Biomedical Engineering Reference
In-Depth Information
are always produced by MMA. After the first oscillation the method changes to GP
until convergence is reached.
10.5.2
Finite Element Model
The contact formulation used allows the study of several porous coating lengths. In
the present work two coating lengths were considered, a totally coated stem and a
half coated one (similar to the Tri-Lock prosthesis shown in Figure 10.4). A friction
coefficient of
ϑ
=
0
.
6 is assumed for coated surfaces while uncoated surfaces are
modeledasfrictionlesscontact.
To simulate several daily activities a multiple load case with three loads were
considered [42]. With the multiple load case the three loads are applied sequentially
in order to reproduce successive activities. In Figure 10.8 it is possible to see load
directions and intensities.
To have a suitable finite element mesh of the stem-bone assemblage, it is
necessary to have accurate geometries of femur and femoral stem. The femur
mesh was built using a geometry based on the ''standardized femur'' [43]. Stem
shape changes in all optimization iterations. In order to assure similar stem meshes
in all iterations a meshing algorithm was developed. Therefore, the finite element
mesh discretization has a total of 7176 eight-node brick elements, 5376 for the
femur and 1800 elements for the femoral stem.
The bone is a nonhomogeneous structure where one can distinguish between
trabecular and cortical bone. Additionally, long bones have a cavity in the bone shaft
filled with marrow. Therefore, the femur model considers marrow, trabecular, and
1
3
2
1
F
h
2
3
F
a
F
x
(N)
F
y
(N)
F
z
(N)
Load case
F
h
224
972
−
2246
1
F
a
−
768
−
726
1210
F
h
−
136
630
−
1692
957
2
F
a
−
166
−
457
−
383
−
382
F
h
796
−
1707
547
z
3
F
a
y
−
669
x
Figure 10.8
Load cases.
