Objectives: The purpose of this study was to investigate the effect of preheat temperatures on polymerization contraction stress and mechanical properties of three resin composites. Methods: Three resin composites (Filtek Supreme XT, GC Kalore, and Gradia Direct X) at room temperature, 37°C, and 60°C were investigated. Stress development and maximum contraction stress of the composites were evaluated. Directly after preheating, samples were light-cured for 40 seconds and the force recorded for 15 minutes. Subsequent calculations were done to account for the system’s compliance and to obtain the shrinkage stress of the composites. In addition, composite discs (5 mmø and1 mmthick) were light-cured for 40 seconds at the preheat temperature. Hardness, elastic modulus, and creep of composites were investigated using a nano-indentation system (UMIS 2000). The results were analyzed using Two-way Analysis of Variance (2-way ANOVA) and Tukey’s Post-Hoc test (α = 0.05). Results: The results indicated that preheating composites to 37°C and 60°C increased the polymerization contraction forces, but did not significantly affect hardness, elas

The loss of interfacial integrity was identified as one of the major causes for replacement of resin composite restorations. Preheating procedure has been proven to enhance flowability and adaptation of resin composites and increase their degree of conversion. The purpose of this study was to investigate polymerization contraction stress produced in resin composites after preheating to 37˚C and 60˚C, and measure microleakage of Class V restorations restored with preheated composites. Three resin composites (GC Kalore, Gradia Direct X, Filtek Supreme XT) at room temperature, 37˚C, and 60˚C were investigated. Maximum contraction stress of the composites (n = 5) was evaluated in a modified lowcompliance device. Samples were light-cured for 40 seconds and the maximum force was recorded during 15 minutes. Calculations were done to adjust for the system's compliance and obtain linear shrinkage values of composites. Data were analyzed by Multivariated Analysis of Variance (MANOVA) and Tukey's test for multiple comparisons (α = 0.05). Seventy-two Class V cavities were prepared on the buccal surfaces of extracted premolars and divided into 9 groups. The teeth were restored with composites at 3 temperatures and were thermo-cycled between 5˚C and 55˚C with a one-minute dwell-time for 1000 cycles. The teeth were sealed with wax and nail vanish before placed in 0.5% toluidine blue dye for 24 hours. The teeth were embedded in self-curing resin and sectioned bucco-lingually with a slow-speed diamond saw, providing 3 sections per restoration. Microleakage was rated by two evaluators using a 0-4 ordinal scale at the occlusal and cervical margins under light microscope. Microleakage data were analyzed with Kruskal-Wallis ANOVA and Mann-Whitney U test (α = 0.05). Results indicate that preheating composites to 37˚C and 60˚C significantly increased polymerization contraction stress of composites (p < 0.05). A significantly greater amount of leakage was found at the cervical margins (p < 0.05). For all tested materials, preheating composites to 60˚C resulted in significantly less microleakage at the cervical margin.

Silicon nitride (Si3N4) ceramic is an attractive material for dental applications, especially used as a dental core material, due to its unique properties including high fracture toughness, high strength, high wear resistance and non-cytotoxicity. In this study, the Si3N4 ceramic was fabricated by a non-pressure sintering technique at a relatively low sintering temperature of 1650˚C in nitrogen atmosphere. Borosilicate glass and 5 wt% ZrO2-added borosilicate glass were used for coating on the Si3N4 core surface because of their compatibility in thermal expansion, high chemical resistance and bio-inert. The specimens were then fired in electric tube furnace at 1100˚C. The Vickers microhardness of borosilicate glass and 5 wt% ZrO2-added borosilicate glass veneering materials were measured and compared with the commercial dental veneer porcelain as a control (VITA VMK 95). The cytotoxicity of the Si3N4 ceramic and the veneering materials were tested by MTT assay, using human gingival fibroblasts (HGF) and periodontal ligament fibroblasts (HPDLF). The results indicate that the Si3N4 ceramic and Si3N4 ceramic veneered with borosilicate glass or 5 wt% ZrO2-added borosilicate glass veneering materials tested in this study are not toxic to oral tissue and can be used to produce dental prostheses.

White Silicon nitride (Si3N4) ceramic has unique characteristics. Because of its high fracture toughness, strength, and biocompatibility, it can therefore be used to fabricate dental restorations. The purpose of this study was to produce partially-sintered block of Si3N4 for fabrication of CAD/CAM dental restorations. The related properties of this novel Si3N4 were evaluated including sintered shrinkage, flexural strength and fracture toughness. Partially sintered Si3N4 ceramic blocks were prepared by heating at 1,400°C for 2 h under N2 gas. After full sintering at 1,650 o C for 2 h, the linear shrinkage value was recorded at 19.88±0.56%. The flexural strength and fracture toughness were measured, the results were 891.21±37.25 MPa and 6.33±0.30 MPa•m 1/2 , respectively. These results showed that flexural strength and fracture toughness of Si3N4 were more than 800 MPa and 5 MPa•m 1/2 , the white Si3N4 developed in this study can be used to fabricate multi-unit dental restorations According to ISO 6872.

Objectives This study aimed to evaluate fatigue resistance of cast-on implant abutment using three alloys. Materials and Methods Forty specimens of implant-supported crowns were prepared; Group 1 (TA) stock titanium abutments, Group 2 (GS) abutment cast with 40% gold alloy, Group 3 (GP) abutment cast with palladium alloy, and Group 4 (CN) abutment cast with nickel–chromium alloy. Specimens were cyclic loaded at 20 Hz, starting from 200 N (5,000 cycles), followed by stepwise loading of 400, 600, 800, 1,000, 1,200, 1,400, 1,600, and 1,800 N (30,000 cycles/step). Specimens were loaded until failure or reached 245,000 cycles. Statistical Analysis The withstand cycles were analyzed using one-way analysis of variance and Weibull survival analysis. Fracture surfaces were examined using scanning electron microscopy. Results The results of withstand cycles were TA (189,883 ± 22,734), GS (195,028 ± 22,371), GP (187,662 ± 22,555), and CN (200,350 ± 30,851). The statistical analysis showed no significant difference between the groups (p = 0.673). Conclusion Although CN has higher Weibull characteristic strength which means greater durability, its lower Weibull modulus demonstrated less structural reliability. Consistent failures at implant fixture level were also found in CN group.