Evaluation of the stress in the implant and the surrounding bone; from the different vertical misfits on the implant supported overdenture and the different cantilever designs : A three-dimensional finite element analysis

Aim: The purpose of this study is to investigate with finite element analysis the distal bar extension, bar framework material types and bar framework abutment misfit, stress in implant and its surrounding bone tissue in mandibular overdenture prostheses. Material and methods: A prosthesis with a bar holder was designed on three implants placed in the canine region and midline level in the full toothless lower jaw. The design of the distal cantilever length design of the bar holder is modeled as 0 mm, 8 mm, 14 mm extensions. The vertical incompatibility of the bar holder framework with the abutment is modeled as 0 µm, 100 µm, 200 µm. Twenty-seven (3X3X3) different models were obtained from three different bar framework materials (Ti Grade5, Type 4 Au, Cr Co). The occlusal forces were applied perpendicular to the midpoint of rigid comestibles (food stuff) with a radius of 1 cm and 150 N. Food stuff was applied unilaterally to the 1. molar tooth central fossa in the left posterior region. The loading is planned in such a way as to stimulate the masticatory force. Results: It has been found that compression and strech/strain stresses in the cortical bone were higher than the stresses in trabecular bone. As the distal cantilever extension increases, the stress value in the cortical bone concentrates throughout the bone around to the terminal implant to which the force is applied, from the other terminal implant, and the compression value is always concentrated around the terminal implant where the force is applied. It was analyzed that the stress values consisted in the bone around the implant in different framework materials and at different vertical compatibility degrees were similar. As a result of the increase in the amount of distal cantilevers, it was analyzed that the Von Mises values seen in the implants increased in the terminal implant on the side where the force was applied. With the distal extension, the Von Mises value in the terminal implant, where the force was not applied, decreased to about half. While von mises values in implants with different vertical misfit degrees were similar, they were found to be higher in materials with high elastic modulus in different framework materials. The highest stress value in the 14 mm group of distal cantilever extension of the bar framework was found. By changing the elastic module of the framework material, Von Mises values have changed. Conclusions: When the systems with bar holders are planned to be designed with a distal cantilever extension, other studies should be carried out to determine the ideal extension amount. Within the limitations of this study, it can be concluded that increasing of cantilever length in mandibular overdentures did not cause distinct increasing in stress, especially around the implant adjacent to cantilever. The implant circumference of the stiffness of the bar framework with vertical misfit had minimal effect on the bone, but the mechanical components were more clearly affected. Further studies should be carried out for the real effects of framework mismatch on implants. Harder alloys caused higher stress values under all conditions. Key words: Bar attachment overdenture, distal kantilever, finite element analysis, dental implant