**2. History**

In the early years of endovascular techniques, the main method for the treatment of wide-neck aneurysms was surgical. Attempts at embolization presented significant risks of coil herniation, migration, parent artery occlusion and poor mid-term morphological results with high recurrence rates. A great effort has been made by engineers and manufacturers to develop coils that present a better arrangement inside the aneurysm sac and fulfil two important conditions: a good apposition with the aneurysm wall, and a stable three-dimensional conformation, so that loops do not herniate through the aneurysm neck.

The first issue was approached by the development of coils with a geometry intended to fill the aneurysm sac, even if it is irregular. A good apposition with all regions of the cavity may improve the aneurysm embolization ratio and increase the stability of the coil mass, preventing migration. The second issue was approached through the development of coils with a memory of their three-dimensional shape, so that they may be used to create a

© 2012 Maldonado and Bonafé, licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

relatively predictable cage that would keep the subsequent coils inside. As one may imagine, the above properties are relatively antagonistic. A coil that penetrates irregular spaces and has a good position to the aneurysm wall is also a coil that may herniate through a large aneurysm neck. In this context of technical difficulty, balloon and stent-assisted techniques have been used to provide protection for the parent artery as well as to treat coil mass herniation.

Intracranial stents also serve as scaffolding for neo-endothelization, providing additional reduction of the flow into the aneurysm. Consequently, their use improves intrasaccular thrombosis and decreases the risk of recanalization.

Even if these concepts seem attractive, manufacturers were rapidly confronted with technical difficulties, such as the narrowness and fragility of the intracranial vasculature, the need to navigate tortuous vessels, and the obligation to provide materials that were thin enough so that a microcatheter could be placed simultaneously in the vasculature.

The first case of intracranial stenting for treating a brain aneurysm was reported by Higashida et al. in 1997 (Higashida et al., 2005). In that occasion, the authors used a balloonexpandable cardiac stent in combination with Guglielmi detachable coils to treat a fusiform aneurysm of the vertebrobasilar junction. At that time, other authors had already attempted the placement of stent and coils in a fusiform aneurysm in an experimental context in pigs. Soon after, different groups reported a number of strategies using a combination of balloonmounted stents and coils. In 2000, the use of stents for managing coil migration during the treatment of wide neck aneurysms was reported (Fessler et al., 2000, Lavine et al., 2000) and the case series became progressively larger.

The first stent specifically designed for the intracranial area to obtain Food and Drug Administration (FDA) approval was the NeuroformTM (Boston Scientific Corporation, Natick, USA). The device was approved for 'humanitarian device exemption' in 2002. This means that its use was complicit to the additional approval of an Institutional Review Board and was supposed to be limited to use on no more than 4000 individuals per year in the United States of America (USA).

Outside the USA, especially in Europe and in the context of clinical research, other stents became rapidly available. That was the case with the LeoTM (Balt, Montmorency, France), the first self-expanding closed cell design stent released in Europe in 2003, and then the EnterpriseTM (Cordis Neurovascular I., Miami Lakes, USA), which was approved by the FDA in the US in 2007.

Flow diverters are the last technical advances bringing the concept of 'reverse remodeling' for intracranial aneurysm treatment. SilkTM (Balt, Montmorency, France) and PipelineTM (Chestnut Medical Technologies Incorporation, Menlo Park, CA, USA) are in this category. These devices are intended to exclude the aneurysm sac from the parent artery by creating significant flow disruption, so that blood significantly stagnates inside the aneurysm sac and thromboses.
