Hybrid composite pedicle screw - finite element modelling with parametric optimization

Abstract

Abstract As a result of population aging, the number of spine surgeries steadily increases. Metallic pedicle screw systems are commonly used to achieve spinal fusion, and to correct spine misalignments. However, certain disadvantages are linked to these metallic systems: stress shielding can lead to bone degeneration and adjacent disc disease, artefacts prevent precise diagnosis during CT, MRI, or X-ray medical imaging, radiotherapy is complicated due to backscattering, and patient comfort is reduced due to the implant weight. These disadvantages have led to the development of carbon fibre reinforced polyether ether ketone (CF-PEEK) pedicle screws. Carbon fibres improve the structural mechanical properties of the PEEK matrix so that the mechanical requirements of an implant are met. However, with existing CF-PEEK screws the relative motion between screw and bone is increased due to discontinuous fibre reinforcement. Discontinuous fibre reinforced screws complicate precise stiffness control. In this study, a novel screw design consisting of a reinforcing element in the screw centre, and discontinuous short fibre reinforced CF-PEEK in the screw thread and head is presented. By this concept, the stiffness of the pedicle screw can be controlled and tailored by adapting the reinforcing element. This first study concerning hybrid pedicle screws is based upon numerical investigations to optimize hybrid composite screw stability in human bone. Parametric finite element (FE) models of screw and bone were developed. The main purpose of the study was to obtain design recommendations from the optimization, which will be used for future manufacturing of hybrid composite screws. It is further shown that stress shielding could be significantly reduced with this hybrid pedicle screw design as compared to titanium screws, despite the reinforcing element. Additionally, it is confirmed that the worst-case scenario with the highest risk of composite pedicle screw pull-out is shortly after surgery, when osseointegration is still poor. In future studies, mechanical, functional, and visibility tests of the hybrid composite pedicle screw will be conducted.