(194k) Modeling of Intervertebral Disc Tissue Exposed to Pulsed Electric Fields

This work investigates irreversible electroporation and
tissue damage induced within intervertebral discs (IVD). IVD disease is a primary
cause of lower back pain, which impacts approximately 80% of humans during
their lifetime. Therapeutic treatments of IVD disease are mostly palliative,
and IVD structure and function are compromised by surgical interventions,
prompting a need for curative treatments. Irreversible electroporation uses pulsed
electric fields (PEF) to permeabilize and kill cells within diseased tissue,
making way for healthy cells while limiting collateral damage. A threshold
local electric field strength must be reached within the IVD tissue to
accomplish irreversible electroporation. Herein, the magnitude of the electric
field and the thermal gradients are modeled in an IVD for comparison to experimental
results.

In this study, a Comsol Multiphysics
model of an IVD is constructed using a rotated axisymmetric geometry (Fig. 1A)
with a Joule heating physics interface and a time-dependent study. The PEF exposure
chamber is modeled as two plate electrodes sandwiching the tissue. Physical
properties for the IVD come from values in the literature for annulus fibrosus
tissue and cartilage. From simulations, it was found that a linear relationship
between applied voltage and electric field strength exists and is independent
of pulse width. Ohm’s Law predicts this relationship, and the applied voltage
necessary to achieve irreversible electroporation within an IVD that is 6.9 mm
thick is approximately -1.63 kV.

The electric field within
the tissue generates heat at a rate determined by the Joule heating equation. This
heat begins to damage tissue via necrotic cell death and thermal degeneration
of biomolecules at temperatures between 50 ^oC and 60 ^oC. The
temperature of the IVD increases by only a fraction of 1 K, even when exposed
to 1000 consecutive pulses of 1-μs width and -1.7 kV (Fig. 1B). Variations in
geometry of the tissue are expected in practice, and a parametric sweep during
a single 10-ms pulse shows that the applied voltage needed to achieve the
maximum specified temperature of 343.15 K depends linearly on tissue thickness.