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Extending the life of joints

August 2002

by Will Sansom

While a joint replacement procedure can drastically improve the quality of life for patients, the effects arenít necessarily permanent. One in every 20 artificial hips, knees or shoulders wears out before the recipient does.

"The problem is that stress on the artificial joints causes small particles to flake off," said C. Mauli Agrawal, Ph.D., associate professor of orthopaedics. "These particles are smaller than one one-thousandth of a millimeter in size, but they trigger Ďosteolysis,í a term for eating away of the bone. The particles cause something to go wrong with the boneís natural remodeling process."

As particles break off, the prosthesis becomes loose. More wear occurs as the implanted joint moves abnormally.

The key, said Jay Mabrey, M.D., associate professor of orthopaedics, is to "make replacement joints more like the material into which they are being implanted." One way is to coat the joint sockets, balls and shafts with substances to which the body will respond. A coral-like material called hydroxyapatite may be applied. Growth factors also might spur integration of the artificial material into existing bone.

Dr. Mabrey is investigating the design of artificial joints with the goal of eliminating abnormal movement. "We are using a plastic mobile bearing in knee replacements," he said. "Rather than locking into place, the bearing moves. This minimizes wear on the joint, and it is an approach that is new in this country."

Hip joint

Traditionally, physicians replace a joint with a metal bearing and a plastic socket. Over time, minute particles of the plastic socket begin to flake off. These particles eat at existing bone. Eventually, the joint moves abnormally. UTHSC physicians are developing joints out of new materials that are similar to what is already in the body. They also are redesigning artificial joints with the goal of eliminating abnormal movement.

He also is experimenting with joint materials. Metal bearings and plastic sockets have been the standard for years, but last year, Dr. Mabrey became one of the first orthopaedic surgeons in the United States to insert a metal-on-metal replacement hip system.

He is engineering a new shoulder system that contains an extra bearing for an extra center of rotation. "We are nearing the lifetime joint," he said. "We used to tell patients that their hips or knees would last 12 to 15 years at most. Now we are shooting for 40 years or as long as a patient lives. These new designs will last that long."

"That would be a tremendous relief," said Evangeline Harper, who hopes to remain pain-free with her joint replacements. And what if, someday, doctors could prevent the need for joint replacements altogether? "That would be huge," she said.

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