A University of Pennsylvania surgeon announced yesterday that he has successfully transplanted a complete knee joint from a teen-ager who had died in a motorcycle accident into a New Jersey woman whose knee had been destroyed by a tumor.

The experimental surgery, which experts said yesterday has been attempted only a few times previously, was performed Sept. 30, and the patient, Susan Lazarchick, 32, of Absecon, N.J., is doing well.

"I know he saved my leg," Lazarchick said yesterday of her surgeon, Dr. Richard Schmidt, 33, an orthopedic cancer specialist who directs the bone bank at the Hospital of the University of Pennsylvania. "The other option was having my leg amputated. I decided that I would rather go ahead and try this and have the possibility that I'd have my leg," she added.

If the patient's progress continues, "it will certainly be considered a major step forward," said Dr. Henry J. Mankin, chief of orthopedics at Massachusetts General Hospital and an expert on bone transplants. He cautioned that recipients of such transplants sometimes develop progressive joint damage because the grafted joint lacks normal sensation.

Schmidt said in an interview that he decided to try the experimental operation after a donor knee joint became available shortly before Lazarchick was scheduled to have surgery to remove a rapidly growing, grapefruit-sized benign tumor involving her right knee and portions of the adjoining leg bones.

He said the urgency of removing the tumor made it impossible to wait several weeks for a custom-made artificial knee joint and that Lazarchick did not want to have her leg amputated at mid-thigh level, which would have been the standard treatment.

The use of a human joint, rather than a metal one, also will allow Lazarchick to move the knee normally by using her leg muscles, Schmidt said. During the operation, Schmidt attached Lazarchick's thigh muscles to the corresponding tendons of the transplanted knee joint.

"Her muscles will now grow into the tendons and power this knee," he said.

Schmidt said he removed the donated knee joint and adjoining leg bones from an 18-year-old man who had died in a motorcycle accident. The tissue was immediately frozen at -112 degrees Fahrenheit. He said bone preserved in this way can be used surgically as long as a year later.

The freezing process kills bone cells, but the nonliving, mineral structure of the bone and the cartilage lining the joint remain intact, he said.

During the seven-hour operation, Schmidt said, he first removed a 15-inch segment of Lazarchick's right leg, including the tumorous knee joint, the lower six inches of her femur or thigh bone, and the top half of her tibia or shin bone.

He then replaced the missing segment with the donated knee joint and portions of the adjoining bones. A metal rod was inserted in the shaft of the bones to attach Lazarchick's femur to the donated femur, and a metal plate was used to connect her tibia with the donated tibia.

Schmidt said he had previously stripped the skin and muscles off the grafted joint. He also had carefully washed the marrow out of the bones' central cavities to remove blood cells that could provoke rejection of the transplant.

Schmidt said rejection of bone transplants occurs only in about 5 percent of cases if the donated bone has been previously frozen and cleansed of blood cells.

Finally, Schmidt attached Lazarchick's muscles to the tendons of the transplanted knee joint and sewed her skin over it.

Schmidt said he is elated by Lazarchick's progress. He said that her leg healed without becoming infected -- the major potential complication of the transplant operation.

If the joint remains healthy, he said, her case may represent the first successful complete knee transplant. Partial transplants, in which only the top or bottom half of a joint is replaced, are done much more frequently. Schmidt said he performed the first complete shoulder transplant in January 1986, on a 28-year-old New Jersey man with bone cancer and that he is also doing well.

Schmidt said he prefers implanting real joints, rather than metal ones. "It's a versatile tissue," he said. "I can get any bone I want, any size and shape. It allows me the ability to virtually be my own custom manufacturer."

Lazarchick said she began having leg pain and trouble walking several months ago but did not realize that she had a tumor. She said the most difficult part of undergoing the experimental operation was uncertainty about whether her leg could be saved.

"It was scary," she said. "Physically, it was hard, but I had a lot of emotional support."

She said her leg has been in a cast since the operation, and Schmidt said it will remain in one for six to eight more weeks. She was released from the hospital three weeks after the surgery. After the cast is removed, Schmidt said, Lazarchick will wear a leg brace and undergo intensive physical therapy, including electrical stimulation of her muscles to strengthen and retrain them.

Boston's Mankin said that the first reported knee transplant was performed in 1907 and that the operation has been done a few times since then.

He said surgeons who previously attempted knee transplants encountered high rates of a complication known as Charcot's arthropathy, in which the transplanted joint becomes damaged because it lacks the sensation normally provided by nerve endings in joint linings and surrounding muscles.

Schmidt acknowledged that such a complication could develop in his patient but said such joint damage is often painless and need not impair function. He said the transplant will at any rate leave Lazarchick with a healthy femur and tibia, which make it possible to replace the knee joint with an artificial one if necessary.