Comparative evaluation of the enamel bond strength of 'etch-and-rinse' and 'all-in-one' bonding agents on cut and uncut enamel surfaces.

AIMS AND
OBJECTIVES: To compare tensile bond strength of an 'etch-and-rinse' bonding agent (Single bond,3M ESPE, MN, USA) with an 'all-in-one' bonding agent (iBond, Heraeus Kulzer, NY, USA) on cut and uncut enamel surfaces. The null hypothesis tested is that the 'all-in-one' bonding agent matches the 'etch-and-rinse' bonding agent in terms of tensile bond strength to enamel.
MATERIALS AND METHODS: Forty extracted human mandibular teeth were used for the study. Twenty teeth with intact enamel surfaces were divided into two groups of 10 teeth each. The enamel surfaces of the 20 teeth were prepared and assigned to two more groups of 10 teeth each. One group each of intact and prepared enamel surfaces were used to bond with the 'etch-and-rinse' bonding agent [Single bond (SB), 3M ESPE, MN, USA] and the other two groups one each of intact and prepared enamel surfaces were used to bond with the 'all-in-one' bonding agent [ iBond (IB), Heraeus Kulzer, NY, USA]. The tensile bond strength was measured on the universal testing machine (Unitek, 9450 PC, FIE, INDIA) at a cross head speed of 1 mm / minute.
RESULTS: The results were statistically analyzed using a one-way ANOVA and student 't' test. The values for the 'etch-and-rinse' bonding agent SB were significantly higher for both the cut and uncut surfaces, compared to the 'all-in-one' bonding agent IB (P < 0.05). The all-in-one bonding agent resulted in a higher bond strength on the cut enamel surfaces.
CONCLUSIONS: Based on the results, it is advisable to use the 'etch-and-rinse system' in a clinical situation requiring bonding on enamel alone.

INTRODUCTION

The principles of adhesive dentistry date back to 1955, when Buonocore, using techniques of industrial bonding, postulated that acids could be used as a surface treatment, before application of the resins.[1] The fundamental principle of adhesion or bonding to the tooth substrate is an exchange process where inorganic tooth material is exchanged for a synthetic resin.[2] Enamel bonding is one of the most significant advancements in dentistry in the twentieth and twenty-first centuries, and it is the main reason for the clinical success of several restorative procedures. They include the bonding of ceramic veneers to the etched enamel, the use of resin-based composites to close diastemas, the placement of sealants, the extension of class V restorations on to the beveled enamel surfaces, and the placement of orthodontic brackets.[3] The bonding procedure involves the application of a bonding agent prior to the application of a resin composite. Bonding systems are available with different application protocols. Essentially, two approaches are followed at present, an ‘etch-and- rinse’ approach in which a separate etching step is carried out, with 37% phosphoric acid, followed by rinsing with water, prior to the application of a primer / adhesive, and a ‘self-etch’ approach in which there is no separate etching step, rather the bonding is achieved by the application of a single solution. The self-etch solutions contain an acidic monomer, which simultaneously etch, infiltrate, and polymerize. The ‘all-in-one’ self-etch adhesives offer an easy-to-use approach for the clinician, as they encompass all the steps into ‘one’ and are less technique-sensitive. The first evaluation of these new systems has shown a sufficient bond to conditioned dentin, while the bond with the enamel is less effective. As the bonding procedures not only involve bonding to dentin all the time, and the margins may extend to the prepared or intact enamel, their effectiveness in terms of bond strength to enamel has to be ascertained. A compromise of enamel bond strength results in microleakage and subsequent failure of the restoration over a period of time. In the light of these developments, this study is undertaken to ascertain the efficacy of the simplified ‘all-in-one’ bonding system (i-Bond, Heraeus Kulzer,NY,USA) when compared to the ‘etch-and-rinse’ bonding system (Single Bond,3M ESPE, St. Paul, MN,USA), on cut and uncut enamel surfaces. The new parameter tested in this study is the tensile bond strength of the ‘self-etch adhesive’ on prepared and intact enamel surfaces when compared to the tried and tested ‘etch-and-rinse’ adhesive.

MATERIALS AND METHODS

Forty freshly extracted mandibular molars with unattrited, intact crowns, free from decay and fractures, were collected. The teeth were scaled with an ultrasonic scaler to remove tissue tags, plaque, and calculus, and they were polished with pumice and stored in physiological saline till further use. They were randomly divided into four groups of 10 teeth each.

Group I — Uncut teeth (n = 10): Application of SB

Group II — Teeth with cut superficial enamel surfaces (n = 10): Application of SB

Group III — Uncut teeth (n = 10): Application of IB

Group IV — Teeth with cut superficial enamel surfaces (n = 10): Application of IB

For Group I and Group III the samples are prepared in the following manner:

The teeth are polished with pumice and thoroughly cleaned with toothpaste and water. The buccal surfaces of the teeth are etched with phosphoric acid (Scotchbond, 3M ESPE, St. Paul, MN, USA) following the manufacturer's instructions. Metal dies with internal diameters of 3 mm on one side and 8 mm on the other side, prepared especially for this study, are used. The side with the 3 mm diameter is placed on the buccal surface of the tooth and held in place with the help of sticky wax. The corresponding bonding agent(Single Bond / iBond) is applied on the tooth from within the die and cured following the manufacturer's instructions. The composite material (Filtek Z 250, 3M ESPE, MN, USA) is condensed into the die and cured for 20 seconds following the manufacturer's instructions.

For Group II and Group IV, 0.5 mm of enamel on the buccal surfaces of the teeth was reduced, using diamond disks, taking care not to expose the dentin. The same restorative procedure with the corresponding bonding agent for the particular group, as mentioned earlier, was followed.

The teeth were placed in the incubator at 37°C for 24 hours. The teeth were thermocycled in distilled water 100 times, between 5°C and 55°C, with 30 seconds dwell time and 5 seconds transfer time for each. The teeth were sectioned using diamond disks at the cementoenamel junction, to facilitate holding them in the jaws of the Universal Testing Machine.

The tensile bond strength tests were measured using the Universal Testing Machine (Unitek, 9450 PC, FIE, INDIA), at a cross head speed of 1 mm / minute.

The force required to separate the composite material from the tooth surface was registered in newtons, and converted into megapascals as a ratio of newtons to surface area of the bonded surface (MPa = N/m2). The obtained results were statistically analyzed.

RESULTS

The quantitative data were presented as Mean, Standard deviation, Standard error, and range values. One way ANOVA was used for multiple group comparisons followed by an unpaired t-test, for pairwise comparisons.

The mean bond strength values and standard deviations of Group I to Group IV are shown in Table 1. The bond strength values were obtained in Kilo Newtons and converted to Megapascals.

Table 1

Descriptive measurements of enamel bond strengths of different groups

The results of the comparison of enamel bond strengths between Group I and Group III are shown in Table 2.

Table 2

Comparison of enamel bond strengths of group I and III

The mean bond strength values for Group I were 4.94 ± 0.83 MPa and for Group III were 3.62 ± 1.56 MPa. The P value for comparison between Group I and Group III was 0.03, which was less than 0.05, indicating the statistically significant difference between the groups. This indicated that the Single Bond (SB) was more efficient on uncut enamel surfaces compared to the iBond (IB).

The results of comparison of the enamel bond strengths between Group II and Group IV are shown in Table 3.

Table 3

Comparison of enamel bond strengths of group II and IV

The mean bond strength values of Group II were 25.44 ± 5.25 MPa and for Group IV were 8.51 ± 2.45 Mpa. The P value for comparison between Group II and Group IV was 0.001, which was less than 0.05, indicating a statistically significant difference between the groups.

This indicated that the Single Bond (SB) was more efficient on cut enamel surfaces compared to the iBond (IB). However, the tensile bond strength values for IB on cut enamel surfaces were higher when compared with those on uncut enamel surfaces.

DISCUSSION

The present study compared the ‘iBond’ (seventh generation) self-etching adhesive characterized by the single bottle system with the ‘Single Bond‘ (fifth generation) ‘etch-and-rinse’ adhesive.[4]

The parameter chosen was tensile bond strength using the universal testing machine on cut and uncut enamel surfaces.

The iBond was chosen because it was claimed by the manufacturer to be a unique, all-in-one bottle system, incorporating etching, priming, and bonding in a single step, and hence, user friendly and saving time. It was thought necessary to supplement the data as regards its bond strength when compared to the more established ‘etch-and-rinse’ approach of the fifth generation bonding agent, which had been successfully used over a significant period of time and was considered to be the benchmark against which newer systems should be compared.

Considering the multiplicity of uses elaborated by the manufacturer and the fact that not all the margins of the restorations were usually beveled, it was of utmost importance to determine the ability of the new self-etch adhesive material's ability to bond to cut and uncut enamel.[5] Thus, the study included groups involving cut and uncut enamel surfaces.

The rationale behind the bond strength testing was, the higher the actual bonding capacity of an adhesive, the better it would withstand stress and the longer the restoration would survive in vivo. Bond strength testing was relatively easy and fast, and besides, it did not require special equipment.[6]

In the present study, as regards intact enamel surfaces, iBond which is a self-etch adhesive, has resulted in lower bond strength values (3.62 ± 1.56 MPa) as compared to the Single Bond (4.94 ± 0.83 MPa) and there has been a significant difference between bond strength values (P < 0.05); which is in agreement with other studies that reported self-etching adhesives as being less effective on intact enamel surfaces than the ‘etch-and-rinse’ adhesive.[57-9]

The penetration and diffusion of the bonding agent into the demineralized enamel surface was very important for enamel adhesion.[10] The bond strength of the phosphoric acid-etched enamel was mainly attributable to the resin's ability to penetrate between the enamel crystallites and rods.[11] This ultrastructure might have contributed to the high bond strength values obtained with the Single Bond.

The lower adhesion strength of the iBond to intact enamel surfaces might be a result of the less pronounced enamel etching pattern obtained with the self-etch adhesive, because of the relatively inadequate pH of the primer. Intact enamel was less accessible to self-etch adhesives, because it was hypermineralized and could contain more flouride than instrumented enamel. Additionally, changes occurred in the outermost enamel layer after eruption, and a prismless enamel layer could be present, preventing further penetration of the self-etching adhesives.[12]

On cut enamel surfaces, the application of the Single-Bond resulted in bond strength values of 25.44 ± 5.23 MPa and the application of iBond resulted in 8.51 ± 2.45 MPa. There was a highly significant difference between the groups (P < 0.01).

The results of this study are in contrast to the other studies,[12] which report resin tag penetration into the roughened enamel treated with self-etch adhesives, which has been deemed comparable to phosphoric acid–etched enamel. However, some studies have reported that the etching pattern of self-etch adhesives is not as well-defined as that of the total-etch adhesives on roughened enamel surfaces, but result in similar bond strength values.[1314]

The reason for the low bond strength might be related to the difference in the pH of phosphoric acid (Scotch Bond etching gel) used with the Single Bond (0.6)[15] and the pH of the acidic primer in the iBond (1.6);[6] and the absence of a well-defined etching pattern might correspond to the low-bond strength values.

Both the iBond and Single Bond resulted in higher bond strength values on the ground enamel suggesting that some kind of enamel preparation might facilitate better adhesion.

Single Bond is ethanol- / water-based and iBond is acetone- / water-based.

Acetone has a relatively higher vapor pressure (184 mm Hg at 20°C) when compared with ethanol (43.9) and water (17.5) and results in easier solvent evaporation.[16]

It was noted during a study that the acetone-depleted solution presented a notably higher viscosity.[17] Thus, it was plausible that the viscosity of the adhesive system increased as the solvent concentration decreased, and in turn it might affect the ability of the resin monomer to infiltrate the substrate. High frequency of use, associated with higher temperatures, might accelerate the process of solvent evaporation. Furthermore, self-etching primers incorporated a significant amount of water, as a solvent, to promote the ionization of acidic monomers. After solvent evaporation, the adhesive layer could be very thin, and therefore, the mechanical properties might be low.[1819] Another possible explanation for the low bond strength obtained using acetone-based adhesives was the high percentage of acetone (approximately 70%), which might not permit the formation of a uniform film on the bonding surface.[20] This might have possibly contributed to the lower bond strength values obtained with the iBond.

A static bond-strength test is often regarded as clinically less relevant, as such abrupt loading of the adhesive-tooth bond clinically never occurs. Therefore, dynamic fatigue testing is often claimed to better predict the clinical effectiveness of adhesives.[21]

Fracture toughness (KIc) or the strain energy release rates (GIc) are tests that are considered more meaningful to measure the energy or work to separate the adhesive resin from its bond to dentin.[22]

CONCLUSIONS

Within the limitations of this in vitro study the following conclusions are drawn:

The tensile bond strength of the Single Bond is significantly higher compared to that of the iBond, on both cut and uncut enamel surfaces.

It is advisable to follow the established practice of using the etch-and-rinse approach when bonding on enamel alone.

Further studies are required to evaluate the efficiency of the iBond and other ‘all-in-one’ bonding agents in different testing conditions, such as the Microtensile bond strength test, Dynamic fatigue testing, and the Weibull distribution function.

Acknowledgment

The authors would like to thank Dr. Beena Rani Goel, Former Professor and H.O.D., Department of Conservative Dentistry and Endodontics, P.M.N.M. Dental College, Bagalkot, for her active support during the initial stages of the study.