Biomechanical Analysis of Adjacent Segments after Spinal Fusion Surgery Using a Geometrically Parametric Patient-Specific Finite Element Model

Summary

Here, we used a patient-specific finite element model to analyze the mechanical changes in adjacent segments after spinal fusion surgery. The results showed that fusion surgery reduced the overall motion of the lumbar spine but increased the load on and stress in adjacent segments, especially the proximal segment.

Abstract

This study aimed to perform a mechanical analysis of adjacent segments after spinal fusion surgery using a geometrically parametric patient-specific finite element model to elucidate the mechanism of adjacent segment degeneration (ASD), thereby providing theoretical evidence for early disease prevention. Fourteen parameters based on patient-specific spinal geometry were extracted from a patient's preoperative computed tomography (CT) scan, and the relative positions of each spinal segment were determined using the image match method. A preoperative patient-specific model of the spine was established through the above method. The postoperative model after L4-L5 posterior lumbar interbody fusion (PLIF) surgery was constructed using the same method except that the lamina and intervertebral disc were removed, and a cage, 4 pedicle screws, and 2 connecting rods were inserted. Range of motion (ROM) and stress changes were determined by comparing the values of each anatomical structure between the preoperative and postoperative models. The overall ROM of the lumbar spine decreased after fusion, while the ROM, stress in the facet joints, and stress in the intervertebral disc of adjacent segments all increased. An analysis of the stress distribution in the annulus fibrosus, nucleus pulposus, and facet joints also showed that not only was the maximum stress in these tissues elevated, but the areas of moderate-to-high stress were also expanded. During torsion, the stress in the facet joints and annulus fibrosus of the proximal adjacent segment (L3-L4) increased to a larger extent than that in the distal adjacent segment (L5-S1). While fusion surgery causes an overall restriction of motion in the lumbar spine, it also causes more load sharing by the adjacent segments to compensate for the fused segment, thus increasing the risk of ASD. The proximal adjacent segment is more prone to degeneration than the distal adjacent segment after spinal fusion due to the significant increase in stress.

Introduction

Intervertebral spinal fusion surgery is the most commonly used surgical procedure for the treatment of degenerative diseases of the lumbar spine¹. An excellent outcome in the short-term period after surgery can be achieved for more than 90% of patients². However, the results of a long-term follow-up study revealed that some patients developed degeneration of segments adjacent to the fused segment³. Lumbar interbody fusion accelerates degenerative changes in adjacent segments, which is known as adjacent segment degeneration (ASD). According to the literature, the incidence of ASD diagnosed based on....

Protocol

The protocol was conducted in accordance with the Declaration of Helsinki, and the protocol was approved by the Institutional Review Board of the China-Japan Friendship Hospital.

1. Parametric modeling of lumbar spine geometry

1. Extract the initial data (DICOM 3.0 format with a pixel size of 0.33 mm and a layer spacing of 1 mm) for modeling from a CT scan data set of an adult healthy male with no history of trauma, deformity or tumor of the spine (height 180 cm, weig.......

Discussion

In this study, a geometrically parametric patient-specific FE model was established to analyze the biomechanical characteristics of the lumbar spine after PLIF surgery. The results showed that the stress in the facet joints and disc of the fused segment decreased significantly after PLIF surgery, indicating that PLIF could effectively strengthen the stability of the decompressed segment and delay further aggravation of the lesion. The overall mobility of the lumbar spine decreased after PLIF surgery, while the ROM, facet.......

Materials

Name	Company	Catalog Number	Comments
Abaqus	Dassault	https://www.3ds.com/products/simulia/abaqus	Finite element analysis
AutoCAD	Autodesk	https://www.autodesk.com/products/autocad/	An Engineering Computer Aided Design software used to measure the ROM of different vertebral segment
CT scan dataset	China Japan Friendship Hospital		Dataset of an adult healthy male with no history of trauma, deformity or tumor of the spine (height 180 cm, weight 68 kg).The raw data were stored in Dicom 3.0 format with a pixel size of 0.33 mm and a layer spacing of 1 mm.
Hypermesh 2019	Altair	https://altair.com/hypermesh/	Mesh generation
Mimics Research 21.0	Materialise	https://www.materialise.com/en/healthcare/mimics-innovation-suite/mimics	Model construction