It has been reported that when micromovement of an implant occurs, an ingrowth of soft tissue occurs after the implant is embedded; therefore, it is difficult to achieve osseointegration [32-34]. Brunski et al. [35] reported that when immediate loading or early loading is carried out, micromovements of the implant should be controlled to 100 μm or less and excessive movement of the implant not only impairs osseointegration but also encourages the growth of connective tissue. In the experimental results of the present study, the displacement of the contact model, which assumed immediate loading, showed greater values than the fixation model, which assumed delayed loading. That is to say, in the FEA models constructed in the present study, the results support the notion that micromovements are likely to occur during immediate loading and that suppressing these as much as possible is necessary for successful osseointegration.

When assessing stress values of FEA models, it is desirable to do so after confirmation of the validity of the models [11]. Therefore, we first confirmed the validity of the FEA models from comparisons of the correlation coefficients of the displacements under loading conditions in the experimental and contact models. Then, the equivalent stress values and their sites of occurrence were assessed to examine how peri-implant bone is impacted by differences in boundary conditions and loading points.

The equivalent stress values in both the contact and fixation models were smallest for central loading, while buccal and lingual loading showed substantially equivalent values greater than that of central loading. Equivalent stress occurrence sites were observed to be high in bone surrounding the implant neck on the loading side, similar to previous reports [36,37]. Hobo et al. [38] stated that while implants were resistant to vertical pressure, horizontal pressure (bending movements) generated torque in the implants and had more harmful effects; therefore, it would be wise to limit the occlusal contact of the superstructure to vertical pressure and avoid horizontal pressure as much as possible. This was also consistent with reports that lateral loading generated more stress than vertical loading, as is also found in Sato et al.’s report using a geometric analysis [39], and supports the existing clinical concept that a lateral force applied to an implant greatly increases the stress in the surrounding bone.