Study design: The issue of segmental stability using finite element analysis was studied. Effect of ligament and facet (total and partial) removal and their geometry on segment response were studied from the viewpoint of stability.
Objectives: To predict factors that may be linked to the cause of rotational instabilities, spondylolisthesis, retrospondylolisthesis, and stenosis.
Summary of background data: The study provides a comprehensive study on the role of facets and ligaments and their geometry in preserving segmental stability. No previous biomechanical study has explored these issues in detail.
Methods: Three-dimensional nonlinear finite element analysis was performed on L3-L4 motion segments, with and without posterior elements (ligaments and facets), subjected to sagittal moments. Effects of ligament and facet (partial and total) removal and their orientations on segment response are examined from the viewpoint of stability.
Results: Ligaments play an important role in resisting flexion rotation and posterior shear whereas facets are mainly responsible for preventing large extension rotation and anterior displacement. Facet loads and stresses are high under large extension and anterior shear loading. Unlike total facetectomy, selective removal of facets does not compromise segmental stability. Facet loads are dependent on spatial orientation.
Conclusions: Rotational instability in flexion or posterior displacement (retrospondylolisthesis) is unlikely without prior damage of ligaments, whereas instability in extension rotation or forward displacement (spondylolisthesis) is unlikely before facet degeneration or removal. The facet stress and displacement distribution predicts that facet osteoarthritis or hypertrophy leading to spinal stenosis is most likely under flexion-anterior shear loading. Selective facetectomy may restore spinal canal size without compromising the stability of the segment. A facet that is more sagittally oriented may be linked to the cause of spondylolisthesis, whereas a less transversely oriented facet joint may be linked to rotational instabilities in extension.