Comparing the Effect of Precrystallization or Postcrystallization Adjustments on Fatigue Behavior of Lithium Disilicate-Based Ceramics

The aim of this study is to assess the effect of the timing of adjustment on the fatigue flexural behavior and surface characteristics of lithium disilicate-based ceramics. Discs (Ø = 15 and 1.2 mm thickness) of four lithium disilicate-based ceramics (IPS e.max CAD, Ivoclar AG; Rosetta SM, HASS Corp; T-Lithium, Shenzhen Upcera Dental Technology Co Ltd; Goozir, Shenzhen Yurucheng Dental Materials Co) were obtained and randomly allocated into three groups based on the timing of adjustment factor: CTRL—directly crystallized without any additional treatment; PRE—subjected to adjustment precrystallization; POST—adjusted postcrystallization. The specimens underwent cyclic fatigue biaxial piston-on-three-balls flexural testing (n = 10; 20 Hz frequency, 10,000 cycles per step, initial stress of 100 MPa, and step-size of 25 MPa). Fatigue flexural strength (FFS) and the number of cycles for fatigue failure (CFF) were recorded and submitted to factorial (two-way analysis of variance [ANOVA]), survival (Kaplan–Meier and Mantel–Cox tests) and Weibull analysis. Surface roughness data were collected and submitted to two-way ANOVA and Tukey's posthoc tests. Fractographic and topographic features were descriptively and qualitatively analyzed. Both the adjustment (p < 0.0001 and F = 49.18) and material (p < 0.0001 and F = 11.99) factors showed a statistically significant impact on the FFS and CFF, as did their interaction (p = 0.03 and F = 2.51), indicating higher values for the CTRL and PRE groups compared to the POST group, and for the T-LITHIUM, EMAX, and GOOZIR materials compared to ROSETTA. Regarding the Weibull modulus, all groups showed similar statistical values. Surface roughness was also influenced by both factors (p < 0.05—PRE > POST > CTRL). ROSETTA exhibited higher Ra values than EMAX and higher Rz values than both EMAX and TALMAX. Fractures always originated at surface defects in the tensile stress concentration region. Topographic features revealed prominent scratches in the PRE groups due to adjustments, smoother scratches in the POST groups, and polishing scratches in the CTRL groups. Consequently, while precrystallization adjustment increases the roughness of lithium disilicate-based ceramics, it does not affect their fatigue behavior. Meanwhile, postcrystallization adjustment deleteriously affects the flexural fatigue strength. Thus, if needed, adjustments should be made precrystallization of lithium disilicate-based ceramics. Also, different lithium disilicate-based ceramics exhibit varying fatigue behaviors; in this study, Rosetta SM showed reduced properties compared to T-Lithium CAD, IPS e.max CAD, and Goozir Lithium Disilicate.