Introduction

Currently, the development of ceramic biomaterials, such as zirconia for dental applications, is booming. Zirconia restorations are applied using digital systems based on computer-aided design and manufacturing (CAD/CAM).^1,2 Yttria-stabilized tetragonal zirconia polycrystal (YTZP) was initially introduced as a substructure for fully ceramic restorations. Zirconia restorations in dentistry, both for tooth- and implant-supported crowns and bridges, became popular due to their high fracture toughness compared to silica-based ceramics. Zirconia exhibits a flexural strength exceeding 900 MPa, fracture toughness ranging from 4 to 8 MPa√m, and an elastic modulus of 210 GPa.^3-5 In addition to its mechanical superiority, zirconia offers exceptional biocompatibility^,(1,6) high chemical stability,^2,7 and favorable optical properties^.(6,7

However, when zirconia restorations are made on substrates with poor mechanical retention, their clinical performance largely depends on establishing strong adhesion between the cementing agent and the zirconia.^8-10 Adhesive treatments used for other dental ceramics are not effective on zirconia surfaces because it is chemically inert and non-polar, making it resistant to acid attacks due to the absence of vitreous phases in its composition. Furthermore, the lack of silica in its composition prevents chemical bonding with silane agents.^11-13

Therefore, several methods have been proposed for bonding zirconia restorations, including resin-modified glass ionomer cements and both conventional and self-adhesive resin bonding agents. The use of resin bonding agents on zirconia requires chemical preconditioning with monomers containing specific functional groups. The application of products containing phosphate ester monomers, such as 10-methacryloyloxydecyl dihydrogen phosphate (10-MDP), has been extensively documented. When applied after mechanical pretreatment of the zirconia surface, they significantly increase adhesive strength.⁶⁾ Conditioning with 10-MDP monomers is considered a non-destructive chemical treatment that functionalizes the inert surface of YTZP, enabling chemical bonding to the surface.^14-16

In an effort to simplify adhesive protocols, manufacturers have incorporated monomers with specific functional groups, such as 10-MDP and silane agents, into multipurpose adhesive systems called Universal adhesives. These systems are designed to be faster, less technique-sensitive, and easier to use, forming adhesive bonds with different substrates-including metals, vitreous ceramics, dentin, and enamel-without requiring surface pretreatment. ^(16,17) Sandblasting with alumina particles on the zirconia surface, combined with the application of specific functional monomers, remains the protocol established in the literature and recommended by several manufacturers. ^4,18-20

However, the durability of this bond under artificially induced delay conditions remains controversial.^18-21 The establishment of an optimal bonding protocol for zirconia restorations has yet to be defined, making it an area of significant scientific interest and development. Establishing such protocols would improve the clinical performance and longevity of zirconia restorations. Given the above, this study aimed to analyze the immediate shear bond strength and the bond strength after a period of delay in zirconia restorations using adhesive systems containing the 10-MDP molecule in their composition. The null hypothesis to be tested is that no differences will be found in shear bond strength nor durability among the different chemical conditioning agents.

Methodology

Specimen preparation

A partially sintered zirconia disc (Ceramill Zolid, Amann Girrbach AG; Koblach, Austria) was cut using a microcutter (LECO VC50, LECO; St. Joseph, MI, USA) to create rectangular specimens measuring 20 mm x 20 mm x 3 mm. Five specimens were prepared for each group, resulting in a total of 20 specimens. After milling, the specimens were sintered according to the manufacturer's specifications using a Ceramill Therm 3 high-temperature furnace (Amann Girrbach AG; Koblach, Austria). The sintering process was carried out at 1450 °C for 2 hours with a heating rate of 8 °C/min. Upon completion, the samples were allowed to cool gradually to room temperature, following the manufacturer's instructions.

The fully sintered samples were embedded in PP tubes using acrylic resin, with one surface of each test specimen left exposed. The exposed zirconia surfaces were sequentially polished with 220, 400, and 600-grit silicon carbide sandpaper to standardize the surfaces. Between each sanding stage, the zirconia surfaces were rinsed with distilled water for 10 seconds. At the end of the polishing process, the specimens were cleaned in an ultrasonic bath with 99% isopropyl alcohol for 3 minutes.

After cleaning, the specimens were sandblasted with 50-micrometer aluminum oxide particles (Basic Classic, Renfert, Germany) for 15 seconds, maintaining a distance of 10 mm and a pressure of 0.25 MPa. The specimens were then ultrasonically cleaned again for 3 minutes in 99% isopropyl alcohol.

Subsequently, the specimens were randomly divided into four groups (www.randomizer.org) based on the chemical conditioning agent applied: SBU Group - Single Bond Universal (3M ESPE; St. Paul, MN, USA), TBU Group - Tetric N-Bond Universal (Ivoclar Vivadent; Schaan, Liechtenstein), PZM Group - Peak-ZM (Ultradent; South Jordan, UT, USA), and ZP Group - Z Prime (Bisco; Schaumburg, IL, USA). The composition of the chemical conditioning agents used in this study is summarized in Table 1. The chemical conditioning agents were applied to the polished and sandblasted surface of the zirconia specimens in strict accordance with the manufacturer's instructions.

Table 1. Material, manufacturer, composition, lot, and application procedure of the conditioning agents used 

**Information provided by the manufacturer.10-MDP: 10-methacryloyloxydecyl dihydrogen phosphate. Bis-GMA: Bisphenol A-glycidyl methacrylate. UDMA: Urethane dimethacrylate. HEMA: 2-hydroxyethyl methacrylate. DDMA: Decamethylene dimethacrylate. TEGDMA: Triethylene glycol dimethacrylate. BPO: Benzoyl peroxide. BTM: 2-benzotriazolyl-4-methylphenol.

After the application of the chemical conditioning agents, a cylindrical silicone matrix with four holes, each with an internal diameter of 1.4 mm, was positioned on the zirconia surface. Each hole was filled with a conventional dual-cure resin bonding agent (RelyX ARC, 3M ESPE, St. Paul, MN, USA), handled according to the manufacturer’s instructions, and light-cured for 20 seconds using a light-curing unit (Optilight MAX, Gnatus; Ribeirão Preto, Brazil) at an intensity of 1000 mW/cm², which had been previously verified with a radiometer (Bluelight Meter, Ivoclar Vivadent, Schaan, Liechtenstein). Following light-curing, the silicone matrix was removed, revealing four resin bonding agent cylinders. Figure 1 provides a graphic depiction of the methodology steps.

Micro-shear bond strength test

All samples were stored in distilled water at 37°C for 24 hours. Immediately afterward, two of the four resin cylinders from each specimen were subjected to a micro-shear bond strength test. After completing the immediate test, specimens with the remaining two cylinders were then stored in distilled water at 37°C for six months. After this delay period, the remaining two cylinders from each specimen underwent the micro-shear bond strength test.

This test was performed using a mechanical universal testing machine (CMT 2000; MTS SANS, China) at a crosshead speed of 1.0 mm/min. Bond strength (in MPa) was calculated by dividing the load (in Newtons) by the bonded interface area (mm²).

The specimens were randomly organized into groups, with each zirconia block containing two resin cylinders for immediate testing and two for testing after delay (n=10):

SBU-i Group: Single Bond Universal application; immediate shear test.

SBU-e Group: Single Bond Universal application; shear test after 6 months of delay.

TBU-i Group: Tetric Bond Universal application; immediate shear test.

TBU-e Group: Tetric Bond Universal application; shear test after 6 months of delay.

PZP-i Group: Peak ZM application; immediate shear test.

PZP-e Group: Peak ZM application; shear test after 6 months of delay.

ZP-i Group: Z Prime application; immediate shear test.

ZP-e Group: Z Prime application; shear test after 6 months of delay.

Figure 1. Methodological scheme. 

Results

Figure 2 shows the micro-shear bond strength results for all groups tested both immediately and after a 6-month delay.

Figure 2. Micro-shear bond strength values (MPa). 

Different uppercase letters indicate statistically significant differences between the chemical conditioning agents evaluated at 24 hours. Different lowercase letters indicate statistically significant differences between the chemical conditioning agents evaluated after 6 months. Columns under the same horizontal line indicate no statistically significant difference between the immediate and delayed results for each chemical conditioning agent.

According to the two-way ANOVA, the micro-shear bond strength was not significantly affected by the chemical conditioning factor (p = 0.058) nor the delay factor (p = 0.216). However, the interaction between both factors was significant (p = 0.005).

When analyzing the chemical conditioning factor at 24 hours, the highest value was observed for the Z Prime group; however, there were no statistically significant differences compared to the other adhesive strategies (p ≥ 0.426). In turn, when analyzing the Group factor at 6 months of delay, the Z Prime group showed the lowest adhesion values, which were significantly lower than the rest of the groups (p ≤ 0.016). The other groups showed no significant differences among them (p ≥ 0.993).

Upon analyzing the time factor in each of the adhesive strategies used, a significant decrease was observed in the Z Prime group (p < 0.001), while the other groups did not show significant differences when comparing immediate results with those after 6 months of delay (p ≥ 0.216)

Discussion

This study aimed to evaluate the immediate and delayed micro-shear bond strength of a conventional dual-cure resin bonding agent to zirconium oxide surface using different chemical bonding promoters. The results did not show a statistically significant difference in the immediate and delayed adhesion across the different groups, with the exception of Z Prime, where a decrease in adhesive strength was observed between the immediate and delayed values. Therefore, the null hypothesis was partially accepted.

Upon evaluating the immediate bond strength values, these ranged from 17.9 MPa to 19 MPa, showing no statistically significant differences between the groups (p ≥ 0.426). These values are consistent with those reported in the literature ^17,22. Furthermore, the bond strength values after the delayed period ranged from 16.2 MPa to 19.1 MPa, with no significant differences observed among the SBU, TBU, and PZM groups (p ≥ 0.993). These findings can be attributed to the similarity in the composition of the chemical bonding promoters, which are based on the phosphate monomer 10-Methacryloyloxydecyl dihydrogen phosphate (10-MDP) ^18,23-25. This molecule has the ability to form bonds between the divalent phosphoryl groups of the MDP monomer and hydroxyl groups on the zirconia surface ^22,26. The literature reports various clinical protocols for treating the zirconia surface, with no clear superiority when comparing different 10-MDP-based conditioners ^23,27. However, several authors conclude that mechanical treatment, combined with priming using a universal adhesive or a zirconia-specific primer, results in adequate bond strength values in in vitro studies ^10,18.

It has been reported that the presence of the 10-MDP monomer is essential for providing reliable bonding, while its concentration is less important. ²⁷ However, the results of this study showed a significant decrease in adhesion in the group where a zirconia-specific primer was used (p≤0.016). It has been demonstrated that delaying the test specimens leads to a reduction in adhesion values, ^24,28 which may be attributed to the weakening of the bond due to its hydrolysis when stored in water for 6 months. ¹⁷

In this study, a decrease in adhesive values was observed in the ZP group when subjected to a delay through water storage. Additionally, previous studies have reported inferior performance of zirconia-specific primers compared to other bonding promoters, such as universal adhesives ^29,30. This could be attributed to the presence of fillers and monomers more resistant to hydrolytic degradation, such as Bis-GMA ⁽²⁵. Other potential contributing factors include differences in the concentrations and purity of the functional monomers used in each product ⁽²⁹. Furthermore, ZP contains Bis-GMA without an initiation system, which, when not polymerized, could accelerate the hydrolysis of the adhesive layer, thereby reducing bond strength values ³¹.

The literature establishes that values between 15 and 25 MPa in laboratory studies are considered adequate for clinical use, ³²⁾ while values below 13 MPa may be deemed critical. ¹⁹⁾ Therefore, all the materials analyzed in this study meet these standards. However, comparing and extrapolating in vitro data to clinical performance remains challenging, as additional factors must be considered, such as cavity shape, restoration thickness, and finishing, among others, ³³ as well as clinical parameters like secondary caries, material fracture, or gingival health. ³⁴⁾ The authors believe that potential clinical failures may result from the improper application of clinical protocols by professionals. These include overlooking contamination of the zirconia surface with saliva after try-in, ^35,36 incorrect application of bonding agents (such as insufficient rubbing, evaporation, or light curing), ³⁷ or inadequate sandblasting of the restoration's internal surface. ⁽²⁷

The results of this study should be interpreted with caution due to its in vitro nature, as a clinical scenario cannot be fully replicated. The literature also highlights significant variability in in vitro methodologies, making data comparison challenging. ²³⁾ Furthermore, a single bonding agent was used, and only four bonding promoters were analyzed.

Conclusions

Despite the limitations of this study, the results suggest that universal adhesives containing 10-MDP exhibit immediate bond strength comparable to zirconia-specific chemical surface conditioners. After a period of delay, universal adhesives appear to offer greater bond strength stability.