Biomedical Engineering Reference
In-Depth Information
Tabl e 1
Mechanical properties of human tissue
Poisson
coeficient
Young's modulus (MPa)
Elements
Bone structure
7,300
0.3
3D tetrahedrics
Soft tissue
0.15
0.45
3D tetrahedrics
Plantar fascia tendons
350
-
3D tetrahedrics
10
5
Ground
2.10
0.3
quadrilaterals
plantar shear stress, plantar pressure, displacement, etc. In order to achieve it, 3D
foot model file is exported to FEM analysis software, like Ansys
®
or Abaqus
®
,for
example. The next step before simulating is to choose static or flexible dynamics
simulation, either related to static or non-static conditions, enabling different kinds
of foot movements or activities. It is possible to simulate just standing foot position,
where feet are in pure compression (balanced standing position), or one can analyse
different gait phases instances separately, knowing which foot parts are in contact
with ground. The results obtained in each simulation must be looked at, in order to
search for foot areas most affected during gait or run, i.e., search for plantar pressure
maximum peaks or shear stress maximum peaks, which are aspects of most concern
in this context.
FEM simulation starts with the definition of initial conditions, bonding condi-
tions, applied forces or loads and supports. In the case of the foot, it was defined that
bone structure and soft tissues were bonded in the corresponding contact surfaces
and edges. Soft tissue and ground can have three types of bonding conditions:
frictional, bonded or no contact, depending on the simulation goal. Also, there is
the need to apply different initial contact conditions according to the chosen contact
areas.
The FEM software allows completing the 3D foot model with tendons or
ligaments, by adding springs to the bone structure in the plantar fascia or in
calcaneous, representing Achilles tendon, extensor tendons, flexor tendon, peroneus
tertius and longus, plantar fascia tendons and others. As human tissues have
nonlinear behaviour and variable mechanical characteristics, in order to simplify
this computational procedure tissue extrapolation was done. Muscles, fat tissue, and
skin were consider soft tissue, and also no bone constitutes distinction was done for
trabecular and cortical bone, therefore estimating their mechanical properties, such
as Young's modulus and Possion's ratio [
31
]. In addition, bone structure, soft tissue
and tendons were considered to be linearly elastic, isotropic and homogeneous [
9
].
These values are shown in Table
1
. Soft tissue can also be defined as having an
hyperelastic behaviour which is closer to reality [
1
].
For each 3D model a mesh is built, dividing the model into elements, depending
on model complexity, enabling mechanical study. These elements are also defined
in Table
1
. As an example, Fig.
12
shows a model which, due to its complexity,
the generated mesh has more than 173,300 nodes and 97,700 elements, using 3D
tetrahedral elements.
