https://phys.org/news/2018-06-holes.html

But, according to a new study led by the University of Miami Miller School of Medicine's David S. Kushner, M.D., clinical professor of physical medicine and rehabilitation, trepanation was so expertly practiced in ancient Peru that the survival rate for the procedure during the Incan Empire was about twice that of the American Civil War—when, more three centuries later, soldiers were trepanned presumably by better trained, educated and equipped surgeons.

"There are still many unknowns about the procedure and the individuals on whom trepanation was performed, but the outcomes during the Civil War were dismal compared to Incan times," said Kushner, a neurologist who has helped scores of patients recover from modern-day traumatic brain injuries and cranial surgeries. "In Incan times, the mortality rate was between 17 and 25 percent, and during the Civil War, it was between 46 and 56 percent. That's a big difference. The question is how did the ancient Peruvian surgeons have outcomes that far surpassed those of surgeons during the American Civil War?"