Investigation of the Behaviour of the Pedicle Screw - Vertebral Bone Omplex, When Subjected to Pure Pull - Out Loads

The analysis of the failure mechanisms of spinal fixation systems and the stress distribution patterns within vertebral bodies constitutes an indispensable demand for the clinical evaluation of these systems. In this paper the influence of various parameters on the failure of fixation systems due to the pull-out phenomenon of the fixation screws is explored. To achieve this goal a Finite Element Model of the human lumbar vertebral bone and of the transpedicular fixation screw is designed simulating the main characteristics of commercial fixation pedicle screws. The parameters studied include the type of the bone-screw interface as well as some geometrical characteristics of the screws and the vertebral bone. The analysis revealed the different behavior of the pedicle screw- vertebral bone system and the different stress diffusion mechanisms activated according to the interface type. It was, also, concluded that the performance of pedicle screws under pull-out loads could be improved by increasing the thread depth.
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