Still, Binello calls the survival rate of trepanations in Peru "astonishing." "It's a credit to what these ancient cultures were doing," she says.

Some of the differences in survival rates may be due to the nature of the patients' injuries before the surgery, says Emanuela Binello, a neurosurgeon at Boston University who has studied trepanation in ancient China. "The trauma that occurs during a modern civil war is very different from the kind of trauma that would have been happening at the time of the Incas," she says. Many Civil War soldiers suffered from gunshot and cannonball wounds that were quickly treated in crowded and unsanitary battlefield hospitals, which promoted infections.