European Journal of Mechanics B/Fluids 35 (2012) 85–91

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European Journal of Mechanics B/Fluids

journal homepage: www.elsevier.com/locate/ejmflu

Hemodynamic and mechanical aspects of fenestrated endografts for treatment ofAbdominal Aortic AneurysmIdit Avrahami a,∗, Moshe Brand a, Tomer Meirson b, Zehava Ovadia-Blechman b, Moshe Halak c

a Department of Mechanical Engineering & Mechatronics, Ariel University Center of Samaria, Israelb Department of Medical Engineering, Afeka Tel Aviv Academic College of Engineering, Tel Aviv, Israelc Department of Vascular Surgery, The Chaim Sheba Medical Center, Tel Hashomer, Israel

a r t i c l e i n f o

Article history:Available online 21 April 2012

Keywords:Fenestrated endovascular repairAAAFSI

a b s t r a c t

Fenestrated endovascular aneurysm repair (f-EVAR) stent grafts offer an alternative treatment forconventional open heart surgery for patients with juxsta-renal Abdominal Aortic Aneurysms (AAA)that are unsuitable for the common infrarenal endovascular aneurysms repair (EVAR) procedure. Thef-EVAR endograft includes branched stent grafts fixed to the aortic endograft main body via designatedstrengthened fenestrations. Repetitive stresses activated upon the endograft by the pulsatile flow mightcause detachment or fracture of branching. Thus, investigation of the flow forces and stresses on thegraft may help minimize complications and improve the endograft design. The present work investigatesthe flow and stress fields of the fenestrated endograft configuration using a fluid structure interaction(FSI) model in order to evaluate risks for graft fracture or detachments. The results show that the f-EVARdramatically improves the aortic and iliac flow and that elevated cyclic stresses are found at the graft’sbifurcation and branches’ connections.

© 2012 Elsevier Masson SAS. All rights reserved.

1. Introduction

Abdominal Aortic Aneurysms (AAA) is the most common typeof aortic aneurysm, with a mortality rate of 75%–90% in patientspresented with aneurysm rupture. In the US, 15,000 patients dieannually from AAA rupture and many other suffer from AAA-related complications such as stroke due to poor hemodynamicsand thrombus formation [1]. Endovascular aneurysms repair(EVAR) [1,2] is a common procedure for treatment of AAA thatwas proven as an effective alternative for conventional opensurgery [3–5]. However, 30%–50% of AAA patients are unsuitablefor infrarenal EVAR due to short aneurysm neck [1,6]. These AAAare defined as juxta- or supra-renal AAA.

Supra-renal endografts were introduced to accommodate theselimitations [7–9]. The modified endograft includes branchedstented grafts that are fixed to the aortic endograft through desig-nated strengthened fenestrations (Fig. 1). The stented branch graftsare placed over the orifice of the target vessel (Fig. 1b) and fastenedto the strengthened fenestrations [10]. The renal perfusion mightbe strongly affected by the configuration of the fenestrated endo-grafts [11–13]. Long-term maintenance of position is dependenton secure graft fixation. The cardiac and respiratory cycles gener-ate repetitive stresses on the endograft [14] and at the interface

∗ Corresponding author.E-mail address: iditavrahami@gmail.com (I. Avrahami).

0997-7546/$ – see front matter© 2012 Elsevier Masson SAS. All rights reserved.doi:10.1016/j.euromechflu.2012.03.010

between the fenestrated endografts and the branched stents [8].These stresses might cause fracture of the connection to renal ar-teries or lead to detachment or migration of the branching endo-grafts [1,15]. Therefore, in order tominimize complications, carefulinvestigation of the flow forces and stresses in the grafts and arter-ies is needed.

Previous computational studies addressed mostly the fluiddynamics in the aneurysm sac before and after stent grafting, andonly a few of them addresses supra-renal grafts. For example,Sun and Chaichana [16] described a disturbed and recirculatingflow in the pre-stented aneurysm, while after grafting, the flowbecame smoother and more laminar. In addition, they pointed outthat grafting significantly increased wall shear stress and reducedpressure in the aneurysm, and improved flow to renal and iliacarteries.

Other studies coupled the flow in the aneurysm and thearterial wall to address biomechanical risk factors for AAA rupture,such as geometry and wall mechanical properties [17–19] inorder to provide prediction tools for disease progression andaneurism rupture. It was shown that patient-specific factors suchas calcification deposits or collagen fiber orientation in the non-homogenous arterial wall might increase the risk for rupture, aswell as increased iliac or neck angulations.

Computational work that addressed graft dynamics used onlyCFD calculations to evaluate the 3D direction of forces actingon infrarenal endograft by the flow. Morris et al. [14] estimatednumerically the pulsatile forces acting on a symmetric stent graft

86 I. Avrahami et al. / European Journal of Mechanics B/Fluids 35 (2012) 85–91

Fig. 1. Example of fenestrated suprarenal stent graft for treatment of AAAwith narrow neck: (a) assembly of a composite stent graft with two renal branches; (b) angiographdemonstration of renal branch stenting via the stent-graft fenestration; (c) 3D reconstruction of implanted suprarenal stent graft with renal branches using computedtomography (CT) (R, right; H , head; A, anterior).

bifurcation as a factor of bifurcation angle and proximal graftdiameter. Howell et al. [20] showed that pressure-related forcesat the graft bifurcation have a major impact on graft endoleakor migration risk. Figueroa et al. [15,21] concluded that EVARmigration or movement is a result of pulsatile displacement forceswhich are related to endograft geometry and curvature.

The only study that coupled the flow in the stent graft with theendograft structure was done by Li and Kleinstreuer [22]. In thatwork, the impact of blood hemodynamics on the graft’s wall wasanalyzed using staggered fluid–structure interaction. Themaximalstresses calculated by their model were found at the stagnationbifurcation point and reached 1.7 MPa at peak systole. Howevertheir model did not include gravity, and is of an infrarenal graft.

To our knowledge, no published study addressed the fluid–structure coupled stress field developed in supra-renal stent grafts,and in particular at the branch connection of the f -EVAR. Thepresent study presents a numerical calculation of the flow field inthe pre- and post-grafted supra-renal AAA and couples the post-grafted flow field with the fenestrated endograft stent in order toanalyze and evaluate the stress field developed at the branches tobetter predict risks for graft fracture or branch detachments.

2. The numerical model

Two simulations were performed: the first for the fluid domainof the pre-grafted aneurysm (using only CFD) and the second of thepost-grafted case using direct coupling FSI methods to couple thefluid domain with the assembled endograft structure.

The numerical simulations were performed on the geometricalmodel shown in Fig. 2. The geometrical model of pre- and post-grafted aneurysm was based on dimensions taken from CT scansand created in SOLIDWORKS.

Themain body fenestrated endograft is a combination of Nitinolstent chains sewn to a Dacron graft envelope (see Fig. 1a). TheNitinol chains endure external pressure loads, preventing theendograft from collapsing in, while the Dacron graft enduresinternal pressure loads such as blood pressure. In practice, theNitinol chains are active at locationswhere the endograft is pressedagainst the arterial wall, and the Dacron graft is active mainlywhere the endograft is immersed in the aneurysm sac and notin contact with the arteries. In the presented model, we assumedthat for a main body endograft in a supra-renal aneurysm, only theDacron graft contributes to stresses and displacements. Therefore,the main body structure (green in Fig. 2d) was approximated tobe a uniform Dacron shell layer of 0.1 mm thickness with Young’smodulus of E = 3 GPa, Poisson ratio of ν = 0.29 and densityof ρ = 1.4 g/mL. The two renal stent grafts (purple in Fig. 2d)were modeled with linear effective properties of stent grafts asspecified by Li and Kleinstreuer [22] of E = 10 MPa, Poisson ratioof ν = 0.27 and density of ρ = 6 g/mL. These renal brancheswere connected to the main endograft body using rigid links withfully constraint joints. The grafts were connected at all their distalends to arterial sections (red in Fig. 2d) which were assumedhomogenous, linearly elastic and isotropic with E = 1 MPa ν =

0.49 and ρ = 1 g/mL [23]. These arterial sections were fullyfixed at their proximal ends (diagonal stripes in Fig. 2d). Assumingno blood leakage from the endografts, at the outer surface of theendograft a uniform pressure of 6 mmHg was set, modeling theintra-abdominal pressure [24].

The flow and pressure fields in the lumen were calculatedby numerically solving the equations governing momentum andcontinuity in the fluid domain:

∇ · V = 0

ρDVDt

= −∇p + µ∇2V + ρg

(1)

I. Avrahami et al. / European Journal of Mechanics B/Fluids 35 (2012) 85–91 87

7

a b

c

d

Fig. 2. The geometric model and boundary conditions: (a) the geometrical model of pre- and post-grafted fluid domain based on aneurysm anatomic dimensions, (b)imposed inlet aortic pressure and (c) imposed outlet iliac and renal flowrates as a function of time; (d) the structure domain of the endograft assembly: main endograft(green), renal branches stent grafts (purple) and arterial sections (red). Diagonal stripes indicate full constrain at distal ends.

where p is static pressure, t is time, V is velocity vector, ρ and µare density and the dynamic viscosity of blood, and g is the vectorof gravity.

Since the fluid domain changes with wall displacements, theArbitrary Lagrangian–Eulerian (ALE) moving mesh approach wasused to adjust the fluid mesh to the boundary motion [25]. Thus,the local (Eulerian) velocity of the fluid relative to themovingmeshV is:

V = Vabs − Vmesh (2)

where Vmesh and Vabs are the mesh (Lagrangian) and absolutevelocity vectors, respectively. Blood was assumed homogenous,incompressible (with density ρ = 1 g/mL), and Newtonian (withviscosity µ = 3.5 cP). The flow was assumed laminar. Gravity ofg = 9.81 m/s2 was employed.

At the fluid–structure interfaces, displacement compatibility(for no-slip and no-penetration conditions) and traction equilib-rium were applied:

V = U̇; τ f = τs (3)

where V and U̇ are the fluid and solid velocity vectors at theinterface, respectively and τ f and τs are the fluid and solid stresstensors at the interface, respectively.

The boundary conditions were set according to typical physi-ological conditions with pressure of 110/75 mmHg, heart rate of75 BPM, and average flowrate of 480 mL/min directed to each re-nal artery and 1920 mL/min directed to each iliac artery. Time-dependent pressurewas imposed at the inlet as described in Fig. 2band time-dependent outlet flow rates were imposed at each outletas shown in Fig. 2c.

The commercial software ADINA (ADINA R&D Inc., MA) wasused to solve the set of fluid and structure equations usingthe finite-element scheme. The numerical meshes for the fluiddomains of the pre- and post-grafted aneurysm models consistedof about 800,000 tetrahedral elements, each.

As mentioned before, the structural mesh consisted of threedifferent mesh groups (main endograft, two renal stent grafts andarteries). The total mesh model consisted of about 30,000 3D shellelements. Small deformations and small strainswere considered inthe simulation. For each case, four cardiac cycles were calculated(0 s < t < 3.2 s) with a total of 800 time steps per cycle. Theresults of the third cycle were fully periodic.

3. Results

The resulting flow field as calculated for the fluid domains forpre- and post-grafting aneurysm during the third calculated cycle(1.6 s < t < 2.4 s) is shown in Fig. 3. Fig. 3a shows pressuredistribution at peak systole (t = 1.9 s),while Fig. 3b shows velocityvectors plot during diastole (t = 2.4 s). Fig. 3c shows Lagrangiantracking pathlines of 300 particles released in the ascending aortaduring end diastole (t = 2.4 s). The results indicate significantimprovement of flow field with the endograft (lower figures), incomparison to the case without the endograft (upper figures). Inthe case of aneurysmwithout endograft, disturbed flow is observedin the aneurysm sac, especially during diastole where a largerecirculation zone is found, filling the aneurysm and implying poorparticle washout from the aneurysm sac. The non-oriented flowin the aneurysm requires flow particles to change their directionand velocity on their way to the iliac arteries. This leads to lossesas flow enters the iliac arteries, and thus the pressure drop alongthe descending aorta during peak systole reaches 7.5mmHg. In thecase with endograft, the flow field is significantly improved, withsmooth flow in which particles are well directed towards the iliacarteries in a uniform flow profile, even at the last stage of diastole.The pressure drop along the graft is less than 2.5 mmHg.

Fig. 4 shows time-dependent results of the case with a stentgraft. Flow and structure parameters are shown at five timeinstances along the third calculated cycle (t = 1.6, 1.83, 1.9, 1.96,2.0 s). The time instances are shown on the inflow flowrate(purple) and pressure (blue) graphs (Fig. 4a). Due to the anatomiccurvature of the model, the coronal plane chosen for flowparameters is such that it crosses the distal ends of the arteries(Fig. 4b), but a small part of the proximal flow is not shown. Theleft column of Fig. 4c shows velocity vectors of the flow field.For clarity, vector resolution was reduced. As mentioned above,the flow in the graft is well directed towards the renal and iliacarteries. During diastole (t = 1.6, 2.0 s), some recirculation zonesare found where the iliac and renal arteries are twisting. Duringsystole (t = 1.83, 1.9, 1.96 s), the flow in themain graft is uniformand the flow towards the branches is well directed. The velocity inthe right iliac artery is higher than in the left one (up to 75 cm/sat peak systole) due to differences in artery diameter. The highvelocities in the right renal branch (up to 97 cm/s at peak systole)are due to local twist and sharp curvature of the right artery.

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Fig. 3. Results for flow field as calculated for the fluid domains without stent graft (top) and with stent graft (bottom): (a) pressure distribution in the models during peaksystole (t = 1.9 s), (b) velocity vectors plot (in cm/s) during diastole (t = 2.4 s), and (c) traces of particles released in the ascending aorta during end diastole (t = 2.4 s).

These high velocities in the right renal and iliac arteries duringsystole (t = 1.83, 1.9, 1.96 s) induce a pressure reduction of upto 21,000 and 6000 dyn/cm2, respectively (as shown in the secondcolumn at peak systole). During diastole (t = 1.6, 2.0 s), thepressure in the graft is distributed linearly due to gravity. The thirdcolumn of Fig. 4c shows displacement magnitudes in the graft.The highest displacements are found in the arterial sections of theaorta and iliac arteries. In the graft, insignificant displacements arefound with maximal displacement of 0.4 mm during peak systole(t = 1.9 s), near the proximal connection to the aorta. The renalarteries did not expand during the cardiac cycle, because of thehigh velocity and corresponding low pressure calculated in theseregions.

The fourth column of Fig. 4c shows stresses developed at thegraft. Effective stress was calculated at each point according to vonMises criteria. Elevated effective stresses are found in the graftiliac bifurcation, at the renal branches connections, and wherethe Dacron graft is connected with the aorta and with the iliacarteries. Highest stresses are found during peak systole (t =

1.9 s) at the iliac bifurcation. Elevated effective stresses are alsofound at the renal branch connection. Fig. 5 shows time-dependenteffective stresses at these locations. Cyclic stress is found at theiliac bifurcation (blue line) in the range of 400–670 and 220–380kPa at the renal branch connection (purple line). In addition, time-dependent total drag force, acting on the graft by the fluid is shown(green line), with maximal force of 7.36 N at peak systole.

4. Discussion

The present research includes investigation of the time-dependent flow and stress fields developed in a fenestratedendograft and branches, using Fluid–Structure Interaction (FSI)numerical simulations. This study presents simplified models ofsupra-renal endografts. The following factors might influence thecalculations and should be taken into account. Large variationsbetween patients; assumed boundary conditions based on typicalphysiological inlet flow and pressure; assumptions regarding flowdivision between branches; approximated material with linear

properties; and possible numerical errors. However, this modelprovides significant insight on the stresses developed at the renalbranch connection and the flow field in the f -EVAR configuration.

According to the resulted flow field in the aneurysm sac,in a case without endograft, the disturbed flow in the sacduring diastole might lead to thrombus formation, and the poorhemodynamics towards the iliac arteries might impair perfusion.This may explain the flow reduction toward branched arteries inAAA patients [16,17]. The endograft not only reduces pressureinside the aneurysm sac and improves aortic flow, but alsoimproves iliac hemodynamics. These findings agree with previousstudies that discussed endograft efficiency [18,22,26–28]. Thepresent study did not consider stent interference with renalflow; however, according to Sun and Chaichana [13,16], thehemodynamic effect of branched renal stents is insignificant.

The structural analysis of the supra-renal graft reveals minordisplacements in the graft with maximal displacement of 0.4 mmnear the proximal connection to the aorta. These displacements aremainly due to arterial flexibility under pulsation stretching forces.Maximal stresses are found at the graft iliac bifurcation in the rangeof 400–670 kPa. These values are in the range of values calculatedby Li and Kleinstreuer [22] which calculated maximal stress of 560kPa at the bifurcation of an infrarenal stent graft. However, theircalculation ignored gravity, which is an important factor in theselarge scale flow calculations. Moreover, the calculated total forcevalues in the range of 4.6–7.3 N agree with previous studies [14,20] and may explain the main EVAR complications, among themgraft migration, endoleaks, stent fatigue and fracture, and suturerupture [1,15,16].

Other locations with elevated stress were found at theconnections to renal branches, with frequent cyclic stresses of220–380 kPa. These findings suggest possible risk for local fractureor detachments of the branched endografts, as well as a potentialfor tissue overgrowth and occlusion at the renal bifurcations.Fig. 6a and b illustrate clinical examples in which broken ordisconnected branches were observed. In these cases, branchdisconnection allows blood to leak into the aneurysm. This mightincrease the pressure inside the aneurysm and lead to aneurysm

I. Avrahami et al. / European Journal of Mechanics B/Fluids 35 (2012) 85–91 89

a b

c

Fig. 4. Time-dependent results for the fluid and solid domains for the case with stent graft: (a) five time instances of the third calculated cycle; (b) side and front view ofcoronal cross section for flow parameters; (c) velocity vectors and pressure distribution (left columns) and displacement magnitudes and effective stress of the structuraldomain (right columns).

enlargement and rupture [29]. Post-procedural leakage is thereforethe chief cause of aneurysm-related mortality (ARM) and the needfor either open surgical or endovascular reinterventions [1]. Inaddition, twisted renal branch (similar to the right renal branchin our model) might induce elevated velocities and pressurereduction near the connection between the branch stent and the

renal artery. This, in turn, might cause obstruction of the targetartery [11,12,30] as shown in Fig. 6c. In order to allow an effectiveand safe alternative for surgical procedure, fenestrated endograftsfor suprarenal endovascular aneurysm repair should be designedwith great attention. This study proves that specific care shouldbe given to the branching area, to avoid endograft detachment,

90 I. Avrahami et al. / European Journal of Mechanics B/Fluids 35 (2012) 85–91

Fig. 5. Time dependent results of effective stresses at the graft iliac bifurcation (blue) and at the connection to renal branch (purple); and of total drag force on the graft(green). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 6. Examples of clinical failures of renal branches: (a) angiograph demonstration of a fraction in the renal branch stent; (b) a CT image of implanted super-renal stentgraft with a leak from the renal branch fracture; (c) fenestrated stent graft with renal branch occlusion (CT image).

fracture or twists and to prevent interference of malpositionedendografts to the renal flow.

Acknowledgments

The authors would like to thank Dr. Sagi Raz for his assistanceand AFEKA Research Fund for partially supporting this research.

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