Effect of Frame Thickness on Fracture Toughness of Veneered Zirconia All-Ceramic Restorations

Objectives: The chipping of veneering ceramics is problematic in clinical zirconia all-ceramic (ZAC) restorations. We investigated the influences of the presence and thickness of a frame on the fracture toughness of layering ceramic and pressable ceramic used for ZAC restorations.
Methods: Fracture toughness was evaluated according to the ISO 15732 standard. Layering ceramics (Vintage ZR, Shofu) and pressable ceramics (IPS e.max^® ZirPress, Ivoclar Vivadent) used for the ZAC restorations (Cercon^® smart ceramics, DeguDent) were fired without frames or on 0.8- and 1.2-mm-thick zirconia frames. After firing, each sample was embedded, and the cross-sectional surface was ground. The fracture toughness value was measured using a Vickers hardness testing machine, where a diamond indenter was pressed into the specimen surface at a load of 2 kgf. Measurements were obtained at different distances from the zirconia-ceramic junction (every 0.5 mm from 0.5 to 2.5 mm). Data were analyzed using a one-way ANOVA and Tukey’s HSD test (α=0.05). The surface topography was observed using scanning electron microscopy (SEM).
Results: In the layering ceramic group, the fracture toughness values at 0.5 mm of the 1.2-mm-thick frames were significantly lower than those at other points (P<0.0001). No significant differences were observed between any of the distances with 0.8-mm-thick frames and without frames. In the pressable ceramic group, the fracture toughness values at 0.5 mm of 1.2- and 0.8-mm-thick frames were significantly lower than those at other points (P<0.0001). These were not significantly different from the values without frames. SEM images revealed that the 1.2-mm-thick-frame was more prominent in veneering ceramics that had many pores, microcracks, and large crystals in the layering ceramics.
Conclusions: The presence and thickness of a zirconia frame affect the fracture toughness of veneering ceramics. This suggests that a thick frame could be a fracture risk in layering ceramics used for ZAC restorations.