Toggling Effect on Pullout Performance of Pedicle Screws: Review

Abstract

Screw loosening in spine surgery is a clinical complication in patients with poor bone quality. Pedicle screws are subjected to bending moments and axial loads that may cause toggling during daily movements of spine. The purpose of this study was to assess the previous studies related to toggling effect on pullout performance of pedicle screws by surveying the whole literature and to provide some discussion for new studies about pullout performance of pedicle screws after toggling. The search was performed by combining terms of pedicle screw, toggling, screw loosening, fatigue, cyclic loading, and pullout. The retrieved articles dealing with determined terms and also their references were reviewed. Some of these articles were eliminated after review process. Toggling was determined to be crucial for the stabilization performance of pedicle screw because the loosening mechanism of screws was affected directly by cyclic loading. The toggling or cyclic loading affected the holding capacity of pedicle screws negatively, and the possibility of loosening or failure problem for pedicle screws increased with cyclic loading magnitude. Loading conditions, screw properties, test medium, level of spinal region, and cement usage were determined by many researchers as the most important parameters affecting the toggling performance as well as the pullout strength of pedicle screws. The pullout strength of pedicle screws generally decreased with cyclic loading. The parameters of cyclic loading were fairly important for pullout performance of pedicle screws. Screw properties and cement augmentation had critical effects on the stability of screws under cyclic loading, as well.