Spondylolisthesis is a condition where one vertebra slips out of position over another, usually involving L5 slipping over S1. It is caused by a defect in the pars interarticularis that causes instability. There are several types including isthmic, degenerative, traumatic, and dysplastic. Isthmic spondylolisthesis is the most common type under age 50 and involves a stress fracture of the pars interarticularis. Degenerative spondylolisthesis is most common over age 50 and does not involve a fracture. Symptoms include low back pain and leg pain or numbness. Treatment depends on severity but may include rest, bracing, physical therapy, or surgery.