Biomedical Engineering Reference
In-Depth Information
indentation force distribution made previously in this subsection under ''modelling
assumptions'' sufficiently apply. One indication that these may be sufficient is the
reasonable agreement with data provided elsewhere.
Experimental data show that over 80 % of the total relaxation occurs within the
first 100 s of constant indenter displacement. During this time, substantial stress-
relaxation of over 35 % occurred. Figure
5.30
a indicates that a hold phase of 180 s
is sufficient to reach steady-state even at higher compression. Force relaxation
values at the end of the hold phases correspond well to long-term elastic values
obtained through the stepwise indentation procedure described previously.
Deviations of values at cut-off time and long-term values of the fully relaxed
tissues were found to be in the range of 4 %. Some limitations can be found in data
fitting: in Fig.
5.32
it can be observed that the fitted ramp peak value as well as the
lowest points corresponding to load peaks in the time interval between 75 and
150 s of the 40 mm ramp displacement curve were not captured accurately. The
overall theoretical result, however, remains close to experimental data.
In vivo indentation tests performed on the human gluteal region demonstrate
that the viscoelastic component plays an important role in tissue mechanical
behaviour and that a significant amount of force relaxation occurs, influencing
tissue mechanics to a great extent. Our tests have been performed at small strain
rates indicating that viscoelastic properties must already be taken into account
when modelling transient tissue properties in similar applications such as tissue-
support interactions during sitting or in virtual surgical guidance simulation.
Transient tissue properties at small strain rates are in the linear viscoelastic
area, and a nonlinear hyperelastic isotropic model extended with a Prony series
reproduces the observed experimental behaviour. Thus, it is appropriate to
represent gluteal adipose and transversally loaded skeletal muscle stress-relaxation
behaviour in this way.
5.3 B
OSS
-Models
In
Sects. 5.1
and
5.2
the mechanical characterization of human soft tissue of the
gluteal region was discussed. Knowledge of tissue properties is indispensable for
the numerical simulation of the interaction between support devices and the human
body or distinct body regions. To complete the modelling process, appropriate
total or partial models of the human body must be digitalized and contain the
following properties:
• Complete and realistic anatomy
• Gender specific differentiation
• Different body sizes and weights (biological variability)
• Use-oriented patho-physiology (e.g. spine curvature)
