Vertebrate have disc in between them, which keeps them mobile and helps in movement. If the disc is diseased and is causing symptoms, then a decision of fusion may be done, in which case physically the disc is removed, and the bone tags are repaired to cause union. A spacer can also be put between the two vertebrates to keep the gap intact while fusion happens. There are multiple form of bone or other products that can be used to maintain the space as well to promote the fusion between the two vertebrates.