Elsevier

Journal of Biomechanics

Volume 45, Issue 13, 31 August 2012, Pages 2256-2263
Journal of Biomechanics

Computer modeling of deployment and mechanical expansion of neurovascular flow diverter in patient-specific intracranial aneurysms

https://doi.org/10.1016/j.jbiomech.2012.06.013Get rights and content

Abstract

Flow diverter (FD) is an emerging neurovascular device based on self-expandable braided stent for treating intracranial aneurysms. Variability in FD outcome has underscored a need for investigating the hemodynamic effect of fully deployed FD in patient-specific aneurysms. Image-based computational fluid dynamics, which can provide important hemodynamic insight, requires accurate representation of FD in deployed states. We developed a finite element analysis (FEA) based workflow for simulating mechanical deployment of FD in patient-specific aneurysms. We constructed FD models of interlaced wires emulating the Pipeline Embolization Device, using 3D finite beam elements to account for interactions between stent strands, and between the stent and other components. The FEA analysis encompasses all steps that affect the final deployed configuration including stent crimping, delivery and expansion. Besides the stent, modeling also includes key components of the FD delivery system such as microcatheter, pusher, and distal coil. Coordinated maneuver of these components allowed the workflow to mimic clinical operation of FD deployment and to explore clinical strategies. The workflow was applied to two patient-specific aneurysms. Parametric study indicated consistency of the deployment result against different friction conditions, but excessive intra-stent friction should be avoided. This study demonstrates for the first time mechanical modeling of braided FD stent deployment in cerebral vasculature to produce realistic deployed configuration, thus paving the way for accurate CFD analysis of flow diverters for reliable prediction and optimization of treatment outcome.

Introduction

Neurovascular flow diverter (FD) is an emerging paradigm for treating traditionally difficult intracranial aneurysm such as wide-necked or fusiform aneurysms (Nelson et al., 2011). It is a self-expandable, tube-shaped metallic stent with fine braided mesh delivered via a catheter. Owing to its low porosity and high pore density, FD is effective at reducing blood inflow to the aneurysm sac, promoting intra-aneurysm thrombosis while keeping the adjacent arterial perforators unblocked. Endothelialization on the stent inner surface forms a new blood flow conduit within months that bypasses the obliterated aneurysm (Szikora et al., 2010).

While this novel intervention has achieved clinical success in an increasing number of challenging aneurysm cases, serious complications such as delayed post-treatment hemorrhage have also been reported (Byrne and Beltechi, 2010, Siddiqui and Abla, 2012). Several contributing factors have been hypothesized including extended aneurysm occlusion time leading to thrombosis-related aneurysm wall weakening (Kulcsar et al., 2011) and post-treatment pressure increase inside the aneurysm dome (Cebral et al., 2011). These hypotheses essentially attribute the variability of clinical outcomes to the patient-specific modification of hemodynamics, which significantly influences endothelialization, thrombosis and wall remodeling. Therefore, knowledge of detailed hemodynamics modified by FD placement is critical to the prediction of treatment outcome.

Image-based computational fluid dynamics (CFD) analysis can potentially provide important insight for this purpose but requires realistic representations of FD in deployed states. This presents challenges to the numerical simulation of stent implantation, since previous methods do not capture clinically realistic FD deployment processes and cannot reproduce the highly variable deployed configurations.

In light of the increasing clinical need for accurate CFD analysis of FD treatment and the inability of current numerical methods to produce realistically expanded FD geometries, we have developed a virtual FD deployment method using finite element analysis (FEA). In this approach we model the main steps involved in FD stent deployment in patient-specific aneurysm geometry to obtain realistic final FD configuration. Aside from being a prerequisite in accurate CFD analysis, this method can investigate FD deployment strategies employed by operating clinicians, as well as evaluate the mechanical characteristics of FD stents.

Section snippets

Methods

Our FD deployment modeling workflow (Fig. 1) uses these strategies:

  • (1).

    FD stent is modeled by 3D finite beam element because of the slender shape of its helical component strands.

  • (2).

    The entire deployment process is modeled from stent crimping (packaging), fitting into a microcatheter, maneuver and delivery of the stent-microcatheter system, to stent release from the microcatherter.

  • (3).

    Several key components of the FD delivery system are also modeled (Fig. 2). They are found essential to the final deployed

Model validation

First, the grid sensitivity test shows that a beam element size of 0.09 mm was sufficient to accurately capture FD stent’s behavior (Supplement Part 2). Second, experimental validation using a modeled Wallstent (Fig. 6) shows excellent agreement of simulation experiment data. Third, modeling result of an F1 flow diverter in a simple scenario – free release from a sheath in unstrained space – recapitulated time-resolved x-ray imaging of an actual FD coming out of its microcatheter (Fig. 7). Both

Discussion

Initial clinical reports of FD treatment showed high success rates in aneurysm occlusions in six months to one year (Lylyk et al., 2009). However, several complications such as inadequate device apposition, periprocedural distal thrombo-embolization and prolonged occlusion time have been reported to cause morbidity and mortality (D'Urso et al., 2011). At Millard Fillmore Gates Hospital, applications of FD to aneurysms of the posterior fossa also encountered mortality and morbidity despite

Conclusion

The variability of FD treatment outcome requires knowledge of hemodynamic modifications by deployment-specific FD configurations in patient-specific aneurysm geometries. A FEA-based method of simulating mechanical deployment of FD has been developed and tested against available experimental data. The 3D finite beam element model, accounting for interactions between stent strands and between stent and other deployment components, is shown to capture the mechanical responses of braided stents

Acknowledgments

This study is partially supported by Toshiba Medical Systems. We thank Corvidien for Pipeline Embolization Device samples for this study and Debra Zimmer for editorial assistance.

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