Purpose: This in vitro study aims to evaluate the influence of different etching times on fired vs milled Celtra Duo (zirconia-reinforced lithium silicate (ZLS) high-strength glass ceramic, DENTSPLY Sirona Prosthetics, York, Pennsylvania) and the effects on shear bond strength, while determining the type of failure.

Materials and Methods: Zirconia-reinforced lithium silicate (Celtra DUO) CAD/CAM ceramic blocks were cut into 120 pieces measuring 12 mm (H) x 14 mm (L) x 2 mm (W). These were categorized into two groups based on their crystallization stage: fired or milled. All 60 samples, in each group, were sub-categorize into four groups each containing 15 samples. Using 5% HF acid gel (Ivoclar Vivadent), 10 microL of acid gel was added onto the exposed ceramic surface for 10, 20, 30, and 60 seconds then rinsed for the same amount of time they were etched. Samples were left to dry and 0.5 microL of silane (Calibra, Silane coupling agent- DentsplyRTM) was added and left undisturbed for 2 minutes before bonding. Using a surveyor assembly, ensuring ceramic sample and resin (Filte(TM)Z250 resin composite, 3M ESPERTM) rod were perpendicular to each other, the resin rod and ceramic samples were bonded together using self-adhesive resin cement (Calibra Ceram Adhesive Resin Cement, Dentsply). After bonding, all samples were stored in distilled water and placed in an incubator for 24 hours at 37°C. The microtensile bond strength test was employed at a crosshead speed of 0.5 mm/min (InstronRTM, Canton, Massachusetts). Failures were evaluated using a low-power optical microscope to assess the location and type of failure and classified as adhesive, cohesive, or combined. Three-way ANOVA and the Fisher's PLSD tests were applied to test for significant differences in shear bond strength values in relation to the crystallization stage and etch times (p<0.05).

Results: There were significant differences in shear bond strength between condition (Milled vs fired) (p < 0.05) and etch time (p < 0.05). The interaction between condition and etch time was also significant (p < 0.05). The highest mean shear bond strength was recorded for Fired, 20 seconds (3.6 +/- 2.1 MPa). The lowest shear bond strength was recorded for Milled, 60 seconds (1.1 +/- 1.0 MPa). As for the Shear bond strength, regarding the etch time, there was a statistical significance between etch times 20,30 (p < 0.05) and 20,60 (p < 0.05). Bond failures were recorded for all 120 samples revealing 85 adhesive failures while 35 were combined (adhesive and cohesive).

Conclusions: The null hypothesis of this study was rejected because there were differences in shear bond strength amongst fired vs milled samples, along with statistical significance in shear bond strength amongst the different etch times. Firing the Celtra Duo after it has been milled improves the shear bond strength when treated with 5% HF acid gel for 20 seconds, followed by conditioning with silane.