A Biomechanics-Aware Robot-Assisted Steerable Drilling Framework for Minimally Invasive Spinal Fixation Procedures

Abstract

In this paper, we propose a novel biomechanics-aware robot-assisted steerable drilling framework with the goal of addressing common complications of spinal fixation procedures occurring due to the rigidity of drilling instruments and implants. This framework is composed of two main unique modules to design a robotic system including (i) a Patient-Specific Biomechanics-aware Trajectory Selection Module used to analyze the stress and strain distribution along an implanted pedicle screw in a generic drilling trajectory (linear and/or curved) and obtain an optimal trajectory; and (ii) a complementary semi-autonomous robotic drilling module that consists of a novel Concentric Tube Steerable Drilling Robot (CT-SDR) integrated with a seven degree-of-freedom robotic manipulator. This semi-autonomous robot-assisted steerable drilling system follows a multi-step drilling procedure to accurately and reliably execute the optimal hybrid drilling trajectory (HDT) obtained by the Trajectory Selection Module. Performance of the proposed framework has been thoroughly analyzed on simulated bone materials by drilling various trajectories obtained from the finite element-based Selection Module using Quantitative Computed Tomography (QCT) scans of a real patient's vertebra.

Fig. 1:

(a) Conceptual illustration of a conventional PSF using rigid pedicle screws inside an osteoporotic vertebra; (b) Conceptual comparison between the linear and hybrid (linear and/or curved) drilling trajectories using a rigid drill and the proposed steerable drilling robotic system.

Fig. 2:

The proposed biomechanics-aware robot-assisted steerable drilling framework signal flow diagram. This framework is composed of a patient-specific biomechanics-aware Trajectory Selection Module which accepts a QCT scan and outputs a desired trajectory based on finite element analysis to be executed by the Semi-Autonomous Robotic Drilling Module. Figure also shows the step-by-step workflow within each module.

Fig. 3:

The six designed drilling trajectories within the T12 vertebra to thoroughly analyze the performance of the Selection and Robotic Drilling Modules. For the considered curved HDTs, ${\kappa }_1=0$ = 0, ${\kappa }_2$ is 1/0.0695 m=14.388 m⁻¹. and ${\kappa }_3$ is 1/0.035 m=28.571 m⁻¹.

Fig. 4:

(a) Side and (b) Top views of the loading condition for the FE-analysis.

Fig. 5:

Stress distribution of the six considered trajectories.

Fig. 6:

Strain distribution of the six considered trajectories.

Fig. 7:

Experimental set-up including A: a 7-DoF KUKA LBR robot arm integrated with the CT-SDR aligned with a test specimen; B: View of the CT-SDR tool and the load cell used for force measurement; C&D: Shows the behavior of the nitinol guide tubes when extended from the rigid straight stainless steel; E: A Detailed view of the flexible cutting instrument, including drill tip, flexible shaft, and the concentric guiding tubes.

Fig. 8:

Results of the curved drilling trajectories performed with the CT-SDR system. Red and Dark Blue: Tests performed to match the 69.5 mm radius of curvature trajectories with a target insertion depth of 35 and 27 mm, respectively. Green and Light Blue: Tests performed to match the 35 mm radius of curvature with a target insertion depth of 35 and 27 mm, respectively. Average insertion depth and radius of curvature is shown in the sub-images for each trajectory set. Trajectories $T_{12}$ and $T_{13}$ are shown reflected over the y-axis to allow for better visibility.

Fig. 9:

The Kuka end-effector position together with the forces in the x-direction (as shown in Fig. 7) measured by the six-axis sensor and Kuka embedded load cells in each control mode during drilling the trajectory $T_{12}$ with 35 mm straight depth using the Kuka robot and 14.388 m⁻¹ curvature using the CT-SDR.

Fig. 10:

The Kuka end-effector position together with the forces in the x-direction (as shown in Fig. 7) measured by the six-axis sensor and Kuka embedded load cells in each control mode during drilling the trajectory $T_{11}$ with 35mm straight depth using the Kuka robot and the straight CT-SDR extension.