When Incan healers scraped or cut a hunk of bone out of a person's head, they meant business. Practitioners of this technique, known as trepanation, demonstrated great skill more than 500 years ago in treating warriors' head wounds and possibly other medical problems, rarely causing infections or killing their patients, two anthropologists find.

Trepanation emerged as a promising but dangerous medical procedure by about 1,000 years ago in small communities near the eventual Inca heartland in Peru's Andes mountains, say Valerie Andrushko of Southern Connecticut State University in New Haven and John Verano of Tulane University in New Orleans. Incan healers later mastered certain trepanation methods, performing them safely and frequently.

"Far from the idea of 'savages' drilling crude holes in skulls to release evil spirits, these ancient people were highly skilled as surgeons," Andrushko says.

The researchers' new investigation, published online April 3, will appear in the American Journal of Physical Anthropology.

Prehistoric trepanation in this part of South America consisted of four techniques, the scientists say. Practitioners cut out squares of bone, bored holes in the skull, scraped away bone to create an opening or made circular incisions to remove a plug of bone. Inca surgeons specialized in the latter two methods. Excavations, however, have not yielded trepanation instruments.

In pre-Inca times, only one-third of skull surgery patients survived the procedure, as indicated by short- or long-term healing around cranial openings. Survival rates rose to between 80 and 90 percent during the Inca era, from A.D. 1400 to 1532. Few skulls showed signs of infection near surgical holes.

Most recipients of skull surgery displayed one trepanation hole. A substantial minority exhibited from two to seven such openings.

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