Limiting interpedicular screw displacement increases shear forces in screws: A finite element study

Abstract

Background context: Screw loosening has been reported for non-fusion devices. Forces on pedicle screwscould be related to kinematic parameters as the interpedicular displacement (ID), which consists of thedisplacement between superior and inferior screw heads from full extension to full flexion.Purpose: To investigate the relationship between ID and screw loosening for different designs of posteriorimplants using a finite element model.Methods: An L3-sacrum previously validated spine FE model was used. Three-rod designs were consid-ered in L4-L5 segment: a rigid screw-rod implant, a flexible one and a specific design with a sliding rodproviding limited restrain in ID. In order to simulate intermediate configurations, the friction coefficientbetween the sliding rods and connectors were varied. The sacrum was rigidly fixed. Rotations (flexion-extension, lateral bending and axial rotation) were applied to L3, for each modeled configuration: intact,injured, injured with different implants. Model consistency was checked with existing experimentalin vitro data on intact and instrumented segments. Screw loads were computed as well as ID.Results: In flexion-extension, the ID was less than 2 mm for rigid (R) and flexible (F) constructs and 5.5 mmfor intact spine and the sliding implant (S3). Screw’s shear forces were 272 N, 153 N, 43 N respectivelyfor R, F and S3 constructs.Conclusions: Implants that allow ID presented lower screws loads. A compromise between the abilityof the implant to withstand compressive forces, which requires longitudinal stiffness, and its ability toallow ID could be important for future implant designs in order to prevent screw loosening.