Dural Entrapment by Facet Joint after Unilateral Biportal Endoscopic Lumbar Decompression Surgery : Incidence, Risk Factors, and Biomechanical Considerations

Objective: Dural entrapment by the facet joint is an uncommon but potentially symptomatic finding following unilateral biportal endoscopic (UBE) lumbar decompression. Its incidence, associated factors, and underlying mechanisms remain poorly defined. This study aimed to investigate the incidence, clinical course, and anatomical and geometric factors associated with dural entrapment following UBE lumbar decompression. Methods: This retrospective cohort study included 181 patients (257 spinal segments) who underwent UBE lumbar decompression at a single institution between January 2023 and January 2024. Patients with prior lumbar surgery, deformity, severe instability, or incomplete imaging were excluded. Clinical and radiological parameters, including facet joint gap, facet joint angle, facetectomy ratio and dural sac-to-canal ratio, were evaluated. Because of the limited number of events, Firth's penalized logistic regression was applied using two complementary models - a preoperative susceptibility model and a postoperative geometric association model - with receiver operating characteristic (ROC) analysis. Results: Dural entrapment occurred in 15 of 257 segments (5.8%), of which 13 (86.7%) were ipsilateral. Symptomatic entrapment was observed in 2 cases (0.8%), one requiring revision surgery, while most followed a benign radiological course. In the preoperative susceptibility model, degenerative spondylolisthesis (DS; adjusted OR 49.14, p