Biomedical Engineering Reference
In-Depth Information
Fig. 7.45 Optimized cushion shape contours regarding direct stress minimization underneath the
ischium of a the transversal plane strain model and b the sagittal plane strain model. Optimized
contours are highlighted in red for visualization
in this case is depicted in Fig.
7.45
b. Model regions where stress was intended to
be reduced are highlighted in green.
Each shape reduces tissue stress beneath the ischium in the particular plane
strain case. For both configurations, the optimized shapes approximate the outer
body anatomy of the buttocks and upper legs. Using minimization of shear stress
lateral to the ischial bone as optimization criteria results in a modified cushion
shape as depicted in Fig.
7.46
.
This optimized solution is trivial since cushion material is laterally reduced to
prevent vertical tissue displacement and thus create shear stress. Direct stress
beneath the ischium, however, increases as the cushion center supports nearly the
entire body weight.
7.2.3.3 Comparison of Cushion Shapes
To compare the optimized transversal shape (Fig.
7.45
a) with other cushion designs,
the plane strain buttocks model was simulated using block-shaped cushions of 35 and
100 mm thickness. Direct stress and mean direct stress of a finite region beneath the
ischial tuberosity were evaluated and compared, Fig.
7.47
. Direct stress was reduced
by over 50 % compared to the initial cushion shape and approx. 25 % compared to the
cushion with an increased thickness of 100 mm.
The optimized support shapes as derived in the previous subsection are based
on plane strain modelling. To verify these results for the real three-dimensional
buttock anatomy the optimized two-dimensional geometries are used to generate
3D-cushion geometry. The resulting shape combines information of both
optimized contours. In the generation process, the surface of the optimized
transversal shape was extruded along the contour of the optimized sagittal shape,
Fig.
7.48
a.
The
obtained
3D-shape,
Fig.
7.48
b,
was
meshed
with
three-
dimensional continuum elements.
