Inca surgeons in ancient Peru commonly and successfully removed small portions of patients' skulls to treat head injuries, according to a new study.

The surgical procedure—known as trepanation—was most often performed on adult men, likely to treat injuries suffered during combat, researchers say.

A similar procedure is performed today to relieve pressure caused by fluid buildup following severe head trauma.

Around the ancient Inca capital of Cuzco, remains dating back to A.D. 1000 show that surgical techniques were standardized and perfected over time, according to the report.

Many of the oldest skulls showed no evidence of bone healing following the operation, suggesting that the procedure was probably fatal.