Its almost as if the back portion had been broken off and tried to heal back - but never did. Actually, there is good evidence to suspect that this is exactly what has occurred. Spondylolysis isn't something people are born with, but it appears that it first shows up sometime in childhood. Interestingly, boys who are football linemen and girls who are gymnasts seem to be affected the most. The current thought is that the spondylolysis is probably a stress fracture that never completely healed.
Spondylolisthesis is the term used to describe when one vertebra slips forward on the one below it.
This usually occurs because there is a spondylolysis in the vertebra on top. There are two main parts of the spine that keep the the vertbrae aligned - the disk and the facet joints. When a spondylolysis occurs, the facet joint can no longer hold the vertebra back. The intervertebral disk may slowly stretch under the increased stress and allow the upper vertebra to slide forward. In the vast majority of cases, the stretching only allows a small amount of forward slip and there is no real danger in an adult that the slipping will continue until the upper vertebra slips off.