# Tensile Test of Human Lumbar Ligamentum Flavum: Age-Related Changes of Stiffness
## 4 Discussion  
Hypertrophy of the ligamentum flavum plays an important role in the onset of LSS [7]. LSS is a degenerative disease more likely to occur in the elderly, and the incidence rate increase with age. Thus, aging seems to correlate with ligamentous hypertrophy. Indeed, [PERSON] et al. reported the age-related hypertrophy of ligamentum flavum mediated by oxidative stress [18]. Since the mechanical properties of the ligamentum flavum can be thought to change as ligamentous hypertrophy occurs, we hypothesized that the mechanical properties of ligamentum may change with aging also. Although there are several reports in the literature studying anatomical and histological age-related changes of the ligamentum flavum, there are only a small number of studies reporting the relationship between mechanical properties and aging. In anatomical research, it has been reported that the ligamentum flavum loses its homogenous appearance with age. In elderly patients, small cysts, fat infiltration, and calcification are seen in the ligaments which may lead to thickening and changes in mechanical property [6]. [PERSON] et al. reported the thickening of the ligamentum is an age-dependent phenomenon [19]. On the other hand, [PERSON] et al. reported that the thickness of the ligamentum flavum does not differ by gender and does not correlate with age [20]. [PERSON] et al. reported that since the thickness of the ligamentum flavum can increase by buckling due to disc height decrease with no histological change, hypertrophy and thickness is somewhat different [21]. Hypertrophy does result in thickening of the ligament; however, thickening of the ligament does not always mean the occurrence of ligamentous hypertrophy. Histologically, the normal ligamentum flavum consists of approximately 80% elastic fibers and 20% collagen fibers [8]. The viscoelasticity of the ligament decreases with advancing degeneration. This is because fibrous tissue formation and subsequent hypertrophy occur in the elastic fibers. [PERSON] et al. reported that fibrosis is the main cause of ligamentum flavum hypertrophy, and fibrosis is caused by the accumulation of mechanical stress with the aging process [22]. Furthermore, loss of elastic fibers and increase of calcification in the ligaments leading to loss of elasticity was associated with age [9; 10]. As the elastic fibers gradually disappear a relative increase of collagen fibers and subsequent fibrosis of the ligament are seen leading to hypertrophic changes [11; 13]. Recently, as causes for hypertrophy, angiogenesis following mechanical stress and inflammation promoted by angiopoietin-like protein 2 have been reported [14]. Summarizing the results of anatomical and histological studies, it appears that the hypertrophic change of the ligamentum flavum is associated with age due to the effects of mechanical stress and chronic inflammation. When we look at the mechanical properties of ligamentum flavum, there are several cadaveric studies measuring the mechanical properties by several methods. These reports revealed the importance of the posterior ligaments in braking performance during flexion, retroflexion, and rotation of the lumbar spine [23; 24; 25; 26]. However, there are only a few reports that verify changes in the mechanical properties associated with age [1; 16; 17]. Cadaveric studies demonstrated that the ligamentum flavum loses its elasticity with aging, supporting the histological reports [1; 16]. Based on this prior knowledge, we conducted tensile testing of the fresh human ligamentum flavum. As previously reported, the stress s(\(\sigma\))-strain (\(\varepsilon\)) relationship showed a sigmoidal curve before breakage [23; 24]. We calculated the stiffness of the ligamentum flavum from the stress (\(\sigma\))-strain (\(\varepsilon\)) relationship diagram. Our results show that stiffness of the ligamentum flavum increases with age, regardless of gender or type of disease.