At present, glass-ceramic restorations are highly popular due to their excellent esthetic appearance and long-term durability. ¹ Nonetheless, defects, chipping, and fracture of these restorations are common. Replacement of the restoration may be required in some cases. Although restoration replacement leads to more favorable clinical and esthetic results, it is not always the best choice because it may cause pulpal damage, is costly and wastes time. ² , ³ Small chipping or fracture of ceramic restorations may be repaired by composite resin instead of replacing the restoration. This method is more conservative and cheaper than restoration replacement and can be performed within single session. The remaining part of the restoration remains intact and the tooth does not need any further preparation. Thus, pulpal irritation is prevented in restoration repair. However, achieving a durable bond between the composite resin and ceramic is challenging. ⁴ , ⁵ , ⁶ , ⁷ , ⁸ , ⁹ A combination of techniques is often employed to prepare the glass-ceramic surface aiming to enhance the bond of composite resin to ceramic by micromechanical interlocking and chemical bonds. The suggested techniques for this purpose include sandblasting, tooth preparation, surface roughening by bur, silica coating, etching with hydrofluoric (HF) acid, laser irradiation, and application of silane and adhesives. ⁵ The commonly used technique for the repair of glass-ceramics involves HF acid etching and the application of silane and adhesive. Application of HF acid following surface roughening enables micromechanical retention. Furthermore, it removes the glassy matrix and reveals the crystalline structure of ceramic. ¹⁰ , ¹¹ Application of silane as primer enhances the adhesion of the resin to ceramic. ⁸ , ⁹ Low-viscosity silane results in surface wetting and facilitates microscopic penetration of resin into the porous structure of ceramic and enables adhesion by creating covalent bonds. ⁴ Adhesives, mainly conventional adhesives, are applied following the application of silane. Recently, a new generation of adhesives, namely universal adhesives were introduced on the market, which can be used in self-etch, total-etch, and selective-etch modes. Universal adhesives can bond to the tooth structure and indirect restoration surfaces such as ceramic, resin, and metal. ⁸

Kalavacharla et al. reported that silane present in the composition of universal adhesives has no significant effect on bond strength and a separate step of silane application cannot be omitted. ¹² Laser irradiation is among the novel surface treatment techniques. Furthermore, the application of universal adhesives is a more recent technique compared to other methods and evidence is inconclusive regarding their efficacy in the process of ceramic repair. On the other hand, silane is present in the composition of some universal adhesives such as Single Bond Universal (3M), which questions the need for a separate silane application step. This study aimed to assess the effect of mechanical and chemical surface treatments on repair bond strength of composite to ceramic. Furthermore, the efficacy of these novel techniques was compared with that of conventional surface treatment (HF acid etching and application of conventional adhesive). The effect of different surface treatments was evaluated by measuring the repair bond strength of composite to ceramic after 24 h of immersion in distilled water. Moreover, this study aimed to assess the effect of silane application, along with universal adhesive. The null hypotheses were (a) the efficacy of different mechanical surface treatments (laser and HF acid etching) would not be significantly different, (b) application of silane would have no significant effect on repair bond strength, and (c) the effects of conventional and universal adhesives would not be significantly different on repair bond strength.

It is an experimental study with the following in vitro steps

Fabrication of ceramic blocks

Eighty ceramic blocks (Lucite-reinforced glass-ceramic; IPS Empress; IvoclarVivadent, Liechtenstein) were fabricated in a laboratory. In brief, wax molds were fabricated measuring 6 mm × 6 mm with 5-mm thickness and measured with a caliper. After spruing, the molds were poured with dental stone. Ceramic ingots were heated in a furnace at 870°C for 40 min. After removal and cooling, they were sandblasted with aluminum oxide particles, measured again, and glazed. All samples were mounted in acrylic resin such that one surface of the ceramic block remained exposed with 1-mm thickness. The surface of all samples was polished with 320-grit silicon carbide abrasive paper under running water with equal hand pressure for ten times. They were then rinsed with water for 10 s and dried with air spray.

Ceramic surface treatment

The samples were randomly divided into eight groups of 10 for the assessment of three independent variables Figure 1. Figure 1

Flowchart of Materials and methods

One-way ANOVA revealed that the interaction effect of study variables on SBS was statistically significant. Table 1shows the SBS in different groups.{Table 1}

Comparison of groups revealed that application of HF acid, compared to laser irradiation (irrespective of the type of adhesive and use/no use of silane), significantly increased the SBS (P < 0.001). The highest SBS was noted in “HF acid + universal adhesive” and “HF acid + silane + conventional adhesive” groups (P < 0.5). In application of HF acid and universal adhesive, the group without silane yielded higher SBS (P = 0.003). The lowest SBS was noted in “laser + universal adhesive” and “laser + conventional adhesive” groups Figure 2. Figure 2

Shear bond strength (MPa) of study groups

This study aimed to assess the effect of different ceramic surface treatments on repair bond strength of composite to ceramic. Comparison of HF acid and laser for surface treatment of ceramic revealed that application of HF acid yielded a higher bond strength between the composite and ceramic. Some researchers have reported that HF acid etching is the most effective method for ceramic surface treatment because it increases surface roughness and micromechanical retention. ¹³ , ¹⁴ In fact, HF acid creates the highest irregularities on the surface. It reinforces the siloxane bonds and creates the best microstructure for micromechanical bonding (which is the main reason for the superiority of HF acid to laser). ¹⁵ , ¹⁶ These results were in line with those of Özdemir et al. ¹⁷ The reduction in repair bond strength of ceramics treated with laser, compared to those treated with HF acid, can be attributed to the unequal and lower microscopic depth of porosities created and excessive destruction of the matrix phase and crystals or layers damaged by high temperature. ⁷ Ebrahimi Chaharom et al. ¹⁸ showed that the bond strength was not significantly different in HF and laser groups, which was different from our findings. It might be related to the type of composite they had used which were silorane-based composite.

Comparison of use and no use of silane in all groups, except for the universal adhesive group (which contains silane in its composition), revealed superior SBS in silane groups. Thus, it may be stated that the highest SBS is achieved when silane is applied. Similarly, Ozden et al., in 1994 indicated that the highest SBS was noted after using silane in combination with a mechanical preparation method. ¹⁹ Goracci et al., ²⁰ Albaladejo et al., ²¹ and Perdigão et al. ²² reported that silane application is an effective method to increase the bond strength. This can be due to the ability of silane in increasing surface wetting and creating covalent bonds. Silane enhances the adhesion in the presence of epoxy resin polymers; it increases both the chemical bonds between the inorganic substrate and polymer and the surface wettability. ²³ The surface modifiers such as silane have excellent properties for the bond of organic and inorganic materials and organic-inorganic hybrid components which contain silicon-carbon bonds. In resin-based composites, silane can change the filler surface as a coupling agent for bonding of composite to composite and provide a strong chemical bond between them. ²⁴ Kim et al., ⁸ also concluded that separate application of silane improves the repair bond strength. Kupiec et al. ²⁵ pointed to the key role of silane in the process of repair and stated that the highest bond strength was noted in air abrasion plus HF acid group. In this group, silane was applied before the use of adhesive. Sattabanasuk et al. ⁴ demonstrated that silane significantly increased the bond strength in un-etched samples, which was in agreement with our results.

In contrast to our study, Cho et al., ²⁶ and Celik et al. ²⁷ stated that the application of silane had no significant effect on bond strength. The reason can be the difference in the concentration and type of materials used in different studies. Thus, it may be concluded that in most studies, irrespective of the concentration of materials and other factors, the highest SBS is achieved when compounds containing silane are used. This finding highlights the ability of silane to increase surface wetting and create covalent bonds between the OH-groups of inorganic substrates such as glass. In general, silane enhances the adhesion in the presence of epoxy resin polymers. It enhances the chemical bond between inorganic substrates and polymer and increases the surface wettability. ²³ The chemical bond of silane depends on the presence of silica in the ceramic surface, which is rare in the composition of aluminum ceramics and complicates achieving a chemical bond to them. ²⁸

The current findings revealed that in the application of HF acid and silane, the SBS results were superior in the group where the conventional adhesive was used because universal adhesive can decrease water evaporation or surface wetting and prevent the bonding of silane to glass ceramic. ¹² Furthermore, in the application of HF acid without silane in our study, universal adhesive yielded higher SBS values. Sattabanasuk et al. evaluated the efficacy of universal and conventional adhesives for the repair process and showed that the application of universal adhesive containing silane could not yield a higher SBS, especially when compared to separate applications of silane. ⁴ Both the aforementioned studies confirmed the optimal efficacy of silane in universal adhesives. However, Yoshihara et al. showed that the silane present in the composition of universal adhesives was not much stable. ⁶

The current results showed that the efficacy of conventional and universal adhesives was not significantly different and “HF acid + universal adhesive” and “HF acid + silane + conventional adhesive” yielded the best results with regard to SBS. On the other hand, using universal adhesive simplify the repairing procedure (by omitting the silanization procedure of ceramic). Obtaining a strong bond by the use of universal adhesives is related to their inherent properties since they contain methacryloyloxy decyldihydrogen phosphate (MDP) monomer in their composition. They form chemical bonds due to the presence of MDP monomer in their composition, ²⁹ which justifies their optimal etching and bonding ability. The MDP monomer is a phosphate monomer, which enhances the bond between ceramic and composite. The MDP monomer forms highly resistant bonds to metal oxides and silane, and can also bond to SiO2-based ceramics. ² , ⁵ , ³⁰ , ³¹ In fact, MDP is a bifunctional adhesive monomer, which bonds to oxides with its hydrophilic phosphate end and bonds to resin monomers with its hydrophobic methacrylate end. ⁸ , ³²

Considering the current findings, favorable SBS between composite resin and glass ceramics requires micromechanical (HF) and chemical retention (silane). So, for repairing glass-ceramics by conventional adhesives we need HF and silane ceramic treatment but by universal adhesives, just HF treatment.

Under the limitation of this study we can conclude that the repair SBS was higher in groups treated with HF acid compared to laser. Ceramic surface treatment with HF plus silane plus conventional adhesive yielded a higher SBS as well as HF plus Universal adhesive. Thus, the application of silane as a separate step can be omitted in repair of ceramic restorations with universal adhesives.

Acknowledgments

The authors thank Tehran University of Medical Sciences, International Campus for financial support.

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 nonfinancial in this article.