Total loss of Bioss!

Missing 12 (UR2) lateral incisor tooth. After extraction, socket allowed to heal for 4 months before CBCT scan taken. Narrow ridge of 4mm in width but good length available. Large flap raised and released for tension free closure. Site was expanded with osteotomes and implant of 3.6mm x11mm placed. Buccal plate intact with no threads visible but augmented the area with BiOss and BioGide. Very good primary stability. Membrane was tacked high up in the sulcus with 2 titanium tacks. Rochette bridge used as provisional. Periapical taken. At suture removal 10 days later, excellent healing and no swelling or discomfort. At exposure appointment- 5 months later , the entire implant unscrewed when removing the cover screw!!! This has never happened to me. More, not one small particle of BiOss was seen? Where did the augmented material go? Pt has had no discomfort at all or aware of any loss of bone in the area. Why has this happened? Possible causes? Thank you

A few questions first, how often was follow up? Any purulence noted at any follow up? Any X-rays and Pics available?
Possible reasons for failure could be

  1. Too thin of buccal bone left after expansion which just died off due to inadequate blood supply.
  2. Fibrous encapsulation rather than osseous healing

Just to get you started on possible causes.

3,6mm implant on 4mm bone is too risky, better use 3mm - 3,25mm implant