Biomechanical analysis of the tandem spinal external fixation in a multiple-level noncontiguous lumbar fractures model: a finite element analysis

Abstract

Objective: This study aimed to investigate the biomechanical characteristics of the tandem spinal external fixation (TSEF) for treating multilevel noncontiguous spinal fracture (MNSF) using finite element analysis and provide a theoretical basis for clinical application.

Methods: We constructed two models of L2 and L4 vertebral fractures that were fixed with the TSEF and the long-segment spinal inner fixation (LSIF). The range of motion (ROM), maximum stresses at L2 and L4 vertebrae, the screws and rods, and the intervertebral discs of the two models were recorded under load control. Subsequently, the required torque, the maximum stress at L2 and L4 vertebrae, the screws and rods, and the intervertebral discs were analyzed under displacement control.

Results: Under load control, the TSEF model reserved more ROM than the LSIF model. The maximum stresses of screws in the TSEF model were increased, while the maximum stresses of rods were reduced compared to the LSIF model. Moreover, the maximum stresses of L2 and L4 vertebrae and discs in the TSEF model were increased compared to the LSIF model. Under displacement control, the TSEF model required fewer moments (N·mm) than the LSIF model. Compared to the LSIF model, the maximum stresses of screws and rods in the TSEF model have decreased; the maximum stresses at L2 and L4 in the TSEF model were increased. In the flexion condition, the maximum stresses of discs in the TSEF model were less than the LSIF model, while the maximum stresses of discs in the TSEF model were higher in the extension condition.

Conclusion: Compared to LSIF, the TSEF has a better stress distribution with higher overall mobility. Theoretically, it reduces the stress concentration of the connecting rods and the stress shielding of the fractured vertebral bodies.

Keywords: biomechanics; finite element analysis; multilevel noncontiguous spinal fracture; tandem spinal external fixation; thoracolumbar fracture.

FIGURE 1

Clinical application of TSEF (A) Components of TSEF. (B) The overall diagram of TSEF. (C,D) X-Ray images of clinical application of TSEF. The bone substitute showed in this figure was Bicera™ (Wiltrom Ltd, Taiwan.).

FIGURE 2

Models establishment and validation. (A) Normal lumbar model. (B–D) Validation of normal lumbar validation. (E–G) Model of MNSF fixed by LISF. (H–J) Model of MNSF fixed by TSEF.

FIGURE 3

Restriction of two fixations on lumbar mobility under the six conditions. (A) ROM of the normal, TSEF and LSIF models under flexion and extension conditions. (B) ROM of the normal, TSEF and LSIF models under left and right bending conditions. (C) ROM of the normal, TSEF and LSIF models under left and right rotation conditions.

FIGURE 4

Maximum von mises stress analysis of LSIF and TSEF under the six conditions. (A) Heatmap of the Maximum von mises stress on LSIF and TSEF under the six conditions. (B) Cloud plots of the Maximum von mises stress on LSIF and TSEF under the six conditions.

FIGURE 5

Maximum von mises stress on L2 and L4 in LSIF and TSEF under the six conditions.

FIGURE 6

Maximum von mises stress analysis of intervertebral discs under the six conditions. (A) Heatmap of the Maximum von mises stress on intervertebral discs under the six conditions. (B) Cloud plots of the Maximum von mises stress on intervertebral discs under the six conditions.

FIGURE 7

Comparison of LSIF and TSEF under displacement control. (A) Required torque for the two fixations to reach the ROM of flexion 7.02° and extension 5.29°. (B) Maximum von mises stress on screws and rods of the LSIF and TSEF models under the displacement control. (C) Maximum von mises stress on fractured vertebra bodies (L2 and L4) in LSIF and TSEF under the displacement control. (D) Maximum von mises stress on discs in LSIF and TSEF under the displacement control. σ²: variance.