The spinal shortening technique is often used in congenital spinal deformities, benign and malignant invasive tumours, spinal fracture and dislocation, tethered cord syndrome and other diseases [13, 31,32,33]. Among them, pedicle subtraction osteotomy or Smith-Peterson osteotomy was considered safe because of the smaller osteotomy range, smaller spinal shortening, and bone-bone interface contact to improve the fusion rate and stability . However, for patients with tumour invasion, fracture and dislocation and rigid spinal deformity, TES often needs to be cured [7, 35]. Due to the possibility of cage fusion rate, fusion segmental stability, screw-rod fracture, and adjacent segmental disease after spinal surgery, a superior fixation method can bring long-term benefits to patients on the premise of fewer postoperative neurological complications. Although there are many reports about instrument failure after TES, there are few biomechanical studies on spinal reconstruction. For example, Yoshioka et al.  reported that 11 of 22 surgical patients had cage subsidence and 1 had rod breakage. Shinmura et al.  reported that 26 of the 61 patients who underwent TES surgery from 2010 to 2015 had instrument failure. Bone non-union and instrument failure would lead to neurological dysfunction, decline of quality of life and increase the burden of patients. Therefore, it is particularly important to study the biomechanical characteristics of each tissue structure after spinal shortening to provide a better theoretical basis for surgeons and to reduce the rate of postoperative revision. Recent experimental studies suggested that spinal shortening over 1/2 of the vertebral body can lead to serious postoperative complications . Therefore, this study designed five models within the safe distance of spinal shortening, with models A-E shortening the vertebral body by 0%, 10%, 20%, 30% and 50% height, respectively, and analysed the biomechanical characteristics under different spinal shortening lengths.
Then, we evaluated the ability of the model to resist the motion of the operated segments by using the structural stiffness of the model. In this study, the structural stiffness of models A-E increased significantly and had a similar trend. Because the biomechanical assessment of intervertebral fusion was not possible, successful intervertebral fusion was considered according to the FDA definition of bridging trabecular bone between fusion segments, a translational activity less than 3 mm, and an ROM less than 5 degrees . Liang et al. thought that the difference was considered significant when the results differed by more than 20% [19, 38]. In this study, the ROM of the fusion segments of models A-E were less than 5° in all directions, indicating that intervertebral fusion was successful. The results of our study showed that, regardless of the fixation method, the restriction of internal fixation devices on the model was most significant in extension, followed by lateral bending, and minimal in rotation, which was similar to the results of previous biomechanical studies [39, 40]. This may be because the posterior structures of the adjacent vertebrae, including the lamina and facet joints and ligaments, were not damaged during the operation, so they can still play a role in maintaining the stability of the vertebrae in future activities. In this study, although the ROM of models A-E tended to increase slightly in flexion, the maximum difference in ROM was not more than 0.02°, while in other directions, the ROM tended to decrease with increasing shortening distance, which was more obvious in extension and rotation. Although it is a kind of static analysis result, it still showed an overall trend. Among them, the model with a 50% reduction in vertebral height provided only 6% more stability in rotation than before spinal shortening but 59% more stability in extension.
Spinal fusion surgery accelerates the onset of ASD by increasing intervertebral mobility and mechanical stress, resulting in severe back pain, radicular symptoms, or neurogenic intermittent claudication, which can seriously affect people’s daily lives [41,42,43]. It has been reported that the incidence of a second operation for ASD was 4% per year, 16.5% at 5 years, but up to 36.1% at 10 years . In this study, the ROM and IDP of the adjacent segment (L5/S) showed good consistency among each model, and they reflected the motion state of the adjacent segment after the operation. In extension, the ROM and IDP of L5/S decreased slightly with increasing shortening distance, while the situation was opposite under other loading conditions. Although the ROM of each model was consistent under lateral bending, the change in IDP was more obvious than that in ROM. The IDP of model E differed from that of the intact model and model A by 6.7% and 7.4% in lateral bending, respectively. Although generally speaking, with an increase in the shortening distance, the ROM and IDP of L5/S of model A-E showed an increasing trend; they were not all larger than those of the intact model. Cho et al. reported that in their study, the risk of ASD in the proximal adjacent segment was the highest, which was higher than that of the distal adjacent segments , so there may be some possibility that the ROM of distal adjacent segments was smaller than the intact model in some loading conditions. However, the trend of ROM and IDP between models in this study can also give us an important hint.
In vertebral fusion, we should not only consider the possibility of bone graft nonfusion and adjacent segment disease but also consider the risk of vertebral bone destruction and cage subsidence at the interface between the cage and vertebral body. In this study, except for the extensional loading condition, the endplate stress of all surgical models at the interface between the cage and L4 upper endplate was greater than that of the intact model, and the stress showed a downwards trend in models A-E. The reason may be that the internal fixation and posterior structure of the vertebral body limit the extension movement of the spine, thus affecting the VMS of the endplate. It was previously reported that the failure strength of cortical bone was between 90 and 200 MPa . Our study found that although the stress of the endplate increased in all surgical models as a whole, the result was much smaller than that of 90 MPa, within the range of failure strength of vertebral cortical bone. Because of the wide range of total vertebrae resection, it greatly affected the stability of the spine, which mainly depended on the internal fixation to maintain the stability of the spine. Sciubba et al. reported that in their study, the rod breaking rate of 23 patients who underwent TES was as high as 39.1% . Our study found that although the stress trends in the extension of rods of models A-E were different from that of the other directions of activity, generally speaking, the stress of rods increased with an increasing shortening distance. However, in this study, the maximum stress (172.5 MPa) of rods under flexion was much lower than the yield strength of titanium alloy (825–895 MPa). In rotation, the stress of rods of model D was greater than that of model E. The possible reason was that with the increase in the shortening distance, the upper and lower articular processes of the adjacent vertebrae contact each other, thus limiting the movement between the vertebral body. As a result, the stress of the fixed rods was reduced.
Although the results of this study show that spinal shortening technique has certain advantages in the stability of fusion segments and the reduction of endplate stress, the spinal shortening technique still has certain limitations in clinical application. In children, the growth of non-fusion segments during the growth of the spine may reestablish the tension of the spinal cord, it may affect the annual 1 mm growth retardation of the spinal fusion segments, and even lead to crankshaft deformities . Moreover, the operation of spinal shortening is relatively complex, the operation time is long, and the blood loss is large, and it is possible to undergo a second operation because of instrument failure reported by Aldave et al. [48, 49]. And there may be spinal cord injury caused by too long intraoperative shortening distance or too short shortening distance leading to treatment failure. We must consider the adverse consequences of these spine-shortening techniques when making surgical decisions for patients, and personalize the plan to maximize patient benefit compared with other surgical methods.
There were some limitations in this study. The finite element model data in this study were only based on a 24-year-old adult male, and this study was without statistical analysis, which is a common defect of finite element analysis. At the same time, this study simplified the finite element model, and the material properties of each structure were assumed to be isotropic, which cannot more accurately reflect the biomechanical changes of the lumbar structure. Second, previous studies on the mechanism of acute spinal shortening and spinal cord injury were often based on animal experiments, in which the selected segments were equivalent to the human thoracolumbar vertebrae or the middle and lower thoracic vertebrae to simulate the actual situation in the human body 11–13, 50]. The analysis results of this study were based on L1-S vertebrae, and the safe distance at the cauda equina level may be larger than that of the thoracolumbar segment. In addition, because of the risk of spinal shortening surgery, serious neurological complications may occur after operation, so the previous research results were based on animal experiments. Therefore, there is a lack of research data in the human body. However, biomechanics can simulate the physiological conditions and surgical process of normal people, thus avoiding unnecessary risks, but this study did not carry out biomechanical studies on the spinal cord and spinal nerves, which was another limitation of our study. In the future, we plan to conduct more reasonable and rigorous biomechanical studies to verify our results.