The Alignment and Compensation (AC) Classification of High-Grade Spondylolisthesis

STUDY DESING: Cross-sectional reliability study. OBJECTIVE: To propose and validate a novel biomechanical classification for high-grade spondylolisthesis (HGS) that incorporates compensatory mechanisms into the assessment of sagittal alignment patterns. BACKGROUND: Existing classifications of HGS are primarily descriptive, rely on parameters often distorted by sacral endplate dysplasia, offer limited guidance for surgical decision-making and do not completely capture the spectrum of compensation in sagittal alignment. The new classification system defines four alignment types: Type 1: Proportioned sagittal alignment (SVA ≤50 mm, LSA ≥90°, IP ≤T11, LL Apex ≤L3). Type 2: Compensated lower lumbar hypolordosis, with cranialization of IP and/or LL Apex. Type 3: Compensated lumbosacral kyphosis (LSA <90° despite SVA ≤50 mm). Type 4: Sagittal malalignment with exhausted compensation (SVA >50 mm). METHODS: Fourteen (14) spine surgeons from 12 countries evaluated 29 anonymized cases of HGS using the new classification. Each case was presented with full-spine and lumbosacral anteroposterior and lateral standing radiographs. Classifications were repeated in two separate rounds. Interobserver reliability was assessed with Fleiss' Kappa, while intraobserver reliability was evaluated with Cohen's Kappa and simple percentage agreement. RESULTS: Interobserver reliability was almost perfect (Fleiss' Kappa=0.916; 95% CI: 0.888-0.945; overall concordance 82.8%). Intraobserver reliability was equally high (mean Cohen's Kappa=0.958; 95% CI: 0.926-0.991; mean agreement 97.0%). CONCLUSION: This novel biomechanical classification provides a reproducible framework to describe HGS without reliance on approximated parameters. By integrating SVA, LSA, IP, and LL Apex, it captures compensated and decompensated states of malalignment with direct therapeutic implications. The system demonstrated excellent reliability and may serve as a foundation for future surgical guidelines.