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.

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

"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.