This slip is called Spondylolisthesis. Spondylolysis is the commonest cause of spondylolisthesis, and is sometimes referred to as isthmic spondylolisthesis, since the defect is in the isthmus. But there are other causes of spondylolisthesis. More rarely, any infection or tumor affecting the posterior bony ring including the facet joints, can also cause instability and spondylolisthesis. The commonest cause of spondylolisthesis is spondylolysis, and it is the cause of spondylolysis that is the subject of intense debate.
Some people feel that it is an inherited defect of the pars interarticularis.
Spondylolysis has been reported in an infant, although it is rare before age 4. However, certain other observations point to a repetitive trauma etiology. Incidence goes up with age, and incidence is higher in children involved in certain kinds of sports like gymnastics, weight-lifting and football that put a lot of stress on the back. In gymnastics, the hyperextension position of the lumbar spine places excessive stress on the back, leading to stress fractures in the pars interarticularis. Most children with spondylolysis, and even some children with spondylolisthesis are aymptomatic, and may grow up not even aware that they have the conditioin.