Mol Biol Rep (2013) 40:7053–7059 DOI 10.1007/s11033-013-2826-6
Comparison of tensile bond strengths of four one-bottle self-etching adhesive systems with Er:YAG laser-irradiated dentin Qianzhou Jiang • Minle Chen • Jiangfeng Ding
Received: 5 January 2013 / Accepted: 25 October 2013 / Published online: 5 November 2013 Ó Springer Science+Business Media Dordrecht 2013
Abstract This study aimed to investigate the interaction of current one-bottle self-etching adhesives and Er:YAG laser with dentin using a tensile bond strength (TBS) test and scanning electron microscopy (SEM) in vitro. Two hundred and thirteen dentin discs were randomly distributed to the Control Group using bur cutting and to the Laser Group using an Er:YAG laser (200 mJ, VSP, 20 Hz). The following adhesives were investigated: one two-step total-etch adhesive [Prime & Bond NT (Dentsply)] and four one-step self-etch adhesives [G-Bond plus (GC), XENO V (Dentsply), iBond Self Etch (Heraeus) and Adper Easy One (3 M ESPE)]. Samples were restored with composite resin, and after 24-hour storage in distilled water, subjected to the TBS test. For morphological analysis, 12 dentin specimens were prepared for SEM. No significant differences were found between the control group and laser group (p = 0.899); dentin subjected to Prime & Bond NT, XENOV and Adper Easy One produced higher TBS. In conclusion, this study indicates that Er:YAG laser-prepared dentin can perform as well as bur on TBS, and some of the one-step one-bottle adhesives are comparable to the total-etch adhesives in TBS on dentin. Keywords One-bottle self-etching adhesives Tensile bond strength Scanning electron microscope
Q. Jiang (&) J. Ding Key Laboratory of Oral Medicine, School and Hospital of Stomatology, Guangzhou Medical University, 59 Huangsha Thoroughfare, Guangzhou 510140, China e-mail: [email protected] Q. Jiang M. Chen Guangzhou Medical University, 195 Dongfengxi Road, Guangzhou 510182, China
Introduction In the adhesive dentistry field, a great deal of effort has been directed toward simplifying adhesive procedures. To this end, self-etching systems, which do not need previous smear layer removal, have opened up a new horizon. The self-etching systems simultaneously promote demineralization and resin infiltration through the dental surfaces. Recently, one-step one-bottle self-etch adhesives that combine etching, priming, and bonding into a single step have been developed. However, these adhesive systems are generally reported to show reduced bonding properties compared with two-step self-etching adhesives [1]. Nonetheless, simplification of the bonding steps, shorter application time and less technique sensitivity are very attractive to clinicians. The erbium: yttrium–aluminum–garnet (Er:YAG) laser is a new type of laser-powered hydrokinetic system, which is now beginning to be studied and applied in dentistry [3– 8]. Use of the Er:YAG laser is currently accepted as an alternative method for cavity preparation in dental hard tissues. Cavities prepared with an Er:YAG laser present irregular margins and rough walls and a rugged floor, which differs from the basic principles of cavity preparation established by Black [2]. The surface irregularity of laserirradiated dentin seems to be better for adhesive restorations. However, reports concerning the bonding capability of the irradiated dentin have been controversial [3, 4]. To the best of our knowledge, few studies have been undertaken to examine the bond strength of one-bottle selfetching adhesives to Er:YAG laser irradiated dentin [4, 5]. The purpose of this study was to investigate the effect of an Er:YAG laser on dentin bonding and compare the tensile bond strength (TBS) of four one-bottle self-etching adhesives to the Er:YAG laser irradiated dentin, using the total-
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etch adhesive Prime & Bond NT as a control. This study explored the potential and feasibility of using these newly developed products for laser dentin bonding. Scanning electron microscopy (SEM) morphological analysis of treated dentin surfaces was also performed to verify of the adhesive effect. The null hypotheses tested were that (1) Er:YAG laser irradiation has no effect on TBS of the dentin bonding and (2) there is no difference between the five adhesives.
Materials and methods One hundred and seven freshly extracted human molars and premolars with no cavities or fillings were selected and kept in 4 °C distilled water. The molars were obtained under a protocol approved by the Ethics Committee of the Guangzhou Medical University and with the informed consent of the donors. The roots were separated and the crowns were cut in the mesiodistal direction using a diamond bur with water spray. The methods used in this research were referred to in Bahrami’s [6] study, and the schema was shown on Fig. 1.
Fig. 1 Steps in the preparation of the samples a root section and mesio-distal section of the crowns with a cylindrical diamonds drill; b crowns embedded in a self-curing resin; c dentin exposure and polishment; d adhesive procedure and placement of two-piece Teflon matrix; e inverted cone of composite resin restorations; f, g TBS test after 24 h of storage in distilled water
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Dentin specimen preparation Lingual or buccal crown sections were embedded in selfcuring resin and then the enamel surfaces were removed. The specimens were polished flat using 150-, 240- and 400-grit silicon carbide waterproof abrasive papers under running water to insure enamel-free dentin surface. Finally, the dentin surface was polished with 600-grit silicon carbide waterproof abrasive paper under running water for 60 s to standardize the smear layer. A circular mark was made in the center of each dentin surface with an indelible felt-tip pen. Then, 213 samples were made and randomly divided into two groups: control group and laser group. Control group The felt-tip mark on the dentin was erased from 108 teeth using a diamond bur SF-41 (MANI, Japan), and the dentin surfaces were submitted to 37 % phosphoric acid and Prime & Bond NT in Group 1 (23 blocks), G-Bond plus in Group 2 (23 blocks), XENOV in Group 3 (20 blocks), iBond Self-Etch in Group 4 (21 blocks), and Adper Easy One in Group 5 (22 blocks).
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Table 1 Adhesives used in this study Adhesives
Ma components
G-Bond plus (GC)
Phosphoric acid ester monomer, 4-MET, DMA, water, acetone, nano silica filler, initiator
XENO V (Dentsply)
Bifunctional acrylic amides, acrylamido alkylsulfonic acid, ‘inverse’ functionalizedphosphoric acid ester, acrylic acid, water, tertiary butanol, butylated benzenediol, CQ, initiator, stabilizer UDMA, 4-MTEA, glutaraldehyde, acetone, water, CQ, photoinitiators, stabilizers
1005001791
1.38
010112
2.21
Adper Easy One (3 M ESPE)
HEMA, Bis-GMA, methacrylated phosphoric esters, 1,6 hexanediol dimethacrylate, methacrylate,functionalized polyalkenoic acid, silica filler, ethanol, water, initiators, stabilizers
442083
2.3
Apply adhesive to tooth surface for a total of 20 s, dry the adhesive for 5 s, and light cure for 10 s.
Prime&Bond NT (Dentsply)
Di-and Trimethacrylate resins, PENTA, nanofillersamorphous silicon dioxide photoinitiators, stabilizers, cetylamine hydrofluoride, acetone
1008000657
—-
Application of 37 % phosphoric acid gel for at least 15 s; rinsing for 15 s and drying by air blow; application of adhesive and leaving the surface wet for 20 s; gently air dry for 5 s; light-curing for 10 s
i Bond Self Etch (Heraeus)
Lot no.
pH
1003051 1.5
Application protocol Dispense one drop of liquid into well. Apply dried dentin for 10 s. Subject to a strong stream of air to dry for 5 s and light irradiation for 10 s. Apple XENO V sufficiently, wetting the surface uniformly. Then gently agitate the adhesive 20 s. Evaporate solvent thoroughly by air blow until there is no more movement of the adhesive, but for at least 5 s. Avoid pooling. Cure for 20 s. Apply adhesive to tooth surface foe a total of 20 s, dry the adhesive, and light cure for 20 s.
CQ camphorquinone, 4-MET 4-methacryloxyethyl trimellitic acid, DMA dimethacrylate, HEMA 2-hydroxyethyl methacrylate, Bis-GMA bisphenol-A-diglycidyl methacrylate
Laser group The felt-tip mark on the dentin was erased from 105 teeth using an Er:YAG laser Smart-2940DEr:YAG laser (DEKA,Italy), and the dentin surfaces were irradiated under the following conditions: 200 mJ/pulse; very short pulse (VSP) mode, 20 Hz, output power of 4 W, water spray (5–10 ml/min), a focal distance of 1 mm, focused mode and non-contact hand pieces. All of the settings used in this study were selected in accordance with the laser manufacturer’s instructions. The irradiations were made with the beam aligned perpendicular to the surface and moved in a sweeping fashion by hand during the exposure period. After irradiation, the dentin was submitted to 37 % phosphoric acid and Prime & Bond NT in Group 6 (20 blocks), G-Bond plus in Group 7 (23 blocks), XENOV in Group 8 (18 blocks), iBond Self Etch in Group 9 (21 blocks), and Adper Easy One in Group 10 (22 blocks).
Surface treatments Each sample was subjected to the bonding procedure according to the group, following the manufacturer’s instructions (Table 1). The composition of the adhesive used in this study is also listed in Table 1. The bonding area was 7.07 mm2. A split bisected metal matrix was placed over the center of the dentin surface, which formed an inverted conical cavity that was 3 mm in diameter at the
bottom, 5.6 mm at the top, and 3 mm deep. The composite resin Filtek Z-250 (3 M ESPE, Germany) filled the matrix in three increments, filling the first increments no more than 1 mm thick. Each matrix was polymerized for 20 s with mini L.E.D. (Satelek, France). When the matrix cavity was completely filled, the matrix was opened and separated, which left an inverted composite resin cone adhered to the dentin surface.
TBS test After 24 h of storage in distilled water at 37 °C, the samples were submitted to a TBS test in a universal testing machine (SANS, China) at a speed of 0.5 mm/min until fracturing occurred. The average values (in MPa) and standard deviation were obtained for each group. The data were submitted to SPSS 11.0 for statistical analysis using Dunnett T3 coupled to a general linear model. A 5 % level of statistical significance was used for all testing.
SEM evaluation of treated dentin surface The morphological changes after treatment with bur cutting or laser ablation, as well as the treated dentin surface, were observed under a scanning electron microscope. All specimens were immersed in 2 % glutaraldehyde in 0.1 mol/L
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sodium hydroxide buffer at pH 7.4 for 48 h. They were then serially dehydrated in graded ethanol solutions (25, 50 75, 95 and 100 % ethanol) at 45-min intervals, mounted on aluminum stubs, sputter-coated with gold–palladium alloy, and finally examined by a JSM-5610LV (JEOL Ltd., Japan) (Fig. 1).
Results TBS test The mean TBS values and standard deviations are summarized for each experimental group in Table 2. The mean TBS values ranged from 5.02 MPa (group 2) to 14.18 MPa (group 6). The Test of Homogeneity of Variances confirmed that the equal variances of values in all groups were not assumed (p \ 0.001). The general linear model analysis demonstrated significant differences for the factors adhesive system (p \ 0.001) and no significant interaction between the control group and laser group (p = 0.121). Comparisons of the means with the Dunnett T3 test found differences, as shown in Table 2. SEM evaluation of treated dentin surface Bur-prepared dentin showed a homogeneous surface presence of a smear layer and closed dentinal tubules (Fig. 2a). Dentin irradiated with an Er:YAG laser showed a scaly, irregular and rugged appearance. Open dentinal tubules were clearly visible with more prominent peritubular dentin (black arrows) than intertubular dentin (white arrows), although some dentinal tubules were still partially occluded (Fig. 2g). The SEM micrographs illustrate different relationships between adhesive systems and dental structures. Fig. 2b–f shows the results of etch patterns to bur-prepared dentin; Fig. 2h–l presents the results of etch patterns to lased dentin. All the tubules were opened, and the smear layers were completely removed with 37 % phosphoric acid etching after bur preparation (Fig. 2b). Bur-prepared dentin surface was supplied to G-bond plus, iBond Self Etch and Adper Easy One. The dentin surface was still mostly covered with a smear layer, as well as
adhesive particulate, on which a few dentinal tubules are visible (Fig. 2c, e, f). Etch patterns for XENOV on burprepared dentin displays more and wider opened dentinal tubules, and the smear layers were partially removed (Fig. 2d). The tubular openings with phosphoric acid etching after laser irradiation were larger in size compared to those with laser irradiation. More homogeneous surfaces had no protrusion of peritubular dentin from the surrounding intertubular dentin, and fibrous structure is noted in the inner wall of dentinal tubule orifices (Fig. 2h). XENOV and Bond Self-Etch supplied to lased dentin surface, which made the surface less irregular and rugged (Fig. 2j, k).
Discussion Among innovations for teeth preparation, the Er:YAG laser has been the most promising laser for cavity preparations ever since Hibst and Keller [7] first used it in dentistry in 1989. The Er:YAG laser is an instrument for dental hard tissue ablation with a 2.94 lm emission wavelength, which coincides with the strongest absorption peak of water and OH- groups in hydroxyapatite of dental hard tissue. Such features are important for laser ablation, which vaporizes the water present in the tissue, leading to microexplosion and then removal of the organic and inorganic portion of dentin [8]. This laser has been shown to be safe for dental pulp and adjacent tissues as the irradiated area produces little elevation of temperature (\5.5 °C) [9]. In addition, it has an antibacterial effect [10]. The patient can remain comfortable during dental treatment because of reduced noise and decreased pain sensitivity, which sometimes precludes the need for anesthetics [11]. Exclusively, the application of erbium lasers can change the chemical composition of dental structure to achieve acid-resistant surfaces and, consequently, reduced susceptibility to secondary cavities [12]. The microcrater-like surface obtained by this laser is rough and rugged with no smear layer in SEM studies of laser-irradiated dentin. Peritubular dentin was observed to be more prominent than intertubular dentin (Fig. 2g). We compared the bond strength of five different adhesive systems applied to dentin prepared either by a high-
Table 2 Tensile bond strengths (MPa) Prime & bond NT a
Control group
10.67(5.78)
Laser group
14.18(6.20)AB
G-Bond plus abc
XENO V
iBond self-etch b
5.02(1.87)
9.53(5.70)
9.39(3.37)
5.05(2.62)ACD
9.27(4.92)C
6.90(2.67)BE
Adper Easy One 12.75(4.29)c 11.90(5.65)DE
Values are the means (standard deviation). Groups identified with the same superscript letters in the same row are statistically significant in TBS (p \ 0.05)
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7057 b Fig. 2 SEM micrographs of treated dentin surface (2,5009) (Scale
bar 10 lm). b–f results of etch patterns to bur prepared dentin; h– l results of etch patterns to lased dentin. a Bur prepared dentin shows a homogeneous surface presence of a smear layer and closed dentinal tubules. b All the tubulars were opened and smear layer was completely removed with 37 % phosphoric acid aching after bur prepared. c, e, f Bur prepared dentin surface supplied to G-bond plus, i Bond Self Etch and Adper Easy One respectively, still mostly covers with a smear layer as well as particulate of the adhesive, which a few dentinal tubules can be visible. d Etch patterns for XENOVon bur prepared dentin displays more and wider opened dentinal tubules than c, e and f, and smear layer was partially removed. g Dentin irradiated with Er:YAG laser (200 mJ, 20 Hz) showed a scaly, irregular and rugged appearance. Open dentinal tubules were clearly visible with more prominent peritubular dentin (black arrows) than intertubular dentin (white arrows), although some dentinal tubules were still partially occluded. h The tubular openings with 37 % phosphoric acid aching after laser irradiation were larger in size compared to those with laser irradiation. More homogeneous surface has no protrusion of peritubular dentin from the surrounding intertubular dentin and fibrous structure is noted in the inner wall of dentinal tubule orifices. i–l Application of G-bond plus, and Adper Easy One to lased dentin surface respectively, didn’t alter the lased appearance a lot except for covering with particulate of the adhesive. j, k XENOV and Bond Self Etch was supplied to Lased dentin surface respectively, making the surface less irregular and rugged
speed air turbine handpiece or Er:YAG laser. In our study, the use of an Er:YAG laser did not significantly impact the ability of the five tested adhesives to dentin (p = 0.121), which agrees with the first null hypothesis. Why the
irregularities and open dentinal tubules of lased dentin as shown in Fig. 2g, which seemed to be suitable to resin restoration, did not obtain a better adhesive strength? There are several possible explanations for this result. It has been reported that Er:YAG laser irradiation produces changes in the composition and conformation of the organic matrix at the dentin surface, which results in partial collagen degradation and 3–5 lm of denatured dentin subsurface. It is well known that a hybrid layer will be adequately formed only in the presence of preserved collagen structure; otherwise, monomer permeation would not occur. When the remaining denatured collagen fibrils fuse together, with cross-banding lost, adequate resin diffusion into the interfibrillar collagen spaces is prevented, thus compromising bond strength [13]. As suggested by Cardoso et al. [14], salient irregularities on the lased dentin surface may reduce the bond strength by preventing uniform stress distribution at the adhesive–dentin interface. Moreover, the presence of irregularities on the dentinal surface leads to a non-uniform thickness of the adhesive layer, which diminishes bonding effectiveness. The one-step self-etching approach is an alternative based on the use of non-rinse acidic monomers that simultaneously condition, prime and bond tooth tissues. Compared to phosphoric acid, self-etching adhesives have several advantages. Most importantly, self-etching adhesives prevent excessive decalcification, which is characteristic of phosphoric acid etching [15]. Second, the combination of etching, priming and bonding reduces technique sensitivity because it eliminates the dependence on moist bonding, which is characteristic of etch-and-rinse
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adhesive systems, thus also reducing the clinical application time. Another important quality of the self-etch approach is that infiltration of monomers occurs simultaneously with the self-etching process. In this way, the possibility of discrepancies between both processes and, consequently, the presence of an unprotected collagen fiber area is significantly reduced as well as nanoleakage [16, 17]. This study shows the different bonding propertos of five different adhesives (including one two-step total-etching adhesive and four one-step self-etching adhesives) to bur-prepared dentin or laser irradiated dentin, with rejection to the second null hypothesis. Lased dentin responds differently to acid etching, which is more acid-resistant compared to surfaces prepared with bur [12]. Peritubular dentin has lower water and higher mineral content than intertubular dentin; therefore, the Er:YAG laser mechanisms based on micro-explosions due to water vaporization were not as effective on peritubular dentin as they were on intertubular dentin. Additionally, the crystal that melted into the peritubular dentin may have created an acid-resistant barrier against resin infiltration owing to the high inorganic content of peritubular dentin in contrast to the porous layer created on the less mineralized intertubular dentin [18]. Consequently, it is possible that weak acids present in the self-etching system cannot sufficiently modify the surface to promote adhesive penetration, which leads to a reduction in the adhesive strength of lased dentin. However, we found an opposite result. XENOV (pH = 1.38) and Adper Easy One (pH = 2.3) performed to the level of Prime & Bond NT in bonding to lased dentin. Depending on etching aggressiveness, self-etch adhesives can be subdivided into strong (pH B 1), intermediary strong (pH & 1.5) and mild (pH & 2.0) [16, 19]. None of the self-etch systems applied in this research were strong self-etch adhesives. As observed in the SEM micrographs, all of the self-etch adhesives displayed different interaction patterns compared to those with phosphoric acid treatment. For example, the self-etch adhesives showed less widening and opening of dentinal tubules (Fig. 2c–f) and less flattening of the rugged surface (Fig. 2i–l). However, this finding did not seem to explain the good performances of XENOV and Adper Easy One. Interaction patterns on dentin perhaps should not be the primary determinant of bond strength. ‘Mild’ self-etch adhesives only partially dissolve the dentin surface, which leaves a substantial number of hydroxyapatite crystals within the hybrid layer. Specific carboxyl or phosphate groups of functional monomers can then chemically interact with this residual hydroxyapatite [20]. Another significant advantage of a mild self-etch system is the preservation of some hydroxyapatite crystal around collagen fibers [15]. This characteristic may protect
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the collagen against hydrolysis and, therefore, early degradation of the bond [21]. To some extent, it also accounted for the high TBS of the mild self-etch system Adper Easy One. Knowledge of the composition, characteristics and mechanisms of adhesion of each adhesive system is of fundamental importance to the clinical adoption of ideal bonding strategies. It has been reported that HEMA treatment following acid treatment produced a higher bond strength than Er:YAG laser treatment in irradiated dentine [22]. The higher resin bonding effect of HEMA has been suggested to be the result of both physical and chemical reactions in the dentine. There is preliminary evidence that HEMA may partially regenerate the a-helix structure of the acid-etched dentine collagen by Fouriertransformed microscopy [23]. Moreover, HEMA was reported to enhance the wettability of the bonding agent, which promotes higher resin bonding [24]. Thus, the effect of HEMA on dentine bonding may have help Adper Easy One to achieve high TBS. However, because of the lack of clear information on the exact volume of monomer in each product, it cannot be determined whether the monomers were actually a co-mixture of monomers, which makes it difficult to establish more correlation with the TBS results. 4-MET can chemically interact with hydroxyapatite within a clinical time period, and this interaction has been connected to better resistance to degradation by preventing micro- and nanoleakage [25]. Why does G-bond plus containing 4-MET present the lowest TBS of the five adhesives? One reason may be the special application of a strong stream of air for 5 s after adhesive coating. The air-blowing of the adhesive might help to remove interfacial water, thus improving the bonding effectiveness. However, this procedure is somewhat controversial [26]. It has been stated that a strong air stream may increase the adhesive thickness in the cavity angles and denude part of dentin. A thin, uniform layer of bonding resin is a critical, elastic intermediary for absorbing stresses of polymerization shrinkage. Contamination of the dentinal surface with the presence of air voids produced by a strong air stream will make bonding unpredictable under clinical conditions.
Conclusions Er:YAG laser treatment has the great advantage of yielding decontaminated surfaces and performs as well as bur treatment on TBS. For the development of the seventh adhesive system, some of the one-bottle self-etch adhesives possess a good TBS to the level of total-etch adhesives. Considering the advantages of the one-bottle self-etch
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adhesives, Er:YAG laser prepared dentin applied to onebottle self-etch adhesives can be an alternative choice in restorative dentistry. Acknowledgments This study was funded by a grant (20121214022) from Liwan District Minsheng science and technology project. The authors thank Shan-Bin MOU from Wuhan University of Technology for assistance in specimen SEM evaluations and Qian ZHAO from Guangzhou Medical University for supplementary of experiment design.
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Comparison of tensile bond strengths of four one-bottle self-etching adhesive systems with Er:YAG laser-irradiated dentin Qianzhou Jiang • Minle Chen • Jiangfeng Ding
Received: 5 January 2013 / Accepted: 25 October 2013 / Published online: 5 November 2013 Ó Springer Science+Business Media Dordrecht 2013
Abstract This study aimed to investigate the interaction of current one-bottle self-etching adhesives and Er:YAG laser with dentin using a tensile bond strength (TBS) test and scanning electron microscopy (SEM) in vitro. Two hundred and thirteen dentin discs were randomly distributed to the Control Group using bur cutting and to the Laser Group using an Er:YAG laser (200 mJ, VSP, 20 Hz). The following adhesives were investigated: one two-step total-etch adhesive [Prime & Bond NT (Dentsply)] and four one-step self-etch adhesives [G-Bond plus (GC), XENO V (Dentsply), iBond Self Etch (Heraeus) and Adper Easy One (3 M ESPE)]. Samples were restored with composite resin, and after 24-hour storage in distilled water, subjected to the TBS test. For morphological analysis, 12 dentin specimens were prepared for SEM. No significant differences were found between the control group and laser group (p = 0.899); dentin subjected to Prime & Bond NT, XENOV and Adper Easy One produced higher TBS. In conclusion, this study indicates that Er:YAG laser-prepared dentin can perform as well as bur on TBS, and some of the one-step one-bottle adhesives are comparable to the total-etch adhesives in TBS on dentin. Keywords One-bottle self-etching adhesives Tensile bond strength Scanning electron microscope
Q. Jiang (&) J. Ding Key Laboratory of Oral Medicine, School and Hospital of Stomatology, Guangzhou Medical University, 59 Huangsha Thoroughfare, Guangzhou 510140, China e-mail: [email protected] Q. Jiang M. Chen Guangzhou Medical University, 195 Dongfengxi Road, Guangzhou 510182, China
Introduction In the adhesive dentistry field, a great deal of effort has been directed toward simplifying adhesive procedures. To this end, self-etching systems, which do not need previous smear layer removal, have opened up a new horizon. The self-etching systems simultaneously promote demineralization and resin infiltration through the dental surfaces. Recently, one-step one-bottle self-etch adhesives that combine etching, priming, and bonding into a single step have been developed. However, these adhesive systems are generally reported to show reduced bonding properties compared with two-step self-etching adhesives [1]. Nonetheless, simplification of the bonding steps, shorter application time and less technique sensitivity are very attractive to clinicians. The erbium: yttrium–aluminum–garnet (Er:YAG) laser is a new type of laser-powered hydrokinetic system, which is now beginning to be studied and applied in dentistry [3– 8]. Use of the Er:YAG laser is currently accepted as an alternative method for cavity preparation in dental hard tissues. Cavities prepared with an Er:YAG laser present irregular margins and rough walls and a rugged floor, which differs from the basic principles of cavity preparation established by Black [2]. The surface irregularity of laserirradiated dentin seems to be better for adhesive restorations. However, reports concerning the bonding capability of the irradiated dentin have been controversial [3, 4]. To the best of our knowledge, few studies have been undertaken to examine the bond strength of one-bottle selfetching adhesives to Er:YAG laser irradiated dentin [4, 5]. The purpose of this study was to investigate the effect of an Er:YAG laser on dentin bonding and compare the tensile bond strength (TBS) of four one-bottle self-etching adhesives to the Er:YAG laser irradiated dentin, using the total-
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etch adhesive Prime & Bond NT as a control. This study explored the potential and feasibility of using these newly developed products for laser dentin bonding. Scanning electron microscopy (SEM) morphological analysis of treated dentin surfaces was also performed to verify of the adhesive effect. The null hypotheses tested were that (1) Er:YAG laser irradiation has no effect on TBS of the dentin bonding and (2) there is no difference between the five adhesives.
Materials and methods One hundred and seven freshly extracted human molars and premolars with no cavities or fillings were selected and kept in 4 °C distilled water. The molars were obtained under a protocol approved by the Ethics Committee of the Guangzhou Medical University and with the informed consent of the donors. The roots were separated and the crowns were cut in the mesiodistal direction using a diamond bur with water spray. The methods used in this research were referred to in Bahrami’s [6] study, and the schema was shown on Fig. 1.
Fig. 1 Steps in the preparation of the samples a root section and mesio-distal section of the crowns with a cylindrical diamonds drill; b crowns embedded in a self-curing resin; c dentin exposure and polishment; d adhesive procedure and placement of two-piece Teflon matrix; e inverted cone of composite resin restorations; f, g TBS test after 24 h of storage in distilled water
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Dentin specimen preparation Lingual or buccal crown sections were embedded in selfcuring resin and then the enamel surfaces were removed. The specimens were polished flat using 150-, 240- and 400-grit silicon carbide waterproof abrasive papers under running water to insure enamel-free dentin surface. Finally, the dentin surface was polished with 600-grit silicon carbide waterproof abrasive paper under running water for 60 s to standardize the smear layer. A circular mark was made in the center of each dentin surface with an indelible felt-tip pen. Then, 213 samples were made and randomly divided into two groups: control group and laser group. Control group The felt-tip mark on the dentin was erased from 108 teeth using a diamond bur SF-41 (MANI, Japan), and the dentin surfaces were submitted to 37 % phosphoric acid and Prime & Bond NT in Group 1 (23 blocks), G-Bond plus in Group 2 (23 blocks), XENOV in Group 3 (20 blocks), iBond Self-Etch in Group 4 (21 blocks), and Adper Easy One in Group 5 (22 blocks).
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Table 1 Adhesives used in this study Adhesives
Ma components
G-Bond plus (GC)
Phosphoric acid ester monomer, 4-MET, DMA, water, acetone, nano silica filler, initiator
XENO V (Dentsply)
Bifunctional acrylic amides, acrylamido alkylsulfonic acid, ‘inverse’ functionalizedphosphoric acid ester, acrylic acid, water, tertiary butanol, butylated benzenediol, CQ, initiator, stabilizer UDMA, 4-MTEA, glutaraldehyde, acetone, water, CQ, photoinitiators, stabilizers
1005001791
1.38
010112
2.21
Adper Easy One (3 M ESPE)
HEMA, Bis-GMA, methacrylated phosphoric esters, 1,6 hexanediol dimethacrylate, methacrylate,functionalized polyalkenoic acid, silica filler, ethanol, water, initiators, stabilizers
442083
2.3
Apply adhesive to tooth surface for a total of 20 s, dry the adhesive for 5 s, and light cure for 10 s.
Prime&Bond NT (Dentsply)
Di-and Trimethacrylate resins, PENTA, nanofillersamorphous silicon dioxide photoinitiators, stabilizers, cetylamine hydrofluoride, acetone
1008000657
—-
Application of 37 % phosphoric acid gel for at least 15 s; rinsing for 15 s and drying by air blow; application of adhesive and leaving the surface wet for 20 s; gently air dry for 5 s; light-curing for 10 s
i Bond Self Etch (Heraeus)
Lot no.
pH
1003051 1.5
Application protocol Dispense one drop of liquid into well. Apply dried dentin for 10 s. Subject to a strong stream of air to dry for 5 s and light irradiation for 10 s. Apple XENO V sufficiently, wetting the surface uniformly. Then gently agitate the adhesive 20 s. Evaporate solvent thoroughly by air blow until there is no more movement of the adhesive, but for at least 5 s. Avoid pooling. Cure for 20 s. Apply adhesive to tooth surface foe a total of 20 s, dry the adhesive, and light cure for 20 s.
CQ camphorquinone, 4-MET 4-methacryloxyethyl trimellitic acid, DMA dimethacrylate, HEMA 2-hydroxyethyl methacrylate, Bis-GMA bisphenol-A-diglycidyl methacrylate
Laser group The felt-tip mark on the dentin was erased from 105 teeth using an Er:YAG laser Smart-2940DEr:YAG laser (DEKA,Italy), and the dentin surfaces were irradiated under the following conditions: 200 mJ/pulse; very short pulse (VSP) mode, 20 Hz, output power of 4 W, water spray (5–10 ml/min), a focal distance of 1 mm, focused mode and non-contact hand pieces. All of the settings used in this study were selected in accordance with the laser manufacturer’s instructions. The irradiations were made with the beam aligned perpendicular to the surface and moved in a sweeping fashion by hand during the exposure period. After irradiation, the dentin was submitted to 37 % phosphoric acid and Prime & Bond NT in Group 6 (20 blocks), G-Bond plus in Group 7 (23 blocks), XENOV in Group 8 (18 blocks), iBond Self Etch in Group 9 (21 blocks), and Adper Easy One in Group 10 (22 blocks).
Surface treatments Each sample was subjected to the bonding procedure according to the group, following the manufacturer’s instructions (Table 1). The composition of the adhesive used in this study is also listed in Table 1. The bonding area was 7.07 mm2. A split bisected metal matrix was placed over the center of the dentin surface, which formed an inverted conical cavity that was 3 mm in diameter at the
bottom, 5.6 mm at the top, and 3 mm deep. The composite resin Filtek Z-250 (3 M ESPE, Germany) filled the matrix in three increments, filling the first increments no more than 1 mm thick. Each matrix was polymerized for 20 s with mini L.E.D. (Satelek, France). When the matrix cavity was completely filled, the matrix was opened and separated, which left an inverted composite resin cone adhered to the dentin surface.
TBS test After 24 h of storage in distilled water at 37 °C, the samples were submitted to a TBS test in a universal testing machine (SANS, China) at a speed of 0.5 mm/min until fracturing occurred. The average values (in MPa) and standard deviation were obtained for each group. The data were submitted to SPSS 11.0 for statistical analysis using Dunnett T3 coupled to a general linear model. A 5 % level of statistical significance was used for all testing.
SEM evaluation of treated dentin surface The morphological changes after treatment with bur cutting or laser ablation, as well as the treated dentin surface, were observed under a scanning electron microscope. All specimens were immersed in 2 % glutaraldehyde in 0.1 mol/L
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sodium hydroxide buffer at pH 7.4 for 48 h. They were then serially dehydrated in graded ethanol solutions (25, 50 75, 95 and 100 % ethanol) at 45-min intervals, mounted on aluminum stubs, sputter-coated with gold–palladium alloy, and finally examined by a JSM-5610LV (JEOL Ltd., Japan) (Fig. 1).
Results TBS test The mean TBS values and standard deviations are summarized for each experimental group in Table 2. The mean TBS values ranged from 5.02 MPa (group 2) to 14.18 MPa (group 6). The Test of Homogeneity of Variances confirmed that the equal variances of values in all groups were not assumed (p \ 0.001). The general linear model analysis demonstrated significant differences for the factors adhesive system (p \ 0.001) and no significant interaction between the control group and laser group (p = 0.121). Comparisons of the means with the Dunnett T3 test found differences, as shown in Table 2. SEM evaluation of treated dentin surface Bur-prepared dentin showed a homogeneous surface presence of a smear layer and closed dentinal tubules (Fig. 2a). Dentin irradiated with an Er:YAG laser showed a scaly, irregular and rugged appearance. Open dentinal tubules were clearly visible with more prominent peritubular dentin (black arrows) than intertubular dentin (white arrows), although some dentinal tubules were still partially occluded (Fig. 2g). The SEM micrographs illustrate different relationships between adhesive systems and dental structures. Fig. 2b–f shows the results of etch patterns to bur-prepared dentin; Fig. 2h–l presents the results of etch patterns to lased dentin. All the tubules were opened, and the smear layers were completely removed with 37 % phosphoric acid etching after bur preparation (Fig. 2b). Bur-prepared dentin surface was supplied to G-bond plus, iBond Self Etch and Adper Easy One. The dentin surface was still mostly covered with a smear layer, as well as
adhesive particulate, on which a few dentinal tubules are visible (Fig. 2c, e, f). Etch patterns for XENOV on burprepared dentin displays more and wider opened dentinal tubules, and the smear layers were partially removed (Fig. 2d). The tubular openings with phosphoric acid etching after laser irradiation were larger in size compared to those with laser irradiation. More homogeneous surfaces had no protrusion of peritubular dentin from the surrounding intertubular dentin, and fibrous structure is noted in the inner wall of dentinal tubule orifices (Fig. 2h). XENOV and Bond Self-Etch supplied to lased dentin surface, which made the surface less irregular and rugged (Fig. 2j, k).
Discussion Among innovations for teeth preparation, the Er:YAG laser has been the most promising laser for cavity preparations ever since Hibst and Keller [7] first used it in dentistry in 1989. The Er:YAG laser is an instrument for dental hard tissue ablation with a 2.94 lm emission wavelength, which coincides with the strongest absorption peak of water and OH- groups in hydroxyapatite of dental hard tissue. Such features are important for laser ablation, which vaporizes the water present in the tissue, leading to microexplosion and then removal of the organic and inorganic portion of dentin [8]. This laser has been shown to be safe for dental pulp and adjacent tissues as the irradiated area produces little elevation of temperature (\5.5 °C) [9]. In addition, it has an antibacterial effect [10]. The patient can remain comfortable during dental treatment because of reduced noise and decreased pain sensitivity, which sometimes precludes the need for anesthetics [11]. Exclusively, the application of erbium lasers can change the chemical composition of dental structure to achieve acid-resistant surfaces and, consequently, reduced susceptibility to secondary cavities [12]. The microcrater-like surface obtained by this laser is rough and rugged with no smear layer in SEM studies of laser-irradiated dentin. Peritubular dentin was observed to be more prominent than intertubular dentin (Fig. 2g). We compared the bond strength of five different adhesive systems applied to dentin prepared either by a high-
Table 2 Tensile bond strengths (MPa) Prime & bond NT a
Control group
10.67(5.78)
Laser group
14.18(6.20)AB
G-Bond plus abc
XENO V
iBond self-etch b
5.02(1.87)
9.53(5.70)
9.39(3.37)
5.05(2.62)ACD
9.27(4.92)C
6.90(2.67)BE
Adper Easy One 12.75(4.29)c 11.90(5.65)DE
Values are the means (standard deviation). Groups identified with the same superscript letters in the same row are statistically significant in TBS (p \ 0.05)
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7057 b Fig. 2 SEM micrographs of treated dentin surface (2,5009) (Scale
bar 10 lm). b–f results of etch patterns to bur prepared dentin; h– l results of etch patterns to lased dentin. a Bur prepared dentin shows a homogeneous surface presence of a smear layer and closed dentinal tubules. b All the tubulars were opened and smear layer was completely removed with 37 % phosphoric acid aching after bur prepared. c, e, f Bur prepared dentin surface supplied to G-bond plus, i Bond Self Etch and Adper Easy One respectively, still mostly covers with a smear layer as well as particulate of the adhesive, which a few dentinal tubules can be visible. d Etch patterns for XENOVon bur prepared dentin displays more and wider opened dentinal tubules than c, e and f, and smear layer was partially removed. g Dentin irradiated with Er:YAG laser (200 mJ, 20 Hz) showed a scaly, irregular and rugged appearance. Open dentinal tubules were clearly visible with more prominent peritubular dentin (black arrows) than intertubular dentin (white arrows), although some dentinal tubules were still partially occluded. h The tubular openings with 37 % phosphoric acid aching after laser irradiation were larger in size compared to those with laser irradiation. More homogeneous surface has no protrusion of peritubular dentin from the surrounding intertubular dentin and fibrous structure is noted in the inner wall of dentinal tubule orifices. i–l Application of G-bond plus, and Adper Easy One to lased dentin surface respectively, didn’t alter the lased appearance a lot except for covering with particulate of the adhesive. j, k XENOV and Bond Self Etch was supplied to Lased dentin surface respectively, making the surface less irregular and rugged
speed air turbine handpiece or Er:YAG laser. In our study, the use of an Er:YAG laser did not significantly impact the ability of the five tested adhesives to dentin (p = 0.121), which agrees with the first null hypothesis. Why the
irregularities and open dentinal tubules of lased dentin as shown in Fig. 2g, which seemed to be suitable to resin restoration, did not obtain a better adhesive strength? There are several possible explanations for this result. It has been reported that Er:YAG laser irradiation produces changes in the composition and conformation of the organic matrix at the dentin surface, which results in partial collagen degradation and 3–5 lm of denatured dentin subsurface. It is well known that a hybrid layer will be adequately formed only in the presence of preserved collagen structure; otherwise, monomer permeation would not occur. When the remaining denatured collagen fibrils fuse together, with cross-banding lost, adequate resin diffusion into the interfibrillar collagen spaces is prevented, thus compromising bond strength [13]. As suggested by Cardoso et al. [14], salient irregularities on the lased dentin surface may reduce the bond strength by preventing uniform stress distribution at the adhesive–dentin interface. Moreover, the presence of irregularities on the dentinal surface leads to a non-uniform thickness of the adhesive layer, which diminishes bonding effectiveness. The one-step self-etching approach is an alternative based on the use of non-rinse acidic monomers that simultaneously condition, prime and bond tooth tissues. Compared to phosphoric acid, self-etching adhesives have several advantages. Most importantly, self-etching adhesives prevent excessive decalcification, which is characteristic of phosphoric acid etching [15]. Second, the combination of etching, priming and bonding reduces technique sensitivity because it eliminates the dependence on moist bonding, which is characteristic of etch-and-rinse
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adhesive systems, thus also reducing the clinical application time. Another important quality of the self-etch approach is that infiltration of monomers occurs simultaneously with the self-etching process. In this way, the possibility of discrepancies between both processes and, consequently, the presence of an unprotected collagen fiber area is significantly reduced as well as nanoleakage [16, 17]. This study shows the different bonding propertos of five different adhesives (including one two-step total-etching adhesive and four one-step self-etching adhesives) to bur-prepared dentin or laser irradiated dentin, with rejection to the second null hypothesis. Lased dentin responds differently to acid etching, which is more acid-resistant compared to surfaces prepared with bur [12]. Peritubular dentin has lower water and higher mineral content than intertubular dentin; therefore, the Er:YAG laser mechanisms based on micro-explosions due to water vaporization were not as effective on peritubular dentin as they were on intertubular dentin. Additionally, the crystal that melted into the peritubular dentin may have created an acid-resistant barrier against resin infiltration owing to the high inorganic content of peritubular dentin in contrast to the porous layer created on the less mineralized intertubular dentin [18]. Consequently, it is possible that weak acids present in the self-etching system cannot sufficiently modify the surface to promote adhesive penetration, which leads to a reduction in the adhesive strength of lased dentin. However, we found an opposite result. XENOV (pH = 1.38) and Adper Easy One (pH = 2.3) performed to the level of Prime & Bond NT in bonding to lased dentin. Depending on etching aggressiveness, self-etch adhesives can be subdivided into strong (pH B 1), intermediary strong (pH & 1.5) and mild (pH & 2.0) [16, 19]. None of the self-etch systems applied in this research were strong self-etch adhesives. As observed in the SEM micrographs, all of the self-etch adhesives displayed different interaction patterns compared to those with phosphoric acid treatment. For example, the self-etch adhesives showed less widening and opening of dentinal tubules (Fig. 2c–f) and less flattening of the rugged surface (Fig. 2i–l). However, this finding did not seem to explain the good performances of XENOV and Adper Easy One. Interaction patterns on dentin perhaps should not be the primary determinant of bond strength. ‘Mild’ self-etch adhesives only partially dissolve the dentin surface, which leaves a substantial number of hydroxyapatite crystals within the hybrid layer. Specific carboxyl or phosphate groups of functional monomers can then chemically interact with this residual hydroxyapatite [20]. Another significant advantage of a mild self-etch system is the preservation of some hydroxyapatite crystal around collagen fibers [15]. This characteristic may protect
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the collagen against hydrolysis and, therefore, early degradation of the bond [21]. To some extent, it also accounted for the high TBS of the mild self-etch system Adper Easy One. Knowledge of the composition, characteristics and mechanisms of adhesion of each adhesive system is of fundamental importance to the clinical adoption of ideal bonding strategies. It has been reported that HEMA treatment following acid treatment produced a higher bond strength than Er:YAG laser treatment in irradiated dentine [22]. The higher resin bonding effect of HEMA has been suggested to be the result of both physical and chemical reactions in the dentine. There is preliminary evidence that HEMA may partially regenerate the a-helix structure of the acid-etched dentine collagen by Fouriertransformed microscopy [23]. Moreover, HEMA was reported to enhance the wettability of the bonding agent, which promotes higher resin bonding [24]. Thus, the effect of HEMA on dentine bonding may have help Adper Easy One to achieve high TBS. However, because of the lack of clear information on the exact volume of monomer in each product, it cannot be determined whether the monomers were actually a co-mixture of monomers, which makes it difficult to establish more correlation with the TBS results. 4-MET can chemically interact with hydroxyapatite within a clinical time period, and this interaction has been connected to better resistance to degradation by preventing micro- and nanoleakage [25]. Why does G-bond plus containing 4-MET present the lowest TBS of the five adhesives? One reason may be the special application of a strong stream of air for 5 s after adhesive coating. The air-blowing of the adhesive might help to remove interfacial water, thus improving the bonding effectiveness. However, this procedure is somewhat controversial [26]. It has been stated that a strong air stream may increase the adhesive thickness in the cavity angles and denude part of dentin. A thin, uniform layer of bonding resin is a critical, elastic intermediary for absorbing stresses of polymerization shrinkage. Contamination of the dentinal surface with the presence of air voids produced by a strong air stream will make bonding unpredictable under clinical conditions.
Conclusions Er:YAG laser treatment has the great advantage of yielding decontaminated surfaces and performs as well as bur treatment on TBS. For the development of the seventh adhesive system, some of the one-bottle self-etch adhesives possess a good TBS to the level of total-etch adhesives. Considering the advantages of the one-bottle self-etch
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adhesives, Er:YAG laser prepared dentin applied to onebottle self-etch adhesives can be an alternative choice in restorative dentistry. Acknowledgments This study was funded by a grant (20121214022) from Liwan District Minsheng science and technology project. The authors thank Shan-Bin MOU from Wuhan University of Technology for assistance in specimen SEM evaluations and Qian ZHAO from Guangzhou Medical University for supplementary of experiment design.
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