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Hip Science - Hip Replacement

Using space technology, researchers are developing artificial bones for pain-free hip replacement implants.

by Steve Price and Dr Tony Phillips

79-year-old Bob Hayes has heard all the statistics.

There are more than 300,000 knee and hip replacement surgeries performed each year in the United States. Sixty-five percent of hip replacements and seventy-two percent of knee replacements are received by people over the age of 65. Because the U.S. population is aging, the number of hip fractures is expected to exceed 500,000 annually by the year 2040. The average hospital stay for a knee or hip replacement: 5 days followed by four weeks using a walker.

Bob, a retired veterinarian from Golden, Colorado, knows the statistics because he's one of them. Between 1978 and 1999 Bob had two hip replacements and five revisions. "I kept three or four of them as mementos," he laughs. "I've been thinking of using them as bookends."

Bob's sense of humour is still intact, but the pain is no laughing matter. "You go until you can't stand it anymore," he says, "and then you have surgery again." And again and again.

"The problem that faces medicine today is that the current implants last only about ten years," explains Dr. Frank Schowengerdt, a friend of Bob's and director of the Centre for Commercial Applications of Combustion in Space (CCACS) at the Colorado School of Mines. (CCACS is a Commercial Space Centre managed by NASA's Space Product Development program.) "Surgeons cut out the old joint and glue in a new one," continues Schowengerdt. "Time along with wear and tear cause the glue to deteriorate."

Learn more about hip replacement surgery from MEDLINEplus.

A normal hip (left) and an artificial hip implant (right).

Bob recalls his own experiences: "The glue would loosen and the joint would pinch a nerve. The pain was intense."

Putting an end to that kind of suffering is what motivates Schowengerdt and colleague Dr. John Moore. They're working at CCACS to make better artificial bones from ceramics - implants so much like the real thing that they could actually meld with living bone. Such implants wouldn't come loose and need to be replaced so often.

Most artificial bones nowadays are made from hydroxyapatite, which has the same chemical formula as bone itself. Synthetic hydroxyapatite, however, is neither as porous as real bone nor as strong.

Pores are important, says Schowengerdt. They are conduits for blood flow (blood is generated in bone marrow) and they allow bones to be strong without being too heavy. Pores also provide a way for living bone to attach itself permanently to an implant. "If we get good bone growth into the pores of an implant, then we've won," says Schowengerdt. It won't matter if the glue comes loose 10 years later.

Researchers have also tried sea coral as a bone substitute. "It's porous enough," says Schowengerdt, "but it lacks strength. Sea coral is mostly used for cranial restructuring."

The solution, according to Schowengerdt, is ceramics. He and Moore believe it's possible to synthesize ceramic materials with the right combination of strength and inter-connected pores to mimic real bone. Indeed they've developed a process in their Colorado laboratory that looks promising.

"Making ceramic bones isn't like making a ceramic coffee cup," says Schowengerdt. "The process is completely different." Ordinary ceramics are made from powders mixed together with a binding agent. They're baked in an oven (about 1000 C), which evaporates the binder and leaves behind a grainy matrix stronger than the original powders. The chemical formula remains unchanged. Unlike coffee-cup makers, however, "we fire our ceramics at a much higher temperature, so that the powders react to form new substances."

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Living bones are porous.

For example, one of the most promising ceramics starts as a powdery mixture of calcium and phosphate compounds (CaO and P2O5). Schowengerdt and Moore ignite the mixture, which burns at 2600 C. CaO and P2O5 react to produce tricalcium phosphate (Ca3(PO4)2), a substance remarkably similar (chemically) to real bone. The reaction also yields heat and gaseous by-products that naturally form bubbly pores.

It's a good start, says Schowengerdt, but there's more to do. For one thing, real bones are porous (weak) on the inside and solid (strong) on the outside. "What we've made is like the weak interior of a bone; it doesn't yet have a strong outer layer. We need to learn to control our process to mimic the stratification of actual bones."

Their technique, called self-propagating high-temperature synthesis or "SHS," is indeed hard to control. "During the firing process, the ceramic is molten. Gases rise and liquids sink. There's a lot of convective motion that make the reaction unpredictable," says Schowengerdt. "To understand this process, we really need to do our experiments in a weightless environment where gravity-driven convection is minimized."

CCACS researchers have flown furnaces onboard NASA's KC-135 'vomit comet' - a parabolic-flight aircraft that provides brief periods of weightlessness. They observed dramatic differences between ceramics prepared in normal gravity (1-g) and those prepared in flight. For instance, the low-gravity ceramics had larger and better-connected pores.

What happened? No one is sure because those brief periods of weightlessness didn't allow enough time for probing and tinkering. That's why Schowengerdt and Moore are looking forward to March 2003, when a new materials processing facility named "Space-DRUMSTM" (a device that holds floating molten ceramics motionless using sound waves) is slated to be installed on the International Space Station. By remote control from Earth and with the aid of astronauts, they'll be able to conduct their tests in low gravity for much longer times than ever before.

"We don't intend to mass produce ceramic bones on the ISS," notes Schowengerdt. "That would be way too expensive. But if we can learn more about the role of gravity in pore formation, we might be able to duplicate our successes in space here on Earth."

Millions of people will benefit from fewer surgeries and less pain if this research produces marketable ceramic bone replacements. But there could be a problem: what to do with all those obsolete implants?

Bob Hayes has an answer: "They make great bookends."


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First Science 2014