Patient-Specific Finite Element Models of Posterior Pedicle Screw Fixation: Effect of Screw's Size and Geometry.

Marco Sensale,Tanguy Vendeuvre,Michel Rochette,Christoph Schilling,Thomas Grupp,Enrico Dall’Ara

doi:10.3389/fbioe.2021.643154

Abstract

Pedicle screw fixation is extensively performed to treat spine injuries or diseases and it is common for thoracolumbar fractures. Post-operative complications may arise from this surgery leading to back pain or revisions. Finite element (FE) models could be used to predict the outcomes of surgeries but should be verified when both simplified and realistic designs of screws are used. The aim of this study was to generate patient-specific Computed Tomography (CT)-based FE models of human vertebrae with two pedicle screws, verify the models, and use them to evaluate the effect of the screws’ size and geometry on the mechanical properties of the screws-vertebra structure. FE models of the lumbar vertebra implanted with two pedicle screws were created from anonymized CT-scans of three patients. Compressive loads were applied to the head of the screws. The mesh size was optimized for realistic and simplified geometry of the screws with a mesh refinement study. Finally, the optimal mesh size was used to evaluate the sensitivity of the model to changes in screw’s size (diameter and length) and geometry (realistic or simplified). For both simplified and realistic models, element sizes of 0.6 mm in the screw and 1.0 mm in the bone allowed to obtain relative differences of approximately 5% or lower. Changes in screw’s length resulted in 4–10% differences in maximum deflection, 1–6% differences in peak stress in the screws, 10–22% differences in mean strain in the bone around the screw; changes in screw’s diameter resulted in 28–36% differences in maximum deflection, 6–27% differences in peak stress in the screws, and 30–47% differences in mean strain in the bone around the screw. The maximum deflection predicted with realistic or simplified screws correlated very well (R2 = 0.99). The peak stress in screws with realistic or simplified design correlated well (R2 = 0.82) but simplified models underestimated the peak stress. In conclusion, the results showed that the diameter of the screw has a major role on the mechanics of the screw-vertebral structure for each patient. Simplified screws can be used to estimate the mechanical properties of the implanted vertebrae, but the systematic underestimation of the peak stress should be considered when interpreting the results from the FE analyses.

Highlights

In the lumbar spine, pedicle screw fixation is the most widespread technique to achieve spinal fusion and stabilization (Verma et al, 2016)
Other studies focused on the vertebra-screws interactions and proposed finite element (FE) models validated with experimental measures: FE models were found to be good predictors of pull-out strength and stiffness obtained by experimental tests better than apparent density estimated from Computed Tomography (CT) images (Abbeele et al, 2018; Chevalier et al, 2018; Widmer et al, 2020)
The screw size and other insertion-related parameters have been tested with linear FE models (Qi et al, 2011; Newcomb et al, 2017), with non-linear FE models (Chen et al, 2003; Bianco et al, 2017, 2019; Molinari et al, 2021), or assuming the bone as heterogeneous material with elastic properties driven by the local bone mineral density (BMD) (Matsukawa et al, 2016, 2020; Biswas et al, 2019; Molinari et al, 2021)

Summary

Introduction

Pedicle screw fixation is the most widespread technique to achieve spinal fusion and stabilization (Verma et al, 2016). Despite the extensive use of pedicle screws in the current clinical practice, screw loosening and screw breakage are recurring mechanical complications of spinal fixation that can bring to a revision surgery in about 6% of cases (Prud’homme et al, 2015; Bredow et al, 2016) For this reason, surgeryrelated parameters should be optimized to improve the outcomes of this surgery. To the authors’ knowledge, a comprehensive assessment of the effect of the mesh size and the sensitivity of the models to the screw size and geometry, in terms of stress in the screw, strain in the heterogeneous bone, and deflection of the screw within the bone, has not been reported in the literature yet This gap in the literature makes it difficult to compare the outcomes from different studies and understanding the potential of the FE models in evaluating the biomechanics of the implanted vertebrae

Objectives

Methods

Results