Load-bearing characteristics of lumbar facets in normal and surgically altered spinal segments.

An in-vitro experimental study was carried out to measure the induced loading on human lumbar facets due to varying amounts of compressive axial load. Testing was done on L2-L3 and L4-L5 spinal motion segments obtained from cadavers at autopsy. The compressive loading was applied to the spinal specimens in first a neutral position and then in an extended position. The motion segments were tested in a normal state and after facetectomy of the left facet. Contact pressures were quantified using pressure-sensitive film. This film was inserted between the articulating surfaces of the two facet joints prior to placing axial loads on the motion segments. In addition to recording pressure, the film was also used to quantify contact area. This provided sufficient information to compute the total facet loadings for all cases studied. Results of this study show how facet loads, peak pressures and contact areas change with respect to the various loads, positions and states studied. In particular this study shows that the absolute facet loads remain relatively constant with increasing segmental compressive loads such that the facet load expressed as a percent of load applied to the segment decreases with increasing axial loads. It also shows that the contact area moves cranially at L2-L3 and caudally at L4-L5 with increasing loads in extension. Furthermore, this study indicates that the load on the remaining facet is reduced substantially after a facetectomy although peak pressure increases. Finally, this study demonstrates the substantial difference in facet loadings between L2-L3 segments and L4-L5 segments.