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Research Papers

Hemodynamics of Flow Diverters

[+] Author and Article Information
Ronak Dholakia, Chander Sadasivan, David J. Fiorella, Henry H. Woo

Department of Neurological Surgery,
Stony Brook University Medical Center,
Stony Brook, NY 11794

Baruch B. Lieber

Professor
Department of Neurological Surgery,
Stony Brook University Medical Center,
HSC T12, Room 080,
100 Nicolls Road,
Stony Brook, NY 11794-8122
e-mail: Baruch.lieber@stonybrook.edu

1Corresponding author.

Apart from the device comparison section on Hemodynamics of Stents and Flow Diverters, portions of this paper appear as part of a book chapter in “Flow Diversion of Cerebral Aneurysms,” 2016, Min S. Park, Phil Taussky, Felipe C. Albuquerque, and Cameron G. McDougall, Eds., Thieme Medical Publishers, New York, www.thieme.com (Reprinted with Permission).Manuscript received June 29, 2016; final manuscript received September 21, 2016; published online January 19, 2017. Assoc. Editor: Victor H. Barocas.

J Biomech Eng 139(2), 021001 (Jan 19, 2017) (10 pages) Paper No: BIO-16-1270; doi: 10.1115/1.4034932 History: Received June 29, 2016; Revised September 21, 2016

Cerebral aneurysms are pathological focal evaginations of the arterial wall at and around the junctions of the circle of Willis. Their tenuous walls predispose aneurysms to leak or rupture leading to hemorrhagic strokes with high morbidity and mortality rates. The endovascular treatment of cerebral aneurysms currently includes the implantation of fine-mesh stents, called flow diverters, within the parent artery bearing the aneurysm. By mitigating flow velocities within the aneurysmal sac, the devices preferentially induce thrombus formation in the aneurysm within hours to days. In response to the foreign implant, an endothelialized arterial layer covers the luminal surface of the device over a period of days to months. Organization of the intraneurysmal thrombus leads to resorption and shrinkage of the aneurysm wall and contents, eventually leading to beneficial remodeling of the pathological site to a near-physiological state. The devices' primary function of reducing flow activity within aneurysms is corollary to their mesh structure. Complete specification of the device mesh structure, or alternately device permeability, necessarily involves the quantification of two variables commonly used to characterize porous media—mesh porosity and mesh pore density. We evaluated the flow alteration induced by five commercial neurovascular devices of varying porosity and pore density (stents: Neuroform, Enterprise, and LVIS; flow diverters: Pipeline and FRED) in an idealized sidewall aneurysm model. As can be expected in such a model, all devices substantially reduced intraneurysmal kinetic energy as compared to the nonstented case with the coarse-mesh stents inducing a 65–80% reduction whereas the fine-mesh flow diverters induced a near-complete flow stagnation (∼98% reduction). We also note a trend toward greater device efficacy (lower intraneurysmal flow) with decreasing device porosity and increasing device pore density. Several such flow studies have been and are being conducted in idealized as well as patient-derived geometries with the overarching goals of improving device design, facilitating treatment planning (what is the optimal device for a specific aneurysm), and predicting treatment outcome (will a specific aneurysm treated with a specific device successfully occlude over the long term). While the results are generally encouraging, there is poor standardization of study variables between different research groups, and any consensus will only be reached after standardized studies are conducted on collectively large datasets. Biochemical variables may have to be incorporated into these studies to maximize predictive values.

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Figures

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Fig. 1

Simplified schematic of the therapeutic mechanism of flow diversion treatment for aneurysms. (a) An aneurysm is (b) treated by implantation of a flow diverter, which reduces flow activity within the aneurysm and (c) promotes clot formation within the aneurysm over hours to days; concurrently, a new arterial lining, called neointima, starts growing over the device. (d) Over weeks to months, the vessel remodels itself by resorbing the aneurysm along with completion of neointimal coverage of the device. While the aneurysm which is essentially a “dead-end” clots off, the natural pressure gradient maintains blood flow through side branches covered by the mesh.

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Fig. 2

(a) The different mesh structures of a flow diverter (Pipeline, right) and an intracranial stent (Enterprise, left) contribute to the difference in function of the devices. (b) Theoretical variation of porosity and pore density based on deployment diameter for a “typical” flow diverter with 48 wires, about 30 μm wire diameter, in-air device diameter (dashed line) of 4 mm and approximately 70% porosity. These plots are characteristic of braided flow diverters. (c) Numerical simulation of a flow diverter deployed across a cavernous aneurysm shows the local variability that can exist in the pore structure at the aneurysm neck (inset: black border demarcates the aneurysm neck).

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Fig. 3

(a) Computational fluid dynamics results (velocity magnitudes on top and vectors on bottom) of intra-aneurysmal flow during systole in a simplified sidewall aneurysm. The flow enters the distal neck in the unstented (control) case and forms a vortex within the aneurysm. Flow activity progressively reduces after implantation of a high-porosity stent (Neuroform) and a low-porosity stent (Pipeline). (b) Average velocity measures (as percentage of unstented cases) from a few literature reports. Decreasing porosity is seen to reduce flow activity; data toward the bottom of the plot are from sidewall-type geometries while those toward the top are from bifurcation-type geometries.

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Fig. 4

Micro-CT slice of Pipeline in the aneurysm model (left) and the same slice after segmentation with the device wires cropped (right)

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Fig. 5

(a) CAD model in position across the aneurysm neck (top row), and expanded view of the anisotropic tetrahedral mesh generated using meshing tools available within adina (bottom row): (a) Pipeline and (b) FRED devices

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Fig. 6

Peak systolic speed distribution in the aneurysm (left) and velocity vector map (right) for (a) control, (b) Neuroform, (c) Enterprise, (d) LVIS, (e) FRED, and (f) Pipeline

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Fig. 7

Instantaneous intra-aneurysmal kinetic energy throughout the cardiac beat for the five commercial neurovascular devices

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Fig. 8

Comparison of intra-aneurysmal mean kinetic energies among five commercial neurovascular devices: (a) absolute values and (b) percent reductions as compared to control

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Fig. 9

The effect of measured porosity and pore density on reduction in intra-aneurysmal mean kinetic energy

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Fig. 10

Systolic velocity magnitudes in a right carotid superior hypophyseal (top) and a right carotid cavernous (bottom) aneurysm before (Pre) and after (Post) flow diversion treatment; the reductions in mean intraneurysmal kinetic energy (KE) are noted for each case

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