Fracture resistance of super speed-sintered zirconia crowns after mastication simulation

Super-speed sintering protocols for multi-yttria-layered zirconia can reduce sintering time to 15 minutes; however, the impact on the materials' long-term fatigue resistance and fracture strength after thermomechanical loading remains uncertain.

The purpose of this in vitro study was to assess the impact of sintering protocols (conventional versus super-speed) and thermomechanical aging on the fracture resistance and durability of monolithic and multi-yttria-layered yttria-partially stabilized zirconia (Y-PSZ) crowns.

Sixty complete coverage zirconia crowns were milled and divided into 3 groups (n=20): monolithic 3 mol% Y-PSZ (IPS e.max ZirCAD LT) sintered conventionally (CS; 8 to 9 hours), multi-yttria-layered 5Y/3Y-PSZ (IPS e.max ZirCAD Prime) sintered conventionally, and multi-yttria-layered 5Y/3Y-PSZ (IPS e.max ZirCAD Prime Block) processed with a 15-minute super-speed protocol (SS). All crowns were milled for a standardized mandibular first molar preparation and adhesively luted to fiber-reinforced resin dies with adhesive resin cement (Multilink Automix). Half of the specimens in each group (n=10) were subjected to thermomechanical fatigue loading (1.2 million cycles at 110 N; 10 000 thermocycles at 5 °C to 55 °C). All surviving specimens and the other half of the specimens underwent load-to-fracture testing in a universal testing machine (1 mm/minute crosshead speed) using a 4-mm stainless steel conical indenter applied to the central fossa. Scanning electron microscopy and stereomicroscopic images were obtained for microstructural and fracture pattern analyses. The data were analyzed using 2-way ANOVA and Tukey post hoc tests (α=.05).

All specimens survived thermomechanical loading. At baseline, 3Y-PSZ demonstrated significantly higher fracture resistance (4108 N) than both CS and SS 3Y/5Y-PSZ (3382 N, 3288 N, respectively; P<.001). Thermomechanical loading had no significant effect on fracture resistance (P=.051). 3Y/5Y-PSZ sintering protocol had no significant impact on fracture resistance at baseline (P=.888) and after aging (P=.240). SEM analysis showed a uniform grain structure in monolithic 3Y-PSZ and a heterogeneous, layer-dependent microstructure in multilayer zirconia, with less distinct grain boundaries after speed sintering. Fatigued specimens exhibited predominantly cohesive fractures originating at the loading contact area, with greater fragmentation observed in multilayer zirconia than in monolithic 3Y-PSZ.

Super-speed sintering of multi-yttria-layered zirconia crowns did not significantly impair their mechanical stability or fatigue resistance relative to conventional sintering. The monolithic 3Y-PSZ material was the most resistant to fracture, but all the zirconias survived the fatiguing process.