Is L5/S1 interbody fusion necessary with concurrent iliac fixation for long segment spinal fusion constructs?—a systematic review of biomechanical studies

Introduction

There are many known complications of multilevel spinal fusion constructs, one of which is pseudarthrosis (1-4). In the setting of long segment fusions in the thoracolumbar spine, pseudarthrosis often occurs at the lumbosacral junction (LSJ) due to the significant biomechanical forces at the base of the construct (5). L5–S1 interbody fusion is often performed to provide anterior column support and improve fusion rates (6,7). Biomechanical studies have shown L5–S1 interbody fusion and anterior column support improve fusion rates by decreasing S1 screw strain (8-12). More recently, with improved iliac fixation techniques, it is common to further stabilize the LSJ with iliac or S2-alar-iliac screws in multilevel fusions over 4 levels (11,13). However, with both pelvic fixation and L5–S1 interbody fusion achieving a similar goal of decreasing S1 screw strain, recent clinical studies have suggested interbody fusion may not be necessary in the presence of iliac fixation to achieve successful fusion in long constructs (5).

This review sought to answer the following question: Is the mantra for L5–S1 interbody “anterior column support” a relic of the pre-iliac fixation era? Specifically, (I) what are the biomechanical studies demonstrating the advantage of concurrent L5–S1 interbody fusion and iliac fixation? (II) How does iliac fixation compare to L5–S1 interbody fusion in stabilizing the L5–S1 segment. We present this article in accordance with the PRISMA reporting checklist (available at https://jss.amegroups.com/article/view/10.21037/jss-25-132/rc).

Methods

A systematic review of MEDLINE via the PubMed database (available to public) and EMBASE was performed by two independent reviewers (M.T. and A.J.S.) using Covidence software (https://www.covidence.org/) to identify all relevant cadaveric studies that have been published in English since 1996, addressing the biomechanical advantage/disadvantage of L5/S1 fusion in long construct fusion. The MeSH search terms used were as follows: “Lumbosacral Junction”, “Long Fusion”, “Biomechanics”, “Adult Spinal Deformity”, “Long Fusion to Sacrum”, “Sacropelvic Fixation”, “Lumbar Interbody Fusion”, “Long Constructs”, “Iliac Screw”, “Anterior Lumbar Interbody Fusion”, “Transforaminal Lumbar Interbody Fusion”, “Iliac Fixation” and “Pelvic Fixation”. The Boolean operator “AND” was used to maximize the sensitivity of the search. Each search term was used in conjunction with “AND” and “Biomechanics”. Titles and abstracts were screened for relevance.

Exclusion and inclusion criteria

The PRISMA protocol was followed for data selection and analysis (Figure 1) (14). Inclusion criteria were as follows: (I) human cadaveric studies that had undergone multilevel spinal fusion; (II) fusion comprising the L5–S1 junction; (III) studies that measured biomechanical differences with and without iliac fixation and interbody fixation at L5–S1 (Figure 2). Exclusion criteria were as follows: (I) technical notes, case reports, literature reviews, finite element analyses, human studies and non-human cadaveric studies; (II) pathologies different from adult spinal deformity (ASD) such as osteoporosis/osteopenia, infection, tumoral entities, ankylosing spondylosis, achondroplasia, etc.; (III) constructs less than 4 levels; (IV) studies that focused on spinal levels separate from the L5–S1 junction; (V) studies not focused on biomechanical outcomes.

Figure 1 PRISMA diagram depicting study selection process following PRISMA guidelines.

Figure 2 Illustrative depiction of L5–S1 interbody cage (A) versus interbody cage with pelvic fixation (B) versus pelvic fixation alone (C).

Data extraction

The following data were summarized from each publication: author; country of origin; year of publication; surgical procedure information (type of procedure, number of fused levels); interbody cage type; study design. Descriptive statistics of biomechanical outcomes such as: load bearing, range of motion (ROM), flexion, extension, lateral bending, axial rotation, rod strain, sacral screw strain and compression were used as the main findings/outcomes from the selected articles.

Both reviewers (M.T. and A.J.S.) independently screened abstracts and titles after removing 141 duplicates and 13 abstracts. A total of 22 full-text articles were selected for a thorough read-through, and after reviewing references from the articles, a total of 7 articles were included in the final analysis. Disagreements were resolved by a third independent researcher (J.D.L.).

Results

There were seven cadaveric studies that met our inclusion/exclusion threshold. The PRISMA diagram for how studies were selected for the analysis is shown in Figure 1. The main outcomes focused on in this study were ROM at the L5–S1 joint, S1 screw strain and posterior rod strain. The results are summarized below in Table 1.

Iliac screws versus interbody cage

All seven included studies compared iliac screw fixation with L5–S1 interbody cage placement. When assessing S1 screw strain or L5–S1 ROM, 3 studies showed iliac fixation was superior to interbody cages, 4 studies showed iliac fixation and interbody cages to be equivocal, and no studies showed interbody cages were superior to iliac fixation. Cunningham et al. found that both iliac screws and anterior lumbar interbody fusion (ALIF) decreased S1 screw strain, however, iliac screws were more effective (P<0.05). The authors recommended either iliac fixation or interbody placement for long segment constructs with UIV L2 or above (8). Hlubek et al. found no difference between in S1 screw strain between iliac fixation alone versus ALIF, but observed increased screw strain with TLIF alone. Iliac screw fixation alone was found to increase rod strain compared to the ALIF cohort. ALIF with pedicle screws showed comparable S1 screw strain as iliac fixation with TLIF, however, there was decreased rod strain in the ALIF group (9). Khashan et al. found that ALIF alone was inferior to iliac fixation in terms of S1 screw strain and lumbosacral ROM (15). Kleck et al. found that iliac fixation decreases S1 screw strain but increases rod strain. ALIF further decreases rod strain, however, this effect is maximally seen in sagittal motion, but not in axial motion (12). Sutterlin et al. found that iliac fixation reduces S1 screw strain more than TLIF, but comparably to AxialLIF placement (11). Fleischer et al. found that AxialLIF and iliac fixation were equivalent in S1 screw strain reduction, and approximately double the reduction seen in ALIF (10). Lee et al. found no difference in L5–S1 ROM between ALIF and iliac fixation, although SI motion increased with ALIF (6).

Iliac screws with interbody cage

Five of the seven included studies assessed the use of iliac screw fixation concurrently with L5–S1 interbody cage placement. When assessing S1 screw strain or L5–S1 ROM, all 5 studies showed no statistically significant improvement with concurrent iliac and interbody fixation, although 4 studies reported a trend towards decreased L5–S1 strain or ROM. Hlubek et al. found that ALIF cage decreased iliac screw-induced rod strain when added to iliac fixation (P<0.02), opposed to TLIF cage placement. There was a trend toward decreased S1 screw strain, but did not reach statistical significance (9). Khashan et al. found that addition of ALIF to iliac fixation further decreased S1 screw strain, but did not reach statistical significance, concluding that ALIF does not replace the need for iliac fixation (15). Kleck et al. found that addition of TLIF to iliac fixation increased rod strain, however, the addition of ALIF provided two times decreased rod strain compared to TLIF (P<0.01) (12). Sutterlin et al. showed that AxialLIF with iliac fixation resulted in the greatest reduction of S1 screw strain, but did not reach statistical significance (11). Lee et al. found that addition of ALIF to iliac fixation decreased L5–S1 ROM and added stability in T4-pelvis constructs only, but not in T10-pelvis of L2-S1 constructs (6).

Discussion

L5–S1 interbody fusion has long been recommended at the LSJ in the setting of long spinal deformity construct for “anterior column support”. This was particularly relevant with long instrumented fusions ended at S1. With the advent of modern iliac fixation, recent studies have suggested that L5–S1 interbody fusion may not be necessary for successful arthrodesis (5). Moreover, the addition of L5–S1 interbody fusion comes at the additional cost of operating room time, implant costs, and potentially a separate anterior approach if an ALIF is performed. The purpose of this study is to assess the biomechanical evidence for concurrent L5–S1 interbody cage placement and iliac fixation.

In this systematic review, 7 human biomechanical studies were identified which studied lumbosacral biomechanics and compared interbody fixation, iliac fixation, and both interbody and iliac fixation. When comparing interbody fixation to iliac fixation alone, 3 studies showed iliac fixation was superior to interbody fixation, 4 studies showed iliac fixation and interbody fixation to be equivocal, and no studies showed interbody fixation was superior to iliac fixation. Five of seven studies compared concurrent interbody fixation and iliac fixation to iliac fixation alone. When assessing S1 screw strain or L5–S1 ROM, all 5 studies showed no statistically significant improvement with concurrent iliac and interbody fixation, although 4 studies reported a trend towards decreased L5–S1 strain or ROM. When assessing posterior rod strain, iliac fixation was noted to significantly increase rod strain compared to ALIF. When ALIF was combined with iliac fixation rod strain significantly decreased compared to iliac fixation alone. Interestingly, in both studies that reported a decrease in rod strain after adding ALIF to a construct with IF, Hlubek et al. and Kleck et al., the decrease in strain completely counteracted the observed IF-related increase in strain, in all directions (9,12).

In biomechanical studies of the spine, particularly in cadaveric studies, it is important to understand how ROM, and biomechanical forces such as screw strain and rod strain are measured. Studies primarily utilized uniaxial strain gauges placed on rods and screws to measure screw and rod strain with different forces (8-12). Strain gauges were utilized in slightly different ways, for example Kleck et al. uses epoxy resins with an independent testing protocol (12), while Sutterlin et al. recapitulated the methodology of Fleischer et al. (10,11). Axial loading was introduced in different ways, with Khashan et al. using a custom “sliding ring” cable-driven system to replicate axial loading conditions (15), while Hlubek et al. loaded specimens with 400N pure compression following bending tests (9). For flexibility testing, the majority of studies used the pure-moment flexibility testing protocol, or a very similar protocol. While many similarities in testing methodology between studies, subtle differences in both the application of biomechanical forces on the cadaveric construct, as well as recording of measurements, may account for some variation in testing results.

A recent systematic review of human clinical studies by Cavagnaro et al. identified 12 studies with a total of 1,216 patients long constructs crossing the LSJ (5). They found that iliac fixation decreased risk of pseudarthrosis, while interbody fixation did not. The results of this study are in agreement with this, as there does not appear to be a significant biomechanical difference on L5–S1 ROM or S1 screw strain with concurrent iliac fixation compared to iliac fixation alone. Prior studies have additionally shown that motion across the L5–S1 segment is reduced with ALIF placement or sacroiliac joint fusion in the setting of long segment fusions, however, these studies did not directly compare this reduction in motion to iliac fixation (16-18). From these cadaveric studies, it does appear that ALIF may reduce L5–S1 ROM more than SI joint fusion, however, a direct competitive study is warranted.

There are several limitations to this study that warrant discussion. Several different types of interbody fixation were studied in the articles identified in this review. Three studies used ALIF, while two studies compared ALIF and TLIF, and two studies compared ALIF and axial lumbar interbody fusion. While the purpose of this review was not to compare the biomechanical properties of different types of interbody fixation, ALIF tended to be more effective in stabilizing the LSJ than TLIF. Hlubek et al. found that ALIF with pedicle screws was comparable to TLIF with iliac screws in decreasing S1 screw strain. The methods of quantifying the biomechanics of the LSJ are somewhat heterogenous across the studies identified. More rigorous biomechanical and clinical studies are needed to clarify particular scenarios where interbody fixation, iliac fixation, or both, are needed. As the current study focused on cadaveric experiments, clinically relevant parameters such as health-related quality of life (HRQOL) measures, pain and other patient-reported outcomes (PRO) could not be assessed from the current scope. Finally, this study is focused on the biomechanics of the different fixation strategies. Two clinical benefits of interbody fusion not assessed in these studies are the increased surface area for arthrodesis and ability to correct deformities through the disc space.

Achieving successful arthrodesis at the LSJ is a key goal of multilevel spinal fusion. Iliac fixation and interbody fixation are both strategies to decreased S1 screw strain and L5–S1 ROM, which can in turn increase the rate of successful arthrodesis. The results of this study suggest that from a biomechanical perspective, the addition of interbody fixation does not add significant biomechanical stability to iliac fixation alone. In our practice, interbody fixation at L5–S1 is included typically if the following are needed: a wide decompressive laminectomy for stenosis, a posterior column osteotomy for deformity correction, or if a hyperlordotic ALIF is needed for lordosis correction through the disc space.

Conclusions

Based on the current biomechanical literature, ALIF and IF are both similar in their reduction in ROM about the L5–S1 joint, as well as S1 screw strain. There does not appear to be significant biomechanical benefit of concurrent ALIF and IF. However, the benefit of reduced rod strain and potentially lower instrumentation failure rate must be considered in the setting of longer surgical operations with more potential adverse surgical outcomes.

Acknowledgments

The authors acknowledge Ms. Armstrong-Davies for her illustrative work for this manuscript.

Footnote

Reporting Checklist: The authors have completed the PRISMA reporting checklist. Available at https://jss.amegroups.com/article/view/10.21037/jss-25-132/rc

Peer Review File: Available at https://jss.amegroups.com/article/view/10.21037/jss-25-132/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-25-132/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.

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. Kim HJ, Yang JH, Chang DG, et al. Adult Spinal Deformity: Current Concepts and Decision-Making Strategies for Management. Asian Spine J 2020;14:886-97. [Crossref] [PubMed]
2. Good CR, Auerbach JD, O'Leary PT, et al. Adult spine deformity. Curr Rev Musculoskelet Med 2011;4:159-67. [Crossref] [PubMed]
3. Lee CS, Hwang CJ, Lee SW, et al. Risk factors for adjacent segment disease after lumbar fusion. Eur Spine J 2009;18:1637-43. [Crossref] [PubMed]
4. Nishimura Y, Hara M, Nakajima Y, et al. Outcomes and complications following posterior long lumbar fusions exceeding three levels. Neurol Med Chir (Tokyo) 2014;54:707-15. [Crossref] [PubMed]
5. Cavagnaro MJ, Orenday-Barraza JM, Khan N, et al. Is L5/S1 interbody fusion necessary in long-segment surgery for adult degenerative scoliosis? A systematic review and meta-analysis. J Neurosurg Spine 2022;36:997-1004. [Crossref] [PubMed]
6. Lee BS, Walsh KM, Healy AT, et al. Biomechanics of L5/S1 in Long Thoracolumbosacral Constructs: A Cadaveric Study. Global Spine J 2018;8:607-14. [Crossref] [PubMed]
7. Edwards CC 2nd, Bridwell KH, Patel A, et al. Long adult deformity fusions to L5 and the sacrum. A matched cohort analysis. Spine (Phila Pa 1976) 2004;29:1996-2005. [Crossref] [PubMed]
8. Cunningham BW, Sefter JC, Hu N, et al. Biomechanical comparison of iliac screws versus interbody femoral ring allograft on lumbosacral kinematics and sacral screw strain. Spine (Phila Pa 1976) 2010;35:E198-205. [Crossref] [PubMed]
9. Hlubek RJ, Godzik J, Newcomb AGUS, et al. Iliac screws may not be necessary in long-segment constructs with L5-S1 anterior lumbar interbody fusion: cadaveric study of stability and instrumentation strain. Spine J 2019;19:942-50. [Crossref] [PubMed]
10. Fleischer GD, Kim YJ, Ferrara LA, et al. Biomechanical analysis of sacral screw strain and range of motion in long posterior spinal fixation constructs: effects of lumbosacral fixation strategies in reducing sacral screw strains. Spine (Phila Pa 1976) 2012;37:E163-9. [Crossref] [PubMed]
11. Sutterlin CE 3rd, Field A, Ferrara LA, et al. Range of motion, sacral screw and rod strain in long posterior spinal constructs: a biomechanical comparison between S2 alar iliac screws with traditional fixation strategies. J Spine Surg 2016;2:266-76. [Crossref] [PubMed]
12. Kleck CJ, Illing D, Lindley EM, et al. Strain in Posterior Instrumentation Resulted by Different Combinations of Posterior and Anterior Devices for Long Spine Fusion Constructs. Spine Deform 2017;5:27-36. [Crossref] [PubMed]
13. Finger T, Bayerl S, Onken J, et al. Sacropelvic fixation versus fusion to the sacrum for spondylodesis in multilevel degenerative spine disease. Eur Spine J 2014;23:1013-20. [Crossref] [PubMed]
14. Page MJ, Moher D, Bossuyt PM, et al. PRISMA 2020 explanation and elaboration: updated guidance and exemplars for reporting systematic reviews. BMJ 2021;372: [Crossref] [PubMed]
15. Khashan M, Camisa W, Berven S, et al. Stand-alone anterior interbody fusion for substitution of iliac fixation in long spinal fixation constructs. Arch Orthop Trauma Surg 2018;138:479-86. [Crossref] [PubMed]
16. McGrath KA, Schmidt ES, Loss JG, et al. Assessment of L5-S1 anterior lumbar interbody fusion stability in the setting of lengthening posterior instrumentation constructs: a cadaveric biomechanical study. J Neurosurg Spine 2022;36:900-8. [Crossref] [PubMed]
17. Mushlin H, Brooks DM, Olexa J, et al. A biomechanical investigation of the sacroiliac joint in the setting of lumbosacral fusion: impact of pelvic fixation versus sacroiliac joint fixation. J Neurosurg Spine 2019;31:562-7. [Crossref] [PubMed]
18. Mushlin HM, Shea P, Brooks DM, et al. The Effect of Sacroiliac Fusion and Pelvic Fixation on Rod Strain in Thoracolumbar Fusion Constructs: A Biomechanical Investigation. Spine (Phila Pa 1976) 2021;46:E769-75. [Crossref] [PubMed]