| A three-dimensional, comprehensive and realistic FEA code that incorporated the geometric, material and contact non-linearities was used for studying the biomechanics of a lumbar motion segment under compression, flexion, extension, axial torsion and combined loads. A new algorithm for modeling of nonlinear facet articulation problem is presented. Experimental and analytical results on gross segment response from literature were used for validation of analytical results. The stress and displacement distribution and the load-sharing among intervertebral disc, ligaments and facets of the lumbar motion segment, with and without posterior elements were evaluated under compression, flexion, extension and axial torsion. Sensitivity study with respect to ligament, disc and facet parameters was performed. The role of ligaments and facets in lumbar spine stability was evaluated.; Levels of magnitudes for the stresses and displacements were similar to the reported values from experimental and analytical studies. Disc was found to play the major role in resisting compressive and small extension loads on spine, ligaments were important in flexion, while facets provided most of the resistance to axial and large extension rotations. Central regions of endplates and vertebral body were susceptible to fracture under large compressive loads. Disc fiber strains are largest in axial torsion but they are unlikely to fail under torsion alone, unless preceded by facet degeneration or removal. Supraspinous and interspinous ligaments were likely to sprain under large flexion, while capsular ligament failure was most likely under large axial rotations. Facet degeneration, hypertrophy and osteoarthritis may occur under large extension, axial torsional or flexion-anterior shear loading. The stability study predicts that facet hypertrophy may precipitate spinal stenosis, particularly under flexion-anterior shear loading. Localized facet excision may restore spinal canal size without compromising segmental stability. The parametric study revealed that weakening of nucleus and increase in wedge angle of the disc result in change in internal displacement trends within the disc leading to load re-distribution. Decrease in fiber stiffness and annulus degeneration result in lower stresses and larger deformations in the disc annulus. Facet and ligament response were sensitive to their geometric parameters. A more sagittally oriented facet may be linked to the etiology of spondylolisthesis, whereas a less transversely oriented facet may be linked to the etiology of rotational instabilities in extension. |
