Occurs when changes from ageing and general wear and tear make it hard for your joints and ligaments to keep your spine in the proper position. The vertebrae move more than they should, and one vertebra can slide forward on top of another. If too much slippage occurs, the bones may begin to press on the spinal nerves or compress the small joints at the back of the spine (facet joints). This causes pain in the back which is made worse with extension (leaning backwards) and may lead to back spasms.
Click the button to a video explaining Spondylolithesis: