Elsevier

World Neurosurgery

Volume 112, April 2018, Pages e125-e133
World Neurosurgery

Original Article
Biomechanical Role of the Thoracolumbar Ligaments of the Posterior Ligamentous Complex: A Finite Element Study

https://doi.org/10.1016/j.wneu.2017.12.171Get rights and content

Highlights

  • In this study, we verify the significant role of the supraspinous ligament in maintaining the stability of the injured thoracolumbar spine by studying the range of motion and the instantaneous axes of rotation in finite element models.

  • The current study was based on the clinical sequential manner from the facet capsular ligament, part of interspinous ligament, supraspinous ligament, and entire interspinous ligament, to the ligamentum flavum.

Objectives

To investigate the effect of sequential ligament failure on the range of motion (ROM) and location of the instantaneous axes of rotation (IAR) of the thoracolumbar spine (T12–L1) finite element (FE) model, and to verify the role of the supraspinous ligament (SSL) in maintaining the stability of the injured thoracolumbar spine.

Methods

An FE model of the fractured thoracolumbar spine was developed and validated against published data. The posterior ligamentous complex (PLC) in the fractured T12–L1 segment was then reduced in a sequential manner from the facet capsular ligament (FCL), part of the interspinous ligament (ISL), SSL, and entire ISL, to the ligamentum flavum (LF). The ROM and IAR of the T12–L1 segment were measured at the fracture and at each reduced ligament step under 4 directions of flexion, extension, lateral bending, and rotation, and 4 bending motions of 1.5, 3.0, 4.5, 6.0 Nm.

Results

The FE model showed a consistent increase in the ROM and location of the IAR as the ligaments were removed sequentially. Furthermore, failure of the SSL had the most significant influence on the change in the ROM and IAR in flexion. In extension, removal of the FCL caused the largest shift.

Conclusions

The SSL is a significant ligament that allows the PLC to maintain the stability of the thoracolumbar spine during injury.

Introduction

The thoracolumbar junction, and the T12–L1 segment in particular, are susceptible to traumatic fracture.1 Studies have shown complications after spinal fractures.2, 3, 4 Traumatic bone fracture of T12–L1 leads to spinal instability, which in turn decreases the patient's quality of life. The motion of an intact spinal segment is well managed by the coordination of muscles, discs, facets, and ligaments. Any injury to these structures influences the function of the spine. A posterior element failure commonly occurs alongside an anterior structure injury; this is particularly the case with the posterior ligamentous complex (PLC).5, 6, 7

The PLC is composed of the interspinous ligament (ISL), supraspinous ligament (SSL), facet capsular ligament (FCL), and ligamentum flavum (LF). The Thoracolumbar Injury Classification and Severity (TLICS) scoring system suggested that PLC integrity was a key part of the stability assessment of the thoracolumbar spine.8, 9 However, PLC integrity was assessed only as a whole in TLICS, which affects its application in a clinical setting. Although different methods have been used to describe the problem of the effect of failure of a particular PLC ligament on spinal stability, no consensus has been reached.

In some previous spinal kinematic studies, researchers analyzed changes in the range of motion (ROM) and locations of the instantaneous axes of rotation (IAR) of the spinal segments to assess the role of each ligament by sequentially cutting the ligaments.10, 11, 12 However, the method of sequentially reducing the ligaments was not realistic in these experiments. Some recent studies showed that the realistic sequential injury order of the PLC in a clinical setting is the FCL, part of ISL, SSL, entire ISL, and LF and that the SSL is the pivotal ligament that allows the PLC to maintain the stability of the thoracolumbar spine.13, 14, 15 There has been no biomechanical finite element (FE) study of thoracolumbar spine fractures that follows the above realistic ligament failure order. The objective of the current study was to assess the ROM and IAR caused by various ligament failures and pure motion and to determine whether SSL rupture provides the key to PLC incompetence.

Section snippets

Materials and Methods

Computed tomography scan data from a healthy 30-year-old man using a 512 × 512 pixel matrix at 1-mm intervals were used to construct a 3-dimensional FE model of the human T12–L1 segment. Informed consent was obtained from the volunteer. These data were then imported into Mimics version 19.0 (Materialise, Leuven, Belgium) to obtain a simplified skeletal model. After that, a 3-dimensional solid model was constructed using Geomagic version 2012 (Geomagic, North Carolina, USA). The intervertebral

ROM Analysis for the Intact Model

The predicted ROMs were 3.56°, 4.45°, 4.64°, and 1.85° for flexion, extension, lateral bending, and rotation under 7.5 Nm of pure bending motion. Figure 3 illustrates the comparison of the predicted ROMs with published experimental results.17, 18 The current FE model successfully fell within the range of the aforementioned experimental data for the T12–L1 segment.

ROM Analysis for Models with Fracture

As shown in Table 2, the fracture of the L1 model caused a slight increase in the ROM under 6.0 Nm of pure bending motion, with 1.04°

Discussion

In this study, the 2 important physical parameters of the ROM and IAR were considered in an FE model of the thoracolumbar spine, which contained various conditions of ligament failure by means of a stepwise reduction of the ligaments. In TLICS, the integrity of the PLC was described as a key point for stratifying patients into surgical and nonsurgical treatment groups. Previous biomechanical studies also showed the significance of PLC integrity in spine stability and attempted to reveal the

Conclusion

The location of the ROM and IAR was changed in all planes of motion in intact, fracture, and ligament failure models. Particular ligament failure caused a much more significant shift of the ROM and IAR. Furthermore, after removal of the SSL, the ROM, and IAR of the T12–L1 segment increased sharply under flexion motion. In extension, excision of the FCL caused the most remarkable increase. The changes in the ROM and IAR reveal the mechanism of unstable motion of a fractured spinal segment under

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  • Cited by (0)

    Conflict of interest statement: This work was supported by the National Natural Science Foundation of China (81371988, 81501933), Major scientific and technological project of medical and health of Zhejiang Province (WKJ-ZJ-1527), Wenzhou Science and Technology Project (Y20170080), and Zhejiang Provincial Natural Science Foundation of China (LY17H060008).

    Cong-Cong Wu and Hai-Ming Jin contributed equally to this work and are co–first authors.

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