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<0.001) and preoperative ipsilateral facet joint gap (adjusted OR 4.77, p=0.007) were independently associated with entrapment (combined AUC 0.875). In the postoperative geometric association model, DS (adjusted OR 34.87, p<0.001) and postoperative ipsilateral facet joint gap (adjusted OR 4.12, p=0.001) showed independent associations (combined AUC 0.897). The facetectomy ratio was not associated with entrapment (p=0.222).

CONCLUSION

Dural entrapment after UBE lumbar decompression is uncommon and usually follows a benign course, although symptomatic cases may occur. DS and the ipsilateral facet joint gap were the principal associated factors, with the absolute facet gap width - rather than the relative extent of bone resection - being the more relevant determinant. These findings, together with the marked ipsilateral predominance, support a geometric basis for this phenomenon and may inform preoperative risk assessment.