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

P.S.D Patel,D.E.T Shepherd,D.W.L Hukins

doi:10.2174/1874155x01307010035

Abstract

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 > 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.

Highlights

Osteoporosis weakens bone; as a result, it is sometimes considered as a contra-indication for screw fixation, especially in the spine [1,2,3,4,5]
The results show that there was no significant difference (p > 0.05) between the pullout strength of screws without and after toggling for cancellous and cortical screws from either normal or OP bone models
The pullout strength of the cortical screw from the 0.16 g.cm-3 density foam was greater with toggling than without

Summary

Introduction

Osteoporosis weakens bone; as a result, it is sometimes considered as a contra-indication for screw fixation, especially in the spine [1,2,3,4,5]. Pedicle screws that are used for fixation of implants to the vertebrae of the spine are subjected to bending and rotational moments [13,14,15]. It has been claimed that pedicle screws are subjected to a bending moment, that would cause toggling, as well as to an axial force [18, 19]. There have been several studies of pullout of pedicle screws under a wide range of conditions, from many different materials [20,21,22,23,24]. Few studies have considered the effect of screw toggling on pullout strength in OP vertebrae and the effect of toggling on pullout strength is not clear [25, 26]

Methods

Results

Conclusion