On the mechanical behaviour of intervertebral discs.

The response to compression, shear, bending, and axial rotation of an intervertebral disc is studied by using a theoretical disc model. The annulus fibrosus is modelled by ten fibre layers with alternating fibre inclination. The nucleus pulposus as well as the substance filing the space between the fibre layers is assumed to consist of an incompressible fluid. The end-plates are assumed to bulge at action of the pressure in the fluid phase. Geometry of fibre layers and end-plates are chosen to agree as closely as possible with what is representative for lumbar discs. The results show considerable increase of the different stiffnesses at increasing levels of the axial load. They also show quite large fibre strains, well over 10% at motions within normal physiological limits.