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Zirconia-based ceramic has been increasingly used in dentistry due to its good biocompatibility, superior mechanical properties, and adequate esthetics (1). Despite having the excellent properties, its glass-free, polycrystalline structure makes the traditional hydrofluoric acid etching followed by silane application ineffective in surface conditioning. Especially when luting a non-retentive zirconia restoration, optimizing adhesive cementation is critical for a successful outcome. Among the several alternative surface treatments suggested for improved resin bonding to zirconia (2-4), the application of functional monomers with a chemical affinity to metal oxides, such as carboxylic and phosphoric acid esters was proposed as a simple and noninvasive procedure (5).

To create a chemical coupling between zirconia and resin cement, an organophosphate monomer, 10-meth acryloyloxydecyl dihydrogen phosphate (MDP) has been contained in various luting resins and primers. Although some study supports the use of MDP-containing primers as a substitute for airborne-particle abrasion (6), a combination of micromechanical interlocking and chemical bonding seems to be effective in achieving reliable and durable adhesion between zirconia and resin composite (7, 8)

MDP is also incorporated in multipurpose universal adhesives, which have been introduced for either direct or indirect use based on a simplified bonding strategy. Several recent studies have reported promising results with the use of universal adhesives on zirconia bonding (9-11). Notwithstanding the proven efficacy of universal adhesives as a priming agent for a zirconia restoration, instructions for clinical procedures are unclear without being specific about light-irradiation as well as application and drying methods. This ambiguity might contribute to the varying results from the different application protocols (12, 13).

For a stable structure with polymerized monomers in an adhesive applied to the substrate, proper light-curing of the adhesive is required prior to composite placement. An unpolymerized adhesive allows a flowable layer and pressure from the overlying composite may be enough to extrude the adhesive from the bonding interface, leaving little material needed for promoting adhesion (14). Moreover, compositional changes in the bonding interface caused by the uncured adhesive mixed with the resin cement may adversely affect the polymerization of the luting resin (15).

The purpose of this in vitro study was thus to evaluate the shear bond strengths of a resin cement to the air-abraded zirconia surface treated with the different surface conditioning protocols. The null hypothesis tested was that light-activation of priming agents prior to resin cement application would not affect early bond strengths and bonding durability between a resin cement and zirconia ceramic.

Two commercial universal adhesives (All-Bond Universal, ABU, Bisco Inc., Schaumburg, IL, USA; Single Bond Universal Adhesive, SBU, 3M ESPE, Seefeld, Germany) were selected in comparison with a conventional zirconia primer Z-Prime Plus (ZPP, Bisco Inc.) for this study. Their manufacturers, compositions, and application protocols are summarized in Table 1

1. Preparation of zirconia ceramic specimens

The study design for shear bond strength testing is illustrated in Figure 1. A total of 36 zirconia blocks with dimensions of 10 x 15 x 2 mm were cut from pre-sintered blanks (Ceramill Zolid, Amann Girrbach AG, Koblach, Austria), wet-polished with 600-to 800-grit silicon carbide (SiC) papers, and sintered in an oven (Ceramill Therm, Amann Girrbach AG) at 1450℃ for 2 hours according to the manufacturer’s instructions. The sintered blocks were embedded in round silicone rubber molds using poly(methyl methacrylate) resin, ensuring that one surface of the zirconia block remained uncovered for bonding. The exposed surface of all specimens was polished up to 1200-grit with wet SiC paper and cleaned in ultrasonic bath with 90% isopropyl alcohol for 3 minutes. Air abrasion was performed in a chairside air-abrasion device (MicroEtcher II, Danville Materials Inc., San Ramon, CA, USA) with 50 μm Al₂O₃ at 0.25 MPa for 15 seconds at a distance of 10 mm (8). The specimens were ultrasonically cleaned again in 90% isopropyl alcohol for 3 minutes, rinsed with deionized water, and finally air-dried.

Figure 1. 
A study design for shear bond strength testing.

Table 2 summarizes the shear bond strengths of resin cement to the zirconia surface treated with the different priming protocols prior to cementation. Two-way ANOVA showed statistically significant differences for the priming agent and thermocycling, and for the interaction between these factors (all p < 0.001).

Before thermocycling, the light-cured specimens of ABU and SBU showed higher bond strengths than the uncured ones of the materials (p < 0.05), whereas no significant difference was found between the pre-cured and uncured specimens of ZPP (p > 0.05). Group SBU-LC exhibited the highest bond strength when the comparisons were made within the pre-cured groups, but the differences not being statistically significant (p>0.05).

After aging by thermocycling, there were significant decreases in the bond strength for all specimens treated with the universal adhesives ABU and SBU (p<0.05). ZPP, however, showed no significant difference after 5,000 thermal cycles (p>0.05) and attained higher bond strength values for both the pre-cured and uncured specimens when compared to the two universal adhesives (p<0.05). Regarding the effect of light-curing, no significant difference in the bond strength was found between the pre-cured and uncured specimens of ZPP and ABU (p>0.05), while after aging the bond strength values of the uncured specimens were still inferior to those of the light-cured ones for SBU (p<0.05).

Fisher’s exact test indicated that there were no significant differences in failure modes among the groups regardless of light-curing and thermocycling (p > 0.05). Figure 2 shows the failure pattern distribution (%) as assigned using the light microscope. Although all modes of failures were observed in all test groups except ABU-NC, in which no cohesive failures were found, adhesive failures were predominant. Prevalence of adhesive failures appeared to increase with aging for the uncured groups, which was not shown in the pre-cured groups. Furthermore, after thermocycling, the uncured groups demonstrated a higher tendency of adhesive failures compared to the pre-cured groups.

Figure 2. 
Distribution Of Failure Pattern (%) In The Test Groups.

This in vitro study examined the effect of pre-curing the universal adhesives on the shear bond strengths of a luting resin to zirconia ceramic and compared it with that of the conventional zirconia primer. Since an adhesive functional monomer is not necessary in a resin cement if it is present in a primer for bonding to zirconia (18) and the present work focused on the bonding performance of MDP-containing priming agents, a composite-based resin cement (acidic monomers-free) Duo-Link was selected as the luting resin. The pre-cured two universal adhesives ABU and SBU yielded the superior initial bond strengths and the pre-cured SBU maintained the higher bond strength than the uncured one even after artificial aging. However, there were significant decreases in the bond strength after thermocycling, irrespective of light-curing or uncuring the two adhesives. Hence, the null hypothesis, which assumed pre-curing priming agents would not affect early bond strengths and bonding durability between a luting cement and zirconia ceramic, has to be partially rejected.

According to the respective manufacturer's instructions of the three priming agents, ABU should be light-cured for 10 seconds, SBU agitated for 20 seconds, and ZPP air-dried for 3-5 seconds before resin cement placement. Because of the lack of clarity in the clinical protocols, all the details about the application were standardized, thus enabling the light-curing to only influence the results. Considering that pre-curing improved 24-h bond strengths of ABU and SBU in this study, light-curing prior to cementation would be beneficial to zirconia bonding when using the universal adhesives as a priming agent. The inferior bonding performance of the uncured adhesives can be attributed to the collapse of the uncured layer and the resultant defective bonding interface caused by pressure during resin cement placement as described by Seabra et al. (14). It is commonly recommended that an adhesive applied on the internal surface of a restoration be kept unpolymerized before the restoration seated. As the thickness of a polymerized adhesive varies depending on both the adhesives type and the surface geometry of a restoration (19), a pooled adhesive may cause problems with restoration fit. However, from the clinical standpoint, co-curing of a luting resin and an underlying adhesive through opaque zirconia ceramic with limited light transmission leads to insufficient hardening of the adhesive joint, consequently contributing to early debonding failure of adhesive zirconia restorations (15, 20). Even though pre-curing failed to provide a durable bond between the two universal adhesives and zirconia, the tendency of adhesive failures to increase with thermocycling in the uncured adhesives may imply the formation of an unfavorable clinical bond without pre-curing.

Unlike the universal adhesives, light-curing the zirconia primer ZPP does not seem to significantly influence the early adhesion and long-term stability of resin-zirconia interface in the present study, which is in contrast to a previous study that demonstrated increased shear bond strengths with light-polymerized, two coats of ZPP (14). Although an uncured adhesive layer is seemingly unable to form a fully polymerized stable interphase due to its thinning-away between a luting resin and a restoration as well as light attenuation underneath zirconia ceramic as aforementioned (20, 21), this may be less apparent in ZPP, when considering the comparable bond strength results obtained in the cured and uncured ZPP. This result can be partly explained by a lower contact angle and better wettability of ZPP on zirconia surface than those of a universal adhesive (6).

As regard the long-term stability of the bond, the use of ZPP for conditioning zirconia can be advantageous rather than applying the universal adhesives, reflecting the differences in the chemical composition of the materials. Aside from the common ingredients such as MDP, HEMA, Bis-GMA, and ethanol in the three priming agents tested, water is incorporated in the universal adhesives for the ionization of acidic functional monomers. The addition of water renders the materials more susceptible to hydrolytic degradation especially under acidic aqueous conditions. Water mixed with acidic monomers becomes acidified and can hydrolyze the ester bonds of the unreacted methacrylate monomers and polymer networks, thereby compromise the integrity of resin-zirconia bond over time (22). The pH values for the two universal adhesives range from 2.4 ± 0.05 to 3.0 ± 0.05 (23). Meanwhile, ZPP has a carboxylate functional monomer BPDM in addition to MDP, which was reported to cooperate in the development of a chemical bond with metal oxide in the substrate (24). The synergistic coupling of these two monomers might result in the stable bonding in the current study. The increased hydrophobicity created by incorporating Bis-GMA in ZPP was claimed to offer a high priming efficacy to allow uniform spreading and improved adaptation of a resin cement (25). However, the presence of the extra resin does not appear to contribute to the bonding durability obtained with ZPP, because the other two universal adhesives are basically Bis-GMA-based.

SBU contains a silane primer to expand its use to silica-based ceramics. Although not significant, SBU exhibited the highest bond strength (23.8 ± 3.5 MPa) after 24-h storage, which was significantly decreased after additional thermocycling. The superiority in the initial bond strength is thought to be due to that the silane promoted wettability of the zirconia surface and thus enhanced spreading of the resin cement into the primed surface (26). However, while the increased wettability of the bonding surface could be a positive factor for the initial adhesion to zirconia, hydrophilic constituents from the silane film formed on the zirconia surface would be eventually detrimental to a long-lasting bonding with resin (27). Therefore, one of the critical steps during the application of any water-based adhesive, including universal adhesives, is evaporating the undesirable ingredients such as water, solvent and other reaction products from the bonding surface (28, 29). Meanwhile, Vitrebond copolymer in SBU was found to play a role in the improved initial bond strength and bonding durability of a resin cement to zirconia ceramic owing to its moisture-stabilizing effect (30-32). However, according to the reduction in the bond strength after 5,000 thermal cycles in this study, the inclusion of Vitrebond copolymer is not likely to result in any additional improvement in bond stability.

The degree of hydrophobicity and hydrophilicity of a primer-treated zirconia surface can be determined using contact angle measurements (33). Since the bond durability and interfacial characteristics of universal adhesives rely on the photo-irradiation conditions such as light intensity and irradiation time (34) and the bonding performance is related to the surface hydrophobicity of primer-coated zirconia (35), the surface characteristics of zirconia before and after light-curing priming agents obviously needs further investigation.

Adhesion is susceptible to mechanical, chemical, and thermal degradation in the oral environment (36). Even though bond strengths attained by the universal adhesives significantly decreased after thermocycling, they were still a bit higher than the suggested minimum value (10-13 MPa) for clinically acceptable bonding (37). However, considering the service life of zirconia restorations and the harsh conditions of the oral cavity (38), 5,000 thermal cycles used in the present study does not seem sufficient to simulate clinical conditions. Although a standardized, reliable thermal cycling protocol has not been developed yet, long-term water aging is required to evaluate the stability of the resin-zirconia interface formed by these new materials.

Within the limitations of this in vitro study, it can be concluded that when universal adhesives are used for surface treatment of zirconia ceramic, they should be light-cured prior to application of a resin cement, which may lead to significant improvement in early bond strengths. A conventional zirconia primer, Z-Prime Plus seems to provide a durable bond between a resin cement and zirconia. When adhesively cementing zirconia restorations, universal adhesives can be advantageously used for the both substrates tooth and zirconia without a need for a separate primer. However, despite the benefits of the simplified clinical technique, they may not show good performance in terms of bond stability compared to a conventional zirconia primer.