In my case, I lost bone from cracked roots (old root canals)in molars for example. Infection develops over time. The infection eats away at the bone. Sometimes this infection does not show up on an xray. So, by the time the tooth is extracted there is significant bone loss. If you then choose to go the implant route a bone graft is required (artificial bone used). All of my many unfortunate dental troubles were long before Prolia.