The Effect of Silane Treatment of a Resin-Based Composite on Its Microtensile Bond Strength to a Ceramic Restorative Material

Abstract

The purpose of this study was to investigate the effect of silane treatment of a resin-coated resin-based composite (RBC) base material on its microtensile bond strength (μTBS) to a computer aided-design/computer-aided manufacturing (CAD/CAM) ceramic restorative material. RBC blocks (4 mm × 7 mm × 10 mm) were prepared, and the adherend surfaces were prepared as follows: no resin coating + no silane treatment (Group I), no resin coating + silane treatment (Group II), resin coating only (Group III), and resin coating + silane treatment (Group IV). The resin coating was applied with Clearfil SE Bond and flowable RBC to the adherend surface. Each treated sample was bonded to a ceramic block using Panavia V5. After storage in 37 °C water for 1 week, microspecimens were fabricated, and the μTBS was tested. The failure mode of each specimen was determined using stereomicroscopy and scanning electron microscopy. The µTBS of Group II (20.2 ± 4.0 MPa) was not significantly different from that of Group I (17.6 ± 5.9 MPa) (p > 0.05), and the µTBS of Group I was significantly smaller than that of Group III (21.3 ± 7.2 MPa) (p < 0.05). The µTBS of Group IV (24.9 ± 3.8 MPa) was significantly greater than that of Group III (p < 0.05). Our findings indicate that silane treatment of the resin-coated surface can enhance the µTBS. Silane treatment of RBC base material was not effective when prepared without resin coating.

1. Introduction

Direct restorations using resin-based composites (RBC) are now the first choice for carious molars. This method only removes the caries-infected tooth substrate, allowing the caries-affected dentin to remain [1]. Calcium phosphate crystals, such as brucite and whitlockite, are deposited in the tubules of caries-affected dentin, thus blocking external stimuli and reducing damage to the dentin–pulp complex [2].

By contrast, indirect restorations, such as inlay and onlay restorations, are selected to restore large defects caused by caries removal. Recent computer-aided design/computer-aided manufacturing (CAD/CAM) technology has made it possible to perform single-appointment restorations of indirect restorations as well as direct RBC restorations [3]. However, because indirect restoration requires cavity preparation with a resistance form, it is accompanied by the removal of caries-affected dentin and healthy dentin [4]. Deep and large defects can often be filled with a base material, thereby reinforcing the thin and fragile tooth structure [5]. In particular, RBC base materials have higher compressive strength than mineral trioxide aggregate (MTA)-based base materials, and thus can be expected to have a higher reinforcing effect [6].

Resin coating or immediate dentin sealing can protect the dentin–pulp complex on the exposed dentin surface immediately after cavity formation [7,8,9]. Furthermore, studies have reported that the bond strength of resin-based luting cement to resin-coated dentin is higher than that to dentin without a resin coating [10,11,12,13]. The adhesion of composite materials to crown restorations has been severally studied, and almost all of these studies have focused on the adhesion between restorative material and luting cement. These results have suggested that silane treatment of the surface of silica-based restorative materials is effective for enhancing bond strength to silica-based indirect restorations [14,15]. In clinical practice, the surfaces of cavities reinforced with a base material contain a mixture of exposed dentin and base material, and it is not possible to apply a resin coating only to the exposed dentin surfaces. Therefore, the resin coating is applied to the entire surface of the cavity. The effect of resin coating on the bond strength to luting cement has rarely been reported [6]. The effect of pretreatment with a silane coupling agent on the surface of RBC base material is also unknown.

The purpose of this study was to investigate the effect of silane treatment of a resin-coated RBC base material on its microtensile bond strength (μTBS) to a CAD/CAM ceramic restorative material. The null hypothesis of this study was that there is no effect of silane pre-treatment, regardless of whether the RBC was resin-coated.

2. Materials and Methods

2.1. Materials Used in This Study

The materials and their respective manufacturers, compositions, and batch numbers are described in

Table 1. Details of the materials used in this study.

Brand Name	Manufacturer	Composition *	Lot No.
[Base material]
Clearfil® Majesty® ES Flow (Low) (Shade A3)	Kuraray Noritake Dental, Tokyo, Japan	Barium glass, silica filler, TEGDMA, methacrylic acid monomer, photopolymerization catalysts, stabilizers, colorants, etc.	0003AA
[Resin-coating material]
Clearfil® SE Bond **	Kuraray Noritake Dental, Tokyo, Japan	Primer: HEMA, 10-MDP, hydrophilic dimethacrylate, water, photoinitiator Bond: Bis-GMA, 10-MDP, HEMA, hydrophobic dimethacrylate, silanized colloidal silica, photoinitiator	000077
Clearfil® Majesty® ES Flow (High) Shade A3	Kuraray Noritake Dental, Tokyo, Japan	Barium glass, silica filler, TEGDMA, methacrylic acid monomer, photopolymerization catalysts, stabilizers, colorants, etc.	940030
[Silane coupling agent]
Clearfil® Porcelain Bond Activator	Kuraray Noritake Dental, Tokyo, Japan	3-MPS	260010
[Ceramic primer]
Clearfil® Ceramic Primer Plus	Kuraray Noritake Dental, Tokyo, Japan	3-MPS, 10-MDP, ethanol	7N0005
[Luting cement]
Panavia® V5	Kuraray Noritake Dental, Tokyo, Japan	Tooth Primer: 10-MDP, original multifunctional monomer, new polymerization accelerator, HEMA, water, stabilizer Paste: Bis-GMA, TEGDMA, aromatic multifunctional monomer, aliphatic multifunctional monomer, new chemical polymerization accelerator, dl-camphorquinone, photopolymerization accelerator, surface treated barium glass, fine particulate filler	7M005 (Tooth Primer) 720007 (Paste)
[CAD/CAM ceramic block]
IPS Empress CAD for CEREC and InLab (HT/A2 I12)	Ivoclar Vivadent, Schaan, Liechtenstein	Leucite-reinforced CAD/CAM glass ceramic (Silicon dioxide, aluminum oxide, potassium oxide, sodium oxide, pigments)	S28661

* Bis-GMA, 2,2-bis[4-(2-hydroxy-3-methacryloyloxypropoxy)phenyl]propane; HEMA, 2-hydroxyethylmethacrylate; TEGDMA, triethylene glycol dimethacrylate; 3-MPS, 3-(methacryloyloxy)propyltrimethoxysilane; 10-MDP, 10-methacryloyloxydecyl dihydrogen phosphate. ** Also known as “Clearfil Megabond” in Japan.

. Clearfil^® Majesty ES Flow (Low) (Kuraray Noritake Dental, Tokyo, Japan) was used as the base material. A combination of Clearfil^® SE Bond (Kuraray Noritake Dental, Tokyo, Japan) and Clearfil^® Majesty ES Flow (High) (Kuraray Noritake Dental, Tokyo, Japan) was used as the resin coating material. Panavia^® V5 (Kuraray Noritake Dental, Tokyo, Japan) was used as the luting agent. Prior to the application of Panavia V5, Panavia V5 Tooth Primer (Kuraray Noritake Dental, Tokyo, Japan) and/or a mixture of Clearfil SE Bond Primer and Clearfil^® Porcelain Bond Activator (Kuraray Noritake Dental, Tokyo, Japan) was used for pre-treatment.

2.2. Preparation of Specimen Blocks from the RBC Base Material

A schematic illustration of the preparation method for specimen blocks is shown in Figure 1. Specimen blocks were prepared according to previous studies [16]. Silicone molds (4 mm × 7 mm × 10 mm) were placed on a slide glass (Matsunami Glass Industries, Kishiwada, Japan) and filled with the RBC base material (Clearfil Majesty ES Flow Low, Kuraray Noritake Dental, Tokyo, Japan). They were then light-cured using a light emitting diode (LED) light-curing unit (G-Light Prima, GC, Tokyo, Japan) from both above and below for 20 s each. A total of eight blocks were prepared. The light intensity was controlled using a hand-held dental radiometer (Bluephase^® Meter II; Ivoclar Vivadent, Schaan, Liechtenstein) to ensure that the light output was at least 1500 mW/cm² [17]. Only one side (4 mm × 7 mm) of the block was abraded using 600-grit SiC paper, and two blocks were randomly assigned into the following four experimental groups.

Groups I and II (No coating): An RBC block was inserted in a 4 mm × 7 mm × 20 mm silicone mold, and the remaining space was filled with a hydraulic temporary sealing material (Caviton EX pink, GC). After storage in water at 37 °C for 1 week, the RBC block was removed from the mold. The hydraulic temporary sealing material adhered to the surface of the RBC block was then rinsed with tap water for 10 s and dried thoroughly.

Groups III and IV (Resin-coated): The adhered surface of the RBC block was pre-treated using Clearfil SE Bond Primer for 20 s, sufficiently dried, coated with the adhesive agent Clearfil SE Bond, gently air-dried, and finally light-cured for 20 s. The surface was further coated using Clearfil Majesty ES Flow High and light-cured again for 20 s. The unpolymerized layer of the coated surface was removed using an alcohol cotton pellet. The coated blocks were stored in the same manner as the blocks in Groups I and II.

2.3. Preparation of Specimens from the CAD/CAM Block

Leucite-reinforced ceramic blocks (IPS Empress CAD, HT/A2, I12, Ivoclar Vivadent, Liechtenstein) were cut to the same size as the composite resin blocks using a low-speed water-cooled diamond saw (Isomet, Buehler, Lake Bluff, IL, USA) to yield eight blocks. The cut surface (4 mm × 7 mm) was ground using 600-grit SiC paper to prepare a uniform and flat surface. The abraded surfaces were then subjected to alumina airborne particle abrasion with 50 µm particles (JET BLAST II, J. Morita, Kyoto, Japan) for 5 s at a pressure of 0.2 MPa and a perpendicular distance of 10 mm from the target surface. The blocks were then cleaned using 37% phosphoric acid (K-Etchant GEL, Kuraray Noritake Dental, Tokyo, Japan) for 10 s, thoroughly rinsed with water spray, and dried with compressed air. A ceramic primer (Clearfil Ceramic Primer Plus, Kuraray Noritake Dental, Tokyo, Japan) was applied to the surface and thinned with compressed air (Figure 2).

2.4. Bonding Procedures

A self-etching primer (Panavia V5 Tooth Primer, Kuraray Noritake Dental, Tokyo, Japan) was applied to the RBC blocks and air-dried for 20 s. In Group II and Group IV, a mixture of Clearfil SE Bond Primer and Clearfil Porcelain Bond Activator was applied and then air-dried (

Table 2. Surface treatment for samples in each group.

	Resin Coating	Self-Etching Primer	Silane
Group I (Control)	−	+	−
Group II	−	+	+
Group III	+	+	−
Group IV	+	+	+

). The specimen surfaces were bonded to the ceramic block using Panavia V5 and light-cured from the four proximal sides for 20 s each (total curing time of 80 s). They were then pressed for 3 min using finger pressure and then left at room temperature for 15 min. All bonding procedures were performed by one operator (A.H.).

2.5. µTBS Testing

After storage in water at 37 °C for 1 week, specimens were serially sectioned perpendicularly to the adhered surface using a low-speed water-cooled diamond saw (Isomet) to obtain rectangular 1 mm × 1 mm stick-shaped microspecimens. The microspecimens were attached to a microtensile jig using cyanoacrylate glue (Model Repair II Blue, Dentsply-Sankin, Ohtawara, Japan) and subjected to a tensile load using a microtensile tester (Bisco, Schaumburg, IL, USA) at 1.0 mm/min crosshead speed until failure. The width and thickness of each specimen were measured to the nearest 0.01 mm using a digital caliper (CD-15 CPX, Mitutoyo, Kawasaki, Japan) to determine the µTBS (MPa) by dividing the recorded force (N) at the time of fracture by the bond area (mm²) (Figure 3). The µTBS of each specimen was calculated as

µTBS = F/A,

(1)

where F is the tensile load force at failure (N) and A is the cross-sectional area of the bonded specimen (mm²). The µTBS (N/mm²) was expressed in MPa.

2.6. Statistical Analysis

The adequate minimum sample size for µTBS testing was calculated as follows:

n = 2{(Z_α/2 + Z_β)² × σ²}/Δ²,

(2)

where n is the minimum sample size, α is the probability of falsely rejecting a true null hypothesis (α = 0.05, Z_α/2 = 1.96), β is the probability of failing to reject a false null hypothesis (β = 0.20, Z_β = 0.84), σ is the standard deviation, and Δ is the difference among groups. According to the calculation, at least 16 specimens were needed in each group.

The data from the bond strength test were subjected to a two-way analysis of variance (p < 0.05) for the two factors “resin-coated” and “silane-treated”. The critical value was 0.05. All statistical analyses were carried out using SPSS statistical software (IBM SPSS 18; SPSS Inc., Chicago, IL, USA).

2.7. Analysis of the Fracture Surface

The failure mode was determined at a 50× magnification with a stereomicroscope (ZEISS Stemi 508, Zeiss, Oberkochen, Germany). After stereomicroscopic observation, the representative specimens in each group were selected for scanning electron microscopy (SEM). After being mounted on stubs, the specimens were Au-Pd-coated using a cooled sputter coater (MSP-20-UM Automatic Magnetron Sputter, Vacuum Device, Mito, Japan). The coated specimens were examined using SEM (SU6600, Hitachi, Tokyo, Japan) at 15 kV at 80× magnification. The fracture patterns were classified according to the following categories (Figure 4):

Pattern A: Cohesive failure in the ceramic.

Pattern B: Mixed failure in the ceramic and in the luting cement.

Pattern C: Mixed failure at the adhesive interface between the ceramic and luting cement, and within the luting cement.

Pattern D: Cohesive failure in the luting cement.

Pattern E: Mixed failure within the luting cement (or coating material) and at the RBC interface.

Pattern F: Failure at the interface between the luting cement and coating material.

Pattern G: Cohesive failure of the luting cement (or coating material) and the RBC.

Pattern H: Failure at the interface between the coating material and RBC.

Pattern I: Cohesive failure within the RBC.

3. Results

3.1. µTBS

The µTBS results for each group are shown in

Table 3. Microtensile bond strength (MPa) and failure mode analysis.

Group	Resin Coating	Silane	µTBS * Mean (S.D.)		A	B	C	D	E	F	G	H	I	Total
I	−	−	17.6 (5.9) B	NS	0	0	20	6	2	0	0	0	0	28
II	−	+	20.2 (4.0) b		0	0	14	5	0	0	1	0	0	20
III	+	−	21.3 (7.2) A	S	0	0	6	13	7	2	0	0	0	28
IV	+	+	24.9 (3.8) a		0	0	4	15	4	0	1	0	0	24

* The uppercase letters (‘A’ and ‘B’) indicate the absence of a significant difference with and without silane groups (p > 0.05); the lowercase letters (‘a’ and ‘b’) indicate the absence of a significant difference between silane-treated groups (p > 0.05).

. The results of two-way ANOVA showed a significant difference between the µTBS in the presence and absence of the resin coating (p = 0.028) and presence and absence of silane treatment (p = 0.001). There was no interaction between the two factors (p = 0.160). Therefore, comparisons were made using the Student’s t-test with and without the resin coating and with and without silane treatment. The bond strength of Group I (17.6 ± 5.9 MPa) was significantly lower than that of Group III (21.3 ± 7.2 MPa). The bond strength of Group IV (24.9 ± 3.8 MPa) was significantly greater than that of Group III (21.3 ± 7.2 MPa).

3.2. Failure Modes

Table 3 shows the failure modes, and Figure 5 shows a typical example of a failure surface after a bonding test. In Group I and Group II, failure was most frequently observed at the interface between the ceramic and luting cement, and within the luting cement. In Group III and Group IV, cohesive failure in the luting cement was the most common mode of failure.

4. Discussion

The purpose of this study was to investigate the effect of silane treatment of a resin-coated RBC base material on its microtensile bond strength to a CAD/CAM ceramic restorative material. To achieve this objective, we evaluated the effect of silane treatment on RBCs with and without a resin coating.

We used a combination of Clearfil SE Bond and Clearfil Majesty ES Flow for the resin coating material [18]. Clearfil SE Bond is the “gold standard” two-step, self-etch, filled adhesive [19], which has very high bonding performance on both enamel and dentin [20,21]. The mechanical properties of the adhesive itself are also good because of its filler loading (~10%) [22]. De Carvalho et al. [23] reported that an additional coating with flowable RBCs can significantly improve the microtensile bond strength compared to immediate dentin sealing with a resin adhesive system alone.

When the cavity is reinforced with a base material, the cavity surface is composed of a mixture of exposed dentin and RBC. In clinical situations, it is difficult to apply a resin coating only to the surface of the exposed dentin, and thus the resin coating is applied to the entire cavity wall, including the surface of the base material. The results of this study indicated that resin coating to the RBC base material enhanced the µTBS. Therefore, the application of the resin-coating technique to an RBC base material is effective, as it is the dentin surface [24].

Regarding the failure mode after µTBS testing, almost all specimens in Group I (without resin coating) showed Pattern C (mixed failure at the adhesive interface between the ceramic and luting cement, and within the luting cement). The results indicated that the adhesion between the surface of the base material and the luting cement is good, even if resin coating is not applied to the RBC base material. In addition, almost all specimens in the resin-coated group (Group III) showed Pattern D (cohesive failure in the luting cement), suggesting that the resin coating did not adversely affect the adhesion between the surface of the base material and the luting cement.

The Porcelain Bond Activator used in this study is a silane coupling agent, whose main component is 3-methacryloxypropyltrimethoxysilane (3-MPS). Silane coupling agents have an organofunctional terminal group for bonding to organic resins, and a hydrolysable alkoxyl group for bonding to inorganic elements, as well as a hydrocarbon chain spacer. The hydrolysable alkoxyl group is activated by an acid to form silanol. Silanol is attached to the hydroxy group on the surface of an inorganic substrate, producing a strong bond called a polysiloxane bond (Si–O–Si)_n [15,25,26]. Furthermore, the acidic functional monomer 10-MDP contained in Clearfil SE Bond Primer might accelerate the hydrolysis of the silane coupling agent to form silanol groups (Si–OH), and efficiently bind to the inorganic filler by forming a siloxane structure [27].

In our previous study [16], we used a bulk-fill flowable RBC (Bulk Base, Sun Medical, Moriyama, Japan) as the RBC base material, a 1-bottle thin-film coating material (Hybrid Coat II, Sun Medical) as the resin coating material, and a 4-META/MMA-TBB resin (Super Bond, Sun Medical) as the luting cement. However, we were unable to determine the effectiveness of silane treatment on the resin-coated surfaces because Hybrid Coat II is an unfilled resin coating material.

Our results revealed the effectiveness of silane treatment on resin-coated surfaces. Therefore, the null hypothesis in this study was rejected. Our results agreed with those of Staxrud et al. and Çakir et al., who reported a significantly higher bond strength for the repair of RBCs when silane was used in combination with Clearfil SE Bond beforehand than when Clearfil SE Bond was used alone [28,29]. The resin coating material used in this study (Clearfil Majesty ES Flow High) contained barium glass and silica as fillers. According to the manufacturer’s report, the filler loading of the material was 71 wt%. Therefore, silane treatment prior to resin coating is theoretically effective for bonding resin-based materials to the surface of cured composite resins [30]. We have previously investigated the effect of additional silane treatment on the µTBS between RBC post-and-core build-up material and an RBC CAD/CAM block. The findings indicated that additional silane treatment prior to the application of adhesive did not enhance µTBS [31]. Therefore, the results of this study were similar to those of our previous study.

This study showed that the resin-coated surfaces using Clearfil SE Bond and Clearfil Majesty ES Flow should be treated with silane before the luting of indirect restorations. On the other hand, the effectiveness of silane treatment was also dependent on the material used for resin coating, as discussed in our previous study. Therefore, in the clinical situation, treatment of the resin-coated surface should be performed based on the component of the resin coating material.

The results of this study were obtained over a short period of time; therefore, adhesion durability, including thermal cycling, needs to be investigated.

5. Conclusions

Considering the limitations of the current in vitro study, we did not find the contribution of silane treatment to improve the bond strength of the composite resin cement to the resin-based composite base material; however, the bond strength was significantly improved by silane treatment of the resin-coated surface. Therefore, in a clinical situation, treatment of the resin-coated surface should be tailored to the composition of the resin coating material.