Object Increasing use of endovascular techniques is shadowing the most recent developments of microvascular neurosurgery. This paper is focused on our large experience on open microsurgery of intracranial aneurysms. Our strategy is to perform simple, clean, and fast microsurgery while preserving normal anatomy. Methods Microneurosurgical techniques, initially introduced by Prof. Yasargil, have been modified by the senior author (JH) during surgery on more than 3000 aneurysm patients at two neurosurgical departments in Finland, Kuopio and Helsinki, which have altogether treated over 10000 aneurysm patients since 1936. Here we review our experience in treatment of both anterior and posterior circulation aneurysms. Results Most of the anterior circulation aneurysms are treated using the lateral supraorbital (LSO) approach, a less invasive frontally located modification of the classical pterional approach, while only distal anterior cerebral artery aneurysms (6 % of our patients) are operated using interemispheric approach. In the posterior circulation, basilar bifurcation aneurysms are approached either via the subtemporal or the transsylvian route depending on the height of the basilar tip, while most of the vertebral and posterior inferior cerebellar artery (PICA) aneurysms, if located at least 10 mm above the foramen magnum, can be treated by simple lateral suboccipital craniotomy without far lateral extension. The midbasilar region is the most difficult to approach by open surgery, and extensive skull base approaches are needed. Conclusion In our experience, microsurgical clipping is an effective and long-lasting method for treating intracranial aneurysm. With proper treatment strategy, microsurgical technique, and adequate training complication rate can be kept low.

Key Words lateral supraorbital approach, microsurgical technique, cerebral aneurysm, anterior circulation, posterior circulation, skull base surgery Running title: Principles for aneurysms surgery