Surgeons completed the replacement of part of a women's spine with an artificial substitute here today, but they expressed concern about her chances of surviving the rigors of the unique 16-hour operation.

In the operation at the University of Maryland hospital, Dr. Charles C. Edwards replaced four of the vertebrae of Jessie Thomas, 33, with a specially made metal and plastic device that he designed.

She was listed in "guarded" condition late tonight.

The vertebrae were removed June 17 in a 15-hour operation in which Edwards and his team removed a canteloupe-sized malignant tumor from the Baltimore woman's body. Although bone was removed, the spinal cord was left intact.

It was the degree to which the effects of the earlier operation had healed that prolonged yesterday's operation and jeopardized the patient's life, Edward said.

The healing "is a good sign for the patient" if she survives, said Edwards, adding, "We had to take apart an extremely large area which had already healed and formed scar tissue."

Because part of Thomas' spine has been missing for the past 46 days, said Edwards, her kidneys rotated out of position, and the kidneys, spinal cord and part of her heart had become attached to the same mass of fat and scar tissue.

Edwards said his team of surgeons had to separate the vital organs in the 236-pound patient -- an extremely delicate and dangerous procedure -- before they could even begin to think about implanting the artificial device.

"There was a great deal of bleeding," said the surgeon, who said the operation involved cutting three feet of incisions in Thomas' body.

Speaking at a mid-morning press conference here, Edwards said Thomas' condition and chances of survival are as tenuous as those of a patient who has been severely injured in many areas of the body.

He said the question during the next few days would be whether such organs as her kidneys, heart and lungs would survive the strain imposed by the surgery.

Thomas was lucid enough an hour after surgery, at about 2:30 this morning, to properly answer some simple questions, said Edwards. While the best Thomas can probably hope for now is life in a wheelchair, her outlook prior to the surgery was far bleaker.

Had the tumor not been removed -- and such large growths normally can't be successfully removed -- it would have killed her, probably within a year, said Edwards.

Thomas' lower body was paralyzed by the tumor before she came to the university hospital last May. The tumor had reached the point where it cut off her body's blood supply when she was turned on her stomach during tests at the hospital.

Had the tumor been removed, but the spine replacement not attempted, said Edwards, Thomas would have been condemned to a brief life, strapped immobile on a special frame on which she would be rotated every few hours, much like a chicken on a spit.

Thomas experienced that sort of life for the five weeks between operations, said Edwards.

"She was not in much pain, but you can imagine her feelings. She was lying on a flat cot, which looks like a stretcher, canvas between two rods, looking at the ceiling for four hours and then looking at the floor for two hours and then after those two hours she'll start looking at the ceiling again. It goes on interminably," said Edwards.

If all goes well, he said, Thomas will be placed in a cast in about two weeks and then start sitting up and begin physical therapy. She could go home in a month to two months.

The surgeon said that although the operation would have been easier had Thomas weighed less, doctors decided not to attempt to reduce her weight while she was in the hospital.

Food, said Edwards, provided Thomas with one of her few pleasures, and the physicians decided to allow her to maintain her great weight rather than deprive her.

The device placed in Thomas' back was custom designed and manufactured at a cost of about $1,600. It is made of an alloy of chrome, cobalt and molybdenum, stainless steel and plastic.

The main part of the device consists of a cylindrical object with a groove running down one side to cradle the spinal cord.

Inside the cylinder are two rods which, with the use of a specially designed tool, were slid from each end of the cylinder into holes prepared in the vertebrae at the top and bottom of the gap in Thomas' spine. Special glue and set screws were used to hold the rods in place.

The insertion of the device was one of the most delicate parts of the operation. Surgeons, who had one practice session on a cadaver, had to slip the main portion of the device behind the spinal cord without damaging the cord. Then they had to align the top and bottom of the six-inch alloy cylinder with the existing vertebrae. The spinal cord itself was then to run down a shallow groove in the front of the device.

The two screw-like rods had to be placed on the two outer corners of the front portion of the vertebrae, less than two inches apart, so that they would run parallel to the spinal cord and just in front of it.

Edwards said one of the delays in the operation occurred because the device was improperly inserted the first time, and it took several hours to remove the glue -- which sets in three minutes -- before attempting the placement a second time.

The other two parts of the device are long, thin, threaded, stainless steel rods that are attached with hooks to the top and bottom vertebrae, and then tightened, protecting the spinal cord and hopefully preventing the spine from rotating.

Edwards said that although Thomas' spinal cord was preserved during the first operation, it was probably so damaged by the pressure of the tumor that it is highly unlikely she will ever again use her legs.

Yesterday's surgery, he said, should not be viewed as giving hope to those paralyzed in accidents or those who suffer from back trouble.

Such devices might be used on patients in this country a few times a year, said Edwards. The operation "brings us very close to the margins of what can be done."

The real value of the operation is "more philosophical," said Edwards. "It shows that procedures we're not familiar with can be designed." The surgeon, who has successfully completed about 11 unique bone reconstructions, said doctors can now rebuild "almost any part of the skeletal system."

Hospital officials said they had no real idea what Thomas' hospital care and surgery will cost, but a hospital source put the figure at more than $25,000, part of which will be covered by private insurance.

Edwards said he does not know what he will bill for his surgical services, but said with a smile, "If billed at actual value, it would be substantially more than anyone at this place is charging."