gms | German Medical Science

Objective: Despite technical advances such as computer navigated instrumentation, malpositioning of pedicle screws is still a major issue in spine surgery. Although, robots can potentially be smart tools if equipped with sensors, existing robotic systems simply retain the trajectory for screw placement precisely. In doing so they are imaging based and comprise the same potential risk of malpositioning associated with computer navigation.

The aim of this work was to evaluate a concept of intelligent, sensor based pedicle drilling to predict a potential cortical breach.

Methods: We hypothesized that a robotic arm equipped with force-torque sensors could drill with a maximum inaccuracy of < 1mm within the cortical borders of the lumbar spine. The system was able to detect the resistance at the drill tip while traversing the cortical and spongious bone. Subsequently, trajectories (n=20) were drilled in various segments of a human lumbar spine specimen. Pre- and post-drilling high-resolution CT scans of the trajectories were acquired and co-registered. Per each drilled trajectory, the axial force applied to the drill tip was extracted and correlated with the candidate trajectory.

Results: Seventeen datasets were available for accuracy analysis. Intentionally, in 4 of 17 trajectories a medial breach was created and 7/17 trajectories a lateral breach. The remaining 6/17 trajectories were fully passed through the pedicle. The pose of the drill trajectory could be identified as accurate as 0.25±0.11 mm (n= 17), with available accuracies during lateral and medial breach to be 0.27±0.06 mm (n = 7) and 0.34±0.11 mm (n = 4) respectively. Available accuracy in the pedicle was 0.15±0.08 mm (n = 6).

Conclusion: Conclusively, the maximum error of drill tip estimation was < 0.5 mm, indicating a potentially very powerful approach that operates independently of an image-guidance model (including inherent problems of image co-registration, optical tracking, instrument calibration etc.). The force-based measurement can accurately detect the position of the drill tip by identifying the trabecular bone matrix – an individual fingerprint – of the drill path according to the high-resolution CT scan. Detection of a cortical bone breach within 0.5 mm represent a novel and clinically highly relevant safety feature. By adding sensor technologies, robots can become "smart" and lead the way to the next level of robot-assisted spine surgery.