Statement of problem: The manufacturer of newly introduced pressable ceramic claimed that the strength can be increased by subjecting the material to a thermal tempering cycle at 9% below pressing temperature after its pressing. However, the effect of this thermal tempering protocol on the fracture resistance of different heat-pressed glass ceramics is still not known.

Purpose: This in-vitro study aimed to investigate the effect of thermal tempering on the fracture resistance and microstructural features of two heat-pressed glass ceramics.

Materials and methods: A total of 28 glass-ceramic crowns were assigned into two groups according to the ceramic type (n =14): Group (A): Lithium Disilicate glass-ceramic crowns and Group (B): Zirconia Reinforced Lithium Silicate glass ceramic crowns. Crowns of each group were divided into two equal subgroups (n=7) according to the subsequent thermal tempering temperature: Subgroup (T0): crowns were not subjected to thermal tempering and subgroup (T1): crowns were subjected to a temperature of 9% below pressing temperature. Fracture resistance of all samples were tested. A two-way ANOVA test was used to study the effect of ceramic type, thermal tempering and their interactions on fracture resistance.
Results: When no thermal tempering was applied, IPS E.max press showed statistically significantly higher mean fracture resistance value than Celtra press (1819.04 N and 1286.4 N respectively). Thermal tempered Celta press crowns showed the highest fracture resistance value (1951.7 N).
Conclusions: Lithium disilicate crowns demonstrated higher fracture resistance than Zirconia-Reinforced Lithium Silicate (ZLS) crowns without tempering. Thermal tempered ZLS crowns demonstrated increase in fracture resistance.