Abstract
Introduction and Objective
Home-based monitoring of fracture healing has the potential of reducing routine follow-up and improve personalized fracture care. Implantable sensors measuring electrical impedance might detect changes in the electrical current as the fracture heals. The aim was to investigate whether electrical impedance correlated with radiographic fracture healing.
Materials and Methods
Eighteen rabbits were subjected to a tibial osteotomy that was stabilized with an external fixator. Two electrodes were positioned, one electrode placed within the medullary cavity and the other on the lateral cortex, both three millimeters from the osteotomy site. Transverse electrical impedance was measured daily across the fracture site at a frequency range of 5 Hz to 1 MHz using an Analog Discovery 2 Oscilloscope with Impedance Analyzer. Biweekly x-rays were taken and analyzed blinded using a modified anterior-posterior (AP) radiographic union score of the tibia (RUST). Each animal served as its own control by performing repeated measurements from time zero until the end of follow-up.
Results
At 5 Hz measurements, a linear mixed model revealed an average impedance at day zero of 10670 +/− 272 Ohm (p<0.001) and a change in impedance from day 0 to day 7 of −3330 +/− 152 (p<0.001). The slope from day 0–7 was estimated as −548.6 +/− 26 (p<0.001) and was steeper than the slope after day 7 which was estimated to −85.6 +/− 4 (p<0.001). This indicates that the impedance decreased quicker before day 7 and slower after day 7. The coefficient of variation for difference between RUST scores, from double intra-rater measurements of 15 radiographs with a minimum of 22 days between, was 1.3. Spearman's correlation coefficient between impedance and RUST score at the 5 Hz was −0.75 (p<0.001).
Conclusions
This osteotomy model showed that the electrical impedance can be measured in vivo at a distance from the fracture site with a consistent change in impedance over time. This is the first study to demonstrate a significant correlation between increasing radiographic union score and decreasing impedance. Further studies are warranted to investigate how these new and important results can further be translated into larger animal studies.
