The apical vertebral rotational correction of the lumbosacral fractional curve: implications for coronal alignment in degenerative lumbar scoliosis

Introduction

Scoliosis is an abnormal lateral curvature of the spine (1), the existing pathology is a three-dimensional spinal deformity combining sagittal, coronal, and horizontal or axial dimensions. All the abnormal planes should be paid attention since each plane is important and interacts with each other. Degenerative scoliosis as a result of progressive degeneration of structural spinal elements leading to spinal column malalignment has been reported ranges from 2% to 32% in the adult population (2,3). Each year, 266 million people (3.63%) worldwide suffer from degenerative lumbar scoliosis (DLS), with Europe having the highest estimated incidences (4). DLS has become a serious health care concern due to the aging population and the greater emphasis on quality-of-life over cost considerations in current healthcare system. It is not merely a cosmetic condition; it also cause substantial pain and impairment (5).

Initially, surgical deformity correction in patients with DLS highlighted sagittal alignment as a goal priority (6-8). While the growing of the literature about the importance of coronal alignment with health-related quality of life has been widely studied. Correction and maintenance of the coronal alignment played a key role on the proper global alignment to achieve appropriate clinical outcomes (9-16). However, there is one least-mentioned plane that theoretically constitutes a major component of scoliosis termed the forgotten axial plane by Illés et al. (17). This plane was thought to be the first failed plane of the intervertebral disc in the spinal degenerative cascade (17-20). Failure of this plane, especially at the foundation—the lumbosacral junction, leads to misalignment of the other two planes, which strongly relate to health status scores (19,20). Recent research has shown that the development of degenerative scoliosis is linked to the parameters of the spine and pelvis (21,22). Coronal correction of the lumbosacral fractional (LSF) curve provided a significant impact on the postoperative coronal alignment (20,23,24), the apical vertebral rotational correction at this curve is not determined. To the best of our knowledge, limited studies specifically mentioned about the correlation between the amount of axial rotation of vertebrae and global coronal alignment. This study aimed to analyse the correlation between preoperative and postoperative axial apical vertebral rotation (AVR) at the LSF and global coronal alignment in DLS. We present this article in accordance with the STROBE reporting checklist (available at https://jss.amegroups.com/article/view/10.21037/jss-25-152/rc).

Methods

Study design and patient population

This is a retrospective study based on a Phramongkutklao Hospital and College of Medicine database. The study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments. The study was approved by the institutional review board of Phramongkutklao Hospital and College of Medicine (IRB number: R077h/65_Exp; IRBRTA 0298/2566) and informed consent was obtained from all individual participants or their legal guardian(s). All methods were carried out in accordance with relevant guidelines and regulations. Consecutive patients with DLS (Cobb angle >20°) who underwent operative management between January 2017 and January 2022 were corrected. Patients with true DLS (type 1 in Aebi’s classification) who underwent primary spinal deformity correction, instrumented fusion via a posterior-only approach and minimum 2-year follow-up met the inclusion criteria. The exclusion criteria were fusion levels equal to or less than three, a history of hip or knee arthroplasty, and an absolute discrepancy in leg length >20 mm.

Surgical techniques

All patients underwent an open posterior only standard approach technique by dual surgeons. Pedicle screws were placed bilaterally at each level of the construct. The number of interbody fusion levels depended on the unique pathoanatomic characteristics of each patient such as the curve type, global alignment status, etc. An interbody cage filled with harvested local bone was unilaterally inserted on the concave side of the LSF intended to correct the L4 and L5 vertebral body tilt. Contralateral side, the index intervertebral disc space was released and filled with the harvested bone graft. 42 patients were fused to the pelvis [S2-alar-iliac (S2AI)] (29.1%), while 63 patients were fused to S1 (43.7%). Fifteen patients were used iliac accessory kickstand rods supplementally (10.4%). To achieve more deformity correction, spinal osteotomies grade 2 according to Schwab’s classification (25) were performed in all patients.

Data collection and radiographic analysis

The demographic data, the direction of the thoracolumbar/lumbar curve apex, the orientation of L4 coronal tilt, radiographic parameters including coronal balance distance (CBD), spinopelvic parameters [pelvic tilt (PT), sacral slope (SS), pelvic incidence (PI)], main thoracolumbar/lumbar and LSF Cobb angles, the amount of the fractional apical vertebral rotation (fAVR), and L4 and L5 tilt angles were independently performed by two fellowship-trained spine surgeons who were blinded to each other’s results and to clinical data. All measurements were obtained preoperatively and at 6 weeks, 1 year, and 2 years postoperatively using standing full-length spine radiographs analyzed with radiographic software (Figures 1,2). Coronal malalignment was classified into three types based on the Bao classification (9). In addition to type C, the direction of L4 coronal tilt to the C7 plumbline was further classified it into two types, according to Zhang et al. (26).

Figure 1 Sequential radiographic evaluation showing measurement of fAVR at the lumbosacral fractional curve before and after surgery. (A) Pre-operative standing AP full-length spine radiograph. (B) 6-week postoperative radiograph. (C) 1-year postoperative radiograph. (D) 2-year postoperative radiograph. (E) Measurement of fAVR and L4/L5 tilt angles at 2 years. ΔfAVR, change in fractional apical vertebral rotation; AP, anteroposterior; fAVR, fractional apical vertebral rotation; L, left.

Figure 2 Pre- and postoperative full-length spine radiographs illustrating apical vertebral rotation assessment and instrumentation planning. (A) Pre-operative radiograph analyzed with digital software. (B) Post-operative radiograph. (C) Pre-operative planning image showing vertebral edge (large red circle), pedicle area (orange dotted circle), and planned screw positions (small red circles). (D) Post-operative image showing final screw placement which red circle indicate planned pedicle screw positions.

The L4 and L5 vertebras were referred to as the apical rotational vertebras on the LSF (18,27). To improve the accuracy of the apical vertebral rotational measurement, we used a digital software tool (Surgimap, Nemaris Inc., New York, NY, USA) (28). The fAVR was measured by a digital anteroposterior (AP) standing full-length spine radiograph both preoperatively and during the 6-week postoperative period (Figure 2).

Statistical analysis

Statistical analysis was performed using SPSS software version 26. We conducted descriptive analysis on the clinical and radiological variables. All values were measured as mean and standard deviation. The Pearson correlation coefficients were used to test the association between the correction of fractional curve rotation and the correction of Cobb angles, global coronal alignment, as well as the L4 and L5 tilt angles. Levels of association strengths were classified according to Evan’s classification (very weak r<0.2, weak r=0.2–0.39, moderate r=0.40–0.59, strong r=0.60–0.79, very strong r>0.80). According to the Bao classification, subgroup analysis was done to analyse the effect of the fAVR on global coronal alignment.

Results

Demographic data

A total of 144 patients were included in this study. The mean follow-up was 27.1±10.2 months. Thirty patients were males (20.5%) and 114 patients were females (79.5%). The average age was 67±6.9 years. The mean PI was 53.1°±9.6°, with an equal distribution of low-PI (<50°) and high-PI (≥50°) patients. According to the Bao classification, most of the patients were classified as type A (118 patients, 81.9%), followed by type C (19 patients, 13.1%) and type B (7 patients, 4.8%) (Table 1).

Radiographic parameters

Radiographic analysis revealed that the mean fAVR was 12.7°±4.3°. The mean PI was 53.1°±9.6°, the mean PT was 27.1°±7.9°, the mean LL was 31.8°±15.2°, the mean Cobb angle of the main curve was 27.8°±3.6°, the mean Cobb angle of the LSF was 17.8°, and the mean coronal distance was 2.4 cm.

Relationships between fAVR and coronal alignment

The ΔfAVR was strongly correlated with the correction of coronal alignment (r=0.6; P<0.05) and moderately correlated with the correction of Cobb angle and L4 and L5 tilt angles (r=0.51, 0.417 and 0.403 consecutively; P<0.05) (Table 2). On the contrary, we found no correlation between the fAVR and other pelvic parameters such as PI, PT, and SS.

The subgroup analysis revealed a strong correlation between ΔfAVR and the correction of coronal alignment in low-PI patients, and a moderate correlation in high-PI patients. According to the Bao classification, type C is the type showing the highest correlation between the ΔfAVR and the correction of coronal alignment, followed by type B and type A (r=0.87, 0.656 and 0.506 respectively, P<0.05) (Figure 3).

Figure 3 The subgroup analysis of the correlation between fAVR correction and the correction of coronal alignment according to Bao classification: Bao A (blue), Bao B (red), and Bao C (yellow). AVR, apical vertebral rotation; fAVR, fractional apical vertebral rotation.

Discussion

DLS typically results from asymmetrical degeneration of intervertebral discs coupled with arthrosis of facet joint complexes. Theoretically, the first failure in the spinal plane is a horizontal or axial plane, followed by a coronal and/or sagittal plane, depending on the severity of intervertebral disc degeneration (17). Consequently, the rotatory subluxation of multiple lumbar functional spinal units developed and progressed, leads to various pathoanatomical appearance and a myriad of symptoms (18). In general, the curves in DLS are composed of a main curve and a fractional compensatory curve. The fractional curve usually located at the lumbosacral junction (L4–S1) often presents as a compensatory curve; some however, it can present as the main curve, leads to proximal thoracolumbar/lumbar compensatory curve. The LSF in DLS have a significant impact on the pre- and postoperative coronal alignments, which affect clinical outcomes (23-25). Comparison to other curve locations, the residual LSF was associated with the poorest self-reported scores in patients with prior surgery (9).

A study by Illés et al. showed that axial plane deformity is very important. They found that the degree of axial plane deformity seems to be the best way to predict the outcome of scoliosis, besides the coronal and/or sagittal views (17). Karam et al. showed that the AVR in adolescent idiopathic scoliosis (AIS) patients caused the coronal malalignment measured by frontal odontoid-hip axis angle (OD-HA) (29). Although the pathogenesis of AIS and DLS are different, this study emphasized the importance of axial plane deformity to avoid the progression of global coronal malalignment. The multivariate analysis in their study showed that the frontal OD-HA angle was determined mainly by the AVR, not by the severity of the Cobb angle or the location of spinal segment deformity. However, the combination of the high grade of axial vertebral rotation and the distal location of structural scoliosis made AIS patients suffer a greater degree of frontal malalignment. Recently, Zuckerman et al. emphasized that in adult spinal deformity (ASD) patients, the LSF curve was often opposite and harder to correct than the maximum coronal Cobb angle, especially in the cases of coronal malalignment and combined malalignment. LSF curve correction was more pronounced in Bao type C patients. A higher LSF curve required more transforaminal lumbar interbody fusions (TLIFs) to improve correction. Both the LSF curve and Cobb angle influenced radiographic outcomes, but neither significantly impacted clinical complications or patient-reported outcomes (16). Therefore, the axial deformity is strongly associated with global coronal malalignment.

There are several methods described various technique to evaluate the vertebral rotation and other parameters (22). This study, we intend to use digital AP standing full-length spine radiographs because by performing on patients in a weight-bearing position, it allows for the analysis of compensatory mechanisms that represent the true shape and position of the vertebrae in a functional standing position. In addition, this investigation is quite simple and reachable both by clinicians and patients without high radiation exposure. A novel method in this current study for estimating three-dimensional AVR using a two-dimensional coronal Cobb angle demonstrated high reliability as in a previous study (19).

Our study found that the amount of ΔfAVR was strongly correlated with the correction of coronal alignment (r=0.6; P<0.05) but moderately correlated with the correction of L4 and L5 tilt angles (r=0.417, 0.403 respectively; P<0.05). This finding could imply that the correction of the global coronal malalignment was found to be determined by the axial rather than the frontal correction. The global coronal balance control was found to be related to the severity of the axial vertebral rotation at the lumbosacral junction as a spinal segment foundation. Type C, which is one of the risk factors for postoperative coronal imbalance in ASD (30), had the strongest correlation between correction of the axial rotational curve at the LSF curve and correction of the coronal alignment. The second strong correlation is type B and type A is the third with r values of 0.87, 0.656, and 0.506, P<0.05 respectively. Therefore, these findings highlight the necessity to emphasize on the apical axial vertebral rotation of the LSF curve, to restore or avoid the global coronal malalignment (Figures 2,3).

According to our findings, correcting the change in fAVR (ΔfAVR) plays a crucial role in improving global coronal alignment (Figure 4). However, no significant correlations were found between fAVR and other pelvic parameters such as PT, or SS, implying that the impact of axial rotation correction is more localized to the spinal alignment rather than the spinopelvic parameters. The correction of coronal malalignment in DLS correlates to the correction of apical vertebral rotational at the LSF curve. With gravity, the spine is evenly distributed in three dimensions. Correcting one plane, the axial rotation, might affects the other plane through the coupling mechanism. When axial rotation is corrected, it alters the orientation and load distribution on these structures, which in turn influences curvature in the sagittal plane and alignment in the coronal plane (29). Consequently spontaneous resolution of the compensatory curve, even though some degrees, to keep the overall global balance (31,32).

Figure 4 A digital AP standing full-length spine was used to illustrate coronal balancing following apical vertebral rotation correction using the kickstand rod technique; the images are preoperative (A) and 2-week postoperative (B). AP, anteroposterior; L, left.

Interbody fusion with or without posterior fixation is surgical advance for spinal deformity correction. Nowadays, no one technique is superior to the others. The comparable fusion outcome and ability to correct deformity inconclusive so far to which interbody fusion technique is better (33). We corrected the LSF curves by inserting polyetheretherketone (PEEK) cages at the concave side by TLIF technique and releasing as well as filling packed locally bone graft at the contralateral side. The packed bone graft could correct the abnormal intervertebral planes at the index spinal level by ligamentotaxis with the spacer. This gives rise to stable intervertebral disc space vertically (33). While usage of TLIFs provided better correction of the AVR at LSF curves (20), additional surgical strategies such as usage of iliac accessory kickstand rods to improve more adequately correct LSF curves and balance correction of lumbosacral and major thoracolumbar curves were proven to provide efficiency as demonstrated in our study.

This study has several limitations. First, the sample size is relatively small, which could affect the statistical power of subgroup analyses. Additionally, the follow-up period was limited to 6 weeks postoperatively, and longer-term outcomes of AVR correction and global coronal alignment were not assessed. The use of digital radiographic software for measuring AVR, while reliable, may have introduced measurement variability, as manual input is still required. Future studies with larger patient cohorts, multi-center participation, and extended follow-up are needed to validate these findings and further explore the clinical impact of AVR correction in DLS patients. Although this study demonstrated strong correlations between ΔfAVR and radiographic parameters of coronal alignment, clinical outcomes such as Oswestry Disability Index (ODI), Scoliosis Research Society-22 (SRS-22), or Visual Analog Scale (VAS) scores were not analyzed due to incomplete data across the cohort. In a subset of 58 patients with available ODI and SRS-22 data, improvements in ΔfAVR tended to correlate with better ODI and SRS-22 function subscores, although these did not reach statistical significance (P=0.08 and P=0.10, respectively). Therefore, the functional significance of improving ΔfAVR remains undetermined. Future prospective studies incorporating standardized patient-reported outcome measures are necessary to determine whether correction of ΔfAVR translates into sustained improvements in function and quality of life.

Conclusions

This is the study to demonstrate a correlation between the AVR at the LSF curve and the global coronal alignment in DLS patients. We found that the ΔfAVR is associated with the improvement of the global coronal alignment. This study revealed the influence of axial alignment on the spinal global balance and emphasized that axial alignment is the foundation of spinal global correction. Further research should focus in greater detail on the clinical relevance of these findings.

Acknowledgments

None.

Footnote

Reporting Checklist: The authors have completed the STROBE reporting checklist. Available at https://jss.amegroups.com/article/view/10.21037/jss-25-152/rc

Data Sharing Statement: Available at https://jss.amegroups.com/article/view/10.21037/jss-25-152/dss

Peer Review File: Available at https://jss.amegroups.com/article/view/10.21037/jss-25-152/prf

Funding: None.

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://jss.amegroups.com/article/view/10.21037/jss-25-152/coif). The authors have no conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. The study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments. The study was approved by the institutional review board of Phramongkutklao Hospital and College of Medicine (IRB number: R077h/65_Exp; IRBRTA 0298/2566) and informed consent was obtained from all individual participants or their legal guardian(s).

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

References

1. The American Heritage dictionary of the English language. 4th edition. Boston, MA, USA: Houghton Mifflin; 2000. Available online: https://search.library.wisc.edu/catalog/999898536102121
2. Aebi M. The adult scoliosis. Eur Spine J 2005;14:925-48. [Crossref] [PubMed]
3. Kotwal S, Pumberger M, Hughes A, et al. Degenerative scoliosis: a review. HSS J 2011;7:257-64. [Crossref] [PubMed]
4. Ravindra VM, Senglaub SS, Rattani A, et al. Degenerative Lumbar Spine Disease: Estimating Global Incidence and Worldwide Volume. Global Spine J 2018;8:784-94. [Crossref] [PubMed]
5. Ploumis A, Transfledt EE, Denis F. Degenerative lumbar scoliosis associated with spinal stenosis. Spine J 2007;7:428-36. [Crossref] [PubMed]
6. Lafage V, Schwab F, Patel A, et al. Pelvic tilt and truncal inclination: two key radiographic parameters in the setting of adults with spinal deformity. Spine (Phila Pa 1976) 2009;34:E599-606. [Crossref] [PubMed]
7. Daubs MD, Lenke LG, Bridwell KH, et al. Does correction of preoperative coronal imbalance make a difference in outcomes of adult patients with deformity? Spine (Phila Pa 1976) 2013;38:476-83. [Crossref] [PubMed]
8. Glassman SD, Berven S, Bridwell K, et al. Correlation of radiographic parameters and clinical symptoms in adult scoliosis. Spine (Phila Pa 1976) 2005;30:682-8. [Crossref] [PubMed]
9. Bao H, Yan P, Qiu Y, et al. Coronal imbalance in degenerative lumbar scoliosis: Prevalence and influence on surgical decision-making for spinal osteotomy. Bone Joint J 2016;98-B:1227-33. [Crossref] [PubMed]
10. Plais N, Bao H, Lafage R, et al. The clinical impact of global coronal malalignment is underestimated in adult patients with thoracolumbar scoliosis. Spine Deform 2020;8:105-13. [Crossref] [PubMed]
11. Ploumis A, Simpson AK, Cha TD, et al. Coronal Spinal Balance in Adult Spine Deformity Patients With Long Spinal Fusions: A Minimum 2- to 5-Year Follow-up Study. J Spinal Disord Tech 2015;28:341-7. [Crossref] [PubMed]
12. Chapman TM Jr, Baldus CR, Lurie JD, et al. Baseline Patient-Reported Outcomes Correlate Weakly With Radiographic Parameters: A Multicenter, Prospective NIH Adult Symptomatic Lumbar Scoliosis Study of 286 Patients. Spine (Phila Pa 1976) 2016;41:1701-8. [Crossref] [PubMed]
13. Buell TJ, Smith JS, Shaffrey CI, et al. Multicenter assessment of surgical outcomes in adult spinal deformity patients with severe global coronal malalignment: determination of target coronal realignment threshold. J Neurosurg Spine 2021;34:399-412. [Crossref] [PubMed]
14. Jiang Z, Liu Z, Li C, et al. The correction range of lumbosacral curve vertebral body tilt in degenerative scoliosis for achieving postoperative coronal balance. BMC Musculoskelet Disord 2025;26:401. [Crossref] [PubMed]
15. Ransom SC, Pennington Z, Brown NJ, et al. Assessing the Fractional Curve for Proper Management of Adult Degenerative Scoliosis. Neurospine 2024;21:458-73. [Crossref] [PubMed]
16. Zuckerman SL, Chanbour H, Hassan FM, et al. The Lumbosacral Fractional Curve vs Maximum Coronal Cobb Angle in Adult Spinal Deformity Patients with Coronal Malalignment: Which Matters More? Global Spine J 2024;14:1968-77. [Crossref] [PubMed]
17. Illés TS, Lavaste F, Dubousset JF. The third dimension of scoliosis: The forgotten axial plane. Orthop Traumatol Surg Res 2019;105:351-9. [Crossref] [PubMed]
18. Kobayashi T, Atsuta Y, Takemitsu M, et al. A prospective study of de novo scoliosis in a community based cohort. Spine (Phila Pa 1976) 2006;31:178-82. [Crossref] [PubMed]
19. Faraj SSA, Boselie TFM, Vila-Casademunt A, et al. Radiographic Axial Malalignment is Associated With Pretreatment Patient-Reported Health-Related Quality of Life Measures in Adult Degenerative Scoliosis: Implementation of a Novel Radiographic Software Tool. Spine Deform 2018;6:745-52. [Crossref] [PubMed]
20. Theologis AA, Lertudomphonwanit T, Lenke LG, et al. The role of the fractional lumbosacral curve in persistent coronal malalignment following adult thoracolumbar deformity surgery: a radiographic analysis. Spine Deform 2021;9:721-31. [Crossref] [PubMed]
21. Murata Y, Takahashi K, Hanaoka E, et al. Changes in scoliotic curvature and lordotic angle during the early phase of degenerative lumbar scoliosis. Spine (Phila Pa 1976) 2002;27:2268-73. [Crossref] [PubMed]
22. Ferrero E, Lafage R, Diebo BG, et al. Tridimensional Analysis of Rotatory Subluxation and Sagittal Spinopelvic Alignment in the Setting of Adult Spinal Deformity. Spine Deform 2017;5:255-64. [Crossref] [PubMed]
23. Campbell PG, Nunley PD. The Challenge of the Lumbosacral Fractional Curve in the Setting of Adult Degenerative Scoliosis. Neurosurg Clin N Am 2018;29:467-74. [Crossref] [PubMed]
24. Liu H, Li Z, Helil B, et al. Matching correction of main and compensatory curves is critical for immediate postoperative coronal balance in correction of severe adult idiopathic scoliosis. Eur Spine J 2021;30:3233-42. [Crossref] [PubMed]
25. Bourghli A, Boissière L, Konbaz F, et al. On the pedicle subtraction osteotomy technique and its modifications during the past two decades: a complementary classification to the Schwab's spinal osteotomy classification. Spine Deform 2021;9:515-28. [Crossref] [PubMed]
26. Zhang J, Wang Z, Chi P, et al. Directionality of Lumbosacral Fractional Curve Relative to C7 Plumb Line, A Novel Index Associated with Postoperative Coronal Imbalance in Patients with Degenerative Lumbar Scoliosis. Spine (Phila Pa 1976) 2021;46:366-73. [Crossref] [PubMed]
27. Nathan H. Osteophytes of the Vertebral Column: An Anatomical Study of Their Development According to Age, Race, and Sex with Considerations as to Their Etiology and Significance. The Journal of Bone & Joint Surgery 1962;44:243-68.
28. Eijgenraam SM, Boselie TF, Sieben JM, et al. Development and assessment of a digital X-ray software tool to determine vertebral rotation in adolescent idiopathic scoliosis. Spine J 2017;17:260-5. [Crossref] [PubMed]
29. Karam M, Ghanem I, Vergari C, et al. Global malalignment in adolescent idiopathic scoliosis: the axial deformity is the main driver. Eur Spine J 2022;31:2326-38. [Crossref] [PubMed]
30. Barile F, Ruffilli A, Paolucci A, et al. Risk factors for postoperative coronal imbalance after surgical correction of adult spinal deformities: a systematic review with pooled analysis. J Neurosurg Spine 2023;38:558-72. [Crossref] [PubMed]
31. Hasegawa K, Dubousset JF. Cone of Economy with the Chain of Balance-Historical Perspective and Proof of Concept. Spine Surg Relat Res 2022;6:337-49. [Crossref] [PubMed]
32. Guo X, Guo Z, Li W, et al. Scoliosis in dysplastic spondylolisthesis: a clinical survey of 50 young patients. BMC Musculoskelet Disord 2022;23:335. [Crossref] [PubMed]
33. Choi T, Moghamis IS, Alhammoud A, et al. The effectiveness of interbody fusion devices in adult spine deformity. Seminars in Spine Surgery 2022;34:100990.