Spondylolisthesis

Spondylolisthesis is a condition in which a vertebra of the lumbar spine slips out of place. As the spine tries to stabilize itself, the joints between the slipped vertebra and adjacent vertebrae can become enlarged, pinching nerves as they exit the spinal column. Spondylolisthesis may cause not only low back pain but severe sciatica leg pain.

From Wikipedia - Free Encyclopedia

Spondylolisthesis (not to be confused with spondylosis or spondylolysis) is an anteroposterior translatory movement of two spinal vertebrae in relationship to each other caused by instability between the two involved vertebrae.

The instability can be caused by degenerative changes of the facet joints, or by congenital or traumatic disruption of the pars interarticularis of the upper of the two vertebrae. It occurs most commonly in the lumbar spine.