Validation of a novel smart drilling system to monitor bone impedance during transpedicular screw placement: a pilot study

Transpedicular screw placement is a high-risk procedure routinely performed in spine surgery. To decrease the rate of complications, it is necessary to find innovative solutions to assist the surgeon during screw insertion so as to avoid the chance of mispositioning. In this study, we developed a new drilling system able to estimate the mechanical properties of drilled tissues. Several investigations show that cortical bone requires a high level of thrust force and torque during drilling compared to trabecular bone. To implement an algorithm for bony breakthrough detection, a new drilling system has been built together with a mechanical support to drill the pedicle along a pre-planned trajectory. The mechanical support is equipped with a smart rotative drill that embeds force and position sensors. Ten human vertebral segments have been used to test the surgical platform, for percutaneous bone drilling. 10 transpedicular holes from L1 to L5 have been performed bilaterally. The holes were further evaluated by computed tomographic scans to measure bone density in the cortical and in the trabecular layers. To compare bone density with the bony mechanical impedance two new parameters (DHU and DPAI) have been introduced. The results show that in 18 out of 20 cases the D values of bone density and mechanical impedance, related to the same bone transition, differ less than 10%. The proposed system is thus able to evaluate the variation of bone density of the cortical and the trabecular layer using impedance. Therefore, it is possible to use the described system to increase the accuracy of transpedicular screw placement.