Spondylolisthesis is the displacement of one spinal vertebra compared to another. While some medical dictionaries define spondylolisthesis specifically as the forward or anterior displacement of a vertebra over the vertebra inferior to it (or the sacrum), it is often defined in medical textbooks as displacement in any direction. Spondylolisthesis is graded based upon the degree of slippage of one vertebral body relative to the subsequent adjacent vertebral body. Spondylolisthesis is classified as one of the six major etiologies: degenerative, traumatic, dysplastic, isthmic, pathologic, or post-surgical. Spondylolisthesis most commonly occurs in the lumbar spine, primarily at the L5-S1 level with the L5 vertebral body anteriorly translating over the S1 vertebral body. Types Olisthesis (synonym olisthy) is a term that more explicitly denotes displacement in any direction. Forward or anterior displacement can specifically be called anterolisthesis. Anterolisthesis commonly involves the fifth lumbar vertebra. Backward displacement is called retrolisthesis. Lateral displacement is called lateral listhesis or laterolisthesis. A hangman's fracture is a specific type of spondylolisthesis where the second cervical vertebra (C2) is displaced anteriorly relative to the C3 vertebra due to fractures of the C2 vertebra's pedicles. Anterolisthesis Classification Anterolisthesis can be categorized by cause, location and severity. By causes Degenerative anterolisthesis (a.k.a. type 3) is a disease of the older adult that develops as a result of facet arthritis and joint remodeling. Joint arthritis, and ligamentum flavum weakness, may result in slippage of a vertebra. Degenerative forms are more likely to occur in women, persons older than fifty, and African Americans. Traumatic anterolisthesis is rare and results from acute fractures in the neural arch, other than the pars. Dysplastic anterolisthesis (a.k.a. type 1) results from congenital abnormalities of the upper sacral facets or inferior facets of the fifth lumbar vertebra, and accounts for 14% to 21% of all anterolisthesis. Isthmic anterolisthesis (a.k.a. type 2) is caused by a defect in the pars interarticularis but it can also be seen with an elongated pars. Pathologic anterolisthesis (a.k.a. type 5) is caused by either infection or a malignancy. Post-surgical/iatrogenic anterolisthesis (a.k.a. type 6) is caused by complications after surgery. By location Anterolisthesis location includes which vertebrae are involved, and may also specify which parts of the vertebrae are affected. Isthmic anterolisthesis is where there is a defect in the pars interarticularis. It is the most common form of spondylolisthesis; also called spondylolytic spondylolisthesis, it occurs with a reported prevalence of 5–7 percent in