It consists in a forward shifting of a vertebra over the next one. According to the extent of the shift, it can be classified into 5 grades with grade I being the mildest and grade 5 being the most severe. It can be produced by the degeneration of the tissues that stabilize the vertebrae (degenerative cause), by weakness or lack of union in an area of the vertebra that causes a poor support (isthmic spondylolysis), or by trauma and other less common causes. The resulting consequence is the narrowing of the canal and the foramens through which the nerves emerge, compressing them and causing back and leg pain, as well as alterations in sensitivity and mobility. The initial treatment is a conservative treatment with medications and physical therapy, but when there is no response, surgical intervention is necessary to perform decompression of the nerves and stabilization of the affected segment (lumbar spinal fusion).