- Open Access
Hemodynamic performance within crossed stent grafts: computational and experimental study on the effect of cross position and angle
© The Author(s) 2018
- Received: 18 April 2018
- Accepted: 12 June 2018
- Published: 19 June 2018
Background and aims
The crossed limbs stent graft technique is regularly employed to treat abdominal aortic aneurysm patients with unfavorable aneurysm necks or widely splayed common iliac arteries. This article numerically evaluates the hemodynamic performance of the crossed limbs strategy by analyzing numerical simulations and conducting experiments using two series of idealized bifurcated stent grafts with different cross angles and cross positions.
Results demonstrated that the absolute helicity at outlets decreased with increased cross angles and increased with decreased cross positions. The time-averaged wall shear stress remained approximately unchanged, whereas the oscillating shear index and relative resident time decreased slightly when the cross angle increased and cross position decreased in iliac grafts. Additionally, both numerical and in vitro experimental results indicate the displacement force acting on the stent graft gradually increased as cross angles increased and cross positions decreased. Results further indicated that strip areas with a high oscillating shear index and high relative resident time, which may be vulnerable to thrombosis formation, exist along the outer surface of the iliac artery grafts.
Given this information, the optimal crossed limbs configuration may contain a small cross angle and low cross position; however, low cross positions may increase the risk of migration.
- Bifurcated stent graft
- Helical flow
Clinically, bifurcated stent grafts comprised of modular components that allow graft-guided blood flow to the two iliac arteries are most commonly used in endovascular aneurysm repair (EVAR) [1, 2]. The conventional surgery procedure employs proximal neck deployment (with an attached ipsilateral iliac limb) first, followed by contralateral iliac limb deployment. This conventional EVAR procedure becomes more complex and difficult when abdominal aortic aneurysm (AAA) patients have an unfavorable anatomy such as widely splayed common iliac arteries or severe aneurysm necks [3, 4]. Surgeons sometimes employ the “crossed limbs” technique to overcome such problems, in which the limbs of a bifurcated stent graft (BSG) are rotated into the so-called “ballerina position” at the discretion of the surgeon [4–6].
Several studies have reported that the migration force acting on the stent graft due to blood flow is much lower for the crossed limbed strategy relative to the conventional strategy [5, 7]. The crossed limbs strategy generates helical flows within the iliac artery grafts [5, 6], resembling patterns typically observed in the human aorta [8, 9]. Helical flow patterns have been documented to suppress atherogenic lipid deposition within the arterial wall, enhance oxygen supply to arteries, and reduce platelet adhesion [10, 11], thereby protecting the arterial wall from atherosclerosis and thrombosis formation [8, 12–15]. The crossed limbs strategy is thus believed to be advantageous for AAA repair treatment.
Geometry and meshing
Computational meshes were modeled using ANSYS ICEM CFD (ANSYS Inc., Canonsburg, PA). Surface meshes contained a mixture of tetrahedral and hexahedral volume meshes. The maximum and minimum mesh sizes were 0.8 and 0.05 mm, respectively. Each mesh contained approximately 1.7 million cells.
The finite volume method was adopted to solve the mass and momentum conservation equations using ANSYS Fluent CFD (ANSYS Inc., Canonsburg, PA). These calculations were performed in 200-step cycles, with a step time of 0.005 s. A pressure-based solver was used with a second-order upwind scheme for momentum spatial discretization. The residual continuity and velocity were both assigned values of 1.0 × 10−5. Five pulsatile cycles were computed to obtain a periodic solution, and a sixth was used as the final solution. MATLAB (MathWorks) and Tecplot (Tecplot) were used in post-processing to analyze data and observe results.
Quantities of interest
TAWSS, OSI, and RRT distributions
In-vitro experiment: aortic perfusion model
A BSG was inserted into the circuit with its proximal portion anchored to a strain gauge load cell via rigid connectors. The BSG used had a diameter of 26 mm at the proximal end, and 15 mm at the distal end. The BSG length was 180 mm which included the iliac graft length of 140 mm. The BSG was secured with ligatures and placed on the outside surface of the connectors to ensure that the displacement force in the vertical direction could have a maximal transfer to the load cell. The proximal and distal BSG portions were connected to the silicone tube by a soft rubber tube with highly elasticity, allowing slight displacements of the BSG, so the displacement force measurements would not be influenced. The measurement range of the load cells was 0–10 N, and calibration was performed with weights. A pressure transducer was inserted into the circuit to monitor the pressure within the iliac graft. A force monitor was connected to the load cell to display and record force values.
Displacement force measurements were conducted at perfusion pressures of 60, 80, and 100 mmHg. Perfusion pressure zero leveling and in situ calibration were performed before each measurement. Displacement force measurements were recorded every 20 s under steady flow conditions. Displacement forces are presented as mean values.
Flow-induced displacement forces (N) acting on the BSG under various cross angles and perfusion pressures
Flow-induced displacement forces (N) acting on the BSG under various cross position ratios and perfusion pressures
Cross position ratio
The crossed limbs AAA repair strategy has been often used in AAA patients with unfavorable aneurysm neck angulation or widely splayed common iliac arteries [3, 4]. Two geometric features, the cross angle α and cross position l, affect the hemodynamic performance of this strategy. This article described the construction of two crossed limbs series models with various cross angles and positions and subsequently the comparative numerical investigation of flow patterns in these models for hemodynamic performance evaluation in terms of the helical flow strength, TAWSS, OSI, RRT, and displacement force.
This study revealed that double helical blood flows, consisting of a dominant left-handed helical flow and smaller right-handed helical flow, were generated. This closely resembles results reported by Shek et al. [5, 6]. The discrepancy between the current results and theirs is that their models had a nonplanar feature whereas the current ones did not. As it is believed helical blood flows in the arterial system have physiological functions, protecting the arteries by suppressing the accumulation of atherogenic low density lipoproteins within the arterial wall , enhancing O2 supply to the artery , and reducing platelet/monocyte adhesion [10, 11], the crossed limbs strategy is beneficial for AAA treatment from the perspective of helical flow generation. The results obtained in this study indicate that the intensity of helical flow strength produced in the crossed limbs strategy decreased with increasing cross angles and decreasing cross position ratios. Small cross angles and low cross positions should thus be considered when implementing the crossed limbs strategy.
These results also showed that the TAWSS on the iliac artery grafts remained approximately equal as the cross angle increased, whereas the OSI and RRT decreased. These hemodynamic indicators demonstrated the same tendencies, but to negligible extents (< 3%), with cross position ratio increases. Despite these minor differences, significant high OSI and RRT strip areas appeared on the outer surface of the cross parts. High OSI strip areas were also observed in the crossed stent graft study performed by Shek et al. . It has been widely recognized that high OSIs and RRTs lead to thrombosis by stimulating platelet aggregation, activating platelets, and increasing the residence time of procoagulant microparticles [28–30]. These strip areas are therefore vulnerable to thrombosis formation, potentially resulting in long-term stent graft failure.
Clinically, stent graft migration remains a well-recognized complication [26, 31]. According to the study by Li et al. , migration behavior could be influenced by several factors including the iliac bifurcation angle, endograft size, blood pressure, endograft wall compliance, iliac branch curvature, and neck length. For instance, in the study by Li et al. , the displacement force was found to increase nonlinearly with the iliac angle and blood pressure. By performing numerical simulations and conducting in vitro experiments, the results of this study not only showed that the perfusion pressure could be a significantly influential factor of migration behavior, but also that the cross positions and angles of the crossed limbs strategy are significant factors affecting BSG migration behavior. Lower cross positions and larger cross angles correlate to stronger displacement forces acting on BSGs. These results could be ascribed to the causative relationship between large cross angles or low cross positions and increases in the projected iliac artery wall surface area onto the transverse plane. Lower cross positions and larger cross angles can therefore increase the risk of stent graft migration in the crossed limbs strategy.
In the present study, although idealized crossed limbs geometric models with various cross angles and positions were constructed for simplification, it remains reasonable to draw hemodynamic parameter variation trends from the simplified results. For example, the graft wall was assumed to be rigid when it is not; however, numerical studies have shown that stent graft deformation under blood pressure is not apparent owing to its high stiffness (10 MPa) [17, 19]. Therefore, although the above simplifications might affect the accuracy of the simulation results, the major conclusions should remain the same.
In summary, the strip areas of high OSI and RRT on the outer surface of the iliac artery grafts might be vulnerable to thrombosis formation. A minor cross angle and a low cross position may be optimal configurations for “crossed limbs” strategy implementation; however, low cross positions may increase the risk of migration.
ML and AS participated in the design of the study and drafted the manuscript; AS and XD coordinated the study and helped draft the manuscript. All authors read and approved the final manuscript.
We would like to acknowledge the funding agencies for the support of the work.
The authors declare that they have no competing interests.
Availability of data and materials
The datasets used and/or analysed during the current study are available from the corresponding author on reasonable request.
Consent for publication
We consent for publication of this work.
Ethics approval and consent to participate
This work was supported by the National Natural Science Foundation of China (Nos. 11472031, 11572028), National Key Research and Development Plan (2017YFB0702501) of China and the Natural Science Foundation of Jiangsu Province (BK20161366).
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