The spine can become unstable for a variety of reasons. In young people, a condition called spondylolysis can lead to spondylolisthesis. In older individuals, wear and tear of the spine can cause a degenerative spondylolisthesis. When these conditions occur, the vertebral bodies are not properly held in place, and prone to "slip" forward over the vertebral body below itself.
Treatment of spondylolisthesis depends on two factors: how significant the slip is, and how quickly it is progressing. Mild cases of this condition can be managed with simple treatments, more significant cases may need surgical stabilization. Most often the unstable vertabrae are fused in place. In the most serious cases, the vertebral body is returned to its normal position ('reduced') and subsequently fused.