Spinopelvic compensation and functional outcomes after short-segment lumbar fusion in patients with residual sagittal imbalance

Objective To evaluate whether postoperative spinopelvic compensation is associated with functional improvement in patients with residual sagittal imbalance after short-segment lumbar fusion. Methods Clinical data from 104 patients with lumbar degenerative disease who underwent short-segment lumbar fusion and had sagittal vertical axis (SVA) >5 cm at the 2-year follow-up were retrospectively analyzed. Patients were divided into functional improvement ( n = 64) and non-improvement ( n = 40) groups according to whether their Oswestry Disability Index (ODI) improvement reached the minimal clinically important difference (MCID) of 12 points. Visual Analog Scale (VAS), Japanese Orthopaedic Association (JOA) score, ODI, and spinopelvic parameters, including pelvic incidence minus lumbar lordosis (PI-LL) and pelvic compensation index (PCI), were compared between groups. Logistic regression was used to examine postoperative parameters associated with ODI improvement. Baseline-adjusted separate models were supplemented by a joint PI-LL/PCI sensitivity model. Results Interobserver ICCs ranged from 0.913 to 0.987. VAS and JOA scores improved significantly in both groups, while the 2-year ODI was significantly better in the functional improvement group ( P < 0.001). Mean PI was approximately 51° and did not change significantly over time (mean change 0.04°; P = 0.563). The functional improvement group had higher postoperative lumbar lordosis, pelvic tilt, and PCI, as well as lower PI-LL. In the joint model, each 5° increase in postoperative PI-LL was associated with lower odds of reaching MCID (OR = 0.229, 95% CI 0.116–0.454; P < 0.001), while each 0.10 increase in PCI was associated with higher odds (OR = 2.643, 95% CI 1.291–5.411; P = 0.008). Joint-model VIFs ranged from 1.010 to 1.037. The apparent and optimism-corrected AUCs for the joint model were 0.866 and 0.845, respectively. Conclusion Adequate spinopelvic compensation, reflected by higher PCI and lower postoperative PI-LL, was associated with favorable functional outcomes despite residual sagittal imbalance.