During daily activities, spinal posture and thoracic kyphosis influence the load distribution in vertebral body; repetitive cyclic loading will cause collapse of fractured vertebral body; these can lead to thoracolumbar kyphosis . It has been reported postoperative KR may be of greater significance than postoperative vertebral body re-collapse [16, 17]. Therefore, our study analyzed the risk factors of postoperative KR. Our study showed that correction loss progressed most rapidly within 12 months after surgery; correction loss progressed much slowly after implant removal. Therefore, it’s necessary to separately discuss the risk factors of KR before or after implant removal. The threshold of 5° was use to define in this study according to the studies of Lee et al. and Chen et al. . There is a difference between realignment failure and kyphosis reoccurrence. The definition of failure is an increase of 10° or more in local kyphosis in the latest follow-up radiographs compared with the measurement on the initial postoperative radiographs or implant failure . This is more influenced by the improper selection of surgical indications. And it has to be treated surgically. If that's the case, it would be too late to deal with it. Defining the threshold as 5° can help the surgeons to take measures to prevent failure in advance. Therefore, this value can have a greater feasibility.
Previous studies showed that age will primarily affect bone remodeling toward resorption; this was because vertebral body contained numerous trabecular bone [18, 19]. All these studies supported our results that age > 49 years was a risk factor of KR before implant removal. There was a lack of studies on the correlation between BMI and KR. Our study showed that BMI > 24.0 was a risk factor of KR before implant removal; BMI > 25.17 was a risk factor of KR after implant removal; these implied that BMI works differently at different stages after surgery. Bone healing of fractured vertebra has not been fully achieved before implant removal . Therefore, a high BMI will increase the load on instruments and even lead to the deformation of implants. The fractured vertebra has healed before implant removal; therefore, the KR after implant removal may be mainly caused be the loss of UIVA correction; in this period, a higher BMI with longer action time is needed to result in space collapse, reduction of buffering capacity and degeneration of upper intervertebral discs . Chung et al.  reported that diabetes mellitus could increase the risk of fragility fractures, which was often associated with osteoporosis. Our study showed that diabetes mellitus was not a risk factor of KR. This was because the most thoracolumbar fractures of patients with diabetes mellitus involved more than one vertebra, which was much severer than single-level thoracolumbar burst fractures. According to the inclusion and exclusion criteria, our study included a small number of patients with diabetes mellitus, which might influence our results.
Fractured endplates and nucleus pulposus were intruded in to fractured vertebral body during burst fracture; this could result in collapse of the cancellous framework in the vertebral body . During SSPI-f, annulus fibrosus of the collapsed intervertebral disc was distracted; then the wall of fractured vertebral body was reduced by the annulus fibrosus attached to the periphery of the endplate; however, because the annulus fibrosus was not sufficiently attached to the center of the endplate, the fragmented nucleus, endplates and collapsed cancellous framework were not sufficiently reduced . Similarly, our study showed that AVH% > 49.5%, PVH% > 85.5% were protective factors of KR before implant removal. This was because low AVH% and low PVH% usually meant a severer vertebral body compression. These would lead to large bone defect created inside the fractured vertebra after reduction, which had been reported as an important reason of internal fixation failure and KR . Additionally, the results in our study showed that AVH% > 61.5% was a protective factor of KR after implant removal, which implied that the loss of anterior vertebra height had a long-term impact on therapeutic effect. Preoperative CA, RA and VWA were not the risk factors of KR in our study. The possible explanation might be that thoracolumbar burst fractures usually involved anterior and posterior columns, both anterior and posterior walls of vertebra were fractured; therefore, compared with vertebra height, the variation of vertebra angle could not accurately the severity of thoracolumbar burst fractures .
The fractured endplates and nucleus pulposus were intruded into the fractured vertebral body during burst fracture, which would result in collapse of the cancellous framework in the vertebral body . Similarly, Oner et al.  reported that even with the fracture of end plates, the upper discs were intact in most of the patients. It could be seen from the trend of parameters in our study that the loss of correction caused by the collapse of upper intervertebral disc of fractured vertebra was a slow process, which was not due to the adjacent disc injured by direct high-energy violence. Therefore, superior endplate fracture, the decrease of preoperative UIVA and preoperative LIVA were not the risk factors of KR.
Our results showed that preoperative VAS score could not be used to predict the risk of KR. The reason was that VAS score had strong subjectivity with many influence factors, which made its accuracy limited [3, 6]. Pellise et al.  pointed out that LSC score was positively correlated with correction loss. However, our study showed that LSC score was not a risk factor of KR. The possible explanation might be that most of the previous studies discussed the curative effect of skip-level fixation without inserting additional screws into the fractured vertebra. Our study analyzed the treatment effect of SSPI-f, which could increase stiffness and reduce the failure rate of instrumentation with the insertion of two screws in the fractured vertebra; this seemed to reduce the influence of LSC score on the treatment effect . TLICS was comprehensive not only in determining injury prognosis, but also in guiding treatment . Our study found that TLICS was not a risk factor of KR. The reason might be that the mean TLICS was more than 4 points in our study; the fractured vertebras of patients with higher TLICS were highly unstable, which was usually treated with long-segment fixation and should be excluded from our research.
There were limitations in our study. First, it was a retrospective study that developed the detailed prediction model; however, our study was a multi-center study with strict inclusion and exclusion criteria, which was more meaningful compared with prior risk factor analysis. Second, magnetic resonance imaging (MRI) was not fully discussed in our study, which made the evaluation of soft tissue around fractured vertebra very difficult; this might lead to the omission of significant risk factors.