Mechanism of Facet Load Transmission as a Hypothesis for Low-Back Pain

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Low-back pain has a complex and multi-faceted etiology. The articular facets have been shown to be load-bearing structures and may be a site for low-back pain. The aim of this paper is to establish the mechanism for the transmission of axial load across a facet joint and to propose a facet-related hypothesis for low-back pain. The mechanism of load transmission was studied by two methods. Lumbar segments were instrumented with an intervertebral load cell (IVLC) to measure disc load so that facet load could be deduced. The applied load was moved 10 mm anteriorly and 12.5 mm posteriorly from the center of the vertebral body. The facets then were separated from the body and loaded axially to determine their stiffness in tension and compression and to observe the fallure mode of the joint. It was shown optically that compressive loading of the isolated facet joints was equivalent to spinal extension and tensile loading to spinal flexion. Lastly, a finite element model of a lumbar motion segment was developed to simulate the transmission of facet load and to study the effects of disc degeneration on facet loads. Results of the study on six lumbar segments revealed that the normal facets carried 3–25%. If the facet joint was arthritic, the load could be as high as 47%. Experiments on isolated facet joints revealed that they behaved as a stiffening spring in compression and were weak in tension. The resistance supplied by the capsular ligaments was an order of magnitude less than that in compression. When loaded to failure in compression, the inferior lumbar facets rotated posteriorly relative to the superior facets of the vertebra below and caused the capsule to rupture at about 6 kN without bony fracture. The transmission of compressive facet load occurs through contact of the tip of the inferior facet with the pars of the vertebra below. The data also show that an overloaded facet joint will cause rearward rotation of the inferior facet, resulting in the stretching of the joint capsule. The finite element model predicted an increase in facet load due to a decrease in disc height. The following hypothesis is proposed: Excessive facet loads stretch the joint capsule and can be a cause for low-back pain. Further study is needed to prove this hypothesis.

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