Correction of rotational deformity and restoration of thoracic kyphosis are inversely related in posterior surgery for adolescent idiopathic scoliosis

The three-dimensional coupling mechanism in scoliosis and its consequences for correction

INTRODUCTION

In idiopathic scoliosis, the anterior spinal column has rotated away from the midline and has become longer through unloading and expansion of the intervertebral discs. Theoretically, extension of the spine in the sagittal plane should provide room for this longer anterior spinal column, allowing it to swing back towards the midline in the coronal and axial plane, thus reducing both the Cobb angle and the apical vertebral rotation.

METHODS

In this prospective experimental study, ten patients with primary thoracic adolescent idiopathic scoliosis (AIS) underwent MRI (BoneMRI and cVISTA sequences) in supine as well as in an extended position by placing a broad bolster, supporting both hemi-thoraces, under the scoliotic apex. Differences in T4-T12 kyphosis angle, coronal Cobb angle, vertebral rotation, as well as shape of the intervertebral disc and shape and position of the nucleus pulposus, were analysed and compared between the two positions.

RESULTS

Extension reduced T4-T12 thoracic kyphosis by 10° (p < 0.001), the coronal Cobb angle decreased by 9° (p < 0.001) and vertebral rotation by 4° (p = 0.036). The coronal wedge shape of the disc significantly normalized and the wedged and lateralized nucleus pulposus partially reduced to a more symmetrical position.

CONCLUSION

Simple extension of the scoliotic spine leads to a reduction of the deformity in the coronal and axial plane. The shape of the disc normalizes and the eccentric nucleus pulposus partially moves back to the midline.

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.

Patient and surgical predictors of 3D correction in posterior spinal fusion: a systematic review

BACKGROUND

Restoration of three-dimensional (3D) alignment is critical in correcting patients with adolescent idiopathic scoliosis using posterior spinal fusion (PSF). However, current studies mostly rely on 2D radiographs, resulting in inaccurate assessment of surgical correction and underlying predictive factors. While 3D reconstruction of biplanar radiographs is a reliable and accurate tool for quantifying spinal deformity, no study has reviewed the current literature on its use in evaluating surgical prognosis.

PURPOSE

To summarize the current evidence on patient and surgical factors affecting sagittal alignment and curve correction after PSF based on 3D parameters derived from reconstruction of biplanar radiographs.

METHODS

A comprehensive search was conducted by three independent investigators on Medline, PubMed, Web of Science, and Cochrane Library to obtain all published information on predictors of postoperative alignment and correction after PSF. Search items included "adolescent idiopathic scoliosis," "stereoradiography," "three-dimensional," "surgical," and "correction." The inclusion and exclusion criteria were carefully defined to include clinical studies. Risk of bias was assessed with the Quality in Prognostic Studies tool, and level of evidence for each predictor was rated with the Grading of Recommendations, Assessment, Development, and Evaluations approach. 989 publications were identified, with 444 unique articles subjected to full-text screening. Ultimately, 41 articles were included.

RESULTS

Strong predictors of better curve correction included preoperative normokyphosis (TK > 15°), a corresponding rod contour, intraoperative vertebral rotation and translation, and upper and lower instrumented vertebrae selected based on sagittal and axial inflection points. For example, for Lenke 1 patients with junctional vertebrae above L1, fusion to NV-1 (1 level above the neutral vertebra) achieved optimal curve correction while preserving motion segments. Pre-op coronal Cobb angle and axial rotation, distal junctional kyphosis, pelvic incidence, sacral slope, and type of instrument were identified as predictors with moderate evidence. For Lenke 1C patients, > 50% LIV rotation was found to increase spontaneous lumbar curve correction. Pre-op thoracolumbar apical translation and lumbar lordosis, Ponte osteotomies, and rod material were found to be predictors with low evidence.

CONCLUSIONS

Rod contouring and UIV/LIV selection should be based on preoperative 3D TK in order to achieve normal postoperative alignment. Specifically, Lenke 1 patients with high-lying rotations should be fused distally at NV-1, while hypokyphotic patients with large lumbar curves and truncal shift should be fused at NV to improve lumbar alignment. Lenke 1C curves should be corrected using > 50% LIV rotation counterclockwise to the lumbar rotation. Further investigation should compare surgical correction between pedicle-screw and hybrid constructs using matched cohorts. DJK and overbending rods are potential predictors of postoperative alignment.

Digital measurement and correlation analysis of coronal and sagittal anatomic parameters in the radiographs of adolescent patients with idiopathic scoliosis

OBJECTIVE

To use digital software to measure the morphologic and anatomic parameters of adolescent idiopathic scoliosis (AIS). Differences and correlations among different parameters were compared to provide an anatomic basis for the selection of treatment methods and preoperative evaluation of AIS.

METHODS

Spinal radiographs were taken from 300 boys and girls (age, 10-18 years) suffering from idiopathic scoliosis in four grade-A hospitals in Inner Mongolia. After screening, 120 cases with complete imaging data were assessed. Imaging data were transferred to a work station (Dr Wise™). The anatomic indices of the Cobb Angle, CVA, AVT, TS, CA, CPT, CSI, FPT, CCA, TK, LL, SS, PT, and PI were measured.

RESULTS

There were significant differences in AVT between different grades and types of scoliosis (F = 34.079, P = 0.000; χ² = 23.379, P = 0.000). AVT was a protective factor, and the smaller the AVT, the less severe was the scoliosis. Compared with adolescents with mild or moderate scoliosis, the Cobb angle of adolescents with severe scoliosis was negatively correlated with CCA, LL, and SS (r =  - 0.641, p < 0.05; r =  - 0.695, p < 0.01; r =  - 0.814, p < 0.01).

CONCLUSIONS

Some of the anatomic parameters in the coronal and sagittal planes of adolescents with idiopathic scoliosis were significantly different according to the severity and type of scoliosis. Significant correlations were found between more anatomic indices in adolescents with severe scoliosis than in adolescents suffering from mild or moderate scoliosis.

Convex-concave and anterior-posterior spinal length discrepancies in adolescent idiopathic scoliosis with major right thoracic curves versus matched controls

PURPOSE

The apical deformation in adolescent idiopathic scoliosis (AIS) is a combination of rotation, coronal deviation and passive anterior lengthening of the spine. In AIS surgery, posterior-concave lengthening or anterior-convex shortening can be part of the corrective maneuver, as determined by the individual surgeon's technique. The magnitude of convex-concave and anterior-posterior length discrepancies, and how this needs to be modified to restore optimal spinal harmony, remains unknown.

METHODS

CT-scans of 80 pre-operative AIS patients with right convex primary thoracic curves were sex- and age-matched to 80 healthy controls. The spinal length parameters of the main thoracic curves were compared to corresponding levels in controls. Vertebral body endplates and posterior elements were semi-automatically segmented to determine the length of the concave and convex side of the anterior column and along the posterior pedicle screw entry points while taking the 3D-orientation of each individual vertebra into account.

RESULTS

The main thoracic curves showed anterior lengthening with a mean anterior-posterior length discrepancy of + 3 ± 6%, compared to a kyphosis of - 6 ± 3% in controls (p < 0.01). In AIS, the convex side was 20 ± 7% longer than concave (0 ± 1% in controls; p < 0.01). The anterior and posterior concavity were 7 and 22 mm shorter, respectively, while the anterior and posterior convexity were 21 and 8 mm longer compared to the controls.

CONCLUSIONS

In thoracic AIS, the concave shortening is more excessive than the convex lengthening. To restore spinal harmony, the posterior concavity should be elongated while allowing for some shortening of the posterior convexity.

The Sagittal Plane in Spinal Fusion for Adolescent Idiopathic Scoliosis

Sagittal balance is widely recognized as the primary determinant of optimal outcomes in adult spinal deformity. In adolescent idiopathic scoliosis (AIS), coronal correction risks being obtained at the expense of sagittal malalignment after posterior spinal fusion. Apical lordosis, often underestimated on two-dimensional imaging, is the primary deforming factor in AIS. Failure to restore thoracic kyphosis and lumbar lordosis during posterior spinal fusion contributes to problematic early surgical complications, including proximal or distal junctional kyphosis and failure. Although adolescent patients often compensate for sagittal imbalance in the short-term and mid-term, late sequelae of iatrogenic sagittal imbalance include flatback syndrome, disk degeneration, cervical kyphosis, and late decompensation. Objective criteria using spinopelvic parameters and preoperative three-dimensional planning can guide sagittal plane correction during PSF for AIS. Technical caveats can help avoid sagittal plane complications, including instrumentation level selection, anchor type, and anatomic protection of adjacent levels. Other surgical techniques to optimize restoration of thoracic kyphosis include higher implant density, stiffer rod material, Ponte osteotomies, and deformity correction technique.

Rod Link Reducer system in adolescent idiopathic scoliosis: a retrospective observational trial

PURPOSE

The Rod Link Reducer (RLR) (Globus Medical, PA, USA) allows direct three-dimensional correction of the spine deformity follows the direct vertebral rotation (DVR) theories. The purpose of this retrospective study is to compare RLR with traditional correction technique (TCT) in two cohorts of patients with adolescent idiopathic scoliosis (AIS).

METHODS

Fifty-four patients (M:F = 1:8) between 2018 and 2020 were included. The first group (n = 22) was treated by RLR while the second one (n = 32) by TCT. All spines were classified as per the Lenke system. Length of hospitalization, days in intensive care unit (ICU), operative time, and blood loss were recorded. SRS-30 and SF-36 questionnaires were administered pre-operative and post-operative. We collected radiological data: pre-operative and post-operative Cobb angles, coronal and sagittal balance, trunk and thoracic height.

RESULTS

RLR and TCT groups are homogeneous in age (p = 0.317), sex ratio (p = 0.347), and Risser stage (p = 0.222). Between both groups there was no significant statistical difference in haemoglobin value, hospitalization length, days in ICU, operative times, SF-36, SRS-30, NRS, and perceived satisfaction. RLR group shows a better improvement of correction of main thoracic (MT) curve (RLR 54.2% ± 15.9%/TCT 38.1% ± 20.4%, p = 0.031). Nevertheless, RLR group shows a worse thoracic kyphosis correction (RLR 16.82° ± 9.13°/27.12° ± 12.13°, p = 0.015).

CONCLUSION

RLR system allows a more effective MT curve correction than TCT systems, but it seems to give a hypokyphosis effect.

Comparison of four correction techniques for posterior spinal fusion in adolescent idiopathic scoliosis

INTRODUCTION

When performing posterior spinal fusion for adolescent idiopathic scoliosis (AIS), it is of major importance to address both coronal and sagittal deformities. Although several techniques have been described, few data exist comparing them. Our objective was to compare four techniques (in situ bending (ISB), rod derotation (RD), cantilever (C) and posteromedial translation (PMT)) for the correction of spinal deformity in AIS including thoracic deformity.

MATERIAL AND METHODS

We conducted a multicenter retrospective study including 562 AIS patients with thoracic deformity with at least 24-month follow-up. Radiographic analysis was performed preoperatively, postoperatively and at last follow-up. The main outcomes were main curve correction and thoracic kyphosis restoration (TK).

RESULTS

Coronal correction rate was significantly different among the four treatment groups (ISB 64% vs C 57% vs RD 55% vs PMT 67%, p < 0.001). Multivariate regression revealed that correction technique did not influence correction rate, whereas implant density, convex side compression and use of derotation connectors did. TK increase was significantly higher in the PMT group (average + 13°) than in DR (+ 3°), while ISB (-3°) and cantilever (-13°) resulted in TK decrease (p < 0.001). Multivariate analysis revealed that TK increase was only influenced by the reduction technique (p < 0.001) and preoperative TK (p < 0.001).

DISCUSSION

The four techniques had the same ability to correct spinal deformity in the coronal plane. Three factors were identified to improve correction rate: implant density, convex compression and use of derotation connectors. On the other hand, PMT was more effective in restoring TK, particularly in hypokyphotic patients.

Comparison of different strategies on three-dimensional correction of AIS: which plane will suffer?

PURPOSE

There are distinct differences in strategy amongst experienced surgeons from different 'scoliosis schools' around the world. This study aims to test the hypothesis that, due to the 3-D nature of AIS, different strategies can lead to different coronal, axial and sagittal curve correction.

METHODS

Consecutive patients who underwent posterior scoliosis surgery for primary thoracic AIS were compared between three major scoliosis centres (n = 193). Patients were treated according to the local surgical expertise: Two centres perform primarily an axial apical derotation manoeuvre (centre 1: high implant density, convex rod first, centre 2: low implant density, concave rod first), whereas centre 3 performs posteromedial apical translation without active derotation. Pre- and postoperative shape of the main thoracic curve was analyzed using coronal curve angle, apical rotation and sagittal alignment parameters (pelvic incidence and tilt, T1-T12, T4-T12 and T10-L2 regional kyphosis angles, C7 slope and the level of the inflection point). In addition, the proximal junctional angle at follow-up was compared.

RESULTS

Pre-operative coronal curve magnitudes were similar between the 3 cohorts and improved 75%, 70% and 59%, from pre- to postoperative, respectively (P < 0.001). The strategy of centres 1 and 2 leads to significantly more apical derotation. Despite similar postoperative T4-T12 kyphosis, the strategy in centre 1 led to more thoracolumbar lordosis and in centre 2 to a higher inflection point as compared to centre 3. Proximal junctional angle was higher in centres 1 and 2 (P < 0.001) at final follow-up.

CONCLUSION

Curve correction by derotation may lead to thoracolumbar lordosis and therefore higher risk for proximal junctional kyphosis. Focus on sagittal plane by posteromedial translation, however, results in more residual coronal and axial deformity.

CORRECTION OF THE SPINAL SAGITTAL PLANE: THE APPLICATION OF CONVERGENT OR DIVERGENT SCREWS

ABSTRACT Objective To present a new principle for correction of the sagittal plane of the spine through the convergent or divergent placement of monoaxial pedicle screws in this plane, associated with compression or distraction, to provide lordotizing or kyphotizing leverage force. Method A statistical mechanical study of twenty-eight fixations in synthetic spine segments was performed. In fifteen pieces, pedicle screws were applied to the ends of the segments with positioning convergent to the center of the fixation. They were attached to the straight rods and subjected to compression force. The other thirteen segments were fixed with pedicle screws in a direction divergent to the center of the fixation, attached to the straight rods, and subjected to distraction force. Results To create kyphosis in the 15 synthetic segments of the spine, the mean pre-fixation Cobb angle was - 0.7° and the mean post-fixation angle was +15°. To create lordosis in the 13 segments, the mean pre-fixation Cobb angle was +1° and the mean post-fixation angle was +18°. The difference was confirmed by statistical mechanical tests and considered significant. However, there is no relevant difference between the mean angles for lordosis and kyphosis formation. Conclusions It was concluded that the correction of the sagittal plane of the spine by applying the new instrumentation method is efficient. A statistical mechanical test confirmed that the difference in Cobb degrees between pre- and post-fixation of the synthetic spine segments was considered significant in the creation of both kyphosis and lordosis. Level of evidence II C; Statistical mechanical study of synthetic spine segments.