Biomechanical comparison of posterior intermediate screw fixation techniques with hybrid monoaxial and polyaxial pedicle screws in the treatment of thoracolumbar burst fracture: a finite element study

Huan Liu,Hongwei Wang,Jun Liu,Changqing Li,Yue Zhou,Liangbi Xiang

doi:10.1186/s13018-019-1149-2

Abstract

BackgroundTo compare the biomechanical characteristics of different posterior intermediate screw fixation techniques (ISFTs) with hybrid monoaxial pedicle screws (Mps) and polyaxial pedicle screws (Pps) used in thoracolumbar burst fractures.MethodsFixation techniques are compared with regard to the von Mises stress (VMS) of the instrumentations and intradiscal pressures (IDPs) of the adjacent segments by finite element method (FEM).ResultsThe redistributed ROM of the fixation models with Pps fixed at the lowest segment was twice of the other fixation models in flexion and extension. The largest value of maximal VMS of a pedicle screw was located at the lowest pedicle screws when Mps are fixed at the lowest segment. The largest value of maximal VMS of the rods was decreased when more Pps are fixed at the models. Maximal IDPs of the upper adjacent segments were all larger than those of the lower adjacent segments. The maximal IDPs of the fixation model with MPs fixed at the lowest segment were larger than the other fixation models in flexion and extension.ConclusionsPolyaxial pedicle screws could be placed at the upper or the median segment for the facilitated efficient application of the connecting rod. We should focus on the adjacent segmental degeneration especially the upper adjacent segment in the fixation model with Mps fixed at the lowest segment.

Highlights

The focus of our research is to find how to provide sufficient biomechanical stability with hybrid monoaxial pedicle screws (Mps) and polyaxial pedicle screws (Pps) on the premise of ensuring convenient placement of rods
MMM 6 Mps fixated at three levels; PPP 6 Pps fixated at three levels; PMM 2 Pps fixated at upper level, 4 Mps fixated at lower two levels; MPM 4 Mps fixated at upper and lower two levels, 2 Pps fixated at median level; MMP 4 Mps fixated at upper two levels, 2 Pps fixated at lower level; MPP 2 Mps fixated at upper levels, 4 Pps fixated at lower two levels; PMP 2 Mps fixated at median level, 4 Pps fixated at upper and lower levels; PPM 4 Pps fixated at upper two levels, 2 Mps fixated at lower level
Our previous study suggested that the intermediate screw fixation technique can significantly increase the stability of the spine in both the Mps fixation group and Pps fixation group

Summary

Introduction

Posterior short-segment pedicle screw fixation is widely used for the management of traumatic thoracolumbar burst fractures [1,2,3], posterior intermediate screw fixation technique (ISFT) at the fracture level can help improve and maintain the kyphosis correction, and the biomechanical stability can be increased [4,5,6,7,8,9,10,11,12,13,14,15,16,17,18]. Biomechanical characteristics of fixation techniques including MMM (6 Mps fixated at three levels), PPP (6 Pps fixated at three levels), PMM (2 Pps fixated at upper level, 4 Mps fixated at lower two levels), MPM (4 Mps fixated at upper and lower two levels, 2 Pps fixated at median level), MMP(4 Mps fixated at upper two levels, 2 Pps fixated at lower level), MPP(2 Mps fixated at upper levels, 4 Pps fixated at lower two levels), PMP (2 Mps fixated at median level, 4 Pps fixated at upper and lower levels), and PPM (4 Pps fixated at upper two levels, 2 Mps fixated at lower level) were compared using finite element methods, redistributed ROM, VMS of instrumentations, and IDPs of the adjacent segment under displacement loading which was evaluated. To compare the biomechanical characteristics of different posterior intermediate screw fixation techniques (ISFTs) with hybrid monoaxial pedicle screws (Mps) and polyaxial pedicle screws (Pps) used in thoracolumbar burst fractures

Methods

Results

Conclusion