Biomedical Engineering Reference
In-Depth Information
Fig. 26.9
Illustration of the interpretation of the invariants
I
1
and
I
2
of the
right
(
left
) Cauchy-
Green tensor as related to change in length or surface area of an infinitesimal volume element
of a textile with much higher resistance to area stretch in the plane of the membrane
than to elongations in uniaxial stress state. Similar observations were reported from
the multiaxial characterization of fetal membranes (Bürzle et al.,
2012
), indicating
that this behavior might be common to several biological membranes.
Attention was given to the definition of the reference configuration in both, the
mechanical test set-up and the
in vivo
physiological reference state. The influence
of setting a threshold membrane tension value for the analysis of uniaxial and biax-
ial data was demonstrated. Liver capsule, as many other biological membranes, is
subjected to a preload due to the homeostatic pressure at their reference physiolog-
ical state. Thus, a mechanical model describing the response of the capsule in the
vicinity of that reference state might be defined with respect to a preloaded config-
uration, i.e. the state corresponding to the
in vivo
membrane tension. This approach
was adopted here for determining a constitutive model of Glisson's capsule. It is
interesting to note that the scatter of the experimental results is drastically reduced
in case the physiological membrane tension is taken as threshold force value.
26.4.3 Diagnostic Relevance of Aspiration Measurements
As demonstrated with corresponding finite element investigations, the aspiration
technique cannot detect moderate changes in consistency of parenchyma, mainly
due to the shielding effect of the capsule. However at least for dense lesions with
a high connective tissue content, which are larger than the aspiration hole and not
located deeper than 15 mm, a trend to a stiffer response was observed. The reduced
sensitivity of the technique motivated the analysis in terms of relative stiffness, i.e. to
assess the base properties of normal parenchyma, such as a stiffness-level, and then
to assess the suspected lesion site relative to its peripheral parenchyma character-
ized from the reference measurement. This strategy leads to a real significant result:
lesions with a high connective tissue content, a larger diameter than the aspiration
hole, and not located deeper than 15 mm beneath the capsule lead to a detectable
