The Effect of “Toggling” on the Pullout Strength of Bone Screws in Normal and Osteoporotic Bone Models

P.S.D. Patel, D.W.L. Hukins, D.E.T. Shepherd*
School of Mechanical Engineering, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK.

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© 2013 Patel et al

open-access license: This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 International Public License (CC-BY 4.0), a copy of which is available at: ( This license permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

* Address correspondence to this author at the School of Mechanical Engineering, University of Birmingham, Edgbaston, Birmingham, B15 2TT,UK; Tel: 01214144266; Fax: 01214143958; E-mail:


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 and osteoporotic (density 0.16 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 > 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.

Keywords: Bone, osteoporosis, pullout, screws, strength, toggling.