Effect of different radial hole designs on pullout and structural strength of cannulated pedicle screws

Cannulated pedicle screws are designed for bone cement injection to enhance fixation strength in severely osteoporotic spines. However, the screws commonly fracture during insertion. This study aims to evaluate how different positions/designs of radial holes may affect the pullout and structural strength of cannulated pedicle screws using finite element analysis. Three different screw hole designs were evaluated under torsion and bending conditions. The pullout strength for each screw was determined by axial pullout failure testing. The results showed that when the Von Mises stress reached the yield stress of titanium alloy the screw with four radial holes required a greater torque or bending moment than the nine and twelve hole screws. In the pullout test, the strength and stiffness of each screw with cement augmentation showed no significant differences, but the screw with four radial holes had a greater average pullout strength, which probably resulted from the significantly greater mean maximum lengths of cement augmentation. Superior biomechanical responses, with lower stress around the radial holes and greater pullout strength, represented by cannulated pedicle screw with four radial holes may worth recommending for clinical application.