Proteomic and Histological Analysis of Ligamentum Flavum Hypertrophy and Degeneration in Lumbar Spinal Stenosis: Clinical, Surgical, and Therapeutic Implications

1. INTRODUCTION Lumbar spinal stenosis (LSS) is a highly prevalent condition worldwide, characterized by the narrowing of the spinal canal. Epidemiological studies indicate a high incidence of LSS, influenced by demographic shifts and the increasing burden of age-related musculoskeletal disorders. The etiology of LSS is primarily classified as acquired (degenerative) or congenital, affecting approximately 103 million individuals globally. The degenerative form becomes increasingly common with advancing age. Anatomically, degenerative LSS is subdivided into central, lateral, and foraminal stenosis, with the highest prevalence observed at the L4-L5 level. Key etiological factors include thickening and deformation of the ligamentum flavum (LF), often resulting from reduced disc height, facet joint hypertrophy, or a combination of both. Among these factors, ligamentum flavum hypertrophy (LFH) is recognized as the primary cause of lumbar spinal canal stenosis (LSCS).

The LF is a crucial structure connecting the laminae of adjacent vertebrae, composed of approximately 80% elastic fibers and 20% collagen fibers. It plays a vital role in forming the posterior boundary of the spinal canal, preventing excessive flexion of the vertebral column, and maintaining spinal stability. Several studies indicate that individuals with hypertrophic LF exhibit a reduction and disorganization of elastic fibers, accompanied by an increase in collagen fibers, suggesting that LFH is driven by a fibrotic process. Identifying risk factors for LFH remains challenging and is a subject of ongoing debate in the literature; age and mechanical stress are currently considered the most significant contributors.

Few studies have focused on elucidating the molecular mechanisms underlying LSS and LFH or on identifying specific diagnostic and prognostic biomarkers. As a result, the pathophysiology and molecular basis of LSS and LFH remain poorly understood, underscoring the need for precise molecular characterization and the development of targeted treatments based on specific molecules.

Zhao et al. identified a significant increase in thrombospondin-1 (THBS1) expression in LFH using proteomics and single-cell RNA sequencing in clinical samples. Laboratory experiments demonstrated that THBS1 activates the Smad3 signaling pathway via transforming growth factor β1 (TGF-β1), enhancing the expression of fibrotic markers COL1A2 and α-SMA. A bipedal murine model confirmed the crucial role of THBS1 in LFH development. Additionally, sestrin2 (SESN2), a stress-responsive protein, was shown to suppress THBS1 expression, preventing fibrosis in LF cells. These findings suggest that mechanical overload increases THBS1 production, triggering the TGF-β1/Smad3 pathway and leading to tissue hypertrophy. Suppressing THBS1 expression could provide a novel therapeutic approach for LFH.

In another study, Wang et al. found that wild-type amyloid transthyretin (ATTRwt) was present in LF samples from patients undergoing decompression surgery, with amyloid load positively correlating with LF thickness and lumbar LF burden in a dose-dependent manner.