The term Spondylolysis refers to a stress fracture in the pars interarticularis. A stress fracture, similar to that of shin splints in a competitive runner, is not a complete break in the bone. Rather, a stress fracture is seen with repetitive unnatural forces causing microscopic cracks in the bone. In severe cases the pars interarticularis can be completely broken, leading to a separation of the front and back segments of the spine. This causes a forward slippage of the upper vertebral body known as Spondylolisthesis.