Stress Distribution and Abutment Screw Loosening in All-Ceramic and Metal-Ceramic Implant Prostheses: A Computational Finite Element Study

Abstract

Background. Implant-supported prostheses are widely used for replacing lost teeth and restoring chewing function. However, complications such as screw loosening, particularly in prostheses with external hex connections and molars, remain common. This study compares the biomechanical behavior of titanium abutment screws in metal-ceramic and all-ceramic implant prostheses with cantilevers and traditional bridges with pontics in the posterior maxilla using finite element analysis (FEA). Materials and Methods. Four FEA models of implant-supported restorations were designed. Group 1 included a PFM (Porcelain Fused to Metal) bridge with a pontic, where implants were placed in positions 4 and 6. Group 2 involved an all-ceramic bridge with a pontic, with implants in the same positions. Group 3 focused on a mesial cantilever design with PFM restorations, with implants placed in positions 5 and 6. Lastly, Group 4 used a mesial cantilever design with an all-ceramic restoration, also with implants in positions 5 and 6. Each model underwent axial loading along the longitudinal axis and vertical loading under centric occlusion to assess the implant-supported junction and abutment screw loosening. Additionally, biomechanical behavior under different prosthetic materials was examined. Results. The findings indicated that the highest stress was concentrated on the screw head and the implant-abutment connection, with the greatest displacement observed in tooth number 6, particularly in the cantilever models. The all-ceramic models showed minimal screw-abutment separation, with the largest being 11.1 micrometers in the cantilever model. The metal-ceramic models also exhibited minimal separation, with the highest being 11.5 micrometers in tooth number 6. Tooth number 6 consistently demonstrated the greatest potential for screw loosening. Conclusion. The study concluded that monolithic zirconia bridges are suitable and acceptable for use in implant-supported prostheses, especially when properly reinforced at the connector points. The findings suggest that well-designed prostheses can reduce stress concentrations and the likelihood of screw loosening, thereby enhancing the durability of implant-supported restorations. Further detailed studies and observational research are recommended to validate these results across different parts of the dental arch and in real-world clinical scenarios.