Fracture Toughness Testing of Dental Restoratives: a Critical Evaluation

A comparative study of mechanical properties of yttria stabilized zirconia monolithic and bilayer configuration for dental application

Zirconia multilayer presents promising prospects, but there is scarce information about its microstructural and mechanical characterization. Therefore, this study sought to produce them in-house and to perform their characterization by comprising four groups of specimens to assess the biaxial flexural strength, microhardness, fracture toughness, phase characterization and quantification, fractography, and microstructural features. Weibull analysis was performed to determine the Weibull modulus and characteristic strength. The results showed that bilayers 3YSZ and 5YSZ presented intermediate mechanical properties when compared to 3YSZ and 5YSZ monolithic controls (680 MPa, 464 MPa, 885 MPa, 594 MPa, respectively). Fractographic analysis revealed that the failure origin was not at the interface in the bilayer groups, but residual stress was present between the layers. Hardness and fracture toughness were not affected by the interface.

Interfacial fracture toughness of different surface treatments on zirconia-reinforced lithium silicate glass-ceramics

This study investigated the influence of different surface treatments on unfiring or firing zirconia-reinforced lithium silicate (ZLS) glass-ceramics. Celtra Duo and IPS e.max CAD blocks were cut and process following manufacturer protocols. The specimen surface was treated with seven different protocols. Two ceramic blocks with the same surface treatment were bonded with luting agent and prepared for mini-interfacial fracture toughness tests (mini-iFT). The specimens were tested after 1-week storage. The data was statistically analyzed using two-way ANOVA and Dunnett's T3 comparison (α=0.05). The highest mini-iFT of both Celtra Duo unfired and fired was shown in the HF+S group, which was not significantly different from HF+S+UA. For IPS e.max CAD, the mini-iFT was higher in the groups treated with hydrofluoric acid. Additional adhesive after silane application did not significantly improve bonding effectiveness. Therefore, surface treatment with hydrofluoric acid and silane is recommended for both unfiring and firing ZLS glass ceramics.

Fracture toughness of dental incremental composite-composite interfaces at elevated temperatures

The aim of the present laboratory study was to mechanically characterize the interface between two dental resin-based composite (RBC) increments, and to investigate if elevated temperatures have an influence on the quality of the interface mimicking clinical filling procedure. Four RBCs (CLEARFIL MAJESTY™ Posterior, Kuraray (CMP)/Filtek™ Supreme XTE, 3M (FSX)/Grandio®SO, VOCO (GSO)/VisCalor® bulk, VOCO (VCB)) were tested with a fracture toughness test using Chevron notched beams (K_I,CNB) at 23, 37 and 54 °C. K_I,CNB specimens (3 × 4x25mm) with a V-shaped notch at the incremental interface were loaded until failure in a 4-point bending set-up. Failure modes were characterized using light microscopy, microstructural interface was analyzed using SEM. Statistical analysis was performed using Kolmogorov-Smirnoff test, two-way ANOVA and Tukey Post-Hoc test (p = 0.05). Mean K_I,CNB ranged between 0.73 ±0.14 MPam^0.5 (VCB, 23 °C) and 1.11 ± 0.11 MPam^0.5 (FSX, 23 °C). The tested conventional highly filled RBCs presented fracture toughness at the incremental interface comparable to the cohesive strength of the bulk materials. VCB showed reduced interfacial fracture toughness at 23 and 37 °C, but performed well at elevated temperature of 54 °C.