Evaluation of the effects of the distance and the vertical level of the terminal implant in bar retained, four-implant supported overdenture treatments on the stresses in the peri-implant, using finite element analysis

Objective: The aim of this study is to investigate the effects of the distance and the vertical level of the terminal implant in relation to its neighboring implant in bar retained, four-implant supported overdenture treatments on the stresses and strains in the surrounding bone, using 3D finite element analysis. Method: Four main groups were constructed by moving the neighboring implant mesially at 1 mm intervals resulting in distances of 3.8, 4.7, 5.6 and 6.5 millimeters. For each of these four main groups, seven sub-groups were created by setting the left terminal implant's vertical position 1, 2 and 3 millimeters both coronally and apically with respect to its neighboring implant. The remaining group was set to have the same vertical position with respect to its neighboring implant. A vertical load of 100 N was applied to the distal part of the bar's left side cantilever. The von Mises stresses, maximum and minimum principal stress and strains for each of the 28 models were computed. Results: The models with coronally positioned implants showed similar or decreased levels of compressive stress, and strains in the surrounding bone, compared to the models in which the terminal implant was at the same vertical position in relation to its neighboring implant. The tensile stress and strain levels around the terminal implant showed a significant increase when it was placed farther coronally. The models in which the terminal implant was positioned apically resulted in reduced levels of strain and stress. Increasing the distance between the terminal implant and its neighboring implant, in general, resulted in significantly increased levels of strain and stress in the surrounding bone. Conclusion: The current study, with its limitations inherent to finite element analysis, suggests that in four-implant supported, bar retained overdenture treatments, unideal vertical positioning of the terminal implant due to reasons like bone irregularities and defects might not cause excessive levels of stress and strain in the surrounding bone.