Characterization of the electrical conductivity of bone and its correlation to osseous structure.

The interaction of osseous tissue with electric fields is an important subject. The electrical stimulation of bone promotes osteogenesis, while bone impedance has been proposed as a measure of osteoporosis, to follow fracture healing, or as a method to improve safety of surgical procedures. However, a deeper understanding of the electrical properties of bone and their relation to the architecture of osseous tissue is required to extend the range of use of electrical measurements to clinical studies. In this paper we apply electrical impedance spectroscopy to study the conductivity of fresh bovine tibia and we correlate the measured conductivities with its structural properties. Impedance was measured using a custom-made cell and a potentiostat. Bone conductivity was determined at 100 kHz, where the phase shift was negligible. A good agreement (R2 = 0.83) was found between the measured conductivity and the bone volume fraction, determined on microCT images. Based on this relationship, an equivalent circuit model was created for bone samples. The results of this ex-vivo study are comparable to previous in-vivo observations reporting bone resistivity as a function of bone density. This information can be used to construct a map of the tissue resistivity directly derived from clinical images.

A Neuromonitoring Approach to Facial Nerve Preservation During Image-guided Robotic Cochlear Implantation.

HYPOTHESIS: A multielectrode probe in combination with an optimized stimulation protocol could provide sufficient sensitivity and specificity to act as an effective safety mechanism for preservation of the facial nerve in case of an unsafe drill distance during image-guided cochlear implantation. BACKGROUND: A minimally invasive cochlear implantation is enabled by image-guided and robotic-assisted drilling of an access tunnel to the middle ear cavity. The approach requires the drill to pass at distances below 1  mm from the facial nerve and thus safety mechanisms for protecting this critical structure are required. Neuromonitoring is currently used to determine facial nerve proximity in mastoidectomy but lacks sensitivity and specificity necessaries to effectively distinguish the close distance ranges experienced in the minimally invasive approach, possibly because of current shunting of uninsulated stimulating drilling tools in the drill tunnel and because of nonoptimized stimulation parameters. To this end, we propose an advanced neuromonitoring approach using varying levels of stimulation parameters together with an integrated bipolar and monopolar stimulating probe. MATERIALS AND METHODS: An in vivo study (sheep model) was conducted in which measurements at specifically planned and navigated lateral distances from the facial nerve were performed to determine if specific sets of stimulation parameters in combination with the proposed neuromonitoring system could reliably detect an imminent collision with the facial nerve. For the accurate positioning of the neuromonitoring probe, a dedicated robotic system for image-guided cochlear implantation was used and drilling accuracy was corrected on postoperative microcomputed tomographic images. RESULTS: From 29 trajectories analyzed in five different subjects, a correlation between stimulus threshold and drill-to-facial nerve distance was found in trajectories colliding with the facial nerve (distance <0.1  mm). The shortest pulse duration that provided the highest linear correlation between stimulation intensity and drill-to-facial nerve distance was 250  µs. Only at low stimulus intensity values (≤0.3  mA) and with the bipolar configurations of the probe did the neuromonitoring system enable sufficient lateral specificity (>95%) at distances to the facial nerve below 0.5  mm. However, reduction in stimulus threshold to 0.3  mA or lower resulted in a decrease of facial nerve distance detection range below 0.1  mm (>95% sensitivity). Subsequent histopathology follow-up of three representative cases where the neuromonitoring system could reliably detect a collision with the facial nerve (distance <0.1  mm) revealed either mild or inexistent damage to the nerve fascicles. CONCLUSION: Our findings suggest that although no general correlation between facial nerve distance and stimulation threshold existed, possibly because of variances in patient-specific anatomy, correlations at very close distances to the facial nerve and high levels of specificity would enable a binary response warning system to be developed using the proposed probe at low stimulation currents.