If teeth have been missing for a long period of time then there are no teeth loading the bone. The bone will shrink and oftentimes you are left with a knife edge ridge which is not wide enough to place implants. When this occurs we need to re-grow the bone width (ie: ridge augmentation) so we can place large, stable implants.
The process is the same principle as any bone grafting. Bone graft material is grafted in the site and covered up with a membrane to allow the healing to take place undisturbed. During this time the bone graft is replaced with natural bone. After 8 months of healing your Periodontist can return to place your implants.