The aim of this retrospective study was primary to evaluate the clinical and radiological outcomes of SSFK for OTBF, and to compare the outcomes with those of LF. Our results showed that both SSFK and LF could effectively reduce pain, improve function and correct kyphotic deformity for OTBF. However, compared with LF, SSFK had the advantage of reducing operative time and blood loss.
Although more and more OTBF happen nowadays due to an increasing aged population, the optimal surgical treatment for this type of fracture in aged patients remains a matter of discussion . The goal of surgical treatment for OTBF patient without neurological deficit is to correct kyphotic deformity, provide sufficient biomechanical stability for early mobilization, while reduce surgical invasiveness and related complications. However, some challenges should be taken into consideration when choosing the surgical procedure, including poor bone quality, old age, medical comorbidities and possible perioperative morbidities.
Compared with anterior or combined approaches, posterior approach does not pose risks to chest or abdominal organs, and it is correlated with less surgical invasiveness and lower complication . Further, most spine surgeons are more familiar with posterior approach. In biomechanically, LF with two or more levels above and below the fracture is a better choice for thoracolumbar burst fracture, which can provide greater mechanical stiffness and reduce likelihood of segmental collapse and implant failure [4, 19]. However, except disruption of spinal motion segments, LF is correlated with more severe surgical invasiveness. In our study, compared with SSFK group, the operative time and blood loss were significant higher in the LF group. In addition, LF construct may correlated with adjacent or non-adjacent vertebral fracture. Short-segment pedicle fixation has some advantages including less surgical invasiveness, preserving motion segment and reducing adjacent segment stress. While, unacceptable increasing instrumentation failure and kyphotic correction loss were reported after traditional four-screw short-segment (one level above and below the fracture) fixation for thoracolumbar burst fracture . A finite element analysis for simulating burst fracture demonstrated four-screw short-segment posterior fixation permitted a greater range of motion (ROM) in flexion compared with intact condition, and the stability could be enhanced with the increased number of instrumented levels .
Six-screw pedicular fixation involving the placement of two pedicle screws at the fractured vertebra has been proposed to improve postoperative stability, meanwhile, retain the own advantages of short-segment fixation. Baaj et al.  reported adding bilateral index-level screws to short-segment constructs could improve stability by 25%, especially for flexion and lateral bending restriction, although the stability remained less than that provided by long-segment construct with or without index-level pedicle screws. Dobran et al.  even reported that six-screw short-segment construct for unstable thoracolumbar fracture resulted in a kyphosis correction and in a maintenance of sagittal alignment as a long-segment construct. However, the included patients in their study were relatively young, and the results were not based on the osteoporotic population. Schulze et al.  reported significant migration of pedicle screws following fixation of osteoporotic vertebrae placed under flexion/extension cyclic loading, and the anchoring effect holding the screw in place was decreased in osteoporotic cases. Although intermediate pedicle screws were used in the fractured vertebra, significant kyphotic correction loss and mechanical failure were observed in cases with osteoporotic thoracolumbar fracture . In addition, the intravertebral area of osteoporotic burst fracture enlarged by positional or instrumental reduction is nearly empty, and such insufficient anterior column support may not endure vertical physiological strength with stand-alone posterior pedicle construct, even LF construct was used [24, 25].
The reconstruction of weight-bearing anterior column could reduce posterior instrumentation strain and sequentially reduce instrumentation failure and kyphotic correction loss, especially for osteoporotic thoracolumbar burst fracture. With the successful application of PKP in the treatment of osteoporotic vertebral compressive fracture, some authors even applied it to burst fracture . Biomechanical study showed cement augmentation could supported the anterior column especially in flexion, however, did not reduced ROM in extension . Hence, kyphoplasty of the affected vertebra combined with posterior instrumentation was proposed by some authors in order to achieve circumferential fixation through a single posterior approach . By using this method, better VAS score reduction, ODI improvement and kyphotic correction could be achieved compared with simple PKP . In a finite element analysis, Liao et al.  reported short-segment fixation combined with intermediate screws and anterior column cement augmentation provided the strongest stability among different types of posterior short-segment fixation. In our study, SSFK construct could provide sufficient stability, and no instrumentation failure or revision surgery was occurred in any patient.
Cement leakage is one of risks during vertebral augmentation for burst fracture due to the rupture of the posterior vertebral body wall and spinal canal occupancy . Wang et al.  reported the cement leakage rate was 14.3% after using PKP for very severe osteoporotic vertebral compression fracture (vertebral body collapse to less than one third of their original height) with spinal canal compromise. However, in a study of comparing the results between osteoporotic burst and compression fractures with PKP, Li et al.  reported cement leakage rate was 47.8 and 36.6%, respectively, and all the leakages were minor and without neurological deficit. They conclude that osteoporotic burst fracture with asymptomatic spinal canal compromise was not a contraindication for PKP. We found the osteoporotic burst fractured vertebra exhibited an “eggshell” like change after postural and instrumental reduction, which created a relatively safe cavity for padding of thick and doughy bone cement. Further, due to posterior fixation supplementation, it is not necessary to inject bone cement into the fractured vertebral body as much as possible. In our study, the cement leakage rate (20.83%) was relatively low, and there was no case of leakage causing neurological deficit and organ compression. Hence, bone cement augmentation used in our study was a safe measure.
There are several limitations to the current study. Retrospective analysis reduced the evidence level. A small sample size was another weak point, limiting its statistical power. The follow-up duration in this study was relatively short. An extended observation period is beneficial for better evaluation of clinical efficacy, correction loss and instrumentation failure. Hence, a high-quality randomized controlled study of with larger sample size and longer follow-up duration is required to confirm our finding in the future.