Objectives . To evaluate the effect of the ceramic material and connector size on the fracture load and fracture behavior of fixed partial dentures (FPDs).
Materials and methods . Five ceramic materials were used: a yttria partially stabilized tetragonal zirconia polycrystal (Y-TZP – In-Ceram YZ, Vita), a glass-infiltrated alumina/zirconia (ICZ – In-Ceram Zirconia, Vita), a polycrystalline alumina (AL – In-Ceram AL, Vita) and two feldspathic ceramics (VM7 and VM9, Vita). Forty FPDs were produced according to the manufacturer’s instructions and divided in four groups ( n = 10): (1) YZ-9: Y-TZP/VM9 with 9 mm 2 connector cross-section; (2) YZ-16: Y-TZP/VM9 with 16 mm 2 connector cross-section; (3) AL-16: AL/VM7 with 16 mm 2 connector cross-section; (4) ICZ-16: ICZ/VM7 with 16 mm 2 connector cross-section. Steel models simulating prepared abutment teeth were constructed to design the FPDs, and the CEREC InLab system was used to produce the frameworks. The FPDs frameworks were veneered with a uniform thickness of 1.2 mm in the crowns and pontic area and 6.0 mm around the connector. A layer of bonding agent (Effect Bonder, Vita) was applied and sintered in the Y-TZP frameworks before porcelain veneering. The FPDs were cemented in the metal dies with zinc phosphate cement. All FPDs were tested in a universal testing machine with the load being applied by a stainless steel sphere in the center of the pontic with 1 MPa/s stress rate until failure and in a 37 °C deionized water bath. Fracture surfaces were examined using stereomicroscope and SEM. Results were statistically analyzed using Kruskal–Wallis and Tukey ( α = 0.05).
Results . The mean fracture load ( N ) and standard deviation values and statistical grouping for the experimental groups were: YZ-9: 2206 ± 186b; YZ-16: 4526 ± 406a; AL-16: 1973 ± 239b; ICZ-16: 2310 ± 134b. YZ-16 showed the highest mean fracture load value. There was no significant difference between YZ-9, AL-16 and ICZ-16 groups. All fractures started at the gingival side of the connector propagating throughout the pontic towards the occlusal surface. Fractography analyses showed that a surface flaw grew to critical stress intensity factor in the veneer porcelain, deflected at the veneer–core interface and fractured catastrophically through the core material.
Conclusions . Considering the materials and the limitations of this study, Y-TZP FPDs with 16 mm 2 connector presented the highest mean fracture load value. Y-TZP PFDs with 9 mm 2 connector (YZ-9) showed similar fracture load value to AL-16 and ICZ-16 groups. Therefore, it can be concluded that Y-TZP showed the best mechanical behaviour and the connector size had a significant effect on the fracture load of Y-TZP FPDs.