Spondylolisthesis is the slipping of all or part of one vertebra forward on another vertebra.
The most common type of spondylolisthesis is isthmic spondylolisthesis and it is commonly seen in children and young adults. A stress fracture can occur at the pars interarticularis due to repetitive movements of extension and rotation which leads to an increase in shear forces in the lumbar spine. As shear forces increase, pressures on the facet joints increase. These forces are then transmited to the pars interarticularis. Since the pars interarticularis is small, it is unable to absorb repetitive shock which leads to the stress fracture.