Influence of Prosthetic Screw Designs with and Without Titanium Base on the Fracture Resistance of Implant-Supported Zirconia Frameworks

This study aimed to evaluate the fracture resistance and patterns of implant-supported frameworks with and without a titanium base (Ti-base), using different prosthetic screw designs, and to determine how firing cycles followed by thermomechanical aging affect fracture resistance. Implant-supported zirconia frameworks were fabricated and allocated to three main groups: Ti-base with conventional screws, direct-connection with flat-seat screws, and direct-connection with conical screws. Each main group was further divided into two subgroups (n = 10). The control subgroup (C) did not undergo firing cycles or thermomechanical aging, whereas the experimental subgroup (E) underwent three consecutive firing cycles to simulate clinical zirconia veneering, followed by thermomechanical aging consisting of 5000 thermocycles and 1,250,000 chewing cycles. Fracture resistance differed significantly between screw designs in the control and experimental groups (p = 0.006 and p < 0.001, respectively). The combined effect of firing cycles and thermomechanical aging did not significantly affect fracture resistance in Ti-base and conical screw designs, whereas the flat-seat screw design showed a significant increase (p = 0.002). The failure pattern involved frequent deformation of prosthetic screws and multi-unit abutments (MUAs) in direct-connection designs. In contrast, the Ti-base design prevents such deformation, a benefit not present in direct-connection designs.