Jie-jie was born with two left arms and doctors took off the one that had lain across his chest. But the chief surgeon, Chen Bochang, said they were unsure how well his remaining left arm, which is further up on his shoulder, would respond to therapy.

"The surgery really went much better and smoother than expected because we found the nerves and blood vessels for the arm were formed just as they would be for a normal arm," Dr Chen told reporters following the three-hour operation.

He said Jie-jie would require long-term physical therapy to gain function in his remaining left hand, which has no palm and flexes in either direction.

"We're hoping to exchange information with doctors who've dealt with similar cases anywhere in the world," added Dr Chen, head of the orthopaedics department at Shanghai children's medical centre. "This is so rare that we have virtually no information to go on."

Jie-jie's parents are poor farmers from Anhui province, west of Shanghai. His mother said she was relieved the surgery was over but worried for her son's future.

"I'm very happy but there are some fears that I just can't let go of," she said. "I worry about how he will grow, whether this will have a big impact on his growth."

Dr Chen said doctors did not know what caused the growth of extra limbs, although many speculated that they began as limbs of a conjoined twin that never developed. He ruled out environmental factors such as pollution, saying that couldn't fully explain similar cases elsewhere. No reliable figures existed on the frequency of such cases, Dr Chen said, partly because many foetuses with more than four limbs were aborted or miscarried.

In most cases where the foetus survived, it was clear which limb was less developed and should be amputated. Jie-jie's case was especially rare because both left arms were almost equally well developed. He said doctors decided to remove the one closest to the chest because it did not fully extend.