Biomedical Engineering Reference
In-Depth Information
as well as on viscoelastic tissue behaviour. The exclusive use of long-term tissue
properties, however, seems justified since pressure sores are clinically observed to
develop over minutes to hours while human tissue stress induced experimentally
recedes within seconds to minutes, to reach a nearly constant level (Palevski 2006;
Silver-Thorn et al. 1999), also cf.
Sect. 5.2.4
. The effect of tissue viscoelasticity
may thus be secondary, especially during recumbent body interaction where, due
to constant body loading, tissue creep effects dominate.
Similar to human soft tissue property evaluation, the foam support materials
presented here partially showed distinctive time dependent material behaviour.
They were, however, mainly characterized by their long-term, elastic material
response, as described in
Sect. 4.2.1.2
.
In particular cases where strong visco-
elastic effects were observed, materials were characterized via creep and relaxation
experiments.
Another crucial issue in terms of numerical stress evaluation, besides
mechanical material characterisation, was adequate anatomic modelling of the
pelvis, especially the region forming the pelvis floor. The pelvis region is a region
particularly prone to pressure sores. Simulation has shown that the stress distri-
bution at the tissue-sacrum bone-interface is strongly influenced by the compli-
ance of the pelvic diaphragm musculature. Thus, adequate anatomical modelling
of this region is mandatory. MR-imaging shows that the gluteal tissue during
compressive loading, with the body in a recumbent position, tends to displace
towards the abdominal cavity, deforming the pelvic diaphragm musculature. This
amplifies the relative movement of tissue to adjacent bony structures and the
resulting stress additionally depends upon the magnitude of displacement of the
pelvic diaphragm musculature.
In the following, body supports and anti-decubitus systems are introduced
together with introductory and historical remarks regarding pressure sore aetiology.
6.2 Mattresses and Anti-Decubitus-Systems
6.2.1 Diagnostic Findings of Decubitus
According to the most recent definition by the national pressure ulcer advisory
panel (NPUAP, Washington, DC, USA), pressure ulcers (also decubitus or pressure
sores) are localized injuries to the skin and/or underlying tissue. In the process of
sore formation, sustained mechanical loading plays a preeminent role. According to
a survey conducted in Germany among institutions of geriatric care, the incidence
of pressure sores is 3 % amongst people requiring external care and assistance in
personal hygiene and eating at least once a day. This figure increases to up to 20 %
for those in need of intensive care (Leffmann et al. 2002; Halboom 2005).
An estimated 400,000 people per year in Germany develop a form of pressure
ulcer that requires treatment. Specifications on prevalence in German hospitals are
10 %, in geriatric clinics and nursing homes around 30 %. Figures indicate an
