Comparative evaluation of the bonding efficacy of sixth and seventh generation bonding agents: An In-Vitro study.

AIMS: To compare the shear bond strength of sixth generation and seventh generation bonding agents to dentin.
MATERIALS AND METHODS: EIGHTY HUMAN MAXILLARY PREMOLARS WERE REDUCED TO EXPOSE FLAT SURFACE OF DENTIN AND DIVIDED INTO FOUR EQUAL GROUPS, WHICH WERE BONDED USING FOLLOWING BONDING AGENTS: Sixth generation bonding agents, Adper SE Plus and Xeno III and Seventh generation bonding agents, Adper Easy One and Xeno V. Composite cylinders were then built using a plastic mould on these prepared dentinal surfaces. Samples were stored in distilled water for 24 hours and tested for shear bond strength with universal testing machine. Shear force was applied perpendicular to the long axis of composite cylinder at adhesive-tooth interface until debonding occurred. The data so obtained were tabulated and analyzed statistically using independent-samples t test and analysis of variance (ANOVA) test.
RESULTS: The seventh generation adhesives showed significantly higher shear bond strength to dentin compared to sixth generation adhesives (P < 0.01). The highest value of shear bond strength was obtained from Adper Easy One system, while Adper SE Plus gave the lowest shear bond strength values.
CONCLUSIONS: Seventh generation adhesives are more advantageous than sixth generation adhesives in dentin bonding as it requires less time, fewer steps, and better bond strength.

INTRODUCTION

Dental adhesive systems have evolved through several generations with changes in their chemistry, mechanism, number of steps, application techniques, and clinical effectiveness. Compared with etch-and-rinse adhesives, several advantages have been ascribed to self-etching adhesives. Firstly, self-etching adhesives involve a less technique-sensitive procedure, since the etch-and-rinse phase is omitted, which may cause collapse of vulnerable demineralized collagen network after acid etching.[1] Secondly, the simultaneous demineralization and resin infiltration should lead to an optimally infiltrated hybrid layer.[2] However, recent observations of nanoleakage beyond hybrid layer have shed some doubt on complete resin infiltration.[3] Thirdly, mild self-etching adhesives are assumed to cause less post-operative pain, as they use smear layer as a bonding substrate, leaving residual smear plugs that cause less dentinal fluid flow than etch-and-rinse adhesives.[3] Lastly, mild self-etching adhesives leave hydroxyapatite crystals available for chemical bonding of functional monomers to calcium, which may contribute to interface stability.[4]

One-Step self-etching adhesives are becoming increasingly more popular due to their easy and fast application procedure. Reduction of different steps in application procedure also entails fewer errors during application of the adhesive. This is often referred to as the “low technique sensitivity” of one-step self-etching adhesives.

In light of these developments, present in-vitro study was undertaken to investigate and compare the bonding efficacy of these newer simplified sixth and seventh generation bonding system, since in-vitro laboratory bond strength testing is used as screening tool to understand and predict the clinical behavior of these new products in a short period of time and lesser cost.

MATERIALS AND METHODS

Eighty freshly extracted non-carious, intact human premolar teeth indicated for extraction as a part of orthodontic treatment were selected for this study. The collected teeth were debrided of blood and saliva and stored in saline solution. The specimen teeth were utilized within three months of extraction. These teeth were polished with slurry of pumice and water. The root portion of teeth were cut off, and only the coronal portion was embedded in cold cure acrylic resin with the help of custom-made metallic mould of dimensions 2 × 2 cm. Teeth were mounted horizontally. The labial surface of each tooth was reduced with a high speed handpiece using #245 carbide bur under constant water spray in order to expose flat surface of dentine. The prepared samples were randomly divided into four experimental groups with 20 specimens in each group. For identification purpose, each group's acrylic block was painted with different color:

Group 1: Sixth generation bonding agent, Adper SE Plus (3M ESPE), red color acrylic blocks.

Group 2: Sixth generation bonding agent, Xeno III (Dentsply, India), blue color acrylic blocks.

Group 3: Seventh generation bonding agent, Adper Easy One (3M ESPE), peach color acrylic blocks.

Group 4: Seventh generation bonding agent, Xeno V (Dentsply, India), pink color acrylic blocks.

Tooth surface was rinsed and blotted dry. Bonding agents were applied onto surface with a microbrush and light-cured according to the manufacturer's instruction. Then, the composite resin Z 350 (3M ESPE) was placed in a 2 layer increment using plastic mould (2 × 2.5 mm) and was light-cured for 40 seconds. Each composite cylinder was also cured for an additional 40 seconds after removal of the mould. All the specimens were stored in distilled water for 24 hours prior to shear bond testing.

Specimens were mounted on the universal testing machine (Unitek, 9450 PC, FIE, INDIA), force applied by the machine on each specimen was at a crosshead speed of 1 mm/min in a compression mode using a blade parallel to the adhesive-dentin interface. The bonded composite cylinder was positioned horizontally, so that the shearing blade is perpendicular at composite-dentin interface. Each specimen was loaded until failure. Shear force required to debond the specimen was recorded. Debonding stress in megapascal was then calculated by the ratio of maximum load in Newton to the surface area of prepared resin cylinder (MPa = N/mm2). The data so obtained were tabulated and analyzed statistically using independent-samples t test and analysis of variance (ANOVA) test.

RESULTS

There was significant difference in the shear bond strength of all the four different self-etching adhesives tested (P < 0.01). The seventh generation adhesives showed significantly higher shear bond strength to dentin compared to sixth generation adhesives. The highest value of shear bond strength was obtained from Adper Easy One system, while Adper SE Plus gave the lowest shear bond strength values. Graph 1 shows comparison of mean shear bond strength of all four groups. Graph 2 shows comparison of mean shear bond strength of sixth and seventh generation bonding agents.

Graph 1

Comparison of shear bond strength of all four groups

Graph 2

Comparison of shear bond strength of sixth and seventh generation bonding agents

DISCUSSION

The integrity of bond between dentin and resin adhesive systems plays an important role in its success in clinical dentistry by providing good marginal adaptation, thereby preventing microleakage, recurrent caries, and pulpal irritation. In some studies, the shear test may give somewhat higher values than the tensile test, but with the same ranking of products. However, shear stress is considered to be more representative of the clinical situation[5] and relatively easier to perform. When composites are bonded to tooth volumetric shrinkage that occurs under polymerization generates stress on the bonded opposing walls in box-like cavities. An estimated shear bond strength of 17-21 MPa has been proposed as the critical value needed to withstand these stress of polymerization contraction of composite material.[67] The present study compared shear bond strength of sixth generation (Adper SE Plus, 3M and Xeno III, Dentsply) and seventh generation (Adper Easy One, 3M and Xeno V, Dentsply) adhesives to dentin. The manufacturing company was kept same to eliminate bias between the groups. Also, in order to exclude possible influences of different restorative resins on the bond strength, all bonding agents in this study were used in combination with Z-350 (3M) composite resin.

Bond strength studies are quite rough categorizing tools for evaluating the efficacy of bonding materials. The literature reports large variations in bond strength data, which are thought to be caused by differences in protocol.[8] Several factors influence in vitro bond strength to dentin, such as type and age of teeth, degree of dentin mineralization, dentin surface being bonded, type of bond strength test (shear or tensile), storage media, environmental relative humidity in substrates, complex nature of testing procedures, sensitivity of handling and manipulation of these systems and composite restorative material.[9] These variations could be responsible for high standard deviation and wide ranges obtained in the present study.

According to the results obtained in present study, it was observed that group 3 (Adper Easy One) gave highest mean shear bond strength to dentin compared to all other groups followed by group 4 (Xeno V), whereas group 1 (Adper SE Plus) gave lowest mean of shear bond strength followed by group 2 (Xeno III).

Adper SE Plus (3M) and Xeno III (Dentsply) have pH values of less than 1 and 1.4, respectively, thus are characterized as ‘strong self-etch’ and ‘intermediary strong self-etch’ adhesives. Such low-pH self-etching adhesives have often been documented with rather low bond strength values, especially to dentine due to their initial high acidity that causes deep demineralization.[1011] Conversely, for Adper Easy One Bond (3M), the pH is much higher 2.3, thus it is considered to be a ‘mild self-etching adhesive.’ Xeno V is also considered as mild self-etch adhesive.

“Mild” self-etch system demineralizes dentin only to a depth of 1 μm, keeping residual hydroxyapatite still attached to collagen. Nevertheless, sufficient surface-porosity is created to obtain micromechanical interlocking through hybridization. The thickness of hybrid layer is, however, much smaller than that produced by strong self-etch or etch-and-rinse approach, but has been proven to be minor in importance with regard to actual bonding effectiveness.[12] The preservation of hydroxyapatite within the submicron hybrid layer may serve as a receptor for additional chemical bonding. Such mild self-etching adhesives are found to have higher bond strength.[811] This might be a possible explanation for low bond strength of Adper SE plus and Xeno III compared to Adper Easy One and Xeno V.

Water is an indispensable component of self-etch agents, in order to ionize the acidic monomers and trigger demineralization process.[812] Strong self-etch agents are likely to contain higher amounts of water. However, a concern is the effect of residual water that remains within the adhesive interface, which hardly can be completely removed.[12] Adper SE Plus has only water as solvent, which might explain its low bond strength due to residual water, which can cause phase-separation, polymerization inhibition, and reduced shelf-life. Also, water is a poor solvent for organic compounds such as monomers.[13] These difficulties can be overcome by addition of a secondary solvent such as ethanol, which aids in displacement of residual water as well as carrying the polymerizable monomers into the opened dentin tubules.[14] Adper Easy One, Xeno V, and Xeno III have additional solvent ethanol in it.

Xeno III has to be mixed prior to application, which remains a technique-sensitive step. Errors in technique and operator variability lower bond strength. Inadequate primer drying time decreases bond strength for self-etch adhesives, especially water-based system. These technique-sensitive factors could undo the benefits. Also, Xeno III has high viscosity, which can further decrease bond strength. All these factors might explain the high variability in shear bond strength of Xeno III in this study.

Even though the bond strength of Adper SE Plus in the current study was low, the results were in agreement with previous studies.[1315] The results of dentin shear bond strength with Xeno III in this study are in agreement with the literature.[1617] In the present study, Adper SE Plus adhesive showed lower bond strength values compared to Adper Easy One and Xeno V; there was significant difference between bond strength values, which is in agreement with a recent study[15] that reported two-step self-etching adhesives bonded better with enamel and one-step self-etching adhesives bonded better with dentin.

However, certain studies contradict these results stating that two-step self-etch system showed better performance than single bottle system.[1218] Two-step self-etch adhesive systems have been reported to yield higher bond strengths compared to one-step self-etch adhesive systems, may be due to the proportions of their chemical constituents. Both contain functional monomers, cross-linking monomers, solvent, inhibitors, and activators, but in different proportions. The one-step self-etch adhesive systems generally have less cross-linking monomers. These cross-linking monomers provide most of the mechanical strength; therefore, there is a potential for lower bond strength, but this is product-specific and may not apply to the recently introduced systems.[15] The amount of monomers, diluents, and filler load differs between products according to manufacturer's technology, which is not well-described in adhesive composition. Also, little is known about the shrinkage and stiffness of these filled adhesives after polymerization. These factors could affect the shear bond strength significantly, but is not listed by manufacturer as final formulation is proprietary secret.

The result of the present study showed that there was significant difference in the in-vitro dentin shear bond strength among the self-etching adhesives tested, but there is no common factor, which accounts for the differential performance of the systems tested. However, due to the inherent limitation of an in-vitro study, the bonding and sealing ability of these self-etching adhesive systems to dentin warrant further investigation.

CONCLUSION

Within the limitations of this study, it can be concluded that seventh generation adhesives showed significantly higher mean shear bond strength to dentin than sixth generation adhesives. There was significant difference in the shear bond strength to dentin of all the four different self-etching adhesives used in this study. The highest value of shear bond strength was obtained from Adper Easy One from seventh generation system. Adper SE plus from sixth generation system gave the lowest shear bond strength values compared to all other groups. Based on these results, it appears that seventh generation adhesives are more advantageous than sixth generation adhesives in dentin bonding as it requires less time, fewer steps, and better bond strength.