Spondylolysis occurs when there is a stress fracture of one or more of the one bones of the spine (vertebrae). this most commonly occurs on the fifth lumbar (low back) vertebrae. The pars interarticularis is a part of the spine in the lower back that joins the front and back of a spinal segment. Spondylolysis is confirmed by seeing a defect in this area. If the defect increases and the vertebra moves forward, this is spondylolisthesis. If the displacement is great, the vertebrae may pinch or stretch nerves, and surgery may be required.