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

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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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Acknowledgements

This work is supported by the Orthotek  Laboratory, Shanghai University.

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Authors and Affiliations

  1. Department of Orthopedic Surgery, The First Affiliated Hospital of Nanchang University, No. 17 Yongwaizheng Street, Donghu District, Nanchang, 330006, Jiangxi Province, People’s Republic of China

    Jia-Ming Liu, Yang Zhou, Xuan-Yin Chen, Shan-Hu Huang & Zhi-Li Liu

  2. Department of Orthopedic Surgery, Jiujiang No. 1 People’s Hospital, Jiujiang, 332000, People’s Republic of China

    Yu Zhang

  3. Orthotek Lab, School of Mechatronic Engineering and Automation, Shanghai University, No. 149 Yanchang Road,Jing’an District, Shanghai, 200072, People’s Republic of China

    Zi-Kai Hua

Authors
  1. Jia-Ming Liu
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  2. Yu Zhang
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  3. Yang Zhou
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  4. Xuan-Yin Chen
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  5. Shan-Hu Huang
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  7. Zhi-Li Liu
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Contributions

Jia-Ming Liu and Yu Zhang contributed equally to this study and share the first authorship.

Corresponding authors

Correspondence to Zi-Kai Hua or Zhi-Li Liu.

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Conflict of interest

The authors declare that they have no conflict of interest.

Funding

This work is supported by the fund of Department of Science and Technology Program of Jiangxi Province, China (No. 20162BCB22022, 20162BCB23057).

Ethical approval

This study was approved by the medical research ethics committee of our hospital.

Informed consent

Informed consent was obtained from all individual participants included in the study.

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Liu, JM., Zhang, Y., Zhou, Y. et al. The effect of screw tunnels on the biomechanical stability of vertebral body after pedicle screws removal: a finite element analysis. International Orthopaedics (SICOT) 41, 1183–1187 (2017). https://doi.org/10.1007/s00264-017-3453-y

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  • DOI: https://doi.org/10.1007/s00264-017-3453-y

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