Old vertebral compression fractures are often caused by compression fractures after trauma or in elderly patients who are not diagnosed and treated in a timely fashion and in whom these fractures are often accompanied by kyphotic deformity. The clinical symptoms are primarily persistent low back pain and neurological symptoms caused by spinal cord compression; these include lower limb numbness and decreased muscle strength. When conservative treatment is ineffective, spinal cord compression can be relieved by surgery to restore the sagittal balance of the spine; this can effectively improve clinical symptoms and correct the kyphotic deformity [9–12].
VCR is a traditional orthopedic surgery for spinal osteotomy. Through posterior resection of spinous process vertebral plate, pedicle screws are placed on the upper and lower two vertebrae; then, through resection of the entire vertebral body and upper and lower intervertebral discs, titanium mesh is placed and fixed, followed by screw-rod fixation. VCR surgery effectively corrects severe kyphotic deformity and achieves reconstruction of the vertebral body; however, due to the difficulty of surgery, there are often problems such as longer operation time, larger amounts of blood loss, and greater degree of trauma.
PUVCR, a modified modification of VCR, exposes the vertebral body through a unilateral approach, removes half of the vertebral body and the upper and lower intervertebral discs, shortens the operation time, and reduces the blood loss while restoring the sagittal sequence of the spine [13]. Some studies suggest that PUVCR can completely decompress and effectively restore deformity in the treatment of old thoracolumbar fractures [1, 13, 14]. Nevertheless, there is a lack of effective biomechanical verification as to whether only half of the vertebral body and the upper and lower intervertebral discs has any effect on spinal biomechanics; there also remains debate as to the appropriate length of fixation. For these reasons, in this study, we analyzed the effect of VCR and PUVCR on vertebral stability using finite element analysis.
Finite element analysis reconstructs the shape of the spine using computer simulation, and then directly interprets changes of spinal biomechanics caused by internal fixation by loading its material parameters and loading conditions, and then guiding the clinical surgical plan [15]. Finite element analysis can be repeatedly applied, saving medical resources, while safely and efficiently simulating some difficult surgical procedures. For these reasons, it has become a hot area of research.
Studies have analyzed lumbar ROM after Ponte osteotomy and showed that this technique leads to 20% instability, while total discectomy leads to further instability [16]. Due to decreased stability, additional fixation is required to enhance stability, and pedicle screw fixation is the most common fixation method in spinal fixation. Liu et al. analyzed the stress of screw rod and found that VMS of the pedicle screw was the smallest when the directional pedicle screw was applied (136.9 mPa) and the VMS of the pedicle screw was the largest with hybrid application (382.6 mPa) [17]. VMS of the rod was the largest when a polyaxial pedicle screw was applied (439.9 mPa) and the VMS of rod was the smallest when a directional pedicle screw was applied (341.7 mPa). Because most patients with old vertebral compression fractures and kyphotic deformities are elderly and have low relative bone mineral density, we applied directional pedicle screws for simulated fixation, and this can reduce the stress on the screw-rod and prevent its pullout. Natarajan et al. showed that the application of a 6-mm screw-rod system resulted in stronger fixation than a 5-mm screw-rod system, increasing the size of the system from 5 mm to 6 mm, increasing the flexion/extension moment by 8%, the torsion moment by 14%, and the lateral bending moment by 24% [18]. We applied a 6-mm screw-rod system to simulate two surgical modalities in the hope of increasing fixation strength.
Fusion fixation reduces thoracolumbar motion, as shown in previous studies [15, 19]. Lu and Lu found that the short-segment fusion fixed model decreased by 52–94% compared with the intact model [20].
There are few studies on ROM values after VCR surgery. In our test, the ROM of all models decreased relative to the intact model, with the LVCR model showing the largest decrease of 97.28% in extension and SVCR model showing the smallest decrease of 14.90% in lateral bending. The thoracolumbar ROMs of long-segment fixed models (LVCR, LPUVCR) were significantly smaller than those of short-segment fixed models (SVCR, SPUVCR), while the ROM of the VCR model was relatively smaller than that of the PUVCR model; however, the difference was not significant. This finding suggests that the ROM of thoracolumbar segment is greatly affected by fixation; however, it has little relative relationship with the amount of vertebral body and soft tissue removed.
We found that the site with the largest VMS in internal fixation was at the screw-rod connection site in all models, which explains why internal fixation is prone to failure at the screw-rod connection site. Similar findings were reported previously [21–23]. PUVCR showed slightly higher screw and rod stresses than VCR in all loading cases except in short segment lateral bending. In terms of titanium cages, although the maximum stress site varied in various models due to the length of fixation, the maximum value of VMS was at the contact surface site between the titanium cage and the vertebral body, suggesting that the risk of prosthesis displacement is the greatest at the contact surface between the titanium cage and the vertebral body. Compared with VCR, the stress of PUVCR titanium cage increased by 41.9%, 5.2%, 3.2%, and 2.2% in forward flexion, extension, lateral bending, and rotation, respectively, while in long-segment fixation, the stress of PUVCR titanium cage increased by 4.5%, 1.2%, and 25.2% in forward flexion, extension, and rotation, respectively, compared with VCR. In the position of the superior and inferior endplates, the stress in the short segment was significantly greater than the stress in the long segment, and this may explain why the short segment is more prone to subsidence of the anterior prosthesis. Nevertheless, the amount of vertebral body removal did not appear to affect the change of the area of stress concentration, whether it was the VCR model (LVCR, SVCR) or the PUVCR model (LPUVCR, SPUVCR). The site with higher stress was located in the contact position between the titanium cage and the endplate, which also suggested that settlement of the prosthesis could occur at the contact site.
This study has some limitations. First, we used CT scans of normal subjects to construct the old thoracolumbar fracture model, and this may have had an impact on the test. Second, we only used one person to establish the model. Due to individual differences, there may be structural variations in vertebral bodies and ligaments. In future studies, model reconstruction using several patients can be further performed, resulting in more objective results.