Preoperative ligamentum flavum thickness as a predictor of the progression of postoperative hypertrophy at the cranial adjacent level after posterior lumbar interbody fusion: a retrospective observational study in Japan

Study Design: Retrospective observational study.Purpose: To determine whether preoperative ligamentum flavum (LF) thickness at the cranial adjacent level predicts postoperative LF hypertrophy progression at the same level 1 year after posterior lumbar interbody fusion (PLIF), and whether preoperative LF thickness at L2/3 also predicts this progression.Overview of Literature: Adjacent segment disease (ASD) is a recognized complication of PLIF.LF hypertrophy at adjacent levels contributes to symptomatic ASD; however, it remains unclear whether preoperative LF thickness predicts postoperative hypertrophy progression.Methods: This retrospective study included 51 patients with lumbar spinal stenosis who underwent PLIF and had preoperative and 1-year postoperative computed tomography scans.LF thickness was measured at the cranial adjacent level and at L2/3.Additional preoperative radiographic and clinical variables were also assessed.Patients were classified into progression and nonprogression groups based on the median change in LF thickness at the adjacent level, and intergroup comparisons were performed.Correlation analysis and linear regression with backward elimination were performed to identify predictors of LF hypertrophy progression.Sensitivity analyses using ridge and least absolute shrinkage and selection operator regression were conducted to confirm robustness.Results: LF thickness at the cranial adjacent level increased from 3.4 mm preoperatively to 4.0 mm at 1 year (p<0.001).In univariate analysis, preoperative LF thickness at the cranial adjacent level, LF thickness at L2/3, and facet joint degeneration were associated with postoperative LF thickening.In multivariable regression, only LF thickness at L2/3 (coefficient=0.37,p<0.001) and cranial adjacent facet joint degeneration (coefficient=0.32,p=0.030) remained predictors (adjusted R 2 =0.27).No other preoperative variable showed an association.Conclusions: Preoperative LF thickness at L2/3 and cranial adjacent facet degeneration predicted postoperative LF hypertrophy progression after PLIF.Both inherent predisposition and mechanical stress may contribute to adjacent-level LF thickening.