The Dynamic Viscoelastic Properties of the Rat Lumbar Disc Are Decreased Following Nucleus Pulposus GAG Reduction

Abstract

The intervertebral disc has the important biomechanical function of dissipating energy during spinal loading. With degeneration, the disc experiences, among other changes, a loss of mechanical function and degradation of its composition. Using a rat model of early disc degeneration by injection of Chondroitinase ABC (ChABC), glycosaminoglycan (GAG) content in the nucleus pulposus (NP) was reduced which altered neutral zone mechanical properties. The contribution of decreased NP GAG content to the dynamic viscoelastic properties has yet to be determined. The advantage of dynamic viscoelastic testing is that it provides both viscous and elastic stiffness values as a function of loading frequency. These methods have been employed previously in a rabbit disc regeneration model, in ligament under three modes of loading, and in NP under oscillatory shear and compression. Therefore, the objective of this study was to determine the viscoelastic behavior of a rat lumbar disc at several equilibrium strains and to quantify the impact of GAG reduction on this behavior. Our hypotheses were: 1) elastic stiffness would be greater, and viscous stiffness and loss angle would be lower with increased frequency, and 2) both elastic and viscous stiffness would be lower in the reduced GAG discs at all frequencies.