Micro-tensile bond strength of different adhesive systems on sound dentin and resin-based composite: An in-vitro study.

AIM: To analyze the difference in the micro-tensile bond strength of specimens made with two different adhesive systems and compare them with two homogenous substrates.
MATERIALS AND METHODS: Sixty permanent mandibular molars were mounted in acrylic blocks and sectioned with exposed dentin surfaces. Samples were then divided into four groups. To Group-I Adper Single Bond 2 and to Group-II Adper Self-Etch plus bonding agents were applied. For Group-I and Group-II beams consisted of resin composite in the upper half and dentin in the lower half. In Group-III beams were made of only dentin. In Group-IV beams were made of only composite. Fifteen specimens of each group were taken for the micro-tensile bond strength test. STATISTICAL ANALYSIS: The results are analyzed using one-way analysis of variance and Critical Difference test.
RESULTS: The interface bonded with the two adhesive systems had lower micro-tensile bond strength than those of dentin and resin composite and the self-etching adhesive Adper Self-Etch plus had comparable bond strength with total-etch adhesive Adper Single Bond 2.
CONCLUSION: The bond strength values for current adhesive systems cannot be compared to the micro-tensile bond strength of dentin and resin composite, and self-etching adhesives have comparable bond strength with total-etch adhesives.

INTRODUCTION

Bonding of resin-to-dentin is far more difficult and less predictable than bonding to enamel.[1] Dental adhesive systems have evolved through several “generations” with changes in chemistries, mechanisms, application techniques and clinical effectiveness.[2] The bonding mechanism to dentin is effective and predictable when the smear layer is dissolved; collagen fibers are exposed and formation of a hybrid layer after infiltration of resin monomers.[3] The adhesive system interacts with the dentin tissue following two different strategies.[2] They can either remove the smear layer, that is, etch and rinse technique or maintain it as the substrate for bonding, that is, self-etch technique.[4] Two step self-etch adhesive showed better bonding ability than one-step self-etch adhesives.[5] The objective of the present in-vitro study was to analyze the difference in the micro-tensile bond strength of specimens made with two different adhesive systems (total-etch and two-step self-etch) and compare them with two homogenous substrates.

MATERIALS AND METHODS

Sixty intact human permanent molar teeth were collected, cleansed thoroughly, and samples were then stored in distilled water for use in this study.

The teeth were then partially embedded in a glass box using chemically activated acrylic resin; leaving their crown exposed. This was done to facilitate specimen fixation and sectioning. The occlusal enamels were removed perpendicular to the long axis of each tooth, using a slow speed diamond disk with a water coolant, so that the superficial dentin was exposed. Then the surrounding enamel was removed using diamond bur. The exposed middle dentin surfaces were wet polished with a 600 grit silicon carbide paper to create a standardized smear layer before adhesive system application. Adhesive systems used in this study were - Adper Single Bond 2, Adper Self-Etch plus. The composite resin used in this study was filtek Z-350. Samples were then divided into four groups of 15 each.

Group-I (Adper Single Bond 2)

Dentin surfaces were acid etched by 35% phosphoric acid for 15 s. Then it was rinsed with water for 10 s. An excess amount of water was removed using a paper point. Immediately after blotting 2–3 consecutive coats of adhesives were applied for 15 s, with gentle agitation using fully saturated applicator. Then it was gently air dried for 5 s to evaporate the solvent. It was light cured for 10 s, using a light emitting diode light curing unit. After bonding procedure, six increments of approximately 1 mm thickness of Z-350 composite resin were placed on the bonded surface of the dentin. Each increment was light cured for 40 s. At the end of this procedure, resin composite block of approximately 6 mm high was obtained.

Group-II (Adper Self-Etch plus)

At first one drop of liquid “A” and one drop of liquid “B” dispensed into the mix well. Then the applicator brush was wetted with liquid “A” and applied to the dentin so that a continuous red-colored layer was obtained on the surface. Then, the liquid “B” was scrubbed into the entire wetted surface of the bonding area. The red color disappeared quickly, indicating that the etching components have been activated. For proper etching, the dentin surface was scrubbed continuously for 20 s with moderate finger pressure. After that, it was air dried for 10 s to evaporate water. Then, a second coat of liquid “B” was applied to the entire bonding surface. It was lightly air-dried to adjust film thickness and consistency and light cured for 10 s. After bonding procedure, six increments of approximately 1 mm thickness of Z-350 composite resin were placed on the bonded surface of the dentin. Each increment was light cured for 40 s. At the end of this procedure, resin composite block of approximately 6 mm high was obtained.

Group-III

To evaluate the cohesive strength of dentin, sound extracted tooth was sectioned to obtain square shaped blocks.

Group-IV

To evaluate the cohesive strength of Z-350 composite fresh five square shaped blocks were built with the aid of a plastic matrix in 10 increments of 1 mm. Each increment was light cured for 40 s.

All the specimens were stored in distilled water at 37°C for 24 h. The teeth were then individually fixed to a sectioning block mounted on the acrylic resin. The block was then mounted on the precision saw (Isomet 2000). Each tooth was serially sectioned in the occlusogingival direction producing 3 mm thick slabs. The block was then rotated 90° and serial sectioning was repeated. The resulting specimens were sectioned free from the root. The specimens were termed as beams.

For Group-I and II beams consisted of resin composite in the upper half and dentin in the lower half. In Group-III beams were made of only dentin. In Group-IV beams were made of only composite. Fifteen specimens of each group were taken for micro-tensile bond strength test having cross-sectional area 3 mm × 3 mm.

Micro-tensile bond strength test

The beams were then attached to a custom made Jig Prepared by Electrical Discharge Machine. The beams were attached to the jig by cyanoacrylate glue. The jig with specimen was fixed in a micro-tester. Micro-tensile force was applied parallel to long axis of each specimen at a crosshead speed of 1 mm/min until the beam fractured. The tensile load at which the fracture occurred was recorded. The micro-tensile bond strength value was recorded in units of megapascal (Mpa).

The results are analyzed using one-way analysis of variance (ANOVA) and Critical Difference test to determine possible statistical variation among the groups tested in the study.

RESULTS

Among the adhesive systems tested the mean micro-tensile bond strength values [Table 1] of Adper Single Bond 2 (41.02 ± 3.79 Mpa) was more than Adper Self-Etch plus (39.80 ± 3.35 Mpa). None of the systems tested was able to reach the mean micro-tensile cohesive strength values of sound dentin (101.58 ± 5.72 Mpa) and Filtek Z-350 composite resin (80.27 ± 3.37 Mpa). When comparisons were made using one-way ANOVA, a highly significant difference was observed among different groups [Figure 1]. Then, the data are further analyzed using Critical Difference test. The critical difference was found to be 3.03989. The difference between mean micro-tensile bond strength values of Adper Single Bond 2 and Adper Self-Etch plus was 1.214 [Figure 2]. This was less than critical difference value that is, 3.039. So there was no statistically significant difference in the micro-tensile bond strength of Adper Single Bond 2 and Adper Self-Etch plus. The difference between mean micro-tensile bond strength values of Adper Single Bond 2 and dentin was 60.56 that was more than critical difference. Therefore, it was statistically significant. Similarly, the difference between mean micro-tensile bond strength values of Adper Self-Etch plus and dentin was also statistically significant. Critical Difference test also showed that there was statistically significant difference between the bond strength of Adper Single Bond 2, Adper Self-Etch plus and Filtek Z-350 composite resin.

Table 1

Mean and SD of Group-I and Group-II, Group-III and Group-IV

Figure 1

Comparison of mean micro-tensile bond strength (Mpa) of four groups

Figure 2

Comparison of mean micro-tensile bond strength (Mpa) Adper Single Bond 2 and Adper Self-Etch plus

DISCUSSION

The etch and rinse (total-etch) strategy is characterized by application of a preliminary and separate etching step (usually 37% phosphoric acid gel) that is, later rinsed away.[6] The smear layer is removed by etch and rinse adhesive system during etching[7] and water rinsing causing sound dentin de-mineralization. Etching opens the tubules by funneling their orifices and removes all residual smear plugs. The intertubular dentin is de-mineralized for few microns depending on acid concentrations, gel formations, time and mode of application. The removal of the mineral phase from the superficial dentin allows the exposure of the dentin organic matrix. The resin monomers then infiltrate the delicate network of de-mineralized collagen fibrils, creating the so-called hybrid layer. The penetrations of adhesives into funneled dentinal tubules create resin tags. The bond established by etch and rinse adhesive relied on the micro-mechanical retention[8] between the de-mineralized dentin matrix and polymerized adhesive system.

Some inconveniences have been related to this etch and rinse bonding technique, that is, dentin substance must not be over-dried or over wetted. It requires a number of clinical steps,[9] increased application time and postoperative sensitivity.

To eliminate these inconveniences[10] in bonding procedure but at the same time retain the effectiveness of dentin adhesive, a self-etching approach was identified in the early 90's.[11] In this approach, the rinsing step was eliminated. This no rinse technique lessens clinical operating time.

Additionally, self-etching primers are less likely to result in a discrepancy between the depth of de-mineralization and depth of resin infiltration as self-etching primer de-mineralize and infiltrate dentin simultaneously.[12] This self-etching primer does not remove the smear layer from dentin completely, which results in less postoperative sensitivity than total-etch adhesives.[13]

To improve the bond strength of bonding agent, newer adhesives are introduced in the market that contain bonded zirconia nanofiller.[14] The presence of these filler particles in adhesives help to develop a uniform film, which in turn leads to higher bond strength.

The dentin bond strength was chosen due to the highly variable morphological and compositional nature of dentin plus the presence of smear layer that makes resin-dentin bonding more difficult to attain compared to enamel (Cardoso PEC et al.,1998).[15]

To assess bonding effectiveness of adhesives, bond strength testing is commonly used in the laboratory.[16] However, traditional bond strength test method uses specimens of large surface areas. These tests have been criticized for the uneven distribution of stress at the resin-dentin interface. As many as 80% of the failure occurred cohesively in dentin during testing. The test result cannot provide reliable information on the bond strength of dentin adhesives.

To overcome these difficulties micro-tensile bond strength test was introduced by Sano et al. in 1994. It has better stress distribution in the same specimen during testing as it uses specimens having small cross-sectional areas[17] and presents more reliable results with less variation as compared to shear or tensile tests.[15] The micro-tensile test also allows a higher number of specimens per tooth, raising the accuracy of the test[18] and also it has a lower coefficient of variation and lower occurrence of cohesive fracture is seen along dentin.[19]

The original micro-tensile bond strength test utilizes dumbbell or hour glass specimens that allow tensile stress to be more uniformly directed toward the weakest interfacial region.[19] The disadvantage of this method is that cohesive substrate failures are more liable to occur during testing of specimens with high bond strength.

The present study utilized a modified version of the micro-tensile bond strength test. Here tensile stress was applied to a composite dentin beam that had a small but uniform cross-sectional area throughout the entire length of the beam.[20]

It places stress on the adhesive interface during specimen preparation and handling compared to other methods. This modified testing method allows the study of materials producing more accurate bond strengths.

Bekes et al., 2007[21] in their study concluded that self-etch adhesive agent gives an equal clinical performance in comparison with total-etch adhesive. Yaseen et al., 2009[22] got the similar finding. They concluded that self-etch adhesive systems having comparable bond strength with the total-etch adhesive system may be a good alternative in pediatric dentistry; as it has less number of clinical steps. Their results were comparable to the present study.

Several clinical parameters, among which dentin origin, site and area of bonding have shown significant influence on the mean micro-tensile bond strength of the adhesive system. Practitioners can easily control these parameters by using a standardized protocol for the clinical success of the self-etch adhesive system.[5] Along with micro-mechanical bonding, the self-etch adhesives show the chemical interaction between functional monomers and tooth substrate components which have recently gained attention.[2324] Having eliminated the multiple application stages and being time saving, easy to apply and less technique sensitive, the self-etching adhesives are becoming more popular these days.[25]

It is demonstrated in this study that bond strength values for current adhesive systems cannot be compared to the micro-tensile bond strength of dentin and resin composite. When one considers a bonded restoration, the weakest spot will always be the interface as we are bonding a very heterogeneous structure dentin, to a more homogenous material, the composite resin. Clinically, it is important to consider that the adhesive interface continues to be the weakest point in bonded aesthetic resin composite restoration. So, the clinician should be careful when working on it.

Attaining very high micro-tensile bond strength is not necessarily an indicator of clinical success. Other parameters, such as chemical interaction with the tooth surface and bond stability over long-term, may be important for the clinical success of bonded restorations.[23] The results of the bond strength study should thus be complemented with micro-leakage studies and validated by the findings of in-vivo trials.

CONCLUSION

This study showed that the Interface bonded with the two adhesive systems had lower micro-tensile bond strength than those of dentin and resin composite. And also the self-etching adhesive Adper Self-Etch plus had comparable bond strength with total-etch adhesive Adper Single Bond 2. However, this in-vitro study needs further in-vivo implementation. So, long-term clinical studies are required to evaluate the efficacy and durability of these bonding systems.

Financial support and sponsorship

SCB Dental College and Hospital for providing materials used to do the study.

Conflicts of interest

There are no conflicts of interest.