January 15, 1999

Implant may aid in re-growth of bones

Implant may aid in re-growth of bones

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This is a close-up view of the composite material compound being used to replace lost segments of bone.

A new implant material being used at Vanderbilt University Medical Center is showing promise in helping bones re-grow faster following joint replacement.

The material, hedrocel, aids in the healing process by helping to create a new, dense bone nearly as strong as the original.

"It is a very promising material," said Dr. Michael J. Christie, assistant professor of Orthopaedics and Rehabilitation. "Hedrocel is as strong as the bone that was there and it allows adequate healing between the bone and the implant so we have a long-term attachment.

"The body accepts the material and the bone seems to grow through the material, as we hoped it would. We are using it to replace lost segments of bone, particularly in the cases of failed implants like knee replacements, infections and bone tumors in which a large segment of bone has to be removed."

Hedrocel is a composite material composed of carbon and tantalum, an inert metal. The material was originally used in the aerospace industry to make solid filters for airplanes and spacecraft.

Under a microscope, Christie says it looks very much like the normal spongy material at the end of a bone.

"It's a porous metal material that has holes all the way through it. They aren't holes like you make with a drill, but formed holes, on a microscopic level that allow tissue to actually grow into the gaps between the metal surfaces."

Prior to the availability of hedrocel, surgeons had to either replace an entire bone with a metal piece, which would provide no place to attach the muscle mechanism and would often fail because there was no biologic attachment, or they could use a similarly-sized cadaver bone, which increased the risk of infection.

Christie says studies show that during the first year after implantation using hedrocel, the bone becomes increasingly dense and eventually approaches the density of the surrounding bone.

"Which means in essence that the material is allowing the bone to heal back to its normal structure and function and that is very exciting. Our experience, so far, is that it grows back faster and better than we would have previously thought possible."

In his surgeries using a custom-made implant, Christie says a metal rod goes through the segment of hedrocel and up inside the hollow part of the bone.

"We can get a good fit and then over time the body's bone heals to the hedrocel and actually grows through it. That implant or joint replacement can then better handle the weight pressures and forces that are applied to the bone. The implant will then last longer, which is the real goal."

The new material not only got patient Patsy Sue Smith of Hixson, Tenn., out of a wheelchair, it also saved her leg.

Smith's leg, which had a prior knee replacement, had been broken in a car accident and infection could not be contained. One out-of-state doctor wanted to amputate the leg.

But Christie decided to try the new implant.

After removing the infected bone, Christie inserted a spacer to hold the muscles and tendons in place, while antibiotics killed the infection.

Then Christie and Dr. Herbert S. Schwartz, professor of Orthopaedics and Rehabilitation, designed a custom-made prosthesis using hedrocel.

"We took measurements to decide how much bone was lost and created an implant that matched her other leg. It was designed to fill the defect created by her lost bone."

Now, two years later, Christie says Smith has had a steady increase in function.

"We have been very excited about how she has done. She has worked very hard after not having a knee in for many months to get her motion and strength back and has just done extremely well."

And, he says, x-rays show the bone is growing nicely and adhering to the hedrocel implant.

Smith says the leg and knee work perfectly, with a scar the only sign of the implant inside. Most importantly, she says she has her old life back.

"I can walk and go wherever I want to, even up stairs. It just works like a regular knee," Smith said.

VUMC doctors have used the new implant material in about 10 custom-made devices so far, more than any other medical center in the country, according to Christie. Nationally, more than 500,000 joint replacements are performed each year.

Using the material in hip socket replacements has recently been approved by the FDA.

"The potential for the material is very exciting. There is still more study to be done, but the early signs are very good. So far, we have seen no limitations," Christie said.