Biomedical Engineering Reference
In-Depth Information
systems, commonly used for pressure measurement of seated or recumbent
subjects, to judge anti-decubitus effectiveness of body supports. Such systems
permit rudimentary estimation of mechanical pressure to the skin surface but do
not capture interface shear stress or internal tissue stress. Since internal tissue
stress differs from interface stress, a more comprehensive study is necessary to
establish a possible relation between interface and internal stress. Should a relation
exist, then pressure mapping systems can continue to play a role in body support
evaluation. However, should no relation be apparent, it is possible that an effective
support design cannot be made with the help of such systems.
The simulation has shown that contoured foam, with the same material and
outer dimensions of a homogenous support, does not ensure less tissue stress,
despite the claim. A virtual support material, stiffer than the investigated soft
foams, exhibited a strong effect on tissue stress distribution. It is possible to
evaluate stress evaluation on the skin surface and in deep tissue regions using
different support materials. Thus the simulation approach can further advance anti-
decubitus support design and can prove or disprove sales arguments.
The stress-strain evaluation within the tissue showed direct stress components
to be nearly of the same magnitude and, in addition, to be almost equivalent to the
principal stress. Furthermore, direct stress dominated shear stress by up to three
orders of magnitude (Table
6.2
). The impact of individual stress components on
cell damage and tissue necrosis is uncertain. Several hypotheses regarding the
effects of mechanical loading on tissue have been proposed. Uniform volumetric
compression could lead to capillary occlusion and limit nutritive blood flow to the
microvasculature. In addition, increase of ambient pressure may lead to disturbed
cell metabolism. Shear strain, in contrast, may lead to pathological cell wall
deformation, in turn causing cell breakdown. Damage thresholds, however,
depending on different stress and tissue types, have not been determined. Possibly,
cell breakdown may be much more sensitive to shear loading than to normal
loading. Thus, the finding in the previous investigation that shear stress is sig-
nificantly smaller than direct stress does not automatically imply that shear stress
plays an inferior role in tissue necrosis.
The modelling technique, including the pelvic floor, showed high impact on
internal tissue stress distribution.
6.2.5.2 Viscoelastic Body-Support-Systems
In contrast to E-BSS (
Sect. 6.2.5.1
), the following considerations focus on the
interactive behavior of the BSS and B
OSS
-Model comprising viscoelastic tissue and
support material properties. The material parameters used are provided in Table
4.4
in
Sect. 4.2.1.5
and in Table
5.7
in
Sect. 5.2.5.1
and in Table
5.9
in
Sect. 5.2.5.2
.
Comfort Related Systems: The following FE-analysis is based on the support
systems
E-BSS
introduced
in
Sect. 6.2.5.1
and
the
human
model
BoMo1
(cf.
Sect. 5.3.4
)
.
