On the changes that are described are the changes that are present from birth. Our spine is made up of blocks of bone stacked one above the other like a tall column of 33 bricks. The upper 7 are called cervical vertebrae, next come the 12 thoracic vertebrae (opposite of the chest), next are the 5 lumbar vertebrae (these are opposite the abdomen), next are the 5 sacral vertebrae and lastly 4 are the coccigeal vertebrae of tail bone. The sacral vertebrae do not have any movements between them. The term sacralisation means merger of one of the lumbar vertebrae with the sacrum. The lumbar vertebrae are mobile, so merger with the immobile section causes an increase in stress and strain of the rest of the lumbar vertebrae.
The spinal canal is a canal which has formed within this column of bricks to protect the most important nerve complex of spine called the spinal cord. Normally it is a full tube. In some from birth it is only like a canal and not a tube. This is called spinal bifida. This is often an incidental finding on X- rays and may have nothing to do with your back pain so don't be alarmed by this. The joint between the sacrum and your pelvic bone (above the hip joint) is the sacroiliac joint. Sclerosis means increased density of the bone. This is often so in some patients with arthritis of the spine. Again I do not think you need to worry about this. The term listhesis means horizontal slipping of one brick over the other. In the case your spine slippage of L5 S1 is not there. I think what you need is careful clinical examination by a competent orthopaedic surgeon or a neurosurgeon. Your symptoms and X-ray findings alone cannot help me guide you correctly. The physical findings are equally important. Let your surgeon assess you and then get back to me with the findings. Then I will be in a better position to give a satisfactory reply to your worries.