Research has confirmed the beneficial therapeutic effect of short-segment posterior fixation with pedicle screws in the injured vertebra as treatment for thoracolumbar fractures [6-13]. Biomechanically, the screws at the fracture level function as a push point with an anterior vector, creating a "lordorizing" force that restores the anterior vertebral height and corrects the kyphosis . Some clinical studies have shown that inserting monoaxial or polyaxial pedicle screws at the fracture level could achieve better kyphosis correction, less sagittal alignment correction failure, and better maintenance of anterior vertebral height [7,8,10,12,18]. Short-segment posterior fixation can be achieved by the insertion of either a monoaxial or a polyaxial pedicle screw into the injured vertebra; however, no studies have compared the efficacy of the two options.
Our retrospective study shows that both options markedly improved the outcome of patients postoperatively. This was reflected in considerable improvements whether analysed according to sex, age, fracture injury classification, fracture site, TLISS score, ASIA Score, AO classification, hospital stay, injury-to-operation interval, or associated injury after treatment. However, no statistically significant differences were observed between the two groups（p>0.05. These results support the proposition that inserting either a monoaxial or polyaxial pedicle screw in the injured vertebra is an effective surgical treatment.
In our study, we measured and calculated the prevertebral height ratio, injured vertebral endplate centre ratio, and the kyphosis of the injured vertebrae. As shown in Table 2, the prevertebral height ratio and the kyphosis of the injured vertebrae postoperatively and at the final follow-up were greatly improved. This confirms that inserting a monoaxial or polyaxial pedicle screw in the injured vertebra can correct the deformity through vertebral endplate augmentation with its buttress effect (bending force) as with the rod-sleeve method, which was until recently commonly used in spinal instrumentation [7,19-22]. No statistically significant differences were observed, however, between the two groups. The injured vertebral endplate centre ratios in the postoperative and the final follow-up were greatly improved, and the correction effect of group A was better than that of group B. In addition, significant differences were observed between the two groups (p<0.05). Many reports  have proposed that the screw at the fracture level may provide a mass effect that prevents the vertebra from collapsing. It may also help to support the anterior column, which is vital for the stability of the construct. However, it is not clear why there is a better correction effect in group A compared to group B.
Polyaxial pedicle screw heads are vulnerable to fatigue failure; the region between the screw head and shaft has been found to fail first in many biomechanical studies [23-25]. Further, the use of additional intermediate monoaxial pedicle screws may result in a stiffer construct and a reduced level of von Mises stress on the pedicle screws than on the polyaxial pedicle screw models . In addition, the monoaxial screws can slap the collapsed endplate, reset the endplate fracture, maintain the reduction, reduce the degeneration of the intervertebral disc injury, and perhaps better maintain the stability of the spine; thus, reducing the incidence of back pain. Furthermore, the head of the polyaxial pedicle screw is movable and cannot support the injured vertebral body. The monoaxial pedicle screws inserted at the fracture level showed higher stability in flexion and extension than the similarly placed polyaxial pedicle screws . Moreover, in the operation（as shown in Figure 4), we selected the midpoint of the transverse process on both sides of the injured vertebral body and the intersection of the anterior line of the superior articular process as the needle point. We then placed two locating needles, and kept them at an angle of about 30 degrees to the sagittal plane, the positioning needle, and the cross-section form an angle; the vertebral bodies above and below the injured vertebrae are in the direction of the traditional needle insertion, and the sagittal plane is formed at an angle of about 15 degrees, so that the positioning needle is parallel to the cross-section; The vertebral pedicle screw installed in the manner of needle insertion can support the sling and the role of the vertebral endplate. Thus, our results provided a reference for the treatment of thoracolumbar fractures with severe vertebral endplate damage using invasive intermediate monoaxial pedicle screw fixation.
This study has many limitations. First, the number of patients included in the study was small, and this was a retrospective study. Second, a selection bias may exist because this study included patients referred to our teaching hospitals. Third, it is necessary to discuss several factors including different patient conditions, the variability in bone density, muscle forces, vertebral size, the length and diameter of pedicle screws, and the degree of joint degeneration of the body.