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PHILADELPHIA (CBS) -- Children's Hospital of Philadelphia is getting more recognition for its historic surgery to separate conjoined twins. Abby and Erin Delaney are 2-and-a-half now and back home in North Carolina where they're busy little toddlers.

"They talk a little, say 'mama, dada' and they're starting to have more dedicated babble," says mother, Heather Delaney.

It's been an amazing journey from this. They were born conjoined at the top of their heads.

Their separation surgery in June of 2017 at CHOP is highlighted in a new report published on Wednesday in the New England Journal of Medicine, which describes the innovative devices and intricate high-tech planning surrounding the 11-hour operation.

Neurosurgeon Dr. Greg Heuer was on the team that separated the Delaney twins. One of the biggest innovations used in the case -- a distracter -- inserted when the twins were 3 months old before the actual separation surgery.

"We sort of slowly pushed them apart and changed the anatomy where the two were connected and then that allowed us to do the separation around 10 months of age," said Heuer. "It made it a little bit safer and improved our outcome we think."

Doctor Reveals New Details About Historic Surgery Separating Conjoined Twins At CHOP

And a computer navigation system helped the surgical team map the operation.

"The most difficult part for these girls where they shared some really important big blood vessels, so having to be able to separate those and having the brain recover after we did the separation was really the hardest part," said Heuer.

It's been a long and difficult recovery with several setbacks but the girls are exceeding expectations.

The twins are getting a lot of therapies, including physical, occupational and speech -- and getting stronger every day.

"Now they smile so much, it is the best thing in the world to see all the teeth come out," said Delaney. "They grin and giggle, it just makes everything so worth it."

In the next few years, the sisters will need additional surgery to close the openings in their skulls.

The girls are among the youngest twins joined at the head to be successfully separated.