Multi-Objective Optimisation of a Novel Bypass Graft with a Spiral Ridge.

The low long-term patency of bypass grafts is a major concern for cardiovascular treatments. Unfavourable haemodynamic conditions in the proximity of distal anastomosis are closely related to thrombus creation and lumen lesions. Modern graft designs address this unfavourable haemodynamic environment with the introduction of a helical component in the flow field, either by means of out-of-plane helicity graft geometry or a spiral ridge. While the latter has been found to lack in performance when compared to the out-of-plane helicity designs, recent findings support the idea that the existing spiral ridge grafts can be further improved in performance through optimising relevant design parameters. In the current study, robust multi-objective optimisation techniques are implemented, covering a wide range of possible designs coupled with proven and well validated computational fluid dynamics (CFD) algorithms. It is shown that the final set of suggested design parameters could significantly improve haemodynamic performance and therefore could be used to enhance the design of spiral ridge bypass grafts.

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.

Computational Comparison Between a Classic Bifurcated Endograft and a Customized Model With "Dog Bone"-Shaped Limbs.

PURPOSE: To use computational simulations to compare the hemodynamic characteristics of a classic bifurcated stent-graft to an equally long endograft design with "dog bone"-shaped limbs (DB), which have large diameter proximal and distal ends and significant narrowing at the midsection to accommodate aneurysms with an extremely narrow bifurcation. MATERIALS AND METHODS: A 3-dimensional model was constructed using commercially available validated software. Inlet and outlet diameters were 28 and 14 mm, respectively. The total length of both models was kept constant to 180 mm, but the main body of the DB model was 20 mm shorter than the bifurcated endograft. The iliac limbs of the DB model had a 9-mm stenosis over a 30-mm segmental length in the midsection. Flow was quantified by time-averaged wall shear stress, oscillatory shear index (OSI), and relative residence time (RRT). The displacement forces in newtons (N) and maximum wall shear stress (WSS) in pascals (Pa) were compared during a cardiac cycle at 3 segments (main body, bifurcation, and iliac limbs) of both models with computational fluid dynamics analysis. RESULTS: The DB accommodation was associated with higher forces at the main body (range 3.15-4.9 Ν) compared with the classic configuration (1.56-2.34 N). On the contrary, the forces at the bifurcation (3.81-5.98 vs 3.76-5.54 N) and at the iliac limbs (0.34-0.85 vs 0.49-0.74 N) were comparable for both models. Accordingly, maximum WSS was detected at the iliac sites for both models throughout the cardiac cycle. The highest values were detected at peak systole and equaled 26.6 and 12 Pa for the DB and bifurcated configurations, respectively. The narrow segments in the DB model displayed high stress values but low OSI and very low RRT. CONCLUSION: The DB accommodation seems to correlate with higher displacement forces at the main body and higher stresses at the iliac limbs. Consequently, regular imaging follow-up of the DB design deems necessary to delineate its mid- and long-term clinical performance.

Estimating the hemodynamic influence of variable main body-to-iliac limb length ratios in aortic endografts.

BACKGROUND: We conducted a computational study to assess the hemodynamic impact of variant main body-to-iliac limb length (L1/L2) ratios on certain hemodynamic parameters acting on the endograft (EG) either on the normal bifurcated (Bif) or the cross-limb (Cx) fashion. METHODS: A customary bifurcated 3D model was computationally created and meshed using the commercially available ANSYS ICEM (Ansys Inc., Canonsburg, PA, USA) software. The total length of the EG, was kept constant, while the L1/L2 ratio ranged from 0.3 to 1.5 in the Bif and Cx reconstructed EG models. The compliance of the graft was modeled using a Fluid Structure Interaction method. Important hemodynamic parameters such as pressure drop along EG, wall shear stress (WSS) and helicity were calculated. RESULTS: The greatest pressure decrease across EG was calculated in the peak systolic phase. With increasing L1/L2 it was found that the Pressure Drop was increasing for the Cx configuration, while decreasing for the Bif. The greatest helicity (4.1 m/s2) was seen in peak systole of Cx with ratio of 1.5 whereas its greatest value (2 m/s2) was met in peak systole in the Bif with the shortest L1/L2 ratio (0.3). Similarly, the maximum WSS value was highest (2.74Pa) in the peak systole for the 1.5 L1/L2 of the Cx configuration, while the maximum WSS value equaled 2 Pa for all length ratios of the Bif modification (with the WSS found for L1/L2=0.3 being marginally higher). There was greater discrepancy in the WSS values for all L1/L2 ratios of the Cx bifurcation compared to Bif. CONCLUSIONS: Different L1/L2 rations are shown to have an impact on the pressure distribution along the entire EG while the length ratio predisposing to highest helicity or WSS values is also determined by the iliac limbs pattern of the EG. Since current custom-made EG solutions can reproduce variability in main-body/iliac limbs length ratios, further computational as well as clinical research is warranted to delineate and predict the hemodynamic and clinical effect of variable length ratios.

The shear stess profile of the pivotal fenestrated endograft at the level of the renal branches: A computational study for complex aortic aneurysms.

PURPOSE: This study investigated the impact of the variant angulations on the values and distribution of wall shear stress on the renal branches and the mating vessels of a pivotal fenestrated design. METHODS: An idealized endograft model of two renal branches was computationally reconstructed with variable angulations of the left renal branch. These ranged from the 1:30' to 3:30' o'clock position, corresponding from 45° to 105° with increments of 15°. A fluid-structure-interaction analysis was performed to estimate the wall shear stress. RESULTS: The proximal part of the renal branch preserved quite constant wall shear stress. The transition zone between its distal end and the renal artery showed the highest values compared to the proximal and middle segments, ranging from 8.9 to 12.4 Pa. The lowest stress values presented at 90° whereas the highest at 45°. The post-mating arterial segment showed constantly low stress values regardless of the pivotal branch angle (6.3 to 6.6 Pa). The 45° configuration showed a distribution of the highest stress posteriorly whereas the 105°-angulation anteriorly. CONCLUSIONS: The variant horizontal branch orientation influences the wall shear stress distribution across its length and affects its values only at its transition with the mating vessel. These findings and their potential association with adverse effects deserve further clinical validation.

Studying the flow dynamics in an aortic endograft with crossed-limbs.

PURPOSE: To evaluate the flow phenomena within an aortic endograft with crossed-limbs, comparing to an endograft with the ordinary limb bifurcation. METHODS: An endograft model with crossed-limbs was computationally reconstructed based on Computed Tomography patient-specific data, using commercially available software. Accordingly, its analogue model was reconstructed in the ordinary fashion (ordinary bifurcation). Computational fluid dynamics analysis was performed to determine and compare the flow fields, velocity profiles, pressure and shear stress distribution throughout the different parts of both endograft configurations, in different phases of the cardiac cycle. RESULTS: The flow patterns between the "Ballerina" and the classic endograft were similar, with flow disturbance near the inlet zone at late diastole and smooth flow patterns during the systolic phase. Both configurations presented similar pressure distribution patterns throughout the cardiac cycle. The highest and lowest pressures were demonstrated in the inlet-main body area and the iliac limbs, respectively. Marked differences were observed in the velocity profiles of the proximal limb segments between the two configurations, mostly in the peak- and end-systolic phase. The regions of lower velocities correlated well to low shear values. Differences in the shear stress distribution were noted between the two configurations in the systolic and, predominantly, in the diastolic phase. CONCLUSIONS: There are differences in the velocity profiles and shear distribution between the limbs of the two endograft configurations. The pathophysiologic implication of our findings and their possible association with clinical events, such as thrombus apposition, deserves further investigation.

Geometric factors affecting the displacement forces in an aortic endograft with crossed limbs: a computational study.

PURPOSE: To compare the hemodynamic behavior between an aortic endograft model in the "crossed-limbs" configuration and the customary bifurcated deployment position under the influence of several geometric factors. METHODS: A crossed-limbs graft and its analogue model with uncrossed limbs were computationally reconstructed. The displacement forces acting over the entire endograft and at the bifurcation and iliac sites separately were calculated using a fluid structure interaction simulation under a range of specific geometric characteristics, namely, the lateral and anteroposterior (AP) neck angulation, the iliac bifurcation angulation, and the endograft curvature. RESULTS: The variations of lateral neck angulation caused a constantly higher total displacement force for the crossed-limbs graft, whereas the force at the bifurcation of the two configurations differed only within a narrow range of 30° to 50°. On the contrary, the displacement force at the iliac site was higher in the crossed-limbs configuration only with lateral neck angulation >50°, reaching its highest value at 70°. The variations of AP neck angulation also caused higher total displacement forces in the crossed-limbs graft. Increasing AP neck angulation values caused generally lower forces at the crossed iliac limbs and higher at its bifurcation, respectively, compared to the uncrossed limbs model. Similarly, the influence of high iliac bifurcation angulation and endograft curvature was associated with slightly elevated forces over the entire crossed-limbs graft and its bifurcation, whereas the opposite held true at the iliac site. CONCLUSION: Apart from minor differentiations due to geometric alterations, the customary bifurcated and crossed-limbs endografts present similar hemodynamic performance. Further clinical studies should be conducted to confirm the clinical applicability of these findings.

A computational study of the factors influencing pressure in arteriovenous fistulae venous aneurysms.

PURPOSE: To investigate the factors influencing the hydrostatic pressure exerted within the venous aneurysms (VA) of an arteriovenous fistula (AVF). METHODS: Ideal models of a side-to-end brachial-cephalic AVF were computationally constructed and typical values for the length and the local diameters were considered for both the artery and vein sections of the models. Three VA configurations were reconstructed (spherical, fusiform and curved) and hydrostatic pressure was assessed with respect to different degrees of the outflow vein stenosis, ranging from 25% to 95%, and VA maximum diameters, using validated, commercially available software. RESULTS: The pressure in the VA was steady (1200 Pa) for venous outflow stenoses up to 75%. For stenoses greater than 75% a exponential pressure rise was observed, reaching 1500 Pa for stenoses of 95%. Neither the VA configuration nor its maximum diameter affected the pressure values exerted within the VA or the point of the pressure upstroke. CONCLUSIONS: our study supports the presence of a critical stenotic outflow vein diameter beyond which there is an exponential VA pressure increase, influenced neither by the shape nor the size of the VA. Whether the prompt, non-invasive detection of this finding can contribute or lead to the determination of a criterion for early intervention in VAs before clinical complications are developed, should be investigated by future studies.

Modeling and computational analysis of the hemodynamic effects of crossing the limbs in an aortic endograft ("ballerina" position).

PURPOSE: To evaluate the displacement forces acting on an aortic endograft when the iliac limbs are crossed ("ballerina" position). METHODS: An endograft model was computationally reconstructed based on data from a patient whose infrarenal aortic aneurysm had an endovascular stent-graft implanted with the iliac limbs crossed. Computational fluid dynamics analysis determined the maximum displacement force on the endograft and separately on the bifurcation and iliac limbs. Its analogue model was reconstructed for comparison, assuming the neck, main body, and total length constant but considering the iliac limbs to be deployed in the usual bifurcated mode. Calculations were repeated after developing "idealized" models of both the bifurcated and crossed-limbs endografts with straight main bodies and no neck angulation or curved iliac segments. RESULTS: The vector of the total force was directed anterocaudal for both the typical bifurcated and the crossed-limbs configurations, with the forces in the latter slightly reduced and the vertical component accounting for most of the force in both configurations. Idealized crossed-limbs and bifurcated configurations differed only in the force on the iliac limbs, but this difference disappeared in the realistic models. CONCLUSION: Crossing of the iliac limbs can slightly affect the direction of the displacement forces. Although this configuration can exert larger forces on the limbs than in the bifurcated mode, this effect can be blunted by concomitant modifications in the geometry of the main body and other parts of the endograft, making its hemodynamic behavior resemble that of a typically positioned endograft.

Application of bioengineering modalities in vascular research: evaluating the clinical gain.

Using knowledge gained from bioengineering studies, current vascular research focuses on the delineation of the natural history and risk assessment of clinical vascular entities with significant morbidity and mortality, making the development of new, more accurate predictive criteria a great challenge. Additionally, conclusions derived from computational simulation studies have enabled the improvement and modification of many biotechnology products that are used routinely in the treatment of vascular diseases. This review highlights the promising role of the bioengineering applications in the vascular field.