Date Published: February 4, 2019
Publisher: Public Library of Science
Author(s): Yueh Wu, Chia-Hsien Chen, Fon-Yih Tsuang, Yi-Cheng Lin, Chang-Jung Chiang, Yi-Jie Kuo, Costin Daniel Untaroiu.
The majority of compressive vertebral fractures in osteoporotic bone occur at the level of the thoracolumbar junction. Immediate decompression is often required in order to reduce the extent of neurological damage. This study evaluated four fixation methods for decompression in patients with thoracolumbar burst fractures, and presented the most suitable method for osteoporotic patients. A finite element model of a T7–L5 spinal segment was created and subjected to an L1 corpectomy to simulate a serious burst fracture. Five models were tested: a) intact spine; 2) two segment fixation (TSF), 3) up-three segment fixation (UTSF), below-three segment fixation (BTSF), and four segment fixation (FSF). The ROM, stiffness and compression ratio of the fractured vertebra were recorded under various loading conditions. The results of this study showed that the ROM of the FSF model was the lowest, and the ROMs of UTSF and BTSF models were similar but still greater than the TSF model. Decreasing the BMD to simulate osteoporotic bone resulted in a ROM for the four instrumented models that was higher than the normal bone model. Of all models, the FSF model had the highest stiffness at T12-L2 in extension and lateral bending. Similarly, the compression ratio of the FSF model at L1 was also higher than the other instrumented models. In conclusion, FSF fixation is suggested for patients with osteoporotic thoracolumbar burst fractures. For patients with normal bone quality, both UTSF and BTSF fixation provide an acceptable stiffness in extension and lateral bending, as well as a favorable compression ratio at L1.
Burst fractures of the thoracolumbar segment in the spine typically occur where the lowest thoracic vertebrae connects to the first lumbar vertebrae. A burst fracture occurs when an axial compressive force on the anterior and middle column collapses the bone and causes failure of the anterior and middle supporting columns . The thoracolumbar segment is the most common site for unstable burst fractures, representing approximately 15% of vertebral injuries .
The finite element software ANSYS 16.0 (ANSYS Inc., Canonsburg, PA, USA) was used to create an FE model of an 11-level thoracolumbar spine. As shown in Fig 1A, a T7–L5 spine segment was developed using geometry from a morphologically accurate spinal model that included vertebrae and intervertebral discs (Zygote Media Group, Inc.). The annulus material was based on an incompressible, hyperelastic, 2-parameter (C1, C2) Mooney-Rivlin formulation, and the nucleus pulposus was modeled as an incompressible fluid. The anterior longitudinal ligament, posterior longitudinal ligament, ligamentum flavum, interspinous ligament, supraspinous ligament, and capsular ligaments were assigned properties based on published experimental values and approximated as nonlinear, tension-only springs (ANSYS 16.0) with insertion points approximated to typical anatomy [18,19]. These ligaments were represented as 2-node tension-only link elements, as shown in Fig 1J. The material properties of the T7–L5 model were sourced from literature [18–22] and are shown in Table 1.
The purpose of pedicle screw stabilization is to maintain spinal stability to facilitate bone healing. But the high reported failure rates of instrumented segments, leading to traumatic instability, has led to an unacceptable incidence of anterior column defects . The thoracolumbar junction is a transition zone between the posterior thoracic curve and the anterior lumbar curve and experiences some of the highest stress levels in the spine. This has resulted in a high incidence of burst fractures in the region when compared to other areas of the thoracic or lumbar spine . With advances in implant materials and manufacturing technologies, the success rate of fixation screws is improving and surgeons are increasingly opting for short-segment fixation. Minimizing the number of vertebral segments required for fixation is also an important goal of internal fixation in order to maintain flexibility. However, failure rates of between 20% and 50% have been reported with the use of short-segment fixation for thoracolumbar burst fractures [32–34]. Hence, the objective of this finite element study was to investigate the importance of the number of fixed segments used for treating thoracolumbar burst fractures.
There is no single “gold standard” method for treating thoracolumbar burst fractures, as a number of aspects such as the bone quality and severity of the fracture should be considered before deciding on a treatment method. This study developed models to simulate severe thoracolumbar burst fractures in both normal bone and osteoporotic bone. The results indicated that FSF fixation was the better choice for osteoporotic bone, probably because it provides the greatest mechanical stiffness for initial fixation and can reduce the likelihood of segmental collapse. However, it may also lead to adjacent segment disease in the long term. Both UTSF and BTSF fixation were acceptable options for normal bone. Particularly in patients with normal bone quality that need a greater ROM, UTSF and BTSF fixation provide an acceptable stiffness in extension and lateral bending, as well as a favorable compression ratio at L1.