Introduction
Dental ceramics became one of the mostly used restorative materials in the last years for esthetic and rehabilitation procedures due to various positive properties in topics as wear resistance, biocompatibility, thermal conductivity and optical characteristics (Denry&Kelly, 2008; Kelly&Denry, 2008). However, to obtain an optimal bonding of the ceramic restoration to tooth structure it is required, surface treatment in dental tissues (Peumans et al., 2010), ceramic material (Brentel et al., 2007) and the use of luting agents.
Different kinds of luting agents have been used to bond ceramic restorations to tooth structure (Edelhoff&Özcan, 2007). Resin-based polymerizing cements can perform some chemical and mechanical interaction to tooth and restoration (Peumans et al., 1999; De Munck et al.2005). Furthermore, they present superior bond strength to tooth and ceramics than water-based cements, low solubility, adhesion capability and long-term stability with silica-based ceramics (Sadowsky, 2006; Kramer et al., 2000). Resin cements may be bonded to the tooth using either the etch-and-rinse or self-etch adhesive technique, also other kind of resin cements may not need an intermediate adhesive system to bond the tooth structures and they are called self-adhesive resin cements (Ferracane et al., 2011).
To achieve adhesion between resin cement and glass-ceramic material, there are two main approaches: micromechanical interlocking and chemical bonding (Tian et al., 2014). Mechanical attachment can be promoted by etching the glass-ceramic surface (commonly with hydrofluoric acid (HF)) (Filho et al., 2004) and sandblasting (Moharamzadeh et al., 2008), while silane may produce chemical adhesion (Matinlinna et al., 2004). The combination of both mechanical and chemical bonding is the preferred way to establish an optimal resin-cement/glass-ceramic bond (Blatz et al., 2003; Brentel et al., 2007; Lung&Matinlinna, 2012).
Since 1977, silane coupling agents have been applied as adhesion promoters in dentistry (Eames et al., 1977). Moreover, silane coupling agents must be activated through a hydrolyzation process (SiOR—>SiOH), so they could be able to bond through OH groups (Lung&Matinlinna, 2012). So, many commercial silane-solutions use a molecule with a pre-hydrolyzed silane group in one side and a methacrylate group in the other side (3-Methacryloxypropyltrimethoxysilane, MPS) in order to bond resin and ceramic, and using ethanol and water as solvents in one bottle (Anagnostopoulos et al., 1993).
Likewise, it has been suggested that solutions containing phosphate acid monomers such as MDP (10-methacryloyloxydecyl dihydrogen phosphate) may also improve ceramic-resin bonding (Blatz et al., 2004). Those phosphate acid monomers are employed due to their capability to bond metallic ions present in some ceramics (mainly non-glass ceramics) with methacrylate groups in resin cements (Blatz et al., 2004; Kitayama et al, 2010). Thus, when silane is added along (as in some commercial primers), that solution can enhance bonding in glass ceramics and non-glass ceramics indistinctly. Additionally, these phosphate acid monomers are also incorporated in some enamel-dentin self-etch adhesives to act as couplers between methacrylate groups and calcium present in enamel and dentin (Van Meerbeek et al., 2011). Recently, silane has been also added to those adhesives obtaining a “universal” bonding solution to be used with enamel, dentin, ceramics and some metal alloys, possibly simplifying clinical steps (Van Meerbeek et al., 2003). This broad range of silane-containing solutions are supposed to act as conventional silane couplers, but there is no further evidence that clears if they can improve equally the ceramic-cement bonding and how they interact with different kinds of resin cements.
The aim of this in vitro study is to determine the effect of different silane-containing solutions on ceramic-cement bonding and their interaction with different dual-cure resin cements. The hypothesis set in this study was that ceramic-cement µSBS could be influenced by the usage of different resin cements and silane-solutions.
Materials and methods
One glass-ceramic (IPS e-max lithium disilicate CAD/CAM, color A2, Ivoclar, Vivadent, NY, USA; Lot. N76665) was used as a substrate to bond five different dual-cure resin cements which three are conventional adhesive resin cements (all in A2 shade): RelyX Ultimate (RU) (3M ESPE, St. Paul, MN, USA/Neuss, Germany), RelyX ARC (AR) (3M ESPE), VarioLink II (VL) (Ivoclar Vivadent AG, Schaan, Liechtenstein); and two self-adhesive resin cements: RelyX UNICEM 2 (U2) (3M ESPE), and BiFix (BF) (VOCO GmbH, Cuxhaven, Germany) (Table 1). Three silane-containing solutions were employed as silanization treatment: RelyX Ceramic Primer (AS) –a sole-silane solution– (3M ESPE), Scotchbond Universal (SU) –a built-in-silane multi-mode adhesive– (3M ESPE) and Clearfil Ceramic Primer (CP) –a ceramic primer containing silane and MDP– (Kuraray Noritake Dental Inc., Okayama, Japan) (Table 1). The specimens preparation methodology for the microshear bond strength test (µSBS) was adapted from the one developed by Shimada (Shimada et al., 2002).
Forty five lithium disilicate ceramic plaques measuring 10±0.1mm in length, 6±0.1mm in width and 2±0.1mm in thickness were milled from pre-sintered CAD/CAM blocks on an E4D Dentist System (D4D technologies, LLC, Richardson, TX, USA) using a custom-mill file. They were sandblasted with aluminum oxide (50 μm; Bio-Art, São Carlos, SP, Brazil) at 2 bar pressure for 5 seconds in only one surface at a working distance of 5mm from the ceramic plaque. They were also treated with 10% Hydrofluoric acid gel (HF) (Porcelain Conditioner Dentsply, Petropolis, Brazil) for 20 seconds (Borges et al., 2003) and washed with tap water for 1 minute. Then all plaques were ultrasonically cleaned for 5 minutes in distilled water and air-dried. Each ceramic plaque was prepared to receive 8 resin-cement cylinders.
All the 45 glass-ceramic plaques were divided into 3 groups of 15 plaques each to be treated with one of the three silane-solutions. Each group was subsequently divided in 5 groups of 3 plaques to receive each of the cements listed above. All materials’ employment protocol is described on Table 2. Each treated glass ceramic plaque was placed in a silicon mold with eight cylindrical-shaped compartments measuring 1.6±0.1mm in diameter and 1±0.1mm in thickness. All resin cements were “calibrated”, squeezing out the first amount of material until it was delivered evenly from either chambers or dispensers. Then, the resin cement was injected into each compartment on the treated ceramic plaque (8 cement cylinders on each ceramic plate) (Table 2) and light-cured (Optilight Max, Gnatus, Brazil; light output: 600 mW/cm2) for 40s. A 1mm glass slide was placed between the light tip and the cylindrical resin cement specimen. The ceramic-plaque/resin-cement assemblies were removed from the molds after 5 minutes, summarizing 24 cylinders for each group. The specimens were water rinsed, dried and stored in 100% relative humidity at 37°C for 24 h.
Specimens were attached to a holding device with cyanoacrylate glue (Super Bonder Loctite, Henkel) and placed on a universal testing machine (Instron 4411, Instron Corporation, Canton, MA, USA) in order to do microshear bond test (μSBS) using a 500-N load cell. A shear load was applied to the base of the resin cement cylinder with a thin wire (0.20 mm diameter) placed strictly parallel to the ceramic plaque surface at a crosshead speed of 0.5 mm/min. Values of each specimen were expressed in MPa, and then a group mean was calculated (24 measures). All data were statistically analyzed by Two-Way ANOVA (resin cement vs. silane containing solution) and Tukey test at a significance level of 5% (p<0.05). The hole methodology procedure is illustrated on Scheme 1.
For failure pattern evaluation, all specimens were mounted on aluminum stubs with the aid of carbon ribbon, sputter coated with gold/palladium powder (SCD 050; Balzers, Schaan, Liechtenstein) and then examined using a scanning electron microscope (SEM) (JSM 5600 LV; JEOL, Tokyo, Japan) operating at 15 kV. Photomicrographs of representative areas of the fractured surfaces were taken at different magnifications for fracture pattern evaluation and then classified according to the following criteria: AD-CC, adhesive failure between ceramic and resin cement/adhesive system (when employed); C-AS, cohesive failure in adhesive system (when employed); C-CE, cohesive failure within the ceramic; C-RC, cohesive failure in resin cement and MIX, mixture of various patterns in the same specimen.
Results
Two-way ANOVA statistical analysis revealed that the factors “Resin cement” and “silane-solution” significantly influenced the µSBS (p=0.00001). Likewise, significant differences were found in the interaction between the two factors (p=0.00001). All μSBS mean values, data distribution and standard deviation per each group are illustrated in Figure 1.
Spreading data ranged from 2.8 MPa to 48.4 MPa as shown in Fig. 1. Tukey test (p<0.05) showed that treating the glass-ceramic with a conventional silane (AS) produced significantly higher µSBS values compared with the ceramic primer (CP) and the universal adhesive (SU). When treated with AS, cements RelyX ARC (AR) and VarioLink II (VL) obtained the statistically highest results, but VL was not significantly different from RelyX Ultimate (RU), RelyX UNICEM 2 (U2) and BiFix (BF) (p>0.05) (Fig. 1). When treatment SU was applied, cements VL and RU obtained the significantly highest values among all cements (p<0.05). However, RU was statistically not different from U2 and AR (p>0.05). The lowest µSBS mean for this treatment was obtained by BF that by the way was not significantly different from AR (p>0.05) (Fig. 1). When specimens were treated with CP, AR presented the significantly lowest mean within the groups (p<0.05) (Fig. 1). Cements RU, U2 and VL showed significantly higher values when treated with AS than when treated with SU and CP (p<0.05). BF registered lower results when treated with SU than with AS and CP (p<0.05). In the case of AR, AS produced the statistically highest values, followed by SU and CP (p<0.05) (Fig. 1).
Results from SEM fracture pattern analysis are presented in Fig. 2. Representative photomicrographs from all groups are shown from Figure 3 through 7. All groups treated with AS presented C-RC failure pattern and it was also recurrent in most CP groups. Generally, fracture patterns AD-CC and MIX were mostly prevalent in groups with lower µSBS values. In general SU groups showed the most variable failure pattern within all groups (Figs. 3 through 7).
Discussion
As it is well known two major interfaces are involved in the luting process of an indirect glass-ceramic restoration: tooth/luting agent and luting agent/ceramic material. On this regard, it is clinically relevant to achieve an optimal bonding performance in both interfaces. Thus, the present investigation aimed to study the ceramic/cement interface and two of the main factors involved with it: silanization and its interaction with different resin-cements.
To assess bonding performance of ceramic/cement interface, a well-established µSBS method was used (Shimada et al., 2002). This method uses a thin wire close and parallel to the bonding area instead of a blade, in order to better distribute the stress in the surrounding specimen area (Dehoff et al., 1995). Also, the usage of a micro-specimen leads to a greater uniformity on that stress distribution and avoids better the undesirable cohesive failure in substrate, producing more “realistic” failure patterns than when using bigger specimens (Della Bona et al., 2000; Della Bona et al., 2003; Pisani-Proenca et al., 2006).
Overall, statistical evaluation revealed significant differences between the groups (Tukey test, p<0.05), as well as that factors “resin cement” and “silane solution” did influenced ceramic/cement bonding performance, so the hypothesis set in this study must be accepted (Two-way ANOVA, p=0.00001).
Three conventional adhesive and two self-adhesive resin cements were used as luting agents. A conventional silane (AS), a built-in silane universal adhesive (SU) and a ceramic primer containing silane and MDP were employed as silanization agents.
Regarding “silane solution”, when specimens were treated with AS all cements tested presented their statistically highest results compared with the other treatments, so in this conditions, all cements performed pretty well (Fig. 1). This can be illustrated with the fact that with AS, cements obtained a relatively uniform data distribution and so a stable behavior with this treatment could be presumed with the only exception of BF that showed a non-uniform data distribution (Fig. 1). RelyX ceramic primer (AS), contains a pre-hydrolyzed silane (3-Methacryloxypropyltrimethoxysilane, MPS) mixed in one bottle with water and ethanol (according to material MSDS). So, MPS present in AS is a ready-for-use silane and capable to bond methacrylate groups via OH- groups and also it may improve the wettability of the ceramic surface and promote better interaction between ceramic and cement as it has been previously suggested (Lung&Matinlinna, 2012; Meng et al., 2011). In fact, a good interaction between resin-cement and ceramic surface could be presumed for AS groups in this study as all specimens presented C-RC failure mode (Figs. 2, 3a and b, 4a and b, 5a and b, 6a and b, 7a and b), sign of a more intimate contact and higher ceramic-cement bond strength according to previous findings (Pollington et al., 2010). Another important factor in this regard, is that silanes form three different oligomer layers when applied as a sole-coat (Ishida&Koenig, 1980) but just the first one is capable to bond with ceramics (Berg&Jones, 1998). So, it is important to get the thinnest layer possible and eliminate the solvent and other by-products of bonding reaction to improve adhesion (Roulet et al., 1995). In our study, a single silane layer was applied and dried (following manufacturers’ instructions) with the intention of eliminating most of solvents and by-products and improve chemical adhesion. Moreover, sandblasting and HF etching performed an all ceramic plaques may have promoted mechanical interlocking and enhancing adhesion (Tian et al., 2014). This combined mechanical/chemical bonding is considered to be the most accepted glass-ceramic pre-treatment and it has been reported that it provides the highest ceramic/cement bonding (Brentel et al., 2007; Kitayama et al., 2010; Shimada et al., 2002). With those issues in mind and in light of our results, it can be affirmed that this protocol (AS) continues to be the preferred glass ceramic pre-treatment.
A universal ceramic primer (CP, containing silane and MDP) and a silane-containing multi-mode adhesive system (SU, containing also MDP and silane) were also employed. Kern and Wegner (Kern&Wegner, 1998) first reported that resin cements containing phosphate monomers could form a water-resistant chemical bond with zirconia (Wegner&Kern, 2000). Furthermore, literature reports that acidic monomers rapidly hydrolyze silane coupling agents, enhancing the polysiloxane bonding network needed to form a chemical union of resin cement with zirconia and also with glass ceramics (Kitayama et al., 2010; Tanaka et al., 2008). Probably, with this intention is that phosphate monomers and silane are mixed as in SU and CP. Conversely in the present study, most groups treated with SU and CP obtained lower values than AS, so the supposed combined benefit of silane and MDP didn’t perform better than the sole-silane (Fig. 1). This may be due in part, to the addition of MDP to the silane-solution, in the sense of having less silane quantity per area in contact with ceramic surface and available to perform the link with resin cement (Ikemura et al., 2011) (in contrast with the sole silane solution) and also hindering the elimination of solvents and other byproducts obstructing the formation of complete condensation reactions needed to form siloxane network (Shen et al., 2004). According to this, is that some “spaces” could be observed in SEM images of some groups employing SU and CP, suggesting incomplete polymerization areas possibly produced in part by water release (silane condensation reaction byproduct) within the interface (Figs. 3d and f, 5d, 6d, 6f and 7d). Furthermore, the addition of other components may affect negatively the adhesion process and may promote the formation of a thicker silane layer with more oligomer presence that may be also detrimental hindering an intimate interaction and consequently bonding performance (Tian et al., 2014; Roulet et al., 1995; Monticelli et al., 2006).
CP and SU behaved relatively similar within the cements, just BF and AR obtained different values between those treatments (Fig. 1). In the case of BF, it obtained lower results with SU than with AS and CP and it also was the lowest on SU group together with AR (Fig. 1). Thus, it could be presumed that some incompatibility between SU and BF may have caused bond strength decrease and produced a very variable failure pattern prevalence (Fig. 2) as well as some possible incomplete polymerization areas visible on SEM images (Fig. 5c and d). Also, BF-SU shows a pretty uniform data distribution on a low range (Fig. 1). Several studies have confirmed some incompatibilities between dual-cure (or chemically-cure) composites and some acidic monomers present in some simplified adhesives and ceramic primers (Chen&Suh, 2013; Schittly et al., 2010; Suh et al., 2003). This occurs primarily because the acidic monomers in the oxygen inhibition layer can deactivate the tertiary amines (needed to produce chemical polymerization), affecting the chemical polymerization system on self-curing and dual-curing resin cements (Suh et al., 2003). Hence, highly acidic one-step adhesives or primers greatly decrease ceramic/cement bond strength when using self-cure or dual-cure resin cements with it, as probably happened in this case (Shittly et al., 2010).
The other cement that behaved different with SU and CP was AR. This cement particularly obtained the highest values for AS, lower for SU and the lowest for CP (Fig. 1). It was also the lowest mean value among all cements when treated with CP. Hence, it could be presumed that also some incompatibility as the one mentioned before may exist between AR and CP, as it behaved very well with AS and quite normal with SU but not very well with CP. Also its data distribution is relatively constant on a low range (Fig. 1). That incompatibility may be due to the fact that CP is a ceramic primer containing an acidic monomer (MDP) with a pH value of approximate 2.5 (according to manufacturer) and AR a dual-cure resin cement containing an amine and benzoyl peroxide as chemical polymerization system. That situation matches the one described previously about incompatibility between acidic primers/adhesives and dual-cure resin cements. Additionally, the more prevalent failure mode was AD-CC and it is associated with low µSBS values. Also it could be observed on SEM images, some areas of apparent rejection and maybe incomplete polymerization between the primer layer and the resin cement (Fig. 6d and f).
In the case of RU, manufacturer claims that this cement is designed to work well with SU as it has integrated a “dark cure activator” so its self-curing portion would not be affected with acidic one-bottle adhesives. In the case of U2, as it is a self-adhesive resin cement it already has acidic monomers on its composition, so its chemical polymerization component may be acidic-resistant already. In the present study RU showed a pretty uniform behavior with the three silane-solutions and also the most homogenous data distribution together with U2 (Fig. 1). So it could be presumed that RU and U2 really did not suffer greater affectation when used along with acidic one-bottle solutions. In fact, instructions for use of SU recommend the employment of that activator (available as a separate product) when uses SU along with another dual-cure/self-cure resin cement than RU. As one of our objectives was to investigate the interaction between the pure cements and the silane-solutions, we did not use that activator with any of the cements used on the study.
Interesting was that despite this theoretical problem, just BF-SU and AR-CP showed some kind of decrease on µSBS values. In the case of VL, it obtained also a high mean value with the three solutions. It presented more variable data distribution when used with SU and CP signalizing a less constant behavior in those conditions but nothing as critical as AR (Fig. 1). This may be remarkable because VL uses also benzoil peroxide/amine as chemical polymerization mechanism (as AR) and so a major affectation would be expected when used with SU and CP. So it can be inferred that VL’s polymerization depends more on the physical polymerization system than on the chemical polymerization component, different than AR and possibly explaining why it was not equally affected by acidic solutions. So, that negative interaction may be material dependent, as each specific composition may influence the cements’ behavior against simplified acidic solutions containing silane. This is clinically relevant as the main purpose of those one-bottle/universal solutions is to simplify clinical steps and diminish the quantity of clinical steps to be used in the cementation process. So the clinician may pay attention to this interaction issues in order to avoid any problem that could affect bonding performance when combining different silane-containing-solutions with different dual-cure resin cements.
Conclusions
Within the limitations of this in vitro study we can conclude the following:
- 1. The sole-silane solution performed better than the silane-containing primer and the silane-containing universal adhesive when used as silanization agents.
- 2. In general, both alternative silane-solutions (SU and CP) performed relatively well and very similar between each other.
- 3. Compatibility between those alternative silane-solutions (as they are acidic solutions) and dual-cure resin cements may be material dependent as some dual-cure resin cements may use an acid-sensitive chemical polymerization system and some other may not.
Acknowledgements
The authors want to thank University of Costa Rica, FOP-UNICAMP, CAPES and Brazilian authorities for all the support given in many ways to Dr. Murillo-Gómez to perform his researching activities in Brazil. These results were used by Dr. Murillo-Gomez as part of his master’s degree thesis.
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