Muhammad Ibrahim's skull was reconstructed on Tuesday at Medical City hospital in Dallas, Texas, USA, using pieces of bone that were removed during the surgery in 2003 to separate him from his brother, Ahmad.
Only skin and tissue covered large portions of their brains after the separation surgery, requiring the twins to wear protective helmets.
Dr Kenneth Salyer, a craniofacial surgeon, said after the four-hour procedure that it would take at least three months for the bone to solidify.
But it would take another few months before the skull can withstand children's activities. Meanwhile, Muhammad must continue wearing the helmet.
The Dallas hospital doctors said
Ahmad's progress was a slow
Ahmad had a similar procedure in February, but Salyer said he had not been regenerating bone as rapidly as doctors would have liked.
The boys were born connected at the top of their heads in southern Egypt in June 2001.
A year later, they were brought to Dallas by Salyer's World Craniofacial Foundation, a nonprofit organisation that helps children with head and face deformities.
The surgeries were expected to be the twins' last major operations. They are scheduled to return to Egypt with their parents in the autumn.