A comparison of monoaxial and polyaxial pedicle screws in thoracolumbar fracture

Purpose: To evaluate the use of short-segment Methods: All patients who underwent short-segment posterior fixation with monoaxial pedicle screws or polyaxial pedicle screws in the injured vertebra of a thoracolumbar fracture (T12-L2) in our hospital between June 2012 and December 2018 were categorised into two groups: monoaxial pedicle screws group (group A) and polyaxial pedicle screws group (group B). We compared the Thoracolumbar Injury Severity Score (TLISS), American Spinal Injury Association (ASIA) score, the fracture level, Arbeitsgemeinschaft für Osteosynthesefragen (AO) classification, hospital stay, injury-to-operation interval, and associated injuries between the two groups. In addition, the prevertebral height ratio, the injured vertebra Cobb angle, and the injured vertebral endplate centre ratio were measured preoperatively, postoperatively, and at the final follow-up. Results: There were 63 patients (21 males and 42 females) with an average age of 44.7 years. Compared with group B, the injury vertebral endplate centre ratio significantly increased postoperatively and at the final follow-up (P<0.05) in group A. Conclusion: Short-segment posterior fixation with monoaxial or polyaxial pedicle screws via the fracture level for thoracolumbar fracture can achieve kyphosis correction, reduce sagittal alignment correction failure, and maintain anterior vertebral height. The insertion of monoaxial pedicle screws at the fracture level after thoracolumbar vertebral fracture has a flick up effect on the central vertebral body of the injured vertebrae, which is beneficial to the recovery of the vertebral endplate.


Introduction
Spine fractures are common, accounting for about 14% of all fractures [1], and the thoracolumbar spine is reportedly the most common site of spinal fractures due to trauma, accounting for 60-70% of spine fractures [2,3]. Thoracolumbar fractures can cause an unstable spine leading to pain, motion disability, and even full paralysis [4]. Posterior pedicle screw fixation, to maintain the stability of the spine and avoid further nerve damage, has been popular globally [5]. Posterior pedicle screws can be inserted in either the fractured vertebra itself or the adjacent vertebrae. Many orthopaedic specialists removal. Patients with significant osteoporosis, endocrine system disease, vertebral tumour, tuberculosis, ankylosing spondylitis, and other destructive vertebral structural diseases were excluded from the study.

Clinical observation indicators and efficacy evaluation
Perioperative parameters: Injury-to-operation interval (days), associated injury, and hospital stay (days) on first admission.
Imaging parametersX-rays of the ortho and lateral positions of the spine were taken preoperatively, three days postoperatively, and on removal of internal fixation. Measurements taken include the height of the leading edge of the vertebral body adjacent to the injured vertebra and that of the injured vertebra, the height of the centre of the injured endplate and injured vertebra, the Cobb angle, and the vertebral changes for the two internal fixation methods between three days postoperatively and when the internal fixation device was removed. We also assessed whether the internal fixation fractured or loosened.

Measurement and calculation of prevertebral height ratio (Figure 1)
The sum of the heights of the leading edges of the vertebral bodies above and below the injured vertebra was compared with twice the height of the leading edge of the injured vertebra.

Measurement and calculation of injured vertebral endplate centre ratio (Figure 2)
The sum of the heights of the upper and lower endplates of the injured vertebrae was compared with twice the height of the centre of the injured endplate.

Measurement and calculation of kyphosis of the injured vertebrae (Figure 3)
The angle between the upper endplate of the injured vertebra and the extension of the lower endplate of the injured vertebra.

Statistical methods
The statistical software SPSS 20 was used to analyse the data of the monoaxial and polyaxial pedicle screw groups preoperatively, three days postoperatively, and at the removal of internal fixation. The measurement data were expressed as mean ± standard deviation (x ± s). Data between groups were compared using an independent sample t-test. P<0.05 was considered statistically significant.

General information
There were 33 patients in group A (24 males and nine females), and 30 patients in group B (18 males and 12 females). X 2 test analysis (P > 0.05) revealed no significant intergroup differences in terms of.
In addition, there were no significant differences in hospital stay, injury-to-operation interval, and associated injuries between the two groups (P > 0.05) ( Table 1).

AO Classification and Fracture level
Of the 33 patients in group A, there were ten cases of A1 type, two cases of A2 type, and 12 cases of A3 type. There were six cases of B1 type and three cases of B2 type. The fracture sites were T12 in two cases, L1 in 17 cases and L2 in 14 cases. Of the 30 cases in group B, the fractures were classified into six cases of A1 type, four cases of A2 type, nine cases of A3 type, six cases of B1 type, and five cases of B2 type. The fracture sites were zero cases of T12, 21 cases were L1, and nine cases were L2. According to the X 2 test analysis, p > 0.05; therefore, there were no statistically significant differences (Table 1).

TLISS Score
In the Thoracolumbar Injury Severity Score (TLISS) group, in group B, 17 people scored five points, nine people scored six points, and seven people scored seven points. In group A, 16 people scored five points, 11 people scored six points, and three people scored seven points. According to the X 2 test analysis, p > 0.05; there were no statistically significant differences (Table 1).

ASIA Score
In the ASIA score group, there were seven people evaluated as C, 17 evaluated as D, and nine evaluated as E in group A; and there were three people evaluated as C, 16 evaluated as D, and 11 evaluated as E in group B. According to the X 2 test analysis, p > 0.05; therefore, there were no statistically significant differences (Table 1).

Radiographic data
The radiographic data from surgery shows that the prevertebral height ratio and the injured vertebra Cobb angle between the two groups were not significantly different in preoperative, postoperative (three days after surgery), and final follow-up (removal of internal fixation) evaluated by t-test analysis (P > 0.05). In addition, the injured vertebral endplate centre ratios between the two groups were not significantly different preoperatively (P > 0.05). However, the injured vertebral endplate centre ratio between the two groups were significantly different in the postoperative and final followup checks (P < 0.05) ( Table 2).

Discussion
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 [17]. 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][20][21][22]. No statistically significant differences were observed, however, between the two groups. The injured vertebral endplate centre ratios in the postoperative and the final followup 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 [17] 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][24][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 [26]. 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 [27]. Moreover, in the operation(as shown in Fig. 4) 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.
In conclusion, based on our results, short-segment posterior fixation with monoaxial or polyaxial pedicle screws at the fracture level of a thoracolumbar fracture achieved kyphosis correction, reduced sagittal alignment correction failure, and maintained anterior vertebral height. We found short-  Figure 1 The sum of the heights of the leading edges of the vertebral bodies above and below the injured vertebra was compared with twice the height of the leading edge of the injured vertebra((a+c)/2b).

Figure 2
The sum of the heights of the upper and lower endplates of the injured vertebrae was compared with twice the height of the centre of the injured endplate((a+c)/2b).