Hemodynamic parameters within aortal stent-grafts vs. their spatial configuration : a comparison based on computer simulations

Background: It is difficult to find reliable premises which would enable a modification of the construction of stent-grafts to improve their durability and stability. Many systemic conditions make a comparison of homogeneous groups of operated patients a complicated task. Hence, it is helpful to use computer simulations and to verify them in a clinical observation. The hemodynamic parameters within aortal stent-grafts depending on their spatial configuration were compared using a computer simulation. Material/Methods: Computer simulations were made for 6 patients with abdominal aortal aneurysm (AAA) in whom bifurcated aortal stent-grafts were implanted. A basis for a spatial model were angio-CT data. Flow rate parameters were obtained in US-CD. In numerical calculations, CFD - Fluent® 6.2 software was used. Numerical grids (about 300,000 tetrahedral elements) were generated on the basis of three-dimensional geometries of AAA segmented from CT scans. A laminar character of flow was assumed. Blood viscosity was described by Quemada's rheological model. In all patients, two variants of the graft geometry were generated assuming that common long-body and shortbody grafts were applied. The patients’ real anatomical conditions were taken for the simulations. Pressure drop on the graft level and wall shear stress were analyzed. Results: It was found that the short-body graft caused a higher pressure drop along the inlet-outlet segment. The long-body graft offered smaller resistance to blood flow, and, consequently, the shear stress was lower. For the rate around 0.8 m/s, the difference reached 5500 Pa. In both variants, the highest value of shear stresses occurred near the bifurcation area. Conclusions: An increase of the shear stress is more distinct when the short- body graft is used, which can suggest that this part of the graft should be as long as possible

A Novel Attempt to Standardize Results of CFD Simulations Basing on Spatial Configuration of Aortic Stent-Grafts.

Currently, studies connected with Computational Fluid Dynamic (CFD) techniques focus on assessing hemodynamic of blood flow in vessels in different conditions e.g. after stent-graft's placement. The paper propose a novel method of standardization of results obtained from calculations of stent-grafts' "pushing forces" (cumulative WSS--Wall Shear Stress), and describes its usefulness in diagnostic process. AngioCT data from 27 patients were used to reconstruct 3D geometries of stent-grafts which next were used to create respective reference cylinders. We made an assumption that both the side surface and the height of a stent-graft and a reference cylinder were equal. The proposed algorithm in conjunction with a stent-graft "pushing forces" on an implant wall, allowed us to determine which spatial configuration of a stent-graft predispose to the higher risk of its migration. For stent-grafts close to cylindrical shape (shape factor φ close to 1) WSS value was about 267 Pa, while for stent-grafts different from cylindrical shape (φ close to 2) WSS value was about 635 Pa. It was also noticed that deformation in the stent-graft's bifurcation part impaired blood flow hemodynamic. Concluding the proposed algorithm of standardization proved its usefulness in estimating the WSS values that may be useful in diagnostic process. Angular bends or tortuosity in bifurcations of an aortic implant should be considered in further studies of estimation of the risk of implantation failure

The WSS values in a function of shape factor <i>φ</i> for analyzed stent-grafts.

<p>Correlation was calculated with Spearman non-parametric test.</p

The shape factors and WSS values for the analyzed stent-grafts.

<p>The shape factors and WSS values for the analyzed stent-grafts.</p

Reference blood velocity profile from USG-Doppler used as a inlet boundary condition for analyzed stent-grafts.

<p>Reference blood velocity profile from USG-Doppler used as a inlet boundary condition for analyzed stent-grafts.</p

An example for reconstructed spatial configuration of analyzed stent-grafts.

<p>An example for reconstructed spatial configuration of analyzed stent-grafts.</p

Spatial configuration of stent-grafts that are not on the regression line.

<p>With red colour stent-grafts that are not on the regression line. With violet colour corresponding by shape factor, stent-grafts close to the regression line.</p

Geometrical parameters (angular bends and tortuosity) for the selected stent-grafts.

<p>A_L—angular bend in left branch of a stent-graft, A_R—angular bend in the right branch of a stent-graft, T_L—tortuosity of a left branch of a stent-graft, T_R—tortuosity of a right branch of a stent-graft.</p

A scheme of reference cylinder for each stent-graft calculation procedure.

<p>A scheme of reference cylinder for each stent-graft calculation procedure.</p