Influence of Adhesive Layer Thickness of Dentin Adhesives on Microtensile Bond Strength of a Resin Composite to Dentin

Ⅰ. INTRODUCTION

While the importance of hybrid layer is emphasized through many studies, less studies are done about the adhesive layer; that is, the hydrophobic layer overlying the hybrid layer (Kim et al. 2014). As the component overlying the hybrid layer, the adhesive layer may help to protect the bonding interface from polymerization shrinkage stresses and act as a stress absorbing layer (Cadenaro et al. 2008; Kwon et al. 2012).

A number of studies have evaluated the role of the elasticity of the hybrid layer and/or adhesive resin layer in relieving the polymerization stress of resin composites. Although there seems to be no relationship between the thickness of the hybrid layer and tensile bond strength, concern remains that thin hybrid layers may not provide as much stress-breaking function as thicker hybrid layers (El Zohairy et al. 2005). However, one possible solution is to use thicker adhesive layers on top of thin hybrid layers.

Since there is limited information in the dental literature correlating bond strengths and thickness of the adhesive resin layer, this study evaluated the relationship between the thickness of the adhesive resin layer and microtensile bond strength. The hypothesis is that increased thickness of the adhesive layer would result in higher bond strengths by improving stress distributions in the bonding interface.

Ⅱ. MATERIALS AND METHODS

Two commercially available dentin adhesives (All-Bond 2 and All-Bond 3, both from Bisco, Inc.) were used. Tetric Ceram (Ivoclar Vivadent) was used as the composite resin. Their manufacturers, lot numbers are summarized in Table 1.

Table 1.

Materials used in this study

Non-carious human molars were collected after obtaining the patients’ informed consents obtained under a protocol approved by the Ethics Committee of the School of Dentistry, Kyungpook National University. The teeth were stored in 0.5% chloramine in water at 4℃ and used within one month following extraction. The occlusal enamel and roots of the teeth were removed using a slow-speed saw with a diamond-impregnated disk (Isomet, Buehler Ltd., Lake Bluff, IL, USA) under water cooling to form 5-6 mm thick, parallel-sided crown segments (Tay et al. 2003). A 600-grit silicon carbide paper was used under running water to create a thin smear layer on the dentin surface (Mazzitelli et al. 2008).

An adhesive tape with a 10 mm diameter hole was placed on the center of the dentin surface to limit the bonding surface area. The three different thickness (50, 100, and 200 μm) of the adhesive layers were controlled by the thickness of the adhesive tape (Table 2). The teeth samples were then randomly divided into six experimental groups (Table 2). The two dentin adhesives used according to the manufacturer’s instructions (Table 3) except for the control of the adhesive layer thickness. After applying the bonding resins, the dentin surface was then built up with a resin composite (Tetric Cram, Ivoclar Vivadent) to a height of 5 mm on three increments, light curing each increment for 20 s. All the bonded samples were stored in water at 37℃ for 24 h (Kim et al. 2014).

Table 2.

Six experimental groups

Table 3.

Instructions according to the manufacturer

Each bonded specimen was longitudinally sectioned into 0.9 mm-thick slabs with the slow-speed diamond saw (Isomet). Each slab was fixed on a glass platform with sticky wax and serially sectioned into 0.8 mm² (±0.1) sticks, in accordance with the “non-trimming” method of the microtensile test (Mazzitelli et al. 2008; Di Hipólito et al. 2012). The exact dimensions of each stick were measured using a digital caliper to calculate the precise cross-sectional area. For microtensile bond strength testing, three sticks were sectioned from the center of each bonded specimen (Hiraishi et al. 2009). The microtensile bond strength of each bonded specimen was recorded as the average of the three readings (Kim et al. 2015). Sticks with premature bond failure were assigned a null bond strength value and were included in the compilation of the mean bond strength as well as the failure mode assessment (Yip et al. 2001). The bonded composite-dentin sticks were attached to a testing device with cyanoacrylate glue (Zapit, DVA, Corona, CA, USA). The device was attached to a microtensile bond tester (Bisco Inc.) and loaded in tension at a crosshead speed of 0.5 mm/min until failure (Yip et al. 2001).

After fracturing, all specimens were examined under a stereomicroscope (Olympus, Tokyo, Japan) at a magnification of 20× and failure modes were categorized as: (a) adhesive failure along the dentin-resin interface; (b) cohesive failure of the dentin (Cohesive D), and (c) cohesive failure of the resin (Cohesive R) (El Zohairy et al. 2005).

The cross-sectional view of the bonding interface was also examined under a field emission-scanning electron microscope (FE-SEM, JSM-6700F, Jeol, Tokyo, Japan). The specimens were dehydrated through a series of ascending ethanol concentrations (70%, 80%, 95%, three changes in 100%) for 2 hours each, and then left to completely dehydrate in absolute ethanol for an additional 48 hours (Tay et al. 2003). Prior to SEM analysis, the specimens were finally air-dried and sputter-coated with platinum (Hiraishi et al. 2009).

All the data were statistically analyzed by parametric methods at α = 0.05 because they did not meet the homogeneity of variances assumption (Levene’s test). The Kruskal-Wallis test was used to compare the groups, followed by the Mann-Whitney post hoc test, with adjustment of significance levels using the Sidak correction for multiple testing (Yan et al. 2010). All the statistical analyses were performed using SPSS 17.0 for Windows (SPSS Inc, Chicago, IL, USA).

Ⅲ. RESULTS AND DISCUSSION

Figures 1 and 2 show the cross-sectional SEM images of the All-Bond 2 and All-Bond 3-applied groups. According to the photos, the adhesive layers with different thickness, along with the hybrid layer, were well-developed. The formation of the resin tags was also observed.

Figure 1.

Cross-sectional SEM images of the All-Bond 2-applied groups: (a) 50 μm (250×, bar = 100 μm); (b) 100 μm (200×, bar = 100 μm); and (c) 200 μm (2,000×, bar = 10 μm).

Figure 2.

Cross-sectional SEM images of the All-Bond 3-applied groups (2,000×, bar = 10 μm): (a) 50 μm; (b) 100 μm; and (c) 200 μm.

Table 4 shows the microtensile bond strength values, along with the failure pattern analysis results. The influence of adhesive layer thickness on microtensile bond strength to dentin was found to be material-specific. In the case of All-Bond 2, the relatively thick (100 or 200 μm) adhesive layer produced a superior resin bonding to dentin compared with the thin (50 μm) adhesive layer. In contrast, as regard to All-Bond 3, the thickness of the adhesive layer did not significantly affect the microtensile bond strength. These findings may be due to the property or quality of the adhesive layer, rather than that of hybrid layer.

Table 4.

Microtensile bond strength and failure pattern of the six experimental groups

Recent advances in adhesive dentistry have seen a reduction in the application steps of enamel/dentin adhesives to meet the clinical demand for simplified, less technique sensitive bonding procedures (Suh et al. 2003; Kwon et al. 2012). However, the changes in formulation to develop simplified adhesives entails more acidic, and as a result, more hydrophilic adhesives (Tay & Pashley 2003). As the bonding resin of three-step etch and rinse, and two-step self-etch adhesives, do not contain either acidic monomers or solvents, they form a neutral, relatively hydrophobic adhesive layer prior to the placement of the filling (Kwon et al. 2012). On the contrary, two-step etch and rinse, or one-step self-etch adhesives (simplified adhesives), tend to retain a thin, uncured (i.e. oxygen inhibited) acidic, hydrophilic adhesive layer (Rueggeberg & Margeson 1990; Suh 2004).

In this present study, two three-step total-etch dentin adhesives were tested (Table 1). Thus, the adhesive layers for both dentin adhesives were basically hydrophobic. However, All-Bond 2 bonding resin (D/E Resin) contain 30% 2-hydroxyethyl methacrylate (HEMA), which is a small monomer that is in widespread use, not only in dentistry (Van Landuyt et al. 2007). Its popularity in medical applications must be attributed to its relatively good biocompatibility, even though the uncured monomer is notorious for its high allergenic potential. Uncured HEMA presents as a fluid that is well solvable in water, ethanol and/or acetone. Moreover, HEMA has been described to be able to evaporate from the adhesive solutions, though only in very small amounts (Pashley et al. 1998). Another important characteristic of HEMA is its hydrophilicity. Even though this monomer cannot be used as a demineralizing agent, its hydrophilicity makes it an excellent adhesionpromoting monomer. By enhancing wetting of dentin, HEMA significantly improves bond strengths. Nevertheless, both in uncured and cured state, HEMA will readily absorb water. HEMA in the uncured adhesive may absorb water, which can lead to dilution of the monomers to the extent that polymerization is inhibited (Van Landuyt et al. 2007). HEMA fixed in a polymer chain after polymerizing will still exhibit hydrophilic properties and will lead to water uptake with consequent swelling and discoloration (Burrow et al. 1999). Apart from the water uptake, which adversely influences the mechanical strength, high amounts of HEMA will result in flexible polymers with inferior qualities(Van Landuyt et al. 2007).

Therefore, it seems that the hydrophilicity of the All-Bond 2 adhesive layer affected the microtensile bond strength values, depending on the thickness of the adhesive layer (Tay & Pashley 2003). This finding suggests that the thickness of 50 μm did not create a high-quality adhesive layer. On the other hand, the thickness of 200 μm could increase the degree of hydrophilicity of the bonding interface. According the manufacturer, the All-Bond 3 bonding resin, which does not contain HEMA, is more hydrophobic than that of All-Bond 2. Hence, the influence of the adhesive layer thickness of the material was minimal.

Although one reason for difference in the microtensile bond strength in All-Bond 2 may be difference in the monomer conversion of the adhesive layers (Yan et al. 2010), polyHEMA is basically a flexible porous polymer even after polymerization (Van Landuyt et al. 2007). As such, high concentrations of HEMA in an adhesive may have deteriorating effects on the mechanical properties of the resulting polymer. HEMA also lowers the vapor pressure of water, and probably also of alcohol. High amounts may therefore hinder good solvent evaporation from adhesive solutions (Pashley et al. 1998).

Like all methacrylates, HEMA is vulnerable to hydrolysis, especially at basic pH, but also at acidic pH. HEMA is very frequently added to adhesives, not only to ensure good wetting, but also because of its solvent-like nature. This property improves the stability of solutions containing hydrophobic and hydrophilic components and will keep ingredients into solution (Van Landuyt et al. 2005). Although, the incorporation of HEMA into dentin adhesives is sometimes indispensable, the amount should be carefully controlled not to make the materials too hydrophilic (Tay & Pashley 2003). According to a previous study (Kim et al. 2014), dentin surface moisture has a crucial effect on the bond strength of resin materials. This issue was not included in this study; further studies are needed.

Ⅳ. CONCLUSION

The influence of adhesive layer thickness on microtensile bond strength to dentin was found to be material-specific. As for All-Bond 2, the relatively thick adhesive layer produced an enhanced resin bonding to dentin compared with the thin adhesive layer. For All-Bond 3, the thickness of the adhesive layer did not significantly affect the microtensile bond strength. These results seem to have relation with the incorporation of the hydrophilic monomer HEMA into the bonding resins.

Acknowledgments

This research was supported by Kyungpook National University Research Fund, 2012(2013, 2014).

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