Finite element analysis of the lower lumbar neural arch under facet loading.

To evaluate the mechanical effect of the direction of facet load on the isthmus stresses of the lower lumbar neural arch, stress analyses were performed by using three-dimensional finite element models of isolated L4 and L5 vertebrae with particular emphasis on accurate discretization of the posterior elements. The bilateral symmetric or unilateral facet loads, with a constant magnitude, were applied within the sagittal and transverse planes. The largest maximal principal stress in the neural arch (the largest sigma(max)) tended to be qualitatively similar in L4 and L5. At the physiologic range, the largest sigma(max), observed on the anterior surface of the isthmus, was relatively insensitive to the sagittal plane loadings but sensitive to the transverse plane loadings. The values of the largest sigma(max) of L4 and L5 were low at the more frontally directed facet load. The magnitude and direction of the largest sigma(max) depended upon the facet loadings mainly on the ipsilateral side and partly on the opposite side. The results suggest that the direction of facet load, which may be characterized by the facet orientation, can have mechanical effects on the occurrence and direction of isthmic crack.