Viscoelastic properties of the intervertebral disc and the effect of nucleus pulposus removal

The viscoelastic properties of 12 ovine lumbar discs attached to the adjacent vertebrae were investigated using dynamic mechanical analysis. Each specimen was subjected to a sinusoidally varying compressive force of between 36 and 44 N at frequencies in the range 0.1–10 Hz. Storage stiffness, loss stiffness, and phase angle were calculated. The nucleus pulposus was then removed and the dynamic mechanical analysis was repeated for the denucleated specimen. During the testing the intact and denucleated discs were viscoelastic. Storage stiffness for the intact and denucleated discs increased linearly with log frequency. After removal of the nucleus pulposus the storage stiffness can increase or decrease. The experimental procedure may be useful for investigating whether nucleus replacement implants can restore the viscoelastic properties of an intact disc.