Assuming the 20 year old fusion is solid (no pseudoarthroses), the most common problem at around 20 years post-fusion is a proximal junctional kyphosis (PJK). The movable levels above the solid spine (fused) segment tends to take all the stress as a stress riser and simply wears out. Depending on the amount of angulation (kyphosis) is present and how many segments are involved, another fusion might be necessary. You start the diagnostic process with a standard X-ray.
Dr. Corenman