Biomedical Engineering Reference
In-Depth Information
The analyses described previously were performed employing data from a single
individual. Clearly, there is substantial individual variability in the mechanical
properties of soft tissue (see
Sect. 5.2.2.4
)
, and parameters employed in the
described study may not represent human gluteal soft tissue properties in general.
However, to support design optimization regarding tissue stress/strain reduction,
basic design principles can be derived without parameter sets corresponding to a
population differing in age, gender and physical tissue condition. Such design
optimization by means of FE-analysis is based on a relative result comparison since
damage threshold values for individual tissues are still pending in the literature.
Another discussion issue relates to the comparison of tissue deformation
exclusively at tissue interfaces, i.e. at the skin surface and the fat-muscle interface.
Tracking distinct material points beside the tissue boundaries is difficult.
Furthermore, comparison was performed for one single in-plane region only, i.e.
the transversal plane at the ischial tuberosity. Anatomical sites at different trans-
versal positions have not been examined. However, reasonable accordance of
tissue behaviour at sites adjacent to the examined position may be assumed. This
assumption relies on the reasonable accordance at the considered in-plane position
where deformations of the complex tissue anatomy are additionally strongly
affected by adjacent tissue deformation.
6.2.4 Critical Body Sites
Body sites where tissue stress and strain accumulate during static recumbency or
sitting are depicted in Fig.
6.12
. At positions 1 and 2 high direct compressive
stress occurred during recumbency, whereas at position 3 maximum direct stress
occurred during sitting. Lateral to positions 1, 2 and 3 maximum shear stress and
strain occurred at positions 1a, 2a and 3a.
Finite element modelling of the buttocks inevitably faces the problem of how to
represent the region underneath the sacrum and the coccyx as well as at the
posterior border of the body of the ischium. This includes the pelvic diaphragm,
which is spanned with muscle fibers of the musculus levator ani, the musculus
coccygeus, and associated connective tissue, cf. Fig.
6.12
. The representation of
this region should not be underestimated since MR-images taken from an unde-
formed, free-hanging buttock compared to a body-weight loaded buttock indicate
that the pelvic diaphragm musculature relative to the adjacent bone structures
deforms introversive into the abdominal cavity, subject to loading. Depending on
the compliance of this particular model region, simulated stress and strain differed
significantly. Low compliance resulted in low localized tissue stress, high com-
pliance increased stress magnitude up to more than one order of magnitude.
Performing tissue-support interaction simulation, this stress/strain dependence is
exemplarily shown with the buttock simulated on a homogenous soft foam support
and altering the compliance of the pelvic diaphragm region. Within these simulations
low compliance is synonymous with fixed nodes connecting the pelvic diaphragm
