<qa>  What is the forward displacement of a vertebra?Spondylolysis is as stress fracture in the pars interarticularis of the vertebral arch most commonly occurring in the lower lumbar spine. Spondylolisthesis is the forward displacement of a vertebra, especially the fifth lumbar vertebra, most commonly occurring after a break or fracture. These injuries to the spine are extremely common in adolescent athletes and some research reports that spondylolysis and spondylolisthesis account for over 40 of all adolescent athletes low back pain. 