Introduction: Hypertrophy of the ligamentum flavum (LF) is a major pathological contributor to lumbar spinal stenosis. Mechanical stress has been implicated in LF degeneration; however, its direct pathological impact is difficult to isolate in humans because aging, inflammation, and systemic degenerative processes coexist. Although several animal models have been reported, many require complex procedures or special loading conditions, which may limit standardization and reproducibility. This study aimed to investigate whether localized mechanical stress can induce structural and molecular changes in the LF using a reproducible rat model.

Methods: Localized mechanical stress was generated at the L5/6 segment using a lumbar instability procedure without direct injury to the LF. Segmental motion was evaluated using dynamic radiography to confirm mechanical alteration. Structural changes were assessed by elastica van Gieson and toluidine blue staining and Col2a1 immunohistochemistry. Molecular alterations were quantified using real-time polymerase chain reaction analysis of fibrosis- and chondrogenesis-related genes.

Results: The instability group demonstrated a significant increase in segmental range of motion at L5/6 compared with controls. A positive correlation was observed between segmental range of motion and LF cross-sectional area. Histological analysis revealed LF hypertrophy at the stress-concentrated level, characterized by disruption of elastic fibers and increased cartilage-like matrix deposition. Immunohistochemistry showed increased Col2a1-positive cells within the degenerated LF. Gene expression analysis demonstrated upregulation of fibrosis-related genes (Col1a1, Col3a1) and chondrogenic markers (Col2a1, Biglycan), along with downregulation of Elastin.

Conclusion: Localized mechanical instability induced structural and chondrogenic changes in the ligamentum flavum. These findings support a contributory role of mechanical stress in ligamentum flavum hypertrophy. This reproducible rat model provides experimental evidence for the involvement of mechanical stress in LF degeneration and may serve as a practical platform for future mechanistic and therapeutic studies.