Mechanical Interactions - Preventive Biomechanics - page 266

Table 6.2 Stress measures (absolute values) at specific bone sites and corresponding nearest skin
positions specified for homogenous and contoured soft foam, both composed of S AF support
material, with respect to body weight loading
Position Foam homogenous—S AF Foam contoured—S AF

S 33
(MPa 9 10 -3 )

S 13
(MPa 9 10 -3 )

S 33
(MPa 9 10 -3 )

S 13
(MPa 9 10 -3 )

Skin Bone Skin Bone Skin Bone Skin Bone

Sacral bone edge 4.9 10.2 0.17 9.1E-03 4.9 10.9 0.18 8.6E-03

P. s. iliac spine 4.5 9.6 0.31 0.29 4.5 9.3 0.33 0.38

Ischial tuberosity 4.6 8.7 0.15 0.11 4.1 8.2 0.13 0.11

Tail bone 3.3 6.5 0.15 7.4E-02 2.8 6.7 0.16 6.4E-02

laterally, i.e. the bone structures that span the diaphragm musculature. At these
sites except the ischial tuberosity, the direct Cauchy stress component S 33 and the
Cauchy shear stress component S 13 , both considered in global spatial directions,
dominate. At the ischial tuberosity the shear stress component S 23 dominated.
Direct stress magnitudes exhibited their highest values at the bone surface and in
the muscle layer, not in the skin-support interface. This agrees with numerical
results previously reported Todd and Thacker (1994) or Oomens et al. (2003) or
Linder-Ganz and Gefen (2007) and is consistent with surgical findings revealing a
cone shaped necrosis, with the base situated on the bone surface, in a majority of
decubiti cases, (cf. Quintavalle et al. 2006) or Bass and Phillips (2007). Regarding
the skin surface, direct compressive stress S 33 has been shown to exceed shear
stress by one order of magnitude. Adjacent to the bone surface, up to three orders
of magnitude greater direct stress dominated, depending on the position, cf.
Table 6.2 .

Influence of the Support Material and Design: Dominating tissue stress
values at static equilibrium evaluated at the most loaded gluteal body sites are
listed in Table 6.2 . Herein, effects of homogenous and contoured foam support
using the same material as well as consistent outer specimen dimensions were
compared.
The contoured S AF foam support showed a similar stress magnitude as its
homogenous S AF counterpart (Table 6.2 ). Likewise, the Supra material which
showed a stiffer material response in material testing than S AF material, showed
similar tissue stresses. The virtual foam material, however, resulted in an increase
of tissue stress by a factor of 2. The stiffer virtual support material showed tissue
stress at the sacral bone surface to have tripled, compared to values at the skin
surface (Figs. 6.21 a, 6.22 a).
A general characteristic of all body sites considered and of all materials used
was that direct compressive stress did not continuously increase when approaching
the bone. It rather initially decreased passing through the fat tissue to reach its
minimum values at the fat-muscle interface and, proceeding through the muscle
tissue, it again increased to finally exceed the skin values at the bone surface