Doctors at the University of Maryland hospital in Baltimore completed early this morning what was described as the first partial replacement of a patient's spine.

The operation on 33-year-old Jessie Thomas of Baltimore took 16 hours. She was described early today as in good condition.

Dr. Charles Edwards, the attending physician said he was optimistic. He said the only concern was the possibility of infection.

The operation involved insertion of a six-inch metal and plastic device that was designed to fill the gap created when doctors earlier removed five vertebra and a tumor the size of a melon from the woman's back.

A spokeswoman for the hospital said last night that no complications were encountered during the surgery, "only delay."

The principal obstacle apparently was tissue that had grown around the woman's spinal cord since the earlier surgery, which occurred six weeks ago.

The fatty tissue made it more difficult to perform the operation. The length of the operation also made it necessary for the patient to be supplied with more blood, and cross-matching of the blood reportedly added to the delay.

In an earlier interview, Dr. Edwards emphasized that the operation was a spinal transplant, not the total replacement of a spine. He said it did not involve the spinal cord, which had been left intact in the earlier surgery.

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

Although the possibility that Thomas would walk again was considered remote, it was believed that the historic operation might offer her a chance to move again, possibly to sit up and use a wheel chair, and to care for her two children.

The operation was essentially intended to provide the woman with a rigid support in her lower back.

Prior to yesterday's surgery, she had lain motionless for 45 days with no bone connecting the lower and upper halves of her body.

A hospital spokesman called the procedure completed early today "the world's first total lumbar spine replacement."