Electrical impedance characterization of bone fracture

Electrical impedance spectroscopy measurements might be used for real-time monitoring of bone healing. Differences in electrical characteristics of different tissues during fracture healing can provide information of the tissue composition within the fracture region. This concept embraces the digital revolution of orthopaedics allowing for a sensor-based and home-based monitoring of bone healing. Furthermore, real-time monitoring will allow individualized and timely treatment adjustments to prevent bone healing complications. However, electrodes must be placed at a distance to the fracture site in order not to interfere with bone healing. Therefore, we investigated whether longitudinal and transverse electrical impedance measurements from electrodes placed at a distance to the bone defect can detect differences between intact bone and bone defects in vivo. Approval was granted from the Inspectorate of the Animal Experimentation under the Danish Ministry of Justice. Six rabbits were anaesthetized and had both tibias subjected to an osteotomy protocol where first the medial, then the lateral, and final the posterior cortex were removed resulting in a complete 2 mm bone defect. Electrical impedance was measured prior to and after each step of the osteotomy protocol. Recordings were obtained at different frequencies (10 Hz to 1 MHz) from an inner electrode placed into the medullary canal and two different electrodes placed extracortical on the lateral and posterior bone with a distance of 5 mm to the defect. For each rabbit, one tibia had measurements with a free inner electrode and the other tibia had measurements both with a nail and an isolated nail. For all tibias, the intact bone resulted in higher impedance compared with the complete defect, and this difference was most pronounced in the frequency range of 1 kHz to 100 kHz. This applied for all types of internal electrodes including electrode, nail, isolated nail. The isolated nail showed the biggest impedance difference between the intact bone and the complete defect. Incomplete bone defects had lower impedance compared with intact bone, but no consistent pattern for differences in impedance was observed between the different applied defects. Consistent impedance differences between intact bone and complete defects were detected in-vivo in rabbits. Further research is needed to explore whether the presented method of electrical impedance measurements can be used to characterize bone healing over time.