Jessie Thomas has lain motionless for 45 days, with no bone connecting the upper and lower halves of her body as she waited for today, when surgeons will attempt the first partial replacement of a patient's spine.

The six-inch metal and plastic device they will use was designed to fill the gap created when doctors removed five vertebrae and a tumor the size of a cantaloupe from the 33-year-old Baltimore woman's back.

The operation scheduled at the University of Maryland Hospital here would give Thomas a chance at least to move again, perhaps sit up, get about in a wheelchair and care for her two children.

Dr. Charles C. Edwards, head of the surgical team attempting the procedure, said it only "remotely possible" Thomas will ever walk again.

Thomas has been strapped to a special frame that holds her motionless since Edwards and his team performed a unique 15-hour operation July 17 to remove the bone and tumor while keeping her spinal cord intact.

That operation saved her life, at least temporarily.

Although surgeons were able to remove the relatively rare cancerous tumor without destroying the spinal cord, Edwards said the pressure exerted by the massive growth may already have permanently damaged the cord.

The operation is intended, he said, to give Thomas a rigid support in her lower back.

Thomas first came to University Hospital in May, suffering from paralysis she thought was caused by a fall at home.

Instead, a series of tests showed she was suffering from a recurrence of a tumor which had been surgically removed seven years ago.

Under most circumstances such a tumor, involving a large portion of the spine and attached to an outer wall of the heart, would be considered inoperable. It might be treated with radiation.

But following surgery in 1972 for the earlier, much smaller tumor, Thomas received what physicians consider the maximum lifetime dose of radiation, making the current series of operations her only hope for survival.

Edwards said that at the time he removed the tumor and spinal section he had no idea what he was going to do to replace the spine.

"This is not all part of some grand plan," he said.

Edwards said he was faced with doing a "tumor operation and I knew I'd be left with a patient whose upper and lower bodies weren't connected.

"I was asked in the department (of orthopedics, which he heads), 'How are you going to rebuild the spine?' and I could honestly say, 'I don't know,'" Edwards said.

But Edwards, who has successfully completed "10 or 11 other custom reconstructions" of skeletal parts not usually considered replaceable, set about designing a device which would reconnect Thomas' spine.

At a news conference yesterday Edwards was as intent on stressing what he has not come up with as what he has. He said he wants to avoid giving false hope to people who are paralyzed or suffer from back problems.

"We're not talking about a spinal transplant," he said. "We're not talking about (replacing) a total spine. "We're not talking about the spinal cord."

The device Edwards and members of his team designed -- and had built at a cost of $1,625 -- consists of three parts and is made of a cobalt, chrome and molybdenum alloy, stainless steel and polyurethane.

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

Inside, the cylinder there are two rods which, using a specially designed tool, will slide from each end of the device and fit into holes prepared in the remaining vertebrae at each end of the gap in Thomas' spine. Special glue and set screws will hold the rods in place.

The two other parts of the prosethetic device are long, thin, threaded, stainless steel rods which will be attached with hooks at the top and bottom vertebrae to protect the spinal cord and, hopefully, prevent the spine from rotating.

Edwards said that the surgeons plan to pack the area around the device with bone chips raken from Thomas' pelvis during the operation, which is expected to take five hours.

He said his only alternative to designing the device would have been to fill the gap with bone pieces which would then be fused in place.

But such a procedure would leave Thomas immobile for up to two years, "being rotated every two or three hours" in her special frame. "This is where you come to the quality-of-life question."

Using the chips in combination with the artificial device, Edwards said, will help eliminate the problems that might arise as the prosthesis slowly deteriorates over the years.

Edwards, who said he and his team successfully practiced the implant on a cadaver earlier in the week, said he looks forward to Thomas being able to move within a few weeks, and go home in about two months.