Human annulus fibrosus dynamic tensile modulus increases with degeneration

The annulus fibrosus (AF) of the intervertebral disc experiences cyclic tensile loading in vivo at various states of mechanical equilibrium. Disc degeneration is associated with alterations in the biochemical composition of the AF including decreased water content, decreased proteoglycan concentration, and increased collagen deposition that affect mechanical function of the AF in compression and shear. Such changes may also affect the dynamic viscoelastic properties of the AF and thus alter the disc's ability to dissipate energy under physiologic loading. The objectives of this study were to quantify the dynamic viscoelastic properties of human AF in circumferential tension and to determine the effect of degeneration on these properties. Nondegenerated and degenerated human AF tensile samples were tested in uniaxial tension over a spectrum of loading frequencies spanning 0.01Hz to 2Hz at several states of equilibrium strain to determine the dynamic viscoelastic properties (dynamic modulus, phase angle) using a linear viscoelastic model. The AF dynamic modulus increased at higher equilibrium strain levels. The AF behaved more elastically at higher frequencies with a decreased phase angle. Degeneration resulted in a higher dynamic modulus at all strain levels but had no effect on phase angle. The findings from this study elucidate the effect of degeneration on the dynamic viscoelastic properties of human AF and lend insight into the mechanical role of the AF in cyclic loading conditions.