Toward a better understanding of direct vertebral rotation for AIS surgery: development of a multisegmental biomechanical model and factors affecting correction

Direct Vertebral Rotation (DVR) Does Not Improve Clinical and Radiological Results Compared to Differential Rod Contouring (DRC) in Patients Treated Surgically for Idiopathic Scoliosis

Direct vertebral rotation (DVR) is the most widespread method to correct axial vertebral rotation. Differential rod contouring (DRC) also includes derotation, but not to the same extent as DVR. DVR requires additional surgical effort with potential consequences, which are absent in DRC; moreover, the data concerning the clinical benefits of apical derotation are not convincing. In the present study, clinical and radiological outcomes were compared in patients who underwent surgery for adolescent idiopathic scoliosis (AIS), having DVR and DRC vs. DRC only. In total, 73 AIS patients with curves of 40-85°, consecutively operated on by one surgeon, participated in this study and were followed up over 2 years. Scores from the SRS-22 questionnaire were analysed, the angles of trunk rotation (ATR) were measured with an inclinometer and a radiographic assessment of coronal and sagittal spinal profiles was conducted. In 38 cases, only DRC was performed, and in 35 DRC was performed and followed by DVR; the groups did not differ from an epidemiological point of view. Total SRS-22 scores after 2 years were similar in both groups (4.23 (±0.33) in DRC vs. 4.06 (±0.33) in DRC/DVR, p = 0.1). In all components of SRS-22, the differences were minor, with p being way above 0.05. The mean ATR in the DRC/DVR group was slightly smaller (8 ± 4°) than that of the DRC group (10 ± 5°), p = 0.16. Radiographic analysis did not show significant differences. The coronal curve was corrected by 66 ± 12% for DRC and 63 ± 15% for DVR, p = 0.28. Thoracic kyphosis in the DRC/DVR group increased by 1°, whereas in the DRC group the average kyphosis increased by 5° with a p value of 0.07. The complication rates were similar in both groups. This investigation did not show any advantages of the combination of DRC and DVR in scoliosis correction over DRC only, both radiologically and clinically, yet it affected intraoperative parameters, extending the operation time with only a minor increase in blood loss.

Efficacy and Safety of Direct Vertebral Rotation in the Surgical Correction of Scoliosis: A Meta-Analysis

OBJECTIVE

To compare outcomes of scoliosis correction with direct vertebral rotation (DVR) and without DVR (no direct vertebral rotation [N-DVR]) through meta-analysis.

METHODS

MEDLINE and Embase databases were searched from the earliest available date of indexing through April 10, 2018, for studies evaluating outcomes of DVR and N-DVR in scoliosis. Two researchers performed the data extraction independently. Any discrepancies were resolved by a consensus.

RESULTS

Seven comparative studies were identified. There were no significant differences between DVR and N-DVR for apical vertebral translation, thoracic kyphosis, lumbar lordosis, coronal balance, sagittal balance, complication rate, and preoperative major Cobb angles. DVR was more effective than N-DVR for correction of thoracic Cobb angle (P = 0.02, weighted mean difference = -3.46° [95% confidence interval, -5.57°, -1.35°]), lumbar Cobb angle (P < 0.0001, weighted mean difference = -4.37° [95% confidence interval, -6.31°, -2.42°]), and apical vertebral rotation (P < 0.0001, weighted mean difference = -3.28° [95% confidence interval, -4.85°, -1.72°]).

CONCLUSIONS

This meta-analysis showed that postoperative thoracic Cobb angle, postoperative lumbar Cobb angle, and postoperative apical vertebral rotation were better with DVR compared with N-DVR. Further large multicenter studies will be necessary to substantiate these results.

Are we simplifying balance evaluation in adolescent idiopathic scoliosis?

BACKGROUND

Clinical evaluation of the postural balance in adolescent idiopathic scoliosis has been measured by sagittal vertical axis and frontal balance. The impact of the scoliotic deformity in three planes on balance has not been fully investigated.

METHODS

47 right thoracic and left lumbar curves adolescent idiopathic scoliosis and 10 non-scoliotic controls were registered prospectively. 13 spinopelvic postural parameters were calculated from the 3-dimantional reconstructions of X-rays. 7 balance variables describing the position and sway of the center of pressure were recorded using a pressure mat. A regression analysis was used to predict sagittal vertical axis and frontal balance from the 7 balance variables. A canonical correlation analysis was performed between all the postural parameters and balance variables and the significant associations between the postural and balance variables were determined.

FINDINGS

sagittal vertical axis and frontal balance were not significantly associated with the position or sway of the center of pressure (p>0.05). Canonical correlation analysis showed significant associations between the postural variables in the 3 planes and center of pressure position (R²=0.81) and sway (R²=0.62), p<0.05.

INTERPRETATION

Frontal Cobbs, apical rotations, distal kyphosis, pelvic incidence, sacral slope, sagittal vertical axis, and frontal balance contributed to the postural balance in the cohort. The compensatory role of the pelvis and distal kyphosis in sagittal plane was underlined. Multidimensional analyses between the postural and balance variables showed the alignment of the thoracic, lumbar, and pelvis in the 3 planes, in addition to the global head-pelvic position impact on adolescent idiopathic scoliosis balance.

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.

The impact of direct vertebral rotation (DVR) on radiographic outcome in surgical correction of idiopathic scoliosis

INTRODUCTION

Recent developments of spinal instruments allow to address nearly all components of idiopathic scoliosis. Direct vertebral rotation (DVR) maneuver was introduced to correct apical axial vertebral rotation. It is however still not well established how efficiently DVR affects results of scoliosis correction. The object of the study was to evaluate en bloc apical vertebral rotation (DVR) and its impact on coronal and sagittal correction of the spine in patients undergoing surgical scoliosis treatment.

MATERIALS AND METHODS

Thirty-six consecutive patients who underwent posterior spinal fusion with pedicle screws only constructs for idiopathic scoliosis. Fifteen patients (20 curves) were corrected by rod derotation only and 21 patients (26 curves) had both rod derotation and DVR. Curve measurements were performed on x-rays obtained before and postoperatively-coronal curves, kyphosis (T2-T12, T5-T12). Spine flexibility was assessed on prone bending x-rays. Apical axial rotation was determined on CT scans obtained intraoperatively and postoperatively. Rotation angle (RAsag) was measured according to Aaro and Dahlborn.

RESULTS

We observed reduction of RAsag in all patients; however, in DVR group, decrease was greater, by 31.8% comparing to non-DVR group, by 8.6% (p = 0.0003). Mean coronal correction in DVR group was 68.8% and in rod derotation group without DVR 55% (p = 0.002). No significant correlation was found between degree of derotation obtained and coronal correction. In DVR group T2-T12 kyphosis has increased in 28 (65%) patients whereas in non-DVR group in 31 (69%) cases. Mean value of T2-T12 kyphosis growth was 16.7% in DVR and 22.1% in non-DVR group. These differences however did not occur statistically significant.

CONCLUSIONS

Direct vertebral rotation (DVR) maneuver reduces significantly apical rotation of the spine, enhances ability of coronal correction, and it does not reduce thoracic kyphosis.