Spondylolisthesis refers to a slip of one vertebrae on another. This can occur either forwards (anterolisthesis) or backwards (retrolisthesis) and can be due to chronic, degenerative changes with age or in younger patients with a fracture (spondylolysis). This forward slip of the vertebrae can cause narrowing of the space available for your nerves, resulting in compression and back and leg pain, as well as heaviness, numbness/tingling, and weakness of the legs. Treatment depends on symptom severity and duration, varying from nonoperative to operative techniques.