When Dr. Spiegel sampled bone from areas reconstructed with implants 18 months after reconstructive surgery he found that the coral material accounted for only about a third of the bone volume.

Traditionally, surgeons doing reconstructive surgery borrowed bone from elsewhere in the patient's body, generally from the outer layer of the skull, the ribs or the hip.

But surgeons can only borrow limited quantities, and the need to gather bone requires new incisions which in almost 10 percent of patients lead to complications, like infection and pain. "After we take bone from the hip, patients often can't walk well for days or weeks," Dr. Miller said.

Also, he adds, "If you're trying to correct major congenital abnormalities, you really need more bone than you can gather from all three places, anyway." Risks From Cadavers

When desperate for filler, surgeons resort to bone from cadavers or made from synthetic materials, but both these approaches have drawbacks. Cadaver bone carries a small risk of infection with hepatitis or even the AIDS virus, and since it is foreign tissue it can provoke the ire of the immune system. Synthetic materials produce inflammation, which impedes healing and generally increases the risk of infection.

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In contrast, the coral bone does not appear to activate either the body's inflammation or immune responses.

Because it is so inert, Dr. Spiegel said, the coral bone settles in place much faster than even a cadaver substitute. "It is nothing more than a template or a scaffold," he said, "so bone doesn't attack it, it just grows right in. The body recognizes the cadaver bone as foreign and must break it down before laying down new bone."

The marine coral used in the experiments, from the genuses Porites and Goniopora, is heat-treated to convert it from calcium carbonate, the main element in coral, to hydroxyapatite, a calcium-containing compound that is a prime element in bone. In the process, the coral organisms are killed, and only a porous mineral structure remains. Comes in Blocks

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"It's what's in bones normally, which is probably why the body doesn't see it as different," said Dr. Edwin Shors, vice president for research and development at Interpore International of Irvine, Calif., one of a few companies that make the coral bone, which comes in blocks of various sizes.

Although the coral bone is brittle compared to real bone, doctors say they are generally able to carve it to the shapes they need for surgery. Once the surgery has healed, the strength of the resulting bone composite is excellent, Dr. Spiegel said. Last year, at the American Fracture Society's annual meeting, he reported on a three-year follow-up of 19 patients who had fractures repaired with coral bone grafts. All healed solidly, including those in major weight-bearing bones of the leg.

Scientists have experimented with coral as a substitute for bone since the mid-1980's and are now trying to soak the sea bone in chemicals that encourage bone growth to hasten the merger-and-healing process.

"We'd like to put in a coral substitute that is integrated solidly into bone in a few months," said Dr. Miller. In rabbits, Dr. Miller has found that a combination of the coral bone and a naturally occurring bone growth-enhacing liquid called bone morpheogenic protein has worked well. When coral bone grafts used to repair defects in the animals' skulls were pre-treated with the bone-growth protein, 50 percent of the grafts' pores had been filled in by adjacent rabbit bone in just three months, as against 10 percent when coral bone without the growth supplement was used.

The team is currently planning a trial of the treated coral in humans.

"There's a lot of research going on about how to improve the technique," said Dr. Shors. "You'd want the entire implant to be filled with bone very quickly. After that, the ideal might be for the prosthesis to degrade over time." The coral hydroxyapatite grafts degrade by about 2 percent a year. So if a person has a fracture fixed with coral bone in 1991, 100 years later his bone would be nearly reclaimed from the sea.