Numerical Investigation of Frictional Locking Mechanisms and Load Sharing in Pedicle Screw-Rod Assemblies Under Flexion: A Mechanically Consistent Finite Element Analysis

This study investigates the biomechanical stability and load-sharing mechanics of a pedicle screw-rod assembly under flexion using non-linear finite element analysis. A two-step analysis methodology was employed. First, a preload state equivalent to a 10 Nm tightening torque was approximated via an interference fit method using a non-linear Ti6Al4V model, and interface behavior was assessed through power screw theory and literature. Second, the developed model, which demonstrated mechanical consistency with experimental literature, was subjected to flexion loads utilizing non-linear material properties and contacts. Model 2.2 predicted a maximum moment capacity of 22.08 Nm, showing strong agreement with literature. Exceeding the maximum flexion force of 869.5 N triggered a significant internal load shift, where axial force on the screw head decreased while the reaction force on the tulip saddles nearly doubled, initiating macro-slip. Maximum von-Mises stresses exceeding the material's yield strength indicated localized yielding, facilitating a secure mechanical interlock. The model successfully captures the transition from static frictional locking to kinematic instability. By confirming the model's reliability through a comprehensive sensitivity analysis of solver parameters, this study demonstrates that the intrinsic frictional locking capacity, rather than solely geometry, is the primary determinant of system stability. Consequently, this research provides a robust and numerically consistent framework for evaluating the biomechanical performance of spinal implant designs.