Biomedical Engineering Reference
In-Depth Information
Fig. 5.27 a MR-compatible loading device for creep indentation testing, and b transversal MR-
images of the buttock region showing tissue creep deformation at 0.67 and 268 s; positions [1]
and [2] indicate the indenter location and pelvic bone
symmetric with respect to the image plane and the indenter axis. Additionally, when
rotating the image slice information, especially skin, fat and muscle and bone contour
of one half of the image around the indentation axis, it may not deviate excessively
from the existing anatomical geometry. Hence, this approach involves MR-scanning
to find a test point that fulfills the listed requirements.
The simplified model has the further advantage of a short simulation time,
which is advantageous in iterative parameter optimization simulation.
Further tissue parameters obtained from axissymmetric modelling are provided
in the following (Table
5.6
).
5.2.4 In vivo Experiments II: Viscoelasticity
In vivo gluteal tissue properties at steady state elasticity or equilibrium elasticity as
described in
Sects. 5.2.2
and
5.2.3
, respectively, serve as the basis for determining
tissue viscoelastic in vivo behavior. Generally, these properties are shown in the
force-displacement data provided in Fig.
5.15
b and d in terms of force relaxation
and hysteresis, respectively. Here, the relaxation behavior in the force-time dia-
gram is supplied by exponentially decreasing force values as shown in Fig.
5.15
b.
In the force-displacement diagram Fig.
5.15
d, vertical decreasing force values at
constant displacement (up to the relaxation termination point) can be detected.
The subsequent viscoelastic characterization is based on both creep and
relaxation experiments. Creep testing is of particular interest due to tissue creep
processes taking place during tissue-support interaction in the seated or recumbent
body posture. Here, tissue and support materials are both loaded at transiently
constant forces (body mass) and respond with transiently increasing deformation
(cf.
Sect. 3.2.2.2
)
.
