Computational comparison between the Altura aortic

endograft configuration and the classic bifurcated designs

Efstratios Georgakarakos, MD, MSc, PhD

Dept. of Vascular Surgery, Democritus University of Thrace, Greece

School of Medicine, University of Barcelona27th to 29th MARCH 2019

Disclosures: none

School of Medicine, University of Barcelona27th to 29th MARCH 2019

The Altura (Alt) endograft design comprises 2 proximal D-shaped endografts forming

a round circumference in the aortic neck

The 2 conduits land in the cross-limb fashion

This design totally lacks the main-body segment and flow divider

Introduction

Krievins et al, JEVT, 2018

School of Medicine, University of Barcelona27th to 29th MARCH 2019

Aim

Certain geometrical alterations can affect the hemodynamic profile of aortic

endografts (Georgakarakos et al., CVIR, 2014)

Therefore, aim of this computational study was to compare displacement forces and

flow differences between the Altura design and the classic bifurcated or cross-limbs

(Ballerina) design of equal total length

Altura (Alt) Classic Bifurcation (Bif)

Cross-limbs (Cx)

School of Medicine, University of Barcelona27th to 29th MARCH 2019

All computational models: angulation of neck and bifurcation being 35 ◦ και 60◦,

respectively (mean values of most endografts, according to IFU)

Inlet (neck) and outlet (iliacs) diameters of 28mm and 14mm, respectively

Total length for all models of 180mm, with main-body of 80mm for Bif and Cx.

Methodology – model dimensions

School of Medicine, University of Barcelona27th to 29th MARCH 2019

Use of commercially available and validated Finite Volume Analysis Application (ANSYS CFX,

Ansys Inc)

Construction of 3D tetrahedral meshes for estimation of hemodynamic parameters.

Fluid domains of model meshes comprising 307.091 & 356.715 elements, respectively.

Use of representative values for Poisson ratio (0.27), Young modulus (10 MPa),

density(6g/cm3) & endograft thickness (2mm).

Blood: non-Newtonian fluid (Carreau – Yasuda model).

Computational Fluid Dynamics (solving based on conservation of mass and momentum

equations).

Methodology

School of Medicine, University of Barcelona27th to 29th MARCH 2019

Hemodynamic parameters - equations

Displacement forces on models

Wall shear stress (WSS)

Time-averaged wall shear stress

(TAWSS)

Oscillatory shear index (OSI)

Relative residence time (RRT)

School of Medicine, University of Barcelona27th to 29th MARCH 2019

Hemodynamic parameters - interpretation

Displacement forces on models Acting over the entire length of models (sum)

Wall shear stress (WSS) Associated with formation of intraluminal thrombus

Time-averaged wall shear stress

(TAWSS)

integrating each nodal WSS magnitude over the cardiac

cycle

Oscillatory shear index (OSI)

Indicates flow reversal: 0 (min) when WSS is

unidirectional during cardiac cycle, 0.5 (max) when

equal positive and negative WSS areas under the WSS

vs. time curve

Relative residence time (RRT)

Measure of residence time of particles near the vessel

wall. High values present regions with flow recirculation

compared to regions with near-zero RRT

School of Medicine, University of Barcelona27th to 29th MARCH 2019

Methodology – boundary conditions (i.e., feeding the model with data)

Estimation during one cardiac cycle

School of Medicine, University of Barcelona27th to 29th MARCH 2019

Results

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Total displacement forces on endografts over a cardiac cycle

Altura Bifurcated cross-limbs

Alt was associated with lower forces (4.0-5.9N) than Bif (4.17-6.15N) and Cx and Bif (4.4-6.5N)

School of Medicine, University of Barcelona27th to 29th MARCH 2019

Results

Alt inlet presented higher TAWSS (Wall shear stress) (0.14-0.42Pa) than inlet of Cx and Bif (0.07-0.14Pa)

Alt limbs had higher TAWSS (0.42-0.56Pa) than Cx & Bif iliacs (0.14-0.56Pa)

School of Medicine, University of Barcelona27th to 29th MARCH 2019

Results

Alt inlet presented higher OSI (Oscillatory shear index) (0.3-0.4) than inlet of Cx and Bif (0.05-0.15)

All models shared the highest OSI values (0.35-0.5) at the main body

Lowest OSI values were detected at the iliac sites (<0.15)

School of Medicine, University of Barcelona27th to 29th MARCH 2019

Results

Highest RRT values were detected in the main-body of Bif and Cx and D-shaped central segment of Alt

Iliac limbs of all models shared low Relative residence time (RRT) values (<6Pa-1

) along their entire length

School of Medicine, University of Barcelona27th to 29th MARCH 2019

Results

Helicity was more prominent at the cross configurations (Alt, Cx)

Helicity also present at the proximal 2 D-shaped segment of Alt

vs.

vs.

School of Medicine, University of Barcelona27th to 29th MARCH 2019

Conclusions

The D-shaped endograft design is associated with lower

displacement forces than other designs

Better stability to migration theoretically

It presents also higher oscillatory shear index with higher stress values at the central

segment

Hence, this central dual-lumen segment may be more prone to

thrombosis

The Alt design seems to present iliac limbs of equal hemodynamic profile

School of Medicine, University of Barcelona27th to 29th MARCH 2019

Computational results are subject to several limitations:

Idealized geometries instead of patient specific

Model walls: undeformable and rigid rather than elastic

Blood assumed non-Newtonian

Not taking into account the unique structure of Alt; endoskeleton of braided stent

attached to fabric only centrally and caudally

Further parametric simulation studies will provide further information

Findings should be confirmed by in-vitro experiments

Comparative studies should validate our observations in the clinical field!

Not to forget!

School of Medicine, University of Barcelona27th to 29th MARCH 2019

Thank you!