The anisotropic hydraulic permeability of human lumbar anulus fibrosus - Influence of age, degeneration, direction, and water content

Study Design: Experimental investigation to determine the effect of interve rtebral disc degeneration on the kinetic behavior of fluid in human anulus fibrosus. Objectives. To measure the hydraulic permeability co-efficient of anulus fi brosus specimens in the axial, circumferential, and radial directions to de termine the anisotropic permeability behavior of nondegenerate and degenera te human intervertebral discs over a range of ages. Summary of Background Data. Fluid, a major component of normal intervertebr al discs, plays a significant role in their load-supporting mechanisms, Tra nsport of fluid through the intervertebral disc is important for cell nutri tion and disc viscoelastic and swelling behaviors. The hydraulic permeabili ty coefficient is the most imperial property governing the rate of fluid tr ansport. However, little is known about the anisotropic behavior of this ki netic property and how it is influenced by disc degeneration. Methods. Using a permeation testing apparatus developed recently, testing w as performed on 306 axial, circumferential, and radial anulus fibrosus spec imens from the posterolateral region of 30 human lumbar (L2-L3)discs. A new method, Row-controlled testing protocol, was developed to measure the hydr aulic permeability coefficient. Results. The hydraulic permeability coefficient of anulus fibrosus depended significantly on the disc degenerative grade (P = 0.0001) and flow directi on (P = 0.0001). For the nondegenerate group (Grade I), the hydraulic perme ability was significantly anisotropic(P<0.05), with the greatest value in t he radial direction (1.924 x 10(-15) m(4)/ Ns) and the lowest value in the circumferential direction (1.147 x 10(-15) m(4)/Ns). This anisotropic kinet ic (flow) behavior of anulus fibrosus varied with disc degeneration. For th e Grade III specimen group, there was no significant difference in hydrauli c permeability coefficient a mo ng the three major directions (P = 0.37). W ith disc degeneration,the hydraulic permeability coefficient was decreased in the radial direction and increased in the axial and circumferential dire ctions. The variations of hydraulic permeability coefficient from nondegene rate discs (Grade I) to mildly degenerate discs (Grade II) in each directio n were significant (P< 0.05). However, the changes in permeability from Gra de II to Grade III groups were not significant (P> 0.05) except in the circ umferential direction (3.8% P<0.05). Conclusions: The hydraulic permeability of human non-degenerate anulus fibr osus is direction-dependent (i.e., anisotropic), with the greatest permeabi lity in the radial direction. With disc degeneration, the radial permeabili ty of anulus fibrosus decreases, mainly because of decreased water content, and the axial and circumferential permeability coefficients increase, main ly because of structural change, leading to more isotropic permeability beh avior for Grade III discs.