This condition occurs when a lumbar vertebra slips out of place. It slides forward, distorting the shape of your spine. This may compress the nerves in the spinal canal. The nerves that exit the foramen (open spaces on the sides of your vertebrae) may also be compressed. These compressed nerves can cause pain and other problems.
An overall narrowing of the spinal canal and can occur on a more congenital basis where a patient has pre-existing canal tightening which is exacerbated by disc injury or a degenerative process producing a build-up of ligament and joint as well as disc bulge from the anterior side producing spinal stenosis.