Effect of disc degeneration at one level on the adjacent level in axial mode.

Nonlinear three-dimensional finite element models of a ligamentous two motion segments spine specimen (L3-L4-L5) were developed to investigate the effects of disc degeneration, simulated at the L4-L5 level, on the biomechanical behavior of the adjacent intact L3-L4 motion segment. The disc degeneration was simulated by removing the hydrostatic capabilities of the nucleus and making the L4-L5 disc stiffer than a normal disc. The results of the degenerated model were compared with the predictions for a model in which the L4-L5 disc was left intact. The loads on the facets decreased, and intradiscal pressure in the intact L3-L4 disc increased as a result of disc degeneration compared with the intact model. The predicted increase in the intradiscal pressure and the associated increase in the disc bulge in the posterior region over time may trigger the degenerative process at the L3-L4 motion segment. This is in accordance with the Wolff's law; living tissue responds to chronic changes in stresses and strains. The limitations of the present two motion segments model and the potentials of multisegmental models are discussed.