Computer-Aided Engineering Resources and FEM for Biodevices - Advanced Design and Manufacturing Technologies for Biomedical Devices - page 154

8.4.5 Case Study: Effects on Tissue Submitted

to Thermal Ablation

Thermal simulations are also highly interesting, as some surgical procedures involve
the application of high temperatures and some active implantable devices could
potentially affect the temperature of surrounding tissues. Therefore, related thermal
simulations can help with surgical planning and also with risk assessment of some
specific active implantable devices (Díaz Lantada 2009 ).
Here we provide a case study linked to the thermal ablation of the mitral valve
tissue, as an alternative way to annuloplasty (previously discussed in Chaps. 4 and 5 ) ,
when trying to improve the mechanical stiffness of patient's muscular mitral valve
annulus. Localized thermal ablation can lead to small areas of more stiff tissue, but
the whole surgery has to be especially controlled, so as not to damage greater zones
than expected. In this case, the steady state is not so important, what is indeed rel-
evant to simulate is the transitory heating process, in order to obtain an idea of how
long the activation of the device for thermal ablation should be applied.
The CAD structure resembling the left atrioventricular union has been taken
from my Ph.D. thesis. It is a 2 mm width trunk cone and has been meshed using
10-node tetrahedron elements. Density is similar to that from soft tissue, a specific
heat c p = 3,500 J/(kg · K) (Huang and Wilber 2000 ) and a thermal conductivity
k = 0,6 W(m · K) (Koncan et al. 2000 ) have been used. A heat convection coefficient
h = 500 W/(m 2 · K), for taking into account the heat transfer to the surrounding fluids
at 37°C, has been considered.
In any case, it is important to note the complex measurement of heat convection
coefficient in the surroundings from mitral valve. Several different values have been
found in other research (Tangwongsan et al. 2004 ; Shah et al. 2006 ), so here we
have just selected a mean value and simulation can initially just be considered as
estimation. Only empirical results from in vitro/in vivo trials can help to further
adjust such coefficients.
A localized heating load of 2 W has been applied to the zone of the mitral valve
annulus. During the transitory heating, tissue should reach between 50 °C and 100 °C
(Williams et al. 2002 ), and studying the transitory is indeed interesting, as it pro-
vides useful information for the surgical intervention (although prior to that ade-
quate in vitro and in vivo trials in animal models should be made).
After solving, using the thermal flow solver from NX-8.0, post-processing
(Fig. 8.13 ) provides temperature results for t = 1 s, t = 5 s, t = 10 s, and t = 15 s to
assess and prepare the typical surgical treatment. In just 15 s, temperatures above
90 °C are reached, what is in consonance with results and advice from previous
research (Huang and Wilber 2000 ).