Poroelastic analysis of lumbar spinal stability in combined compression and anterior shear.

OBJECTIVE A three-dimensional poroelastic finite element (FE) L2-L3 model was developed to study lumbar spinal instability and intrinsic parameters in the intervertebral disc (IVD). METHODS The FE model took into consideration poroelasticity of the IVD and viscoelasticity of the annulus fibers and ligaments to predict the time-dependent behavior. To simulate a holding task, the motion segment was subjected to a combined loading of constant compressive load (1600 N) and anterior shear (200 N) for 2 hours, and the role of facet joints and ligaments in the biomechanical response was investigated by removal of unilateral/bilateral facets, posterior ligaments (supraspinous and interspinous), and facets and ligaments. RESULTS The results show the stabilizing role of the facets and ligaments in resisting anterior shear and sagittal rotation under combined loading over time. The main pathway of fluid movement was found to permeate through the central region of the endplate, and the fluid diffusion occurred earlier at the posterior nucleus than the anterior nucleus. The fluid loss from the nucleus dictated the time-dependent motion under the sustained loading, whereas the intrinsic properties of ligaments/annulus fibers played a role only in the early stage of the loading. CONCLUSION The predicted results using poroelastic elements provide new insight into the IVD in providing the spinal stiffness under combined loading.