Blood flow and structure interactions in a stented abdominal aortic aneurysm model

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Abstract

Since the introduction of endovascular techniques in the early 1990s for the treatment of abdominal aortic aneurysms (AAAs), the insertion of an endovascular graft (EVG) into the affected artery segment has been greatly successful for a certain group of AAA patients and is continuously evolving. However, although minimally invasive endovascular aneurysm repair (EVAR) is very attractive, post-operative complications may occur. Typically, they are the result of excessive fluid–structure interaction dynamics, possibly leading to EVG migration. Considering a 3D stented AAA, a coupled fluid flow and solid mechanics solver was employed to simulate and analyze the interactive dynamics, i.e., pulsatile blood flow in the EVG lumen, pressure levels in the stagnant blood filling the AAA cavity, as well as stresses and displacements in the EVG and AAA walls. The validated numerical results show that a securely placed EVG shields the diseased AAA wall from the pulsatile blood pressure and hence keeps the maximum wall stress 20 times below the wall stress value in the non-stented AAA. The sac pressure is reduced significantly but remains non-zero and transient, caused by the complex fluid–structure interactions between luminal blood flow, EVG wall, stagnant sac blood, and aneurysm wall. The time-varying drag force on the EVG exerted by physiological blood flow is unavoidable, where for patients with severe hypertension the risk of EVG migration is very high.

Introduction

Aneurysms, an irreversible ballooning of weakened artery segments, occur most frequently in the abdominal aorta. As the aneurysm expands, it may eventually rupture, making it the 13th leading cause of mortality in the US. Alternative to open surgery, where the diseased aorta segment is replaced with a synthetic graft, a minimally invasive technique has evolved over the last decade called endovascular aneurysm repair (EVAR). In EVAR, an endovascular graft (EVG) is guided from the iliac to the affected area where the EVG expands and forms a new artificial blood vessel, shielding the aneurysm from the pulsatile blood flow. The EVG or stent-graft is basically a cylindrical wire mesh embedded in synthetic graft material. While EVAR has shown outstanding success, especially for non-distorted abdominal aortic aneurysms (AAAs), EVG failure may occur due to blood leakage into the aneurysm cavity, which elevates the sac pressure and may cause rupture. This may also be caused by EVG migration when the drag force exerted on the EVG exceeds the fixation force, exposing the aneurysm sac again to the pulsatile blood flow.

So far, experimental studies and computational work have focused on the analysis of blood flow induced wall stresses of either the AAA or the EVG [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12]. For example, Fillinger et al. [1], [2] computed the peak wall stresses in realistic AAA configurations and declared the maximum stress to be the most important indicator of AAA rupture. Di Martino et al. [3], [4] and Finol et al. [9] simulated the interactions between blood flow and aneurysm wall to analyze system parameters which are of concern in AAA-rupture risk assessment. Raghavan et al. [5] and Thubrikar et al. [7] investigated wall stress distribution on three-dimensionally reconstructed models of human abdominal aortic aneurysms. Chong and How [11] measured the flow patterns in an endovascular EVG for abdominal aortic aneurysm repair. Liffman et al. [12], Morris et al. [13], and Mohan et al. [14] investigated numerically or clinically the forces on a bifurcated EVG, assuming a rigid EVG wall.

Typically, the EVG is anchored to the aortic neck due to frictional forces, where arterial ingrowth and/or stent barbs and hooks may play supportive roles. Clearly, a long cylindrical AAA neck with healthy tissue is most desirable for secure EVG placement [15], [16]. Nevertheless, Resch et al. [17] reported that 45% of patients after EVAR showed migration of their EVGs. Other than problematic AAA necks, Volodos et al. [18] added systemic hypertension as a major factor in EVG migration.

So far, most publications focused on AAA-wall stress or EVG-lumen flow separately. However, a stented AAA is a complex and strongly coupled system between blood flow and EVG/AAA wall. Thus, in order to evaluate blood flow patterns, wall stress distributions, sac pressure and EVG drag forces, fluid–structure interaction (FSI) dynamics have to be considered. Presently, no publication dealt with fully coupled FSI dynamics of stented AAAs. The purpose of this paper was to analyze the pulsatile 3D hemodynamics and its impact on EVG placement, EVG/AAA wall stress distributions, sac pressure generation, and EVG drag force in a representative stented AAA.

Section snippets

System geometry

The representative 3D asymmetric stented AAA model is shown in Fig. 1. The interacting materials include the luminal blood, EVG wall, stagnant blood in the AAA cavity, and AAA wall. The EVG is assumed to be a uniform 3D bifurcating shell attached to the proximal neck and iliac artery wall. The cavity between EVG and aneurysm wall is filled with stagnant blood, experiencing a time-dependent pressure as a result of the dynamic fluid–structure interactions. The drag force exerted on the EVG, due

Results

The results are divided into three groups, i.e., the beneficial impact of EVG insertion (Fig. 4, Fig. 5), the luminal blood velocity fields, wall stress distributions and sac pressure levels at three selected time levels during the cardiac cycle (Fig. 6, Fig. 7, Fig. 8), and the effect of blood pressure waveforms on the transient EVG drag force, calculated from the wall shear stress distribution and the net momentum change (see Fig. 9).

Discussion

Although minimally invasive endovascular aneurysm repair is very attractive, post-operative complications may occur, which are the result of excessive fluid–structure interaction dynamics. Thus, the fluid–structure interactions for a representative AAA model with and without a realistic EVG have been investigated in terms of pulsatile blood flow influencing EVG movement, which is transmitted via the stagnant blood in the cavity to the aneurysm wall. Of interest are the beneficial impact of an

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