Can a cadaver bone graft recede over the years and expose the implant's screw under the crown?

All bone grafts (all BONE), get replaced over time.  Like the lining of the stomach, where the tissue is constantly being turned over, bone is broken down and replaced all the time as a source of calcium.This means that all graft material is broken down and replaced with native bone.  Within 3-6 months, human bone graft material (host or cadaver) is completely gone and in its place is real bone.Can it recede over time?  Sure, just like regular bone can.  If handled correctly, this loss of bone can be minimized.

yes and that will happen , but difficult to say about ratio and duration. but on an average with time bone around implant reabsorbs and there r chances of thread exposure.

Dear dentalmit: The only constant in life if that everything must chance. Any treatment is subject to changes over time.  The  main problem with bone grafts is that we typically see up to 40-50% resorption of the graft within the first year.  Even after implants are placed in the draft and it is in service, we still see some resorption overtime.  This  is the reason why surgeons typically overbuild the graft as much as possible. In addition very careful monitoring and cleaning of the area are a must.   At this point,  you should see a qualified periodontist or oral surgeon to evaluate what can be done whether to restore the bone and tissue to the area, which is typically very difficult, or to maintain the situation at this level very carefully. Best of luck