ALTERATIONS IN THE MECHANICAL-BEHAVIOR OF THE HUMAN LUMBAR NUCLEUS PULPOSUS WITH DEGENERATION AND AGING

This study tested the hypothesis that changes in the morphology and co mposition of the nucleus pulposus with age and degeneration have assoc iated changes in its mechanical properties. A torsional shear experime nt was used to determine viscoelastic shear properties of cylindrical samples of human nucleus pulposus with large ranges of grades of morph ological degeneration (normal to severely degenerated) and ages (range : 16-88 years; average: 57 +/- 21.5 years). Viscoelastic shear propert ies were determined from stress-relaxation and dynamic sinusoidal test s. A linear viscoelastic law with a variable-amplitude relaxation spec trum was used to model experimental behaviors of nucleus pulposus spec imens. A statistically significant increase in the instantaneous and d ynamic shear moduli was found with increasing age and grade of degener ation; the values for moduli ranged from 5.0 to 60 kPa. A significant decrease in tan delta was also detected; the values ranged from 0.43 t o 0.33, indicating a decreased capacity for the nucleus pulposus to di ssipate energy. The dynamic modulus and tan delta were also significan tly affected by frequency. It was generally concluded that the nucleus pulposus undergoes a transition from ''fluid-like'' behavior to more ''solid-like'' behavior with aging and degeneration.