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Ligamentum flavum (LF) thickness was measured by using lumbar magnetic resonance imaging in patients with low back or leg pain.This study investigated whether LF thickening is due to hypertrophy or buckling related to disc degeneration and examined the correlations between the thickness of the LF and age, sex, body mass index (BMI), degree of pain, lumbar spinal stenosis (LSS), and disc herniation.“LF thickness” and “LF hypertrophy” are used interchangeably in the literature, although they are not necessarily the same thing. Thickness may increase by buckling without a change in the mass of the LF, and whether LF thickening is due to tissue hypertrophy or buckling remains controversial.The thickness of 896 LFs at the L2–L3, L3–L4, L4–L5, and L5–S1 levels of 224 (mean age, 47.8 ± 16.7 yrs) patients was measured prospectively on axial T1-weighed magnetic resonance images, obtained at the facet joint level. The presence of disc degeneration, spinal stenosis, and disc herniation was evaluated.At all of the levels investigated, LF thickness was significantly greater in patients with grades IV to V degeneration compared with the patients with grades I to III degeneration (P < 0.05). LF thickness at all levels increased significantly with age (P < 0.05). Sex and the degree of pain were not correlated with the thickness of the LF. Patients with a BMI of 25 kg/m2 or greater had the thickest LF at the L3–L4 level (P < 0.01). LF thickness was significantly greater at the L2–L3, L3–L4, and L4–L5 levels in subjects with LSS and significantly greater at all levels in subjects with disc herniation (P < 0.05).Thickening of the LF is correlated with disc degeneration, aging, BMI, LSS, spinal level, and disc herniation. The authors concluded that thickening of the LF is due to buckling of the LF into the spinal canal secondary to disc degeneration more than to LF hypertrophy. Sex and the degree of pain were not correlated with the thickness of the LF.