Mechanical stress on the rod in posterior spinal fusion with and without rod fracture - Finite element analysis

Sep 29, 2026

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Rod fracture after posterior spinal fusion is a frequent complication linked to high reoperation rates. This study quantifies mechanical stress on spinal rods and intervertebral discs across multiple loading directions using 3D CT-based finite element analysis in a simplified L3–S1 short-segment model. Comparisons were made between a no-rod-fracture condition and a unilateral rod fracture condition, aiming to identify motion directions most associated with elevated rod loading.

LanguageEnglish

CategoryHealthcare

FormatPDF

Pages7

Reading Time18 min

AccessFree to download

Abstract

Introduction

Methods

Study patients

Finite element models and loading

Outcome measures

Results

Conclusions

Inoue et al. BMC Musculoskeletal Disorders (2026) 27:2 [https://doi.org/10.1186/s12891-025-09377-2](https://doi.org/10.1186/s12891-025-09377-2) BMC Musculoskeletal Disorders RESEARCH Open Access Mechanical stress on the rod in posterior spinal fusion with and without rod fracture: a finite element analysis Tomohisa Inoue 1, Yoh Kumano2, Jun Sugaya 1, Ken Okazaki 1 and Toru Doi 1* Abstract Background Rod fracture (RF) is a common complication of posterior spinal fusion surgery, with a high reoperation rate. This study evaluated the mechanical stress on the rods and intervertebral discs under various loading conditions in a simplified short-segment lumbar fusion model (L3-S1) with and without a unilateral rod fracture (URF) via 3D-computed tomography finite element analysis (CT/FEA), to clarify the spinal motion directions most associated with the mechanical stress on the rods. Methods Two 3D nonlinear finite element models of L3–S1 posterior spinal fusion were created using the preoperative CT images of 10 patients: (1) a no rod fracture (NRF) model and (2) a URF model with a simulated left RFat L4/5 . A 10-Nm load was applied in six directions (flexion, extension, right and left lateral bending, and right and left axial rotation), and the maximum von Mises stress on the right rod at L4/5 and the minimum principal stress on the L2/3 adjacent and L4/5 intervertebral discs were measured and compared between both models. Furthermore, the maximum von Mises stress on the right rod was also compared among six loading directions. Results Mechanical stress on the L4/5 intervertebral disc was significantly higher in the URF model under all loading directions, but there was no significant difference in the mechanical stress on the adjacent L2/3 intervertebral disc between both models. In the NRF model, mechanical stress on the right rod was significantly higher under left and right axial rotational loadings versus the other loading directions. In the URF model, mechanical stress on the right rod was significantly higher under left lateral bending and axial rotational loading versus flexion loading. Conclusions After posterior spinal fusion surgery, restricting spinal axial rotation and lateral bending toward the fractured side may help reduce mechanical stress on the rods. Keywords Rod fracture, Posterior spinal fusion, Finite element analysis, Mechanical stress *Correspondence: Toru Doi [tooru.doi@gmail.com](tooru.doi@gmail.com) 1Department of Orthopaedic Surgery, Tokyo Women’s Medical University, 8-1 Kawada-cho, Shinjuku-ku, Tokyo, Japan 2Department of Spine Surgery, Tokyo Yamate Medical Center, Japan Community Healthcare Organization, 3-22-1 Hyakunin-cho, Shinjuku-ku, Tokyo, Japan © The Author(s) 2025. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit [http://creativecommons.org/l](http://creativecommons.org/l)icenses/by/4.0/. Inoue et al. BMC Musculoskeletal Disorders (2026) 27:2 Introduction Adult spinal deformity is a common spinal disorder characterized by spinal deformity and malalignment, resulting in chronic low back pain, impaired mobility, and reduced quality of life [1]. Its reported incidence ranges from 1.4% to 32%[2–4], and spinal instrumentatio

FAQs

What was the main goal of the study?

To evaluate how mechanical stress on spinal rods and intervertebral discs changes under different loading directions when comparing models with and without rod fracture.

How were the finite element models created and tested?

Two 3D nonlinear finite element models of L3–S1 posterior spinal fusion were built from preoperative CT images of 10 patients, using a no-rod-fracture model and a unilateral rod fracture model. A 10-Nm load was applied in six directions, and stress metrics were compared.

Which motion restrictions were suggested by the findings?

The study suggests restricting spinal axial rotation and lateral bending toward the fractured side after posterior spinal fusion to help reduce mechanical stress on the rods.

Sep 29, 2026

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