Dental ceramics are the material of choice for many patients requiring esthetic dental restorations due to optimal esthetics, translucency, fluorescence, wear resistance, biocompatibility, and chemical stability.¹ The advent of computer-aided design/computer-aided manufacturing (CAD/CAM) technology enabled the same day delivery of ceramic restorations.² Dental material manufacturers produce monolithic blocks for chairside CAD/CAM restorations, which are dense and homogeneous, and have minimal internal defects. Chairside CAD/CAM materials can be divided into six groups based on their clinical applications and properties: (I) adhesive ceramics (feldspathic and leucite reinforced) which should be etched and bonded to tooth structure; (II) high-strength ceramics such as lithium disilicate and zirconia-reinforced lithium silicate (ZLS) with improved properties compared with adhesive ceramics; (III) flexible ceramics that do not require a porcelain furnace and are bonded to tooth structure using an adhesive; (IV) composite materials with a resin matrix; (V) full-contour zirconia which is cemented on the tooth; and (VI) provisional materials for provisional restorations.²

Celtra Duo is a high-strength ZLS ceramic which has a high content of ultrafine (<1 μm) glass-ceramic crystals and 10% zirconia. It is produced in completely crystalline form by the manufacturer and may be glazed manually or in a ceramic furnace before delivery. It is available in different shades and high and low translucencies. According to the manufacturer, a high number of fine high-glass lithium silicate particles in Celtra Duo is responsible for its excellent optical and mechanical properties, translucency, fluorescence, opalescence, and chameleon effect. It also has optimal marginal adaptation and excellent polishability.³

The chairside CAD/CAM blocks of flexible ceramics include LAVA Ultimate, VITA Enamic, and Cerasmart, which have a resin matrix instead of a glass matrix. According to the manufacturers, these ceramics can tolerate higher forces without fracture. Furthermore, they do not require additional heating after fabrication and can be delivered to patients' right away.⁴ VITA Enamic is a resin-based hybrid ceramic with 14wt% Triethylene glycol dimethacrylate (TEGDMA) and Urethane dimethacrylate (UDMA). It has an integrated dual structure ceramic network reinforced with leucite and zirconia (86wt%). Its mechanical properties are somewhere between those of glass ceramics and filler-rich composite resins. It also has high wear resistance but is fragile.

Treatment of the bonding surface of ceramics has a significant effect on their clinical service. Treatment of ceramic surfaces is imperative to enhance their adhesion, which can be performed mechanically by sandblasting, hydrofluoric (HF) acid etching, and diamond burs or chemically by the use of silane and bonding agents.⁴ Primers are commonly applied to enhance adhesion between dissimilar surfaces. They are substrate specific, and chemical bonding may be obtained to some substrates. Nonetheless, all primers enhance the wettability of bonding surfaces. Silane-based primers are used for metal and ceramic surfaces. Recently, universal primers were introduced which can be applied on various substrates.⁵,⁶

In the use of both Celtra Duo and Vita Enamic ceramics, the surface is first etched with HF acid (5%–9%), and then the silane coupling agent is applied.⁷ Silane coupling agent has a dual function. Its organic functional part reacts with the organic matrix, and its alkoxy groups react with the mineral phase.⁸ The silane commonly used in dentistry is the gamma-methacryloxypropyltrimethoxysilane diluted in ethanol and water, which has a pH of 4–5.⁹ Kitahara et al.¹⁰ assessed the bond strength of Clearfil ceramic primer containing 10– methacryloyloxydecyl phosphate and single-bottle silane (Clearfil porcelain bond activator) reinforced with 5wt% 4-methacryloyloxyethy trimellitate anhydride (4-META). They reported higher bond strength for silane reinforced with 4-META.¹⁰ It has been reported that after application on the ceramic surface, 4-META is converted to 4-methacryloxyethyl trimellitate due to the effect of surface moisture, and the silane is activated, hydrolyzed, and forms silanol groups, which are bonded to ceramic through the formation of siloxane structure. Furthermore, they showed that silane containing 5wt% 4-META yielded higher shear bond strength compared with silane containing 10wt% 4-META, although this difference did not reach statistical significance.

Appeldoorn et al.¹¹ assessed the shear bond strength of composite to porcelain using All-Bond 2 (Bisco), Cerinate Prime (Den-Mat), Clearfil Porcelain Bond (Kuraray), Etch-Free (Parkell), Monobond-S (Vivadent), Porcelite (Kerr/Sybron), Scotchprime (3M), and Silistor (Kulzer GmbH, Wehrheim, Germany) porcelain repair systems. Without etching, the 4-META bonding agent yielded the highest mean bond strength after 24 h of immersion in water and the second highest mean bond strength after 3 months of water storage. Clearfil Porcelain Bond yielded the highest bond strength after 3 months of water storage.

Both dual-cure and light-cure cement can be used for ceramic bonding. However, previous studies on the effect of the addition of 4-META to silane used dual-cure resin cement, and a comparison between the bond strength of dual-cure and light-cure resin cement to ceramics following the addition of 4-META to silane has not been conducted.¹⁰,¹²,¹³

Considering all the above, this study aimed to assess the effect of the addition of 4-META in 0%, 2.5%, 5%, and 10wt% concentrations to silane on microtensile bond strength (μTBS) of light-cure and dual-cure resin cement to VITA Enamic and Celtra Duo ceramics.

This in vitro, experimental study was conducted on VITA Enamic (VITA, Bad Säckingen, Germany) and Celtra Duo (Sirona Dentsply, Milford, DE, USA) ceramics with Allcem Veneer (FGM, Joinville, SC, Brazil) light-cure and Allcem (FGM, Joinville, SC, Brazil) dual-cure resin cement containing 4-META in 0%, 2.5%, 5%, and 10wt% concentrations in 16 groups (n = 4). Table 1 presents the composition of materials used in this study.{Table 1}

Addition of 4-methacryloyloxyethy trimellitate anhydride to silane

In this study, the Silane Bond Enhancer (Pulpdent, Watertown, USA) was used, which is supplied in 1.2 mL bottles. Thus, 0.03 g of 4-META (Polysciences Erope, Eppelheim, Germany) was weighed by a digital scale (A and D, Tokyo, Japan) and added to the contents of the bottle to obtain silane with 4-META (2.5%). To prepare 4-META (5%), 0.06 g of 4-META was weighed and added to 1.2 mL of silane. To prepare 4-MTA (10%), 0.12 g of 4-META was weighed and added to 1.2 mL of silane. It should be mentioned that after complete mixing of powder with silane, the color of silane solution became transparent with a tint of milky white.

Preparation of study groups

A total of 32 VITA Enamic and 32 Celtra Duo ceramic blocks were selected and manually polished with 600-grit silicon carbide abrasive paper under running water to simulate the internal restoration surface after preparation by bur in a CAD/CAM milling machine.¹⁴ Each group of Enamic and Celtra Duo ceramic blocks was randomly divided into eight subgroups (n = 4) and underwent the following surface treatments:

Groups 1 and 9: The ceramic surfaces were etched with 5% HF acid (prepared by addition of 7 units of distilled water to one unit of 40% HF acid [Merck, Darmstadt, Germany]). The etching time was 30 s for Celtra Duo¹⁵ and 60 s for Enamic blocks.¹⁴ They were then rinsed with water and cleaned in an ultrasonic bath containing 99% alcohol for 5 min and dried with air spray. Silane with 4-META (0%) was applied on the surface by a microbrush, allowed 1 min, and dried with air spray for 15 s. Next, Amber APS (FGM, Joinville, SC, Brazil) bonding agent was applied on the surface, air thinned for 10 s, and light cured for 20 s using a curing unit (Bluephase C8; Ivoclar, Vivadent, Liechtenstein). Allcem Veneer light-cure cement was applied on the bonding agent and ceramic with 0.5 mm thickness as instructed by the manufacturer. Each layer was cured for 20 s. This process was repeated until the thickness of the cement layer reached 4 mm.

The process was the same in other groups with the difference that depending on the group, different concentrations of 4-META and different cement types were used.

Thermocycling

All specimens in all groups underwent 2500 thermal cycles in 5–55°C water baths with 30 s of dwell time and 10 s of transfer time.

Bond strength test

Each block was sectioned into five specimens (a total of 20 in each group) measuring 1 mm × 1 mm using a cutting machine (Delta precision section machine, Mashhad, Iran). The specimens were fixed to the jig of a universal testing machine (TB-5T; Kpppa, Sari, Iran) with cyanoacrylate glue (Razi, Tehran, Iran) and subjected to tensile load at a crosshead speed of 1 mm/min until debonding. The cross-sectional area of specimens at the debonding interface was measured by a digital caliper (Mini Digital Calipers with 100 mm Hold Function; Shinwa Rules Co. Ltd., Sanjo, Japan). The μTBS in megapascals (MPa) was calculated by dividing the load in Newtons by the cross-sectional area in square millimeters.

Mode of failure

The debonding area was inspected under a stereomicroscope (Dewinter Technologies, Milano, Italy) at ×40. The mode of failure was determined as follows:

Type 1: Cohesive within the ceramic

Statistical analysis

Data were analyzed by SPSS version 24 (IBM Co., Armonk, NY, USA). Considering the small sample size (presence of four blocks and five sticks in each block and the similarity of the sticks), data were analyzed by the nonparametric Kruskal–Wallis and Mann–Whitney tests. The modes of failures were analyzed by the Chi-square test. P < 0.05 was considered statistically significant.

Bond strength

Table 2 and Table 3 present the raw data for μTBS of Celtra Duo and Vita Enamic. Table 4 shows the mean μTBS of the groups. As shown, the μTBS of Celtra Duo was significantly higher than that of Enamic in bonding to light-cure cement using 4-META (2.5%) (P = 0.003). No other significant difference was noted (P > 0.05).{Table 2}{Table 3}{Table 4}

Comparison of μTBS based on different concentrations of 4-META separately for each ceramic type and cement type by the Kruskal–Wallis test revealed a significant difference among Enamic groups with different concentrations of 4-META bonded to dual-cure cement (P < 0.001) and light-cure cement (P = 0.049). Furthermore, a significant difference was found in the μTBS of Celtra Duo groups with different concentrations of 4-META bonded to dual-cure cement (P = 0.019). No other significant differences were noted (P > 0.05). Pairwise comparisons of the Enamic groups (by the Kruskal–Wallis test) with the different concentrations of 4-META bonded to dual-cure cement revealed significant differences between 0% and 2.5% (P = 0.034), 0% and 5% (P = 0.021), and 0% and 10% (P = 0.01) groups, such that the μTBS increased with an increase in concentration of 4-META. No other significant differences were found (P > 0.05). Pairwise comparisons of the Enamic groups with the different concentrations of 4-META bonded to light-cure cement revealed significant differences between 0% and 10% (P = 0.013) and 2.5% and 10% (P = 0.01) groups, such that the μTBS was significantly higher in 4-META (10%). No other significant differences were found (P > 0.05).

Comparison of μTBS based on the cement type with different concentrations of 4-META and ceramic type by the Kruskal–Wallis test revealed a significant difference between the μTBS of dual-cure and light-cure groups in the use of 4-MTA (2.5%) for bonding to Celtra Due ceramic, such that the μTBS was significantly higher to light-cure cement (P = 0.017). No other significant differences were found (P > 0.05).

In comparison of μTBS based on the ceramic type, cement type, and concentration of 4-META, the lowest mean μTBS was recorded in the Enamic ceramic group with 4-MTA (0%) and dual-cure cement (14.26 MPa), and the highest mean μTBS was recorded in Enamic ceramic with 4-META (10%) and light-cure cement (18.59 MPa); this difference was statistically significant as shown by the Kruskal–Wallis test (P < 0.001).

Mode of failure

Table 5 presents the frequency of different modes of failure in the four groups. Of 80 Celtra Duo dual-cure specimens, type 3 failure occurred in 77 (93.3%) and type 4 failure occurred in 3 (3.8%) specimens. In each of the Celtra Duo light cure, VITA Enamic dual cure, and VITA Enamic light-cure groups, 78 specimens (97.5%) showed type 3 failure, and 2 specimens (2.5%) showed type 4 failure. Types 1 and 2 were not seen in any group. No significant difference was found among the four groups in the mode of failure (P = 0.952).{Table 5}

The results showed that the lowest mean μTBS was recorded in Enamic ceramic group with 4-META (0%) and dual-cure cement (14.26 MPa), and the highest mean μTBS was recorded in Enamic ceramic with 4-META (10%) and light-cure cement (18.59 MPa) (P < 0.001). A significant difference existed between Celtra Duo and Enamic ceramics in the use of light-cure cement and 4-META (2.5%), such that the μTBS of Celtra Duo was significantly higher than that of Enamic (P = 0.003).

Kitahara et al.¹⁰ reported the bond strength of silane reinforced with 4-META to be equal to that of Clearfil Porcelain Bond Activator + Photo Bond and higher than that of Clearfil Ceramic Primer Plus (Kuraray, Japan), which can be attributed to the conversion of 4-META to 4-MET due to surface moisture and activation of silane, its subsequent hydrolysis, and formation of silanol groups that reinforce the bond strength. Soleimani et al.¹² reported an increase in the bond strength of Clearfil Porcelain Bond Activator silane following addition of 4-META under heat treatment. The present results were in agreement with those of the abovementioned two studies. Furthermore, higher bond strength was found in Enamic ceramics cemented with light-cure cement and 4-META (10%) compared with 0% (P = 0.013) and 4-META (10%) compared with 2.5% (P = 0.01). Although the μTBS was also higher in other groups with different concentrations of 4-META compared with 0%, the differences did not reach statistical significance. Unlike the present study, Kitahara et al.¹⁰ reported higher microshear bond strength in 4-META (5%) compared with 10%; although this difference was not significant. Such variations can be attributed to the different types of tests and the use of different ceramic types. Chang et al.¹⁶ reported that the application of 4-META enhanced the bond strength in general, which was consistent with the present findings.

In the present study, comparison of μTBS based on the cement type in different concentrations of 4-META and ceramic types by the Mann–Whitney test revealed a significant difference between the μTBS of dual-cure and light-cure groups in the use of 4-META (2.5%) for bonding to Celtra Duo ceramic, such that the μTBS was significantly higher in use of light-cure cement (P = 0.017). Furthermore, regarding the mode of failure, the results showed that all failures (100%) were adhesive in most groups. The frequency of adhesive failure was the lowest (90%) in Celtra Duo bonded to dual-cure cement with 4-META (5%).

Asmussen¹⁷ showed that light-cure polymers, especially those cured with visible light, had comparatively high resistance to indentation. This finding was probably due to the relatively fast release of polymerization-initiating radicals from light-cure materials, leading to a high degree of conversion of double bonds. In addition, the low content of inhibitor in light-cure materials may have caused a high proportion of converted double bonds to be engaged in crosslinking of the polymer. The same was reported by Taylor et al.¹⁸ Thus, higher frequency of mixed failure in Group 7 can be attributed to lower cross-linking of dual-cure cement and its lower resistance to indentation. Baratto et al.¹⁹ reported that cohesive failure was more frequent in the use of dual-cure resin cement, whereas mixed and adhesive failures had a higher frequency in the use of light-cure cement; they reported a significant difference in the mode of failure among the groups. De Carvalho et al.²⁰ demonstrated that mixed failure (adhesive failure between ceramic and cement and cohesive failure within the cement) had the highest frequency.

Comparison of μTBS of Enamic to light-cure cement revealed the highest μTBS in use of 4-META (10%) and the lowest in 4-META (0%). Salz et al.²¹ showed that increasing the concentration of 4-META in solvent decreased the pKa, and resultantly, the solution became more acidic and its solubility increased. Thus, the enhanced bond strength of Vita Enamic by the addition of 4-META to silane can be attributed to its increased solubility and surface roughness.

Comparison of μTBS of Celtra Duo with light-cure cement revealed the highest μTBS in the use of 4-META (2.5%) and the lowest in 4-META (0%). Abdulkader et al.¹⁵ showed that the shear-bond strength of Celtra Duo to resin cement was affected by the type of surface treatment.

Comparison of μTBS based on the type of ceramic, cement, and 4-META concentration revealed that the addition of 4-META to silane caused a significant improvement in μTBS to Enamic ceramic; however, in Celtra Duo groups, no significant difference was noted in any group compared with 4-META (0%). This finding was in line with the results of Straface et al.²² who found that Enamic showed greater surface roughness after HF acid etching compared with other ceramics such as ZLS, and therefore, can provide a potentially greater surface area for the effect of 4-META. Thus, future studies are recommended to assess the surface roughness of these two ceramics immediately after acid etching and before addition of silane using scanning electron microscopy. Bjelopavlovic et al.²³ evaluated the retentive strength of Vita Mark II, Empress CAD, e.max CAD, Vita Suprinity, Vita Enamic, and Celtra Duo ceramics and showed that type of ceramic affected the retentive strength, and application of specific materials resulted in higher retentive strength of some ceramic types.

In response to the question of whether silane coupling agent is imperative for bonding of ceramic to resin, it should be mentioned that mixing of silane with an acidic agent or its heating right before application is imperative for its activation. Thus, the conventional silane coupling agents are mixed with bonding agents containing acidic monomers for simplified application. According to Kitahara et al.,¹⁰ 4-META converts to 4-MET in the presence of moisture, and the hydrolysis of silane molecules is enhanced by the creation of an acidic environment with a pH of 3.

In the present study, the addition of 4-META (10%) to silane in bonding of VITA Enamic to light-cure cement caused a significant improvement in bond strength; the increase in bond strength was significant in addition of all concentrations of 4-META to silane in bonding to dual-cure cement. Thus, minimum (2.5%) concentration of 4-META can be used in the application of dual-cure cement, and 4-META (10%) can be used in the application of light-cure cement for bonding to VITA Enamic ceramic to enhance the bond strength. This finding was in line with the results of Soleimani et al.,¹² who recommended the addition of 4-META to silane. Considering the higher stiffness of Celtra Duo ceramic compared with Enamic,²⁴ lower bond strength of Celtra Duo can be attributed to the formation of more defects during sectioning with diamond discs.²⁵ Furthermore, considering the higher surface roughness of Enamic²² compared with Celtra Duo after acid etching with HF acid and dissolution of glass phase, it may be concluded that the application of 4-META on the Enamic ceramic affects the bond to the ceramic phase at a greater depth, and the micromechanical bond of polymer phase and resin cement would lead to greater effect of 4-META on Enamic, compared with Celtra Duo.

This study had an in vitro design. Thus, the generalization of results to the clinical setting must be done with caution. Further studies are required on the shelf-life of silane containing 4-META.

According to the results of this pilot study, the addition of 4-META (10%) to silane caused a significant improvement in μTBS to light-cure cement. The addition of 4-META in all concentrations significantly improved the μTBS to Enamic ceramic in the use of dual-cure cement; however, it had no significant effect on μTBS of Celtra Duo. Nonetheless, the results should be interpreted with caution due to the relatively small sample size.

Financial support and sponsorship

Nil.

Conflicts of interest

The authors of this manuscript declare that they have no conflicts of interest, real or perceived, financial or non-financial in this article.