Tzenakis et al. [42] reported that retightening of the screw is strongly recommended, but the appropriate timing of retightening is still not clear. Cho et al. [39] 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 [26]. Seol et al. [37] 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 [39]. 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.