Haemodynamic performance of AFX and Nellix endografts: a computational fluid dynamics study

Haemodynamic Analysis of Branched Endografts for Complex Aortic Arch Repair

This study aims to investigate the haemodynamic response induced by implantation of a double-branched endograft used in thoracic endovascular aortic repair (TEVAR) of the aortic arch. Anatomically realistic models were reconstructed from CT images obtained from patients who underwent TEVAR using the RelayPlus double-branched endograft implanted in the aortic arch. Two cases (Patient 1, Patient 2) were included here, both patients presented with type A aortic dissection before TEVAR. To examine the influence of inner tunnel branch diameters on localised flow patterns, three tunnel branch diameters were tested using the geometric model reconstructed for Patient 1. Pulsatile blood flow through the models was simulated by numerically solving the Navier–Stokes equations along with a transitional flow model. The physiological boundary conditions were imposed at the model inlet and outlets, while the wall was assumed to be rigid. Our simulation results showed that the double-branched endograft allowed for the sufficient perfusion of blood to the supra-aortic branches and restored flow patterns expected in normal aortas. The diameter of tunnel branches in the device plays a crucial role in the development of flow downstream of the branches and thus must be selected carefully based on the overall geometry of the vessel. Given the importance of wall shear stress in vascular remodelling and thrombus formation, longitudinal studies should be performed in the future in order to elucidate the role of tunnel branch diameters in long-term patency of the supra-aortic branches following TEVAR with the double-branched endograft.

Modeling and Computational Comparison of the Displacement Forces Exerted between the AFX Unibody Aortic Stent Graft and its Hybrid Combination with a Nitinol-based Proximal Aortic Cuff

BACKGROUND

The bifurcated AFX (Endologix, Inc, Irvine, CA, USA) aortic stent-graft is the sole unibody endograft for the management of Abdominal Aortic Aneurysms (AAA). In order to improve the AFX central sealing and clinical efficacy in challenging cases, a replacement of the central chromium-cobaltium AFX extension with a Nitinol-based proximal aortic cuff has been suggested. Yet, comparative data regarding the hemodynamic performance of this design is missing. Aim of this study was to compare the displacement forces (DF) acting on the hybrid AFX-Endurant design, with the classic AFX and Endurant endografts, in angulated and non-angulated cases based on patient-specific Computational Fluid Dynamics (CFD) simulations.

METHODS

3D endograft models of 11 treated AAA cases were reconstructed from Computed Tomography Angiography (CTA) imaging data: 5 cases of AFX, 3 cases of the combination AFX-Endurant and 3 cases of the classic Endurant design. The DF on the main-body, the iliac limbs, and the entire stent-graft was calculated by processing the velocity and pressure fields generated by pulsatile CFD simulations.

RESULTS

The range of total DF (acting on the whole endograft structure) in the AFX, hybrid AFX-Endurant and Endurant group was 2.5-5.2N, 2.0-5.9N and 1.9-2.9N respectively, with the maximum total DF being lower for Endurant. The DF on the main-body of the classic and hybrid AFX cases were higher than the right and left iliac limbs (2.5-4.9N vs. 0.6-5.3N and 0.7-3.6N respectively). Conversely, the DF on the main-body of the Endurant cases was comparable to the force exerted on the right and left limbs. When separating the cases with respect to their neck angulation, the DF on all endograft parts (main-body, limbs) and on the endograft as a whole were lower for the hybrid AFX-Endurant group compared to the classic AFX and Endurant groups, for cases with almost straight neck.

CONCLUSION

The off-label use of the hybrid AFX-Endurant stent-graft does not seem superior to the conventional AFX or Endurant endografts in angulated cases but was associated with lower DF than AFX or Endurant in non-angulated cases. The clinical value and utility of these findings remain to be elucidated.

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.

Efficiently Simulating an Endograft Deployment: A Methodology for Detailed CFD Analyses

Numerical models of endografts for the simulation of endovascular aneurysm repair are increasingly important in the improvement of device designs and patient outcomes. Nevertheless, current finite element analysis (FEA) models of complete endograft devices come at a high computational cost, requiring days of runtime, therefore restricting their applicability. In the current study, an efficient FEA model of the Anaconda™ endograft (Terumo Aortic, UK) was developed, able to yield results in just over 4 h, an order of magnitude less than similar models found in the literature. The model was used to replicate a physical device that was deployed in a 3D printed aorta and comparison of the two shapes illustrated a less than 5 mm placement error of the model in the regions of interest, consistent with other more computationally intensive models in the literature. Furthermore, the final goal of the study was to utilize the deployed fabric model in a hemodynamic analysis that would incorporate realistic fabric folds, a feature that is almost always omitted in similar simulations. By successfully exporting the deployed graft geometry into a flow analysis, it was illustrated that the inclusion of fabric wrinkles enabled clinically significant flow patterns such as flow stagnation and recirculation to be detected, paving the way for this modelling methodology to be used in future for stent design optimisation.

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.

Endograft Specific Haemodynamics After Endovascular Aneurysm Repair: Flow Characteristics of Four Stent Graft Systems

OBJECTIVES

The implication of haemodynamics in the occurrence of complications after endovascular aneurysm repair (EVAR) has been raised in the literature. Different aortic stent graft configurations may lead to different haemodynamic properties. The current study deals with the post-operative haemodynamic variability between four stent graft systems with different structure, material, and type of fixation.

METHODS

Computed tomography data of 32 patients were used, equally distributed among the four endograft groups, namely the AFX, Endurant, Excluder, and Nellix. Velocity, wall shear stress (WSS), and helicity statistics were calculated, in regions around the flow division where disturbances are expected. The haemodynamic data were compared between and within the groups.

RESULTS

The morphology of AAAs pre-operatively did not vary significantly among the four groups. Before the flow division, lowest velocity was observed in Endurant cases and highest in Nellix cases. Endurant induced the lowest peak WSS and Nellix the highest (p = .03). The helicity levels were low in AFX and Nellix cases and high in Endurant and Excluder cases. After the flow division, the trend in the results was preserved. Nellix induced the highest velocity and WSS, followed closely by Excluder and AFX. There was a significant increase of helicity before and after flow division in AFX (p <0.001, R² = 0.09) and Nellix (p <0.001) cases.

CONCLUSIONS

It has been shown that different types of endografts induce variable haemodynamic conditions around the flow division. The parallel limb structure, featured by Nellix, seems to induce favourable flow conditions in terms of velocity and WSS, while helical flow before the flow division is suppressed. High WSS is generally considered to be a desirable flow characteristic in endovascular devices, whereas helicity extremes (very low or high) are potentially a negative sign. Endurant, with the stiffer material and the short neck structure, was associated with the lowest blood velocity and WSS values but preserved high helicity levels. The AFX and Excluder, which include the same material, induced similar haemodynamic conditions.

Abdominal aortic aneurysm endovascular repair: profiling post-implantation morphometry and hemodynamics with image-based computational fluid dynamics

Endovascular aneurysm repair (EVAR) has disseminated rapidly as an alternative to open surgical repair for the treatment of abdominal aortic aneurysms (AAAs), because of its reduced invasiveness, low mortality and morbidity rate. The effectiveness of the endovascular devices used in EVAR is always at question as postoperative adverse events can lead to re-intervention or to a possible fatal scenario for the circulatory system. Motivated by the assessment of the risks related to thrombus formation, here the impact of two different commercial endovascular grafts on local hemodynamics is explored through 20 image-based computational hemodynamic models of EVAR-treated patients (N=10 per each endograft model). Hemodynamic features, susceptible to promote thrombus formation, such as flow separation and recirculation, are quantitatively assessed and compared with the local hemodynamics established in image-based infrarenal abdominal aortic models of healthy subjects (N=10). The hemodynamic analysis is complemented by a geometrical characterization of the EVAR-induced reshaping of the infrarenal abdominal aortic vascular region. The findings of this study indicate that: (1) the clinically observed propensity to thrombus formation in devices used in EVAR strategies can be explained in terms of local hemodynamics by means of image-based computational hemodynamics approach; (2) reportedly pro-thrombotic hemodynamic structures are strongly correlated with the geometry of the aortoiliac tract postoperatively. In perspective, our study suggests that future clinical follow up studies could include a geometric analysis of the region of the implant, monitoring shape variations that can lead to hemodynamic disturbances of clinical significance.