[A new approach to the biomechanics of the normal and pathologic lumbar disk using the finite element method]

The model was created based on the L2-L3 intervertebral disc considering it to be symmetrical on a longitudinal axis and a mid-transverse plane and could be subjected to a longitudinal load of 400 N in compression. The analysis of this two-dimensional problem by using the finite element program S.A.M.C.E.F. utilizes a quadrangular isoparametric and axi-symmetric element. The results took into account the vertical deflection and radial displacement of the nodal points and also the deformation diagram and stress distribution (Von Mises comparison) of the three regions studied: nucleus pulposus, annulus fibrosus and the end plate. This model has been applied to three pathological states: fissuring of the annulus and nucleus herniation, simulation of an enucleation, and a fracture of the central area of end plate with Schmörl's nodule formation. In each case, a deformation and a stress distribution modified relatively to those observed in normal discs were obtained. For instance, in the case of herniation the maximum stress equals 3.700 N/mm2 in comparison with 1.350 N/mm2 found in normal cases. This simplified model is one of the first which reproduces some pathological states of intervertebral discs and facilitates the understanding of the biomechanics of these states.