Benjamin S. Carson Sr., M.D., director of pediatric neurosurgery at The Johns Hopkins Hospital, led the team that separated 11-month-old Zambian boys Luka and Joseph Banda, who were born as type 2 vertical craniopagus twins--joined at the head but facing opposite directions. The separation was performed December 31.

Carson has done two previous conjoined twin separation surgeries, one in South Africa and one at Hopkins. He credited the current success to "surgical rehearsals" with a computerized, three-dimensional virtual "toolbox" that allowed him to "see" computerized reconstructions of the twins' brains in spectacular detail. Holding mock instruments, Carson was able to peel away layers of brain tissue, probing and separating blood vessels. "One actually gets all the surface topography in mind and practice before you really do the operation," says Carson.

The virtual toolbox, called BrainBench, was developed by CieMed, a joint project between Hopkins and the National University of Singapore established in 1994. BrainBench combines many two-dimensional images of a patient's brain obtained by CT or MRI into a single, three-dimensional virtual brain. The effect is like recombining slices of bread back into the original loaf.

The workbench station has a computer monitor suspended so it projects brain images down onto the work area. A physician puts on special glasses and looks down through a chest-high glass screen at the image of the patient's brain. The glasses let the surgeon see the brain in three dimensions and BrainBench's computer program lets the operator manipulate the image of the 3-D brain image as in a real operation.

When Carson later operated on the Banda twins, he found that their skulls were like one long tube, with no separation between one brain and the other. Even though the twins' brains touched, they shared no gray matter. They did, however, share intricate blood vessels, which flowed into each child's brain. Surgeons had to sever each vessel, decide which boy would receive it, and create a new connection for blood flow and drainage.

The surgery was performed at South Africa's Ga-Rankuwa Hospital, 30 miles north of Pretoria. Neurosurgeon Sam Mokgokong, who invited Carson to lead the team, said the children continue to improve. The boys will wear helmets to protect their heads from injury and will undergo reconstructive surgery to form a bony covering that will create their individual skulls.

"We won't know for certain about the presence or absence of deficits until the twins are older," says Carson, "but we are thrilled at their progress so far."

This is the third time that Carson, 46, has separated conjoined twins. In 1994, he traveled to South Africa at the invitation of Mokgokong to separate the Makwaeba girls at the Pretoria hospital. The 7-month-old twins, who also were joined at the head, died within hours of the surgery because they were symbiotic, sharing heart and kidney functions.

Prior to that, in 1987, Carson and a 70-member team successfully separated 7-month-old boys from Germany. The Binder twins were joined at the backs of their heads and shared the sagittal superior sinus, the major blood drainage system of the brain. This was the first time that doctors separated conjoined twins using circulatory bypass, induced hypothermia and deliberate cardiac arrest to preserve brain function during surgery. The children survived; however, they were left with severe neurological deficits.

Conjoined twins form when a single fertilized egg fails to divide completely to create two distinct entities. Twins joined at the head occur approximately once in every 2 million live births.

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