Biomechanics of the intrinsically optimal design of the intervertebral disc.

The spinal shock-absorbing disc needs to have the flexibility to enable the spine to bend and twist. At the same time under loading, its lateral and axial deformations have to be contained, so that it does not herniate and impinge on the spinal-chord. The disc is composed of a fluid-like nucleus pulposus (NP) contained within an annulus. Hence when the disc is loaded, the NP gets pressurized and stresses the surrounding annulus. Now, because its elastic modulus is stress-dependent (i.e. E-E 0 = ksigma, where k is a constitutive parameter ), the annulus stiffens under loading. In this way, the flexible disc is able to sustain its loading with minimal deformation and thereby contain its deformation. In this paper, we have carried out a stress and deformation analysis of the spinal disc, and demonstrated that its deformations are invariant with the load intensity and only dependent on its dimensions and its constitutive property parameter k. Thus, we demonstrate that the intrinsic design of the spinal disc makes it an optimal structure.