Comparative Evaluation of the Bonding Efficacy of Multimode Adhesive, Two-Step Self-Etch Adhesive, and a Total-Etch System to Pulpal Floor Dentin – An In vitro Study

Abstract

Background:

Postendodontic restoration with composite resins has gained popularity due to the development of more reliable adhesives with simplified bonding procedures and less technique sensitivity. Pulpal floor dentin is a complex biological structure and a more challenging bonding substrate. Hence, appropriate selection of the bonding agent for these regions of the tooth is important.

Aims:

To compare the shear bond strength of a new generation of dental adhesives referred to as multimode adhesives, two-step self-etch adhesive, and a total-etch system to pulpal floor dentin.

Materials and Methods:

Sixty human mandibular molars were sectioned with a slow speed diamond disc to expose the pulp chamber and divided into four groups of 15 teeth each, based on the bonding agent used. Group A: Single bond universal (self-etch); Group B: Single Bond Universal (etch-and-rinse), Group C: Clearfil SE Bond, Group D: Adper Single Bond 2. Using a split Teflon mold, composite cylinders were built on the prepared dentinal surfaces. The shear bond strength was then evaluated with the universal testing machine.

Results:

Two step self-etch adhesives recorded higher shear bond strength than total-etch and multimode/universal adhesives applied in both self-etch and etch-and-rinse techniques.

Conclusion:

The self-etching/priming system bonded more strongly to the pulpal floor dentin than multimode adhesive and total-etch system.

Introduction

The quality of the permanent coronal restoration is an important parameter that can adversely affect the outcome of root canal treatment. Therefore, it is necessary to have adequate seal and adhesion between the final restoration and tooth structure. With the development of more reliable adhesives with simplified bonding procedures and less technique sensitivity, there has been an increase in the use of resin-based composite to restore the endodontically treated teeth. Biggest advantage of adhesive restorations is that they bond directly to the tooth structure, reinforce the weakened tooth structure,[1] and reduce microleakage of the resin composite core. Composite core build up provides high bond strength to tooth structure and increased resistance to fracture.[2]

Bonding to enamel is more reliable, whereas bonding to dentin represents a greater challenge and has proved to be more difficult and less predictable as a result of inherent characteristics of this substrate.[3] The pulpal floor dentin is a complex biological structure and bonding systems may behave differently when compared to coronal dentin. Furthermore, the chemical substances and irrigating solutions as NaOCl[4,5] and ethylenediaminetetraacetic acid[6] used during biomechanical preparation of root canals could modify the dentine structure[7-9] and interfere on dentin/resin bonding. Hence, selection of appropriate bonding agent is an important criterion for restoration of endodontically treated teeth.

Adhesive systems can be classified as total-etch or self-etch adhesives, in versions of three steps, two or just one step. Recent developments have focused on the simplification of the multistep bonding process and minimizing of the errors during the application of the adhesive. According to Belli et al,[1] clinicians should choose an adhesive system that is simple, easily retrievable, and technique insensitive.

A new generation of one-bottle dental adhesives is currently being used. These are referred to as universal or multi-mode adhesives (UA) in the literature. These materials are simplified adhesives, usually containing all bonding components in a single bottle. UA is indicated as either self-etching (SE) or etch-and-rinse (ER) adhesives.

This study aims to evaluate the shear bond strength of different adhesive systems to pulpal floor dentin.

Materials and Methods

The study protocol was approved by the Ethics Committee of Mar Baselios Dental College, Kothamangalam with reference number IEC/5/2014/MBDC on January 20, 2015. The study design does not include any patient related data.

Materials used in the study:

1. Filtek Z350XT Nanohybrid restorative material (3M ESPE, St. Paul, MN, USA)

2. Bonding agent:

   • Clearfil SE bond (Kuraray, Tokyo, Japan)

   • Single bond universal/SBU (3M ESPE, St. Paul, MN, USA)

   • Adper single bond 2 (3M ESPE, St. Paul, MN, USA).

3. Gel etchant: 37% phosphoric acid (d-tech).

In accordance with similar studies,[2,10] a total of sixty freshly extracted, noncarious, intact human mandibular molars extracted for periodontal reasons were used for this study. The teeth were then randomly and equally divided into four groups based on the bonding agent used, with an effective sample size of 15 per group.

All teeth were cleaned with ultrasonic scalers and stored in saline. The teeth were cut horizontally 3 mm above the cemento-enamel junction with a slow speed diamond disc to expose the pulp chamber. Pulp tissue was removed carefully using a spoon excavator. The specimens were rinsed with distilled water to remove debris, and then air-dried. The remaining apical part of each tooth was mounted on a metallic mold of 1 × 1 × 1 inch dimension using acrylic resin [Figure 1]. The pulpal floor dentine was prepared and bonded according to the manufacturer’s instructions.

Figure 1.

Prepared Specimens (n=15)

Experimental groups

Group A: One step self-etch adhesive system, SBU- self-etching

The adhesive was applied to tooth surface for 20 s and gently air dried for 5 s followed by light curing for 10 s.

Group B: Etch-and-rinse adhesive, SBU-etch-and-rinse

After acid-etching with 37% phosphoric acid for 15 s, the tooth surface was rinsed for 15 s. The excess moisture was removed with absorbent paper, and adhesive was actively applied for 20 s. The solvent was evaporated with air stream for 5 s, followed by light curing for 10 s.

Group C: Two-step self-etch system (Clearfil SE Bond)

Primer was applied for 20 s and gently dried for 5 s. The adhesive was applied to primed tooth surface and then air-thinned to make uniform bond, followed by light curing for 10s.

Group D: Total-etch system (Adper single bond 2)

The tooth surface was etched with 37% phosphoric acid for 15 s and rinsed for 15 s. After drying excess water, 2 consecutive coats of adhesive was applied for 15 s and gently air dried for 5 s, followed by light curing for 10s.

Composite build up and shear bond strength testing

Following application of adhesive, a Teflon mold was placed over the bonded surface at the predesigned location in the mould holder [Figure 2]. Filtek Z350 XT (3M ESPE) was placed in increments into the circular punch hole measuring 2 mm × 3 mm in a Teflon sheet and cured layer by layer for 20 s. Once the composite was cured, the split teflon mold was gently lifted out and specimens were obtained. All specimens are then stored at 37°C in water for 24 h.

Figure 2.

Split Teflon mould

Specimens were then mounted on a Universal Testing Machine (Tinius Olsen H10KS Universal Testing Machine) individually and then subjected to shear bond strength evaluation at a cross-head speed of 1 mm/min [Figure 3]. The shear bond strength values were then subjected to statistical analysis.

Figure 3.

Tinius Olsen Universal Testing Machine

Statistical analysis

The one-way analysis of variance was used to determine and compare the means of the shear bond strength among the multimode adhesive, two-step self-etch adhesive and total etch system to pulpal floor dentin. Post hoc analysis was done for intergroup comparisons whenever a significant difference between the means was revealed by SPSS 17.0 software. Differences were statistically significant when P < 0.05.

Results

Shear bond strength values of all the groups in terms of mean, standard deviation are given in Table 1. The results revealed that samples of Group C (Clearfil SE Bond) had the maximum shear bond strength (22.0167 MPa). Strength in the decreasing order was as following;

Table 1.

Mean shear bond strength values obtained in each group

Groups	n	Mean	SD	SE
Group A	15	20.7059	0.96711	0.24971
Group B	15	18.7913	1.11229	0.28719
Group C	15	22.0167	2.03518	0.52548
Group D	15	20.2315	1.09017	0.28148
Total	60	20.4364	1.76990	0.22849

SD: Standard deviation; SE: Standard error

• Clearfil SE Bond > SBU-SE > Adper Single Bond 2>SBU-ER

• Intergroup comparisons were done using post hoc analysis [Table 2].

Table 2.

Post hoc analysis: Tukey’s multiple comparison test

(I) groups	(J) groups	Mean difference (I−J)	SE	Significant
Group A	Group B	1.91467	0.50010	0.002
Group A	Group C	−1.31080	0.50010	0.053
Group A	Group D	0.47447	0.50010	0.779
Group B	Group C	−3.22547	0.50010	0.000
Group B	Group D	−1.44020	0.50010	0.028
Group C	Group D	1.78527	0.50010	0.004

SE: Standard error

Discussion

With the development of more reliable bonding systems, coronal restoration of endodontically treated teeth using resin composites has become a common procedure in dental practice. The advantage of bonding properties coupled with a composite core build up can achieve satisfactory form for resistance and retention as well as adequate mechanical properties.[11]

Bonding systems may behave differently to pulpal floor dentin when compared to coronal dentin due to the difference in the morphological characteristics of different regions of superficial, deep and pulp chamber dentin. When compared to coronal dentin, the diameter of tubules in pulpal floor dentin is much smaller and the tubule density is high.[12] These morphological differences will have an effect on the conductance of fluid within dentin and thereby on the bond strength. However, the bond to radicular and pulp chamber dentin does seem to vary quite a lot depending on the bonding agent used.[13] Hence, it is essential to give importance to the selection of bonding agent used for these regions of the tooth.

In this study Clearfil SE Bond (Kuraray Noritake Dental, Tokyo, Japan) is used as representative of classic two-step self-etch adhesives as it is considered as the gold standard for this category of adhesive systems.[14] It also offers high bond strength to tooth substrates in laboratory[15-17] and clinical studies.[18,19]

The introduction of universal adhesives or multimode adhesives is yet another recent trend in adhesive dentistry. These can be used with total-etch, self-etch, or selective etch techniques.[20] However, in the literature, there is limited information on the bonding effectiveness of these adhesive systems to pulpal floor dentin.

Laboratory bond strength evaluations have been conducted using “shear,” “tensile,” or “push-out,” and “pull-out” methodologies.[21,22] Despite the concerns expressed about static bond strength testing, these tests remain a popular way to screen the bonding performance of new adhesive systems.[15] In this study, shear bond strength evaluation was chosen, as it is the most commonly used simple evaluation procedure to test the adhesion of dental adhesives.[23,24]

Results of the current study showed that, bond strength values achieved were the highest for group C (Clearfil SE Bond) followed by Group A (SBU-SE), Group D (Adper Single Bond 2), and Group B (SBU-ER). The results are in accordance with previous studies which reported higher bond strength for two step self-etch adhesives when bonded to pulpal floor dentin.[2,11,25] This could be attributed to the fact that pulpal floor dentin is usually not contacted by any cutting instruments so it is largely devoid of smear layer. Acid conditioning of primer in Clearfil SE bond appeared sufficient to demineralize the dentin and envelop collagen fibers and hydroxyapatite crystals.[3,14,22] The presence of highly hydrophilic 10-MDP monomer in its composition enables an intensive and stable chemical bond with hydroxyapatite and improve wetting of the moist tooth surface.[18] Furthermore, fillers present in Clearfil SE bond increase bond strength and improve mechanical properties of bonding agent.[3,26] Furthermore, the high degree of conversion of two-step self-etch adhesives resists water aging and improves the initial bond strength and durability of the resin–dentin bond.[27]

Single bond universal used in this study is constituted of 10-MDP which is a phosphate monomer that renders the adhesive an acidic characteristic. Therefore, it demineralizes dentin only partially, leaving hydroxyapatite partially attached to collagen, enabling a chemical bond between the MDP and hydroxyapatite through nanolayering.[28,29] This might help in improving the long-term stability of the adhesion formed. According to the manufacturer, SBU is additionally comprised of a polyalkenoic acid copolymer (Vitrebond™ copolymer), which, provides satisfactory bonding to dentin under moist or dry conditions.[30]

As a self-etch adhesive, SBU may be considered as a mild self-etch adhesive because of its relatively high pH (pH = 2.7). Unlike the bur-cut dentin, pulp chamber dentin surfaces are free of smear layers unless it is not prepared during endodontic treatment. Hence, the pulp chamber dentin may be considered as a favorable substrate for self-etch adhesives with mild pH.[31]

In the total-etch adhesive system Adper Single Bond 2 (group D), intratubular resin-tag formation and resin infiltration into demineralized intertubular dentine are the major elements that contribute to the bond strength. In the case of pulpal dentin, the diameter of tubules is much smaller and the tubule density is high; hence, there is little intertubular dentin between tubules to permit hybrid layer formation.[32] Furthermore, the use of strong phosphoric acid could remove inorganic material more aggressively, especially since the surface is not covered by a smear layer initially.

The reason for lower bond strength values of SBU-ER in this study could be attributed to the fact that pulpal floor dentin is rich in organic components and fewer in mineral. Using 37% phosphoric acid will end in over etching and collapsing of collagen fibers resulting in decreased bond strengths. This indicates that the application of the acid as a separate clinical step is not essential to improve the bond strength results when using the universal adhesive tested in this study. Few studies reported that previously acid etching dentin does not significantly increase the bond strength of self-etch adhesive to dentin.[33,34]

In this study, there was no statistically significant difference in the shear bond strength values of one-step SBU-SE group and two-step self-etch adhesive, Clearfil SE Bond. Therefore, when bonding to pulpal floor dentin, one step universal adhesive (SE) can be considered as it is easy to apply, requires fewer steps and less time than the multistep adhesive systems. Furthermore, their bond strength lies in the optimal range for clinical success.

Limitations of the study

In this study, the pulpal floor dentin has been chosen as substrate; however, in clinical situation, the eventual bonding success will be derived from the surrounding walls as well. Hence, further studies are needed to investigate the bond strengths of these adhesive systems under clinically acceptable conditions.

Conclusion

Based on the results of this study, two step self-etch adhesive recorded the highest shear bond strength, followed by one-step self-etch adhesive, total-etch adhesive and universal adhesive in etch and rinse mode. Within the limitations of this study, it may be concluded that, pulpal floor dentin may be a favorable substrate for self-etch adhesives and that the etching conditions needs to be modified when bonding to pulpal floor dentin. Further studies are needed to investigate the bond strengths and longevity of these.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.