다양한 pH를 가지는 Universal adhesives와 이원 중합 레진 세멘트 간의 전단결합강도

INTRODUCTION

Recently, there is an increasing demand for tooth-colored indirect restorations because they offer many advantages such as enhanced mechanical, esthetic, and functional properties when compared with direct restorations. Usually, luting cements are used in the dental procedure for indirect restorations such as inlays, onlays, crowns, endodontic posts (1, 2). There are lots of products for cementation, and it is very common clinical situations using dental adhesive systems and resin cements from different manufacturers together. To prevent adverse reaction between products, it is necessary to have knowledge of bonding agents.

The dental adhesive systems may be classified as etch-and-rinse and self-etch adhesives on the basis of enamel and dentin conditioning with phosphoric acid (3). Depending on the number of procedures, the self-etch adhesive systems can be also divided into two steps and one step systems. These adhesives can also be classified into three categories based on their initial pH-value as mild (pH>2.5), moderate (pH of approximately 1), and strong (pH<1), depending on their acidic monomer (4). Acidic functional monomers play an essential role in the bonding performance of self-etch adhesives, as they may be capable of conditioning enamel and/or dentine substrates. Currently, new and simplified adhesive systems, namely, the universal adhesives or multi-mode adhesives are available in the market (5, 6). These systems can be used with the etch-and-rinse, self-etch, or selective-etch strategies. However, these universal adhesives also contain an acidic functional monomer that affects pH of universal adhesives. Although no adverse reaction will occur with the majority of the clinical situation, some self-etch adhesives are recently found to be incompatible with some resin cements (7-9).

Resin cements are classified as light-cured, chemical-cured, or dual-cured according to the curing mechanism. Dual curing stands for both light curing and chemically curing, and due to limitations of curing depth, many of resin cements are released as dual-cured resin cement. For chemical curing, dual-cured resin cements contain benzoyl peroxide (BPO) as initiator and the tertiary amine catalytic component as a catalyst. The interaction between acidic adhesive monomers and the tertiary amines of the resin cements results in acid-base reactions, leaving unreacted layers (10, 11). This will result in slow or no polymerization, depending upon the acidity and concentration of the acidic resin monomers and decrease bonding performance. To overcome these phenomena, using adhesives with high pH or resin cements without the tertiary amine could be used. Among the universal adhesives that have recently been released, there is All-Bond Universal (Bisco Inc.) that has the pH of 3.2. According to the manufacturer, it is reported that incompatibility does not appear when used with resin cement containing the tertiary amine because the pH is ultra-mild. In the current commercial market, resin cements not containing the tertiary amine such as RelyX ultimate (3M ESPE) and NX3 Nexus^TM (Kerr) have been introduced.

Many previous papers have reported the incompatibility between simplified adhesive and dual-cure resin cement, however there are few reports about incompatibility between universal adhesives and resin cements without the tertiary amine. Therefore, the aim of this study was to assess the bonding performance of three universal adhesives with different pH, with two dual-cured resin cements, containing the tertiary amine or not.

MATERIALS AND METHODS

2. Experimental Design and Specimen Preparation

The teeth were assigned randomly to two groups according to the resin cements (n = 40): Calibra (CB; Dentsply, Konstanz, Germany) which is conventional resin cement, and RelyX Ultimate (RXU; 3M ESPE, St Paul, MN, USA) which is tertiary amine free cement. Each group was subdivided into four groups according to the adhesives (n = 10): All-Bond Universal (ABU; Bisco Inc., Shaumburg IL, USA), Single Bond Universal (SBU; 3M ESPE, St Paul, MN, USA), G-Premio Bond (GP; GC Corp., Tokyo, Japan), and Scotch-bond Multipurpose (SBM; 3M ESPE, St Paul, MN, USA). The adhesives were applied according to the manufacturer’s instructions. The resin cements and adhesives used were listed in Table 1 and Table 2. After that, the samples were fixed in ultradent jig (Ultradent Products Inc., South Jordan, UT), which has a cylinder (diameter 2 mm, height 3 mm) coated with Teflon. Then resin cement was applied in the mold; excessive cement was removed and light cured for 20 seconds using a light-emitting diode (LED) light curing unit under 1200 mW/cm² (BluephaseG2, Ivoclar Vivadent Inc., Amherst, NY, USA).

Table 1.

Adhesives used in the study

Table 2.

Resin cements used in this study

3. Shear Bond Strength (SBS)

After storing the specimen in distilled water at 37℃ for 7 days, the specimens were tested in shear mode, using the shear bond testing machine (Bisco, Schaumburg, IL, USA) at a crosshead speed of 1.0 mm/min until failure. Fig. 1 b showed the diagram of a shear bond test. The SBS was calculated by dividing the load at failure by the bonding surface area.

Figure 1.

Dimensions of dentin-cement specimen and a shear bond strength test

Results

Table 3 presents the results. For RXU groups, there were no significant differences among the adhesive systems (p > 0.05). For CB groups, ABU, and SBM groups showed significantly higher bond strength than did the GP and SBU groups (p < 0.05). Among the adhesives, only GP showed significantly different bond strength between the resin cements.

Table 3.

SBS(MPa) after 7 days’ storage

Table 4 present the results of fracture mode analysis. Adhesive failure was predominant in all groups and there were no significant differences.

Table 4.

Failure mode after 7 days’ storage

Figure 2.

SBS between cements and adhesive

DISCUSSION

When universal adhesives that have a lower pH are used in association with resin materials containing components responsible for chemical curing like the tertiary amine, there is an interaction of residual acidic monomers, in the adhesive inhibition layer, with the tertiary amine, resulting in unploymerized layer (8, 11, 12). This layer can affect the bonding durability.

Results of the present study showed that different adhesives influenced the shear bond strength when used in association with conventional resin cements with the tertiary amine. SBM was used as a control group, because it’s 3-step etch and rinse system and is not affected by resin cements with the tertiary amine. When ABU was used with CB, higher bond strength was observed (p<0.05). In contrast, when CB was used with SBU or GP, lower bond strength was observed. This can be explained by the pH of the ABU (pH 3.2), which is higher than that of other adhesives.

Additionally, the hydrophilicity of the adhesive could affect the compatibility with the dual-cured resin cement. Resin cements are basically hydrophobic after cured. When dental adhesives are light cured, hydrophilic nature in adhesive layers becomes hydrophobic. However, even after light curing, unpolymerized acidic monomers in self-etch or universal adhesive can be remained and residual acidic monomers can make adhesive layer hydrophilic. Therefore, when hydrophobic resin cements are applied to the hydrophilic adhesive layer, the wettability of resin cements decrease (13). In this study, ABU group of mild pH presumed that lower residual acidic monomer remained showed higher bond strength.

In addition to using adhesives with mild pH as a method to reduce the incompatibility between self-etch adhesives and dual-cured resin cements, there have been clinical efforts. Sanares et al. predicted that multistep dentin adhesives, requiring the placement of an additional layer of light-curing resin that does not contain acidic functional groups, might be more compatible with self-curing composites (8). Schittly et al. included the application of an intermediate adhesive resin which bonding resin from Clearfil SE bond after pretreatment with Xeno III (pH 1), and this showed better bond strength than did Xeno III alone (9). In this study, resin cements without the tertiary amine have also have been found reducing incompatibility between adhesives with various pH.

In the present study, when using RXU, there was no significant difference between the adhesives. According to the manufacturer, RXU contains another dual-cure activator. This activator was introduced by Yamauch et al. who reported that bond strengths of adhesive systems containing acidic monomers to dentin may be significantly improved by using a ternary initiator system that consists of sulphinic acid or its salts in addition to the existing peroxide-amine systems (14). These components react with acidic resin monomers to produce either phenyl or benzenesulfonyl free radicals that initiate the polymerization reaction via the self-curing mechanism of the adhesive bonding resin (15). Hence, RXU showed reasonable bond strength, regardless of the adhesive. In failure mode analysis, there were no significant differences between adhesives. However, when using RXU with ABU or SBU, there was a mixed or cohesive failure compared to CB with ABU or SBU. These results were consistent with SBS test.

Based on the results of the present study, the clinician can avoid incompatibility between universal adhesives and dual-cured resin cements by using adhesives with mild pH and cements that are tertiary amine-free. Nevertheless, these findings should be interpreted with caution because adhesives and cements used in the present study contain different components that can independently affect the bond strength. Additionally, the long-term performance of the bonded interfaces needs to be evaluated further.

CONCLUSION

Within the limitations of this study, RXU resin cement was compatible with universal adhesives regardless of pH of the adhesives. ABU was compatible with CB resin cement.

Therefore, in dental practice, amine free dual-cured resin can be used with universal adhesives with various pH. And adhesives with mild pH can be used with used with dual-cured resin cements with the tertiary amine.

Acknowledgments

This study was supported by Clinical Research Grant, Pusan National University Dental Hospital (2016).

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