INTRODUCTION

Direct resin composite is commonly used as a restorative material in daily dental practice because of its clinical advantages, such as adhesion to tooth structures, ease of manipulation, superior mechanical properties, excellent esthetics and placement in more conservative tooth preparations. Consistent with the philosophy of minimally invasive dentistry, when a resin composite restoration exhibits secondary caries or a marginal defect, repair of the restoration is recommended rather than total replacement with complete removal1). Reparing resin composite restorations can prevent further loss of sound tooth structure2,3) and can result in lower costs and less chair-side time4). With most composite restoration repairs, the bonding substrates are usually composed of enamel, dentin and resin composite.

In order to make the bonding procedures quicker and easier5), one-step self-etching adhesives (1-SEAs) have been introduced6). The 1-SEA contains hydrophobic components and water with a relatively high concentration of solvent in order to maintain them in one solution7). Hydroxyethylmethacrylate, HEMA, a hydrophilic monomer, is also advisable for maintaining the resin monomers and water in one solution8). However, because of its hydrophilic property, high water

sorption of HEMA-containing 1-SEA causes a reduction in the mechanical properties of the resin polymers rendering the adhesive prone to hydrolytic degradation9). The water sorption in 1-SEAs is dependent upon their HEMA concentration10). Therefore, HEMA-free 1-SEAs have been introduced with reduced water sorption and improved stability of their mechanical properties over a longer period of time11). On the other hand, absence of HEMA from 1-SEAs causes phase separation of the hydrophilic and hydrophobic components, leading to droplet formation in the adhesive layer7,12).

The application of a silane coupling agent has been recommended for repairing composite resin restorations because the silane coupling agent can improve adhesion to resin composite by interaction with silica filler particles in the resin composite13,14). However, it is unclear how bonding performance to enamel and dentin might be affected when a silane coupling agent is added to a 1-SEA. Recently, universal 1-SEAs have been developed, which can be used to on various substrates including enamel, dentin, glass ceramic, zirconia, noble and non-precious alloys, and resin composites without the addition of any extra primer15). Most of the related bonding studies have been performed to separately-prepared substrates of enamel, dentin or restorative materials16-19), however there have been few studies published to date on their bonding performance to each susbtrate when total-bonding to an enamel-dentin-resin composite substrate.

Initial and long-term bond strengths of one-step self-etch adhesives with silane coupling agent to enamel-dentin-composite in combined situationTeerapong MAMANEE1, Masahiro TAKAHASHI1*, Masatoshi NAKAJIMA1*, Richard M. FOXTON2 and Junji TAGAMI1,3

1 Cariology and Operative Dentistry, Department of Oral Health Sciences, Graduate School of Medical and Dental Sciences, Tokyo Medical and Dental University, 1-5-45 Yushima, Bunkyo-ku, Tokyo 113-8549, Japan

2 Division of Conservative Dentistry, King’s College London Dental Institute at Guy’s, King’s and St Thomas’ Hospitals, King’s College London, Floor 25, London Bridge, London, SE1-9RT, UK

3 Global Center of Excellence Program, International Research Center for Molecular Science in Tooth and Bone Diseases, Tokyo Medical and Dental University, 1-5-45 Yushima, Bunkyo-ku, Tokyo 113-8549, Japan

Corresponding author, Masatoshi NAKAJIMA; E-mail: nakajima.ope@tmd.ac.jp, Masahiro TAKAHASHI; E-mail: takahashi.ope@gmail.com

This study evaluated the effect of adding silane coupling agent on initial and long-term bond strengths of one-step self-etch adhesives to enamel-dentin-composite in combined situation. Cervical cavities were prepared on extracted molars and filled with Clearfil AP-X. After water-storage for one-week, the filled teeth were sectioned in halves to expose enamel, dentin and composite surfaces and then enamel-dentin-composite surface was totally applied with one of adhesive treatments (Clearfil SE One, Clearfil SE One with Clearfil Porcelain Bond Activator, Beautibond Multi, Beautibond Multi with Beautibond Multi PR Plus and Scotchbond Universal). After designed period, micro-shear bond strengths (µSBSs) to each substrate were determined. For each period of water-storage, additive silane treatments significantly increased µSBS to composite (p<0.001). On the other hand, they significantly decreased µSBS to dentin (p<0.001), although did not have adverse effect on µSBS to enamel (p>0.05). Moreover, the stability of µSBS was depended on materials and substrates used.

Keywords: Micro-shear bond strength, Bond durability, Silane coupling agent, Combined substrate

*Authors who contributed equally to this work.Color figures can be viewed in the online issue, which is avail-able at J-STAGE.Received Feb 6, 2015: Accepted May 2, 2015doi:10.4012/dmj.2015-050 JOI JST.JSTAGE/dmj/2015-050

Dental Materials Journal 2015; 34(5): 663–670

Table 1 Compositions, lot no. and application of the materials used in this study and chemical abbreviations

Materials Lot No. Composition Application

Clearfil SE One(Kuraray Noritake Dental, Tokyo, Japan)

000024A

MDP, Bis-GMA, HEMA, hydrophobic DMA, sodium fluoride, silanated

colloidal silica, accelerators, initiators, CQ, ethanol, water (pH 2.3)

Apply 10 s, air dry 5 s, light cure 10 s

Clearfil Porcelain Bond Activator(Kuraray Noritake Dental)

00273Aγ-MPS, hydrophobic aromatic

dimethacrylate

Apply a mixture of SE One and Clearfil Porcelain Bond Activator

(1:1) for 10 s, air dry 5 s, light cure for 10 s

Beautibond Multi(Shofu, Kyoto, Japan)

111101Bis-GMA, TEGDMA, carboxylic acid-

based monomer, phosphonic acid-based monomers, acetone, water, other (pH2.4)

Apply 15 s (agitate 5 s), air dry 5 s, light cure 10 s

Beautibond Multi PR Plus(Shofu)

111101 γ-MPS, ethanol, otherApply after applied Beautibond Multi, immediately air dry 5 s,

light cure 10 s

Scotchbond Universal Adhesive(3M ESPE, St. Paul, MN, USA)

482153MDP, DMA, HEMA, polyalkenoic acid

copolymer, silane, ethanol, water, filler, initiators (pH 2.7)

Apply(scrub) 20 s, air dry 5 s, light cure 10 s

Clearfil AP-X(Kuraray Noritake Dental)

01143ABis-GMA, TEGDMA, barium glass filler,

silica filler, photo initiator, catalyst, accelerator, pigment, other

Light cure 40 s

Bis-GMA: Bis-phenol A diglycidylmethacrylate; CQ: camphorquinone; DMA: dimethacrylate; HEMA: 2-hydroxyethyl methacrylate; MDP: 10-methacryloyloxydecyl dihydrogen phosphate; MEP: methacryloyloxyethyl dihydrogen phosphate; 4-MET: 4-methacryloyloxyethyl trimellitic acid; γ-MPS: γ-trimethoxysilylpropyl methacrylate; TEGDMA: triethylene glycol dimethacrylate.

Therefore, the bonding performance of 1-SEAs with a silane coupling agent to enamel, dentin and resin composite in the combined situation is still a question to be answered. This study was undertaken with the purpose of evaluating the effect of adding silane coupling agent on the initial and long-term bond strengths of HEMA-containg and -free one-step self-etch adhesives to enamel, dentin and resin composite in the combined situation, and compare their bonding performances with those of universal 1-SEA. The null hypothesis was that there is no differences in bond strengths to enamel, dentin and resin composite between the adhesives with and without silane coupling agent after each storage period.

MATERIALS AND METHODS

Adhesive materials usedA HEMA-containing 1-SEA and silane coupling agent: Clearfil SE One (SO; Kuraray Noritake Dental, Tokyo, Japan) / Clearfil Porcelain Bond Activator (PB; Kuraray Noritake Dental), a HEMA-free 1-SEA and silane coupling agent: Beautibond Multi (BM; Shofu, Kyoto, Japan) / Beautibond Multi PR Plus (BP; Shofu), and a silane-incorporated 1-SEA: Scotchbond Universal (SU; 3M ESPE, St Paul, MN, USA) were used in this study

(Table 1).

Specimens preparationExtracted non-carious human third molars were collected and stored in 4oC in water containing 0.1%wt of thymol in order to inhibit bacterial growth for a maximum of 6 months until use. Before beginning the experiments, the teeth were retrieved from the disinfectant solution and stored in water, with daily changes for two weeks to remove the disinfectant. The use of human teeth was approved by the Human Research Ethics Committee, Tokyo Medical and Dental University, Japan.

Preparation for micro-shear bond strength test The method of specimen preparation for the micro-shear bond strength (µSBS) test is illustrated in Fig. 1. Three quarters of the root was removed using a model trimmer. Then, in each tooth, two cervical cavities (2×3 mm in diameter and 2 mm in depth) in both proximal surfaces were prepared using a flat-end cylindrical diamond stone at the proximal CEJ area and filled with resin composite (Clearfil AP-X, Kuraray Noritake Dental). After storage in water at 4oC for one week, the composite-filled teeth were sectioned mesio-distally at the central groove and parallel to the longitudinal axis

664 Dent Mater J 2015; 34(5): 663–670

Fig. 1 Schematic illustration of specimen preparation and micro-shear bond strength test.

of the tooth into approximately 2 mm-thick slabs using a slow-speed diamond saw (Isomet, Buehler, Lake Bluff, IL, USA) under water lubrication, exposing the enamel, dentin and resin composite. The specimen surfaces were polished with 600-grit silicon carbide (SiC) paper under running water for 30 s to create a standardized smear layer and a flat surface. The adhesives with or without silane coupling agent (1. SO: SO−, 2. SO+PA: SO+, 3. BM: BM−, 4. BM+BP: BM+, 5. SU) were totally applied to the enamel, dentin and resin composite surfaces in a combined situation according to the manufacturers’ instructions (Table 1).

Before the adhesive was light-cured, a micro-bore Tygon tube (Saint-Gobain Performance Plastics, Nagano, Japan) with an internal diameter and height of 0.79 mm and 0.5 mm, respectively, was placed on each bonded substrate of enamel, dentin and resin composite, and then the adhesive was light-cured. Resin composite was then carefully placed into the tube followed by light-curing for 40 s (Optilux 501, Kerr, Middleton, WI, USA). After storage at the room temperature (23oC) for 1 h, the tube was carefully removed using a sharp blade. The bonded specimens were randomly divided into 3 subgroups according to the storage period in tap water at 37oC: 24 h, 6 months and 12 months. For the 6- and 12-month subgroups, the water was periodically changed every week with 37oC water. After the designated period, each specimen was attached to the testing device (EZ-test 500N, Shimadzu, Kyoto, Japan) and subjected to micro-shear bond testing at a crosshead speed of 1 mm/min, as previously described by Shimada et al20).

Failure mode analysisAfter testing, the specimens were mounted on brass tablets and gold sputter coated. Fractured areas were observed using a scanning electron microscope (SEM,

JSM-5310, JEOL, Tokyo, Japan) at magnifications of 100 and 500 for failure mode determination. The failure mode was classified as one of the following: cohesive failure within substrate (enamel, dentin or resin composite), adhesive failure or mixed failure.

SEM observation of adhesive interfacesCombined surfaces of enamel, dentin and resin composite were prepared as previously mentioned for the µSBS test. After the adhesive procedure, the light cured surfaces were covered with resin composite and further light cured for 40 s. All the specimens were stored in water at 37°C for 24 h. The filled specimens were sectioned perpendicularly to adhesive interface using a slow-speed diamond saw to obtain four slabs of 2 mm thickness for each adhesive. The specimens were then embedded in epoxy resin (Epoxicure Epoxy Resin, Buehler, IL, USA). The exposed adhesive interfaces of the combined enamel, dentin and composite resin substrates were sequentially polished with 600-, 800- and 1000-grit SiC papers under running water. This was followed by polishing sequentially with 6 μm, 3 μm, 1 µm and 0.25 µm diamond pastes (DP-Paste, Struers, Copenhagen, Denmark), and cleaning with an ultrasonic device between each diamond paste polish. After drying, the specimens were mounted on brass tablets and sputter-coated with gold. The adhesive interfaces were then observed using SEM.

SEM observation of tooth surfaces untreated/treated with 1-SEAsIn order to evaluate the etching patterns of each adhesive materials, enamel slabs from the buccal surfaces of the molar teeth and dentin slabs from the mid coronal dentin of the molar teeth were prepared using a low speed diamond saw. After polishing the enamel and

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Fig. 2 Means and standard deviations of micro-shear bond strengths (µSBSs) on enamel substrate.

Fig. 3 Means and standard deviations of micro-shear bond strengths (µSBSs) on dentin substrate.

Fig. 4 Means and standard deviations of micro-shear bond strengths (µSBSs) on composite substrate.

dentin slabs with 600-grit SiC paper, the specimens were randomly divided into one untreated control group and five treated groups. After adhesive application, the treated surfaces were immediately rinsed with 50% acetone for 5 min to remove the adhesive. Then, they were dehydrated in ascending grades of ethanol according to the following protocol (25%, 50%, 75% for 20 min each, and then 95% for 30 min and 100% for 60 min). After the final ethanol immersion, the specimens were dried by soaking in hexamethyldisilazane (HMDS) for 10 min. After drying for 24 h at room temperature, the specimens were mounted on brass tablets and gold sputter coated. The surfaces of the specimens were observed using SEM.

Statistical analysisThe µSBS data were analyzed using a two-way ANOVA and Bonferroni test for multiple comparisons. The failure mode data were analyzed using the Chi-squared test. All statistical analyzes were performed at a confidence level of 95% using PASW version 18 (SPSS, Chicago, IL, USA).

RESULTS

Micro-shear bond strength 2-way ANOVA revealed that the adhesive material and storage period significantly affected the µSBS to all substrates (p<0.001). The interaction between adhesive material and storage period was also significant for the dentin and resin composite substrates (p=0.01 and p<0.001, respectively), but not for enamel (p=0.99). The means µSBSs and standard deviation values for all groups are presented in Figs. 2–4.

The 24 h µSBS result on the enamel substrate showed that there were no significant differences in bond strengths between SO−, BM− and SU (p=1.00), and adding silane coupling agent had no adverse effect on the bond strength values of both SO and BM adhesive systems (p=1.00 and p=0.73, respectively). After 12-month of water storage, SO−, SO+, BM−, BM+ and SU groups exhibited a significant decrease in bond strength (p=0.03, p=0.03, p=0.01, p=0.04 and p=0.002, respectively ).

For the dentin substrate, there were no significant differences in bond strengths between SO− and BM−, SO− and SU, and BM− and SU (p=0.29, p=0.06 and p=1.00, respectively), and adding silane coupling agent significantly decreased the 24 h bond strengths in both SO and BM adhesive systems (p<0.001). After 12 months of water storage, the bond strengths of all the experimental groups were maintained except for BM+ (p=0.001).

For the resin composite substrate, adding silane coupling agent significantly increased the 24 h bond strength for both SO and BM adhesive systems (p<0.001). SU showed comparable bond strength with SO+ and BM+ (p=1.00). After 12 months of water storage, all the experimental groups exhibited significant reductions in bond strength (p<0.001), except for SO+.

Failure mode analysisThe percentages of failure patterns for all groups are shown in Table 2.

Cohesive failure in enamel predominantly occurred in all the experimental groups with each storage period.

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Fig. 5 SEM images of untreated and treated enamel and dentin surface with the tested adhesives (×3,500). Upper: enamel surface; lower: dentin surface. Both treated enamel and dentin surfaces showed no differences

in etching pattern between with and without silane coupling agent for SO and BM adhesive. SU showed similar etching patterns in enamel and dentin to SO− and BM−. The treated enamel and dentin surface of all experimental groups showed similar etching pattern in which still remained smear layer. In the case of dentin, there were smear plugs (arrows) occluding the dentinal tubules (arrow head).

Table 2 Percentage of failure patterns for each adhesive group at each storage period on enamel, dentin and resin composite substrate

SO SO+ BM BM+ SU

A : M : C A : M : C A : M : C A : M : C A : M : C

Enamel24 h

6 months 12 months

00 : 30 : 7000 : 0 : 10000 : 20 : 80

AaAaAa

00 : 20 : 8000 : 20 : 8000 : 20 : 80

AaAaAa

00 : 10 : 9000 : 10 : 9000 : 10 : 90

AaAaAa

00 : 40 : 600 : 0 : 100

00 : 10 : 90

AaAaAa

00 : 20 : 8010 : 10 : 8000 : 50 : 50

AaAaAa

Dentin24 h

6 months 12 months

40 : 60 : 010 : 90 : 020 : 80 : 0

AaABaAa

20 : 80 : 060 : 40 : 070 : 30 : 0

AaAabBa

10 : 90 : 0010 : 90 : 0010 : 80 : 10

AaBaAa

30 : 60 : 1050 : 50 : 0

100 : 0 : 0

AaABaBb

20 : 80 : 020 : 80 : 010 : 90 : 0

AaABaAa

Composite24 h

6 months 12 months

80 : 20 : 0100 : 0 : 0100 : 0 : 0

AaAaAa

0 : 0 : 10000 : 20 : 8000 : 20 : 80

BaBaBa

00 : 100 : 00100 : 0 : 00100 : 0 : 00

CaAbAb

00 : 100 : 000 : 0 : 10040 : 60 : 0

CaBCbCc

00 : 70 : 3020 : 80 : 070 : 30 : 00

CaDa

ACb

Comparisons are valid for each row and column of each substrate. The results of SU were compared with SO− and BM− on enamel and dentin substrate while compared with SO+ and BM+ on composite substrate. Means represented by the same upper and lowercase letters indicate statistically no significant differences in row and column respectively (p>0.05). A: Adhesive failure, M: Mixed failure, C: Cohesive failure in substrate.

On the dentin substrate, in the 24-h group, mixed failure predominantly occurred in all the experimental groups. After 12 months water storage, SO+ and BM+ with silane coupling agent exhibited an increase in adhesive failures. For resin composite, SO− and BM− without silane coupling agent showed adhesive failure after 6- and 12-month water storage. On the other hand, when adding silane coupling agent, SO+ showed predominantly cohesive failures in composite for all storage periods, while BM+ and SU exhibited an increase in adhesive failures after 12 months of water storage.

SEM observations of treated enamel and dentin surfaces with 1-SEAs and adhesive interfaces of enamel, dentin and composite substratesThe untreated/treated enamel and dentin surfaces with

1-SEAs are shown in Fig. 5 and the adhesive interfaces of enamel, dentin and composite substrates are shown in Fig. 6. For the SO and BM systems, there were no observable differences in the etching patterns of the enamel and dentin surfaces between the with and without silane coupling agent groups, while adding silane coupling agent thinned the adhesive layer at the interfaces of the enamel, dentin and composite substrates. The treated enamel surface of all experimental groups showed similar etching pattern in which still remained smear layer. In the case of dentin, all treated dentine surface also showed similar etching pattern as still remained smear layer with smear plugs occluding the tubules.

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Fig. 6 Representative SEM images of adhesive interface (×3,500). Upper: enamel substrate; middle: dentin substrate; lower: composite substrate. AL: Adhesive layer; CR:

composite; E: enamel; D: dentin. The thicknesses of adhesive layer were about 5 µm in SO−, BM− and SU for all substrates, while were less than 1 µm in SO+ and BM+ with silane coupling agent. In adhesive-dentin interface images, dentinal tubules were observed (arrow head).

DISCUSSION

Silane coupling agents are widely used in dental practice for bonding to silica-based restorative dental materials21). The silane coupler is moisture sensitive and activated at a low pH to form reactive silanol groups with hydrolizing alkoxy groups21,22). In the case of a composite repair, reactive silanol groups of a silane coupler can form a siloxane bond with the hydroxyl groups of silica-based glass fillers in resin composite21). In addition, silane coupling agent also creates a multilayer of chemisorbed and physisorbed silane layer on the surface of fillers23-25). The chemisorbed silane layer is bonded to the surface by covalent bond while the physisorbed silane layer is loosely bound layer over the chemisorbed silane layer via hydrogen bond and Van der Waals attraction26). However, it has been reported that hydrolytic degradation of silane multilayer causes the reduction in bond strength27). The application techniques of the silane coupling agent are different between SE One (SO) and Beautibond Multi (BM) systems used in this study. For the SO system, which contains HEMA, the silane coupling agent is mixed with the SO adhesive in order for the acidic functional monomer to activate the silane coupler before application to the bonding substrate. On the other hand, for the BM system, which is a HEMA-free adhesive, it is difficult to mix the silane coupling agent into the miscible solution because of the promotion of phase separation of hydrophobic and hydrophilic components within the BM adhesive. Therefore, the silane coupling agent is applied using an agitation technique on the substrate to be bonded after application of the BM adhesive.

As a silane coupling agent contains a lot of solvent, the addition of a silane coupling agent would change the composition of 1-SEAs through dilution, which might affect the etching ability and/or viscosity of the adhesive agents28). However, SEM observations of treated-enamel and dentin surfaces with SO and BM revealed that there were no differences in etching pattern between the with and without silane coupling agent formulations. On the other hand, following SEM observation of the adhesive interfaces to enamel and dentin, the adhesive layers with SO+ and BM+ were observed to be thinner than those of SO− and BM−. These results would indicate that addition of the silane coupling agent lowered the viscosity of adhesive agent due to dilution by the solvent present in the silane coupling agent. The diluted adhesives with lowered viscosities might result in a reduction in the concentration of the multifunctional hydrophobic monomers on the bonding substrate after air drying, leading to lower mechanical properties of the adhesive layer29,30).

In this study, the bonding specimens were obtained by axially sectioning human molars, therefore the enamel surface was composed of parallel enamel prisms. The inherent strength of enemal parallel to prism orientation is stronger than traverse to prism orientation31). In this study, the predominant cohesive failure in enamel substrate would be due to prism orientation of bonded enamel surface. After 12-month water storage, enamel bond strengths of all the experimental groups significantly decreased with cohesive failure in enamel. These results would indicate reduction of mechanical properties of enamel subsurface which might be due to alteration of mineral content and/

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or deterioration of enamel organic matrix32-34).For the dentin substrate, adding silane coupling

agent significantly reduced the 24-h dentin bond strengths of the SO and BM systems, in which all the groups showed mixed failures. These results might be due to a reduction in the mechanical properties of the adhesive layers because of dilution of the adhesive by adding the silane coupling agent. Moreover, water contamination from the dentin surface might have more adverse effects on the dentin bond strengths of the SO and BM systems with silane coupling agent than without silane coupling agent, because adding silane coupling agent resulted in the formation of a thin adhesive layer due to its lower viscosity. After 12-month water storage, the dentin bond strengths of SO−, SO+ and BM− were stable compared with those of the 24-h group, while that of BM+ significantly decreased along with an increase in the number of adhesive failures after 12-month water storage. For BM, a HEMA-free adhesive, adding silane coupling agent with a lot of solvent would promote droplet formation in the adhesive layer by phase separation7,12,35) along with water contamination from the dentin surface, which might cause deterioration of the adhesive layer over time.

For the resin composite substrate, SO− and BM− without the addition of a silane coupling agent obtained relatively high 24-h bond strengths of 33.3 and 39.7 MPa, respectively. Clearfil AP-X resin composite, aged by 1-week water storage after polymerization, was used as substrate to be bonded in this study and utilizes barium glass filler, as well as silica filler, in order to provide radiopaque capacity. It has been reported that the phosphate group in an acidic functional monomer such as MDP, can interact with the oxide film on the surface of aluminum oxide-based and zirconium oxide-based ceramics36,37). The phosphorus monomer in SO and BM might contribute to bonding to the barium glass filler particles in resin composite, because barium forms an oxide film on the surface by an oxidation reaction in the presence of air38). On the other hand, after 12-month water storage, the bond strengths of SO− and BM− to Clearfil AP-X resin composite decreased significantly with increase in the number of adhesive failures. These results would indicate that bonding ability of SO− and BM− to barium glass filler deteriorated over time.

When silane coupling agent was added, 24-h bond strengths of SO+ and BM+ to Clearfil AP-X were significantly higher than those of SO− and BM− without silane coupling agent. These results are in agreement with the previous studies, which have demonstrated that using silane coupling agent improved bonding performance to resin composite because silane coupling agent can interact with silica fillers13,14). After 12-months water storage, SO+ showed stable bond strength to Clearfil AP-X. This result would indicate that the interaction of SO+ with silica fillers in resin composite was stable in long-term water storage. On the other hand, BM+ significantly decreased the bond strength over time with increasing of adhesive failure. SO+ and BM+ uses same silane coupler of γ-MTS. For BM+, the

decreasing of bond strength to Clearfil AP-X over time might be due to aging deterioration of the adhesive layer with droplet formation by phase separation7,12,35).

ScotchBond Universal (SU) has been introduced as universal 1-SEA which can bond to ceramics and alloy as well as enamel and dentin. The bonding performance of SU to enamel and dentin was similar to those of SO− and BM−. SU uses MDP with HEMA, and contains silane coupler in its formulation, which is expected to increase bonding ability to silica-based materials. The silane coupler of SU is thought to be activated by incorporated acidic functional monomer in one-bottle. SU exhibited comparable bond strength to Clearfil AP-X in the 24-h group with SO+ and BM+. However, bond strength of SU to Clearfil AP-X significantly decreased over time, in which SU exhibited lower bond strength in the 12-month group than SO+ and BM+. This result was in agreement with the previous study about bonding of SU to zirconium oxide-based ceramic, in which the bond strength reduced after 6 months of water storage19). Therefore, the result of present study would indicate that the bonding of SU to barium filler as well as silica filler in resin composite was unstable during long-term water storage. Generally, when a silane coupler is incorporated with an acidic solution into one component, shelf life lowers due to a hydrolysis reaction39,40). Presumably, silane coupler in SU might not effectively interact with the silica filler in resin composite. Further research on the effect of storage duration of universal 1-SEAs on bonding to resin composite should be investigated.

CONCLUSION

Within the limitations of this study, it was concluded that adding silane coupling agent to 1-SEAs had no adverse effect on µSBSs to enamel with cohesive failure in enamel. For dentin substrate, adding silane coupling agent significantly decreased µSBSs with thinner adhesive layers. Additionally, adding silane coupling agent significantly increased the initial µSBSs to resin composite, however, the effect of adding silane coupling agent on bonding durability to resin composite was dependent upon the adhesive materials.

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