The Effect of “Toggling” on the Pullout Strength of Bone Screws in Normal and Osteoporotic Bone Models
This study tests the hypothesis that screw toggling will reduce the pullout strength of bone screws. Pullout strength of cortical (cylindrical) and cancellous (tapered by 4°) bone screws were measured without and after toggling (movement caused by a force perpendicular to the screw axis) by ± 1 mm in polyurethane foam intended to mimic normal (density 0.32 g.cm -3 ) and osteoporotic (density 0.16 g.cm -3 ) bone. Toggling had no significant effect in decreasing the strength of fixation of cortical or cancellous screws inserted in the normal and osteoporotic bone models. Analysis of the screws that had been toggled showed that there was no significant difference (p &gt; 0.05) between the pullout strength of cancellous and cortical screws (when pulled out from the same material). The results provide insights into some mechanical aspects of the pullout strength of bone screws and will aid understanding of the mechanism of screw toggling in vivo.