Spare Parts for the Human Body Shop
by David Pescovitz
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Professor Lisa Pruitt also conducts research on artificial polymers and grafts for damaged cartilage.
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From garage doors to auto shocks, mechanical parts fail from repeated use. The same holds true for artificial hips, shoulders, and knees. Swapping the natural joint for an artificial one the first time is traumatic enough, but eventually the implants wear out as well. UC Berkeley mechanical engineer Lisa Pruitt leads an effort to lengthen the lifetime of artificial parts in the human body shop.
"Everybody seems to either be afflicted with some pain in the knees, hips, or shoulders, or knows someone who has had a joint replacement due to arthritis," says Pruitt. "So our laboratory develops materials technology to improve fatigue and wear resistance and mechanical integrity in the artificial joints."
If the cartilage that allows the bone ends of a joint to move freely and painlessly becomes seriously damaged or diseased, the joint may need to be replaced. Currently, replacement joints often consist of titanium, cobalt chrome alloy and a polymer called polyethylene. For example, in an artificial hip, the stem of the implant that connects to the bone may be titanium while the articulating "ball" at the end is fabricated from cobalt chrome alloy. A polymer cup is substituted for the socket.
"The polymer gives us the best lubricity and mimic of cartilage in the joint," Pruitt says. "But as the joint moves from walking or other load-bearing activities, the hard metal will eventually scratch the plastic or rub off polymer debris that ends up in the joint space."
Over time, this debris can loosen the joint from the bone, causing intense pain. At that point, the patient must undergo "revision" surgery to either repair the artificial joint or replace it with a new one.
"Today's artificial joints are designed to last 15 to 20 years," Pruitt says. "That's OK if you get the replacement when you're 70, but not ideal if you're a lot younger. Revisions never last as long as the primary replacement, so the goal is to get 30 years or more out of these devices."
In Pruitt's medical polymers group, an interdisciplinary team of researchers from mechanical engineering, materials science, biology, and medicine, and a surgeon from UC San Francisco, are developing new techniques to optimize the polymer. The aim is to prevent the plastic from flaking off due to abrasion or crack from repeated cyclic motion. The trick, she explains, is making changes to the microstructure of the material that improves one, or possibly both, characteristics without any sacrifice.
In a recent collaboration with the Harvard Medical School and Brigham and Women's Hospital, the researchers treated a polymer with gamma radiation to produce a material called a highly crosslinked polyethylene with impressive wear resistance. Then they applied a high pressure process to the new material, increasing the crystallinity and thereby the fatigue resistance as well. Once samples are created, they're mechanically tested in Pruitt's lab, put under the microscope, and x-rayed "to correlate the ideal mechanical properties with the specific microstructures."
In the last year, Pruitt and her colleagues have presented preliminary data suggesting that some of their experimental materials not only provide the same phenomenal level of wear resistance of highly crosslinked polyethylene but also show up to 20 percent improvement in fatigue properties.
"They could give us the best of both worlds--fatigue resistance and wear resistance," Pruitt says.
Of course, the new materials still must undergo intense testing and characterization before they will be considered for clinical use. To that end, Pruitt's laboratory is building a custom test system that enables the joints to be subjected to millions of cycles of use while the researchers adjust the kinematics, for example how much an artificial knee rolls in the joint versus sliding.
"In our laboratory, we hope for clinical outcomes in the near future," Pruitt says. "And that's what I like most about this research. We take an interdisciplinary approach to solving a real problem for humanity."
Lisa Pruitt's home page
Medical Polymers Group
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Updated 5/1/05.
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