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Computational Analysis of Transcatheter Bioprosthetic Valve Design
Gunning, P., McNamara, L.M.
Department of Mechanical & Biomedical engineering, NUI, Galway, Ireland
E-mail: p.gunning1@nuigalway.ie
Abstract
Transcatheter Aortic Valve (TAV) replacement is
an endovascular alternative to conventional heart valve
surgery whereby a new aortic valve is implanted
through a minimally invasive approach. TAV
replacement substantially lowers patient risk and
results in a shorter recovery time, particularly in the
treatment of elderly patients or patients at high risk.
However despite this the long term outcomes of TAV
are as yet unknown. The objective of this study is to
develop a comprehensive computational model of an
implanted TAV for preclinical assessment of the lifetime
behavior of the implant.
1. Introduction
TAV’s consist of a bioprosthetic valve, composed
of animal derived pericardium, which is mounted onto a
self expandable frame and deployed into the aortic
annulus against the native stenosed valve. Successful
TAV function is dependent on a number of factors
including deployment geometries and tissue-frameannulus
interactions. Preexisting calcium deposition on
stenosed valves can cause distortion of TAV geometries
resulting in paravalvular leakage, while incorrect tissueframe-annulus
interactions can lead to leaflet stress
concentrations and possible device embolisation. The
durability of TAV has not been proved by long term
clinical studies and their fatigue resistance may be
affected by degradation of the leaflets occurring
primarily due to calcification and leaflet tearing. 1 The
objective of this study is to develop a comprehensive
computational model of the TAV in the physiological
environment to investigate the stress distribution across
the leaflets and monitor leaflet fatigue preclinically.
2. Methodology
A generic TAV model has been developed
consisting of three symmetrical leaflets in a relaxed
configuration, similar to previous studies 2 . A mesh was
generated for the initial relaxed geometry in ABAQUS
using 2048 large strain elements with the assumption of
a uniform 0.25 mm pericardium thickness. Leaflet tissue
was assumed to be linear isotropic for this preliminary
model. Contact between the leaflets was modeled using
the master-slave contact pair interaction with the
coaptation surfaces defined as the ventricular side of the
leaflets. Nodes situated on the leaflet-frame attachment
line were constrained in all three transitional degrees of
freedom with no radial frame displacement assumed.
Leaflets were subjected to a uniform transvalvular
pressure of 120mmHg exerted on the aortic side of the
leaflets and leaflet stress distribution and deformation
were analyzed from an unloaded to a fully loaded
physiological state.
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3. Results
Stress distribution across the valve was higher in
the circumferential direction than in the radial direction
at frame attachment sites, particularly at the distal end
of the commissures edges, see Figure 1. Concentrated
peak stresses were observed at this location in both
directions. Twisting of the valve free edge was observed
resulting in pinwheeling of the valve.
(A)
(B) (C)
Figure 1: FE models (A) Open valve in relaxed
configuration and (B) & (C) Circumferential and Radial
stress distribution at valve closure.
4. Discussion
Preliminary results indicate high stresses and
pinwheeling of the valve, resulting in increased flexion
and peak stresses at the leaflet free edge at coaptation,
which may have adverse effects of leaflet fatigue.
Future studies will include biaxial cyclic testing of
pericardium derived from TAV’s prior to and 3 months
after in vivo implantation to examine the change in
mechanical properties of the TAV leaflet tissue due to
the in vivo environment. Tissue properties from these
studies will be implemented using a generalized Fungelastic
model 3 to provide a realistic material model of
the TAV leaflet, and FE analysis will be performed to
monitor the long term effects of tissue properties of the
function of the valve.
A probabilistic finite element tool is also being
developed to allow for the variation of input factors
such as valve crimping, dilation conditions and suture
density and monitor their affects on the stress
distribution on the valve leaflets and durability.
This research will provide a preclinical design tool, for
the development of next generation transcatheter
bioprosthetic valves.
5. References
1. Mirnajafi, A. (et al) J. Biomed. Mater. Res. Part A.
94: 205-213, 2010 2. Smuts, A.N. (et al) J. Mech.
Behav. Biomed. Mater. 4: 85-98, 2011, 3. Sun W. (et
al) Biomech Model Mechan (2005) 4:190-199