Spondylolisthesis. Coloured X-ray of a section through the lower back of a 42-year-old patient with spondylolisthesis. This is the backward displacement of a vertebra (spinal bone) relative to the one above it. Here it is between the L5 and S1 vertebrae. This condition can be due to a hereditary bone disorder, but can also be caused by trauma. It typically causes back pain and the displaced vertebra may put pressure on nearby spinal nerves or the spinal cord, causing sciatica (leg pain), loss of bladder and/or bowel control and eventually paralysis of the lower limbs.