Biomedical Engineering Reference
In-Depth Information
Figure
6.11
a shows that loading causes the tissue to displace vertically and
laterally. Maximum vertical in-plane displacement of the foam (index ''F'') was
Du
F
= 35 mm and for tissue (index ''T'') Du
T
= 22 mm.
To more objectively compare the quality of the experimental versus the
simulation results, the single in-plane boundary edges of deformed skin/fat, muscle,
bone and foam have been extracted from the MRI-image and the simulation result. In
this process, reconstruction from the MRI-image was done by digitalizing the
boundary edges using Mimics
. Similarly, deformed FE-model geometry was
exported utilizing the HyperView
capabilities of the Altair HyperWorks
package.
In-plane boundary model edges were extracted by trimming the deformed geometry
with the transversal plane running through the position marker point. Boundary
edges of MR-image and simulation were superposed on a squared (260 9 420 mm),
indexed, 5 mm pitch grid (Fig.
6.11
b). The displacement deviation is depicted in
dark grey. Intervals with 5 mm pitch were set to provide discrete points on all
boundary curves to establish a measure of correlation.
AP
EARSON
-Correlation of R
2
= 0.999 was found comparing the lateral bulk
deviation of the foam support resulting from the simulation with the MRI-
finding. Here, equal vertical spacing was maintained. Deformation of fat tissue in
the
experiment
compared
to
the
numerical
result
shows
a
correlation
of
R
2
= 0.998 with constant vertical spacing, and R
2
= 0.996 with constant hori-
zontal spacing.
6.2.3.5 Discussion
The skeletal striated gluteal muscle with its coarse heterogeneous parallel structure
of bundled fibres exhibits anisotropic material properties. As a first approximation,
however, and for the purpose of mechanical modelling, passive gluteal muscle
tissue is assumed to be isotropic and homogenous. The fibre direction of gluteal
muscle is assumed to play an inferior role during mainly transversal loading, as
present with the body in a supine position.
The mechanical properties of soft tissue may only be simulated accurately if the
microstructure can correctly be physically described. Then, phenomena regarding
relevance can be judged and decisions about whether modelling of structures on
the micro scale is reasonable and a possible macroscopic description justified.
Irrespective of the question of availability of mechanical properties of cellular
structures, numerical simulation of real applications such as buttock/support
interaction has not until now been feasible on the micro scale, due to lack of
computational capacity. However, modelling tissue on the macroscopic level using
the aforementioned assumptions provides results which reasonably agree with
experimental findings, as shown in this study.
In-plane boundary edges of simulated skin/fat and muscle tissue in the
deformed state strongly correlate with MRI findings. These findings give reason to
use previously derived tissue parameters in full body simulations involving
complex tissue loading at finite strains.
