Thermal and electrical characterization of the intervertebral disc

Low back pain can now be treated by radiofrequency (RF) heating of the intervertebral disks. So as to develop realistic electrothermal models of the spine, we need to measure the thermal conductivity (K_m) and diffusivity (α) of the nucleus pulposus, as well as the electrical conductivity of the nucleus pulposus and annulus fibrosus. Measurements were performed in excised porcine intervertebral disks. Thermal parameters were measured by a self-heated spherical thermistor technique. Measurement of electrical conductivity (σ) was performed based on the four-electrode technique that injects current through the two outermost electrodes while the innermost electrodes are used to measure the voltage difference. The measurement of thermal conductivity (K_m=0.61W/m. °C) and diffusivity (α=1.93×10^-7 m²/s) of the nucleus pulposus showed that these values are close to those of water (K_m=0.60 W/m. °C and α=1.45×10^-7 m²/s). The measurement of electrical conductivity in the annulus fibrosus was σ=0.85±0.07 (S.m^-1). In the nucleus pulposus the measurements of electrical conductivity were σ=1.17±0.08 (S.m^-1) at 50 Hz and σ=1.36±0.1 ( S.m^-1 ) at 10 kHz.