Spatial Spinal Fixation: A Transformative Approach Using a Unique Robot-Assisted Steerable Drilling System and Flexible Pedicle Screw

TL;DR

The paper addresses fixation failures from rigid spinal instrumentation by introducing Spatial Spinal Fixation (SSF), which leverages a Concentric Tube Steerable Drilling Robot (CT-SDR) and a Flexible Pedicle Screw (FPS) to enable planar and out-of-plane placements across the vertebral body. The SSF framework integrates biomechanics-informed trajectory selection with semi-autonomous drilling and a morphable FPS to realize I- and J-shaped trajectories, including complex I-I, I-J, and J-J configurations. Experimental validation in L3 vertebral phantoms demonstrated high trajectory fidelity, achieving an average curvature radius near 50 mm with an average error of about 1.14%, while enabling faster drilling times compared to some tendon-driven systems. The work suggests SSF can access high-BMD regions and improve fixation stability, with planned future validation in animal and cadaver models and image-guided autonomous procedures.

Abstract

Spinal fixation procedures are currently limited by the rigidity of the existing instruments and pedicle screws leading to fixation failures and rigid pedicle screw pull out. Leveraging our recently developed Concentric Tube Steerable Drilling Robot (CT-SDR) in integration with a robotic manipulator, to address the aforementioned issue, here we introduce the transformative concept of Spatial Spinal Fixation (SSF) using a unique Flexible Pedicle Screw (FPS). The proposed SSF procedure enables planar and out-of-plane placement of the FPS throughout the full volume of the vertebral body. In other words, not only does our fixation system provide the option of drilling in-plane and out-of-plane trajectories, it also enables implanting the FPS inside linear (represented by an I-shape) and/or non-linear (represented by J-shape) trajectories. To thoroughly evaluate the functionality of our proposed robotic system and the SSF procedure, we have performed various experiments by drilling different I-J and J-J drilling trajectory pairs into our custom-designed L3 vertebral phantoms and analyzed the accuracy of the procedure using various metrics.

Figures (4)

• Figure 1: Conceptual illustration of the proposed SSF approach. Top image illustrates the CT-SDR drilling to reach areas of high BMD within the vertebral body in a J-J trajectory configuration. The bottom image illustrates the FPS being fixated within the aforementioned drilled configuration.
• Figure 2: Experimental set-up with CT-SDR, KUKA Robotic Manipulator, and Aurora Magnetic Tracking System. (A) A Close up view of the vertebra phantom with sawbone insert. (B) View of the FPS with magnetic tracking marker placed within the cannulated region. (C&D) Motion of the CT-SDR's drilling tip when actuated.
• Figure 3: The conceptual design and a fabricated FPS with major design features and parameters labeled on them. The figure on the left illustrates the FPS while it is flexing to matched a proposed J-shape curved trajectory with major design features labeled. The figure on the right illustrates a fabricated FPS in the straight configuration with major design parameters labeled.
• Figure 4: X-ray view of the performed SSF and implanted FPS along paths drilled into the vertebral phantom by the robotic framework. Note: All MATLAB plots/results were created with the 7 mm OD FPS, X-ray images were created with a 7 mm and 6 mm OD FPS.