Percutaneous monoplanar screws versus hybrid fixed axial and polyaxial screws in intermediate screw fixation for traumatic thoracolumbar burst fractures: a case–control study

From January 2017 to June 2021, a consecutive series of 100 patients with single-segment TTBFs without neurological deficits who underwent PSISF with 6 MSs (MS group) or hybrid 4 fixed axial screws and 2 polyaxial screws (HS group) were retrospectively enrolled. Sex, age, cause of injury, fracture level, AO Spine Injury Classification (AO classification) [24], Thoracolumbar Injury Classification and Severity Score (TLICS score) [25], Load Sharing Classification (LSC score) [26], number of spinal canal encroachment and time of last follow-up were recorded.

Inclusion criteria: (1) between 18 and 60 years of age; (2) with a single-segment vertebral fracture of the thoracolumbar spine involving T11-L2; (3) fresh traumatic fracture; (4) TLICS score greater than or equal to 4; (5) underwent all preoperative, postoperative and follow-up imaging examinations in our hospital; (6) intact pedicles of the injured vertebra; and (7) patients who signed informed consent forms.

Exclusion criteria: (1) time from trauma to surgery more than 14 days; (2) pathological fractures (including tuberculosis, primary or metastatic tumours, etc.); (3) infection; (4) prior spine surgery; (5) congenital spinal deformities; (6) osteoporosis; and (7) incomplete clinical data.

The study was reviewed and approved by the Ethics Committee of General Hospital of Central Theater Command ([2022]060-01) and was performed in conformity with the Declaration of Helsinki. Observational studies were reported using STROBE guidelines [27].

All operations were performed by the same team, and the chief surgeon was an experienced deputy chief physician in the same department. The patients were placed in the prone position on the Jackson surgical table, and the abdomen was suspended, the operating table was bent into a V shape, manual reduction was performed by applying firm pressure on the spinous process adjacent to the injured vertebra. After marking the puncture points approximately 1.5 cm lateral to the markers positioned at the pedicles of the injured vertebra and two adjacent vertebrae, 6 Jamshidi needles were inserted into the corresponding pedicle with proper orientation and to the appropriate depth. After successful puncture, the needle core was withdrawn. After the long guide wire was placed into the anterior medial third of the vertebral body, the screw was implanted. In the MS group, 6 MSs (Shanghai SANYOU, China) were implanted parallel to the upper endplate of the injured vertebra and two adjacent vertebrae. In the HS group, 4 fixed axial screws (Shandong WEGAO, China) were implanted parallel to the upper endplate of the adjacent vertebrae of the injured vertebra, and 2 intermediate short polyaxial screws (Shandong WEGAO, China) were implanted parallel to the pedicle of the injured vertebrae. The connecting rod, with an appropriate length and radians, was inserted after insertion of all 6 screws. In the MS group, during the distraction process, the intermediate MSs were exerted an upward force on the upper endplate of the injured vertebra, aiming to better restore the anterior and middle columns. In the HS group, indirect reduction of the restoration of the anterior column of the injured vertebra by applying appropriate compression to the fixed axial screw-rod system at both ends. All pedicle screws were placed via a minimally invasive percutaneous and inserted into the corresponding segment vertebral body. All above operations were conducted under the guidance of C-arm fluoroscopy.

For all patients, functional exercises for both lower limbs were started 1 day after surgery. They were encouraged to regularly ambulate 3 days after surgery. All participants were followed up postoperatively for at least 12 months.

Radiologic evaluation indicators, perioperative indicators, and clinical assessment indicators were recorded and assessed.

Radiologic evaluation indicators included the anterior vertebral height ratio (AVHR), kyphosis Cobb angle (KCA), vertebral wedge angle (VWA) and spinal canal encroachment rate (SCER). The AVHR, KCA and VWA were measured on lateral images of the thoracolumbar spine (Fig. 2a). The SCER was measured on CT images (Fig. 2b). Anteroposterior and lateral radiographs, as well as CT, were obtained for the thoracolumbar spine before surgery, at 1 week and 3 months after surgery, and during the final follow-up at our hospital for all patients. Additionally, MRI of the thoracolumbar spine were conducted before surgery and at 1 week after surgery for all patients. The effect of correction was compared and analysed in terms of AVHR, KCA, VWA and SCER. Changes in the corrective effect were evaluated on all lateral radiographs and CT of the thoracolumbar spine after the surgery.

Perioperative indicators included the time from admission to surgery, operation time, intraoperative bleeding volume, time to ambulation after surgery and length of hospital stay.

Clinical assessment indicators included the visual analogue scale (VAS) score, Oswestry Disability Index (ODI) score and complications (spinal cord or nerve root injury, wound haematoma, infection, internal fixation failure, etc.). Pain from the lower back was evaluated using the VAS score and functional outcome was evaluated using the ODI score. The VAS score and ODI score were calculated preoperatively, 1 week and 3 months postoperatively and at the last follow-up. Complications, including intraoperative and postoperative complications, were recorded. Loosening, breakage and pulling out of internal fixation (including screw, rod and locking cap) were defined as failure.

All measured and evaluated data were performed by two experienced physicians in spine surgery respectively. The final values were determined as the average of the measurements performed by the two physicians.

All of the statistical analyses were conducted with SPSS 26.0 (IBM, New York, USA). The categorical variables that were reported as numbers were compared by using the χ² test or Fisher’s exact test. The variables with continuous data were reported as the mean ± standard deviation. Comparisons between two groups were evaluated by independent sample t tests, and comparison of before–after changes in each group was using paired-samples t test. A P value < 0.05 indicated statistical significance.

In the study, a total of 100 patients were recruited with an average age of 46.11 ± 12.3 years. Fifty-one patients underwent fixation with 6 MSs (MS group) and 49 patients underwent fixation with hybrid 4 fixed axial screws and 2 polyaxial screws (HS group). In terms of sex, age, cause of injury, fracture level, AO classification, TLICS score, LSC score, number of spinal canal encroachment and time of last follow-up were not statistically different between the MS group and the HS group (P > 0.05, Table 1).

No statistically significant differences in the time from admission to surgery, operation time, intraoperative bleeding volume, time to ambulation after surgery or length of hospital stay were found between the MS and HS groups (P > 0.05, Table 2).

As shown in Table 3, no significant differences in the AVHR, KCA, VWA or SCER were found between the two groups preoperatively (P > 0.05). One week postoperatively, 3 months postoperatively and at the last follow-up, the AVHR, KCA, VWA and SCER were obviously improved in the two groups. A significant difference between the preoperative and postoperative values were found at every time point (*P < 0.05). The MS group had better correction in the AVHR, KCA and VWA than the HS group after surgery (*P < 0.05). During the follow-up period, correction loss of AVHR, KCA and VWA was found between two groups and correction loss was clearly observed in the HS group at the last follow-up (*P < 0.05). The MS group presented greater improvement in the SCER than the HS group at the last follow-up (*P < 0.05).

Compared to those preoperatively, the postoperative VAS and ODI scores were significantly improved in all the patients in both groups (*P < 0.05), and the scores collected at each follow-up visit did not differ significantly between the two groups (P > 0.05) (Table 4). In the HS group, two cases of internal fixation failure were observed at the last follow-up and the overall failure rate was 4.1%. There was 1 case of rod loosening that developed 13 months after surgery, and one case of screw breakage developed 12 months postoperatively. In two cases, good fracture healing and correction maintenance were found. After implant removal no more problems occurred. There were no severe complications such as spinal cord or nerve root injury, wound haematoma or infection, in either group. Typical case images of the MSs fixation were shown in (Fig. 3).

There were several limitations of this study. First, its retrospective design and lack of randomization could potentially lead to selection bias. Therefore, our findings should be confirmed in additional prospective studies. Second, the study sample size was relatively small. Thus, multicentre relevant studies are needed. Third, we did not assess results of correction loss, functional outcome, and degeneration of adjacent segment of the injured vertebra after removing the internal fixation. As such, further research in this area is warranted.