New materials could mean longer lasting artificial hips

SAN DIEGO, April 4 - Swiss researchers are developing new materials that are more "friendly" to the body's lubricating proteins and could make future hip replacements last longer. Each year, more than 300,000 people undergo surgery to replace damaged and deteriorating hips. But even artificial hip implants slowly deteriorate and eventually can break down due to wear, often because of inadequate lubrication.

Nicholas Spencer, of the Swiss Federal Institute of Technology in Zurich, reported on progress in developing longer-lasting artificial hip joints here today at the 221st national meeting of the American Chemical Society, the world's largest scientific society.

Artificial hip joints have an average life span of 10-15 years, according to Spencer.

Hip replacement joints, frequently constructed of titanium alloys, consist of a ceramic ball that rotates in a polyethylene (plastic) socket. Friction between the ball and socket leads to wear and tear on the polyethylene, according to Spencer. In a natural hip, he said, the "synovial" fluid surrounding the joint lubricates its cartilage. The proteins in this fluid provide most of the lubrication.

In an artificial hip, synovial fluid still surrounds the joint. But it doesn't provide the same level of lubrication it does for natural hip joints.

Polyethylene used in replacement joints differs from cartilage in one important respect - it repels water. Spencer figured that the water-repelling, or hydrophobic, nature of the polyethylene interferes with the proteins' natural lubricating function.

To test his hypothesis, Spencer treated polyethylene to make it more water-loving, or hydrophilic, then assessed the amount of friction experienced when it rubbed against a ceramic object immersed in fluid modeled after the body's synovial fluid. He found that the material had a lot less friction than untreated polyethylene.

When the proteins in the fluid come in contact with the artificial plastic surface, they "essentially turn inside out and flatten in order to stick as tightly as possible to the surface," Spencer said. "This makes them much less effective as lubricants and increases friction and wear in the hip implant."

"Now that we've established that the nature of the polyethylene surface plays a role in how effectively it uses the proteins in synovial fluid as lubricants," Spencer said, "we're trying to find new materials that have similar mechanical properties but are a lot less hydrophobic." He is currently investigating polyethylene derivatives.

The paper on this research, COLL 413, will be presented at 5:00 p.m., Wednesday, April 4, at the Wyndham Emerald Plaza, Opal Room.

Nicholas Spencer, Ph.D., is a professor of surface science and technology at the Swiss Federal Institute of Technology in Zurich, Switzerland.

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