Morphology and clinical importance of epidural membrane and periradicular fibrous tissue in lumbar spinal stenosis.

Purpose: Compared to the ligamentum flavum (LF), morphology of the epidural membrane (EM) and the periradicular fibrous tissue (PRFT) has been largely ignored in studies of lumbar spinal stenosis (LSS). The aim of this prospective study was to elucidate the morphologies and clinical importance of the EM and PRFT in LSS.

Methods: Before starting this study, neural compressive EM (c-EM) and PRFT (c-PRFT) were defined as follows based on our microsurgical experience and a literature review. The c-EM is a constriction band or membrane obstructing dural tube expansion, and the c-PRFT is a fibrous tissue that compresses the nerve root and/or restricts its mobility. This study enrolled 134 patients who underwent microscopic decompression at L4/5. The morphologies of each patient's EM and PRFT were observed and recorded. Specimens were obtained from randomly selected patients for histological evaluation.

Results: The EM and PRFT exhibited a wide morphological spectrum, from a fine strand to a substantial membrane. The c-EM alone was observed in four cases, the c-PRFT alone in 37 cases, and both in three cases. The c-PRFT was more frequently observed in patients with degenerative spondylolisthesis than in those without olisthesis (P < 0.05). Several cases exhibited interesting histological findings including many small arteries, chondrometaplasia, ganglion-like cyst formation, and hyalinized collagen fibers.

Conclusions: Some EM and PRFT transform into degenerative and substantial fibrous tissues during the process of symptomatic LSS development. Such morphological and histological changes can cause dural tear, symptomatic epidural hematoma, and/or inadequate decompression.