Depending on what type they do. I would assume, assume being the key word here lol, that if they used a bone graft then they would need to remove as much material as possible and also scrape and prep the bone so that it is clean and would give the bone the best chance to graft to one another. I know in mine that is how they did it, but I have seen and heard of them doing as you said, leaving some material. In this case I would be inquiring about pseudoarthrosis or mal/non union. Again it is all speculative as it is a forum and I am not a doctor