Clinically, it is generally considered that the crown length increases proportionally when the length of the implant body decreases because of alveolar bone resorption. However, most previous studies performing FEA of short implants have analyzed them with a standard crown length [38]. In this study, the distance from the tip of the implant body to the occlusal plane was standardized to make the analysis condition more applicable to the clinical situation. We set the condition that the crown length would increase as the length of the implant body decreased, and the analysis was then performed. As the crown–implant ratio increases, the rotational moment increases and the stress generated at the implant neck also increases [39]. Previous research reported no significant difference in the survival rate and bone resorption when the crown–implant ratio ranges from 2:1 to 3:1, but when an implant of 4.0 mm length is inserted, it is assumed that the ratio increases [39, 40]. Another report suggested that clinical outcomes are significantly worsened if the crown length exceeds 15.0 mm. In consideration of these reports, analysis of the 4.0 mm length implant was carried out with a crown length of 16.0 mm and a crown–implant ratio of 4:1 [41].

Regarding the FEA of short implants, it is already known that stress concentrates on the cervical cortical bone regardless of the length of the implant body [42]. The present study found that the stress distribution in the cervical cortical bone increased as the length of the implant body decreased. In addition, BL implants showed a maximum stress value similar to TL implants that were 2 mm shorter than the BL design. As such, it was found that the difference in design between TL and BL implants has a greater influence on stress distribution than the 2-mm difference in length. Good clinical results have been reported for BL implants with a length of 6.0 mm. Furthermore, the appropriate crown length and crown–implant ratio have never been evaluated clinically for a 4.0-mm-long TL implant; however, in our in vitro experiment, this implant was placed under severe loading conditions and the maximum stress values were similar between the 6-mm-long BL implant and the 4-mm-long TL implant. Therefore, it is suggested that the 4-mm-long TL implant may be mechanically useful [43]. Although the cases to consider the use of short implants should be selected carefully, it was suggested that the risk of failure can be reduced by the design and material selection of the implant body. However, especially, in extra short implants, these stress concentration increases mechanical risks such as fracture of the implant body and screw loosening. The results in this study are under the limited conditions to compare the stress distribution in the surrounding bone. In the future, in addition to need to evaluate the stress distribution of the component, we believe that it will be necessary to compare with the accumulated clinical results.