Now as we age, a lot of degenerative changes occur in the spine. You get disc degeneration. We get vertebral fractures. We get ligamentous laxity. When we are without estrogen the ligaments are less elastic. We get deterioration of the facet joints in the vertebral body. And overall what you can have setting up is an instability in the spine. The spine meetings at orthopedic surgery focus a lot on spinal instability. And one of the things that happens when you get spinal instability is you get slipping, subluxation, slipping of one vertebral over another and this is called degenerative spondylolisthesis.