A new toggling model in measurement of the pullout performance of pedicle screws and its experimental investigation

Pedicle screw are used in surgery operations for removal of one or more segments of the spine due to tumor and the treatment of scoliosis, trauma and fractures of the spine. The pullout strengths of pedicle screws are important for the success of these operations. The most important problem defined with pedicle screws is that the screw comes out at the early stage of stabilization. It is extremely important to investigate the causes of pedicle screws coming out at an early stage. Toggling may cause the screw to loosen and come out of the highly osteoporotic or osteoporotic spine. Toggling is the main loading of the spine and may cause loosening of the screw. In literature, the effect of toggling loading on the pull-out performances of pedicle screws was investigated, but the loading conditions were not similar to the anatomical loading conditions in these studies. This study is the first to examine the effect of toggling loading on the pullout performance of pedicle screws under the anatomical loading conditions with reference to the relevant standard of ASTM. In this thesis research, toggling test model is designed and produced. These models represent the removal of the middle segment of the three-segment vertebra and bilateral fixation with rods by placing two screws in the upper and lower segments. Within the scope of the study, one type of screw and two types of rod were used in the designed models. The reason for this is the selection of the screw-rod system which is resistant to toggling loads after the toggling loading is examined. Polyethylene (PE) and Polyurethane (PU) foam blocks were prepared in the models in order to mimic the bilayer structure of the bone. When PE blocks were produced on the basis of three different wall thicknesses in order to mimic cortical bone, PU foams were produced on the basis of three different densities in order to mimic trabecular bone. Thus, different cortical bone thicknesses and trabecular bone qualities were taken into account. After a certain number of toggling loading was applied to new designed models, pullout experiments were performed. At the end of the experiments, the effect of toggling was clearly understood in the certain models. As a result, two different rod systems were compared in the models in which the effect of toggling was determined and the screw-rod system resistant to short-term and long-term toggling loading was chosen.