EFFECTS OF ETCHING DURATION ON THE SURFACE ROUGHNESS, SURFACE LOSS, FLEXURAL STRENGTH OF E.MAX CAD GLASS CERAMIC AND SHEAR BOND STRENGTH TO A RESIN CEMENT Background: Long-term retention of ceramic restorations is dependent on the bond strength of the luting resin to both the tooth and porcelain substrates. In order to achieve successful bonding, the surface of the porcelain substrate must be modified to increase the surface roughness, and this can be achieved chemically by hydrofluoric (HF) acid etching. However, prolonged HF acid etching has shown to have a weakening effect on the evaluated lithium disilicate glass-ceramics. Therefore, it is essential to quantify the required etching duration of HF acid to minimize the possible deleterious effects on ceramic strength while maximizing the bond strength to tooth structure. Objectives: To evaluate the effects of HF acid etching duration on the surface roughness, surface loss, flexural strength, and shear bond strength of IPS e.max CAD (Ivoclar Vivadent) lithium disilicate-based glass ceramic to a resin cement. Hypothesis: The differences in HF acid etching durations will not have a significant effect on the surface roughness, surface loss, flexural strength, or shear bond strength of IPS e.max CAD to a resin cement. Methods: 168 specimens were prepared from IPS e.max CAD blocks. All specimens were polished and sonically cleaned in distilled water. Specimens were fired in the vacuum pump furnace according to the manufacturer’s instructions. Specimens were then divided into 4 groups, according to etching durations, then further divided into 3 subgroups, according to the properties tested. Group A was not etched (control), Groups B, C and D were etched with 5-percent HF acid (IPS Ceramic Etching gel, Ivoclar Vivadent) for 20 s, 60 s and 90 s respectively. The morphologies of both etched and non-etched surfaces in specimens of subgroup 1 of each etching group (n = 16/group) were observed under scanning electron microscopy (SEM). In addition, non-contact surface profilometry (Proscan 2000) was used to calculate the surface loss and to examine the surface roughness of the etched ceramic surfaces and roughness values (Ra, Rq) were documented for each group. Furthermore, etched specimens of subgroup 2 (n = 16/group) were silanated (Monobond Plus, Ivoclar Vivadent) and cemented with a resin cement (Multilink Automix, Ivoclar Vivadent). The shear bond strength (SBS) was measured using a universal mechanical testing machine. For each etching group, subgroup 3 specimens (n = 10/group) were loaded to failure in a three-point bending test to measure their flexural strength values using a universal mechanical testing machine. Data for surface roughness, surface loss, and flexural strength were analyzed using one-way analysis of variance (ANOVA), to identify the significant effects of different HF acid etching durations. Data for shear bond strength test were analyzed using two-way ANOVA to test the effects of etching duration, storage for 24 hours/thermocycling, and their interaction. All pair-wise comparisons from ANOVA analysis were made using Fisher’s Protected Least Significant Differences to control the overall significance level at 5 percent. Results: Difference in HF etching durations did not have a significant effect on surface roughness values Ra or Rq (p = 0.3408; p = 0.3245) respectively, but had a significant effect on surface loss (p = 0.0006). SBS values were not significantly different between experimental groups (p = 0.4650); however, SBS values after 24-h storage were significantly higher than that found after thermocycling (p = 0.0166) among different etching durations. Finally, different HF etching durations did not have a significant effect on flexural strength values (p = 0.1260). Conclusion: Within the limitations of this study, different HF etching durations did not have a significant effect on surface roughness, flexural strength, or shear bond strength of IPS e.max CAD. However, the different etching durations significantly affected the surface loss of the lithium disilicate glass ceramics.