The procedure known as trepanation, in which a hole is scraped or drilled in the skull, is an ancient form of neurosurgery that has been performed since the late Stone Age. Exactly why ancient peoples performed trepanation has remained a matter of debate: some researchers argue that it was performed for medical reasons, as it is today, while others believe it was done for magical or religious reasons.

A new study by two American anthropologists now provides evidence that the Incas performed trepanation to treat head injuries; that the procedure was far more common than was previously thought; and that the Incan practitioners of trepanation were highly skilled surgeons with a detailed knowledge of the anatomy of the skull.

Valerie Andrushko, of Southern Connecticut State University in New Haven, and John Verano, of Tulane University in New Orleans, examined a total of 411 skulls excavated from 11 burial sites in the prehispanic Inca capital of Cuzco in the southern highlands of Peru, where the first trepanned skull specimen was found by E. George Squier in 1865.

Of these 411 skulls, 66 exhibited perforations of varying shape and size. More than half were circular, but some were oval or irregularly circular, and one (above) was rectangular. The diameters of the circular holes ranged from approximately 0.3 - 7.3 cm. In all, 109 holes were observed in the 66 skulls, and there was one individual with 7 trepanations.

Andrushko and Verano argue that the Incas performed trepanation primarily to treat head injuries incurred during battle, because the holes are most often found at the front of the skull to the left, consistent with injuries caused by a right-handed opponent during face-to-face combat, and because adult males are overrepresented in the sample. The procedure was evidently used to treat mastoiditis (an infection of the region of the temporal bone behind the ear) as well.

The authors also show that the success rate of the procedure improved with time, as the Inca empire progressed and made advances in medicine. The earliest specimens, dated to around 1,000 A.D., showed no signs of bone growth around the perforations, suggesting that the procedure was often fatal. But specimens dating to around 400 years later suggest a survival rate of around 90%.

By that time, the procedure appears to have been stabdardized, as its practitioners operated only on certain parts of the skull, and avoided other areas, which were likely to result in damage to the meninges (the thick membranes which envelop the brain and spinal cord) or the blood vessels found within the layers of the outer membrane. 

The later specimens suggest that there was a very low frequency of infection, and lead author Andrushko says that balsam, and plants containing compounds called saponins, which have soap-like properties and are antiseptic, may have been used to reduce the risk of infection, and that coca, wild tobacco and maize beer may have been used to alleviate the pain.

Thus, this new study provides strong evidence that the Incas performed trepanation for specific medical conditions, and may finally settle the debate about why people in ancient civilizations performed trepanation. It also suggests that the Incas were highly skilled surgeons with a detailed knowledge of cranial anatomy and an awareness of the medicinal properties of various species of wild plants.

Related:

• An illustrated history of trepanation
• Graphic trepanation film clip
• Ancient Greek neurosurgery

Andrushko, V. A. & Verano, J. W. Prehistoric trepanation in the Cuzco region of Peru: A view into an ancient Andean practice. Am. J. Phys. Anthropol. doi: 10.1002/ajpa.20836 [Abstract]