Influence of bone quality and pedicle screw design on the fixation strength during Axial Pull-out test: A 2D Axisymmetric FE study.

Pedicle screw fixations are widely used to provide support and improve stability for the treatment of spinal pathologies. The effectiveness of treatment depends on the anchorage strength between the screw and bone. In this study, the influence of pedicle screw half-angle and bone quality on the displacement of fixation and stress transfer are analyzed using a 2D axisymmetric finite element model. The pedicle screw proximal half-angle is varied between 0° and 60° in steps of 10°, along with two different distal half-angles of 30° and 40°. Three bone models are considered for cancellous bone to simulate various degrees of bone quality, namely, poor, moderate and good. The mechanical properties of cortical bone are kept constant throughout the study. The material properties and boundary conditions are applied based on previous studies. Frictional contact is considered between the bone and screw. Results show that, the displacement of fixation is observed to be minimum at a proximal half angle of 0° and maximum at an angle of 60°, independent of bone quality. The highest implant displacement is observed in case of poor bone quality. All the bone model showed similar patterns of stress distribution, with high stress concentration around the first few threads. The highest peak von Mises stress is obtained at a proximal half-angle of 60°. Furthermore, the stress transfer increased with increase in proximal half-angle and bone quality, with maximum stress transfer at a proximal half-angle of 60°. It appears that, this study might aid to improve the design of pedicle screw for treatment of degenerative spinal diseases.Clinical Relevance- This study analyses the impact of bone quality on pedicle screw design.