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

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.