I just recently got dentures, but i did not have a bone graft. The bone in your jaw shrinks once you get dentures, it seems weird that you would need this before the dentures. I know that before i got dentures, my jaw bone was more shrunken than many people's because i had many missing teeth, which made it shrink. But i didnt have to get the grafts...Maybe there needs to be more bone so that there is a bigger ridge (i think that is what that is called), because the top dentures stay on due to suction around the gum, and if the ridge is too small, they wont suction. I agree, you should ask why this is necesary. Your jawbone will shrink with the dentures..maybe they are concerned it will shrink too much. In my case, they said i would need a bone graft if i were to get teeth implants later on, but not for dentures. i hope everything goes well!