The development of flow diverting stent

Intracranial aneurysms are pathological dilatations of cerebral arteries, leading to an increased risk of rupture with possibly lethal consequences. Before the therapy of neuroradiologic inter-vention was widely accepted, surgical clipping was the main treatment to prevent the serious complication of rebleeding for the intracranial aneurysms. The endovascular neurosurgery-interventional neuroradiology begins with the first catheterization of intracranial vessels by Luessenhop and Valasquez in 1964 [25,44]. They successfully insert a silastic microcatheter into brain arteries. In the mid-1960s, a new microcatheter called para-operational device(POD) was introduced by Frei, Yodh, Driller, Montgomeryet al[25]. They utilized magnetic field to pull and bend the micromagnet tipped microcatheter, which can inject embolic substances into the arteriovenous malformations to perform endovascular electrothrombosis. However, POD did not become popular due to the later improvement of endovascular microcatheters.

The subsequent development of the treatment of endovascular pathology for cerebral aneurysm can be classified into three stages: balloon era, coil era and flow diverter era.

1.1.1 Balloon era

In 1974, an English article from a Russian neurosurgeon named Fedor Serbinenko appearing inJournal of Neurosurgeryastounded the field of endovascular neurosurgery [61]. He treated more than 300 patients with detachable and non-detachable balloons [25] which were made in a small laboratory [71]. The idea is to fill the balloon with solidifying substance and navigate it into the aneurysm sac. In his 1974 article, Serbinenko suggested the treatment of cerebral aneurysm by employing two balloons, one distal and the other proximal to the orifice of the aneurysm [71]. The process is illustrated in Figure1.1. First, two balloon catheters are delivered

Fig. 1.1 The process of the deployment of balloon [65]

into the carotid artery(Fig.1.1A). The non-detachable balloon is in front of the detachable one and would guide it into the aneurysm sac. After the detachable balloon is inflated with the silicone polymer, the non-detachable balloon presses it to be positioned in the sac and aids it to be detached from the catheter (Fig.1.1B). Finally, the aneurysm sac is deployed with the detachable balloon (Fig.1.1C).

Over the next 15 years, Serbinenko’s concept was widely applied in neuroendovascular centers [30,18]. With the later development of coils, it was gradually supplanted in the early 1990s [25].

1.1.2 Coil era

In 1988 and 1989, Hilalet alfirst introduced the pushable coil for the endosaccular treatment of aneurysms, but these coils were stiff [25]. The endovascular aneurysm treatment was revolu-tionized by Guglielmiet alwho proposed an electrolytically detachable platinum coils [27,26]

in 1991, as illustrated in Fig.1.2a. The coil which is soft, retrievable and detachable, received approval from US Food and Drug Administration in 1995. In subsequent years, numerous kinds of coils were developed with the improvement of softness, length, shape as well as other aspects.

The standard coil embolization technique is limited by its inability to occlude wide-neck aneurysms [7]. Moret et al proposed a balloon assisted, so called remodeling technique, consisting of a temporary balloon to avoid coil protrusion into the parent artery [50]. The non-detachable inflated balloon is used to block the aneurysm neck during coil placement [74], as shown in Fig.1.2b.

1.1 The development of flow diverting stent 3

(a) Simple coiling (b) Balloon assisted coiling (c) Stent assisted coiling Fig. 1.2 Three types of coiling^a.

ahttp://www.neuroradiologist.com.au/services/intracranial-brain-aneurysm-sydney/

Another similar solution for the treatment of the wide-neck aneurysm is the stent assisted coiling, which became popularized in early 2000s [72]. The first self-expandable stent for intracranial was the Neuroform^TMdevice¹[70]. Stents are first delivered and positioned in the parent artery across the aneurysm neck. Then they act as a scaffold when coils are delivered through a microcatheter navigated into the aneurysm through the stent struts [25]. Fig.1.2c illustrates the supporting the stent assisted coiling .

1.1.3 Flow diverting stent era

Stent was first used as a scaffold to prevent the protrusion of coils into the parent vessel. With the development of technology, intracranial flow diverting stent can be used independently for the endoluminal parent vessel reconstruction, as seen from Fig1.3. It was found that the device deployed in cerebral aneurysm produces hemodynamic and biological effects [70]:

• Flow redirection: the stents could divert flow "away" from the aneurysm "back" into the parent vessel [20].

• Sac thrombosis: clinical observations of broad-based saccular intracranial aneurysms show spontaneous sac thrombosis after stent placement [70,68].

Theoretical in vitro studies suggested that stent porosity is the most important metrics [5], which is defined as thearea percentage of metal over the neck. Low porosity stents lead to a reduction up to 90% of the original flow inside the aneurysm sac [33,43].

1Boston Scientific, Fremont, CA, USA

(a) Flow diverter (b) Stented aneurysm

Fig. 1.3 Flow diverter stented in aneurysm

Among the multiple choices, research and clinical studies have been focused on two mainly used flow diverting stent (FDS) used in medical center. Pipeline embolization device(PED)²is a self-expanding, flexible cylindrical mesh-like device [53,24]. It is interwoven by 48 struts with a diameter of 25µm. The diameter of the stent is between 2.5 and 5mmand the length is between 10 and 35mm. Silk³is another a self-expanding stent [53,24], which is also braided by 48 struts with a diameter of 35µm, but the material is nitinol. The diameters is 2−5mm and the length is 15−40mm.