The Role of Lumbar Sagittal Balance on Clinical Results after Posterior Interbody Fusion for Patients with Isthmic Spondylolisthesis: A Retrospective Study.

INTRODUCTION: Isthmic spondylolisthesis is a common cause of low back pain and functional disability, often associated with spinal instability and altered sagittal spinopelvic alignment. Restoration of sagittal balance has been suggested to influence post-operative outcomes; however, its exact role remains controversial. This study aimed to evaluate the impact of lumbar sagittal balance on clinical outcomes following posterior lumbar interbody fusion (PLIF) in patients with isthmic spondylolisthesis. MATERIALS AND METHODS: A retrospective cohort study was conducted on 50 patients with isthmic spondylolisthesis who underwent PLIF between January 2024 and December 2025. Clinical outcomes were assessed using the Oswestry disability index (ODI) at pre-operative, 3-month, 6-month, and 1-year follow-up intervals. Radiographic evaluation included measurement of spinopelvic parameters - pelvic incidence (PI), pelvic tilt (PT), and sacral slope (SS) - preoperatively and postoperatively. Statistical analysis was performed using paired t-tests and correlation analysis, with P < 0.05 considered significant. RESULTS: The mean age of patients was 52.63 ± 8.71 years, with a female predominance (80%). The most commonly involved level was L4-L5 (58%). Significant improvement in functional outcomes was observed, with mean ODI scores improving from 14.13 ± 1.53 preoperatively to 7.58 ± 0.73 at 1 year (P < 0.001). Spinopelvic analysis demonstrated no significant change in PI (27.25 ± 3.30° vs. 27.40 ± 3.10°, P > 0.05). However, SS increased significantly (19.6 ± 5.57°-24.2 ± 4.85°, P < 0.001), while PT decreased significantly (8.93 ± 3.45°-6.10 ± 2.95°, P < 0.001). Correlation analysis revealed a significant positive association between SS and ODI improvement (r = 0.32, P < 0.05), whereas PI and PT showed weak, non-significant correlations. CONCLUSION: PLIF provides significant clinical and radiological improvement in patients with isthmic spondylolisthesis. Restoration of sagittal alignment, particularly increased SS, is associated with better functional outcomes, although spinopelvic parameters may not act as independent predictors in all cases. Further prospective studies are required to validate these findings.