Seven-month-old twins who were connected by their pelvis and lower spine were separated by doctors at Le Bonheur Children's Hospital in Memphis, Tenn., doctors said,.

The twins, Joshua and Jacob Spates, had separate hearts, heads and limbs.

Bill Warner, an orthopedic surgeon in the hospital's spine clinic, said the boys have health problems that will require continuing treatment, but he expects Joshua will be able to walk with braces and hopes that Jacob will do the same.

The outlook is bright as far as them being functional in the community, Warner said. Joshua and Jacob were classified as pygopagus twins.

Anesthesiologists needed to flip the babies over without getting their wires tangled, so they practiced on a pair of Cabbage Patch dolls sewn together, which caused a great deal of laughter but proved to be the key to surgical success, the Associated Press reported.

Conjoined twins are rare, estimated to range from 1 in 50,000 births to 1 in 100,000 births.

Two contradicting theories exist to explain the origins of conjoined twins. The older theory is fission, in which the fertilized egg splits partially.

The second and more generally accepted theory is fusion, in which a fertilized egg completely separates, but stem cells (which search for similar cells) find like-stem cells on the other twin and fuse the twins together. Conjoined twins share a single common chorion, placenta, and amniotic sac, although these characteristics are not exclusive to conjoined twins as there are some monozygotic but non-conjoined twins that also share these structures in utero.

Surgery to separate conjoined twins may range from relatively simple to extremely complex, depending on the point of attachment and the internal parts that are shared. Most cases of separation are extremely risky and life-threatening. In many cases, the surgery results in the death of one or both of the twins, particularly if they are joined at the head.