From the study, the RTVs of the control group, which was left unloaded for 10 min after second tightening decreased 7.78% from the tightening torque. Previous studies have shown a 2 to 10% loss of screw preload . These results can be explained by the settling effect as follows . Firstly, the tightening torque is used to overcome the friction of the contacting metal surface connection . Secondly, wear of the contacting implant-abutment surface can cause axial displacement of the abutment into the implant bore and the length of the elongated screw is shortened microscopically, leading to the loss of screw preload [12, 33]. After mechanical cyclic loading, all experimental groups from 50,000 to 2,000,000 cycles showed significant decreases in mean RTVs compared with the initial RTVs (P < 0.05). These results are similar to those of several studies. Mohammed et al.  reported that the post-loading RTVs of internal hex implants were significantly lower than the initial RTVs after 16,000 cycles and Cibirka et al.  found decreasing RTVs in the internal hexagon of Nobel Biocare implants after 5 million cycles of fatigue testing. The decrease in RTVs after cyclic loading can be explained by the micromovement of the joint connection or progressive settling effect from functional loading . It is assumed that mechanical cyclic loading will serve as a proxy for oral functional loading, which can cause micromovement and slipping between the abutment screw thread and the implant, reducing the tensional force and resulting in decreased preload of the screw [34, 35]. Additionally, in functionally loaded implants, the progressive settling effect and the wedge effect cause increased axial displacement of the abutment into the implant connection . Seol et al. , who analyzed the axial displacement of an internal implant-abutment connection after cyclic loading, found that the two-piece abutment of an internal octagon connection showed continuous axial displacement, but the rate of axial displacement was slow after 100,000 cycles. In our study, the decrease in RTVs was also constant after 50,000 cycles. This might imply that the axial displacement has a great effect on the loss of screw preload. Additionally, Kim et al.  indicated that the RTVs of abutment screws are related to the settling value. They found that after cyclic loading, there were statistically significant differences in the settling value and also in RTVs in internal connections, whereas external connections showed no significant changes in settling values and resulted in no significant changes in RTVs. However, the relation between settling values and RTVs varies depending on many factors, such as the material, design, and characteristic of the abutment-implant interface .
Although the reduction in RTVs was observed after mechanical cyclic loading, no screw loosening occurred after 2,000,000 cycles of loading, which it assumed represents 3 years of function in worse scenario setting according to ISO 14801 in vivo . Our study presents similar results to those of Binon and McHugh, who reported that the 30-N-cm insertion torque can maintain screw-joint stability in 3 years of simulated function . The reduction of screw preload after 2,000,000 cycles of loading was 42.74%. This is considered to be relatively low and might be because of the implant-abutment connection design . The eccentric force has little effect on the screw preload under functional loading because the contacting part of the cone connection helps to provide frictional resistance and mechanical stability [7, 8]. Moreover, the Octatorx lobular anti-rotational design help produce little micromovement in the joint system under load . The screw design also has an effect on the screw preload . Paepoemsin et al.  found that the retaining tapered screws of their implants maintained higher preload efficiency than did the flat head screws of the implants before and after cyclic loading (P < 0.05). In our study, RTVs were constant from 50,000 cycles of loading to 1,800,000 cycles. This result is not in agreement with those of Khraisat et al. , who concluded that 1,000,000 cycles of loading significantly affected the RTVs of CeraOne abutment external hex implants compared with 500,000 cycles of loading. This might be because different types of implant-abutment connections were used. In external connections, implant-abutment stability is obtained primarily by the tension of the screws . Therefore, the screw preload in external connections is affected by the cyclic loading more than in internal connections.
Cho et al.  studied the effect of retightening the abutment screw on RTVs in internal connection implants under cyclic loading at 3, 10,100, and every 20,000 cycles up to 100,000 cycles. They found that most of the decrease in RTVs occurred at 10 cycles, and after that, RTVs did not change significantly. The study showed that retightening the abutment screw under cyclic loading resulted in superior RTVs when compared with no retightening . However, retightening the abutment screw might change the shape of the abutment screw and the inner screw thread of the implant . In our study, we demonstrated decreasing the RTVs after cyclic loading without retightening the abutment screw.
Tzenakis et al.  reported that retightening of the screw is strongly recommended, but the appropriate timing of retightening is still not clear. Cho et al.  recommended retightening abutment screws in the early stage of functional loading, at the first week in vivo, in internal type implants because they found no significant difference in RTVs between 20,000 and 100,000 cycles of loading. In our study, the RTVs decreased in the early stage at 50,000 cycles of loading and then remained constant to 1,800,000 cycles. The result can be used to estimate that the implant should also be retightened in the early stage of cyclic loading under a worst-case situation . Seol et al.  stated that to minimize screw loosening, the screw should be retightening at 1 month in vivo after constant axial displacement at 100,000 cycles of loading. Nevertheless, different implant-abutment connections, designs, and materials influence the screw preload differently. Therefore, it may be summarized that re-tightening is a theoretical recommendation based on in vitro studies that have found it beneficial in terms of RTV reduction.
This study cannot compare the RTVs to the other implant-abutment connections because each implant system has their own manufacturing technique especially the tolerance value between implant and abutment connection, and these values are considered as the confidential data of the company. However, by conforming to the international standard of the medical device for dental implant systems (ISO 13485), the results of this study may be applicable to other combined mandatory index-cone connections of other commercial implant systems. Clinically, the retightening of the abutment screw is recommended after the first 6 months in function because the significant preload loss at the early cyclic loading as shown in our and other studies . The 6-month or yearly regular clinical follow-up after dental implant treatment is suggested. In the clinical situation, the screw loosening leads to implant prosthesis movement and in the worst case, the abutment screw may break. The RTV value which related to the screw and implant-abutment connection loosening in the clinical situation is still not clear and further clinical studies are required.
In addition, further studies are suggested to examine the relation between the settling value  and the preload value of the screw. It would be helpful for evaluating the stability of the implant-abutment connection after functional loading. The decreasing pattern of RTVs after cyclic loading was specific to the design of the implant-abutment connection, so more studies are recommended in other implant systems.