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International Orthopaedics
Published in: International Orthopaedics 6/2017

01-06-2017 | Original Paper

The effect of screw tunnels on the biomechanical stability of vertebral body after pedicle screws removal: a finite element analysis

Authors: Jia-Ming Liu, Yu Zhang, Yang Zhou, Xuan-Yin Chen, Shan-Hu Huang, Zi-Kai Hua, Zhi-Li Liu

Published in: International Orthopaedics | Issue 6/2017

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Abstract

Purpose

Posterior reduction and pedicle screw fixation is a widely used procedure for thoracic and lumbar vertebrae fractures. Usually, the pedicle screws would be removed after the fracture healing and screw tunnels would be left. The aim of this study is to evaluate the effect of screw tunnels on the biomechanical stability of the lumbar vertebral body after pedicle screws removal by finite element analysis (FEA).

Methods

First, the CT values of the screw tunnels wall in the fractured vertebral bodies were measured in patients whose pedicle screws were removed, and they were then compared with the values of vertebral cortical bone. Second, an adult patient was included and the CT images of the lumbar spine were harvested. Three dimensional finite element models of the L1 vertebra with unilateral or bilateral screw tunnels were created based on the CT images. Different compressive loads were vertically acted on the models. The maximum loads which the models sustained and the distribution of the force in the different parts of the models were recorded and compared with each other.

Results

The CT values of the tunnels wall and vertebral cortical bone were 387.126±62.342 and 399.204±53.612, which were not statistically different (P=0.149). The models of three dimensional tetrahedral mesh finite element of normal lumbar 1 vertebra were established with good geometric similarity and realistic appearance. After given the compressive loads, the cortical bone was the first one to reach its ultimate stress. The maximum loads which the bilateral screw tunnels model, unilateral screw tunnel model, and normal vertebral model can sustain were 3.97 Mpa, 3.83 Mpa, and 3.78 Mpa, respectively. For the diameter of the screw tunnels, the model with a diameter of 6.5 mm could sustain the largest load. In addition, the stress distributing on the outside of the cortical bone gradually decreased as the thickness of the tunnel wall increased.

Conclusions

Based on the FEA, pedicle screw tunnels would not decrease the biomechanical stability and strength of the vertebral body. A large diameter of screw tunnel and thick tunnel wall were helpful for the biomechanical stability of the vertebral body.
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Metadata
Title
The effect of screw tunnels on the biomechanical stability of vertebral body after pedicle screws removal: a finite element analysis
Authors
Jia-Ming Liu
Yu Zhang
Yang Zhou
Xuan-Yin Chen
Shan-Hu Huang
Zi-Kai Hua
Zhi-Li Liu
Publication date
01-06-2017
Publisher
Springer Berlin Heidelberg
Published in
International Orthopaedics / Issue 6/2017
Print ISSN: 0341-2695
Electronic ISSN: 1432-5195
DOI
https://doi.org/10.1007/s00264-017-3453-y

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