Mechanical Interactions - Preventive Biomechanics - page 285

E-BASS 2 (female model): In Fig. 6.49 direct stress S 33 and shear stress S 13
distributions in the ischial region are compared for the upright and the reclined
seat positions using the E-BASS 2 system. Figure 6.50 depicts stress values along
node paths 1 (see Fig. 6.45 ) for both back rest positions. It shows that direct and
shear stress are of nearly the same magnitude whereby stress in the reclined
position generally tends to be increased. Maximum direct stress is approximately
8-fold higher, compared to shear stress.
6.4 FE-Simulation Versus Experiments
To validate finite element simulation results of mechanical tissue interaction
shown in Sects. 6.2 and 6.3 , they are compared to results from animal and human
experiments conducted from 1959 to 1994, listed in the literature. These findings
were discussed in Sect. 3.5 (cf. Fig. 3.36 b) where the respective (constant) animal
tissue loading (pressure) was plotted over critical loading time (i.e. time of
constant pressure) where cell damage was observed (experimental pressure-time
damage relations).
From a mechanical point of view, the experiments described previously
represent creep loading (cf. Sect. 3.2.2.2 ) . In contrast to a time rate of deformation
of loaded tissue, the time to cell damage and necrosis is considered.
To compare finite element simulation results with experimental findings, the
simulated mechanical tissue stress was evaluated at the skin surface and at the
bone, based on interaction simulation and viscoelastic tissue material behaviour.
Due to a constant body weight of the B OSS -Models, a creep process can be
assumed during tissue interaction in the seated or recumbent posture.
In Figs. 3.36 b and 6.51 or 6.52 all investigations except those of Reswick and
Rogers (1976), involve experimental animal data. Only Reswick and Rogers
established a time-pressure relationship with human data. They, however, empha-
sized that their data came from an observational study investigating the skin interface
pressure of 980 seated individuals. Their data is almost exclusively based on expe-
rience, rather than on controlled measurements. All experimental curves, presented
below, indicate threshold pressure-time relations where pressure-time values on and
above the individual pressure-time curve were shown to cause tissue damage.
A direct comparison of simulated tissue stress curves with experimental values
is only approximate for several reasons. First, experimental findings involve
different approaches regarding pressure derivation as well as different animal
species and different body sites. This experimental nonconformity may have led to
the deviation in the experimental curves. Secondly, and of particular significance,
simulated tissue stress represents a single loading incident over a certain time span.
The experimental curves, in contrast, represent various loading experiments at
constant pressure assigned to the particular times when cell damage occurs.
The ambiguous situation of experimental findings is reminiscent of similar
findings from clinical health care reports. Pressure sores have been reported to