Mechanical stability of a novel screw design after repeated insertion: Can the double-thread screw serve as a back up?

Abstract We investigated mechanical pull-out behavior and tightening torque of a novel dual core pedicle "6T screw" (6T). The aim of this study was to test if these changes in screw geometry are increasing the strength of the pedicle screw fixation after repeated insertion. Three different types of pedicle screws were inserted in rigid foam blocks. Tightening torque and pull-out strength were measured during two repetitive insertions of a standard 6.5x45mm conical screw. The third insertion into the pilot hole was performed using either standard 6.5x45mm or 7.2x45mm conical screws or the novel 6.5x45mm (6T) screw. Additionally, we performed a surface analysis to investigate the bone/screw interface. The maximal tightening torque at the third insertion of the novel 6T screw was 194% higher compared to the standard 6.5x45mm conical screw and 135% higher compared to the standard 7.2x45mm conical screw. The pull-out strength of the 6T screw showed no significant changes, and surface analysis revealed a compression of the screw-foam interface due to the different internal diameters. The modified geometrical design of the 6T screw seems to have no statistically significant effect on the pull-out strength, although it achieved a higher tightening torque. This might be due to the different pitch angle cutting a new thread into the material and also to the enlarged inner diameter.