Evaluation of self-etching primer on bond strength in feldspathic ceramic

The aim of the study was to evaluate the effect of self-etching silane primer on long-term bonding strength on feldspathic ceramic. Ceramic blocks (IPS Empress CAD, Ivoclar Vivadent) were cut with a low-speed diamond disk (Isomet 1000, Buehler) into 40 blocks of 12x7x6mm, under water cooling, and randomly divided into two groups (n=20), according to the surface conditioning: hydrofluoric acid (HF) (Condac, FGM) or self-etching silane primer (MEP) (Monobond Etch and Prime, Ivoclar Vivadent). After the blocks were bonded to composite resin blocks (12 x 7 x 3mm) (Liss, FGM) with a light cured cement (Variolink Esthetic LC, Ivoclar Vivadent) and LED photoactivation (Radii Cal, SDI) for 40 s on each luting interface line. The sets were cut in sticks with 1-mm2 cross-section composed of ceramic/cement/composite and tested after 24h (baseline) and 90 days aging by stored in distilled water at 37oC. Microtensile bond strength testing (TBS) was performed using a universal machine (DL-1000, EMIC) submitted to traction with speed of 0.5mm/ min until rupture of adhesion. Fracture of specimens were examined under stereomicroscopy to determine the failure pattern. Bond strength (MPa) was calculated, and the failure pattern and interface topography were assessed using scanning electron microscopy (SEM). No significant different TBS was observed between groups HF and MEP (p>0.05). Storage time of the samples did not affect the TBS from the groups. Surface etching pattern with HF produced higher surface alterations than a self-etching primer. Application of MEP may be considered an alternative for silanization for feldspathic ceramic.

Conventionally the gold standard protocol for surface treatment to glass-matrix ceramics is considered by a response of the material to hydrofluoric acid (HF) exposure. HF etching in glass-ceramic materials may act on material components, dissolving the glass-matrix ceramic and improving roughness patterns on the surface, improving the micro-retention for adhesion to resin cement (Choi et al., 2020;Gracis et al., 2015;Murillo-Gómez, Palma-Dibb, & De Goes, 2018;Ramakrishnaiah, Alkheraif, Divakar, Matinlinna, & Vallittu, 2016). Also, the surface treatment includes the hydroxyl groups that have high interaction with silane-coupling agents (Özcan & Volpato, 2015). Silane is a bifunctional molecule that produces the chemical interaction between the silica from the ceramic and methacrylate groups of resin monomers. Thus, the silanization acts on the interaction between ceramic and composite resin-based composite surfaces, bonding both chemically by siloxane bonds (Lung & Matinlinna, 2012;Naves et al., 2010).
Self-etching materials have been introduced to surface treatment of glass-ceramic materials to simplify the clinical steps of treatment by etching and silanization process and is considered a safer procedure rather than the application of HF . The manipulation of HF may expose the clinicians and patients to potential risks, from acid contact to biological risks of damage to soft tissues (Cardenas et al., 2019;Murillo-Gómez et al., 2018;Tribst et al., 2018). Also, the self-etching silane primer clinical could act on preservation of the glass phase of ceramic materials, improving a mechanical performance and acting on maintenance of adequate adhesion between restorative materials or dental substrate and ceramic restorations (Murillo-Gómez, Palma-Dibb, & De Goes, 2018).
According to the manufacturer, the self-etching silane primer materials as Monobond Etch and Prime (MEP, Ivoclar Vivadent, Schaan, Liechtenstein), used in this study has in the composition of material ammonium polyfluoride, which replaces the HF etching of surface and trimethoxypropyl methacrylate that acts on silanization (El-Damanhoury & Gaintantzopoulou, 2018;Murillo-Gómez et al., 2018;Tribst et al., 2018). Thus, according to the manufactures, the self-etching silane primer allows treatment of glass-ceramics without compromising bond strength, although the pattern of surface roughness is smoother than HF etching (Cardenas et al., 2019;Murillo-Gómez et al., 2018;Tribst et al., 2018).
Besides that, the bonded interface is affected by water degradation in the long-term (Lung & Matinlinna, 2012;Murillo-Gómez et al., 2018). However, only a few studies have evaluated the behavior of bonded interface integrity and the bond strength between resin cement and glass-matrix ceramic conditioning with self-etching primer submitted to aging. Thus, the purpose of this in vitro study was to bond CAD-CAM glass-ceramics to a resin cement by self-etching primer or conventional HF etching followed by silanization. The null hypothesis tested was that no significant differences would be found on the bond strength produced by the surface treatments tested among the specimens stored in water for 24 h or 3 months of water storage.
After, the specimens were cleaning ultrasonically (Ultrasonic cleaner, Cristofóli, Campo Mourão, PR, Brazil) with distilled water for 5 min and dried by air jet. Specimens were crystallized following the program recommended by the manufacturer.
Composite blocks (Llis, FGM, Joinville, SC, Brazil) were prepared by a metal matrix with a dimension of 12x7x3mm.
They were prepared using the layering technique (maximum 2-mm-thick layers) and polymerized for 40 s (Radii-Cal, SDI, Bayswater, Victoria, Australia). Composite blocks were prepared with 37% phosphoric acid for 60 seconds and were cleaned ultrasonically (Ultrasonic cleaner, Cristofóli, Campo Mourão, PR, Brazil) with distilled water for 5 min and air dried for 30 s.
Then, the adhesive was applied on the surface for 30 s and dried for volatilization of the solvent.

Experimental design
Blocks were randomly divided into two groups (n=20) according to the silanization procedure (HF+Silane or Monobond Etch and Prime, Ivoclar Vivadent AG, Schaan, Liechtenstein) and the presence of aging by stored water for 24h (b -baseline) and for 90 days (a-aging). Prior to the conventional silanization technique (HF), blocks were etched with 10% hydrofluoric acid (Condacporcelana, FGM, Joinville, SC, Brazil) for 60 seconds, rinsed with water for the same time, and dried with an oil-free air jet. Monobond Plus silane was applied on the surface for 60 seconds, and dried for volatilization of the solvent. For the group with the self-etching primer (MEP), MEP was applied with scrubbing for 20 seconds and left to react for 40 seconds, then the blocks were rinsed with water for the same time, and dried. Research, Society and Development, v. 10, n. 5, e33110514572, 2021 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v10i5.14572

Microtensile Bond Strength Test (TSB)
The procedure of luting was performed after preparation of composite and ceramic blocks as described before. A single operator performed all the bonding procedures. Resin composite blocks were luted with a light cured resin cement (Variolink Esthetic LC, Ivoclar Vivadent AG, Schaan, Liechtenstein) and positioned over the ceramic blocks and kept in position using a device with 45,8 g weight for 5 minutes, after the excess of cement was removed. Simultaneously, the photoactivation was performed the for 40s on each line and luting of the set cement-resin-ceramic, using a LED (Radii Cal, Bayswater, Victoria, Australia) light curing having a irradiance power density of 1200 mW/cm². The set composite-cementceramic was cut using a double-sided 152 x 0.5mm (LC diamond wafering Blades Buehler, Lake Bluff, IL, USA) diamond disk, under water cooling, into 1-mm 2 cross-section beams composed of ceramic/cement/composite. Sticks were stored in Eppendorf tube with distilled water at 37ºC, pH 6,0 for 24h to baseline group (b) and for 90 days for aging group (a), the water was changed every 30 days. One representative specimen from each group was randomly selected for scanning electron microscopy (SEM) analysis (JSM 5600 LV, JEOL) operated at 15 kV. After the bond strength tests each sample was coated with gold, creating a 10nm of thickness layer (Bal-Tec SCD 050). Besides the analysis of failure, four ceramic samples were prepared following the same protocols of etching cited before, for the evaluation of conditioning pattern, using previous parameters to control the SEM analysis.

Statistical Analysis
Data distribution was analyzed using the Shapiro-Wilk test and then by two-way analysis of variance (ANOVA: silanization method x aging). Tests were performed with α = 0.05, using the software SigmaPlot 12.0 (Systat Software Inc., San Jose, CA, USA).

Microtensile Bond Strength Test
The mean values of microtensile bond are shown in Table 1, values ranged between 19.3 and 24.7 MPa (Table 1).
There were no significant differences among the groups (Two-way ANOVA; p=0.322), which means, no significant difference was observed between the silanization protocols applied (p=0.276). Also, no difference was found in the bond strength results for the aging factor (p=0.122). Aging (a) 19.5 (5.7) 24.6 (9.4) Source: Authors (2021).

Failure pattern
The analysis of the type of failure was classified using a stereomicroscope with 50X of magnification (Table 2).
Failure classified as mixed (MIX) was predominant in all analyzed groups. However, after 90 days of water storage samples, it was observed a tendency to increase adhesive failures. Despite the low prevalence the cohesive failures were present in all groups. Representative micrographs of failures found on tested samples from SEM analysis (70x magnification) are shown summarized on Figure 1. Research, Society and Development, v. 10, n. 5, e33110514572, 2021 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org /10.33448/rsd-v10i5.14572 6 primer caused a partial dissolution of the glass matrix, since the specimen showed fewer changes on surface topography with fewer pores and smoother roughness.

Discussion
Based on the findings of this in vitro study, the null hypothesis that no significant differences on bond strength produced by self-etching silane primer and HF for feldspathic ceramics was accepted.
In this study, the bonding effectiveness promoted by MEP and HF, evaluated by microtensile bond strength test, was considered statically similar, as founded in previous studies (Cardenas et al., 2019;Murillo-Gómez et al., 2018). Although, the conventional method of etching with HF promoted higher surface microporosity, and higher dissolution of the glass matrix when compared with HF. When the HF is applied on the surface of glass ceramic, the tetrafluorosilane is formed, which further reacts with HF to form a soluble hydrofluorosilicic acid. Then the vitreous matrix is dissolved and rinsed away, producing microscopically changes on the surface, increase of the surface energy, roughness and pores (Lung & Matinlinna, 2012;Tribst et al., 2018).
Even though, etching procedure using HF and silane is still considered the gold standard method for glass ceramic conditioning (El-Damanhoury, N, Sheela, & Gaintantzopoulou, 2020), promoting high changes on surface topography, which is essential to enhance the micromechanical bonding area, which is not always correlated to high bond strength values for feldspathic ceramic (Murillo-Gómez et al., 2018;Venturini, Prochnow, Rambo, Gundel, & Valandro, 2015).
Despite the etching pattern of the surface of treatments shown in SEM images, ceramic treated to self-etching ceramic primer showed less dissolution of the glass matrix, considering the use of thin ceramic restorations, the use of self-etching primer ceramic represents a less aggressive alternative to the glass phase preserving the mechanical properties and longevity of restorations (Murillo-Gómez et al., 2018). According to reports of previous studies, higher surface roughness does not impact directly in the bond strength values. Thus, feldspathic ceramic known characteristic can explain the role of chemical interaction supported by the silanization procedure, due to the higher amount of glass matrix (Sinthuprasirt, van Noort, Moorehead, & Pollington, 2015). Silanization process can act on the adhesion of dissimilar materials. Silane is a bifunctional compound, which can connect to inorganic and organic materials promoting the adhesion between the organic resin part and inorganic material from ceramics (Matinlinna, Lung, & Tsoi, 2018;Yoshihara et al., 2020).
Previous reports pointed out that the active application mode of self-etching primer could improve the spreading on the surface of ceramic improving the contact angle and wettability proprieties of the ceramic (Tribst et al., 2018). Also, the use of self-etching ceramic primer may prevent the risks of occupational exposure to HF, due to the potential risks of toxicity and damage to soft tissues (Cardenas et al., 2019;Lopes et al., 2019;Tribst et al., 2018). Therefore, the self-etching primer could be considered as an alternative method to etching of glass ceramics, since it is considered a safer procedure, for enabling the simplification of clinical steps and for its promising performance on bond strength evaluation in this investigation, in agreement to previous evaluations (Cardenas et al., 2019;Murillo-Gómez et al., 2018;). This investigation found a major prevalence of mixed failures in both treatment, HF + silane and self-etching primer, the same behavior was found in preliminary investigations (Tribst et al., 2018). It suggests that the silane methacrylate The aging process, by storing the samples for 90 days in water, was not sufficient to compromise the bond strength and evaluation of the degradation process to adhesive interfaces. However, after storage, an increase adhesive failure was noticed, despite the higher prevalence of mixed failures still have been found in all of the analyzed groups. This finding in our study may be associated with hydrolytic degradation of the adhesive interface (Prado et al., 2018). Aging process affects negatively the adhesive interface according to a study that submitted samples for thermocycling at 5000 thermal cycles, which acts on the reduction of bond strength values (Tribst et al., 2018), or when samples are submitted to a year storage period of aging, increase of the adhesive failure values occur, especially on self-etched treatment specimens (Murillo-Gómez & De Goes, 2019).
Our data indicated that, considering the limitations of this in vitro study, especially in relation to the simulation of the aging procedure may not reflect a clinical condition and the dynamic degradation process. Considering such limitation, the self-etching ceramic primer may be recommended for surface treatment to glass ceramic, considering that the bond strength of self-etching silane primer showed similar performance to the conventional treatment. However, further clinical investigations are required to assess the bonding stability after simulation to a critical scenario.

Conclusion
The self-etching silane primer provided a similar ceramic/cement bond strength to HF etching and silane application in both analyses, immediate and after 90 days of aging. Morphological alterations on the ceramic surface were lower by selfetching silane primer in comparison to HF etching. Mixed failures were more prevalent among all the groups.