In fact the abutment screw loosening is due to incomplete seating of the abutment (Fig.10,11 >), which is neglected until the screw becomes loose again (nearly 4 years later).  Using a smaller abutment, the seating is apparently complete (Fig.12).

Ridge Split Last Next 劈开术 Xin Wei, DDS, PhD, MS 1st edition 03/12/2016, last revision 04/10/2021