Impact of multiple firings on fracture load of veneered zirconia restorations.

OBJECTIVES: This investigation evaluated the impact of multiple firings during the veneering process of anterior all-ceramic restorations on the fracture load (FL) of zirconia crowns veneered with different ceramics (ZRT, ZRO, HFZ, STR). METHODS: All-ceramic crowns with anatomically reduced zirconia frameworks were prepared using four different veneering ceramics (N = 192/n = 48 per veneering ceramic). The crowns were fired 2 and 10 times. Half of them were thermocycled (5000 cycles, 5 °C/55 °C, 20s). FL using Voss shear test was measured. Data was analyzed using ANOVA with partial eta squared and post-hoc Scheffé-test as well as t-test and Weibull analysis. RESULTS: Regarding FL, greatest differences were among the number of firings (p<0.001, ηp2 = 0.369). FL was not influenced by the interaction of both number of firings and aging level (p = 0.231) and the interaction of number of firings and veneering ceramic (p = 0.222). Differences were found comparing FL values of ZRT and STR (p<0.001) and of HFZ and STR (p<0.001). No differences occurred among the veneering ceramics after 2 (p = 0.430) and after 10 firings (p = 0.057) in respect to initial FL. For all initially tested veneering ceramics, FL showed higher values after 10 firings (p≤0.001-0.014). When aged, ZRO, HFZ, and STR also provided higher FL values after 10 firings (p≤0.001). For ZRT, FL values differed between initial and aged testing after 2 firings (p = 0.001) with aged specimens providing higher FL values. After 10 firings, none of the veneering ceramics showed differences regarding FL values between initial and aged testing. Weibull modulus was higher for ZRT specimens after 10 firings when tested initially (m = 10.1) and for aged ZRT specimens after 2 firings (m = 11.1). Failures occurred either with chipping of the veneer or total fracture. SIGNIFICANCE: Veneered zirconia restorations tested presented higher FL after 10 firings compared to 2 firings. Aging via thermocycling showed a positive effect on the FL.

Impact of multiple firings on thermal properties and bond strength of veneered zirconia restorations.

OBJECTIVES: This investigation evaluated the impact of multiple firings during the veneering process of all-ceramic restorations on the thermal properties and bond strength between veneering ceramics and zirconia. METHODS: For the measurements of the coefficient of thermal expansion (CTE) prismatic specimens of four different veneering ceramics (ZRT/ZRO/HFZ/STR; N = 40/n = 10 per veneering ceramic) and rod-shaped zirconia specimens (N = 2) were produced. 2 specimens of each veneering ceramic were fired 2, 4, 6, 8, and 10 times simulating dentine-firing. Measurements were performed in a dilatometer and glass transition temperature (Tg) was determined graphically. To analyze bond strength (BS), crack initiation test specimens were prepared using the same veneering ceramics and zirconia (N = 600/n = 150 per veneering ceramic). 30 specimens of each veneering ceramic were fired 2, 4, 6, 8, and 10 times. Half of them were thermocycled (5000 cycles, 5 °C/55 °C, 20s). The crack initiation test was conducted in a universal testing machine. Data was analyzed using ANOVA with partial eta squared statistics and post-hoc Scheffé test as well as Pearson correlation test. RESULTS: CTE was mostly affected by the choice of veneering ceramic (p < 0.001/ηp2 = 0.983). For the leucite-free ceramic, the CTE values were in same value range (p = 0.171) along the number of firings. Tg was not influenced by the number of firings. The highest effect on BS showed veneering ceramics (p < 0.001/ηp2 = 0.055) followed by the number of firings (p = 0.011/ηp2 = 0.023). The global difference in BS was lower for 10 firings compared to 2 firings (p = 0.048). Highest BS were measured for the leucite-free veneering ceramic (ZRT). Artificial aging by thermocycling showed no impact on BS (p = 0.755). Correlations were found for HFZ between the BS and the CTE (r = -0.651/p = 0.021). A correlation between CTE and Tg was observed (r = 0.303/p = 0.029). SIGNIFICANCE: Thermal stability seemed to be more predictable for leucite-free veneering ceramics with superior BS. All BS results exceeded the minimum clinical requirements.