Mechanical Characterization of the Human Liver - Computer Models in Biomechanics - page 349

Experimental observations of the mechanical response of hepatic tissue are es-
sential for determining model parameters of phenomenological model equations and
are useful for verifying the predictive capabilities of model formulations proposed
in the literature. Constitutive equations often aim at describing the in vivo mechan-
ical behavior of tissues and organs when subjected to physiological conditions of
loading and deformation. Mechanical measurements must be characterized by well
defined kinematic and kinetic boundary conditions so to allow the inverse problem
to be solved. Many experimental studies were performed to characterize the me-
chanical behavior of liver tissue, see, e.g., Chui et al. ( 2007 ) and Gao et al. ( 2010 ).
Most of these involved ex vivo measurements on animal tissue. Data from ex vivo
experiments, without physiological blood perfusion, might provide inappropriate
results for mechanical modeling of the liver, as analyzed by Kerdok et al. ( 2006 ).
This motivates the development of experimental techniques for in vivo mechanical
testing. Account on the state of the art in measuring the deformation behavior of
soft biological tissue, in particular liver, is given in Hollenstein ( 2011 ). Among the
procedures proposed for in vivo liver characterization a few works shall be men-
tioned here: in Brown et al. ( 2003 ) an endoscopic grasper is used for experiments
on porcine liver. Ottensmeyer ( 2002 ) and Samur et al. ( 2007 ) present indentation
tests on animal organs. Carter et al. ( 2001 ) performed first intra-operative in vivo
experiments on human liver. They used an indentation device and calculated a value
of approximately 270 kPa for the linear elastic modulus of human liver. In contrast,
dynamic elastography experiments on human liver, Huwart et al. ( 2006 ) and Rou-
vière et al. ( 2006 ), indicated that the linear elastic modulus of healthy human liver
is in the range of 6 kPa.
In our laboratory, first in vivo mechanical experiments on human liver performed
during open surgery using the so called 'aspiration device' were reported in Mazza
et al. ( 2007 ) and Nava et al. ( 2008 ). Data from 23 measurements on 6 healthy organs
were analyzed in Nava et al. ( 2008 ) and a constitutive model for 'average liver tis-
sue' determined. Two main factors affect the corresponding results: (i) the liver was
modeled as a homogeneous deformable solid without consideration of the mechan-
ical resistance of the Glisson's capsule (the connective tissue layer that covers the
organ); (ii) the initial deformation of the liver tissue, due to the compressive force
applied to ensure a good initial contact between aspiration device and liver, was not
considered for the inverse problem.
Results from our latest clinical study with aspiration experiments on the liver dur-
ing open abdominal surgery are presented in this paper. An extensive experimental
campaign with measurements on patients undergoing hepatic resection has been re-
cently completed. In total the livers of 33 patients at the age of 38-82 years have
been tested in vivo at the beginning of the surgical procedure. Measurements were
taken on one normal reference site and one or two target sites with lesion leading to
a total of 207 realizations of the aspiration experiment during open surgery on the
human liver in this study.