MODELING the response of a deformable soft tissue
during cutting is a fundamental scientific problem, an
understanding of which can be used to develop realistic surgical
simulators for providing accurate force feedback to the surgeon
during surgical training for soft-tissue cutting procedures (such
as skin incision and biopsies, which do not use electrocautery
[1], [2] to avoid cellular damage). Scalpel cutting is a
fundamental art required of residents in training in surgery. Most
commonly, scalpel cutting is useful for lower abdominal cutting
of the skin to access the internal organs in open procedures
such as caesarean section (C-section) deliveries. Scalpel cut-
ting in such procedures aids in minimizing the regional tissue
distortion and destruction, and aids in better wound healing and
better scar after the incision is closed by either subcutaneous
sutures or external staples. Before surgeon trainees actually
operate on patients, it is desirable to practice cutting skills on
simulators to familiarize themselves with the real responses of
the soft tissue encountered in a surgical procedure. To enable
realistic simulation, it is necessary to develop models based
on actual experimental data. Our long-term goal is, thus, to
develop a model of soft tissue interaction with a cutting blade
to be used in surgery simulators. The goal of this paper in
particular is to conduct scalpel blade cutting experiments, study
the experimental data during soft tissue cutting and extract the
characterizing parameters via suitable models.
We have conducted ex vivo liver cutting experiments to
measure the cutting force versus cut-length characteristics.
Our results show that the measured force-displacement curves in the
blade cutting experiments exhibited a characteristic pattern: re-
peating units of a segment of linear loading (deformation) fol-
lowed by a segment of sudden unloading (localized crack ex-
tension in the tissue). The prerequisite to high-fidelity surgery
simulation is realistic characterization of these two constituent
phases of the cutting process: the deformation phase and the
crack extension phase. In this paper, we focus in characterizing
the deformation resistance during the deformation phase im-
mediately preceding crack extension in the soft tissue. We also
study the effect of cutting speed, cutting angle, and the depth of
cut on the deformation resistance of the tissue in this paper. In
a subsequent paper, we shall discuss details of a parallel study
on characterizing the crack extension phase.
We have conducted experiments with seven cutting speeds
and two blade angles, namely 45 and 90 with respect to the
horizontal axis. Like most organs, the liver specimens we used
in the experiments are naturally nonuniform in thickness. We
used a stereo camera system and developed image-processing
procedures to determine the time variation of the embedded
blade depth in the tissue as the cutting progressed in the liver
sample with a natural bulge in the thickness direction. The depth
of cut of the blade in the tissue was then used to normalize the
experimentally recorded cutting forces. The depth-of-cut normalized forces were applied in an inverse problem to determine
a normalized local effective modulus (LEM) which is a measure of the deformation resistance of the soft tissue. A feature of