The effect of screw head design on rod derotation in the correction of thoracolumbar spinal deformity: laboratory investigation

Comparative Analysis of Monoaxial and Polyaxial Pedicle Screws in the Surgical Correction of Adolescent Idiopathic Scoliosis

Background: Although several biomechanical studies have been reported, few clinical studies have compared the efficacy of monoaxial and polyaxial pedicle screws in the surgical treatment of adolescent idiopathic scoliosis (AIS). This study aims to compare the radiological and clinical outcomes of mono- and polyaxial pedicle screws in the surgical treatment of AIS. Methods: A total of 46 AIS patients who underwent surgery to treat scoliosis using pedicle screw instrumentation (PSI) and rod derotation (RD) were divided into two groups according to the use of pedicle screws: the monoaxial group (n = 23) and polyaxial group (n = 23). Results: The correction rate of the main Cobb's angle was higher in the monoaxial group (70.2%) than in the polyaxial group (65.3%) (p = 0.040). No differences in the rotational correction of the apical vertebra were evident between the two groups. SRS-22 scores showed no significant differences according to the type of pedicle screws used. Conclusions: The use of polyaxial pedicle screws resulted in coronal, sagittal, and rotational correction outcomes comparable to those associated with the use of monoaxial pedicle screws for surgical treatment using PSI and RD to treat moderate cases of AIS.

Comparison of monoplanar and polyaxial screw fixation systems in percutaneous intermediate fixation for thoracolumbar fractures

Background

The newly developed monoplanar pedicle screws (MPPSs) can mobile in axial plane but fixed in the sagittal plane, which holds potential to combine ease of rod placement with sagittal plane strength theoretically. So far, few clinical studies focused on the outcomes of MPPSs for treatment of thoracolumbar fractures (TLFs). The aim of this study was to compare the efficacy of MPPSs to polyaxial pedicle screws (PAPSs) in percutaneous intermediate fixation of TLFs.

Methods

Seventy-eight patients who sustained TLFs without neurological deficits and underwent percutaneous intermediate fixation using MPPSs (40 patients) or PAPSs (38 patients) with a minimum 1-year follow-up were included in this study. The operation time, blood loss, local Cobb angle (LCA), vertebral wedge angle (VWA), anterior body height ratio (ABHR), visual analogue scale (VAS) and Oswestry Disability Index (ODI) were collected.

Results

No significant differences were observed in baseline demographics, clinical characteristics, operation time or blood loss between the two groups (P > 0.05). The postoperative LCA, VWA and ABHR were significantly corrected compared to these parameters preoperatively in both groups (^#P < 0.05). The postoperative LCA, VWA and ABHR in the MPPS group were significantly better corrected than those in the PAPS group (*P < 0.05). Furthermore, the correction loss of LCA, VWA and ABHR in the MPPS group was significantly lower than that in the PAPS group (*P < 0.05). However, no significant difference in VAS and ODI scores was observed between the two groups.

Conclusions

MPPSs showed similar efficiency as PAPSs in percutaneous intermediate fixation surgical procedures. More importantly, MPPSs achieved better radiological performance than PAPSs in the correction of TLFs and the prevention of correction loss.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12891-022-05129-8.

Efficacy and Radiographic Analysis of Minimally Invasive Posterior Mono-Axial Pedicle Screw Fixation in Treating Thoracolumbar Burst Fractures

Background: The purpose of this study was to evaluate the effectiveness of minimally invasive posterior mono-axial pedicle screws fixation in the treatment of thoracolumbar burst fractures. Methods: In the present study, we analyzed 98 patients retrospectively who had thoracolumbar burst fractures without a neurological deficit. Patients were divided into two groups: mono-axial pedicle screw fixation group (n = 52) and poly-axial pedicle screw fixation group (n = 46). We collected clinical data (visual analog scale (VAS) score for back pain) and included radiographic measurements. Results: Sagittal index was significantly improved at postop and last follow-up in the mono group and the poly group. The mono group was better for reducing and maintaining anterior vertebral height. For the mono group, the mean postoperative regional kyphosis correction rate was 62.31%, and correction loss was 14.18% in late follow-up. For the poly group, the mean postoperative regional kyphosis correction rate was 52.17%, and correction loss was 33.42% in late follow-up. The mono-axial pedicle screw group had a good correction rate and reduced the risks of correction loss. The mean VAS scores for back pain improved by 2.4/2.5 and 3.8/4.2 for the mono and poly groups, respectively. There was no significant difference between groups. Conclusions: The mono-axial pedicle screw fixation was better for reducing and maintaining anterior vertebral height and regional kyphosis. Therefore, the mono-axial pedicle screw is a better optional instrumentation to treat thoracolumbar vertebral fractures.

Comparison of Polyaxial or Poly/Monoaxial Mixed Screw Fixation for Treatment of Thoracolumbar Fractures with O-Arm Navigation: A Case-Control Study

BACKGROUND

Navigation and minimally invasive techniques have revolutionized spinal surgery. There are different types of pedicle screws for navigation-assisted percutaneous internal fixation for thoracolumbar burst fractures, resulting in 2 different types of internal fixation approaches: the PPP method (polyaxial screw fixation for all 3 vertebrae) and the MPM method (polyaxial screw fixation for the injured vertebra and monoaxial screw fixation for the adjacent vertebrae). This study aims to compare these 2 different methods for treatment of thoracolumbar fractures via O-arm navigation.

METHODS

From January 2017 to January 2018, 45 patients were enrolled according to the criteria with a minimal follow-up of 1 year; there were 21 patients in the PPP group, and 24 patients in the MPM group. For follow-up, the demographic features and clinical outcomes were collected and evaluated.

RESULTS

With regard to preoperative indices and demographic features, no significant difference was observed between these 2 groups (P > 0.05). For operation time, intraoperative bleeding, and length of hospitalization, no significant difference was observed either between the 2 groups (P > 0.05). The Cobb angle and anterior vertebrae body height (AVBH) of the injured vertebra were significantly restored in both groups 1 day after surgery (P < 0.05). However, the Cobb angle and AVBH in the MPM group was better corrected than that in the PPP group 1 day, 3 months, and 12 months after surgery, respectively (P < 0.05). In contrast, loss of Cobb angle and AVBH in the PPP group were significantly larger than the MPM group (P < 0.05). Meanwhile, no difference in the Visual Analog Scale and the Oswestry Disability Index were found between the 2 groups (P > 0.05).

CONCLUSIONS

Both MPM and PPP systems are good for treatment of single-segment thoracolumbar fracture. However, MPM showed better restoration effect and postoperative maintained Cobb angle than PPP.

A biomechanical investigation of different screw head designs for vertebral derotation in scoliosis surgery

BACKGROUND CONTEXT

The posterior pedicle screw-rod system, which is widely used to correct spinal deformities, achieves a good correction rate in the frontal and coronal planes but not in the axial plane. Direct vertebral derotation (DVD) was developed to correct axial plane deformities. However, the design of screw head and body connection, in terms of monoaxial, polyaxial, and uniplanar screw, may influence the efficiency of DVD.

PURPOSE

This study compared the efficiency of a newly designed uniplanar screw with that of monoaxial and polyaxial screws in the DVD maneuver.

STUDY DESIGN

A porcine spine model and monoaxial, polyaxial, and uniplanar screws were used to examine the biomechanics of the DVD maneuver.

METHODS

Six T7-T13 porcine thoracic spine segments were used as test specimens in this study. Pedicle screws were inserted in the left pedicles of the T9-T11 spinal segments and then connected with a rod. Three types of pedicle screws with different screw head designs (monoaxial, polyaxial, and uniplanar) were employed in this study. The material testing system (MTS) machine generated a rotational moment through the derotational tube on the T10 (apical body) pedicle screw, which simulated the motion applied during the surgical vertebral derotational procedure. The pedicle strain and the kinematics of the vertebral body and derotational tube were recorded to evaluate the derotational efficiency of different pedicle screw head designs.

RESULTS

The variances of the derotation for the monoaxial, polyaxial, and uniplanar screws were 2.22°±1.43°, 32.23°±2.26°, and 4.75°±1.60°, respectively; the derotation efficiency was 0.65, 0.51, and 0.12, respectively, when the torques of the spinal constructs reached 3 Nm. The rotational variance of the polyaxial screw was statistically greater than that of the monoaxial and uniplanar screws (p<.05). The maximum micro-strains of the pedicles for the monoaxial, polyaxial, and uniplanar screws were 1,067.45±550.35, 747.68±393.56, and 663.55±271.04, respectively, with no statistically significant differences (p>.05).

CONCLUSIONS

The screw head design played an important role in the efficiency and variance of the derotation during the DVD maneuver. The derotational efficiency of the newly designed uniplanar screw was closer to that of the monoaxial screw group than to that of the polyaxial screw group. The polyaxial screw was inferior to DVD owing to a derotational variance between the derotational tube and the apical body that was correlated with the range of motion of the screw head. In the present study, the pedicle strain was similar in all groups. However, the pedicle strain of the uniplanar screw group was lower than that of the monoaxial screw group and was similar to that of the polyaxial screw group when the angle of rotation of the apical body increased.

Novel Screw Head Design of Pedicle Screw for Reducing the Correction Loss in the Patients With Thoracolumbar Vertebral Fractures: A Biomechanical Study

STUDY DESIGN

A biomechanical study.

OBJECTIVE

To study the different biomechanical property among fixed-axis, monoplanar and polyaxial screws in the static and dynamic tests.

SUMMARY OF BACKGROUND DATA

Correction loss is a common phenomenon in the patients with thoracolumbar vertebral fractures who underwent the posterior pedicle screw fixation. The incidence varies with the kinds of fixation instrumentation used. There is higher incidence in polyaxial pedicle screws group than in fixed-axis pedicle screws. Monoplanar pedicle screws, which are mobile in the axial plane but fixed in the sagittal plane, can be a better fixation instrumentation for thoracolumbar vertebral fractures in theory.

METHODS

A total of 30 porcine spinal units (L2-L4) were used for the static and dynamic tests, which were randomized into six groups (A1, A2, A3, B1, B2, and B3). Static test was performed in A1, A2, and A3. In this test, fixed-axis, monoplanar, and polyaxial screws were performed in A1, A2, and A3, respectively. The ultimate load was noted after tested. In addition, dynamic test was performed in B1, B2, and B3, used fixed-axis, monoplanar, and polyaxial screws, respectively. Correction loss (head-shank angle shift and anterior vertebral body height shift) was obtained and analyzed in each mode.

RESULTS

In static test, fixed-axis and monoplanar screws had significantly higher ultimate load than polyaxial screws (P < 0.05) and fixed-axis screws had a little higher ultimate load than monoplanar screws (P < 0.05). In dynamic test, correction loss was minimal in fixed-axis screws, medium in monoplanar screws, and maximal in polyaxial screws. However, the differences were statistically significant in all comparisons but not in the comparison of fixed-axis and monoplanar screws (P > 0.05).

CONCLUSION

The findings from the current study suggest that monoplanar screws can significantly increase the stiffness in axial direction compared with polyaxial screws, and reduce the risks of correction loss. For thoracolumbar vertebral fractures, monoplanar screw is a better optional instrumentation for minimally invasive surgery.

LEVEL OF EVIDENCE

N/A.