Comparative evaluation of effect of One Coat 7 Universal and Tetric N-Bond Universal adhesives on shear bond strength at resin-zirconia interface: An in vitro study.

Background: Success of all-ceramic restorations depends on their cementation process. Bonding crystalline materials like zirconia to an underlying substrate is challenging. Recently, universal adhesives have been developed. They claim to chemically bond to zirconia due to the presence of compounds like 10-methacryloyloxydecyl dihydrogen phosphate. But for majority of these adhesives, few studies demonstrating their effectiveness are available. Aims: This study aimed to assess the effect of two universal adhesives, One Coat 7 Universal and Tetric N-Bond Universal on shear bond strength at zirconia-resin junction. Materials and
Methods: Thirty CAD-CAM milled zirconia cylinders (5 mm × 8 mm) were sandblasted and divided into three groups: Group A (control) received no treatment, while Groups B and C were primed by One Coat 7 Universal and Tetric N-Bond Universal, respectively. They were luted to composite surfaces by a dual-curing resin cement. After 24 h, a shear bond strength test was conducted and failure mode was analyzed. Statistical Analysis: One-way analysis of variance and post hoc Tukey's test were used for statistical analysis.
Results: Both One Coat 7 Universal (24.43 ± 2.66 MPa) and Tetric N-Bond Universal (28.61 ± 3.11 MPa) had significantly higher strength than the control group (9.86 ± 2.00 MPa). Control group underwent 100% adhesive failures. Experimental groups had predominantly mixed/cohesive failures.
Conclusion: Both universal adhesives increased the bond strength at resin-zirconia junction. Tetric N-Bond Universal exhibited the highest strength. Copyright:

INTRODUCTION

In recent years, the demand for esthetic restorations has led to an unprecedented rise in the use of all-ceramic materials. Success of these restorations largely depends on their cementation process.[1] Resin cements are recommended for adhesive ceramic luting.[2] Adhesive resin cementation provides good retentiveness, improves marginal adaptation, prevents microleakage, and increases fracture resistance of the restored tooth and the restoration.[3]

Two major types of all-ceramic materials currently exist, silica-based or glass ceramics and oxide-based or polycrystalline ceramics. Silica-based ceramics have a definite luting protocol; they are first etched by HF and then silanized to create a chemical bond.[3] For polycrystalline ceramics, however, these processes have little effect. Yttrium-stabilized zirconia (Y-TZP) is one such commonly used polycrystalline ceramic. It has high strength, good biocompatibility, and is metastable when alloyed with yttrium oxide. It is routinely used for inlays, onlays, full-coverage crowns, FPDs, etc., However to establish a durable bond with the luting resin, it is necessary to alter the zirconia surface morphology.

Various methods have been suggested to facilitate resin–zirconia bonding. These methods aim to either create micromechanical irregularities for adhesive interlocking, or deposit a layer (silica, plasma, etc.) on the zirconia surface which can then bond with the resin cement.[4]

Literature is ambivalent in terms of which method is best suitable for modifying zirconia and many techniques have been suggested. Air abrasion with alumina particles (sandblasting) is most commonly used for roughening of zirconia. However, it is not sufficient to create a durable bond alone.[5] Tribochemical coating, which involves silica deposition on the zirconia surface, has also been attempted. However, few studies have reported that this technique only provides an air abrasion-like effect.[6]

An innovative method, selective infiltration etching, has also been suggested wherein zirconia is heated in an electric furnace and then infiltrated with molten glass.[7] It alters zirconia ultrastructurally with minimal damage to the surface. Although good results were obtained in vitro, technical feasibility has limited its usage. Several other methods, for example, plasma layer deposition, laser etching, and low fusing micropearls application, etc., have also been proposed.[4] However, their efficacy is debatable and is used sparsely.

None of the aforementioned methods, however, form a true chemical bond with zirconia.

In 2012, Chen et al.[8] demonstrated that the compound 10-methacryloyloxydecyl dihydrogen phosphate (10-MDP), a phosphate ester, can directly bond with the irconia surface. Yoshida et al.[9] had earlier demonstrated that this compound can be used to improve the bonding properties of zirconia. 10-MDP had previously been used to bond base metal alloys[9] and as a part of self-etch adhesives to bond tooth structure with composite resin.[10] Manufacturers utilized this multi-substrate capability and introduced “universal adhesives;” a single formulation having multiple synergistic and cross-linking monomers including phosphate esters like 10-MDP which claim to bond to all substrates.[10]

Various universal adhesives are currently available. Their bonding abilities are mainly determined by the interactions between their constituents. Since all formulations vary in terms of composition, they have different bonding abilities. Hence, it is imperative to study all the adhesives before clinical recommendation. While many studies have successfully demonstrated the efficacy of adhesives such as “Scotchbond Universal” and “All Bond Universal” in terms of resin–zirconia bonding, limited information is available regarding most other adhesives.

Therefore, the present study was undertaken to determine the bonding effectiveness of two well marketed, but relatively less studied 10-MDP-based universal adhesives: “Tetric N-Bond Universal” and “One Coat 7 Universal.”

MATERIALS AND METHOD

Specimen preparation

Thirty cylinders were milled from Arum 4 × 300D (Arum Dentistry, Republic of Korea) machine from 3Y-TZP blanks (Cercon base, Degu Dent, Hanau, Germany). They were then sintered and grinded to achieve a final dimension of 5 mm (diameter) ×8 mm (height). The cylinders were then subjected to air abrasion (Vario Jet, Renfert, Germany) with 50 μm Al2O3 particles at 0.2 MPa for 20 s, rinsed for 5 min with distilled water in an ultrasonic cleaner, and dried.

Adhesive application

The cylinders were divided into three equal groups.

In Group A, no adhesive was applied on the zirconia cylinders and it served as control

In Group B, the cylinders were treated with One Coat 7 Universal (Coltene Whaledent, Switzerland)

In Group C, the cylinders were treated with Tetric N-Bond Universal (Ivoclar Vivadent, Lichenstein).

The adhesives were applied on the intaglio surfaces of the cylinders in a single coat with a microbrush using an active scrubbing motion for 20 s. They were then air-dried for 5 s and light cured for 10 s using an LED light-curing unit (Bluephase N, Ivoclar Vivadent).

Bonding procedures

Acrylic resin blocks measuring 2 cm (width) ×3 cm (length) ×1 cm (height) were fabricated. Cylindrical cavities measuring 5 mm (diameter) ×3 mm (height) were prepared within these blocks.

The cavities were filled with nanofilled composite resin (Brilliant NG, Coltene Whaledent) in increments. Each layer was light cured for 20 s at an intensity of 1200 mW/cm2.

The zirconia cylinders were then cemented to the composite surface with a dual-curing resin cement (Paracore, Coltene Whaledent) by applying light pressure for few seconds. Extra cement was removed and light curing was done.

Shear bond strength testing

The final assemblies were stored in distilled water at 37°C for 24 h. They were then mounted in a universal testing machine (Instron 5000, Instron, USA). The knife-edged blade of the machine was kept as close as possible to the resin–ceramic junction as possible. A shear bond strength test was then conducted at a crosshead speed of 1 mm/min.

Statistical analysis

The values thus obtained were subjected to one-way analysis of variance (ANOVA) and post hoc Tukey tests. Statistical analysis was done using EpiInfo™ for Windows, Version 7.2.2.2 (Centres for Disease Control and Prevention, Atlanta, Georgia, USA). P < 0.05 was taken to be statistically significant.

Failure mode assessment

The failure modes were evaluated under a stereomicroscope (1903 P, Euromex, Holland) at magnifications of ×17.5 and ×45.

The failures were classified as [Figure 1]:

Figure 1

Failure modes in different magnifications (×17.5 and × 45): (a) Adhesive: White opaque roughened zirconia surface. (b) Cohesive: Smooth yellowish resin surface. (c) Mixed: Both white opaque roughened zirconia surface and smooth yellowish resin surface

Adhesive failures:

Failures occurred at the resin cement–zirconia junction. Zirconia cylinders completely detached from the underlying surface. Only the white opaque surface of zirconia was seen which had a roughened appearance due to the effects of sandblasting.

Cohesive failures

Failures occurred within composite resin or resin cement. The fractured ends had a smooth yellowish appearance that was in contrast with the surface observed in adhesive failures.

Mixed failures:

It was a combination of adhesive and cohesive failure. Partially roughened zirconia and partially yellowish smooth resin were visible.

RESULTS

The present study evaluated two parameters, shear bond strength and failure mode.

Shear bond strength

Individual values obtained are depicted in Table 1. Mean shear bond strengths of Groups A, B, and C were 9.86 ± 2.00 MPa, 24.43 ± 2.66 MPa, and 28.61 ± 3.11, respectively.

Table 1

Shear bond strength and failure mode frequency

Specimen number	Group A control (n=10)	Group B One Coat 7 Universal (n=10)	Group C Tetric N Bond Universal (n=10)
Shear bond strength (MPa)
1	8.8	26.95	30.95
2	10.38	22.40	24.17
3	14.97	21.87	23.87
4	10.87	25.95	28.95
5	9.74	25.82	24.82
6	8.8	23.45	31.45
7	8.62	28.95	29.95
8	9.82	20.46	30.46
9	8.23	22.65	31.82
10	8.37	25.81	29.68
Mean±SD	9.86±2.00	24.43±2.66	28.61±3.11
Comparison of P values
Group A: Group B	<0.0001
Group B: Group C	<0.005
Group C: Group A	<0.0001
Failure mode frequency
Failure type	Group A	Group B	Group C
Adhesive	10	3	2
Cohesive	0	4	3
Mixed	0	3	5

SD: Standard deviation

One-way ANOVA followed by post hoc Tukey's test was performed and P < 0.05 was taken to be statistically significant. All the groups had significant differences among them.

Failure modes

Types and frequency of failure modes are depicted in Figure 1 and Table 1, respectively.

Adhesive failures were seen in 100% samples of Group A, 30% samples of Group B, and 20% of samples Group C.

In Group B, 40% and 30% of samples showed mixed and cohesive types of failures, respectively, while in Group C, the frequency of mixed and cohesive failure observed was 30% and 50%, respectively.

DISCUSSION

The present study investigated the effectiveness of two 10-MDP based universal adhesives at resin–zirconia junction. Group A exhibited the lowest while Group C exhibited the highest mean shear bond strength.

Although the exact bond strength needed for a durable resin–zirconia bond has not yet been established, a value of 20 MPa is widely regarded as sufficient.[11] While a roughened zirconia surface is more retentive than a polished one, merely sandblasting is not optimal, as evidenced by the low shear bond strength of the control group.[5]

The higher bond strength values of the experimental groups imply that the universal adhesives had a positive effect on the bonding properties of zirconia. This finding is in accordance with previous studies performed with these adhesives.[121314] Al Jeaidi et al.[12] also reported high bond strength (20.21 ± 1.53 MPa) when using Tetric N-Bond Universal with sandblasted zirconia specimens. One Coat 7 Universal was also studied by Siqueira et al.[13] and Lümkemann et al.[14] The bond strength obtained by them, (36.2 ± 0.9 MPa and 41.4 MPa respectively) were higher than the present study. This difference may be attributed to the use of differences in the resin cement used, sample preparation, and testing method employed. Lümkemann et al.[14] used a 10-MDP-containing cement in their study, which could have further increased the bond strength.

10-MDP is an amphiphilic molecule with a hydrophobic vinyl group at one end and a hydrophilic phosphate group at the other. This phosphate group can directly bond with the zirconia surface and form a bond between oxygen, phosphorus, and zirconia (P-O-Zr),[815] whereas the hydrophobic end polymerizes with the matrix of methacrylate-based resin materials.[10]

To facilitate resin–zirconia bonding, 10-MDP is included as a component either in adhesives (universal adhesive/zirconia primer) or in resin cements. A non-10-MDP-based cement was chosen in the present study to highlight the bonding potential of the adhesives when used with zirconia. The adhesives tend to be less viscous than resin cements and can therefore better penetrate into the irregularities created by an air-abrasion process.[12] The chairside application of universal adhesives may also facilitate intraoral repair of zirconia restorations.[5]

Apart from 10-MDP, One coat 7 Universal contains various other methacrylates, photoinitiators, ethanol as solvent, and water, which is required for monomer disassociation that makes self-etching possible.

Tetric N-Bond Universal also has a similar composition but additionally contains functional monomers such as HEMA (2-hydroxyethyl methacrylate; hydrophilic monomer, promotes wetting of inorganic substrates like dentin), MCAP (methacrylated carboxylic acid polymer; hydrophilic monomer, bonds with hydroxyapatite), and D3MA (decanediol dimethacrylate; hydrophobic cross-linking monomer, bonds to less polar molecules of composite resin).[16]

Group C, which was treated with Tetric N-Bond Universal, had the highest bond strength. This higher bond strength can be attributed to the presence of the hydrophobic monomer, D3MA.

The shear bond strengths obtained were consistent with the mode of failures observed. All samples in the control group underwent adhesive failures. In contrast, both experimental groups had predominantly cohesive/mixed failures. Reduction in adhesive failure mode implied that these adhesives reinforced the resin–zirconia interface, which is believed to be the weakest phase of an indirect restoration.[17]

In the present study, a combination of a mechanical (sandblasting) and chemical method (adhesive application) was used in the experimental groups and yielded satisfactory results for both adhesives (>24 MPa). This was done as per the protocol given by Blatz et al.,[18] i.e., the air abrasion–priming–composite resin concept. It is effective and inexpensive when compared to other methods. Controversy exists regarding optimum alumina particle size for sandblasting zirconia. The protocol used in the present study is based on the recommendations of Kern.[19]

Y-TZP prostheses are bonded to either the tooth structure or to foundation restoration materials like composite resin. Thus, the composite resin was used in this study because it simulates clinical conditions and the weak link of the bonding will be at the interface of interest, i.e., the Y-TZP/resin interface.[20] A large difference exists in the elastic modulus of the two substrates used in the present study – resin and zirconia, which may affect the results of the shear bond strength test. Hence, nanofilled resin was used to decrease this mismatch.[21]

In the present study, bonding efficiency was evaluated by the shear bond strength test as shear stress is commonly experienced by restorations intraorally. Sample preparation is also easier for this test as compared to other tests.[22]

Limitations of the study

Due to the nature of shear tests, nonuniform stress distribution is often observed at bonded interfaces.[23] Thus, obtained values may not reflect actual bond strength values

In vivo conditions were not simulated in the present in vitro study.

CONCLUSION

Within the limitations of the study, it can be stated that 10-MDP-based universal adhesives – One coat 7 Universal and Tetric N-Bond Universal – increased the 24 h shear bond strength at the resin–zirconia junction when used in conjunction with sandblasting. The latter adhesive exhibited higher strength than the former, possibly due to the presence of D3MA.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.