How is a craniotomy performed? Operations to create an opening in the skull date from antiquity. Indigenous peoples of South America and other regions performed trephinations with crude instruments to open the skull. Some individuals survived as there is evidence of bone healing in trephined skull specimens. It is unlikely informed consent was obtained.

Here is the step-by-step process of a fronto-temporal craniotomy to treat a posterior communicating artery aneurysm. Please don't try this at home.

craniotomy - skull
An opening through the frontal and temporal bones is made by making holes in the bone and connecting them with a side cutting saw.
craniotomy - surgery
The patient is anesthetized and the skin incision is drawn. The skin is prepped and draped for sterility.
craniotomy - surgery
The skull is draped.
craniotomy - surgery
The scalp has been pulled upward and the temporalis muscle retracted to expose the skull.

The bone flap has been removed to expose the dura, which lines the inner skull and covers the brain.
craniotomy - surgery
The dural is opened and the frontal lobe retracted backwards with the metal retractor to expose the arteries at the base of the brain.

The left retractor retracts the frontal lobe and the right retractor retracts the temporal lobe exposing the optic nerve (yellow and the internal carotid artery (red).

A clip is placed across the neck of the aneurysm which originates from the carotid artery.

carotid artery and posterior communicating aneurysm
craniotomy - surgery
two clips obliterate the aneurysm

All bleeding is controlled, and the dura is closed. The bone flap is secured to the surrounding skull by small titanium plates and screws. Finally, the scalp is closed with sutures and staples. Craniotomies may last several hours to accomplish a satisfactory result.