Spondylolysis/Spondylolisthesis is caused by a disruption of the facet or zygapophyseal joints and is usually the result of a defect in the pars interarticularis (thin part of the lamina located between the superior and inferior articular process). Spondylolysis therefore represents a defect in the pars interarticularis, which usually represents a stress fracture. Spondylolisthesis describes subluxation of one vertebra relative to the vertebrae below and may occur in association with bilateral spondylolysis.