Nonlinear behavior of the human intervertebral disc under axial load

The nonlinear, rate-independent behavior of human intervertebral discs is studied with a finite element model which incorporates a nonlinear elastic constitutive relation for the annulus fibrosis. The elastic coefficients and a nonlinear constitutive parameter for the annulus fibrosis were obtained by matching experimental results for the disc's load-deflection behavior. The axisymmetric finite element model includes the annulus, end plates, portions of the vertebral bodies and an incompressible nucleus pulposus. Results for annulus bulging, internal pressures, and response variations due to level-to-level changes in geometry are in agreement with available experimental data. It is shown that there are marked differences in the compressive behavior of lumbar and thoracic discs. In addition, two types of degeneration, one characterized by annular tears, the other by a desiccated nucleus, are studied.