Relationship of the Iliac Crest Height with Subsidence After Oblique Lateral Interbody Fusion at L4–5: A Quantitative and Categorical Analysis

<b>Background:</b> This study aimed to evaluate the impact of iliac crest height on clinical and radiological outcomes following oblique lateral interbody fusion (OLIF) at the L4-5 level. <b>Methods:</b> Data of patients who underwent single-level OLIF at the L4-5 level for degenerative spinal stenosis were retrospectively analyzed. The patients were categorized into three groups based on their iliac crest height measured relative to the L4 and L5 pedicles. Categorical and quantitative analyses, including univariate and multivariate logistic regressions, were performed to identify subsidence predictors. Clinical outcomes, including visual analog scale scores for back and leg pain, were assessed over a minimum 2-year follow-up. <b>Results:</b> No significant differences in cage obliquity were observed across the iliac crest height groups (axial angles, <i>p</i> = 0.39; coronal angles, <i>p</i> = 0.79). However, subsidence was significantly more common in patients with higher iliac crest heights, particularly at crest level III, where the subsidence rate reached 43% (<i>p</i> = 0.01). Subsidence was predominantly associated with damage to the L5 endplate, which occurred in 83% of subsidence cases at crest level III. A cutoff value of 12 mm for iliac crest height, above which the risk of subsidence significantly increased, was identified (AUC = 0.688, <i>p</i> = 0.042). <b>Conclusions:</b> Iliac crest height is a critical factor for predicting subsidence following OLIF at the L4-5 level. Surgeons should consider alternative strategies and meticulous preoperative planning in patients with an iliac crest height ≥ 12 mm to reduce the risk of adverse outcomes. Further studies are needed to validate these findings and to explore their long-term implications.