Biomedical Engineering Reference
In-Depth Information
As regards the mechanical behaviour of the calcific elements, an isotropic hyper-
elastic constitutive model based on the Ogden strain energy function [
17
] has been
chosen, by fitting the mechanical response of calcific atherosclerotic tissue published
by Loree and colleagues [
21
].
In order to evaluate the dynamic behaviour of these models and compare the
pathological models to model H, structural dynamic simulations of the cardiac cycle
were performed, as described by Sturla [
16
]. Briefly, standard physiological time-
dependent aortic and ventricular pressures were applied to the inner surface of the AR
wall, after applying a preload of 85mmHg: the aortic pressure was applied to the
structures downstream the AV, while the ventricular pressure was imposed on the
LVOT inner surface. On the AV leaflet surface the difference between the ventricular
and aortic pressure was applied.
2.2 TAV Simulations
In what follows TAV models and simulations will be described in details.
TAV Model
. The balloon-expandable Edwards Sapien
size 26mm (Edwards Life-
sciences Corporations, Irvine, CA, USA) was considered. This device is made of a
stainless steel (AISI 316 LVM) stent, a bovine pericardium valve and a fabric skirt.
The TAV stent model, previously described in the work of Tzamtzis and colleagues
[
22
], was discretized with 169,000 reduced integration tetrahedral elements using the
software Gambit
(Ansys Inc., Canonsburg, PA, USA). A finer mesh was realized
in the regions of the stent that experience higher level of mechanical stresses during
the crimping and deployment phases (e.g. at the joints of the beams). A bilinear
elasto-plastic model based on the Von Mises yielding criterion was used to define the
stent mechanical properties: the model parameters were obtained from experimental
mechanical characterization (E
=
293GPa,
ʽ
=
0
.
3,
˃
y
=
340MPa, E
t
=
715MPa)
[
22
].
The leaflet model was discretized with 10,800 linear 3-node shell elements [
17
].
An isotropic hyperelastic Ogden constitutive model was chosen and the strain en-
ergy function parameters were obtained by fitting uniaxial tensile tests performed on
bovine pericardium specimens at 37
◦
C. In Fig.
2
a the CAD model of the entire TAV
device is shown.
TAV Simulations
. The implantation and function of the TAV model was simulated
through a multi-step procedure, summarized as follows:
1. Crimping simulation of the TAV stent
2. Deployment simulation of the stent within the three AR models
3. Positioning of the TAV leaflet model within the three post-implantation stent
configurations
4. TAV function simulation throughout the cardiac cycle
