Biomedical Engineering Reference
In-Depth Information
the recovery heart performance [
12
]. This method was successfully applied to the
right ventricle [
12
,
13
]. Blood pressures in the left ventricle are higher; the initial
strength of biological grafts is not sufficient and ruptures are likely. In order to make
use of this innovative surgical method, a novel approach was developed. Degradable
scaffolds support the biological grafts until they develop sufficient strength [
22
]. The
implants disappear through degradation; therefore, a second surgery is not needed
to remove them. Developed scaffolds fractured in in vivo studies after one month
[
24
]. This is not sufficient time for healing; therefore implants need enhancement for
higher durability. Since in vitro and in vivo testing is expensive, the finite element
method can be used to speed up the development process.
A flat scaffold that is deformed by hand into an averaged heart curvature before
being sewn onto the myocardium was simulated. It was shown that the scaffold
was highly stressed. Therefore, material damage and breakage of the structure was
likely. The simulation confirms the findings of Schilling et al., where flat scaffolds
fractured in vivo after one month [
24
]. A practicable solution is to preform flat
scaffolds according to the heart curvature and heat-treat them to reduce residual
stresses. Preformed and therefore stress free scaffolds were simulated in a FE model
and deformed according to heart movement. Calculated stresses were significantly
lower compared with the flat scaffold. Bauer et al. measured an approximately 53%
extension in life-time before breakage for the preformed implant, confirming the FE
results [
25
].
Complex FE models of the left ventricle are described in the literature [
31
,
32
].
Some groups include patient-specific fiber orientation maps in their heart models
[
33
], or simulate filling dynamics of the left ventricle [
34
]. These models allow in-
vestigation of cardiac biomechanics and help in understanding heart diseases [
32
,
34
]. However, for the development of myocardial scaffolds, those kinds of models are
too complex. The simulation was limited to modeling the contraction and relaxation
of the heart. To accomplish this, the anterior basal heart curvature was measured in
28 patients and average values were calculated [
25
]. Using these averages, myocar-
dial movement can be mimicked in end diastolic and end systolic states. Only the
translational motion of the heart is considered; the rotational motion is neglected.
This can lead to an underestimation of the implant loading.
Loads that result from a bulging graft were simulated as pressure on the back of
the scaffold in the systolic step. The 0.5mm thick scaffolds showed that the effect
of a bulging graft needs to be taken into account. Simulations without this systolic
pressure led to low stresses, because the scaffold was only deformed due to heart
curvature changes. After inclusion of the systolic pressure, the scaffold was addi-
tionally loaded. Thus, high stresses and larger deformations followed. Physiological
pressures of up to 240mmHg are possible for the left ventricle. To avoid an over-
estimation of that effect, an averaged pressure of 140mmHg was chosen for the
simulation. In order to enhance the finite element modeling, biological grafts must
be included in order to simulate bulging of the graft due to the high blood pressure
and thus the deformation of the scaffold.
Complex heart movement and the effect of a sewn on graft were simplified to a
translational curvature change with systolic pressure. Therefore, it is not possible
