In this condition, damage to bones or joints causes vertebrae to slip forward and distort the spinal cord. There are two types of spondylolisthesis, degenerative and isthmic. Degenerative spondylolisthesis occurs when the joints weaken, allowing a vertebra to slip forward. Nerve roots may become pinched, causing pain to radiate to the legs and feet. Isthmic spondylolisthesis occurs when vertebral bone fractures, allowing a vertebra to slip forward. This can also pinch nerve roots, causing pain to radiate to the legs and feet.