Differential Mechanisms of Ligamentum Flavum Hypertrophy

Study Design: A descriptive study investigating ligamentum flavum (LF) hypertrophy in individuals with and without lumbar spinal stenosis (LSS). Objective: To elucidate the relationship between LF hypertrophy and spinal degeneration in the lumbar spine based on magnetic resonance imaging. Summary of Background Data: LF thickening plays a major role in LSS pathogenesis, initiated by mechanical stress that induces chronic inflammation, tissue damage, scarring, and fibrosis. However, whether LF thickening arises solely from mechanical stress remains uncertain. The connection between spinal degeneration and LF hypertrophy continues to be debated. Methods: LF hypertrophy was defined using a threshold based on the mean and SD of the flavum canal ratio (FCR) measured from MRI images of 1086 young individuals. FCR was measured from L1/2 to L5/s in 1266 patients with LSS and 1000 non-LSS cases. Endplate and disc degeneration were evaluated using Modic changes and the Pfirrmann classification. Results: LF hypertrophy and disc degeneration were significantly more prevalent in the LSS group than in the non-LSS group, with the highest prevalence observed at the L4/5 level in both groups. In spinal degeneration, endplate and disc degeneration were significantly less prevalent at the L1/2 level compared with other levels. The trend analysis revealed a statistically significant pattern: the lower the intervertebral level, the higher the incidence of degeneration in LF hypertrophy. Conclusions: LF thickening occurred in the lower lumbar spine, accompanied by spinal degeneration, whereas thickening in the upper lumbar spine occurred without spinal degeneration. These findings suggested the involvement of different mechanisms in the development of LF hypertrophy. Level of evidence: Level III.