Effects on Aortoiliac Fluid Dynamics After Endovascular Sealing of Abdominal Aneurysms

The Fluid-Dynamics of Endo Vascular Aneurysm Sealing (EVAS) System failure

Purpose

The main objective of this work is to investigate hemodynamics phenomena occurring in EVAS (Endo Vascular Aneurysm Sealing), to understand if and how they could lead to type 1a endoleaks and following re-intervention. To this aim, methods based on computational fluid mechanics are implemented as a tool for checking the behavior of a specific EVAS configuration, starting from the post-operative conditions. Pressure and velocity fields are detailed and compared, for two configurations of the Nellix, one as attained after correct implantation and the other in pathological conditions, as a consequence of migration or dislocation of endobags.

Methods

The computational fluid dynamics (CFD) approach is used to simulate the behavior of blood within a segment of the aorta, before and after the abdominal bifurcation. The adopted procedure allows reconstructing the detailed vascular geometry from high-resolution computerized tomography (CT scan) and generating the mesh on which the equations of fluid mechanics are discretized and solved, in order to derive pressure and velocity field during heartbeats.

Results

The main results are obtained in terms of local velocity fields and wall pressures. Within the endobags, velocities are usually quite regular during the whole cardiac cycle for the post-implanted condition, whereas they are more irregular for the migrated case. The largest differences among the two cases are observed in the shape and location of the recirculation region in the rear part of the aorta and the region between the endobags, with the formation of a gap due to the migration of one or both of the two. In this gap, the pressure fields are highly different among the two conditions, showing pressure peaks and pressure gradients at least four times larger for the migrated case in comparison to the post-implanted condition.

Conclusions

In this paper, the migration of one or both endobags is supposed to be related to the existing differential pressures acting in the gap formed between the two, which could go on pushing the two branches one away from the other, thus causing aneurysm re-activation and endoleaks. Regions of flow recirculation and low-pressure drops are revealed only in case of endobag migration and in presence of an aneurysm. These regions are supposed to lead to possible plaque formation and atherosclerosis.

Computational Comparison between the Altura Aortic Endograft Configuration and the Classic Bifurcated Idealized Designs

BACKGROUND

The Altura (Alt) endograft is a new design, lacking the classic main body with the flow divider. Instead, 2 proximal D-shaped endografts form a round circumference in the aortic neck for secure sealing and land in the iliac arteries in a cross-limb fashion. The aim of this computational study was to compare hemodynamically this model with the classic bifurcated (Bif) and cross-limb (Cx) endograft designs of equal total length.

METHODS

All 3D endograft models were created using the finite volume analysis application ANSYS CFX (Ansys Inc., Canonsburg, PA, USA). The Alt inlet was constructed as 2 opposing D-shaped sections. The flow was quantified by time-averaged wall shear stress (TAWSS), oscillatory shear index (OSI), relative residence time (RRT), and helicity. The displacement forces were also compared for all models with computational fluid dynamics analysis.

RESULTS

The Alt design was associated with lower forces (range 4.0-5.9Ν) than Bif (4.17-6.15 N) and Cx (4.43-6.53 N). The 2-piece inlet site of the separated limbs of Alt has higher TAWSS than the uniform inlet segment of the Cx and the Bif model. Most importantly, the mid-segment and distal segment of the limbs in the Alt design present higher TAWSS in a greater area than the other 2 models. The inlet of the Alt design showed higher OSI than the other accommodations and similar or comparable OSI values along their mid-limb and distal limb segments. The range, location, and values or RRT were comparable between the 3 models. Helicity in the iliac limbs is more prominent in the crossed accommodations (Alt and Cx).

CONCLUSIONS

Only small differences in the hemodynamic indices and displacement forces were detected between the Alt and classic accommodations. From this point of view, the Alt design could be theoretically considered not inferior to other widely used endograft configurations.

Comparison of Fluid Dynamics Variations Between Chimney and Fenestrated Endografts for Pararenal Aneurysms Repair: A Patient Specific Computational Study as Motivation for Clinical Decision-Making

BACKGROUND-AIM

Limited data exist concerning the fluid dynamic changes induced by endovascular aortic repair with fenestrated and chimney graft modalities in pararenal aneurysms. We aimed to investigate and compare the wall shear stress (WSS) and flow dynamics for the branch vessels before and after endovascular aortic repair with fenestrated and chimney techniques.

METHODS

Modeling was done for patient specific pararenal aortic aneurysms employing fenestrated and chimney grafts (Materialise Mimics 10.0) before and after the endovascular procedure, using computed tomography scans of patients. Surface and spatial grids were created using the ANSYS CFD meshing software 2019 R2. Assessment of blood flow, streamlines, and WSS before and after aneurysm repair was performed.

RESULTS

The endovascular repair with chimney grafts leaded to a 43% to 53% reduction in perfusion in renal arteries. In fenestrated reconstruction, we observed a 15% reduced perfusion in both renal arteries. In both cases, we observed a decrease in the recirculation phenomena of the aorta after endovascular repair. Concerning the grafts of the renal arteries, we observed in both the transverse and longitudinal axes low WSS regions with simultaneous recirculation of the flow 1 cm distal to the ostium sites in both aortic graft models. High WSS regions appeared in the sites of ostium.

CONCLUSIONS

We observed reduced renal perfusion in chimney grafts compared to fenestrated grafts, probably caused by the long and kinked characteristics of these devices.

Biomechanical Investigation of Disturbed Hemodynamics-Induced Tissue Degeneration in Abdominal Aortic Aneurysms Using Computational and Experimental Techniques

Abdominal aortic aneurysm (AAA) is the dilatation of the aorta beyond 50% of the normal vessel diameter. It is reported that 4-8% of men and 0.5-1% of women above 50 years of age bear an AAA and it accounts for ~15,000 deaths per year in the United States alone. If left untreated, AAA might gradually expand until rupture; the most catastrophic complication of the aneurysmal disease that is accompanied by a striking overall mortality of 80%. The precise mechanisms leading to AAA rupture remains unclear. Therefore, characterization of disturbed hemodynamics within AAAs will help to understand the mechanobiological development of the condition which will contribute to novel therapies for the condition. Due to geometrical complexities, it is challenging to directly quantify disturbed flows for AAAs clinically. Two other approaches for this investigation are computational modeling and experimental flow measurement. In computational modeling, the problem is first defined mathematically, and the solution is approximated with numerical techniques to get characteristics of flow. In experimental flow measurement, once the setup providing physiological flow pattern in a phantom geometry is constructed, velocity measurement system such as particle image velocimetry (PIV) enables characterization of the flow. We witness increasing number of applications of these complimentary approaches for AAA investigations in recent years. In this paper, we outline the details of computational modeling procedures and experimental settings and summarize important findings from recent studies, which will help researchers for AAA investigations and rupture mechanics.