INTRODUCTION

The earlier dentin adhesives required three independent steps (etching, priming, and bonding) in order to achieve bonding to dentin; these were known as the 3-step systems1). Subsequently, the 2-step systems were introduced by manufacturers. The 2-step systems may be classified into two general categories2): the self-priming systems, which combine the primer and adhesive together used after etching; and the self-etching systems, in which the etching and priming of dentin by the primer is followed by the application of adhesive. With improvements in dental adhesives, the 2-step systems were found to be user-friendly3), less technique sensitive4,5), and performed well both in vitro and clinically6,7).

Although the 2-step self-etching adhesives were already simplified in the application procedure, the 1-step systems, or so called ‘all-in-one’ adhesives, are becoming increasingly more popular in clinical use and their market is still growing8,9). The most significant improvement in the 1-step systems is the possibility to achieve all the steps (etching, priming, and bonding) combined in one bottle but still requiring the light-curing step. Reducing operative procedures makes the 1-step systems more effective in minimizing technique sensitivity10), simultaneous demineralization and resin infiltration11) as well as in reducing postoperative pain caused by the mild self-etching adhesives12).

As adhesive technology is rapidly evolving,

commercial adhesive formulations are frequently replaced. As the only experimental system selected in this study, LLB-2 (Tokuyama Dental, Ibaraki, Japan) is a newly developed 1-step system, which omits the light-curing step after air-blowing on bond applied dentin surface. Self-adhering light-cured flowable composite resins may be considered as a sort of ‘step-less’ system. If the future trend of dental materials is defined by the simplification of operative procedures, the development of newer 1-step systems2,8) or even more simplified systems may be a logical next step for dental manufacturers. Notwithstanding, the short-term bond strength and morphology of newly developed materials need to be assessed initially.

In this study, MG was especially chosen as control group, because of its good in vitro and clinical performance, which has been explained by its chemical composition and good polymerization4,6,13). In previous bond strength testing on dentin, MG showed statistical higher bond strength than the other self-etching systems14), or even no statistical significant difference comparing with that of conventional 3-step system8). Thus, MG was considered as ‘gold standard’ among the self-etching adhesives15) and selected in present study as control group.

The micro-tensile bond strength (µTBS) test16) has been increasingly considered as one of the most reliable methods to evaluate the bonding performance in vitro, which may then be correlated with clinical effectiveness2). The SEM was used on the observations

Bonding performance of a newly developed step-less all-in-one system on dentin

Jiale FU1, Shinichi KAKUDA1, Feng PAN1, Shuhei HOSHIKA1, Shihchun TING1, Anri FUKUOKA1, Yang BAO2, Takatsumi IKEDA1, Yasuko NAKAOKI1, Denis SELIMOVIC3, Hidehiko SANO1 and Sharanbir K. SIDHU4

1 Department of Restorative Dentistry, Division of Oral Health Science, Hokkaido University, Graduate School of Dental Medicine, Kita 13, Nishi 7, Kita-ku, Sapporo 060-8586, Japan

2 Department of Restorative Dentistry, School of Stomatology, Affiliated Stomatological Hospital, China Medical University, No.117 Nanjing Street, Heping District 110002, Shenyang, Liaoning province, China

3 Dental Faculty, Department of Restorative Dentistry and Endodontics, University of Strasbourg, F-67081, Strasbourg, France4 Centre for Adult Oral Health, Queen Mary University of London, Barts and The London School of Medicine and Dentistry, Institute of Dentistry,

Turner Street, London E1 2AD, UKCorresponding author, Jiale FU; E-mail: fullers@126.com

The purpose of this study was to evaluate the short-term µTBS (Micro-tensile bond strength) and microscopic (SEM and TEM) observation of four recent adhesives. One adhesive was an experimental step-less 1-step system (LLB-2, Tokuyama Dental), which is an all-in-one system without the light-curing step in the application process. The other two were self-adhering light-cured flowable composite resin systems FLD (Fusio Liquid Dentin, Pentron Clinical Technologies) and VF (Vertise Flow Dental Restorative Materials, Kerr Corporation), which combine all the bonding steps together. A 2-step self-etching system MG (Clearfil MegaBond, Kuarary Medical) was employed as the control group in this study. The µTBS of MG was the highest (79.0 MPa) followed by that of LLB-2 (63.1 MPa), FLD (23.6 MPa), and VF (13.1 MPa). The microscopic observations showed that MG and LLB-2 had an approximately 20 µm and 5 µm adhesive layer respectively, without bubble or gap-formation at the resin-dentin interface, which were found in FLD and VF.

Keywords: Micro-tensile bond strength, All-in-one system, Step-less system, SEM, TEM

Received Aug 1, 2012: Accepted Dec 4, 2012doi:10.4012/dmj.2012-204 JOI JST.JSTAGE/dmj/2012-204

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Table 1 Chemical formulations and the individual manufacturer’s instructions

Bond System Composition Directions for use

Clearfil MegaBond (Clearfil SE Bond)(Lot: 011528)

Primer: 10-MDP, HEMA, hydrophilic dimethacrylate, photo-initiator, water

Bond: 10-MDP, Bis-GMA, HEMA, hydrophilic dimethacrylate, photo-initiator,

silanated colloidal silica

1. apply the primer for 20 s2. gently air-blow3. apply the bond4. 20 s light-curing

LLB-2(Lot: 2-64)

Adhesive: acetone, water, 3D SR monomer, Bis-GMA, TEGDMA, HEMA, silica fillerResin Composite: CQ, Amine, Peroxid,

Bis-GMA, TEGDMA, filler

1. 10 s wait after application 2. 5 s gentle air & 5 s moderate air3. Restore with resin composite4. 20 s light-curing

Fusio Liquid Dentin(Lot: 201091)

UDMA, TEGDMA, HEMA, 4-MET, Silane treated barium glass, Silica,

Minor additives, Photo Curing System

1. Apply material in 1 mm increments, agitate with tip or brush for 20 s

2. 10 s light-curing

Vertise Flow(Lot: 3363821)

Uncured methacrylate ester monomers, inert mineral fillers, Ytterbium Fluoride,

activator, stabilizers and colorants

1. 5 s maximum air dry2. Apply the material to the preparation with

a dispensing tip3. Brush a thin layer (<0.5 mm) with moderate

pressure for 15–20 s 4. 20 s light-curing

10-MDP: 10-methacryloyloxy methacrylate; HEMA: 2-hydroxyethyl methacrylate; 3D SR monomer: three dimensional surface-reinforcing monomer; Bis-GMA: bisphenol-A-diglycidyl methacrylate; TEGDMA: triethyleneglycol dimethacrylate; CQ: camphorquinone; UDMA: urethane dimethacrylate; 4-MET: 4-methacryloxyethyl trimellitic acid.

of the dentin-resin interface2) and the µTBS-fractured surface7). Recently TEM observation has also been used to examine the interfacial structure in vitro8,14), and it is nevertheless thought to be a very reliable8) with a relatively low artifact incidence17). Therefore, TEM was employed to investigate the interface of adhesives (Fig. 3 & Fig. 4) in present study.

The aim of this paper was to evaluate the bonding performance of a newly developed experimental step-less (1-step) adhesive, two commercial self-adhering light-cured flowable composite resin systems and a control group (2-step self-etching system) on dentin. This was by assessing their micro-tensile bond strength on dentin as well as the micro-morphological and ultra-morphological characteristics at the interface. The null hypothesis of this study was that there are no differences between the experimental step-less adhesive, the two self-adhering light-cured flowable composite resin systems and the control group.

MATERIALS AND METHODS

Teeth usedThe 40 extracted non-carious human molars, collected with patients’ informed consent under a protocol reviewed and approved by the institutional review board of Hokkaido University, were used in the present study. The teeth were stored at 4ºC in an aqueous solution of 0.5% Chloramine-T, and used within 4 months after extraction. The 32 of the 40 teeth were used for the µTBS

test (n=40 sticks, 8 teeth per system and 5 resin-bonded sticks from 1 tooth) with the rest of the 8 teeth used for SEM (resin-dentin interface) and TEM observation.

Flat dentin surfaces were obtained by removing the coronal enamel of each tooth with a gypsum model trimmer under water coolant, leaving the surrounding enamel. The dentin surfaces were then ground with 600-grit SiC paper for 60 s under continuous water-cooling to produce a standardized smear layer prior to bonding.

AdhesivesFour bonding systems were employed in the present study. One was a newly developed experimental step-less 1-step adhesive, LLB-2 (Tokuyama Dental, Ibaraki, Japan), which will be marketed with the commercial name of PrimeFil in the near future with minor modification. In this newly developed system, the light-curing step after air-blowing on the adhesive-applied dentin surface was omitted. The other two were commercially available self-adhering flowable composite resins, VF (Vertise Flow Dental Restorative Materials, Kerr Corporation, Orange, CA, USA) and FLD (Fusio Liquid Dentin, Pentron Clinical Technologies, Wallingford, CT, USA). MG (Clearfil MegaBond, Kuraray Medical INC., Okayama, Japan), a commercial 2-step self-etching system, was selected for the control group. Table 1 shows the chemical formulations and the respective manufacturer’s instructions for use of these four adhesives. All surfaces were built-up with composite resin (Clearfil AP-X, Shade A3, Kuraray Medical Inc.,

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Okayama, Japan) in 5 mm thick increments. Each incremental layer was light cured for 20 s (light output intensity controlled at no less than 550 mW/cm2). The specimens were then stored in distilled water at 37ºC for 24 h.

Micro-tensile bond strength (µTBS) testAfter 24 h water storage in 37ºC, the 40 resin-bonded sticks (1 mm×1 mm approximately) were obtained from 8 teeth of each system using an Isomet diamond saw (Isomet Low Speed Saw, Buehler, Lake Bluff, IL, USA). Sticks were then fixed to a Ciucchi’s jig with cyanoacrylate glue (Model Repair II Blue, Dentsply-Sankin, Otahara, Japan) and subjected to a tensile force at a crosshead speed of 1 mm/min in a desktop testing apparatus (EZ test, Shimadzu, Kyoto, Japan) until failure occurred. The µTBS was expressed in MPa, dividing the applied force (N) at the time of fracture by the bonded area (mm2). The resultant µTBS values from each system were evaluated with one-way ANOVA and the Games-Howell test (n=40, p<0.05).

After µTBS testing, the modes of failure in all specimens were observed using a light microscope (×20). The failure modes were categorized as:

A: Cohesive failure within the adhesive resin only and partial adhesive failure, where remnants of resin were observed on the dentin surface.

B: Adhesive failure at the resin-dentin interface only.

C: Mixed adhesive failure with cohesive failure within dentin as well as cohesive failure in dentin only.

SEM morphological observation 1. Observation of the dentin surface after µTBS testingThe fractured surface on the dentin-side of all specimens from the µTBS test part of the study were dried overnight in desiccators at room temperature, then sputter-coated and observed using FE-SEM (S-4000, HITACHI, Tokyo, Japan) with an accelerating voltage of 10 kV.2. Observation of the resin-dentin interfaceTo observe the morphology of the dentin-resin interface, the newly prepared resin-bonded specimens were made as in the µTBS group and they were sectioned perpendicular to the adhesive interface, using an Isomet saw, to obtain two slabs of 2 mm thickness of each system. The cut surfaces were sequentially polished with 600-, 800-, and 1,000-grit silicon carbide papers under running water. This was followed by polishing sequentially with 6-, 3-, 1-µm diamond pastes (DP-Paste, Struers, Denmark), and cleaning with an ultrasonic device between each diamond paste polish.

After polishing, the specimens were immersed in 1 M hydrochloric acid for 30 s and 5% sodium hypochlorite for 5 min, followed by rinsing with water. After drying, the specimens were sputter-coated with Pt-Pd for 120 s. The resin-dentin interfaces were then observed using a scanning electron microscope (SEM, S-4000, Hitachi, Tokyo, Japan).

TEM observationAfter storage in 37ºC water for 24 h, the specimen was prepared for the TEM observation. Ultra-structural TEM examination procedures included overnight fixation in 2.5% glutaraldehyde containing 0.1M sodium cacodylate buffer at pH 7.4 in 4ºC, and then rinsing in 0.1M sodium cacodylate buffer for 1 min with three changes of solutions. Subsequently, the samples were dehydrated in ascending grades of ethanol (25, 50, 75, 95 and 100%) for 10 min each with three changes every time. The samples were then immersed in 1:1 absolute ethanol-epoxy embedding resin (Epon 812, Polysciences, PA, USA) for 4 h with rotation at 4 rpm (Taab Rotator type N, Taab Laboratories, PA, USA). They were then removed and placed in 100% epoxy embedding resin in new bottles for rotation for a further 8 h to allow resin infiltration. Finally, the resin-infiltrated samples were embedded in molds, and polymerized in an oven at 60ºC for 48 h before ultra-thin sectioning. Sections of 75 nm to 90 nm thickness through the dentin-resin interface were obtained using a diamond knife (Diatome, Bienne, Switzerland) in an ultra-microtome (Ultracut UCT, Leica, Vienna, Austria). The sections were then observed with a Transmission Electron Microscope (H-800, Hitachi) operating at 75 kV. The all images were performed from no-demineralized and no-stained samples.

RESULTS

µTBS TestThe means and SDs of the bond strengths are shown in Table 2. MG showed significantly higher µTBS in comparison with the other three systems (p<0.05). LLB-2 showed significantly higher µTBS in comparison with the other two self-adhering systems (p<0.05). Within the two self-adhering flowable composite resin systems, FLD had statistically higher bonding performance than that of VF (p<0.05).

The fracture modes of MG showed 72.5% of failures were cohesive failures within the adhesive resin (type A); 12.5% fractured at the dentin-resin interface (type B) and 15% were of mixed adhesive failure (type C). LLB-2 showed 55% of type A failures within the adhesive resin; 20% fractured at the dentin-resin interface (type B) and 25% were of mixed adhesive failure (type C). In contrast, 87.5% and 100% failure occurred on the resin-dentin interface (type B) for FLD and VF respectively.

SEM morphological observation1. Observation of the dentin surface after µTBS testingSEM observation of the fractured surfaces after µTBS testing showed that MG (Fig. 1-a) and LLB-2 (Fig. 1-b) showed a typical adhesive layer on the dentin side of the fracture (type A). FLD (Fig. 1-c) showed a typical adhesive failure at the dentin-resin interface (type B); the dentin surface was covered by numerous bubbles. VF (Fig. 1-d) showed adhesive failure at the dentin-resin interface (type B). The empty dentin tubules and the fractured resin tags on the dentin surface could be observed.

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Table 2 Micro-tensile bond strength (µTBS) and fracture modes (n=40/group)

Bond System µTBS mean(SD) MPaFracture mode (%)

A B C

MG 79.0 (16.1) a 72.5 12.5 15

LLB-2 59.4 (20.9) b 55 20 25

FLD 25.2 (6.1) c 12.5 87.5 0

VF 13.0 (9.9) d 0 100 0

Values with different superscript letters are statistically different (p<0.05).Fracture mode categories: A: Cohesive failure within the adhesive resin only with partial adhesive failure, where remnants of resin were observed on

the dentin surface.B: Adhesive failure at the resin-dentin interface only.C: Mixed adhesive failure with cohesive failure within dentin as well as cohesive failure in dentin only.There were 4 premature failed specimens appeared in VF during the µTBS test, which were divided into B, and calculated as 0 MPa in statistical analysis.

Fig. 1 SEM observation of a fractured surface on the dentin side after µTBS test. (a) shows a typical cohesive failure within the composite resin, where remnants of resin were observed on the dentin

surface (MG). The composite resin on the dentin surface can be noted. No dentin surface could be observed. (b) shows a typical cohesive failure within the adhesive layer (LLB-2). The adhesive layer on the dentin surface can

be noted. No dentin surface could be observed. (c) shows a typical adhesive failure in FLD. The dentin surface is visible but covered by smear plugs or tags (arrowed);

the dentin tubules in the central are invisible and covered by numerous bubbles. (d) shows adhesive failure of VF. A smooth surface with/without dentin tags can be observed. Large open dentin

tubules (arrowed) can be noted on the demineralized dentin surface. Some dentin tags have been pulled out and are fractured. The collagen fibril network can be observed around the open dentin tubules.

R: Resin composite, A: Adhesive layer, P/T: plugs or tags, B: bubbles, T: tags.

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Fig. 2 SEM observation of the dentin-resin interface. (a) shows an approximately 20 µm adhesive layer (MG) at the resin-dentin interface. The resin tags can be

observed. (b) shows an approximately 5 µm adhesive layer (LLB-2) at the resin-dentin interface. Long resin tags can be

observed. In (c), the adhesive layer (FLD) is not observed between the dentin and resin; instead, a small gap (arrowed) can be

observed between the dentin and resin. Bubbles at the dentin-resin interface within the resin are noted. A few short resin tags are also noted. The hybrid layer cannot be detected at this magnification.

(d) shows a huge gap between at the dentin-resin interface within VF. Resin tags appear to be pulled out from the dentin tubules. The hybrid layer cannot be detected at this magnification.

R: Resin Composite, A: Adhesive layer, D: Dentin, G1: gap formation.

2. Observation of the resin-dentin interfaceSEM observation of the dentin-resin interface showed that MG (Fig. 2-a) and LLB-2 (Fig. 2-b) had an approximately 20 µm and 5 µm adhesive layer respectively and long resin tags could be observed. FLD (Fig. 2-c) showed bubbles at the dentin-resin interface and a few short resin tags were observed. VF (Fig. 2-d) showed gap formation between at the dentin-resin interface.

TEM observationFigure 3-a shows the resin, adhesive layer, and dentin relationship of MG by TEM observation at low magnification. Figure 3-b shows the resin, adhesive layer, and dentin relationship of LLB-2 by TEM observation at low magnification. A very thin hybrid layer could be observed. The adhesive layer of LLB-2

was approximately 5 µm thick and no bubble formation was found. In contrast, FLD (Fig. 3-c) shows various sizes of bubbles with an average diameter of 2 µm between the dentin and resin. VF (Fig. 3-d) appeared to have gap formation between the dentin and the resin. The empty dentin tubule was observed.

At high magnification, MG (Fig. 4-a) showed a hybrid layer of approximately 0.5 µm thickness. LLB-2 (Fig. 4-b) displayed very fine fillers within the adhesive layer. Uniform and non-uniform fillers could be observed at the base of the adhesive layer. FLD (Fig. 4-c) showed fine separated hydroxyapatite particles dissociated within the adhesive layer (≤0.2 µm). VF (Fig. 4-d) showed gap formation between the dentin and the resin.

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Fig. 3 TEM observation at low magnification. In (a), the resin-dentin interface with MG can be observed. Clearfil Protect Liner F (Kuraray Medical Inc., Okayama,

Japan, Lot: 00072C) was used for this TEM preparation only. The adhesive layer of MG is approximately 20 µm and very fine spherical fillers can be seen within the adhesive layer. No bubble formation could be found.

In (b), the resin-dentin interface with LLB-2 can be observed. The diameter of composite resin spherical fillers is approximately 0.5 µm, and very fine spherical fillers with a diameter of approximately 0.05 µm, may be observed. The adhesive layer of LLB-2 is approximately 5 µm thick and very fine spherical fillers can be seen within the adhesive layer. No bubble formation is detected.

In (c), various sizes of glasses and fillers in FLD are observed. A number of bubbles (arrowed) with a diameter of 2 µm were noted between the dentin and the resin.

In (d), various sizes of inert mineral fillers within VF can be noted. Gap formation (arrowed) between the dentin and the resin as well as the typical empty dentin tubule can be detected.

R: Resin Composite, A: Adhesive layer, D: Dentin, G2: edge of the grid, f: filler.

DISCUSSION

The 2-step self-etching system, MG, was selected as the control adhesive for its good performance, due to its chemical composition and good polymerization4,7). However, the recently developed adhesive systems are more user-friendly. The other three adhesive systems employed in this study represent the latest trend of dental materials, which involve simplified bonding procedures. An experimental 1-step system, LLB-2 (commercial name of PrimeFil in the near future with minor modification from LLB-2), has been designed

to be more simplified and less time-consuming in the application steps. The light-curing step after air-blowing the bond applied to the dentin surface (prior to composite resin placement) is omitted; this is the significant difference in the bonding procedure compared to the routine all-in-one systems18). Thus, at least 10 s may be saved clinically. On the other hand, the other two self-adhering flowable composite resin systems employed in this study were both required to be used directly on prepared teeth. The common characteristic of both commercial flowable self-etching systems were that no additional etching, rinsing, or air-blowing, apart from

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Fig. 4 TEM observation at high magnification. (a) is a higher magnification view of the boxed region of the TEM observation of MG in Fig. 3-a. Very fine fillers

within the adhesive layer can be observed. The thickness of hybrid layer would approximately be 0.5 µm. Detail of the hybrid layer, revealing an upper part which completely demineralized, and a lower part, which hydroxyapatite crystals were still present. The exposed collagen fibrils in the upper part of the hybrid layer appear electron-lucent.

(b) is a higher magnification view of the boxed region of the TEM observation of LLB-2 in Fig. 3-b. Very fine fillers within the adhesive layer can be observed. Uniform and non-uniform fillers are seen at the base of the adhesive layer. The thickness of this hybrid layer would approximately be 0.2 µm. Note that there is a partially demineralized transition zone and some residual hydroxyapatite in the bottom of the hybrid layer due to the intermediately strong self-etching ability of LLB-2.

(c) is a higher magnification view of the boxed region of the TEM observation of FLD in Fig. 3-c. The high magnification image shows fine separated hydroxyapatite particles dissociated in the adhesive layer (≤0.2 µm). On the top of the dentin, the hydroxyapatite shows a typical needle-like shape.

(d) is a higher magnification view of the boxed region of TEM observation of VF in Fig. 3-d. Fillers can be seen on the composite resin side. Gap formation between the dentin and resin can be observed. Some apatites are contained on the edge of the separated resin.

H: Hybrid layer, R: Resin Composite, A: Adhesive layer, D: Dentin, G1: gap formation, f: filler.

light-curing, were recommended by both manufacturer. Thus, commercial PrimeFil and flowable self-adhering light-cured composite resin systems may be considered as step-less system.

Statistically, MG showed the highest bond strength than the other three systems. MG contains HEMA in primer, which has been shown to play a key role19) in osmosis due to its low molecular weight and its

hydrophilicity. Uncured HEMA in the oxygen inhibition layer increases the osmotic pressure, resulting in more water attraction and hence osmotically induced droplet formation. The MG yielded a thick layer of around 20 µm. The thicker the adhesive layer, the lower the water permeability of the adhesive layer to osmotic pressure gradients will be. In addition, MG consists of a separate hydrophobic solvent-free bonding agent, which is known

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to reduce the permeability of the adhesive layer20) and polymerize better21). Therefore, MG did not exhibit phase separation or osmosis-induced droplets in SEM (Fig. 2-a) and TEM (Fig. 3-a) observations.

LLB-2 produced higher bond strengths than the two self-adhering systems. It incorporates a newly developed functional monomer, 3D SR monomer, which is thought to have a positive impact on the bonding performance22) on three aspects: 1) the phosphate monomer of 3D SR monomer could partially self-organize within adhesive and from multifunctional monomer structures with several phosphate groups and polymerizing groups; 2) the phosphate groups in 3D SR monomer are capable to interact with calcium at multiple sites and form ionic bonds; and 3) the air-blowing step on the bond applied dentin surface could be concentrated at the resin-tooth interface and crosslink with calcium.

This monomer has three dimensional structures with functional groups and double bonds. The functional groups can interact with calcium in enamel and dentin2). It could be expected that the 3D SR monomer had the same or higher interaction-potential to calcium than that of mono-phosphates such as MDP2,5). As the molecular size of 3D SR monomer is relatively large compared to MDP or phenyl-P, the 3D SR monomer does not permit deeper penetration into dentin. TEM observation (Fig. 3-b) showed a very thin hybrid layer, which may be caused by the structural characteristics of 3D SR monomer and the intermediately strong acidity of the adhesive as reported previously3). However, the LLB-2 adhesive and the composite resin for LLB-2 were required to be used together. The adhesive of LLB-2 contains a co-initiator, whereas composite resins contain an initiator. Thus, the self-curing interaction between the adhesive and a composite resin may start when they are in contact. The bond strength of LLB-2 between the resin and the dentin could be created by the surface adhesion of 3D SR monomer and this unique self-curing system. However, the long term effect of LLB-2 is unknown and further study regarding the long-term durability of LLB-2 is required.

On the other hand, the micro-tensile bond strength test showed FLD had significantly lower bond strength than that of the step-less all-in-one system (LLB-2), but it performed better than VF. Bubbles were noted in both SEM (Fig. 1-c & Fig. 2-c) and TEM (Fig. 3-c) observations. These bubbles could be attributed to the residual water droplets on dentin surface and the poor permeability of the initial flowable composite resin layer applied on the dentin surface. It is thought that diffusion of water from the dentin through the cured adhesive layer will lead to osmosis and that the cured adhesive resin functions as a semi-permeable membrane20). Osmosis occurs rapidly in HEMA-containing adhesives, due to the strong hydrophilic character and the small dimensions of this monomer19,23). However, under both SEM (Fig. 2-c) and TEM (Fig. 3-c) observation of FLD, bubbles were noted between the composite resin and dentin. The bubbles were embedded in the surface of the dentin-resin structure. There is no water contained in composition

of FLD, thus, it would be suspected that the embedded water droplets maybe come from dentin surface. On the other hand, because of the lower permeability of the flowable composite resin comparing with that of bond resin, it would appear that the bubbles were not able to travel through the self-adhering system although the initial layer was applied as thinly as 1 mm according to the manufacturer’s instructions. Ultimately, the small water droplets probably remained between the dentin surface and the composite resin and appeared as voids20) therefore resulting in low bond performance. Although HEMA has a positive effect on the bond strength of all-in-one systems as previously reported19,23), it is thought that the actual effectiveness of HEMA (the hydrophilic monomer used in the flowable composite resin) would be greatly reduced.

The bond strength of VF was statistically lower than that of both MG, LLB-2 and FLD. Thus, it was not surprising that total dentin-resin failure occurred. TEM observation at low magnification (Fig. 3-d) showed a typical relationship between dentin tubules and resin composite. As the typical empty dentin tubules showed in SEM images (Fig. 1-d & Fig. 2-d), the empty dentin tubule was also observed in TEM (Fig. 3-d). VF is a flowable resin, and this concurs with research showing that flowable resin has poor filler loading of adhesive resin and therefore results in low mechanical strength24).

Vacant dentin tubules and a dentin-resin gap were noted in both the SEM and TEM observations, suggestive of poor bonding ability. Moreover, the non-rinse VF appeared to open the dentin tubules and exposed a micro-porous collagen fibrillar network (Fig. 2-d), which is similar to the effect of an etch-and-rinse approach using phosphoric acid2). In etch-and-rinse (3-step) systems, the calcium phosphates are removed by the rinsing process. However, in the case of VF, these embedded calcium phosphates are thought to be weak structures, thereby seriously weakening the interfacial integrity24). The effect of the inert mineral fillers contained in VF should be investigated.

CONCLUSION

In present study, the three step-less systems showed various bonding performance and statistically lower than that of MG after 24 h. Thus, the null hypothesis that there are no differences between the newly developed step-less 1-step system, self-adhering light-cured flowable composite resin and the 2-step systems was rejected. The further study of long term durability test should be continued.

ACKNOWLEDGMENTS

This study was supported by Grant-in-Aid for Scientific Research (B) #2239035501.

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