Biomedical Engineering Reference
In-Depth Information
Fig. 6.18 Tissue logarithmic shear strain LE
23
at the ischial tuberosity and the sacrum on a path
initiating at skin level (0 mm) and leading to the bone surface (ischial tuberosity: 48 mm;
sacrum: 6.3 mm)
• The fat-muscle interface at the ischial tuberosity region represents a minimum
or an inflexion point in the stress/strain curves and, in addition, a sign change of
shear strain.
Comparison of tissue direct stress components S
11
,S
22
and S
33
, Fig.
6.19
shows
that stress quantities are of similar magnitude. This may be interpreted as a ''nearly
hydrostatic'' state of pressure in the tissue at the investigated body site under body
weight loading. This is especially apparent in tissue regions adjacent to the bone
surface. Results of Fig.
6.19
are based on the homogenous mattress design
employing HR- and VE-foam material. Using highly resilient HR-foam material,
tissue direct stress components show higher congruence compared to viscoelastic
VE-material.
Related Medical Systems: The following four anti-decubitus systems used in
medical care are analyzed by means of FEA. The E-BSS materials are as follows:
• E-BSS 5: Homogenous block-shaped soft foam S
AF
-material (Fig.
6.20
a)
• E-BSS 6: Contoured 3-layer soft foam support with a SUPRA-SAF-SUPRA
material compound (Fig.
6.20
b)
• E-BSS 7: Contoured 3-layered soft foam support with S
AF
-material solely
(Fig.
6.20
b)
Material characteristics of the above introduced materials are highly visco-
elastic for S
AF
material and highly resilient for S
UPRA
material. Regardless of the
particular support material and geometry, numerical simulations show that at the
pelvic bone surface in the supine position under loading, particular anatomic
points accumulate stress. These points are situated at the ischial tuberosity, the
posterior superior iliac spine, the sacral bone edge laterally and the tail bone
