Investigating the optimal design of All-on-Four technique adopting finite element analysis: the aspect of framework material, kind and position of implants

Nowadays, the dental implant surgery is a sophisticate and accurate sector with techniques increasingly innovative such as rapid prototyping, guided implant surgery and stem cell-based approaches. An example is certainly the use of multiple implants, instead of several prosthesis in case of human edentulous condition. The aim of this research is to investigate the mechanical behavior of the All-on-Four technique for different boundary conditions such as the value of load, framework material, type and position of implant. The goal was essentially trying to find out, by the application of structural static Finite Element Analysis (FEM), the best design for this specific treatment. After that, a stress-life fatigue numerical analysis was conducted for the optimal configuration in order to estimate the fatigue life in accordance with both Gerber and Goodman mean stress theory. The coupling involved the implants supported by an arch and a human mandible composed of cortical and cancellous part. After the simulations, it was found that the stress/strain field was very sensitive to the boundary conditions imposed. In particular, the position of the implants and the material framework yielded different responses depending on the implant design. Finally the use of ultrashort implants provided a significant decrease in the developed efforts than the long ones if the first premolar position was assumed. More specific, the stress peaks were in the range 100-225 MPa for the implants, 300-537 MPa for the framework, 50-124 MPa for the cortical bone and 3-35 MPa for the cancellous bone and they were located essentially in the abutment-framework connection as much as in implant neck-bone coupling. The best design saw the presence of ultra-short implant, first premolar position and Co-Cr alloy as framework material. The fatigue test confirmed the stability of the structure even with dynamic loads, but critical spots were present in the framework. In conclusion, the All-on-Four technique is a valid and safe alternative, even in case of ultrashort implants, for human edentulism care.