Standalone Anterior Lumbar Interbody Fusion L5-S1 for Single-Level Degenerative Disc Disease: Implant Parameters Influencing Radiological Results

Abstract

Introduction Degenerative disc disease at the L5-S1 level is a common condition and is often associated with chronic low back pain and, in some cases, radicular symptoms. Anterior lumbar interbody fusion (ALIF) is intended to restore segmental and global lumbar lordosis (LL), improve sagittal alignment, and achieve decompression. The primary objective of this study was to examine the association between implant-related parameters and postoperative radiological outcomes following standalone L5-S1 ALIF. A secondary exploratory objective was to assess how closely postoperative LL at the L4-S1 and L5-S1 levels matched pelvic incidence-based theoretical alignment targets. These targets were used as a reference framework for sagittal alignment assessment and were calculated using established proportional relationships between pelvic incidence and LL. Methods We conducted a retrospective, single-center observational study based on blinded radiological analysis. Sagittal alignment parameters were assessed using EOS imaging, while interbody fusion was evaluated on one-year postoperative CT scans. A total of 69 adult patients underwent standalone L5-S1 ALIF between January 1, 2017, and January 1, 2023, for single-level degenerative disc disease, without prior spinal instrumentation or deformity. Patients were identified using CCAM (Classification Commune des Actes Médicaux) coding, and eligibility was confirmed through individual chart and imaging review. Results Significant postoperative improvements were observed in L1-S1 lordosis (+4.67°), L4-S1 lordosis (+7.1°), L5-S1 lordosis (+9.0°), foraminal height (+3.25 mm), and lumbar distribution index (LDI) (+8.0%) (all p < 0.05). Postoperative L4-S1 lordosis was close to the theoretical target derived from pelvic incidence (mean difference: -0.7° ± 6.6°, p = 0.738), while overcorrection was noted at L5-S1 and undercorrection at L1-S1 (both p < 0.05). Anterior implant height was significantly associated with L4-S1 correction (p = 0.034), as well as with postoperative LDI and foraminal height. One-year CT follow-up showed an 88% fusion rate. Conclusions Standalone L5-S1 ALIF was associated with improvements in sagittal alignment by increasing both global and segmental lordosis and by facilitating indirect foraminal decompression. Anterior implant height was the implant parameter most consistently associated with postoperative radiological outcomes, followed by posterior placement and implant depth, while implant lordosis itself was not significantly associated with postoperative alignment.

Keywords: anterior lumbar inter-body fusion; degenerative disc disease; implant parameters; l5-s1; lumbar lordosis; sagittal balance.

Figure 1. Parameters of anterior lumbar interbody fusion implants

Height (mm), depth (mm), and lordosis angle (°) (A). Radiological correlation of these parameters at the L5-S1 level on EOS imaging (B)

Figure 2. Pelvic parameters and lumbar lordosis measurements on EOS lateral view

a: pelvic incidence (PI, °); b: pelvic tilt (PT, °); c: sacral slope (SS, °); d: L5-S1 lordosis (°); e: L4-S1 lordosis (°); f: L1-S1 lordosis (°)

Figure 3. Preoperative (A) and postoperative EOS measurements (B)

Pelvic incidence (PI, °); pelvic tilt (PT, °); sacral slope (SS, °); L5-S1 lordosis (°); L4-S1 lordosis (°); L1-S1 lordosis (°)

Figure 4. Flow chart depicting patient selection from initial recruitment to final analysis

CCAM: Classification Commune des Actes Médicaux (French coding); OLIF: oblique lateral interbody fusion; ALIF: anterior lumbar interbody fusion; TLIF: transforaminal lumbar interbody fusion

Figure 5. Changes in LDI for each patient between pre- and post-op

LDI <50% corresponds to malalignment with hypo-lordosis of the caudal segment. LDI between 50% and 80% is considered satisfactory alignment. LDI >80% corresponds to malalignment with hyperlordosis of the caudal segment LDI: lordosis distribution index

Figure 6. Preoperative and postoperative deviations from TTL at L5-S1, L4-S1, and L1-S1

This figure displays box plots illustrating the difference (Δ) between measured lordosis and the TTL at the L5-S1, L4-S1, and L1-S1 levels, both preoperatively and postoperatively. Negative values reflect undercorrection relative to TTL, whereas positive values reflect overcorrection. Δ was calculated as measured lordosis minus TTL, with TTL derived from pelvic incidence-based formulas. The figure highlights the magnitude of correction achieved at each level and shows a progressive decrease in correction efficiency at more proximal lumbar segments TTL: theoretical target lordosis

Figure 7. Box plots illustrating the distribution of implant parameters (anterior height, depth, and implant lordosis) in relation to postoperative–preoperative lordosis differentials at each studied spinal level (L5-S1, L4-S1, L1-S1)

Boxplots illustrating the relationship between implant parameters and postoperative changes in lordosis: Δ L5-S1 lordosis (top row), Δ L4-S1 lordosis (middle row), and Δ L1-S1 lordosis (bottom row). Columns represent implant lordosis (°), implant depth (mm), and anterior implant height (mm). Sample sizes for each subgroup are indicated above the boxplots. The dashed horizontal line represents no change (Δ = 0°)

Figure 8. Box-plots illustrating the distribution of implant parameters (anterior height, depth, and cage lordosis) in relation to postoperative-theoretical lordosis differentials at each studied spinal level (L5-S1, L4-S1, L1-S1)

Boxplots illustrating postoperative deviation from the theoretical target lordosis (Δ postoperative – TTL) at three levels: L5-S1 (top row), L4-S1 (middle row), and L1-S1 (bottom row). Columns correspond to implant characteristics: implant lordosis (°), implant depth (mm), and implant anterior height (mm). Sample sizes for each implant subgroup are displayed above the boxplots. The horizontal dashed line represents perfect alignment with the theoretical target (Δ = 0°). Values above the line indicate overcorrection, while values below indicate undercorrection TTL: theoretical target lordosis

Figure 9. Association between preoperative PI and postoperative LDI

Scatterplot illustrating the association between preoperative PI (°) and postoperative LDI, with each point representing an individual patient. The dashed line represents the linear regression fit. The correlation coefficient and corresponding p-value (r = -0.44; p = 0.00019) are displayed in the upper right corner PI: pelvic incidence; LDI: lordosis distribution index