Numerical simulation of a transcatheter aortic valve

In a transcatheter percutaneous aortic valve implantation (TAVI), the interaction of blood, stent and aorta are of central importance to determine implants lifetime.
CADFEM (Suisse) AG and ADMEDES SCHUESSLER GmbH developed together a numerical modeling procedure using commercial software to consider stent crimping and release against the aortic wall followed by a fluid-structure transient simulation during one systolic pulse.

Numerical simulation of a transcatheter aortic valve

Task

In a transcatheter percutaneous aortic valve implantation (TAVI), the interaction of blood, stent and aorta are of central importance in order to determine implants lifetime.

CADFEM (Suisse) AG and ADMEDES SCHUESSLER GmbH developed together a numerical modeling procedure using commercial software to consider stent crimping and release against the aortic wall followed by a fluid-structure transient simulation during one systolic pulse.

Methods

In Nitinol self expandable stents, fatigue analyses are based on the determination of accurate stress cycles in the material. The most relevant steps were simulated using both finite elements and finite volumes discretizations, for the structural and fluid domains respectively, within the ANSYS Multiphysics Workbench platform. Starting from the stent in the undeformed state (Fig. 1a), a first structural analysis delivered its stress and deformation state after crimping and positioning against a stiff aorta wall (Fig. 1b). The pre-stressed structure was then connected to the pericardium valve leaflets (Fig. 1c). The pressure forces transferred from blood to valve leaflets, stent and aorta wall were then taken into account during a weakly coupled transient fluid-structure interaction analysis (Fig. 1d) over a complete systolic blood pulse.

The following effects were taken into account: frictional contacts between stent and wall, nonlinear material behavior for shape memory alloy (super-elastic model), binding points between valve and stent, pericardium orthotropic properties and non-Newtonian blood properties (Carreau model).

 

 

Numerical simulation of a transcatheter aortic valve
Representation of the simulation steps; expanded (a), crimping (b), binding with valve (c), fluid-structure interaction over time (d).

Results

This study delivered preliminary information about the evolution of forces, pressures and stresses during a heart pulse and their effects on working conditions, mechanical integrity and lifetime assessment of the device. Stress - strain curves over a cycle were extracted at the most critical locations in the stent and compared with the fatigue properties of Nitinol.

Discussion

The capability to model valve insertion and positioning inside the aorta and interaction with blood flow during a systolic pulse was demonstrated using a state of the art analysis technique provided by the commercial software ANSYS. The results obtained under the assumption of a perfectly stiff aorta wall (strong stenosis) should however be complemented with further simulations using flexible walls.

The ultimate goal of such simulations is the evaluation of patient-specific configurations, due to the possibility to repeat the analysis with different geometric and material parameter sets. Furthermore, this modeling technique could also be applied for other stent percutaneous interventions such as intravascular stenoses, atherosclerosis or aneurisms.


CADFEM (Suisse) AG

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