The surgeons who led the June separation of Abby and Erin Delaney at Children's Hospital of Philadelphia launched the multistage process with a technique never tried in such a case: They got the twins to begin disconnecting their fused heads on their own, long before the complete-separation surgery.

Distraction osteogenesis has been used for 20 years to correct facial and cranial deformities, but no one had ever used it to help separate craniopagus twins, the rarest type of conjoined siblings. It capitalizes on a biological paradox; when a bone is cut and the two ends are carefully, steadily tractioned apart, the "distraction" spurs bone growth as the tissue seeks to heal.

A year ago, when the twins were just three months old and weighed about five pounds each, CHOP reconstructive surgeon Jesse Taylor and neurosurgeon Gregory Heuer cut the infants' skulls at the juncture point. They installed custom-made, padded distraction devices, then sewed the scalps back together to cover the bone.

Over the next month, the infants' skulls were pulled apart about four-hundredths of an inch per day. The process was painless for the babies, who wound up with a 1-inch band of separation, as well as extra bone and skin that helped with later reconstruction.

The twins also may have developed new cranial blood vessel pathways.

"We knew that the more we could have them remodel their skin, bone, and blood vessels, the better," Heuer said in a recent interview.

"By releasing the bone that connected the twins at an early stage, it probably did alter their vascular connection," said Taylor, who proposed the novel use of bone traction. "There's no way to prove it. It's a hypothesis."

Cranial blood vessels are a crucial obstacle to separation, even for twins like Abby and Erin, who shared only a small spot of brain tissue. One twin's blood supply usually is dominant, and as blood drains from the heads to the hearts and other organs, the dominant twin becomes a biological crutch for the weaker twin. Erin was a crutch for Abby, who began showing signs of vascular stress while still in their mother's womb.

Given that craniopagus twins occur once in about 2.5 million births and only 60 separations have been attempted since 1952, there is little opportunity to test ways to reduce the dangers.

The multistage approach

The world's most experienced craniopagus separation surgeon is James T. Goodrich, director of pediatric neurosurgery at Children's Hospital at Montefiore in New York City. He pioneered an approach that involves operating on each quadrant of the brain over about 10 months. He believes this four-stage separation reduces bleeding and brain swelling, allows time for new venous blood drainage to develop, and averts brain damage.

His original multistaged twins, Carl and Clarence Aguirre of the Philippines, initially were celebrated as the first craniopagus pair ever to avoid neurological damage. But today, Carl requires a wheelchair and has permanent disabilities, including a seizure disorder. Both boys, now 15, still wear protective helmets, according to a recent CNN story.

CNN has followed Goodrich's most recent craniopagus twins, Anias and Jadon McDonald, separated in October 2016 and now 2 years old. The boys have been plagued by wound-healing problems, including cysts, swelling, and infections where surgeons performed the four craniectomies – cutting through scalps, skull, and dura, the protective brain membrane.

‘Lucky or thankful’

Abby and Erin, now 15 months old, have had no such wound complications, partly because surgeons were able to reach their blood vessels by selectively widening the 1-inch channel of separation created by the distraction process. It's too soon to fully assess any neurological problems.

Taylor, who is 43, is circumspect about the innovation.

"The word I would use is lucky or thankful," he said. "You feel a tremendous sense of responsibility to push the envelope and move the surgical science forward. But there's no animal model for this. You have to put it to the test in actual kids. Heather and Riley [the twins' parents] had to take this leap of faith that we knew what we were talking about."

The twins' 11-hour surgery by a 30-member team on June 7 — CHOP's first craniopagus separation — was at times harrowing, Heuer and Taylor readily admit.

Originally, they planned to do two surgeries several weeks apart, culminating with the riskiest step — cutting the sagittal sinus, the major conduit for blood and cerebrospinal fluid.

"But once we separated the first side, it went so well, we decided it was better to continue and not wait," Heuer said.

As expected, the blood loss was life-threatening for both twins, but especially for Abby, who got less of the sagittal sinus after it was divided. Her entire blood volume – about a pint – had to be replaced several times.

"In an adult, blood loss makes the blood pressure drop," Heuer said. "But in a baby, if low blood pressure develops, it's almost too late. So it really takes doctors who are used to lots of blood loss. They know the recipe to put blood products back in, and how to stay ahead of the losses.

"If Abby's bleeding had continued much longer," he added, "we could have lost her. There also comes a point where you have to think about saving one of the twins, and not the other, to stop the blood loss. That was a discussion we had with the parents beforehand."

Heuer, 45, also confided a personal trauma to the Delaneys: Twelve years ago, the neophyte neurosurgeon and his wife lost twins five months into the pregnancy because of untreatable cerebrospinal birth defects. The couple went on to have a healthy daughter, but the loss helps Heuer understand parents' anguish.

"I find solace knowing some good has come from something awful," he said.