. “There was such a good camaraderie between faculty and students.”

As a Ph.D. candidate studying the fracture behavior of polymers, Rimnac was advised by Richard Hertzberg, a professor emeritus of materials science and engineering, and also by the late John A. Manson, who was a professor of chemistry and a polymer scientist.

“I owe much of my success to those two professors,” Rimnac said. “They taught me how to build interdisciplinary teams to work on engineering problems. When I left here, I thought that if I could set up a collaboration like the one John and Dick had, that would be awesome. I’ve been fortunate to be able to bring together people from many different disciplines. That makes our approach to research so much richer.”

Hertzberg gave her another critical assist, Rimnac said, when he helped her land a job as a scientist with the department of biomechanics at the Hospital for Special Surgery in New York City. She held the position from 1983 until her appointment at Case Western in 1996.

The science of joint replacements not only requires knowledge of multiple disciplines, Rimnac said, but it also deals with large numbers. The average person puts three million cycles of wear each year on an artificial joint. This creates problems as implants increase in longevity.

“Some joint replacements last as much as 20 years,” she said. “That means that we are asking materials that cannot repair themselves to survive 60 million cycles of wear.”

A second number is more astronomical. Wear can cause tiny fragments of debris, smaller in size than a human cell, to separate from the implant, especially in a polyethylene sterilized with gamma radiation in air, a technique used until the mid-1990s. As many as 100 million of these particles can be generated each day. They can cause the body’s cells to respond in a way that promotes bone loss near the implant and, ultimately, the failure of the replacement joint.

“The body responds to the device,” said Rimnac, “and the device also responds to the body. It goes both ways.”

In hopes of reducing wear on replacement joints, Rimnac has spent a decade studying the crosslinking of polyethylene. Crosslinking, a technique first used in Japan and South Africa in the 1970s, has been shown in laboratory simulations and clinical studies to reduce wear damage to artificial hip joints, Rimnac said. But this improvement comes with consequences that are not fully understood.

“Crosslinked devices have brought about a major improvement in the performance of joint-replacement devices,” she said. “They reduce the debris from wear to negligible levels. But they can cause loss in ductility and fracture toughness.”

The goal of Rimnac’s current research is to accurately predict the stress and strain response of traditional and crosslinked polyethylene formulations in total hip and total knee replacements. Towards this end, she and her group at Case Western Reserve, along with her collaborator, Steven Kurtz, corporate vice president of Exponent Inc. Engineering and Scientific Consulting in Philadelphia, are evaluating the in vivo performance of retrieved polyethylene components to identify the factors affecting wear damage and fracture. They are also attempting to determine the static and cyclic mechanical properties of polyethylene and to predict the effects on structural performance of changes in design variables.

Rimnac, the 10th person to receive the materials science and engineering department’s Distinguished Alumni Award, said she was “overwhelmed” to be chosen for the honor.

“This is much more than I deserve,” she said. “It is very humbling to be on a wall with such distinguished alums as Robert Stout, Al Pense and Al Romig.”

Stout, who earned an M.S. from Lehigh in 1941 and a Ph.D. in 1944, is a former dean of Lehigh’s graduate school. Alan Pense, who earned his M.S. in 1959 and his Ph.D. in 1962, is university provost emeritus.

Alton Romig, who won the Distinguished Alumni Award when it was last given in 2004, earned the B.S., M.S. and Ph.D. from Lehigh in 1975, 1977 and 1979. He is senior vice president and deputy laboratories director for integrated technologies and systems with Sandia National Labs.

--Kurt Pfitzer


Posted on Friday, October 10, 2008

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