Influence of ceramic thickness and ceramic materials on fracture resistance of posterior partial coverage restorations.

This study evaluated the influence of ceramic thickness and ceramic materials on fracture resistance of posterior partial coverage ceramic restorations. Forty extracted molars were allocated into four groups (n=10) to test for two variables: 1) the thickness of ceramic (1 mm or 2 mm) and 2) the ceramic materials (a lithium disilicate glass-ceramic [IPS e.max] or leucite-reinforced glass ceramic [IPS Empress]). All ceramic restorations were luted with resin cement (Variolink II) on the prepared teeth. These luted specimens were loaded to failure in a universal testing machine, in the compression mode, with a crosshead speed of 1.0 mm/min. The data were analyzed using two-way analysis of variance and the Tukey Honestly Significantly Different multiple comparison test (α =0.05). The fracture resistance revealed a significant effect for materials (p<0.001); however, the thickness of ceramic was not significant (p=0.074), and the interaction between the thickness of ceramic and the materials was not significant (p=0.406). Mean (standard deviation) fracture resistance values were as follows: a 2-mm thickness of a lithium disilicate bonded to tooth structure (2505 [401] N) revealed a significantly higher fracture resistance than did a 1-mm thickness of leucite-reinforced (1569 [452] N) and a 2-mm thickness of leucite-reinforced ceramic bonded to tooth structure (1716 [436] N) (p<0.05). There was no significant difference in fracture resistance values between a lithium disilicate ceramic at 1-mm thickness (2105 [567] N) and at 2-mm thickness. Using a lithium disilicate glass ceramic for partial coverage restoration significantly improved fracture resistance compared to using a leucite-reinforced glass ceramic. The thickness of ceramic had no significant effect on fracture resistance when the ceramics were bonded to the underlying tooth structure.

The effect of tooth-preparation cleansing protocol on the bond strength of self-adhesive resin cement to dentin contaminated with a hemostatic agent.

This study evaluated the effect of tooth-preparation cleansing protocols on the bond strength of a self-adhesive resin cement to dentin contaminated with two different types of hemostatic agents. The occlusal surface of extracted third molars was flattened to expose the dentin surface and prepared for a full crown. Acrylic temporary crowns were fabricated and placed using temporary cement. The specimens were stored at 100% relative humidity for seven days. Following removal of the temporary crowns, the specimens were surface debrided using aluminum oxide abrasion with a particle size of 27 µm at 40 psi. The specimens were randomly assigned to three groups, according to the hemostatic agents: Group I­an agent containing aluminum chloride was applied to the tooth surface; Group II­an agent containing ferric sulfate was applied to the tooth surface and Group III­uncontaminated (control). The contaminated specimens were then further subdivided into three subgroups (A­C; n=12): Group A­tooth surface cleansing with water spray; Group B­tooth surface cleansing with phosphoric acid etch and Group C­tooth surface cleansing with aluminum oxide abrasion with a particle size of 27 µm at 40 psi. Ceramic blocks were treated with a 9.5% hydrofluoric acid-etch and silanized prior to being cemented with self-adhesive resin luting agent (RelyX Unicem) to the prepared dentin. The shear bond strength was determined at a crosshead speed of 0.5 mm/minute. The data were analyzed with two-way ANOVA, followed by the Duncan multiple range test, to determine any significant differences between the testing groups. The microstructure morphology of the tooth surface was evaluated using SEM analysis. The results revealed that there was a significant difference between the bond strength of the control and the contaminated testing groups (p<0.05). A tooth preparation cleansing protocol using particle abrasion with low-pressure aluminum oxide particles provided a significant improvement in bond strength to contaminated dentin, while rinsing with water spray resulted in the lowest mean bond strength of the self-adhesive resin cement to dentin (p<0.05).