The effect of intervertebral disc space narrowing on the contact force between the nerve root and a simulated disc protrusion.

An instrumented probe mounted on the anterior surface of the lumbar spine over an excised lumbar intervertebral disc was used to stimulate a disc protrusion in 12 fresh cadavers. The contact force between probe and nerve root was measured as a function of two independent variables: probe protrusion depth and disc space height. The contact force on the nerve root was found to increase with increasing probe depth. Disc space widening increased the contact force while narrowing the disc space decreased it. A simple mechanical model analysis confirmed that the force exerted on the nerve root by the probe is the result of tension produced in the nerve root as it is deformed by the probe. The mechanical principle that disc narrowing can reduce the pressure on a nerve root produced by a disc protrusion may be an explanation of how chemonycleolysis relieves sciatic pain.