On the Poisson's ratio of the nucleus pulposus

Existing experimental data on the Poisson’s ratio of nucleus pulposus (NP) tissue is limited. This study aims to determine whether the Poisson’s ratio of NP tissue is strain-dependent, strain-rate-dependent or varies with axial location in the disc. Thirty-two cylindrical plugs of bovine tail NP tissue were subjected to ramp-hold unconfined compression to 20% axial strain in 5% increments, at either 30µm/s or 0.3µm/s ramp speeds and the radial displacement determined using biaxial videoextensometry. Following radial recoil, the true Poisson’s ratio of the solid phase of NP tissue increased linearly with increasing strain and demonstrated strain-rate dependency. The latter finding suggests that the solid matrix undergoes stress relaxation during the test. For small strains, we suggest a Poisson’s ratio of 0.12 to be used in biphasic models of the intervertebral disc.