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Recently developed adhesives tend to simplify bonding procedures by reducing the number of application steps required and decreasing technique sensitivity. Overall, these properties help to improve bonding consistency (Van Meerbeek et al., 2003). Additionally, “all-in-one” or “one-step self-etch adhesives” combine all of the application steps into a single step. These simplified one-step adhesives show promising results of immediate strong bonds; however, they have low long-term bonding durability (Van Meerbeek et al., 2011; De Munck et al., 2012).

In recently, a one-step multi-mode adhesive system has been introduced in the dental market. This adhesive system should not be confused with the seventh generation “all-in-one” system because it can be applied using either the etch-and-rinse, self-etch, or selective-etch mode of application (Breschi et al., 2008). This adhesive system also differs from the current self-etch systems because of the incorporation of functional monomers. This universal adhesive contains 10-methacryloyoxydecyl dihydrogen phosphate (10-MDP), which previous studies have confirmed as the best acidic functional monomer for producing a stable and durable interaction with the hydroxyapatite in dentin (Alex, 2015; Fujita et al., 2012; Iwai & Nishiyama, 2012; Van Landuyt et al., 2008). This amphiphilic functional monomer is capable of forming chemical bonds with composite and tooth substrates, and exhibits decreased water sorption and hydrolytic breakdown. As such, this monomer has a high durability (Breschi et al., 2008).

At present, some studies have reported the efficacy and durability of universal adhesives when applied in etch-and-rinse mode or self-etch mode. The influence of additional acid etching on dentin bond strength in etch-and-rinse mode has been controversial. Several authors have reported similar adhesive performance regardless of the modes of application (Muñoz et al., 2013; Perdigao et al., 2012), while one study reported that etch-and-rinse mode enhanced dentin bond strength (Lee et al., 2013). Using 10% sodium hypochlorite (NaOCl), we evaluated whether a universal adhesive could form a stable hybrid layer. It is known that NaOCl attacks the unhybrid collagen fibrils within the interface layer, which makes it possible to indirectly estimate the bonding durability and quality within a short time frame.

The purpose of this study was to evaluate the shear bond strength (SBS) of two universal adhesives using different application modes (etch-and-rinse or self-etch). In addition, the comparisons described above were also evaluated after aging with NaOCl. The null hypotheses tested were as follows: (1) The SBS of universal adhesives are not affected by application mode (etch-and-rinse and self-etch) and (2) after aging, the SBS of universal adhesives does not differ from that of control regardless of application mode (either etch-and-rinse or self-etch mode).

The results of the SBS tests are shown in Table 2. At 24 hours, the mean SBS of adhesive to dentin in etch-and-rinse mode and self-etch mode ranged from 11.62±3.68 to 19.38±3.57 MPa and 11.17±4.12 to 16.29±3.00 MPa, respectively. There were no significant differences between the application modes of the universal adhesives. At 24 hours, SM in etch-and-rinse mode showed a significantly higher SBS value than the universal adhesives (p<0.05). AU in self-etch mode showed a significantly lower SBS value compared with the other adhesives (p<0.05). After aging, the mean SBS of adhesives to dentin in etch-and-rinse mode ranged from 6.31±2.57 to 8.33±2.65 MPa, while in self-etch mode ranged from 7.22±2.77 to 8.70±2.01 MPa. Notably, there was no significant difference among the groups. The SBSs of all adhesives were significantly reduced after aging (p<0.05).

The failure modes of specimens are presented in Fig. 1. The predominant mode of failure for specimens was mixed failure, except for AU in etch-and-rinse mode after aging, which showed the highest rate of adhesive failure (53%). At 24 hours, only AU exhibited adhesive failure. After aging, adhesive failure was seen more frequently in etch-and-rinse mode versus self-etch mode for universal adhesives. The chi-square test (Fisher exact test) did not show significant difference at 24 hours (p=0.560) but after aging showed significant difference in the failure modes (p=0.042) between the application mode

Figure 1. 
Failure modes of adhesives after SBS test.

Representative SEM images of the dentin/resin interfaces from universal adhesives are shown in Fig. 2. The restorative-dentin interface for all the adhesives showed good adaptation to the dentin after aging regardless of etching mode. In both adhesives, the resin tags were longer and denser in etch-and-rinse mode than in self-etch mode. The adhesive layer of SU was thicker in both modes compared with AU.

Figure 2. 
Representative SEM images of aged resin/dentin interfaces created by All-Bond Universal (AU) in etch-and-rinse mode (a) or self-etch mode (b) and Scotchbond Universal (SU) in etch-and-rinse mode (c) or self-etch mode (d). Adhesive layers (A) of SU were thicker than that of AU. In both universal adhesives with etch-and-rinse mode (a, c), denser resin tags (white arrowhead) were observed. C = composite; A = adhesive layer; D = dentin.

Value with the same capital letters in columns indicate no significant difference between adhesives (p>0.05). Value with different small letters in rows indicate a significant difference between etch mode within a universal adhesive (p<0.05).

SM, Scotchbond Multipurpose; SUER, Scotchbond Universal with etch-and-rinse mode; AUER, All-bond Universal with etch-and-rinse mode; CS, Clearfil SE bond; SUSE, Scotchbond Universal with self-etch mode; AUSE, All-bond Universal with self-etch mode.

Previously, self-etch adhesives were not recommended for use in etch-and-rinse mode. Both thickness and resin tag formation of the hybrid layer were improved when phosphoric acid was used prior to the application of a self-etch adhesive. However, bond strength decreased significantly (Van Landuyt et al., 2006; Ikeda et al., 2008; Van Landuyt et al., 2006; Torii et al., 2002). Universal adhesives are designed to bond to dentin via etch-and-rinse mode or self-etch mode using the same single bottle of adhesive. However, universal adhesive was newly introduced to the market, and there is limited information as to whether the different etching modes achieve equivalent bonding performance to dentin. Chen et al. reported that universal adhesives appear to have solved the problem of bonding incompatibility by blending less acidic resin monomers in appropriately reduced concentrations with other resin monomers (Chen et al., 2015). Also, Wagner et al. reported that the addition of a low viscosity monomer, such as HEMA, overcame this incompatibility (Wagner et al., 2014). In the present study, the application modes did not show significantly different SBS in universal adhesives regardless of aging. In each adhesive, the complexes of composition might lead to acceptable multifunctional bonding performance as they have their own characteristics. Therefore, the first null hypothesis can be accepted.

At 24 hours, AU showed lower SBS than SU and CS in self-etch mode. It is thought that AU is less acidic than other adhesives, with a known pH of 3.2. This lower SBS result could be explained by the single application method of universal adhesive in this study, even though the manufacturer of AU recommends the application of two separate coats of adhesive with agitation.

Following aging, there was no significant difference among the groups in the etch-and-rinse groups or self-etch groups. Based on these results, the second null-hypothesis can be accepted, and it can be concluded that the durability of a universal bond is similar between the three-step etch-and-rinse adhesive and 2-step self-etch adhesive systems. Muñoz et al. reported that universal adhesives containing 10-MDP exhibited higher and more stable micro-tensile bond strength (Muñoz et al., 2013). The universal adhesives used in this study also contain 10-MDP, which make it possible for chemical bonds to form between the adhesives and teeth through non-soluble calcium salts. These MDP-calcium salts are deposited in self-assembled nano-layers and are resistant to biodegradation (saboia et al., 2009). In addition, 10-MDP is the most hydrophobic of all the functional monomers (Suh, 2013). As such, 10-MDP may be important for maintaining durability during water sorption and hydrolytic breakdown of the adhesive interface. A previous one-step self-etch adhesive showed acceptable results of immediate bond strength; however, reduced bond strength was observed after long-term aging (Van Meerbeek et al., 2011; De Munck et al., 2012). The lack of hydrophobic bonding resin in one-step self-etch adhesive leads to hydrolytic degradation caused by a semi-permeable membrane (Fukegawa et al., 2006; Yoshida et al., 2012). Because of 10-MDP and a hydrophobic adhesive layer, the universal adhesives used in this study might show similar bond strengths to controls after aging.

In failure mode, mixed failure was seen most frequently in all groups in the present study, except for AU in etch-and-rinse mode (aging group). Adhesive failure was commonly observed with AU in etch-and-rinse mode (aging group). The reason for these results may be because AU contains more solvent than SU (30~60 wt% and 10–15 wt%, respectively), which can lead to more residual solvent being retained in the hybrid and adhesive layers (Tay & Pashley, 2003; Tay et al., 2002; Hashimoto et al., 2000). Consequently, the adhesive interface is more permeable after polymerization and more prone to degradation over time (De Munck et al., 2012; 3M ESPE 2013; Chen & Suh, 2013; Yiu et al., 2005). These results could also be explained by the single application method used with universal adhesives to standardize their application. When a second layer of the adhesive was applied, thicker and more resistant adhesive interfaces were observed (Malacarne et al., 2006).

Scotchbond Universal contains polyalkenoic acid copolymer (PAC), which chemically bonds to the hydroxyapatite in dentin (Mitra et al., 2009). Adhesives containing PAC exhibit higher bond strengths than PAC-free adhesives with the same composition (Ito et al., 2005; De Munck et al., 2005). Muñoz et al. reported that the PAC plays a more important role for etch-and-rinse mode than for self-etch mode (Muñoz et al., 2015). This study also found a lower percentage of adhesive failure for SU compared with AU in etch-and-rinse mode.

To use to multiple modes, the composition of universal adhesives is more complicated than that of the previous generations. It appears that the various components of universal adhesives may lead to various results of SBS. Therefore, clinicians should use the adhesives as per manufacturers’ instructions.

After aging in NaOCl, there was a higher percentage of adhesive failure in universal adhesives in etch-and-rinse mode than in the self-etch mode (43.3% and 16.6%, respectively). Based on these results, the hybrid layer formed using the self-etch technique appears to be more stable against aging in NaOCl. In the present study, application of the etch-and-rinse mode showed increased resin tag formation. This may result in increased adhesive failure of the bonding interfaces, which can subsequently lead to incomplete infiltration of the demineralized collagen network by the bonding resin (Perdigao et al., 2014).

According to Saboia et al., the superoxide radical in 10% NaOCl causes oxidation in unprotected collagen fibrils, which affects bond integrity (Saboia et al., 2009). In their study, the application of 10% NaOCl for three hours led to an accelerated rate of collagen destruction. Furthermore, aging in NaOCl was responsible for only part of the degradation process of the adhesive interface and did not have an effect on degradation of the resin compartment. In the present study, aging in NaOCl for three hours significantly reduced the SBS in all adhesive groups. This result showed that aging in NaOCl affects both the bonding integrity and degradation process of the adhesive interface. However, there is no evidence to indicate that aging in NaOCl is sufficient to activate matrix-bound matrix metalloproteinases and cysteine cathepsins within the demineralized collagen matrix. In this study, we estimated the quality of the hybrid layer using the NaOCl that attacks the unprotected collagen fibrils (in this hybrid layer) within a short time frame. Although aging in NaOCl is a rapid and reliable in vitro aging method for examining the durability of the adhesive interface, it cannot replace long-term aging evaluation (Van Meerbeek et al., 1996).

The clinical implication of this study is that universal adhesives have good bonding performance regardless of the mode of application. Furthermore, these adhesives exhibit a similar quality of bonding with dentin (i.e., bonding durability) compared to conventional multi-step adhesives.

Within the limitation of this in vitro study, it can be concluded that two universal adhesives can be used in both etch-and-rinse and self-etch mode without a substantial difference in bond strength to dentin. Following aging, two universal adhesives exhibited similar bonding performance to controls. Despite improved durability with universal adhesives, in vivo, longevity studies are required to validate these results.