9 OVERVIEW OF CURRENT FLOW-DIVERTING DEVICES

10.1055/b-0037-146683

9 OVERVIEW OF CURRENT FLOW-DIVERTING DEVICES

PEDRO AGUILAR-SALINAS, BARTLEY MITCHELL, DOUGLAS GONSALES, RICARDO A. HANEL, and ERIC SAUVAGEAU

Abstract

The treatment of intracranial aneurysms has impressively evolved over the past 20 years along with the development of less invasive techniques. However, large or giant wide-necked aneurysms remain a challenging and complex pathology to treat by traditional microsurgery or endovascular embolization due to the high rate of recanalization. Therefore, flow-diverting technology has been acknowledged as playing a central role in treating those complex aneurysms. Although the flow-diverting devices have been recently introduced to the endovascular armamentarium, multiple types are already available. All of them have the same mechanism of action but vary slightly in stent design and deployment technique. The Pipeline Embolization Device (PED; Covidien/Medtronic, Irvine, CA), the Silk flow diverter (SILK; Balt Extrusion, Montmorency, France), the Surpass flow diverter (SURPASS; Stryker Neurovascular, Fremont, CA), and the Flow Redirection Endoluminal Device (FRED; MicroVention, Tustin, CA) are discussed in this chapter. The PED is the most widely used stent and the only Food and Drug Administration (FDA)-approved device for clinical use in the United States. The SILK stent was the first flow diverter available. Both stents have 48 braided strands as a mesh design and may require telescoping devices to increase the metal surface coverage. The SURPASS has single-layer mesh that maintains a constant porosity across all sizes available, thus potentially reducing the need of telescoping stents. Conversely, the FRED has a unique double-layer mesh design. It consists of a low-porosity inner mesh and a high-porosity outer stent. The dual-layer system offers an improved scaffolding effect as well as full-stent length fluoroscopic visualization.

9.1 Introduction

The method of treating intracranial aneurysms has impressively evolved over the past 20 years along with the advance of technology and less invasive techniques. Currently, neuroendovascular intervention is considered an effective and safe option after the long-term results of groundbreaking studies were published. ¹ , ² However, the occlusion rates of aneurysms with complex morphology are far from definitive when treated only with coiling or stent-assisted embolization. ³ , ⁴ , ⁵ , ⁶ Flow-diverting devices play a critical role in the neuroendovascular armamentarium with their higher rates of aneurysm occlusion over time, independent of lesion morphology. ⁷ , ⁸ , ⁹ , ¹⁰ These devices work by modifying the blood-flow dynamics through the parent artery, as well as by inducing physiological and biological events to facilitate aneurysm occlusion without endovascular intervention. ¹⁰ , ¹¹ Although this technology has recently been introduced, the development of competing flow-diverting devices has rapidly increased with a growing body of literature to support their efficacy. The aim of this chapter is to provide an overview of the currently available flow-diverting devices. The technique and nuances of each flow diverter will be discussed in detail in the following sections.

9.2 Flow-Diverting Devices

Several features such as the pore density, porosity, metal coverage of the aneurysm neck, and deployment procedure contribute to determine the effectiveness of a flow diverter. However, experimental models suggest that low porosity and high-pore density are the main factors in decreasing the blood flow into the aneurysm with an optimal range between 60 and 76%. ¹² , ¹³ , ¹⁴ Although the principle of action is the same, the currently available flow diverters vary slightly in the aforementioned properties. The following devices will be discussed: the Pipeline Embolization Device (PED; Covidien/Medtronic, Irvine, CA), the Silk flow diverter (SILK; Balt Extrusion, Montmorency, France), the Surpass flow diverter (SURPASS; Stryker Neurovascular, Fremont, CA), and the Flow Redirection Endoluminal Device (FRED; MicroVention, Tustin, CA). Table 9.1 summarizes the main characteristics of the four types of flow-diverting devices.

**Table 9.1** Summary of the main properties of the flow diverters
Flow diverter	FDA/CE mark	Mesh design	Surface coverage (%)	Diameter and lengths (mm)
PED	FDA/CE mark	48 braided strands of 75% cobalt-chromium/25% platinum-tungsten	30–35	D: 2.5–5 L: 10–35
SILK	CE mark	48 braided nitinol strands and platinum filaments	30–35	D: 2–5 L: 15–40
SURPASS	CE mark	Cobalt-chromium and platinum wires	30	D: 2–5 L: 12–50
FRED	CE mark	Dual-layer design: low-porosity inner mesh (48 nitinol wires) and high-porosity outer mesh (16 nitinol wires)	30	D: 3–5.5 L: 7–56a
Abbreviations: CE, Conformité Européenne; D, diameter; FDA, Food and Drug Administration; FRED, Flow Redirection Endoluminal Device; L, length; PED, Pipeline Embolization Device; SILK, Silk flow diverter; SURPASS, Surpass flow diverter.
^a Work lengths (dual-layer segment).

9.2.1 Pipeline Embolization Device

The PED is the only Food and Drug Administration (FDA)-approved flow diverter in the United States and the most widely used worldwide. The first generation received the European CE mark approval in June 2008 and became available in the United States in April 2011 after the Pipeline for Uncoilable and Failed Aneurysms (PUFS) trial. ⁸ The PED is a self-expanding stent, cylindrically shaped, composed of 48 braided strands in a standard pattern, and made of 75% chromium-cobalt and 25% platinum-tungsten alloy microfilaments. The cell sizes range from 0.02 to 0.05 mm. Its porosity consists of 65 to 75% and when deployed properly, it affords 30 to 35% coverage of the arterial wall surface. It is available in lengths from 10 to 35 mm and in diameters from 2.5 to 5.0 mm. However, when expanded, the device may reach 0.25 mm in diameter larger than stated. ¹¹ , ¹⁵

The second generation of this stent is known as the Pipeline Flex Embolization Device (PED Flex). It obtained the CE mark in March 2014 and the FDA approval in February 2015. This new generation keeps the same stent design, material, and configuration of the previous generation but has a new delivery system that makes it almost completely resheathable. ¹⁶ , ¹⁷

9.2.2 Silk Flow Diverter

The SILK device received the European CE mark approval in 2008 and was the first flow-diverting device available in the neuroendovascular field, but it is not available for clinical use in the United States. This is a self-expanding stent designed as a closed-cell mesh cylinder composed of 48 interwoven nitinol strands and 4 platinum microfilaments with flared ends. Its porosity is of 45 to 60%, resulting in a metal surface coverage of approximately 35%. It is available in several diameters from 2 to 5 mm and lengths from 15 to 40 mm. In addition, stents may be placed to vessels from 0.5 mm smaller or up to 0.25 mm larger than the labeled diameter. A new generation has been released, Silk +, designed with a lower porosity, higher radial force, and eight platinum markers. ¹⁸ , ¹⁹

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