Biomechanical study of mandible bone supporting a four-implant retained bridge: finite element analysis of the influence of bone anisotropy and foodstuff position.

PURPOSE: The aim of this work was to study the biomechanical behavior of a "All-on-four" implant-supported prosthesis through a finite element analysis using either isotropic or anisotropic properties of bone. The influence of foodstuff position during mastication was also analysed.
MATERIALS AND METHODS: A three-dimensional finite element model of a mandible with a prosthesis supported by four implants was developed. The geometry of the edentulous mandible and prosthesis was generated from computed tomography. Four MKIII implants (two vertical and two tilted) were modeled. The bone elastic properties used in the anisotropic simulations were orthotropic. The comparison of isotropic and anisotropic models was carried out in the loading condition of mastication with a foodstuff positioned on molar. Three distinct configurations, corresponding to three foodstuff positions, were then studied. MSC/Marc code was used to perform all computations.
RESULTS: Significant differences in stress, strain, and strain energy densities were found in the comparison of isotropic and orthotropic models. Molar position was revealed to be the most critical one, from a stress and strain level point of view, for implants and framework and consequently for peri-implant bone. It was also observed that implant tilting leads to high stress concentrations in bone for the "All-on-four" concept.
CONCLUSION: This study showed that the anisotropic behaviour of bone cannot be neglected in the numerical simulations. The actual design of the prosthesis is not optimal concerning the capacity of all metallic parts to support loads. Finally, it was demonstrated that the tilting of implants induced a high stress level at bone-implant interface.