Mechanical Interactions - Preventive Biomechanics - page 286

Fig. 6.51 Comparison of
simulated tissue direct stress
with experimental literature
data, [7] experimental:
Linder-Ganz (2006--
experimental/numerical--in
rat muscle), [D] simulation:
human supine (skin level),
[B] simulation: human seated
(fat-muscle interface)

develop within minutes to hours. Such discrepancy may be due to varying support
surfaces (employed in different clinical health care institutions), which have
different effects on the tissue depending on subject height and weight, as indicated
by Shelton and Flott (2003).
Figure 6.51 shows simulation results of mechanical tissue-support interaction
between a recumbent (Fig. 6.51 /[D]) and a seated (Fig. 6.51 /[B]) model on a
viscoelastic foam support (thickness: 80 mm) generated by Then and Silber and a
pressure-time cell damage relation (Fig. 6.51 /[7]) given by Linder-Ganz et al.
(2006) (cf. outlines in Sect. 3.5.2 ) . The simulated skin level (compressive direct)
stress (Fig. 6.51 /[D]) at the ischial tuberosity agrees well with experimental
findings (Fig. 6.51 /[7]). Simulated (compressive direct) stress values of the human
fat-muscle interface at the ischial tuberosity, however, are located above the
pressure-time cell damage relation, (in gravitation and vertical loading direction,
the sign has been changed to fit the diagram).
In Fig. 6.52 and in addition to the curves depicted in Fig. 6.51 , all significant
findings regarding pressure-time cell damage provided in the literature from 1959
to 1994 as well as simulated stress values at the human bone surface (Fig. 6.52 /
[A]) and at the skin level (Fig. 6.52 /[C]), both at the ischial tuberosity region
(generated by Then and Silber) are depicted.
Despite the discrepancies in threshold pressure, Fig. 6.52 shows that simulated
direct stress at skin level (Fig. 6.52 /[C] and [D]) agree with the stress range
reported in other investigations. All experimental stress refers to pressure as skin
interface pressure. Linder-Ganz et al. (2006), in addition, used a combined
numerical-experimental approach to determine compressive stress in deeper
muscle tissue regions for long-term pressure exposure. These authors combined
their data on cell death with other experimental data. Data derived by Kosiak
(1961) was especially useful to establish their sigmoid-type pressure relation.
The same order of magnitude between simulation results (Fig. 6.52 /[C], [D]) and
experimental data is not self-evident. The simulated models reflect factors such as the
quality of viscoelastic human skin/fat and muscle tissue properties derived by
material parameter optimization (cf. Sect. 5.2 ) , as well as support material