Do crystallization and refiring protocols affect the translucency, flexural strength, and Weibull reliability...

Nanolithium disilicate is a recently introduced glass-ceramic whose translucency varies with the final temperature of crystallization or refiring. However, the effects of these firing protocols on its optical and mechanical properties remain unclear.

The purpose of this in vitro study was to evaluate the translucency, biaxial flexural strength (BFS), and reliability of nanolithium disilicate when subjected to different firing protocols.

Ninety-eight disk-shaped nanolithium disilicate specimens were randomly divided into 7 groups based on different firing protocols (n=14) providing crystallization at 815 °C (HT, high translucency), 825 °C (MT, medium translucency), 840 °C (LT, low translucency), and 860 °C (MO, medium opacity) and the refiring of HT specimens at 830 °C (HT-MT), 845 °C (HT-LT), and 865 °C (HT-MO). The color coordinates were measured over black and white backgrounds, and the CIEDE2000 formula was used to calculate their relative translucency parameter (RTP). The BFS of the specimens was determined with the piston-on-3 balls method. RTP and BFS data were statistically analyzed using the 1-way analysis of variance and Tukey tests, while their correlation was analyzed with the Pearson correlation test. The Weibull modulus and characteristic strength of the specimens were analyzed with the chi-squared test (α=.05).

HT specimens had the highest, and MO and HT-MO specimens had the lowest RTP (P<.001). The remaining specimens were listed in descending order of RTP as HT-MT, MT, HT-LT, and LT (P≤.030). Except for the HT-MT specimens (P≥.126), HT and MT specimens had the lowest BFS (P≤.026). HT-MT specimens had lower BFS than MO and HT-MO specimens (P≤.006). A significant negative correlation was found between the RTP and BFS (r=-0.641, P<.001). The Weibull modulus of tested specimens ranged between 16.4 (HT) and 23.7 (HT-LT) without significant difference (P=.874). Except for the HT-MT specimens, the HT and MT specimens had the lowest characteristic strength values (P<.001).

The final firing temperature affected translucency, producing either perceptible or imperceptible differences between translucency levels and firing cycles. HT, MT, and HT-MT specimens generally showed lower BFS, suitable for adhesively cemented single-unit restorations, whereas other translucency levels may also be applicable for 3-unit prostheses, excluding molar teeth. Although Weibull moduli were similar, LT, MO, HT-LT, and HT-MO specimens generally had higher characteristic strength, indicating greater reliability.