Original ArticleBiomechanical Role of the Thoracolumbar Ligaments of the Posterior Ligamentous Complex: A Finite Element Study
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
References (30)
- et al.
In-hospital neurologic deterioration following fractures of the ankylosed spine: a single-institution experience
World Neurosurg
(2015) - et al.
Experimental evaluation of the developmental mechanism underlying fractures at the adjacent segment
World Neurosurg
(2016) - et al.
Investigation of thoracolumbar T12–L1 burst fracture mechanism using finite element method
Med Eng Phys
(2006) - et al.
Stepwise reduction of functional spinal structures increase range of motion and change lordosis angle
J Biomech
(2007) - et al.
Biomechanical effects of vertebroplasty on thoracolumbar burst fracture with transpedicular fixation: a finite element model analysis
Orthop Traumatol Surg Res
(2014) - et al.
Biomechanical effect of the extent of vertebral body fracture on the thoracolumbar spine with pedicle screw fixation: an in vitro study
J Clin Neurosci
(2008) - et al.
Superiority of incremental trauma approach in experimental burst fracture studies
Clin Biomech
(2000) - et al.
Comparison of the biomechanical effect of pedicle-based dynamic stabilization: a study using finite element analysis
Spine J
(2013) - et al.
A comprehensive classification of thoracic and lumbar injuries
Eur Spine J
(1994) - et al.
Complete thoracolumbar fracture-dislocation with intact neurologic function: explanation of a novel cord saving mechanism
J Spinal Cord Med
(2017)
The radiologic assessment of posterior ligamentous complex injury in patients with thoracolumbar fracture
Eur Spine J
Correlation of posterior ligamentous complex injury and neurological injury to loss of vertebral body height, kyphosis, and canal compromise
Spine
A new classification of thoracolumbar injuries: the importance of injury morphology, the integrity of the posterior ligamentous complex, and neurologic status
Yearb Orthop
Another diagnostic tool in thoracolumbar posterior ligament complex injury: interspinous distance ratio
Eur Spine J
Instantaneous center of rotation behavior of the lumbar spine with ligament failure: laboratory investigation
J Neurosurg Spine
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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.