Towards Biomechanical Evaluation of a Transformative Additively Manufactured Flexible Pedicle Screw for Robotic Spinal Fixation

TL;DR

Osteoporosis elevates pedicle screw loosening and pullout risk during spinal fixation, undermining outcomes in vertebral fractures. The authors propose a transformative flexible pedicle screw (FPS) paired with a concentric-tube steerable drilling robot (CT-SDR) to exploit curved trajectories and place screws in higher-BMD regions. They design two FPS geometries, fabricate them in Ti-6Al-4V via direct metal laser sintering, and evaluate pullout strength on straight trajectories using Sawbone phantoms per ASTM F543/F1839, with synchronized X-ray imaging to analyze deformation. Results show both FPS variants meet and exceed the required pullout threshold and exhibit a prolonged fixation zone driven by elastic deformation, suggesting reduced loosening and revision risk; future work includes curved-path testing, cadaveric studies, and finite element modeling to optimize designs.

Abstract

Vital for spinal fracture treatment, pedicle screw fixation is the gold standard for spinal fixation procedures. Nevertheless, due to the screw pullout and loosening issues, this surgery often fails to be effective for patients suffering from osteoporosis (i.e., having low bone mineral density). These failures can be attributed to the rigidity of existing drilling instruments and pedicle screws forcing clinicians to place these implants into the osteoporotic regions of the vertebral body. To address this critical issue, we have developed a steerable drilling robotic system and evaluated its performance in drilling various J- and U-shape trajectories. Complementary to this robotic system, in this paper, we propose design, additive manufacturing, and biomechanical evaluation of a transformative flexible pedicle screw (FPS) that can be placed in pre-drilled straight and curved trajectories. To evaluate the performance of the proposed flexible implant, we designed and fabricated two different types of FPSs using the direct metal laser sintering (DMLS) process. Utilizing our unique experimental setup and ASTM standards, we then performed various pullout experiments on these FPSs to evaluate and analyze their biomechanical performance implanted in straight trajectories.

Figures (7)

• Figure 1: Conceptual illustration of the proposed FPS. Left: The proposed FPS is fixed into a curved trajectory within a spinal vertebra. Notably, it is shown to be avoiding areas of low BMD. Right: A RPS fixed into areas of high BMD.
• Figure 2: Experimental Set-Up of the CT-SDR System including the CT-SDR and KUKA LBR. (A): A detailed view of the flexible cutting tool including the steel guide and the drill tip.
• Figure 3: Theoretical design of the FPS emphasizing critical design features based on patient and surgeon necessities.① represents the rounded head of the screw. ② represents the self-tapping threads of the screw, with ③ and ④ representing the flexible and rigid parts of the screw, respectively. ⑤ represents the cannulated region of the screw. (A) illustrates the side view of the full FPS screw with (B) representing the cross-section of the FPS.
• Figure 4: Metal 3D printed flexible pedicle screw. (A) Two different sized flexible pedicle screws fabricated on the build plate before post-processing. (B) The 9 mm OD-6 mm ID screw after post-processing had been done including sandblasting. Critical dimensions for the screw are annotated on the image.
• Figure 5: Experimental set-up for pedicle screw pullout testing. The experimental set-up includes a X-Ray C-Arm machine, an Instron Tensile Testing Machine, and PCF-10 Sawbone samples fitted within a metal frame. It also has a zoomed in view of the Sawbone sample interacting with the Instron machine during the pullout process.