Introduction

Restoring teeth with minimal sacrifice of sound tooth structure currently forms the basis of restorative dentistry. Essential in achieving this goal is the adhesive that provides strong and durable bond to the remaining sound tooth tissues especially when shrinking materials such as resin composites are planned to be used. ₁,₂

The early successful adhesive systems have typically accomplished resin-dentin bonding in three steps respectively are the etching, priming, and application of bonding resin. ₃ Using these systems, the quality of the created bond is greatly influenced by the duration of the etching process, and by the amount of dentin surface humidity following rinsing of the etching acid and prior to resin infiltration. ₄ Therefore, most of the recent researches and developments in dentin adhesion are directed to simplify the bonding procedures and to eliminate all possible technical sensitivities by reducing the number of bonding steps. These developments have been started when the primer and bonding resins were combined together in one bottle. The self-etching primers were then released with the ability to etch and prime the dentin in one step. ₄,₅

The self-etching approach seemed promising as it reduces the chair-side time, and eliminates the critical and difficult standardization of the bonding steps. ₆ The one-step self-etch adhesives were subsequently introduced simplifying the conditioning, priming, and bonding procedures just in a single step. However, the early types of these adhesive systems seemed to achieve lower bond strength values in comparison to the two-step systems. ₇,₈ Newer types of the one-step self-etching adhesive systems have recently been introduced to the market claiming to have higher bond strength via formation of an unusual very thin interaction layer. ₉,₁₀ A conflict was raised regarding the success of these new one-step self-etching (all-in-one) adhesives as it depends on the specific composition of each product. Therefore, it was suggested that these new adhesives are in need for more screening before recommending them for clinical use. ₁₁

The micro tensile test is increasingly used to evaluate the strength of such tooth-adhesive bond. ₁₂,₁₃,₁₄,₁₅ This test provides a purely tensile load on a very small cross-section of the bonding interface regardless the specimens' design. ₁₆ Over such a limited surface, stress distribution is expected to be uniform, thus enabling the test measurements to truly express the interfacial bond strength. ₁₃,₁₄,₁₅ In addition, the micro tensile method has allowed the mapping of bond strength in different regions or at different depths of dental tissues. ₁₇,₁₈

To stand on the efficiency of the new all-in-one adhesives bonding to dentin, this in vitro study aimed to evaluate the micro-tensile bond strength (µTBS) of 2 one-step self-etching adhesive systems to dentin before and after thermal cycling. The bond strength of a 1 two-step adhesive system was also considered to act as control. A microscopic analysis of the fractured surfaces was also carried out to detect the exact mode(s) of bond failure.

Materials and Methods

Thirty caries-free freshly-extracted human molars were selected for this study. The collected teeth were cleaned (Pro-sonic 300 MTH, Sultan Chemists Inc, Englewood, NJ) and stored in de-ionized water that contained antibacterial agent (0.2% sodium azide) for a maximum of one month before trimming their occlusal anatomy (LabMaster, Ray Foster Dental Equipment, Huntington Beach, CA) to prepare flat dentin surfaces. The flattened surfaces were then finished using long cylindrical diamond tips (F31273, EDENTA GmbH, Lustenau) mounted to high speed handpiece. Immediately after finishing and before performing the bonding procedure, the flat dentin surfaces were subjected to thorough cleaning using air-water spray. The prepared teeth were dried with air then equally divided into 3 main groups (n=10 for each adhesive system). The description and manufacturers of the materials used are shown in Table 1.

Figure 1

Table 1: Materials used in this study

For the control group; one 2-step single component adhesive system Excite was used to retain the Tetric Ceram composite to dentin. The flat surfaces of the prepared teeth were etched using 37% phosphoric acid (Vivadent, Schaan, Liechtenstein) for 15 s, washed under copious air-water spray and gently dried using cotton pellets. Two successive coats of the single component resin adhesive were then applied to the moist dentin surfaces using a brush for 10 s and cured by the aid of Hawe-Neos halogen light-curing device (Gentilino, Switzerland) providing intensity of 600 mW/ cm ² for another 20 s after air drying of the excess material. Two one-step, self-etch (G-Bond); and (Adper Prompt L-Pop) adhesive systems were used in the other 2 groups. The self-etch bonding resins were applied to the prepared dentin surfaces and left for 10s before air thinning. Curing of both self-etching adhesives was performed using the same light-curing device for 10s.

The composite material was then incrementally built up to be 2 mm above the flat occlusal surfaces. Each increment was light-cured for 40 s and a rubber mold 8 mm in diameter and 2 mm high, situated over the flat occlusal plane of each prepared tooth, aided in both building and contouring the composite restoration. The restored teeth were incubated in water at 37 1 ° C for 24 h and half of them (5 teeth from each group) were also subjected to thermo-cycling (Th) (Willytec Thermocycler 10714, Munich, Germany) at 5 176; C, 37 ° C and 55 ° C for 5000 cycles with 30-s dwell time resulting in 6 groups. ₁₉,₂₀

Micro-tensile bond strength testing

Assessing the mode of specimens' fracture

Statistical analysis

Results

Means and standard deviations of the recorded micro-tensile bond strengths are shown in table 2. The one-way ANOVA (Table 2) indicated presence of significant differences (p < 0.001) between the bond strength values of different adhesives to dentin before and after thermal cycling. No statistical difference was detected between the 2 one-step adhesives (Tukey's comparisons, P > 0.05) under the same testing conditions (Table 3), APL (16.3 ± 2.7 MPa), APL-Th (13.1 ± 1.2 MPa), GB (18.4 ± 4.9) and GB-Th (14.7 ± 2.4). This difference was not statistically significant (Tukey's comparison, P > 0.05) when the bond strength of unthermal-cycled G-bond was compared with the control Excite (20.9 ± 2.4) or the thermal-cycled G-bond was compared with the thermal-cycled Excite (18.4 ± 1.7). Both adhesive and admix (adhesive/cohesive) types of bond failure were evident as revealed by the stereomicroscope, (Figure 1). However, SEM images (Figures 2a-c) indicated that the majority of bond failures belong to the admix pattern.

Figure 2

Table 2: Microtensile bond strength in MPa of different adhesives

Figure 3

Table 3: Tukey's comparisons between different groups

Figure 4

Figure 1: Percentage of bond failure

Figure 5

Figure 2: SEM Photographs (x 1000) of specimens after µTBS testing demonstrating a mixed failure mode (adhesive/cohesive). (a) GB-bonded specimen. (b) APL-bonded specimen showing cracks in the adhesive layer (pointer) as well as voids at the interface (arrows). (c) Ex-bonded specimen. Hybrid layer (H) Adhesive layer (A)

Figure 6

Figure 7

Discussion

Adhesion to tooth structure usually provides a great opportunity for more conservative restorations. ₂ However, the successful adhesive should fulfill the minimal acceptable level of bond strength that helps to resist both polymerization and thermal stresses at the bonding interface. ₂₂ Recently, a great concern about technique sensitivity and time consuming of adhesives has been developed and this has initiated the development of newer generations with reduced application steps. ₂₃ Accordingly, this in vitro study is concerned with the evaluation of micro-tensile bond strength of two one-step adhesive systems, APL and GB, in comparison to a one two-step type, Ex. The micro-tensile bond strength test was selected for that purpose because of its expected accuracy in comparison to other bond strength tests. ₁₂,₁₃

The recorded data revealed that the µTBS value of GB adhesive was comparable to that of the Ex (Control) and to that of APL, whereas the µTBS value of APL adhesive was lower than that of Ex (Tables 2, 3). These findings coincide with the results of many studies ₂,₇,₂₄,₂₅ and may be explained as follows; the higher bonding efficiency of Ex is almost dependent on the formation of hybrid layer, in addition to the direct penetration of both dentin micro-irregularities and the opened dentinal tubules. ₂₆ Presence of alcohol in the Ex formulation also helps the infiltration of adhesive resin into the collapsed collagen network and accordingly improves the adhesive's bond strength. ₃ Combining both the etching and bonding procedures in one step can deteriorate the bond as recorded in previous studies ₇,₂₄ Those studies related the reduction in bond strength to the presence of etching material and outcomes within the formed dentin/adhesive interaction layer. However, with using GB, functional monomers contained in the bonding material react with hydroxyapatite to form insoluble calcium, forming a thin transitional zone that may be responsible for improving its bond strength. ₉,₂₇,₂₈,₂₉

Subjecting some specimens to thermal cycling before performing such bond strength testing has been recommended in many in vitro studies. ₇,₁₉,₃₀ Application of heat and cold alternatively usually gives a crude indication about the efficiency and longevity of such bonds in service. Similar to other studies, ₇,₁₉,₃₁ a reduction in bond strength values of the tested adhesive systems was found after thermal cycling, but the difference in this study was not statistically significant (Tukey's comparison, P > 0.05) probably because of the relatively short-term application of thermal cycling. The reduction in µTBS values could be related to the fatigue of the already existed bond. Bond fatigue may be developed as a result of thermal stresses that developed at the bonding interfaces because of the differences in coefficient of thermal expansion of materials sharing those interfaces. ₃₂ However, the little effect of both thermocycling and water immersion on the bond strength values of GB could be attributed to the minimal thickness of the adhesive junction. ₂,₅,₂₀

The adhesive and admix (adhesive/cohesive) modes of bond failure were revealed in the results of this study (Figure 1), which coincide with other studies. ₄,₃₃,₃₄,₃₅ The suggestion is that the presence of both etching material and products of the etching process within the adhesive layer weaken mechanical properties of such resin. These remnants own different elastic moduli and coefficients of thermal expansion than those of dentin and composite material. These differences could initiate microcracks within the adhesive layer at the time of specimens' cutting and testing. Therefore fracture of the adhesive body could be expected more frequently with the self-etching adhesives in comparison to the 2 step adhesives those always free of etching remnants. ₇

Conclusion

Within the limitation of this study, the one-step adhesive APL does not perform consistently worse regarding µTBS (16.3 ± 2.7) than the newer one GB (18.4 ± 4.9). The µTBS of GB is also comparable to that of the control, Ex two-step adhesive (20.9 ± 2.4). Thermal cycling has no statistically significant deteriorating effect on the performed bond of different adhesives under investigation, APL-Th (13.1 ± 1.2), GB-Th (14.7 ± 2.4), Ex-Th (18.4 ± 1.7).