Effects of Percutaneous Pedicle Screw Placement and Suboptimal Insertion Depth on Insertion Torque and Pullout Strength: A Biomechanical Analysis

Abstract Introduction: Screw pullout strength is a crucial measure of biomechanical pedicle screw performance and durability in vivo. Factors including screw design, insertion technique, and bone quality can significantly influence this. The effect of insertion depth on the biomechanical properties of percutaneous pedicle screws remains incompletely characterized. Methods: A biomechanical assessment of screw insertion torque and axial pullout strengths was conducted using polyaxial pedicle screws (Viper Prime™, DePuy Synthes, PA, USA) and solid polyurethane foam vertebral bone analogues (20 PCF, Sawbones Inc., WA, USA). Three pedicle screw sizes were evaluated: 5mm diameter (45mm length), 6mm diameter (50mm length), and 7mm diameter (50mm length). A uniaxial linear-torsion testing machine (E10000, Instron Inc, MA, USA) was used to perform insertion and pull out according to ASTM F543 standard. Insertion torque was measured continuously up to full depth. Axial pull-out strength was measured starting with full depth insertion and gradually decreasing to a 10mm gap between screw head and the block surface, in steps of 1mm. Results: Mean maximum insertion torques were 6.17 ± 0.50 Nm, 6.80 ± 2.70 Nm, and 9.85 ± 2.82 Nm for the 5mm, 6mm, and 7mm screws, respectively (p = 0.039). As the gap increased from 0 to 10mm, pull-out strength decreased from 1.62 ± 0.03 kN to 0.98 ± 0.02 kN in the 5mm screw, from 2.20 ± 0.10 kN to 1.71 ± 0.04 kN in the 6mm screw, and from 2.28 ± 0.27 kN to 1.75 ± 0.14 kN in the 7mm screw. Linear regression analysis revealed strong linear relationships (R² = 0.580–0.908, p < 0.01) between gap size and pullout strength for all screw diameters. Statistical analysis confirmed significantly greater pull-out strength for larger screws (p < 0.0001). Conclusion: Larger diameter screws demonstrate significantly greater pullout strength. Increasing screw insertion depth is associated with progressively greater insertion torque and higher pullout strengths across all screw diameters assessed.