Effects of swelling pressure and hydraulic permeability on dynamic compressive behavior of lumbar annulus fibrosus.

The objective of this study was to investigate the effects of swelling pressure and hydraulic permeability on the dynamic behavior of intervertebral disk tissue in confined compression. Normal (served as a control) and trypsin-treated, axial annulus fibrosus (AF) specimens from the porcine lumbar disks were tested and their swelling strain, swelling pressure, equilibrium compressive modulus (HA), dynamic modulus, and hydraulic permeability (k) were determined at 30% and 40% swelling strain levels. The proteoglycan depletion due to trypsin treatment resulted in significantly lower values of the free swelling strain, swelling pressure, equilibrium modulus, dynamic modulus, and higher value of hydraulic permeability for trypsin-treated group, comparing to those for the control group. At the 30% swelling strain level, the equilibrium moduli were 51.84 +/- 14.53kPa (n = 8) for the control group and 15.11 +/- 5.67 kPa (n = 8) for the trypsin-treated group; and the hydraulic permeabilities were 4.50E- 15 +/- 1.60E- 15 m4/Ns and 8.43E-15 +/- 4.29E- 15 m4/Ns for control and trypsin-treated groups, respectively. No statistically significant difference in wet tissue density or dry tissue density was found between control and trypsin-treated groups. There was a significant correlation between swelling pressure and compressive (aggregate) modulus (R2 = 0.824, m = 22). The decrease in measured dynamic modulus for trypsin-treated group was attributed to the reduced swelling pressure (or modulus HA) and increased hydraulic permeability (k) due to PG depletion.