The goals of surgical treatment for thoracolumbar burst fractures are to restore vertebral body height, correct angular deformity, decompress neural tissue, allow rapid mobilization and rehabilitation, prevent development of progressive deformity with neurologic deficit, and limit the number of instrumented vertebral motion segments.2,3,5,32
The ideal surgical management remains controversial, and no evidence based guideline for the most optimal surgical approach or instrumentation technique has been developed.1,10,11
Long segment pedicle screw fixation may be stiffer and impart greater forces on adjacent segments compared with short segment fixation, which may affect clinical performance
and long term outcome but at the cost of sacrificing additional motion segments.20 For this reason, short segment posterior fixation (one level above and below the fracture level) has been used more commonly than long segment posterior fixation for the treatment of thoracolumbar burst fractures.6.19,23 However, some study showed that short segment posterior fixation alone led to a 9% - 54% incidence of implant failure and rekyphosis at long term follow up, and 50% of the patients with implant failure had moderate to severe pain.21 It is important to obtain the best fracture reduction as possible.20 The greater residual kyphotic deformity provides greater anterior vertebral stress on pedicle screws. Thus, the overloading force on the instrument loosens the screw, causing it to break, dislodge, and disconnect, which mostly is seen in short segment posterior fixation.20,21,30
To overcome this situation, some study suggested that pedicle screws be added to the fracture level.28 Because, the stiffness increased an average of 160% when using the additional pedicle screw fixation at the fracture level. Axial testing showed that the six screw construct was 84%stiffer than the four screw construct in flexion and was 38%stiffer than the four screw construct in torsional testing.8
The additional pedicle screw fixation at the fracture level can function as a push point with an anterior vector, creating a lordorizing force that restored anterior vertebral height and the segmental kyphosis. Therefore, short segment posterior fixation with pedicle screw fixation at the fracture level provided better anterior vertebral height restoration and kyphosis correction for thoracolumbar burst fractures.11
Many biomechanical studies demonstrate that the designs of screw head play a significant role in the correction of spinal deformity and have different effects on the stiffness in three dimensions (coronal, sagittal, and axial plane).16 An experimental study has reported by Wang that the mono axial pedicle screw with no motion between screw head and shaft can significantly increase the stiffness in axial direction compared with poly axial pedicle screw, and reduce the risks of correction loss.29 Mono axial pedicle screws can be a better fixation instrumentation for thoracolumbar burst fractures in theory. However, the question arose whether the mono axial pedicle screw fixation could really achieve vertebral height recovery, improve the kyphosis, reduce postoperative kyphosis loss, and reduce the incidence of internal fixation failure better than the poly axial pedicle screw fixation in clinical practice. Hence, we planned this study.
In this study, our results found that there was significant difference in the reduction of anterior vertebral height (sagittal index) and correction of the kyphosis angle (sagittal plane kyphosis) between the mono group and poly group. The mono group was better for reducing and maintaining anterior vertebral height, reducing the kyphosis angle. Because, the mono axial pedicle screw with no motion between screw head and shaft formed a 90°–90° screw-rod construct. But, the poly axial pedicle screw with motion between screw head and shaft could not formed a 90°–90° screw-rod construct. This leaded to a significant difference in the reduction of anterior vertebral height (Sagittal index) and correction of the kyphosis angle (sagittal plane kyphosis) between the mono group and poly group.
Therefore, Mono screw fixation could provide sufficient immobilization to restore spine stability until the fracture healed, thus obtaining satisfactory reduction and maintenance of the fractured vertebrae height.
This study has some limitations. First, this is a retrospective study. Second, 50 patients is a rather small group for such a clinical study. Third, the short follow up time and not taking into account the adjacent intervertebral space height. therefore, the results may be biased.