Original Article Proc IMechE Part H: J Engineering in Medicine 1–8 Ó IMechE 2018 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0954411918755417 journals.sagepub.com/home/pih Investigation of toggling effect on pullout performance of pedicle screws Mehmet Fatih Aycan 1 , Mesut Emre Yaman 2 , Yusuf Usta 1 , Teyfik Demir 3 and Tolga Tolunay 4 Abstract Objective of this study is to assess the pullout performance of various pedicle screws in different test materials after tog- gling tests comparatively. Solid core, cannulated (cemented), novel expandable and solid-core (cemented) pedicle screws were instrumented to the polyurethane foams (Grade 10 and Grade 40) produced in laboratory and bovine vertebra. ASTM F543 standard was used for preparation process of samples. Toggling tests were carried out. After toggling test procedures, pullout tests were performed. Load versus displacement graph was recorded, and the ultimate pullout force was defined as the maximum load (pullout strength) sustained before failure of screw. Anteriosuperior and oblique radiographs were taken from each sample after instrumentation in order to examine screw placement and cement distri- bution. The pullout strength of pedicle screws decreased after toggling tests with respect to the initial condition. While the cemented solid-core pedicle screws had the highest pullout strength in all test materials, they had the highest strength differences. The cemented solid-core pedicle screws had decrement rates of 27% and 16% in Grade 10 and Grade 40, respectively. There are almost same decrement rate (between 5.5% and 6.5%) for all types of pedicle screws instrumented to the samples of bovine vertebra. The pullout strengths of novel expandable pedicle screws in both of early period and after toggling conditions were almost similar, in other words, the decrement rates of it were lower than other types. According to the data collected from this study, polymethylmethacrylate augmentation significantly decreases pullout strength following the toggling loads. Higher brittleness of cured polymethylmethacrylate has adverse effect on the pullout strength. Although augmentation is an important process for enhancing pullout strength in early period, it has some disadvantages for preserving stabilization in a long time. Expandable pedicle screw with polyether- etherketone shell may be good alternative to polymethylmethacrylate augmentation on both primer stabilization and long-term loading application with toggling. Keywords Toggling, bovine vertebra, pedicle screw, polyurethane foam, pullout strength, expandable polyetheretherketone shell Date received: 27 October 2017; accepted: 4 January 2018 Introduction Pedicle screws (PSs) are commonly used medical devices in surgery operations for treatment of spine dis- eases like trauma, tumor resection and scoliosis. 1,2 Pullout strength of the PS is a critical point for success of the operations. In addition, the main aim of the researchers is enhancing pullout strength and preser- ving stabilization of PS. Design parameters of PSs, application methods, augmentation techniques and dif- ferent test materials were studied by many researchers before. 3,4 However, effect of bone mineral density (BMD), size and anatomic structures of vertebra on pullout strength of PSs were investigated in many stud- ies. 5 In biomechanics tests, cadaver (animal or human) and polyurethane (PU) foams standardized with ASTM standards and test protocols are used as a test material. 6 Furthermore, different types of PSs were designed in order to achieve higher pullout strength. 1 Department of Mechanical Engineering, Gazi University, Ankara, Turkey 2 Department of Neurosurgery, Memorial Ankara Hospital, Ankara, Turkey 3 Department of Mechanical Engineering, TOBB University of Economics and Technology, Ankara, Turkey 4 Department of Orthopedics, Yıldırım Beyazıt University, Ankara, Turkey Corresponding author: Teyfik Demir, Department of Mechanical Engineering, TOBB University of Economics and Technology, Sogutozu Street No. 43, Ankara 06560, Turkey. Email: tdemir@etu.edu.tr