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

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.

[Translated article] How do I plan adolescent idiopathic scoliosis surgery? Systematization of a preoperative planning method

Preoperative planning is essential in adolescent idiopathic scoliosis (AIS) surgery to determine the fusion levels and to perform the procedure with greater precision and diligence. However, the protocolized performance of such planning is not as widespread among specialists in training. The aim of this article is to describe in detail the preoperative planning method for AIS used in a specialised paediatric and adolescent spine unit of a referral centre, as well as the logistics and the tactics, supported a free semi-automatic digital measurement and planning software. Three representative cases of different vertebral deformities, treated by posterior spinal fusion after preoperative planning according to the method, are shown. This method is highly suitable for the trainee surgeon as it combines the advantages of traditional and modern methods, and is simple, low-cost, accessible, reproducible and with an educational character.

How do I plan adolescent idiopathic scoliosis surgery? Systematization of a preoperative planning method

Preoperative planning is essential in adolescent idiopathic scoliosis (AIS) surgery to determine the fusion levels and to perform the procedure with greater precision and diligence. However, the protocolized performance of such planning is not as widespread among specialists in training. The aim of this article is to describe in detail the preoperative planning method for AIS used in a specialized pediatric and adolescent spine unit of a referral center, as well as the logistics and the tactics, supported a free semi-automatic digital measurement and planning software. Three representative cases of different vertebral deformities, treated by posterior spinal fusion after preoperative planning according to the method, are shown. This method is highly suitable for the trainee surgeon as it combines the advantages of traditional and modern methods, and is simple, low cost, accessible, reproducible and with an educational character.

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.

Direct vertebral rotation significantly decreases the pullout strength of the pedicle screw: a biomechanical study in adult cadavers

The pullout strength of the pedicle screws after direct vertebral rotation (DVR) maneuver is not known. This biomechanical study was performed to quantitatively analyze the pullout strength of a pedicle screw after DVR maneuver using human cadaveric vertebrae. Thoracic vertebral bodies from three cadavers were harvested and stripped of soft tissues. Thirty pedicles of 15 vertebrae were separated into two groups after bone mineral density measurements. Polyaxial 5.5 mm pedicle screws with appropriate length were inserted with a freehand technique for each pedicle. One Kirschner wire was inserted to the anterior part of each vertebral corpus the half depth of each corpus was embedded into PVC pipes using polyester paste. In the DVR group, each screw was pulled horizontally with 2 kg (~20 N) load over a screwdriver rigidly attached to the screw, and a DVR maneuver was simulated. The control group did not load with a DVR maneuver. Samples were placed on a universal testing machine and pullout loads were measured. The Mann-Whitney U test was utilized, and the P value <0.05 was considered as statistically significant. In the DVR group, the mean pullout strength was 183.35 N (SD ± 100.12), and in the control group, the mean pullout strength was 279.95 N (SD ± 76.26). Intergroup comparisons revealed that DVR maneuver significantly decreases the pullout strength (P = 0.012). The results of this study confirm that the pullout strength of pedicle screw significantly decreases by approximately 35% when DVR maneuver is applied.

Restoration of Thoracic Kyphosis in Adolescent Idiopathic Scoliosis Over a Twenty-year Period: Are We Getting Better?

STUDY DESIGN

A multicenter, prospectively collected database of 20 years of operatively treated adolescent idiopathic scoliosis (AIS) was utilized to retrospectively examine pre- and postoperative thoracic kyphosis at 2-year follow-up.

OBJECTIVE

To determine if the adoption of advanced three-dimensional correction techniques has led to improved thoracic kyphosis correction in AIS.

SUMMARY OF BACKGROUND DATA

Over the past 20 years, there has been an evolution of operative treatment for AIS, with more emphasis on sagittal and axial planes. Thoracic hypokyphosis was well treated with an anterior approach, but this was not addressed sufficiently in early posterior approaches. We hypothesized that patients with preoperative thoracic hypokyphosis prior to 2000 would have superior thoracic kyphosis restoration, but the learning curve with pedicle screws would reflect initially inferior restoration and eventual improvement.

METHODS

From 1995 to 2015, 1063 patients with preoperative thoracic hypokyphosis (<10°) were identified. A validated formula for assessing three-dimensional sagittal alignment using two-dimensional kyphosis and thoracic Cobb angle was applied. Patients were divided into 1995-2000 (Period 1, primarily anterior), 2001-2009 (Period 2, early thoracic pedicle screws), and 2010-2015 (Period 3, modern posterior) cohorts. Two-way repeated measures analysis of variance and post-hoc Bonferroni corrections were utilized with P < 0.05 considered significant.

RESULTS

Significant differences were demonstrated. Period 1 had excellent restoration of thoracic kyphosis, which worsened in Period 2 and improved to near Period 1 levels during Period 3. Period 3 had superior thoracic kyphosis restoration compared with Period 2.

CONCLUSION

Although the shift from anterior to posterior approaches in AIS was initially associated with worse thoracic kyphosis restoration, this improved with time. The proportion of patients restored to >20° kyphosis with a contemporary posterior approach has steadily improved to that of the era when anterior approaches were more common.

LEVEL OF EVIDENCE

3.

Posterior Correction Techniques for Adolescent Idiopathic Scoliosis

Adolescent idiopathic scoliosis represents a complex, three-dimensional deformity of the spine. Posterior spinal fusion is commonly performed in severe cases to avoid the long-term adverse sequelae associated with progressive spinal deformity. The goals of spinal fusion include halting the progression of deformity, optimizing spinal balance, and minimizing complications. Recent advances in short-segment spinal fixation have allowed for improved three-dimensional deformity correction. Preoperative planning and assessment of spinal flexibility is essential for successful deformity correction and optimization of long-term outcomes. Judicious use of releases and/or spinal osteotomies may allow for increased mobility of the spine but are associated with increased surgical time, blood loss, and risk of complications. Appreciation of implant design and material properties is critical for safe application of correction techniques. Although multiple reduction techniques have been described, no single technique is optimal for every patient.

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.

Pullout strength of reinserted pedicle screws using the previous entry point and trajectory

PURPOSE

This study compared the biomechanics of reinserted pedicle screws using the previous entry point and trajectory with those of correctly inserted pedicle screws.

METHODS

The study used 18 lumbar vertebrae (L1-6) from three fresh calf spines to insert 6.5 × 40-mm pedicle screws. A control screw was inserted correctly along the axis of one pedicle, while an experimental screw was reinserted completely using the previous entry point and trajectory in the other pedicle. The experimental screw was removed after being completely inserted in group A and after 80% of the total trajectory inserted in group B. And the experimental screw was removed after 60% of the total trajectory was reached in group C. The biomechanical values of the pedicle screws were measured.

RESULTS

There were no significant differences in pedicle screw axial pullout strength between reinserted screws and correct screws in the 3 groups (P_A = 0.463, P_B = 0.753, P_C = 0.753). Stiffness measurement increased for the reinserted screw compared with that of the control screw. Fracturing was observed between the vertebral body and pedicle.

CONCLUSION

Theoretically, a surgeon can remove the pedicle screw when necessary, inspect the trajectory, and reinsert the screw using the previous entry point and trajectory.