Different surgical techniques in "ALL-ON-FOUR" implants; evaluation of the stress distribution created inthe implant and its surrounding bone with finite element analysis

The purpose of this study is to examine the stresses caused by various surgical techniques in All-on-Four surgery, such as conventional, bicortical, and bicortical combined with nasal floor elevation, on the implant and surrounding bone, using 3D finite element analysis (FEA). An atrophic maxilla bone model was created using tomography of an edentulous adult with atrophic maxilla. The superstructure model, designed from polymethyl methacrylate on a metal bar (chrome-cobalt), was modeled in ALTAIR Evolve software. In all the models, implant diameters were 4 mm, and implant lengths were 13 mm anteriorly and 15 mm posteriorly. The "All-on-four" technique was used to place implants in four different atrophic maxillary models. The implants were designed as anterior and posterior monocortical in the 1st model, anterior and posterior bicortical in the 2nd model, anterior and posterior bicortical in the 3rd model, and anterior and posterior bicortical with nasal floor elevation in the 4th model. By applying force in both vertical and 450 oblique directions, eight linear analyses were performed on the four models developed in this study. For implants and their superstructures, Von Mises values were obtained; for cortical and trabecular bone layers, maximum-minimum Principal stress values were obtained. When the cortical and cancellous bone around the anterior implants were examined, it was observed that the stresses around the implant under oblique and vertical loading conditions were similar in all models and there was no statistically significant difference. When the posterior implants were examined, it was observed that the oblique compression, vertical compression, and vertical tensile forces on the cortical bone and cancellous bone were the most in model 1. According to the VMS analysis results of anterior and posterior implants, higher stress values were observed in model 1 than in model 3 and model 4 under oblique and vertical forces. It was observed that bicortical placement of the implants reduced the stresses on the bone and implant-abutment system but had no significant effect on the stress on the bar. In addition, it was observed that nasal floor elevation did not make a significant difference in terms of distribution and values of forces compared to model 3. Based on the results of this study, the bicortical placement of implants in the All-on- Four technique reduced the stress on the bone and implant when appropriate anatomy was present. In addition, nasal floor elevation can be applied in the atrophic maxilla with the correct indications.