3). Conclusion: The results show that abutment screw loosening is a rare event in singleimplant restorations regardless of the geometry of implant-abutment connection, provided that proper antirotational features and torque are employed. INT J ORAL MAXILLOFAC IMPLANTS 2008;23:681-690Key words: implant complications, implant geometry, meta-analysis, screw loosening, single implants T he stability of the connection between different implant parts is important for the overall success of the reconstruction. This is especially true for singletooth restorations, where a strong interlock between the abutment and implant is necessary. Various mechanisms have been proposed to connect the dental implant abutment to the implant body. Different systems vary in connection geometry, materials, and overall screw mechanics. The implant-abutment connection stability is also influenced by factors such as component fit, machining accuracy, saliva contamination, and screw preload. [1][2][3][4][5][6][7][8] Most in vitro studies, [9][10][11][12] with the exception of one, 13 have demonstrated that internal connections are more mechanically stable than external flat (hex-type) connections. Loosening of abutment screws, particularly with the "classic" external-hex implant systems, has been a well-known technical problem, occurring mainly during the first 2 years after delivery of the crown. [14][15][16][17] The stability of the external implant-abutment connection was subsequently improved by altering the screw alloys and their surfaces and applying proper torque values to establish higher initial preloads. 3,[18][19][20][21]