Effect of different surface treatments and multimode adhesive application on the Weibull characteristics, wettability, surface topography and adhesion to CAD/CAM lithium disilicate ceramic

Abstract This paper aims to evaluate the effect of different surface treatments on surface topography, wettability, and shear bond strength of resin cement to glass ceramic. Methodology: For SBS test, 32 blocks (7x7x2 mm) of lithium disilicate were obtained and randomly divided into eight groups (four blocks per group) according to each surface treatment (HF 20 s, 60 s, 120 s + silanization/S or Scotch Bond Universal/ SBU) and the Monobond Etch & Prime - MEP application followed or not by SBU. On each treated surface ceramic block, up to four dual-curing resin cement cylinders were prepared and light-cured for 40s (N=120/n=15). The specimens were thermocycled (10,000 cycles, 5-55°C, 30 s) and the SBS test (50KgF, 0.5 mm/min) was performed. Furthermore, failure analysis, wettability, AFM, and SEM were carried out. SBS data (MPa) were analyzed using Student's t-test, two-way ANOVA, Tukey's test (5%) and Weibull's analysis. Results: For HF experimental groups, two-way ANOVA presented the factors “etching time” and “bonding agent” as significant (p<0.05). After silane application, the HF groups presented similar bond strength. SBU application compromised the SBS, except for 120s etching time (HF120sS: 23.39ᵃ±6.48 MPa; HF120sSBU: 18.76ᵃ±8.81MPa). For MEP groups, SBU application did not significantly affect the results (p=0.41). The MEP group presented the highest Weibull modulus (4.08A) and they were statistically different exclusively from the HF20sSBU (0.58B). Conclusion: The HF 20s, 60s, 120 s followed by silane, promoted similar resin-bond strength to ceramic and the SBU application after HF or MEP did not increase the SBS.

use of multimode adhesives, such as the Scotch Bond Universal -SBU (3M ESPE / Irvine, CA, USA), which contains MDP monomer, silane, and adhesive system in a single bottle, which could simplify the adhesion of ceramics to resin cement. 15,16 According to et al. 14 (2018)  Talmax silicone/Brazil). After resin polymerization, the ceramic blocks surface was polished again with grit SiC abrasive papers (#600, #800 and #1200) in a polishing machine (Labpol 8-12, Extec, USA) until the excess acrylic resin has been removed. Then, the blocks were randomly divided into eight groups (four blocks per group). On each ceramic block, up to four resin cement cylinders were built-up to complete the 15 cylinders per group (n=15). The groups were randomly divided according to "HF time" and "bonding agent": silanization (HF20sS, HF60sS, HF120sS) or SBU (HF20sSBU; HF60sSBU, Hf120sSBU), and the MEP application (MEP and MEPSBU).

Surface treatments
Firstly, all specimens were immersed in distilled water and were ultrasonically cleaned for 5 min Shapiro-Wilk test was also performed to evaluate the normality.
For SBS, HF experimental groups were analyzed with two-way ANOVA followed by Tukey's test (5%).
Wettability data were analyzed using one-way ANOVA followed by Tukey's test (5%). The MEP groups were compared using Student's t-test (5%). All comparisons were carried out using the MINITAB software (Minitab,

Shear bond strength (SBS)
For HF groups two-way ANOVA revealed that the "etching time" factor (p=0.0001), "bonding agent" factor (p=0.01), and the interaction of both (p=0.0001) were significant (    The Weibull modulus (m) and characteristic strength (σ 0 ) of all groups were statistically different from each other (p=0.0001). The Weibull distributions are graphically presented in Figure 3 and associated parameters are summarized in Table 3. The MEP group presented the highest Weibull modulus (4.08 MPa) A which was higher than HF20sSBU (0.58 MPa) B but similar to the others. Regarding σ 0 , the HF20sSBU       Table 4.

Failure mode analysis
The failure analysis revealed that 92.5% of failures were originated from Mixed 1 mode (predominantly adhesive in resin cement/ceramic interface + cohesive in resin cement) and 7.5% were adhesive in cement/ ceramic interface. The percentage of each failure mode for each group tested is shown in Table 5 The shear bond test was chosen to evaluate the bond strength between the ceramic and resin cement. In addition to being cost-effective and easy to implement, it is often used in studies assessing adhesion between two interfaces. In order to reduce non uniform stress distribution in a shear test, 24 a smaller adhesive area of 2 mm 2 was used in this study. 25 All samples in this study were subjected to thermocycling for 10,000 cycles, which simulates conditions equivalent to one year of clinical use. 26,27 The fatigue process of thermocycling promotes a faster hydrolytic degradation of the interface due to its contraction and expansion stresses as a consequence of different thermal expansion coefficients among different materials, which is considered a significant predictor of the adhesive performance of restorative interfaces 28,29 and for these reasons all groups were submitted to thermocycling. Regarding the wettability, our results showed lower contact angle for HF-exclusively groups. The HF etching results in a higher energy surface, due to the removal of contaminants and the increased roughness of the ceramic surface, supported the greater interaction with silane. 10,22,31 In this study, the contact angle for MEP was higher than those in HF etching groups, and similar to the groups with silane (HF60sS and HF120sS), J Appl Oral Sci. 2020;28:e20200122 10/12 excepting HF20sS, where MEP demonstrated a smaller contact angle. The MEP reduced the surface energy (greater contact angle), which may be an indication that silane molecules contained in the MEP remain effectively linked to the hydroxyl groups available on ceramic surface 34 . According to Moreno,et al. 34 (2019) MEP produces a more superficial conditioning pattern than HF and this may be responsible for decreasing the ceramic wettability. Some authors report that the presence of fluoride contained in the MEP, also seems to reduce the ceramic wettability, 14,17,34 which may justify our findings. Furthermore, it was observed in this study that longer conditioning times (120 s) produced a smaller contact angle. Ramakrishnaiah and others 10,23,30 demonstrated that wettability is directly proportional to surface irregularities, however, despite the greater degradation of the vitreous matrix by prolonged periods of HF, increasing the exposure of hydroxyl groups 10,22 of the ceramic, it can also decrease the interaction of these groups with the silane, and consequently, the hydrophobicity of the ceramic, providing no advantage for adhesion. 17,34 However, further studies are necessary to support this statement.
The second hypothesis that SBU is an effective substitute for silane was rejected. According to our results, HF 20 s and 60 s followed SBU were the resin cement, simplifying the procedure with a single product. 37 Furthermore, MEP has lower toxicity compared to HF, and therefore its use in mouth can be a concern. 38 Other studies also reported that the MEP can be an alternative for HF followed by silanization, without compromising the bond strength between ceramic and resin cement 14,17 providing clinically efficiency and durable adhesion. 39 Lopes,et al. 40 (2019) compared different HF concentrations (5, 9.5, 9.6, and 10%)