Biomedical Engineering Reference
In-Depth Information
Fig. 6.23 a Direct stress along nodal paths at specific buttock sites. b Comparison of direct stress
with the principal stress along a nodal path at the sacral bone
Fig. 6.24 a Nominal strain along a nodal path originating at the skin leading towards the ischial
tuberosity. b Buttock model in cut view at position of the ischial tuberosity on contoured foam
support: contour plot of the nominal direct strain component NE
33
at static equilibrium
stresses, exemplarily depicted for the nodal path at the sacral bone in Fig.
6.23
b.
Thus, it seems that at all investigated tissue sites the stress state could be inter-
preted as a ''quasi-hydrostatic'' state of pressure. Furthermore, von Mises stress
turned out to be more than one order of magnitude smaller than direct stress.
Regardless of the particular support material and geometry at all investigated
sites, nominal direct tissue strain reached peak values within the muscle tissue near
the fat-muscle interface. Shear strain however, was locally minimal at the
fat-muscle interface and reached peak values at the bone surface. Figure
6.24
a
depicts nominal strain values along a nodal path underneath the ischial tuberosity,
originating at the skin surface (point A, Fig.
6.24
b and ending at the bone surface
(point B, Fig.
6.24
b).
Discussion: When exposed to body weight loading, human gluteal tissue
showed maximal stress in tissue adjacent to bone and not at the skin. As has
previously been proposed by Todd and Thacker (1994) and Oomens et al. (2003),
this further questions the usefulness of interface body pressure measurement
