Study Design:​ The nonlinear stress analysis of the antire ligamentous lumbar spine (L1-S1) in single flexion, extension, right lateral, and left lateral moments of up to 15 Nm was performed. In sagittal moments, both disc fiber angles of 27° and 30° were considered. Moreover, in extension moment, the effects of larger gap limit (the distance at which the contact initiates) at all facet joints and of removal of left or left/right L4-L5 facet joints also were studied.

Objectives:​ The overall and intersegmental blomechanics of the lumbar spine under sagittal moments with particular emphasis on the role facets were studied.

Methods:​ The analysis was performed using a nonlinear three-dimensional finite element model.

Results:​ Intersegmental results were nonlinear and varied from one level to the next. Overall, the lumbar flexibility and disc pressures were larger in flexion. Conversely, larger facet forces were computed in extension whereas flexion caused negligible contact forces. Unilateral and bilateral removal of L4-L5 facets in extension reduced the stiffness and increased the disc pressure at the same level while the remaining levels were nearly unaffected. In contrast, larger gap limit for articulation increased stiffness, reduced disc pressure, and increased loads on facets at all segmental levels. Disc fiber layers are most loaded in flexion and least loaded in extension. Large tensile strains occur in disc fibers under flexion and lateral moments. This suggests the vulnerability of disc fibers to failure under movements involving large flexion and lateral rotations.