11.1 Introduction

Our understanding of intracranial vascular pathologies and interactions with therapeutic devices has been rapidly coevolving over the past decade. Computational and experimental models have demonstrated the importance of angioarchitectural and hemodynamic factors on the formation and eventual rupture of intracranial aneurysms (IAs), and this knowledge has in turn informed device design in important ways. Intrasaccular occlusion of aneurysms has long been practiced for aneurysmal lesions, but the geometrical constraints of the technique left certain lesions, such as wide-necked, giant, and fusiform aneurysms, without suitable endovascular options. To address these lesions, technology and technique have pushed the boundary, with progression from balloon-assisted coiling to stent-assisted coiling. Studies of these techniques yielded intriguing insights into mechanisms of action: although stents were initially intended to simply isolate a coil mass from the parent artery, evidence suggested a secondary effect whereby blood flow was redirected, changing the hemodynamic factors that propagated aneurysmal growth. ¹ , ² Standalone stents, whose primary purpose was modification of flow, were hypothesized, developed, proven in animal models, and subsequently shown efficacious in humans. ³

Flow-diverting stents (FDSs) are a paradigm shift in our approach to the treatment of IAs. Flow diverters have the basic form of other intracranial intraluminal stents, but with tighter meshes this additional metal coverage serves to divert blood flow from the aneurysm sac toward the downstream artery. This intraluminal reconstruction results in flow reduction outside of the vessel lumen. ⁴ Over time, this alteration of inflow and outflow jets at the parent artery–aneurysm sac interface induces aneurysm thrombosis. ⁵ , ⁶ FDSs further function through a secondary mechanism: neointimal overgrowth. The dense mesh of the stent wall serves as a scaffold for ingrowth of cells that ultimately covers the stent, reconstructing the parent artery wall and eliminating the aneurysm–parent vessel interface. Neointimal overgrowth, being a latent process, has been shown to spare the origin of branching perforators, likely through a demand mechanism. ⁷ , ⁸ In the short term, aneurysmal thrombosis may result in inflammation, perianeurysmal edema in surrounding brain parenchyma, and even transient neurological findings. ⁹ With time, however, the aneurysm dome tends to shrink and collapse around the device construct, ultimately eliminating local mass effect and related symptoms. ⁴ , ¹⁰

Flow-diverting stents have been used in clinical practice since 2007 for IA configurations that were deemed poorly amenable to traditional coiling. ¹⁰ , ¹¹ , ¹² These include fusiform/dissecting IAs, ¹³ giant/large internal carotid artery IAs, ¹⁴ , ¹⁵ carotid-ophthalmic aneurysms, ¹⁶ as well as blisterlike aneurysms. ¹⁷ Fusiform IAs are typically circumferentially diseased vessels that are difficult to reconstruct with previous techniques, given that there is no good tissue to clip or sew to, and the fusiform segment often cannot be sacrificed or embolized without compromising parent vessel flow. FDSs smoothly reconstruct an endothelial-covered channel in continuation with the parent artery. ⁷ Blisterlike aneurysms are small, broad-based, shallow pseudoaneurysms commonly located at nonbranching sites; they are difficult to treat with conventional endovascular or microsurgical techniques. ¹⁷ , ¹⁸ As experience with flow diverters has increased, practitioners have come to appreciate that flow diversion techniques cause aneurysms to occlude over time, with occlusion rates demonstrated to increase as far as 6 to 12 months after deployment of an FDS. ¹¹ , ¹² This mechanism often seems to be better tolerated physiologically, as well.

We will devote the rest of this chapter to a discussion on the treatment of IAs with Silk flow diverters (SFDs; Balt Extrusion, Montmorency, France), reviewing the published literature and different technical nuances.

11.1.1 Nitinol Alloy

Most modern stents, including the Silk, are constructed using a shape-memory alloy called nitinol (nitinol is a tradename for ni ckel ti tanium N aval O rdnance L aboratory), first introduced as a material for stents in 2003. ¹⁹ Nitinol offers shape-memory effects particularly suited for self-expanding stents (SESs), ¹⁹ , ²⁰ , ²¹ like the Silk, with a unique balance between the desirable features of both conventional stent materials and natural materials (hair, bone, tendon). The elastic deformation of conventional stent-forming metals is limited to approximately 1% strain, and stent elongation typically increases and decreases proportionally with the applied force. On the other hand, natural materials (like hair, tendon, and bone) can be elastically deformed, in some cases up to 10% strain in a nonlinear way ( Fig. 11.1). ²² Nitinol SESs do not require postdeployment heating, imparting a superelastic quality to the stent, thus being crush-recoverable. ¹⁹ , ²⁰ , ²¹ After deployment, these stents exert a gentle chronic outward force and are generally more physiologically compatible than balloon-expandable stents. Despite the high nickel content of nitinol, its corrosion resistance and biocompatibility is equal to that of other implant materials. ¹⁹ , ²⁰ , ²¹

11.1.2 Silk Device Parameters

The SFD received Conformité Européenne (CE) mark approval in 2008. It is a closed-cell mesh cylinder composed of 48 braided 35-µm microfilament strands, 44 nitinol, and 4 platinum. This construct results in metal surface coverage ranging from 35 to 60% with a pore size of 110 to 250 µm ² , depending on the degree of compression/elongation of the stent at deployment. ²³ , ²⁴ The SFD has two flared ends with four sinusoidal radiopaque platinum markers running the length of the stent. The SFD is deployed from its delivery microcatheter (Vasco + 21, Balt Extrusion) through a combination of pushing the delivery wire and retrieving the microcatheter, which allows the operator to fine-tune placement of the stent. The delivery procedure is similar to that for other self-expanding intracranial stents; however, in contrast to others, the SFD allows resheathing and repositioning, even when as much as 90% of its length has been deployed. ²⁰ , ²¹ , ²² , ²³ , ²⁴

The SFD is available in a wide range of diameters (2–5.5 mm) and lengths (15–40 mm) for the treatment of aneurysms of different sizes and locations. The shapes, forms, and available sizes are presented in Fig. 11.2. The original Silk stent, although quite flexible, had a relatively lower radial force than other closed-cell stents, such as the Enterprise (Codman, Raynham, MA), that resulted in the potential for stent migration and even vessel occlusion in stenotic vessels. Adjunctive stenting with other stents with greater radial force was thus sometimes performed (discussed later). ²⁵ , ²⁶ The Silk Plus (Silk +) stent, characterized with 15% more radial force, addressed this point and negated the need for additional stents.

11.2 Reported Obliteration and Complication Rates—Literature Review

A literature review of SFD series is presented in Table 11.1. ⁴ , ¹⁴ , ¹⁷ , ²⁴ , ²⁶ , ²⁷ , ²⁸ , ²⁹ , ³⁰ , ³¹ , ³² , ³³ , ³⁴ , ³⁵ , ³⁶ , ³⁷ Although the ultimate goals of treatment are to prevent hemorrhage and rupture-related morbidity and mortality as well as the alleviation of local aneurysm-related mass effect, these events are, fortunately, rarely encountered during the course of typical clinical follow-up. As a result, aneurysm obliteration and recanalization rates are typically employed as surrogates for aneurysm rupture rate/risk. ³ A review of the data presented in Table 11.1 shows that deployment failure was reported in approximately 3% of cases and deployment difficulty in an additional 11% of cases. Complete aneurysm obliteration was reported in 59 to 90.9% of cases, varying depending on the center, experience of the interventionists, and date of publication. ² , ¹² , ¹⁵ , ²² , ²⁴ , ²⁵ , ²⁶ , ²⁷ , ²⁸ , ²⁹ , ³⁰ , ³¹ , ³² , ³³ , ³⁴ , ³⁵ The series by Velioglu et al ³⁶ featured the longest mean follow-up of 17.5±11.1 months (range: 2–48 months). These authors reported an 87.9% complete obliteration rate. No recanalization was reported. Among the studies reviewed, the mean acute periprocedural complication rate was 12.5% (95% confidence interval [CI]: 8.7–16.3%), and the mean delayed complication rate was 9.9% (95% CI: 6.4–13.4%). Ischemic postprocedural complications included parent artery occlusion in 0 to 8% of cases and embolic events in 0 to 6% of cases. One series, devoted to basilar artery aneurysms treated with SFD, reported a 25% incidence of ischemic events. ²⁹ In-stent stenosis was reported in 1 to 10% of cases, whereas intracranial hemorrhage was reported in 0 to 5% of cases. The overall neurological morbidity and mortality rates reported ranged from 0 to 6% and 0 to 4%, respectively.