Beyond Surgical Factors: Postoperative Thoracolumbar Alignment and L1–2 Disc Degeneration as Independent Drivers of Early-Onset Proximal Junctional Kyphosis

Purpose: This retrospective study investigated the distinct clinical and radiographic drivers of early- versus late-onset proximal junctional kyphosis (PJK) following multilevel thoracolumbar (TL) fusion.Methods: After applying the exclusion criteria (spinal infection, neuromuscular disease, age <50 years), the analysis included 136 patients who underwent ≥4-level TL fusion and were followed up for a minimum of 2 years. PJK was classified as early (≤6 months) or late (>6 months) onset. Patient-related factors, surgical variables, sagittal spinopelvic parameters, and preoperative magnetic resonance imaging findings were analyzed using multivariate logistic regression to identify independent predictors of early PJK.Results: Among 24 patients (17.6%) who developed PJK, the early and late-onset groups included 13 and 11 patients, respectively. The early PJK group exhibited significantly greater preoperative and postoperative TL angles compared with the late group (preoperative: 23.03±13.83° vs. 9.67±9.67°, p=0.024; postoperative: 19.6±6.95° vs. 6.95±6.35°, p<0.001). The Pfirrmann grade of the L1–2 intervertebral disc was significantly higher in the early PJK group (3.92±0.95 vs. 2.81±0.60, p=0.006). No surgical variables differed significantly between the groups. Multivariate analysis confirmed greater postoperative TL angle and more advanced L1–2 disc degeneration as independent predictors of early PJK.Conclusion: Early-onset PJK following multilevel TL fusion is primarily driven by regional biomechanical vulnerabilities, specifically residual postoperative TL kyphosis and advanced adjacent L1–2 disc degeneration, rather than by surgical variables. Meticulous evaluation of regional TL alignment and adjacent disc health during surgical planning is critical for risk stratification and prevention of early junctional failure.