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Introduction
Titanium dental implants are widely used because of their benefits such as optimal mechanical properties and great anchorage in the jawbone. The suitable choice of a particular type of a prosthetic restoration (cement-retained or screw-retained), and an appropriate implant-abutment connection system (external hexagon, internal hexagon, or internal taper) are critical for successful outcomes.1,2 However, mechanical complications such as fracture or mobility of the superstructure or abutment screw have been frequently reported.3,4 These complications may occur due to several reasons, such as the errors accumulated during the multiple steps of prosthesis fabrication, characteristics of the materials, or the surface irregularities of the mating surfaces of implant, abutment, and screw.5–9 Factors such as the connection design, screw design, screw settling, adequate preload by proper tightening torque, precision of mating implant components, and implant diameter are also important in screw joint stability.10–16 Recent studies have reported that the positional stability of the implant-abutment connection is a determinant factor for prosthesis misfit and mechanical complications.17–19
A number of studies have compared the efficacy of different connection mechanisms securing the implant-abutment connection stability, which determines the mechanical integrity, strength and stability of the assembly.9,20–23 Currently, there are over 20 different commercially available geometrical variations of implant-abutment interface, and all manufacturers claim that their designs greatly contribute to implant connection stability.24,25 Most current designs have been derived from 2 basic designs namely the “butt-joint” with an external connection, and internal “cone-in-cone” connection. According to some recent studies, the latter design shows higher resistance against the bending forces applied to the implant-abutment connection and minimizes the risk of screw loosening and fracture that typically occur in use of external butt-joint connections.1,26–28 On the other hand, some studies claim that the abutment screw characteristics, such as the screw material or surface, are the main determinant factors in this respect, and neither the internal nor the external design had any important effect on implant-abutment connection.29,30 An abutment with internal taper connection may have directly machined threads on the abutment body (1-piece abutment), or it may be fixed to the implant by a separate screw (2-piece abutment).1,2 The preference for one type of the abutment over the other (1- or 2-piece) depends on...