Lumbar fusions, cervical fusions and many types of orthopedic procedures require bone grafts to stimulate the bones to heal and to provide support the skeleton by filling in gaps between the two vertebrae.
The bone graft stimulates the bones to knit together. A bone graft is made by taking bone tissue, usually taken from the patint's ilia, crushing it into powder and placing around the fusion site. The surviving bone cells (osteocytes) continue to function normally and knit the bone fragments and vertebra together. Other sources of bone graft material are other living people (an allograft) and cadavars, however self-donation (an autograft) is the most successful and common method.
Some major spine fusions require more bone graft than your own body can supply. In this case the surgeon may mix allograft with autograft.
Some bone grafts are larger pieces of bone used to fill a gap between two bones. For example, if the surgeon needs to replace a vertebra or disc with bone, he may graft a chunk of bone into the space. Because bone is rigid, it will hold the bones apart, while the body grows to the chunk of bone graft at either end. Over time, the entire piece of bone that was grafted will be "remodeled" and replaced by the body with new bone. How long this takes depends on how big a piece of bone was used. It is a slow process that may take years.