Fusing the lumbar spine isn't a simple operation. The vertebrae are difficult to get to and often require moving nerves, blood vessels, and muscles out of the way. Until recently, spinal fusion was done with an open incision from the back of the spine. The muscles were stripped away and then reattached.
With this method of fusion there was the chance for a large blood loss and nerve damage leading to permanent problems. The term fusion disease was used to describe long-term problems from this kind of damage.
Current methods of gaining access to the spine are changing. New tools are making it possible for the surgeon to make a small opening and fuse the spine without cutting muscles or nerves. A tiny TV camera on the end of a long tube is inserted through a small incision. The surgeon does the operation while watching on an X-ray screen.
With this new minimally invasive operation, it's expected that this "fusion disease" will become a problem of the past.