Computational Fluid Dynamics Evaluation of the Cross-Limb Stent Graft Configuration for Endovascular Aneurysm Repair

Abstract

The technique of crossing the limbs of bifurcated modular stent grafts for endovascular aneurysm repair (EVAR) is often employed in the face of splayed aortic bifurcations to facilitate cannulation and prevent device kinking. However, little has been reported about the implications of cross-limb EVAR, especially in comparison to conventional EVAR. Previous computational fluid dynamics studies of conventional EVAR grafts have mostly utilized simplified planar stent graft geometries. We herein examined the differences between conventional and cross-limb EVAR by comparing their hemodynamic flow fields (i.e., in the "direct" and "cross" configurations, respectively). We also added a "planar" configuration, which is commonly found in the literature, to identify how well this configuration compares to out-of-plane stent graft configurations from a hemodynamic perspective. A representative patient's cross-limb stent graft geometry was segmented using computed tomography imaging in Mimics software. The cross-limb graft geometry was used to build its direct and planar counterparts in SolidWorks. Physiologic velocity and mass flow boundary conditions and blood properties were implemented for steady-state and pulsatile transient simulations in ANSYS CFX. Displacement forces, wall shear stress (WSS), and oscillatory shear index (OSI) were all comparable between the direct and cross configurations, whereas the planar geometry yielded very different predictions of hemodynamics compared to the out-of-plane stent graft configurations, particularly for displacement forces. This single-patient study suggests that the short-term hemodynamics involved in crossing the limbs is as safe as conventional EVAR. Higher helicity and improved WSS distribution of the cross-limb configuration suggest improved flow-related thrombosis resistance in the short term. However, there may be long-term fatigue implications to stent graft use in the cross configuration when compared to the direct configuration.

Numerical and Experimental Investigation of Novel Blended Bifurcated Stent Grafts with Taper to Improve Hemodynamic Performance

“…The deflected BBSG was designed based on the “crossed limbs” deployment strategy. When using the “crossed limbs” strategy in EVAR, the iliac grafts usually get crossed, which generates the helical flow [ 2 , 5 ]. The differences between the conventional and novel designed deflected BBSGs are that the division surface of the bilateral iliac grafts for conventional BBSG is located in the symmetry plane of the iliac grafts, whereas the division surface of the bilateral iliac grafts for deflected BBSG is located in the base plane of the iliac grafts.…”

“…To resolve these problems, the deployment technique of intentionally crossing the limbs of the BSG is regularly used [ 2 – 4 ]. With this deployment strategy, the cannulation time and postoperative complication rate in the face of atypical anatomy could be reduced significantly [ 2 , 4 , 5 ].…”

“…Because of these hemodynamic benefits of helical flow, it has been applied by researchers to design vascular devices to avoid intimal hyperplasia and thrombus formation, which are often caused by an unfavorable hemodynamic environment [ 9 ]. In clinical applications, the “crossed limbs” strategy might generate helical flows within the limb of crossed stent grafts [ 3 , 5 ], which is believed to be advantageous in AAA treatment. One significant feature of the arterial system that needs to be addressed is its taper, or the decreasing diameter of an artery as it becomes an arteriole.…”

“…This type of BBSG is believed to improve hemodynamic performance and provide better long-term outcomes. In this study, the deflected BBSG was proposed based on the specific configuration, including the “crossed limbs” strategy and the BBSG, proposed by Morris et al [ 5 , 10 ]. Furthermore, the taper configuration was added to both the conventional and deflected types of BBSGs to obtain the desired high helical flow.…”

Applying Findings of Computational Studies in Vascular Clinical Practice: Fact, Fiction, or Misunderstanding?

“…[16][17][18] Indeed, it seems difficult to compare the hemodynamic or clinical performance of different types of endografts between several patient groups, since certain patient-specific factors, e.g., the configuration of the iliac vessels and the level of peripheral occlusive disease, may confound the effect of the endograft's structural differences on limb patency and incidence of migration and endoleaks. [19][20][21][22] Consequently, under this perspective, subtle geometrical models of unusual or complex endograft patterns, such as the cross-limbs configuration [19][20][21] and fenestrated 22 designs, respectively, can be computationally examined, preceding or even inspiring large-scale clinical studies on the same issue. More intriguingly, as modeling techniques evolve, finite element analysis can be performed for direct comparisons of the mechanical stresses and strains acting on a number of commerciallyavailable device components under a variety of angulations, depicting potential advantages or drawbacks between them.…”