Effect of ceramic cooling protocols and zirconia coloring on fracture load of zirconia‑based restorations

Behnaz Ebadian, Ramin Mosharraf, Mahsa Abbasi


Background: Residual thermal stresses in dental porcelains can cause clinical failure. Porcelain
cooling protocols may affect the amount of residual stresses within porcelain and also
porcelain–zirconia bond strength. The objective of this study was to assess the effect of cooling
protocols on the fracture load of porcelain veneered zirconia restorations.
Materials and Methods: Forty zirconia bars (31 mm × 6.5 mm × 1.35 mm ± 0.1 mm) were
fabricated by computer‑aided design and computer‑aided manufacturing technology. Half of the
specimens were immersed in the coloring agent for 2 min before sintering (yellow group). Thus,
the specimens were divided into two groups of white (W) and yellow (Y) samples (n = 20).
Heat‑pressed ceramic was applied to all bars. After pressing, half of the samples in each group were
immediately removed from the oven (fast cooling) while the other specimens remained in the
partially open door (30%) oven until the temperature reached to 500°C. Samples were thermocycled
for 5000 cycles and subjected to modified four‑point flexural strength test by a universal testing
machine at a crosshead speed of 0.5 mm/min. Two‑way ANOVA, One‑way ANOVA followed by
post hoc Tukey honest significant difference tests were used for data analysis (α = 0.05).
Results: Fractures were cohesive in all samples (within the porcelain adjacent to the interface).
Two‑way ANOVA showed that the effect of cooling protocol on the fracture load of samples was
statistically significant (P < 0.001). In addition, the fracture load of W and Y groups was significantly
different (P < 0.001). The white slow group showed the highest fracture load (179.88 ± 23.43 N).
Conclusion: Slow cooling protocol should be preferably applied for zirconia restorations. Coloring
agent used in this study had a significant negative effect on fracture load.
Key Words: Computer‑aided design, dental bonding, dental porcelain, zirconium oxide

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