Viscoelastic Properties in Fixational Stability of Pedicle Screws in Uhwmpe Block: a Finite Element Study

Pedicle screws are critical components in spinal fixation systems, and the stiffness of the screw-bone interaction plays a crucial role in implant success. There are various ways to investigate screw-bone bonding strength, one of which is vibration-based diagnosis of screw-bone structure. While finite element modeling can reduce the time and give the ability to model different geometries and other conditions, properly modeling the vibration behavior of the screw inside the bone comes with difficulties. This study investigates the influence of insertion-induced prestress and the viscoelastic properties of ultra-high molecular weight polyethylene (UHMWPE) boneanalog material on the natural frequency of a pedicle screwblock assembly using finite element analysis (FEA). Three models were developed: Model I (linear elastic without prestress), Model II (linear elastic with prestress), and Model III (viscoelastic without prestress). A 3D geometry of the screw and UHMWPE block was constructed, and frequency analysis was performed at three insertion depths (<tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$10 \text{mm}, 20 \text{mm}$</tex>, and 30 <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\text{mm)}$</tex>. Simulation results were compared with previously published experimental data. Model I underestimated the natural frequency at all depths around <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$14-30 \%$</tex>, while Model II, accounting for screw insertion-induced radial prestress, improved prediction accuracy, reducing errors down to under 18 %. Model III, which captured UHMWPE's viscoelastic behavior using a Prony series, showed the closest agreement with experimental data, with errors under 7 %. The results highlight the importance of modeling both viscoelasticity and insertion-related prestress to accurately predict dynamic behavior. These findings are useful for improving finite element modeling of modal analysis methods to investigate screw stability in spinal implants.