A comparative assessment of anterior column realignment surgery at 1, 2 and 3 levels

Introduction

Linked with a poor quality of life, adult spinal deformity (ASD) is prevalent in up to 68% of the adult population across the USA (1-6). Approximately 27.5 million people in the United States have been affected by ASD and associated spinal pathologies. One of the most prevalent spinal pathologies associated with ASD that impact quality of life is low back pain, especially in elderly patient populations. Currently, 8% of the elderly population experiences back pain that limits their ability to perform daily life activities (5,6). With the advancements in medicine associated with increased life expectancy, there will likely be an increase in the elderly population. Therefore, it is essential to determine effective surgical treatments to treat adults with ASD, to improve their ability to perform daily activities and quality of life.

The spine sagittal alignment is crucial for maintaining a healthy posture and preventing ASD. Imbalance can lead to excessive load on certain spinal segments that can deteriorate sagittal alignment, requiring surgical intervention to restore the alignment. The anterior column realignment (ACR) is a technique that can be used to restore the sagittal alignment. As ACR is a relatively new technique, the current data on it is sparse and more research is needed to ensure better clinical outcomes. Currently, ACR surgery is commonly performed with 30-degree (30°) hyper-lordotic cage at one level. However, the number of index segments on which ACR should be performed is not fully studied. Different constructs that are explored in this study include two cages of 15-degree (15°) at two levels, or three cages of 10-degree (10°) at three levels. Assessing the biomechanics of the spine, and stress on the implants is necessary to prevent surgical failure, ensure the long-term success of the surgery, and improve patient satisfaction.

The objective of the current study was to explore ACR surgery using three techniques with different lordotic cage sizes. The study compared a 30° hyper-lordotic cage at one level, versus two cages of 15° at two levels, or three cages of 10° at three levels.

Methods

Intact spine model

In this study, ACR simulations were conducted using a previously validated thoracolumbar model (T8-Pelvis). The initial model’s geometry was obtained from computed tomography (CT) scans of a healthy 55-year-old adult male (7-10). Utilizing the CT data, the 3D model’s geometry was reconstructed in MIMICS (Materialize Inc., Leuven, Belgium), and subsequent meshing was performed using IAFE-MESH (University of Iowa, Iowa) and HyperMesh (Altair Engineering, Michigan, USA) software. The Abaqus (Dassault Systemes, Simulia Inc., Providence, RI, USA) software was used to assemble the hard and soft tissues of the model and accordingly material properties were assigned to each component of the model. The pelvis and vertebrae’s cancellous bone were enveloped by an outer layer of cortical bone, each with a thickness of 1 and 0.5 mm, respectively. The intervertebral disc consisted of nucleus pulposus and annulus ground substances with embedded fibers. For ligaments representation, truss elements were used in the Abaqus software.

FE modeling of ACR procedure

To perform ACR surgery in Abaqus at the L3–L4 level, the lamina of the L3 was removed along with the index segment disc and anterior longitudinal ligament (ALL) (11-14). Subsequently, hyperlordotic cage of 30° was placed at the index segment and attached to the respective vertebrae using a “TIE” constraint in the Abaqus software. Following cage insertion, pedicle screws were inserted into the (T10-Pelvis) vertebrae for the posterior rod fixation (13). The dimensions for the pedicle screws are summarized in Table S1. Following pedicle screw insertion, a 5 mm rod was passed through the screw heads. Similarly, ACR procedures were performed at two levels (L3–L5) and three levels (L2–L5) using 15° and 10° lordotic cages, respectively (Figures 1,2). The specific dimensions for each cage type used in the simulations were as follows: 30° cage had an anterior height of 17.25 mm, a posterior height of 9.5 mm, and a length of 50 mm; 15° cage had an anterior height of 12.75 mm, a posterior height of 8 mm, and a length of 50 mm; and the 10° cage had an anterior height of 10.6 mm, a posterior height of 8 mm, and a length of 50 mm. All the cages had a rectangular footprint.

Figure 1 Model for the 30° hyper lordotic cage and the pedicle screw used at the L3–L4 segment.

Figure 2 Side view of multi-level ACR models. (A) L3–L4 (1-level ACR). (B) L3–L5 (2-level ACR) and (C) L2–L5 (3-level ACR). ACR, anterior column realignment.

Material properties

The material properties of all the components of the FE model were acquired from the literature and are summarized in Table S2 (7-10).

Loads and boundary conditions

The encastred boundary condition was used in Abaqus software to fix the pelvis in all the models. All the models were subjected to a compressive load of 300 N load at the thoracic region and 400 N at lumbar levels followed by a 7.5 Nm pure moment applied to the T8 caudal endplate to simulate flexion/extension, lateral bending, and axial rotation (11,12).

Ethical considerations

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 the University of Toledo. The computational model was developed by the informed consent from the subject involved in the study.

Results

Range of motion (ROM)

Instrumented segments (T10–S1) ROM

For L3–L5 (2-level ACR) compared to L3–L4 (1-level ACR), there is a percentage decrease in ROM for flexion by approximately 12.9%, extension by 12.7%, left bending (LB) by 26.3%, right bending (RB) by 26.3%, left rotation (LR) by 26.9%, and right rotation (RR) by 26.9% (Figure 3). This indicates a reduction in the overall ROM with the addition of an extra adjacent level undergoing fixation.

Figure 3 Comparison of instrumented segments (T10–S1) ROM following L3–L4 (1-level), L3–L5 (2-level) and L2–L5 (3-level) ACR surgery with bilateral rod construct. ACR, anterior column realignment; LB, left bending; LR, left rotation; RB, right bending; ROM, range of motion; RR, right rotation.

Similarly, when comparing L2–L5 (3-level ACR) to L3–L4 (1-level ACR), there is a percentage decrease in ROM for flexion by about 17.1%, extension by 16.9%, LB by 21.1%, RB by 21.1%, LR by 18.5%, and RR by 18.5%. These results suggest a further reduction in global ROM with the extension of adjacent segment arthroplasty to three levels.

Adjacent segment (T9–T10) ROM

When comparing L3–L5 (2-level ACR) to L3–L4 (1-level ACR), the percentage differences in ROM are minimal, with flexion, extension, LB, RB, LR, and RR showing changes of less than 1%. This suggests that the addition of an extra adjacent level undergoing arthroplasty has a marginal impact on the adjacent segments ROM. Similarly, comparing L2–L5 (3-level ACR) to L3–L4 (1-level ACR), the percentage differences in ROM remain modest, with variations of less than 1% across all loading conditions (Figure 4).

Figure 4 Comparison of adjacent segment (T9–T10) ROM following L3–L4 (1- level), L3–L5 (2-level) and L2–L5 (3-level) ACR surgery with bilateral rod construct. ACR, anterior column realignment; LB, left bending; LR, left rotation; RB, right bending; ROM, range of motion; RR, right rotation.

Maximum von Mises stress on rods

In flexion loading conditions, the percentage differences reveal a decrease in stress for both L3–L5 (2-level ACR) and L2–L5 (3-level ACR) compared to L3–L4 (1-level ACR), with reductions of approximately 2.3% and 5%, respectively. Extension loading conditions exhibit similar trends, with reductions of 6.1% and 8% for L3–L5 (2-level ACR) and L2–L5 (3-level ACR), respectively. LB and RB also demonstrate stress reductions, with percentage differences ranging from 1.1% to 4.3%. However, LR and RR show a slight increase in stress for L3–L5 (2-level ACR) and L2–L5 (3-level ACR) (Figure 5).

Figure 5 Comparison of maximum stress on rods following L3–L4 (1-level), L3–L5 (2-level) and L2–L5 (3-level) ACR surgery with bilateral rod construct. ACR, anterior column realignment; LB, left bending; LR, left rotation; RB, right bending; RR, right rotation.

Stress on adjacent segment annulus (T9–T10)

In flexion loading conditions, the percentage differences indicate marginal variations in stress, with a slight increase of approximately 0.2% for L3–L5 (2-level ACR) and 0.3% for L2–L5 (3-level ACR) compared to L3–L4 (1-level ACR). Similarly, in extension loading conditions, the percentage differences range from 0.8% to 0.9%, indicating a modest increase in stress for the adjacent levels. LB and RB loading conditions also show marginal increases in stress, with percentage differences ranging from 0.6% to 0.8%. Notably, LR and RR exhibit slightly larger percentage differences, ranging from 0.9% to 1.1% (Figure 6).

Figure 6 Comparison of stress on annulus of (T9–T10) following L3–L4 (1-level), L3–L5 (2-level) and L2–L5 (3-level) ACR surgery with bilateral rod construct. ACR, anterior column realignment; LB, left bending; LR, left rotation; RB, right bending; RR, right rotation.

Intradiscal pressure on adjacent segment nucleus (T9–T10)

In flexion, the percentage differences in pressure are minimal, with a decrease of approximately 0.1% for L3–L5 (2-level ACR) and L2–L5 (3-level ACR) compared to L3–L4 (1-level ACR). In extension, LB, and RB loading conditions, the percentage differences range from 0.7% to 1.2%, indicating marginal variations in pressure for adjacent levels undergoing arthroplasty. Notably, LR and RR loading conditions exhibit slightly larger percentage differences, ranging from 0.5% to 0.7% (Figure 7).

Figure 7 Comparison of intradiscal pressure on T9–T10 nucleus following L3–L4 (1-level), L3–L5 (2-level) and L2–L5 (3-level) ACR surgery with bilateral rod construct. ACR, anterior column realignment; LB, left bending; LR, left rotation; RB, right bending; RR, right rotation.

Comparison of maximum load (N) at the index segment(s)

For L1–L2, the introduction of ACR at L3–L5 (2-level ACR) results in a significant decrease of approximately 21.3% and extending ACR to L2–L5 (3-level ACR) leads to a further reduction of about 17.1%. In the L2–L3 segment, ACR at L3–L5 (2-level ACR) demonstrates a considerable decrease of approximately 31.7%, while ACR at L2–L5 (3-level ACR) shows a notable increase of approximately 19.1%. Notably, the L3–L4 segment experiences a substantial increase in maximum load of approximately 26.6% with ACR at L3–L5 (2-level ACR), while ACR at L2–L5 (3-level ACR) maintains a similar increase of around 26.3%. In contrast, the L4–L5 and L5–S1 segments display significant increases in maximum load, ranging from approximately 49.4% to 59.3%, with the addition of adjacent segment arthroplasty at multiple levels.

Discussion

This study compared ACR surgery at one level (L3–L4) using a hyperlordotic cage of 30° vs. performing ACR at two levels (L3–L5) and three levels (L2–L5) with 15° and 10° cages, respectively.

Currently, ACR surgery is mostly performed with a single level hyper-lordotic cage of 30° at the index segment (10,15-18). This procedure includes laminectomy, resection of the ALL, and placement of a hyperlordotic cage (1,19). The restoration of sagittal alignment can also be restored by performing two level (L2–L3) ACR with 15° cage at each level or by performing three level (L2–L5) ACR with 10° cage at each level. Besides patients’ anatomy and physiology, the choice of surgical approach is also dependent on the surgeons training and expertise. However, currently there are no studies that have compared the influence of performing ACR on one level vs. multiple levels. The current study aimed to perform a comparative analysis of one level (L3–L4) ACR with two levels (L3–L5) and three levels (L2–L5) ACR procedures.

Assessing the data obtained between the one, two and three level models (Table S3), it was shown that during flexion-extension, the global ROM is reduced in the two level (L3–L5) and the three level (L2–L5) ACR models, when comparing to the one level (L3–L4) model. Additionally, there is a marginal increase in the ROM in the adjacent segments in the two level (L3–L5) and the three level (L2–L5) ACR models, when comparing to the one level ACR model.

The stress on the rods exhibits a notable decrease in the two level (L3–L5) ACR model, which can be attributed to the utilization of two cages with a 15° angle, as opposed to a single 30° cage at the L3–L4 level. This reduction in stress on the rods is further pronounced in the three level ACR model, where three cages with a 10° angle each are employed.

Nevertheless, as shown in Table 1, with the incorporation of additional cages in the two level (L3–L5) and the three level (L2–L5) ACR models, there is a consequential increase in the load on the L5–S1 segment, escalating by 55% and 59%, respectively compared to the one level (L3–L4) construct.

The comparative analysis of one level (L3–L4), two level (L3–L5) and three level (L2–L5) ACR models assessed the biomechanical implications of multi-level surgeries. While the reduction in rod stress was evident in the two and the three level ACR models, an increase in load on the L5–S1 segment was observed, raising concerns about potential degeneration and other post-surgical complications. Increased load is one of the most common causes of intervertebral disc herniation is due to degenerative processes. These findings highlight the need for careful consideration when opting for multi-level ACR procedures, as this results in an increase in load on the lower lumber spine. This increased load is associated with higher risks of disc herniation and adjacent segment degeneration.

The limitations of the current study include that cyclic loading was not simulated, thus the time for implant failure could not be predicted. Other limitations in the models in this study include the lack of paraspinal musculature, but this was compensated through applying compressive follower loads to mimic the muscle contractions and effect of body weight on the construct. Future studies should aim to incorporate detailed musculature within models and delve into dynamic testing of the implant constructs to predict the time for rod failure.

Furthermore, the material properties were uniform and simplified in this study. The interactions of different components in the FE model were also simplified. The simplification of using a ‘TIE’ constraint underestimates the micromotion that can occur at the bone-cage interface. Furthermore, the absence of anterior/lateral fixation in the current study may overestimate the load supported by the rods and posterior elements of the spinal column. While the addition of anterior/lateral fixation would likely lower the magnitude of load on these components, the overall trend in stress results may remain consistent. A more nuanced approach considering the complex material behaviors and intricate interactions among model components would enhance the fidelity of biomechanical simulations. Future research could explore advanced modeling techniques to better represent the diverse properties of spinal tissues and implants.

Future work in this field could expand the scope of investigation by exploring the impact of different material properties on rod stress. By systematically varying material characteristics, researchers could gain a more comprehensive understanding of how material selection influences the biomechanics of rod constructs. Additionally, considering patients with osteoporotic bone presents another avenue for further exploration. Investigating the response of spinal fixations in the context of osteoporosis would provide valuable insights into the challenges and considerations specific to this patient population. Additionally, the reliance on a single finite element model limits the generalizability of our findings; future studies should incorporate multiple patient-specific models to enhance clinical translatability.

The current study consistently modeled the ACR procedure with ALL resection and subsequent cage insertion. However, in clinical practice, ALL resection may not be necessary for lower lordotic cages (10° and 15°), particularly when placed via a lateral approach. Therefore, future studies should investigate the biomechanical implications of different cage placements and the impact of ALL preservation versus resection.

Conclusions

This study provides important insights into the biomechanical behavior of rod constructs and offers a comparative analysis of three different configurations, with a focus on the effects of ACR. The findings demonstrate that increasing the number of ACR levels leads to a progressive reduction in ROM across instrumented segments, highlighted by a 12.9% reduction in flexion with 2-level ACR and 17.1% with 3-level ACR. Corresponding decreases in rod stress during flexion and extension, along with modest changes in lateral bending and axial rotation, suggest improved segmental stability with minimal impact on adjacent segments. These results support the biomechanical advantage of multi-level ACR in enhancing spinal stability, which may have meaningful implications for treating ASD. Addressing the limitations of the current study and pursuing future investigations will be essential to validate these findings and further refine ACR strategies for clinical application.

Acknowledgments

None.

Footnote

Data Sharing Statement: Available at https://asj.amegroups.com/article/view/10.21037/asj-25-37/dss

Peer Review File: Available at https://asj.amegroups.com/article/view/10.21037/asj-25-37/prf

Funding: This work was partially funded by a United States National Science Foundation (NSF) Industry/University Cooperative Research Center called the Center for Disruptive Musculoskeletal Innovations (CDMI) (IIP-1916629) at the University of California at San Francisco, CA, the Ohio State University, Columbus, OH, and the University of Toledo, Toledo, OH (www.nsfcdmi.org).

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://asj.amegroups.com/article/view/10.21037/asj-25-37/coif). During the editing of this article, V.K.G. has passed away. The authors feel sorry for this loss. H.E. serves as an unpaid editorial board member of AME Surgical Journal from October 2025 to September 2027. The other 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 the University of Toledo. The computational model was developed by the informed consent from the subject involved in the study.

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. Diebo BG, Shah NV, Boachie-Adjei O, et al. Adult spinal deformity. Lancet 2019;394:160-72. [Crossref] [PubMed]
2. Glassman SD, Bridwell K, Dimar JR, et al. The impact of positive sagittal balance in adult spinal deformity. Spine (Phila Pa 1976) 2005;30:2024-9. [Crossref] [PubMed]
3. Soroceanu A, Burton DC, Oren JH, et al. Medical Complications After Adult Spinal Deformity Surgery: Incidence, Risk Factors, and Clinical Impact. Spine (Phila Pa 1976) 2016;41:1718-23. [Crossref] [PubMed]
4. Godzik J, Haglin JM, Alan N, et al. Retrospective Multicenter Assessment of Rod Fracture After Anterior Column Realignment in Minimally Invasive Adult Spinal Deformity Correction. World Neurosurg 2019;130:e400-5.
5. Safaee MM, Ames CP, Smith JS. Epidemiology and Socioeconomic Trends in Adult Spinal Deformity Care. Neurosurgery 2020;87:25-32. [Crossref] [PubMed]
6. Kanter AS, Shaffrey CI, Mummaneni P, et al. Introduction: Adult spinal deformity: pathophysiology and corrective measures. Neurosurg Focus 2014;36. Introduction. [Crossref] [PubMed]
7. Kumaran Y, Shah A, Katragadda A, et al. Iatrogenic muscle damage in transforaminal lumbar interbody fusion and adjacent segment degeneration: a comparative finite element analysis of open and minimally invasive surgeries. Eur Spine J 2021;30:2622-30. [Crossref] [PubMed]
8. Shah A, Lemans JV, Zavatsky J, et al. Spinal balance/alignment—clinical relevance and biomechanics. Journal of Biomechanical Engineering 2019;141:070805. [Crossref] [PubMed]
9. Palepu V. Biomechanical effects of initial occupant seated posture due to rear end impact injury. The University of Toledo; 2013.
10. Mumtaz M, Mendoza J, Vosoughi AS, et al. A Comparative Biomechanical Analysis of Various Rod Configurations Following Anterior Column Realignment and Pedicle Subtraction Osteotomy. Neurospine 2021;18:587-96. [Crossref] [PubMed]
11. Patwardhan AG, Havey RM, Carandang G, et al. Effect of compressive follower preload on the flexion-extension response of the human lumbar spine. J Orthop Res 2003;21:540-6. [Crossref] [PubMed]
12. Seyed Vosoughi A, Joukar A, Kiapour A, et al. Optimal satellite rod constructs to mitigate rod failure following pedicle subtraction osteotomy (PSO): a finite element study. Spine J 2019;19:931-41. [Crossref] [PubMed]
13. Godzik J, Pereira BA, Newcomb AGUS, et al. Optimizing biomechanics of anterior column realignment for minimally invasive deformity correction. Spine J 2020;20:465-74. [Crossref] [PubMed]
14. Halm H. Pedicle subtraction osteotomy for correction of congenital scoliokyphosis. Eur Spine J 2011;20:995-6. [Crossref] [PubMed]
15. La Barbera L, Wilke HJ, Liebsch C, et al. Biomechanical in vitro comparison between anterior column realignment and pedicle subtraction osteotomy for severe sagittal imbalance correction. Eur Spine J 2020;29:36-44. [Crossref] [PubMed]
16. Januszewski J, Beckman JM, Harris JE, et al. Biomechanical study of rod stress after pedicle subtraction osteotomy versus anterior column reconstruction: A finite element study. Surg Neurol Int 2017;8:207. [Crossref] [PubMed]
17. Kim SM, Kim YC, Kim KT, et al. Surgical sequence in anterior column realignment with posterior osteotomy is important for degree of adult spinal deformity correction: advantages and indications for posterior to anterior sequence. BMC Musculoskelet Disord 2022;23:1004. [Crossref] [PubMed]
18. Berjano P, Cecchinato R, Sinigaglia A, et al. Anterior column realignment from a lateral approach for the treatment of severe sagittal imbalance: a retrospective radiographic study. Eur Spine J 2015;24:433-8. [Crossref] [PubMed]
19. Xu DS, Paluzzi J, Kanter AS, et al. Anterior Column Release/Realignment. Neurosurg Clin N Am 2018;29:427-37. [Crossref] [PubMed]