**5.3 Craniotomy**

The bone opening should be entirely adapted to the location, size, and morphology of the aneurysm. It must be able to reveal the Circle of Willis and be spacious enough to allow the exploration of the main blood vessels. The most commonly used craniotomy for aneurysms of the anterior circulation and of the basilar apex is the frontolateral approach as described by Samii, the classical pterional opening being used in MCA aneurysms and for contralateral clipping in the case of multiple aneurysms. A burr hole is placed at the orbitofrontal angle (keyhole), being careful not to open the orbit or the frontal sinus (if it is large enough to reach this point). The craniotome can then be used to complete the flap. Additional burr holes may be needed. In the classical pterional approach, the sphenoid wing should be drilled as close as possible to the anterior cranial fossa. In the event of a tensioned dura, slight elevation of the head and opening the lumbar drainage will result in proper brain relaxation.
