It is hypothesized that the facet joints may play an important role in the causality behind back pain. Previous biomechanical models lack detailed facet geometry and contact modeling, the inclusion of cartilage, and the modeling of the full lumbar spine. The objective of this study was to more realistically assess facet loading in an existing low back biomechanical model. The second objective of this study was to demonstrate how the model including facets reports load distribution in pushing and pulling tasks.
Several new components were added to an existing biomechanical model: realistic geometry (based on CT & MRI), more accurate facet radii of curvature, articular cartilage thickness, and contact algorithms which defined the contact between each lumbar spine facet. Trials were run once with facets turned on, and repeated with facets turned off.
Resultant disc loads were lower in the model with facets. The model clearly showed facet to lamina contact for many of the trials studied. Facets at L4/L5 and L5/S1 bore about 40% of the total load through those segments.
The nonlinear model performed well. The facets bear a large portion of the load though the lumbar spine. Load transmission percentage results with the facet model are comparable to previous studies. The facets bore a greater amount of load in pushing tasks than in pulling tasks. The direction of the offloading of the intervertebral discs is not as straightforward as previously hypothesized. Including realistic facets in the model does not necessarily result in decreased anterior-posterior shear loads within the disc under these loading conditions.