Viscoelastic properties of the nucleus pulposus of the intervertebral disk in compression

The viscoelastic properties of the nucleus pulposus were measured in compression for 35 specimens dissected from 9 sheep. Measurements on 19 specimens were made on the day of slaughter; the remaining 16 specimens were stored frozen and thawed before testing. A preload of 0.2 N was applied to each specimen and a cyclic compression of 10 microm applied at eight frequencies in the range 0.1-10 Hz at a temperature of 37 degrees C. Freezing appeared to increase the storage modulus, E ', but not the loss modulus, E ", or tan delta=E "/E '. These parameters, E ', E " and tan delta, had values of 64+/-28 kPa, 24+/-11 and 0.33 kPa+/-0.07, respectively. The value of tan delta passed through a minimum at a loading frequency of 0.9+/-0.2 Hz. The water content of the specimens was 80+/-2%.