Combined Anterior–Posterior vs. Posterior-Only Fusion in Adult Spinal Deformity Radiographic, Clinical, and Mechanical Outcomes With Longitudinal Mixed-Effects Analysis

Study Design Retrospective cohort study. Objectives To compare radiographic, clinical, and mechanical outcomes of posterior-only versus combined anterior–posterior fusion in adult spinal deformity (ASD) patients undergoing long-segment thoracolumbar fusion with pelvic fixation. Methods Consecutive ASD patients undergoing instrumented fusion ≥7 levels with pelvic fixation at a single center (2016–2023) were stratified by surgical approach. Longitudinal radiographic changes were analyzed using linear mixed-effects models with difference-in-differences (DiD) estimation. Mechanical complications were evaluated with Kaplan–Meier survival and Cox proportional hazards analysis. Results 207 patients were included (posterior-only, n=115; combined, n=92). Mean age was 62.1 ± 11.1 years (77.8% female), with mean follow-up of 46.2 ± 21.2 months. The combined approach demonstrated significantly greater DiD estimates for lumbar lordosis (−7.8° to −9.2°, all p<0.001), relative pelvic version mismatch (+3.4° to +4.2°, all p<0.001), and GAP score (−1.8 to −2.1, all p<0.001) at all postoperative timepoints. DiD estimates were stable across three covariate specifications. Rod fractures occurred in 14.8% of posterior-only versus 3.3% of combined patients (p=0.008), with superior rod fracture–free survival in the combined cohort (log-rank p=0.019). In the primary Cox model, the combined approach showed a non-significant trend toward lower rod fracture hazard (HR 0.35, 95% CI 0.10–1.24, p=0.10). Patient-reported outcomes improved comparably in both groups. Conclusions In long-segment ASD fusion with pelvic fixation, the combined anterior–posterior approach was associated with greater sustained sagittal correction and lower rod fracture rates, while both approaches provided comparable clinical improvement.