Biomedical Engineering Reference
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Fig. 5.21 3D reconstructed models: a initially undeformed configuration, b fat muscle interface
of a deformed configuration at an indentation displacement of 44 mm
indentation force represents an integral measure of all stresses acting at the
indenter contact surface.
In addition, the experimental scenario is not standardized, the relative axis
directions of the indenter compared to the individual bone anatomy of each
volunteer is not reproducible and can thus not be directly compared.
The small number of volunteers as well as their ''constitution'' do not allow
statistical interpretation of the data. Such conclusions require an appropriately
designed study involving patient anatomies that differ from a ''norm'', i.e. anat-
omies exhibiting extremely thin and thick fat and muscle thicknesses. Conclusions
which have been drawn must thus be considered stimulation for more detailed
investigations.
Further information regarding biological variability of human in vivo soft tissue
behavior can be obtained from the force-displacement data of four female (age:
37-70) and six male (age: 24-65) volunteers, depicted in Fig. 5.20 . It is apparent
that with increasing age, the female curve characteristics reflect a stiffer material
response. In other words, the younger the patient, the softer the tissue. This is not
the case for male volunteers. However, the small number of volunteers again does
not allow statistical interpretation.
5.2.3 Material Identification I: Elasticity
5.2.3.1 3D Reconstruction and FE Model
Surface Reconstruction: To perform material identification for gluteal skin/fat
and muscle tissue, an FE model of the initially undeformed tissue configuration
including the pelvic bone was built. Since the finite element mesh relies on surface
data, these were derived from the MR-images for the relevant buttock region,
cf. Fig. 5.16 , i.e. the right buttock cheek, using the image processing tool M IMICS
(M ATERIALISE , Leuven/Belgium). The reconstructed surface included the initial
undeformed state of fat and muscle and bone, as well as the deformed fat-muscle-
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