Biomedical Engineering Reference
In-Depth Information
point (P, Fig.
26.2
) displacement histories. The course of the apex displacement
exhibits characteristic points, A-D, see Fig.
26.3
. The displacement history can
thus be characterized by relative displacement measures, such as
d
0
:=
d
B
−
d
A
,
d
1
:=
d
A
. The ratio between the maximum
aspiration pressure
P
max
and the induced apex-displacement defines a measure for
the instantaneous stiffness. The ratio between
d
2
and
d
0
might be used to quantify
the degree of 'creep' in the tissue. A few scalar parameters related to the time and
history dependent mechanical behavior of soft biological tissue were analyzed in
previous studies, see, e.g., Bauer et al. (
2009
). The parameter
η
=
P
max
/d
0
will be
reported for the intraoperative liver measurements in this paper.
d
C
−
d
A
,
d
2
:=
d
C
−
d
B
,
d
3
:=
d
D
−
26.2.1.3 Patient Election and Ethical Aspects
Intraoperative aspiration measurements on human livers were conducted under
given approval of the Swiss ethics commission and informed consent by the pa-
tient (SPUK Chirg/Anäst/Patho USZ, Stv-Nr.: 007-2004). Elective were patients of
18 years or older planned for routine hepatectomy with primary or secondary liver
tumors, and benign tumors.
26.2.2 Characterization of Glisson's Capsule
Characterization of the liver capsule aimed mainly at quantifying its influence on the
analysis of the aspiration experiment. In fact, the aspiration measurement does not
provide sufficient information for characterizing both, capsule and parenchyma. An
average capsule model based on uniaxial and biaxial experiments on liver capsule
was implemented in the inverse finite element model described in Sect.
26.2.1.1
.
The present section briefly introduces the methods applied for experimental charac-
terization of Glisson's capsule. The long-term response of 69 bovine and 8 human
liver capsule samples has been recorded in uniaxial tension, and of 18 bovine sam-
ples in biaxial inflation experiments. One healthy human liver could be obtained
postmortem from an 80-year-old subject for mechanical tests from the Institute of
Surgical Pathology, University Hospital Zurich. The decedent gave informed con-
sent for the autopsy. Mechanical tests were complemented with a morphological
and biochemical characterization (Hollenstein,
2011
). The human and the bovine
liver capsule turned out to scale with a factor of about 3 in terms of membrane stiff-
ness, thickness, collagen content, and the diameter of the collagen fibers, whereas
their general microstructure was found very similar. The bovine equibiaxial stress
characteristic, as obtained from the inflation test, were thus used to estimate the
homeostatic point, to calibrate the Rubin-Bodner model (Rubin and Bodner,
2002
),
and to scale the corresponding parameters for inclusion in the finite element analysis
of the aspiration experiment on the human liver.
