Spondylolisthesis. X-ray of the lower back showing the spine with a slipped vertebra (spondylolisthe- sis). The edge of the back is at right. The most common type of spondylolisthesis, as here, is when the fifth lumbar vertebra, the sixth bone seen from the top, slips forward over the top of the sacrum (at lower right). This movement has caused the bony processes on the right of this lumbar vertebra to break. The condition is often due to spondylolysis, where abnormally soft vertebral bones are more liable to slip. Spondylolisthesis, which results in lower back pain, is diagnosed by X-ray and treated by wearing a plaster corset.