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Effect of chemical interaction on the bonding strengths of self-etching adhesives to deproteinised dentine
JJOD-2472; No. of Pages 8 journal of dentistry xxx (2015) xxx–xxx
Available online at www.sciencedirect.com
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Effect of chemical interaction on the bonding strengths of self-etching adhesives to deproteinised dentine Liqun Zhou a,1, Yake Wang a,1, HongYe Yang a, Jinxin Guo a, Franklin R. Tay b, Cui Huang a,* a
The State Key Laboratory Breeding Base of Basic Science of Stomatology (Hubei-MOST) & Key Laboratory for Oral Biomedical Ministry of Education, School & Hospital of Stomatology, Wuhan University, Wuhan, China b Department of Endodontics, College of Dental Medicine, Georgia Regents University, Augusta, GA, USA
article info
abstract
Article history:
Objectives: The present study examined (1) the chemical interaction between three self-
Received 10 November 2014
etching adhesives and sodium hypochlorite (NaOCl)-deproteinised dentine, and (2) the
Received in revised form
influence of NaOCl treatment on bond strength of self-etching adhesives with/without
1 May 2015
adhesive functional monomers to dentine.
Accepted 28 May 2015
Methods: Caries-free dentine disks (control) and those treated with 5.25% NaOCl for 60 s
Available online xxx
were prepared. Xeno V (no functional monomers), G-Bond (containing 4-MET) or S3 Bond (containing 10-MDP) were applied to the NaOCl-treated dentine and either left without
Keywords: Sodium hypochlorite, Self-etching
further treatment, or rinsed with 100% ethanol or distilled water. Attenuated total reflection (ATR) spectroscopy and field-emission scanning electron
Adhesive
microscopy (FE-SEM) were used to evaluate the affinity of functional monomers with
Functional monomer
deproteinised dentine. Chemical interaction between the functional monomers and depro-
Microtensile bond strengths
teinised dentine was evaluated using thin-film X-ray diffraction (TF-XRD). Microtensile bond strength (MTBS) was used to evaluate the mechanical property of the adhesives, either immediately or after thermo-cycling (5–55 8C) for 10,000 cycles. Results: According to the ATR and FE-SEM results, G-Bond and S3 Bond showed stronger affinity to deproteinised dentine than Xeno V even after rinsing with water. TF-XRD showed that chemical interaction between S3 Bond and deproteinised dentine occurred by formation of 10-MDP–Ca salt. Both deproteinisation and thermo-cycling adversely affected the MTBS of Xeno V (P < 0.05) but deproteinisation had no significant influence on S3 Bond. Conclusions: When bonding to NaOCl-treated dentine, self-etch adhesives containing functional monomers (10-MDP) can maintain immediate and aged bond strengths after 10,000 thermal cycles. # 2015 Elsevier Ltd. All rights reserved.
* Corresponding author. Tel.: +86 27 87686130; fax: +86 27 87873260. E-mail address: [email protected] (C. Huang). 1 These authors contributed equally to this work. http://dx.doi.org/10.1016/j.jdent.2015.05.010 0300-5712/# 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Zhou L, et al. Effect of chemical interaction on the bonding strengths of self-etching adhesives to deproteinised dentine. Journal of Dentistry (2015), http://dx.doi.org/10.1016/j.jdent.2015.05.010
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1.
Introduction
Dentine bonding plays an important role in many restorative procedures and is a fundamental aspect of aesthetic dentistry. Dentine bonding systems may be divided into two categories according to their method of application: etch-and-rinse adhesives and self-etching adhesives.1 From the perspective of bonding mechanisms, micro-mechanical interlocking is generally accepted as the primary source of retention for both adhesive types.2 In particular, the formation of hybridised dentine is analogous to tissue engineering, in which dentine collagen fibrils serve as the scaffold for resin infiltration.3 Given their ease of use, acceptable performance in clinical trials of class V restorations and the low incidence of dentine hypersensitivity, self-etching adhesives obtain steadily growing popularity in today’s dental practices.4–9 Apart from micromechanical retention, chemical interactions also contribute to the bonding effectiveness of selfetching adhesives to dentine. These interactions are commonly attributed to the functional resin monomers present in the adhesives, that couple with residual carbonated apatite derived from the self-etched dentine. Yoshida and colleagues10–12 proposed the concept of ‘Adhesion-Decalcification’ and claimed that two phenomena may occur when the functional resin monomers containing carboxyl or phosphate groups come into contact with the partially demineralised dentine substrate. The first phenomenon involves the formation of a Ca–monomer salt if the ionic bond is hydrolytically stable; the second phenomenon involves decalcification and release of calcium and phosphate ions from the tooth surface if the ionic bond is unstable.13 Dentine is an organic–inorganic complex with a microstructure and properties that are the principal determinants of nearly all operations in restorative dentistry.14 The dentine substrate is prone to modification by the chemical agents employed during in dental treatment.15 For instance, sodium hypochlorite (NaOCl), which is the most common endodontic irrigant for chemical debridement of the root canal space, can deproteinise demineralised as well as mineralised dentine.16,17 Nevertheless, the effect of NaOCl collagen deproteinisation on dentine
bonding remains controversial. Lai et al. found that NaOCl deproteinisation adversely lowered the bonding strength of dentine because of its strong oxidising properties.18 By contrast, Hayashi et al. found that NaOCl deproteinisation may improve resin infiltration, thereby resulting in improved dentine bond strength.19 Because of its high alkalinity, NaOCl removes the organic components of dentine, mainly collagen,20 and by doing so, increases the ratio of the inorganic apatite to the organic phase in dentine. It is not known, however, whether this NaOCl-derived shift in phase ratio on dentine surfaces affects the chemical interaction between the functional adhesive resin monomers in self-etching adhesives and deproteinised dentine surface. Thus, the objectives of this study were: (1) to examine the chemical interactions between three self-etching adhesives and NaOCl-treated dentine and (2) to evaluate the effect of NaOCl treatment on the bond strength of self-etching adhesives, with or without the incorporation of functional resin monomers, when these adhesives are bonded to dentine.
2.
Materials and methods
2.1.
Preparation of dentine disks
One hundred and one extracted human non-carious third molars were used in the present study. The teeth were collected after obtaining informed consent from donors under a protocol approved by the Ethics Committee for Human Studies of the School and Hospital of Stomatology, Wuhan University. The teeth were cleaned, stored in 1% chloramine at 4 8C and used within 1 month of extraction. Twenty-seven dentine disks (10 mm 10 mm 1 mm) were cut using a low-speed diamond saw (Isomet; Buehler, Evanston, IL, USA) under water cooling. The surfaces of those disks were sequentially wet-polished with 600-, 800- and 1000grit silicon carbide papers. The disks were then ultrasonically cleaned for 10 min. Twenty-four of the disks were treated with 5.25% NaOCl for 60 s whilst the other three dentine disks were used as controls.
Table 1 – Adhesives investigated in the present study. Adhesive
Manufacturer
Batch No.
Clearfil [S3] Bond
Kuraray Medical Inc., Tokyo, Japan
00168A
G-Bond
GC, Tokyo, Japan
1009071
Xeno V
DENTSPLY Caulk, Milford, DE, USA
1005001791
Composition 10-MDP, HEMA, Bis-GMA, water, ethanol, silanated colloidal silica, CQ, pH 2.7 4-MET, phosphoric-monomer, UDMA, TEGDMA, stabilizer, photo-initiator, silica filler, acetone, water, pH 2 Bifunctional acrylic amides, acrylamido alkylsulfonic acid, ‘inverse’ functionalised phosphoric acid ester, acrylic acid, water, tertiary butanol, butylated benzenediol, CQ, initiator, stabilizer, pH 1.38
Application protocol Apply adhesive; wait for 20 s; air-blow for 5 s; light cure for 10 s. Apply adhesive; wait for 10 s; air-blow for 5 s; light cure for 10 s. Apple adhesive sufficiently, and uniformly; gently agitate 20 s; air blow at least 5 s; Light cure for 20 s.
Please cite this article in press as: Zhou L, et al. Effect of chemical interaction on the bonding strengths of self-etching adhesives to deproteinised dentine. Journal of Dentistry (2015), http://dx.doi.org/10.1016/j.jdent.2015.05.010
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Three self-etching adhesives (with or without functional monomers, Table 1) were applied to 21 NaOCl-treated dentine disks by gently rubbing each dentine surface with a microbrush. After 20 s of adhesive agitation, the samples were airdried and either left without further treatment (referred to as ‘S3_Dentine’, ‘GB_Dentine’ and ‘XE_Dentine’, three specimens per subgroup) or rinsed with 100% ethanol (‘S3_Dentine_EtOH’, ‘GB_Dentine_EtOH’ and ‘XE_Dentine_EtOH’, two specimens per subgroup) or distilled water (‘S3_Dentine_H2O’, ‘GB_Dentine_H2O’ and ‘XE_Dentine_H2O’, two specimens per subgroup, respectively).
2.2. Fourier transform infrared spectroscopy (FTIR) and attenuated total reflection (ATR) spectroscopy FTIR and ATR spectra were carried out with a Nicolet 5700 spectrometer (Thermo Scientific Inc., Madison, WI, USA) and a single-reflection attenuated total reflection (ATR) setup with germanium (Ge) as an internal reflection element (Smart OMNI-Sampler, Thermo Scientific Inc.). The sound dentine disks, NaOCl-treated dentine disks and adhesive-treated dentine disks with and without rinsing, were examined by FTIR spectrophotometer. The specimens were put onto the face of the Ge crystal of the smart OMNIsampler accessory, with the unpolished surface up, adjusted the pointed tip of the standard pressure tower to press onto the mark of samples. Infrared spectra were collected between the range of 4000–675 cm 1 at 4 cm 1 resolution using 32 scans. The three adhesives were tested using FTIR by rubbing them on the surface of slides. FTIR spectra were recorded in the range from 4000 to 400 cm 1 at 4 cm 1 resolution. The specimens were scanned 128 times in each measurement, and the spectrum acquired was the average of all the scans. Background spectrum was automatically subtracted from the acquired spectrum using algorithm incorporated in the OMNIC FTIR software (Thermo Scientific Inc.). The spectral data were expressed as absorbance.
2.3. SEM)
Field-emission scanning electron microscopy (FE-
The surfaces of the sound dentine disks, NaOCl-treated dentine disks and adhesive-treated dentine disks, with and without rinsing were observed using FE-SEM. The prepared samples from Section 2.1 were dried in a drying vessel for 24 h. And then, they were bonded to the copper platform and sputter-coated with Au–Pd alloy. After that, they were observed using FE-SEM at 5 kV (Zeiss, Sigma, Germany).
2.4.
Thin-film X-ray diffraction (TF-XRD)
The sound dentine disks, NaOCl-treated dentine disks and adhesive-treated dentine disks without rinsing were examined by TF-XRD using an X-ray diffractometer (Bruker AXS D8-XRD, Germany) operated under a 40 kV acceleration voltage and 40 mA current with the angle of the incident X-ray beam fixed at 0.58 and a scanning time of 0.028/s for 2u scan.
2.5.
3
Microtensile bond strength (MTBS) evaluation
Eighty-four teeth were sectioned perpendicular to the long axis of each tooth to expose the mid-coronal dentine using the aforementioned diamond saw under water cooling. The exposed dentine surfaces were then polished using 600-grit silicon carbide paper under running water to create standardised surfaces. Half of the teeth were randomly picked out and treated with 5.25% NaOCl for 60 s. According to the manufacturer’s instruction, three selfetching adhesive systems (Xeno V, G-Bond and Clearfil S3 Bond) were randomly applied to either the sound or NaOCl-treated dentine. Resin composite build-ups were then constructed using 4 mm increments of resin composite (Charisma; Heraeus Kulzer, Hanau, Germany). There were 14 teeth in each subgroup. Each tooth was sectioned into 0.9 mm 0.9 mm beams after storage in deionised water at 37 8C for 24 h, and five beams were selected (5 beams per tooth). The beams were assigned for either immediate testing or after thermo-cycling from 5 8C to 55 8C for 10,000 cycles. Thirty-five specimens from each subgroup were used for MTBS evaluation. The exact dimensions of each specimen were measured using a pair of digital calliper. Each specimen was attached to the testing apparatus with cyanoacrylate adhesive (Zapit; Dental Ventures of America, Corona, CA, USA) and loaded under tensile until failure, using an MTBS tester (Bisco, Schaumburg, IL, USA). The MTBS was calculated as the maximum load at failure divided by the cross-sectional area and expressed in megaPascals (MPa).
2.6.
Statistical analysis
Statistical analysis was performed using tooth as the statistical unit; the mean MTBS obtained from the 5 beams of each tooth was used to represent the bond strength of that tooth. Three-way analysis of variance was performed using the kind of self-etch adhesives (Clearfil S3 Bond, G-Bond, Xeno V) as one factor, dentine surface treatments (control, NaOCl for 60 s) as the second factor, and thermo-cycling as the third factor. Tukey test was employed for multiple comparisons. Statistical analysis was carried out using SPSS version 17.0 (SPSS, Chicago, IL, USA). The significance level was set at a = 0.05 for all tests.
3.
Results
3.1.
Deproteinisation of dentine by NaOCl
The typical ATR spectrum of sound human dentine and deproteinised dentine by 5.25% NaOCl for 60 s is shown in Fig. 1. The peaks in these spectra (2000–800 cm 1) have been assigned according to the literature.21,22 The typical bands could be observed such as C O stretching vibration at 1600– 1700 cm 1 for amide I, N–H deformation at 1500–1550 cm 1 for the amide II and N–H deformation at 1200–1300 cm 1 for the amide III band.23 These bands are directly related to polypeptide chains of type I collagen component in dentine.16,24 The spectra suggested a clear weakening of the peaks at Amide I, Amide II and Amide III after NaOCl treatment.
Please cite this article in press as: Zhou L, et al. Effect of chemical interaction on the bonding strengths of self-etching adhesives to deproteinised dentine. Journal of Dentistry (2015), http://dx.doi.org/10.1016/j.jdent.2015.05.010
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5.25% NaOCl for 60 s (C and D), the smear layer was removed, the dentine tubule orifice, peri-tubular dentine and intertubular dentine could be seen clearly on the deproteinised dentine surface.
3.2. Affinity between functional adhesive resin monomers and deproteinised dentine
Fig. 1 – Infrared spectrum of dentine surfaces before and after treated with 5.25% NaOCl for 60 s. The amide bands are typical for polypeptide chains of type I collagen component in dentine, and the phosphate bands are characteristic of the apatite in dentine.
The vibrating peaks at 960 and 1093 cm 1 are assigned to phosphate n1, n3 bands. These bands are characteristic of the apatite in dentine. Because the apatite is insoluble in NaOCl, the phosphate stretching vibration peaks showed constant intensity. The morphology of sound dentine and deproteinised dentine was presented in Fig. 3. The sound dentine (A and B) was initially covered by a smear layer. After treatment with
Fig. 2 showed the infrared spectra of dentine substrate, adhesives and adhesives applied on dentine surfaces. The peak of the adhesives at 1718 cm 1 was assigned to the COstretching vibrations of acrylate or methacrylate groups,25 which represent the characteristic of adhesive. The peaks of dentine at 960 and 1093 cm 1, which are characteristic of the apatite phase PO43 (n1, n3) in dentine. In the Xeno V subgroups, after being rubbed on the dentine surface, there was not any sign of the presence of the peak of the CO-stretching band even in the no-rinsing sample. In the G-Bond subgroups, this peak was observed after applied to the dentine surface even rinsing with water, but weakened after rinsing with ethanol. The intensity of the peak in the S3 Bond subgroups presented the similar tendency with the G-Bond subgroup. The characteristic peaks of the apatite phase PO43 (n1, n3) in dentine decreased in the G-Bond and S3 bond subgroups, indicating that the adhesives on the dentine surfaces blocked the signals of apatite of dentine. Fig. 4 showed the attachment of the adhesives to dentine surfaces. For Xeno V subgroup, thin adhesive layer could be seen on dentine surface without rinsing (Fig. 4A), whilst the adhesive was easily removed by water and ethanol (Fig. 4B and C). By contrast, in the G-Bond (Fig. 4D–F) and S3 Bond (Fig. 4G–I) subgroups, most of the adhesives remained were retained on the surfaces of dentine after rinsing with water (Fig. 4E and H) or ethanol (Fig. 4F and I). In these groups, few dentine tubules
Fig. 2 – The infrared spectra of dentine substrate, adhesives and adhesives applied on dentine surfaces with and without rinsing. Peaks at 1718 cmS1 represent the band of CO-stretching vibration of acrylate or methacrylate groups. G-Bond (B) and S3 Bond (C) had strong affinity to dentine and were difficult to be removed by rinsing with water. There was no sign of the band of CO-stretching vibration in all Xeno V subgroups (A). Peaks at 960 and 1093 cmS1, which are characteristic of the apatite phase PO43S (n1, n3) in dentine, decreased in the G-Bond (B) and S3 Bond (C) groups. Please cite this article in press as: Zhou L, et al. Effect of chemical interaction on the bonding strengths of self-etching adhesives to deproteinised dentine. Journal of Dentistry (2015), http://dx.doi.org/10.1016/j.jdent.2015.05.010
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Fig. 3 – FE-SEM surface examination of sound dentine (A and B: the magnification of A), NaOCl-treated dentine (C and D: the magnification of C). The sound dentine was covered by smear. The dentine tubules were exposed after treated by NaOCl.
were exposed and most of the intertubular dentine remained covered with adhesives (Fig. 4D and G).
3.3. Chemical interactions between functional resin monomers and deproteinised dentine The presence of monomer–Ca salts on the dentine surfaces was identified using TF-XRD26 in Fig. 5. In the S3 Bond group, three weak peaks appeared in the range of 2u = 2.508, 4.848 and 7.168, representing the formation of 10-MDP–Ca salts.26 Peaks that may be assigned to the products of chemical interactions between the functional resin monomer and deproteinised dentine in the G-Bond and Xeno V groups could not be identified in TF-XRD results.
3.4.
Microtensile bond strength
Mean values and standard deviations of the MTBS are represented in Table 2. The variables self-etching adhesives, dentine surface treatments and thermo-cycling all had significant effect on the bond strengths (P 0.001). After treating the dentine with NaOCl, immediate MTBS values decreased significantly only in the Xeno V groups (P = 0.033). No significant reductions in MBTS were observed in the GBond (P = 0.966) and S3 Bond (P = 0.998) subgroups. After thermo-cycling, the MTBS values of the control and NaOCltreated samples decreased dramatically in all the subgroups. For the interaction between these factors, the effect of the type of adhesive was dependent on deproteinisation; there was a statistically significant interaction between ‘‘adhesive’’ and ‘‘deproteinisation’’ (P = 0.02). The effect of the type of adhesive was independent of ageing; there was no statistically
significant interaction between ‘‘adhesive’’ and ‘‘ageing’’ (P = 0.231). The effect of deproteinisation was also independent of ageing; there was no statistically significant interaction between ‘‘deproteinisation’’ and ‘‘ageing’’ (P = 0.446).
4.
Discussion
Functional resin monomers are components of many selfetching adhesives with important functions in bonding.13 The chemical interactions between functional monomers and dentine substrates have been studied in detail.13,25–29 However, few studies had been conducted on the interaction of deproteinised dentine with commercial adhesives containing functional monomers. In the present study, three self-etching adhesives were selected and bonded to deproteinised dentine. The adhesives studied included G-Bond with 4-MET, S3 Bond with 10-MDP and Xeno V without functional resin monomers. The chemical interactions between the functional monomers and dentine were further tested. The affinity of the three adhesives to deproteinised dentine was determined using ATR, which is a non-destructive method for obtaining surface measurements without specimen damage.25 In the present study (Fig. 2), the peak at 1718 cm 1, which is assigned to the –C C–COO– group of acrylates, remained obvious in the G-Bond and S3 Bond groups even after the polymerised adhesives were rinsed with water. This result suggests that G-Bond and S3 Bond has ability to resist water rinsing, indicating their relatively high affinity to dentine. In addition, after applying with G-Bond and S3 Bond, typical peaks characteristic of the apatite phase did not show up, which also evidenced the adhesives remained attached to
Please cite this article in press as: Zhou L, et al. Effect of chemical interaction on the bonding strengths of self-etching adhesives to deproteinised dentine. Journal of Dentistry (2015), http://dx.doi.org/10.1016/j.jdent.2015.05.010
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Fig. 4 – FE-SEM surface examination of dentine applied with adhesives with and without rinsing. Xeno V was applied on the dentine surface (A), but easily removed by water (B) and ethanol (C). G-Bond was applied on the dentine surface (D), but after water (E) and ethanol (F) rinsing, most of the adhesive still remained on the surfaces of dentine, only few dentine tubules were rendered patent. Similarly, S3 Bond (G) resisted dislodgement after rinsing with water (H) or ethanol (I).
Fig. 5 – TF-XRD patterns of sound dentine, NaOCl-treated dentine, and adhesive-treated dentine. In S3 Bond group, three weak peaks appeared in low angles. Peaks characteristic of functional monomers-Ca salts could not be identified from the G-Bond and Xeno V groups.
the dentine surfaces. In contrast, no sign of the –C C–COO– group was observed in the infrared spectrum of the Xeno V group. The absence of this signal indicates the low affinity of Xeno V to dentine surface. However, the band at 1718 cm 1 of Xeno V is much weaker than that of other two adhesives, which may be reason for the absence of the signal after being applied on dentine surfaces. FE-SEM further revealed a distinct layer on the deproteinised dentine after rubbing adhesives on dentine surfaces without rinsing. After rinsing with water or ethanol, dentine surfaces in the Xeno V group appeared cleaner and dentine structures were exposed. The pH of Xeno V is lower than the other two adhesives (Table 1), which may contribute the more exposure of dentine tubules. By contrast, the corresponding layers in the G-Bond and S3 Bond groups were highly stable and bonded to dentine, which might be due to the possible interactions between functional monomers and apatite. When combining the FE-SEM and ATR findings together, it seems that the adhesives containing functional monomers (G-Bond and S3 Bond) show high affinity to dentine. This may be attributed to the components in adhesives and different functional monomers. The properties of the bonding layer were evaluated using TF-XRD to characterise the functional monomer–Ca salts adhering to the deproteinised dentine surface. Only three characteristic but weak peaks were detected in S3 Bond group which contains 10-MDP. These peaks’ positions were in accordance with literatures.26,30 However, no chemical interaction salts were detected in G-Bond group. It has been reported that the chemical bonding capacity of 4-MET was
Please cite this article in press as: Zhou L, et al. Effect of chemical interaction on the bonding strengths of self-etching adhesives to deproteinised dentine. Journal of Dentistry (2015), http://dx.doi.org/10.1016/j.jdent.2015.05.010
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Table 2 – Microtensile bond strength of the three adhesives bonded to sound dentine or deproteinised dentine, with and without thermocycling. Adhesive
Xeno V G-Bond S3 Bond
Control
NaOCl 60 s
No thermocycling
After thermocycling
No thermocycling
After thermocycling
27.38 2.8 a 37.08 3.5 d 28.41 4.4 h
20.56 2.4 b 29.00 4.4 e 21.54 3.4 ij
21.06 2.8 b 34.63 3.9 de 26.72 3.3 hi
13.75 2.1 c 23.50 3.5 f 20.02 2.8 j
Values are means standard deviations in megaPascals (MPa; N = 7 teeth). Different superscript letters indicates significant differences between the values (P < 0.05).
doubtful in a short application time, and 4-MET-Ca salts were unstable.31 For the other hand, this is probably due to the low concentration of the functional resin monomers incorporated in the commercial adhesives, since XRD can only detect relatively large amounts (3–5 wt.%) of a material.25 It is speculated that the typical peaks of these salts may become more visible if pure functional monomers are used for examination.32 In addition, HEMA, which is frequently added to adhesives is also contained in S3 Bond. This water-soluble monomer with hydroxyl group did not prevent the adsorption of 10-MDP onto HAp, yet reduced the demineralisation rate of HAp, and inhibited the formation of 10-MDP-Ca salts.33 It is possible that the ‘HEMA-free/poor’ adhesives containing functional resin monomers may possess easily detected chemical bonding. Microtensile bond strengths were critical aspect of mechanical properties of dentine bonding. Sodium hypochlorite is widely used in endodontics as a root canal irrigant. Deproteinisation of dentine by NaOCl is generally believed to exert negative effects on bond strength because of degradation of the organic matrix and destruction of the stereo structure of fibrillar collagen.34,35 However, in the present study, the immediate bond strength between NaOCltreated dentine and self-etching adhesives with functional resin monomers (4-MET in G-Bond and 10-MDP in S3 Bond) did not decrease. The pH of the self-etching adhesives investigated was 1–3, which is acidic enough to etch dentine and enhance adhesive penetration into dentine micropores to increase micro-mechanical interlocking.36 It should be remembered that the resultant micro-mechanical interlocking is a prerequisite to achieve good bonding. However, the potential benefit of additional chemical interaction should be concerned too.13 Based on the ‘Adhesion-Decalcification’ concept,11,12 chemical interactions between the functional resin monomers and apatite may overcome the detrimental effects of NaOCl on MTBS. According to a previous study,37 when dentine is pretreated with hydroxyapatite paste, G-Bond shows better bonding efficiency than the blank control. Ratios of hydroxyapatite on the surface of dentine treated with NaOCl and hydroxyapatite paste were higher than those in sound dentine, which indicates enhanced opportunities for chemical interaction. The chemical bonding efficiency of the functional resin monomers contributed to their adhesive performance. However, various functional monomers showed different contributions to bond strengths. In this study, adhesives with functional resin monomers (10-MDP) maintained the both the immediate and aged bond strengths of the specimen deproteinised by NaOCl. Nevertheless, adhesives containing 4-MET
as functional monomer only maintained the immediate bond strengths of NaOCl-treated teeth. Such ability was undetected after 10,000 thermal cycles. This may be due to the unstable properties of 4-MET-Ca salts compared with the 10-MDP-Ca salts formed by the interaction between adhesive functional monomers and apatite of dentine.31 Within the limits of the present study, it may be concluded self-etch adhesives containing 10-MDP as functional monomers maintain immediate and aged bond strengths after 10,000 thermal cycles, when bonding to NaOCl-treated dentine. While the 4-MET have a positive effect on the bonding effectiveness of self-etch agents in the initial stage when bonded to deproteinised dentine. This finding may help clinicians achieve satisfactory bonding efficiency when treating deproteinised dentine substrates. In the present study, TF-XRD could not adequately detect low amounts of functional resin monomer–Ca salts to verify the appearance of chemical bonding because of sensitivity limitations. High-resolution XRD may provide more information on the interaction surfaces of dentine and adhesives. Since NaOCl is mostly used in root canal treatment as an antibacterial irrigant and for removal of the organic components of the endodontic smear layer, the bonding efficiency of self-etching adhesives on NaOCl-treated root dentine has to be investigated in future work.
Conflict of interest The authors declare that they have no conflict of interest.
Acknowledgments This work was financially supported by grants from National Natural Science Foundation of China (No. 81070852), National Natural Science Foundation of Hubei Province (No. 2011CDB468) and the Fundamental Research Funds for the Central Universities.
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enamel and dentin: current status and future challenges. Operative Dentistry 2003;28(3):215–35. Liu Y, Tjaderhane L, Breschi L, Mazzoni A, Li N, Mao J, et al. Limitations in bonding to dentin and experimental strategies to prevent bond degradation. Journal of Dental Research 2011;90(8):953–68. Hiraishi N, Breschi L, Prati C, Ferrari M, Tagami J, King NM. Technique sensitivity associated with air-drying of HEMAfree, single-bottle, one-step self-etch adhesives. Dental Materials 2007;23(4):498–505. Van Landuyt KL, Mine A, De Munck J, Jaecques S, Peumans M, Lambrechts P, et al. Are one-step adhesives easier to use and better performing? Multifactorial assessment of contemporary one-step self-etching adhesives. Journal of Adhesive Dentistry 2009;11(3):175–90. Van Landuyt KL, Peumans M, De Munck J, Cardoso MV, Ermis B, Van Meerbeek B. Three-year clinical performance of a HEMA-free one-step self-etch adhesive in non-carious cervical lesions. European Journal of Oral Sciences 2011;119(6):511–6. Peumans M, De Munck J, Van Landuyt KL, Poitevin A, Lambrechts P, Van Meerbeek B. Eight-year clinical evaluation of a 2-step self-etch adhesive with and without selective enamel etching. Dental Materials 2010;26(12):1176–84. Akimoto N, Takamizu M, Momoi Y. 10-year clinical evaluation of a self-etching adhesive system. Operative Dentistry 2007;32(1):3–10. van Dijken JW, Sunnegardh-Gronberg K, Lindberg A. Clinical long-term retention of etch-and-rinse and self-etch adhesive systems in non-carious cervical lesions. A 13 years evaluation. Dental Materials 2007;23(9):1101–7. Fu BP, Sun XM, Qian WX, Shen YQ, Chen RR, Hannig M. Evidence of chemical bonding to hydroxyapatite by phosphoric acid esters. Biomaterials 2005;26(25):5104–10. Yoshida Y, Van Meerbeek B, Nakayama Y, Yoshioka M, Snauwaert J, Abe Y, et al. Adhesion to and decalcification of hydroxyapatite by carboxylic acids. Journal of Dental Research 2001;80(6):1565–9. Yoshioka M, Yoshida Y, Inoue S, Lambrechts P, Vanherle G, Nomura Y, et al. Adhesion/decalcification mechanisms of acid interactions with human hard tissues. Journal of Biomedical Materials Research 2002;59(1):56–62. Van Meerbeek B, Yoshihara K, Yoshida Y, Mine A, De Munck J, Van Landuyt KL. State of the art of self-etch adhesives. Dental Materials 2011;27(1):17–28. Marshall Jr GW, Marshall SJ, Kinney JH, Balooch M. The dentin substrate: structure and properties related to bonding. Journal of Dentistry 1997;25(6):441–58. Hashem AAR, Ghoneim AG, Lutfy RA, Fouda MY. The effect of different irrigating solutions on bond strength of two root canal-filling systems. Journal of Endodontics 2009;35(4):537–40. Zhang K, Tay FR, Kim YK, Mitchell JK, Kim JR, Carrilho M, et al. The effect of initial irrigation with two different sodium hypochlorite concentrations on the erosion of instrumented radicular dentin. Dental Materials 2010;26(6):514–23. Mai S, Kim YK, Arola DD, Gu LS, Kim JR, Pashley DH, et al. Differential aggressiveness of ethylenediamine tetraacetic acid in causing canal wall erosion in the presence of sodium hypochlorite. Journal of Dentistry 2010;38(3):201–6. Lai SCN, Mak YF, Cheung GSP, Osorio R, Toledano M, Carvalho RM, et al. Reversal of compromised bonding to oxidized etched dentin. Journal of Dental Research 2001;80(10):1919–24. Hayashi M, Takahashi Y, Hirai M, Iwami Y, Imazato S, Ebisu S. Effect of endodontic irrigation on bonding of resin cement to radicular dentin. European Journal of Oral Sciences 2005;113(1):70–6.
20. Marshall GW, Yucel N, Balooch M, Kinney JH, Habelitz S, Marshall SJ. Sodium hypochlorite alterations of dentin and dentin collagen. Surface Science 2001;491(3):444–55. 21. Bachmann L, Diebolder R, Hibst R, Zezell DM. Infrared absorption bands of enamel and dentin tissues from human and bovine teeth. Applied Spectroscopy Reviews 2003;38(1):1–14. 22. Antunes A, de Rossi W, Zezell DM. Spectroscopic alterations on enamel and dentin after nanosecond Nd:YAG laser irradiation. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 2006;64(5):1142–6. 23. Chang MC, Tanaka J. FT-IR study for hydroxyapatite/ collagen nanocomposite cross-linked by glutaraldehyde. Biomaterials 2002;23(24):4811–8. 24. Di Renzo M, Ellis TH, Sacher E, Stangel I. A photoacoustic FTIRS study of the chemical modifications of human dentin surfaces. II. Deproteination. Biomaterials 2001;22(8):793–7. 25. Yoshihara K, Yoshida Y, Hayakawa S, Nagaoka N, Torii Y, Osaka A, et al. Self-etch monomer–calcium salt deposition on dentin. Journal of Dental Research 2011;90(5):602–6. 26. Yoshihara K, Yoshida Y, Hayakawa S, Nagaoka N, Irie M, Ogawa T, et al. Nanolayering of phosphoric acid ester monomer on enamel and dentin. Acta Biomaterialia 2011;7(8):3187–95. 27. Iwai H, Nishiyama N. Effect of calcium salt of functional monomer on bonding performance. Journal of Dental Research 2012;91(11):1043–8. 28. Feitosa VP, Sauro S, Ogliari FA, Stansbury JW, Carpenter GH, Watson TF, et al. The role of spacer carbon chain in acidic functional monomers on the physicochemical properties of self-etch dental adhesives. Journal of Dentistry 2014;42(5):565–74. 29. Feitosaa VP, Sauro S, Ogliari FA, Ogliari AO, Yoshihara K, Zanchi CH, et al. Impact of hydrophilicity and length of spacer chains on the bonding of functional monomers. Dental Materials 2014;30(12):E317–23. 30. Yoshida Y, Yoshihara K, Nagaoka N, Hayakawa S, Torii Y, Ogawa T, et al. Self-assembled nano-layering at the adhesive interface. Journal of Dental Research 2012;91(4):376–81. 31. Inoue S, Koshiro K, Yoshida Y, De Munck J, Nagakane K, Suzuki K, et al. Hydrolytic stability of self-etch adhesives bonded to dentin. Journal of Dental Research 2005;84(12):1160–4. 32. Yoshihara K, Yoshida Y, Nagaoka N, Fukegawa D, Hayakawa S, Mine A, et al. Nano-controlled molecular interaction at adhesive interfaces for hard tissue reconstruction. Acta Biomaterialia 2010;6(9):3573–82. 33. Yoshida Y, Yoshihara K, Hayakawa S, Nagaoka N, Okihara T, Matsumoto T, et al. HEMA inhibits interfacial nano-layering of the functional monomer MDP. Journal of Dental Research 2012;91(11):1060–5. 34. Uceda-Gomez N, Loguercio AD, Moura SK, Grande RHM, Oda M, Reis A. Long-term bond strength of adhesive systems applied to etched and deproteinized dentin. Journal of Applied Oral Science 2007;15(6):475–9. 35. Saboia VPA, Neto F, Mazzoni A, Orsini G, Putignano A, Giannini M, et al. Adhesion of a two-step etch-and-rinse adhesive on collagen-depleted dentin. Journal of Adhesive Dentistry 2008;10(6):419–22. 36. Phrukkanon S, Burrow MF, Hartley PG, Tyas MJ. The influence of the modification of etched bovine dentin on bond strengths. Dental Materials 2000;16(4):255–65. 37. Pei DD, Liu SY, Huang C, Du XJ, Yang HY, Wang YK, et al. Effect of pretreatment with calcium-containing desensitizer on the dentine bonding of mild self-etch adhesives. European Journal of Oral Science 2013;121(3):204–10.
Please cite this article in press as: Zhou L, et al. Effect of chemical interaction on the bonding strengths of self-etching adhesives to deproteinised dentine. Journal of Dentistry (2015), http://dx.doi.org/10.1016/j.jdent.2015.05.010
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.intl.elsevierhealth.com/journals/jden
Effect of chemical interaction on the bonding strengths of self-etching adhesives to deproteinised dentine Liqun Zhou a,1, Yake Wang a,1, HongYe Yang a, Jinxin Guo a, Franklin R. Tay b, Cui Huang a,* a
The State Key Laboratory Breeding Base of Basic Science of Stomatology (Hubei-MOST) & Key Laboratory for Oral Biomedical Ministry of Education, School & Hospital of Stomatology, Wuhan University, Wuhan, China b Department of Endodontics, College of Dental Medicine, Georgia Regents University, Augusta, GA, USA
article info
abstract
Article history:
Objectives: The present study examined (1) the chemical interaction between three self-
Received 10 November 2014
etching adhesives and sodium hypochlorite (NaOCl)-deproteinised dentine, and (2) the
Received in revised form
influence of NaOCl treatment on bond strength of self-etching adhesives with/without
1 May 2015
adhesive functional monomers to dentine.
Accepted 28 May 2015
Methods: Caries-free dentine disks (control) and those treated with 5.25% NaOCl for 60 s
Available online xxx
were prepared. Xeno V (no functional monomers), G-Bond (containing 4-MET) or S3 Bond (containing 10-MDP) were applied to the NaOCl-treated dentine and either left without
Keywords: Sodium hypochlorite, Self-etching
further treatment, or rinsed with 100% ethanol or distilled water. Attenuated total reflection (ATR) spectroscopy and field-emission scanning electron
Adhesive
microscopy (FE-SEM) were used to evaluate the affinity of functional monomers with
Functional monomer
deproteinised dentine. Chemical interaction between the functional monomers and depro-
Microtensile bond strengths
teinised dentine was evaluated using thin-film X-ray diffraction (TF-XRD). Microtensile bond strength (MTBS) was used to evaluate the mechanical property of the adhesives, either immediately or after thermo-cycling (5–55 8C) for 10,000 cycles. Results: According to the ATR and FE-SEM results, G-Bond and S3 Bond showed stronger affinity to deproteinised dentine than Xeno V even after rinsing with water. TF-XRD showed that chemical interaction between S3 Bond and deproteinised dentine occurred by formation of 10-MDP–Ca salt. Both deproteinisation and thermo-cycling adversely affected the MTBS of Xeno V (P < 0.05) but deproteinisation had no significant influence on S3 Bond. Conclusions: When bonding to NaOCl-treated dentine, self-etch adhesives containing functional monomers (10-MDP) can maintain immediate and aged bond strengths after 10,000 thermal cycles. # 2015 Elsevier Ltd. All rights reserved.
* Corresponding author. Tel.: +86 27 87686130; fax: +86 27 87873260. E-mail address: [email protected] (C. Huang). 1 These authors contributed equally to this work. http://dx.doi.org/10.1016/j.jdent.2015.05.010 0300-5712/# 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Zhou L, et al. Effect of chemical interaction on the bonding strengths of self-etching adhesives to deproteinised dentine. Journal of Dentistry (2015), http://dx.doi.org/10.1016/j.jdent.2015.05.010
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1.
Introduction
Dentine bonding plays an important role in many restorative procedures and is a fundamental aspect of aesthetic dentistry. Dentine bonding systems may be divided into two categories according to their method of application: etch-and-rinse adhesives and self-etching adhesives.1 From the perspective of bonding mechanisms, micro-mechanical interlocking is generally accepted as the primary source of retention for both adhesive types.2 In particular, the formation of hybridised dentine is analogous to tissue engineering, in which dentine collagen fibrils serve as the scaffold for resin infiltration.3 Given their ease of use, acceptable performance in clinical trials of class V restorations and the low incidence of dentine hypersensitivity, self-etching adhesives obtain steadily growing popularity in today’s dental practices.4–9 Apart from micromechanical retention, chemical interactions also contribute to the bonding effectiveness of selfetching adhesives to dentine. These interactions are commonly attributed to the functional resin monomers present in the adhesives, that couple with residual carbonated apatite derived from the self-etched dentine. Yoshida and colleagues10–12 proposed the concept of ‘Adhesion-Decalcification’ and claimed that two phenomena may occur when the functional resin monomers containing carboxyl or phosphate groups come into contact with the partially demineralised dentine substrate. The first phenomenon involves the formation of a Ca–monomer salt if the ionic bond is hydrolytically stable; the second phenomenon involves decalcification and release of calcium and phosphate ions from the tooth surface if the ionic bond is unstable.13 Dentine is an organic–inorganic complex with a microstructure and properties that are the principal determinants of nearly all operations in restorative dentistry.14 The dentine substrate is prone to modification by the chemical agents employed during in dental treatment.15 For instance, sodium hypochlorite (NaOCl), which is the most common endodontic irrigant for chemical debridement of the root canal space, can deproteinise demineralised as well as mineralised dentine.16,17 Nevertheless, the effect of NaOCl collagen deproteinisation on dentine
bonding remains controversial. Lai et al. found that NaOCl deproteinisation adversely lowered the bonding strength of dentine because of its strong oxidising properties.18 By contrast, Hayashi et al. found that NaOCl deproteinisation may improve resin infiltration, thereby resulting in improved dentine bond strength.19 Because of its high alkalinity, NaOCl removes the organic components of dentine, mainly collagen,20 and by doing so, increases the ratio of the inorganic apatite to the organic phase in dentine. It is not known, however, whether this NaOCl-derived shift in phase ratio on dentine surfaces affects the chemical interaction between the functional adhesive resin monomers in self-etching adhesives and deproteinised dentine surface. Thus, the objectives of this study were: (1) to examine the chemical interactions between three self-etching adhesives and NaOCl-treated dentine and (2) to evaluate the effect of NaOCl treatment on the bond strength of self-etching adhesives, with or without the incorporation of functional resin monomers, when these adhesives are bonded to dentine.
2.
Materials and methods
2.1.
Preparation of dentine disks
One hundred and one extracted human non-carious third molars were used in the present study. The teeth were collected after obtaining informed consent from donors under a protocol approved by the Ethics Committee for Human Studies of the School and Hospital of Stomatology, Wuhan University. The teeth were cleaned, stored in 1% chloramine at 4 8C and used within 1 month of extraction. Twenty-seven dentine disks (10 mm 10 mm 1 mm) were cut using a low-speed diamond saw (Isomet; Buehler, Evanston, IL, USA) under water cooling. The surfaces of those disks were sequentially wet-polished with 600-, 800- and 1000grit silicon carbide papers. The disks were then ultrasonically cleaned for 10 min. Twenty-four of the disks were treated with 5.25% NaOCl for 60 s whilst the other three dentine disks were used as controls.
Table 1 – Adhesives investigated in the present study. Adhesive
Manufacturer
Batch No.
Clearfil [S3] Bond
Kuraray Medical Inc., Tokyo, Japan
00168A
G-Bond
GC, Tokyo, Japan
1009071
Xeno V
DENTSPLY Caulk, Milford, DE, USA
1005001791
Composition 10-MDP, HEMA, Bis-GMA, water, ethanol, silanated colloidal silica, CQ, pH 2.7 4-MET, phosphoric-monomer, UDMA, TEGDMA, stabilizer, photo-initiator, silica filler, acetone, water, pH 2 Bifunctional acrylic amides, acrylamido alkylsulfonic acid, ‘inverse’ functionalised phosphoric acid ester, acrylic acid, water, tertiary butanol, butylated benzenediol, CQ, initiator, stabilizer, pH 1.38
Application protocol Apply adhesive; wait for 20 s; air-blow for 5 s; light cure for 10 s. Apply adhesive; wait for 10 s; air-blow for 5 s; light cure for 10 s. Apple adhesive sufficiently, and uniformly; gently agitate 20 s; air blow at least 5 s; Light cure for 20 s.
Please cite this article in press as: Zhou L, et al. Effect of chemical interaction on the bonding strengths of self-etching adhesives to deproteinised dentine. Journal of Dentistry (2015), http://dx.doi.org/10.1016/j.jdent.2015.05.010
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Three self-etching adhesives (with or without functional monomers, Table 1) were applied to 21 NaOCl-treated dentine disks by gently rubbing each dentine surface with a microbrush. After 20 s of adhesive agitation, the samples were airdried and either left without further treatment (referred to as ‘S3_Dentine’, ‘GB_Dentine’ and ‘XE_Dentine’, three specimens per subgroup) or rinsed with 100% ethanol (‘S3_Dentine_EtOH’, ‘GB_Dentine_EtOH’ and ‘XE_Dentine_EtOH’, two specimens per subgroup) or distilled water (‘S3_Dentine_H2O’, ‘GB_Dentine_H2O’ and ‘XE_Dentine_H2O’, two specimens per subgroup, respectively).
2.2. Fourier transform infrared spectroscopy (FTIR) and attenuated total reflection (ATR) spectroscopy FTIR and ATR spectra were carried out with a Nicolet 5700 spectrometer (Thermo Scientific Inc., Madison, WI, USA) and a single-reflection attenuated total reflection (ATR) setup with germanium (Ge) as an internal reflection element (Smart OMNI-Sampler, Thermo Scientific Inc.). The sound dentine disks, NaOCl-treated dentine disks and adhesive-treated dentine disks with and without rinsing, were examined by FTIR spectrophotometer. The specimens were put onto the face of the Ge crystal of the smart OMNIsampler accessory, with the unpolished surface up, adjusted the pointed tip of the standard pressure tower to press onto the mark of samples. Infrared spectra were collected between the range of 4000–675 cm 1 at 4 cm 1 resolution using 32 scans. The three adhesives were tested using FTIR by rubbing them on the surface of slides. FTIR spectra were recorded in the range from 4000 to 400 cm 1 at 4 cm 1 resolution. The specimens were scanned 128 times in each measurement, and the spectrum acquired was the average of all the scans. Background spectrum was automatically subtracted from the acquired spectrum using algorithm incorporated in the OMNIC FTIR software (Thermo Scientific Inc.). The spectral data were expressed as absorbance.
2.3. SEM)
Field-emission scanning electron microscopy (FE-
The surfaces of the sound dentine disks, NaOCl-treated dentine disks and adhesive-treated dentine disks, with and without rinsing were observed using FE-SEM. The prepared samples from Section 2.1 were dried in a drying vessel for 24 h. And then, they were bonded to the copper platform and sputter-coated with Au–Pd alloy. After that, they were observed using FE-SEM at 5 kV (Zeiss, Sigma, Germany).
2.4.
Thin-film X-ray diffraction (TF-XRD)
The sound dentine disks, NaOCl-treated dentine disks and adhesive-treated dentine disks without rinsing were examined by TF-XRD using an X-ray diffractometer (Bruker AXS D8-XRD, Germany) operated under a 40 kV acceleration voltage and 40 mA current with the angle of the incident X-ray beam fixed at 0.58 and a scanning time of 0.028/s for 2u scan.
2.5.
3
Microtensile bond strength (MTBS) evaluation
Eighty-four teeth were sectioned perpendicular to the long axis of each tooth to expose the mid-coronal dentine using the aforementioned diamond saw under water cooling. The exposed dentine surfaces were then polished using 600-grit silicon carbide paper under running water to create standardised surfaces. Half of the teeth were randomly picked out and treated with 5.25% NaOCl for 60 s. According to the manufacturer’s instruction, three selfetching adhesive systems (Xeno V, G-Bond and Clearfil S3 Bond) were randomly applied to either the sound or NaOCl-treated dentine. Resin composite build-ups were then constructed using 4 mm increments of resin composite (Charisma; Heraeus Kulzer, Hanau, Germany). There were 14 teeth in each subgroup. Each tooth was sectioned into 0.9 mm 0.9 mm beams after storage in deionised water at 37 8C for 24 h, and five beams were selected (5 beams per tooth). The beams were assigned for either immediate testing or after thermo-cycling from 5 8C to 55 8C for 10,000 cycles. Thirty-five specimens from each subgroup were used for MTBS evaluation. The exact dimensions of each specimen were measured using a pair of digital calliper. Each specimen was attached to the testing apparatus with cyanoacrylate adhesive (Zapit; Dental Ventures of America, Corona, CA, USA) and loaded under tensile until failure, using an MTBS tester (Bisco, Schaumburg, IL, USA). The MTBS was calculated as the maximum load at failure divided by the cross-sectional area and expressed in megaPascals (MPa).
2.6.
Statistical analysis
Statistical analysis was performed using tooth as the statistical unit; the mean MTBS obtained from the 5 beams of each tooth was used to represent the bond strength of that tooth. Three-way analysis of variance was performed using the kind of self-etch adhesives (Clearfil S3 Bond, G-Bond, Xeno V) as one factor, dentine surface treatments (control, NaOCl for 60 s) as the second factor, and thermo-cycling as the third factor. Tukey test was employed for multiple comparisons. Statistical analysis was carried out using SPSS version 17.0 (SPSS, Chicago, IL, USA). The significance level was set at a = 0.05 for all tests.
3.
Results
3.1.
Deproteinisation of dentine by NaOCl
The typical ATR spectrum of sound human dentine and deproteinised dentine by 5.25% NaOCl for 60 s is shown in Fig. 1. The peaks in these spectra (2000–800 cm 1) have been assigned according to the literature.21,22 The typical bands could be observed such as C O stretching vibration at 1600– 1700 cm 1 for amide I, N–H deformation at 1500–1550 cm 1 for the amide II and N–H deformation at 1200–1300 cm 1 for the amide III band.23 These bands are directly related to polypeptide chains of type I collagen component in dentine.16,24 The spectra suggested a clear weakening of the peaks at Amide I, Amide II and Amide III after NaOCl treatment.
Please cite this article in press as: Zhou L, et al. Effect of chemical interaction on the bonding strengths of self-etching adhesives to deproteinised dentine. Journal of Dentistry (2015), http://dx.doi.org/10.1016/j.jdent.2015.05.010
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5.25% NaOCl for 60 s (C and D), the smear layer was removed, the dentine tubule orifice, peri-tubular dentine and intertubular dentine could be seen clearly on the deproteinised dentine surface.
3.2. Affinity between functional adhesive resin monomers and deproteinised dentine
Fig. 1 – Infrared spectrum of dentine surfaces before and after treated with 5.25% NaOCl for 60 s. The amide bands are typical for polypeptide chains of type I collagen component in dentine, and the phosphate bands are characteristic of the apatite in dentine.
The vibrating peaks at 960 and 1093 cm 1 are assigned to phosphate n1, n3 bands. These bands are characteristic of the apatite in dentine. Because the apatite is insoluble in NaOCl, the phosphate stretching vibration peaks showed constant intensity. The morphology of sound dentine and deproteinised dentine was presented in Fig. 3. The sound dentine (A and B) was initially covered by a smear layer. After treatment with
Fig. 2 showed the infrared spectra of dentine substrate, adhesives and adhesives applied on dentine surfaces. The peak of the adhesives at 1718 cm 1 was assigned to the COstretching vibrations of acrylate or methacrylate groups,25 which represent the characteristic of adhesive. The peaks of dentine at 960 and 1093 cm 1, which are characteristic of the apatite phase PO43 (n1, n3) in dentine. In the Xeno V subgroups, after being rubbed on the dentine surface, there was not any sign of the presence of the peak of the CO-stretching band even in the no-rinsing sample. In the G-Bond subgroups, this peak was observed after applied to the dentine surface even rinsing with water, but weakened after rinsing with ethanol. The intensity of the peak in the S3 Bond subgroups presented the similar tendency with the G-Bond subgroup. The characteristic peaks of the apatite phase PO43 (n1, n3) in dentine decreased in the G-Bond and S3 bond subgroups, indicating that the adhesives on the dentine surfaces blocked the signals of apatite of dentine. Fig. 4 showed the attachment of the adhesives to dentine surfaces. For Xeno V subgroup, thin adhesive layer could be seen on dentine surface without rinsing (Fig. 4A), whilst the adhesive was easily removed by water and ethanol (Fig. 4B and C). By contrast, in the G-Bond (Fig. 4D–F) and S3 Bond (Fig. 4G–I) subgroups, most of the adhesives remained were retained on the surfaces of dentine after rinsing with water (Fig. 4E and H) or ethanol (Fig. 4F and I). In these groups, few dentine tubules
Fig. 2 – The infrared spectra of dentine substrate, adhesives and adhesives applied on dentine surfaces with and without rinsing. Peaks at 1718 cmS1 represent the band of CO-stretching vibration of acrylate or methacrylate groups. G-Bond (B) and S3 Bond (C) had strong affinity to dentine and were difficult to be removed by rinsing with water. There was no sign of the band of CO-stretching vibration in all Xeno V subgroups (A). Peaks at 960 and 1093 cmS1, which are characteristic of the apatite phase PO43S (n1, n3) in dentine, decreased in the G-Bond (B) and S3 Bond (C) groups. Please cite this article in press as: Zhou L, et al. Effect of chemical interaction on the bonding strengths of self-etching adhesives to deproteinised dentine. Journal of Dentistry (2015), http://dx.doi.org/10.1016/j.jdent.2015.05.010
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Fig. 3 – FE-SEM surface examination of sound dentine (A and B: the magnification of A), NaOCl-treated dentine (C and D: the magnification of C). The sound dentine was covered by smear. The dentine tubules were exposed after treated by NaOCl.
were exposed and most of the intertubular dentine remained covered with adhesives (Fig. 4D and G).
3.3. Chemical interactions between functional resin monomers and deproteinised dentine The presence of monomer–Ca salts on the dentine surfaces was identified using TF-XRD26 in Fig. 5. In the S3 Bond group, three weak peaks appeared in the range of 2u = 2.508, 4.848 and 7.168, representing the formation of 10-MDP–Ca salts.26 Peaks that may be assigned to the products of chemical interactions between the functional resin monomer and deproteinised dentine in the G-Bond and Xeno V groups could not be identified in TF-XRD results.
3.4.
Microtensile bond strength
Mean values and standard deviations of the MTBS are represented in Table 2. The variables self-etching adhesives, dentine surface treatments and thermo-cycling all had significant effect on the bond strengths (P 0.001). After treating the dentine with NaOCl, immediate MTBS values decreased significantly only in the Xeno V groups (P = 0.033). No significant reductions in MBTS were observed in the GBond (P = 0.966) and S3 Bond (P = 0.998) subgroups. After thermo-cycling, the MTBS values of the control and NaOCltreated samples decreased dramatically in all the subgroups. For the interaction between these factors, the effect of the type of adhesive was dependent on deproteinisation; there was a statistically significant interaction between ‘‘adhesive’’ and ‘‘deproteinisation’’ (P = 0.02). The effect of the type of adhesive was independent of ageing; there was no statistically
significant interaction between ‘‘adhesive’’ and ‘‘ageing’’ (P = 0.231). The effect of deproteinisation was also independent of ageing; there was no statistically significant interaction between ‘‘deproteinisation’’ and ‘‘ageing’’ (P = 0.446).
4.
Discussion
Functional resin monomers are components of many selfetching adhesives with important functions in bonding.13 The chemical interactions between functional monomers and dentine substrates have been studied in detail.13,25–29 However, few studies had been conducted on the interaction of deproteinised dentine with commercial adhesives containing functional monomers. In the present study, three self-etching adhesives were selected and bonded to deproteinised dentine. The adhesives studied included G-Bond with 4-MET, S3 Bond with 10-MDP and Xeno V without functional resin monomers. The chemical interactions between the functional monomers and dentine were further tested. The affinity of the three adhesives to deproteinised dentine was determined using ATR, which is a non-destructive method for obtaining surface measurements without specimen damage.25 In the present study (Fig. 2), the peak at 1718 cm 1, which is assigned to the –C C–COO– group of acrylates, remained obvious in the G-Bond and S3 Bond groups even after the polymerised adhesives were rinsed with water. This result suggests that G-Bond and S3 Bond has ability to resist water rinsing, indicating their relatively high affinity to dentine. In addition, after applying with G-Bond and S3 Bond, typical peaks characteristic of the apatite phase did not show up, which also evidenced the adhesives remained attached to
Please cite this article in press as: Zhou L, et al. Effect of chemical interaction on the bonding strengths of self-etching adhesives to deproteinised dentine. Journal of Dentistry (2015), http://dx.doi.org/10.1016/j.jdent.2015.05.010
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Fig. 4 – FE-SEM surface examination of dentine applied with adhesives with and without rinsing. Xeno V was applied on the dentine surface (A), but easily removed by water (B) and ethanol (C). G-Bond was applied on the dentine surface (D), but after water (E) and ethanol (F) rinsing, most of the adhesive still remained on the surfaces of dentine, only few dentine tubules were rendered patent. Similarly, S3 Bond (G) resisted dislodgement after rinsing with water (H) or ethanol (I).
Fig. 5 – TF-XRD patterns of sound dentine, NaOCl-treated dentine, and adhesive-treated dentine. In S3 Bond group, three weak peaks appeared in low angles. Peaks characteristic of functional monomers-Ca salts could not be identified from the G-Bond and Xeno V groups.
the dentine surfaces. In contrast, no sign of the –C C–COO– group was observed in the infrared spectrum of the Xeno V group. The absence of this signal indicates the low affinity of Xeno V to dentine surface. However, the band at 1718 cm 1 of Xeno V is much weaker than that of other two adhesives, which may be reason for the absence of the signal after being applied on dentine surfaces. FE-SEM further revealed a distinct layer on the deproteinised dentine after rubbing adhesives on dentine surfaces without rinsing. After rinsing with water or ethanol, dentine surfaces in the Xeno V group appeared cleaner and dentine structures were exposed. The pH of Xeno V is lower than the other two adhesives (Table 1), which may contribute the more exposure of dentine tubules. By contrast, the corresponding layers in the G-Bond and S3 Bond groups were highly stable and bonded to dentine, which might be due to the possible interactions between functional monomers and apatite. When combining the FE-SEM and ATR findings together, it seems that the adhesives containing functional monomers (G-Bond and S3 Bond) show high affinity to dentine. This may be attributed to the components in adhesives and different functional monomers. The properties of the bonding layer were evaluated using TF-XRD to characterise the functional monomer–Ca salts adhering to the deproteinised dentine surface. Only three characteristic but weak peaks were detected in S3 Bond group which contains 10-MDP. These peaks’ positions were in accordance with literatures.26,30 However, no chemical interaction salts were detected in G-Bond group. It has been reported that the chemical bonding capacity of 4-MET was
Please cite this article in press as: Zhou L, et al. Effect of chemical interaction on the bonding strengths of self-etching adhesives to deproteinised dentine. Journal of Dentistry (2015), http://dx.doi.org/10.1016/j.jdent.2015.05.010
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Table 2 – Microtensile bond strength of the three adhesives bonded to sound dentine or deproteinised dentine, with and without thermocycling. Adhesive
Xeno V G-Bond S3 Bond
Control
NaOCl 60 s
No thermocycling
After thermocycling
No thermocycling
After thermocycling
27.38 2.8 a 37.08 3.5 d 28.41 4.4 h
20.56 2.4 b 29.00 4.4 e 21.54 3.4 ij
21.06 2.8 b 34.63 3.9 de 26.72 3.3 hi
13.75 2.1 c 23.50 3.5 f 20.02 2.8 j
Values are means standard deviations in megaPascals (MPa; N = 7 teeth). Different superscript letters indicates significant differences between the values (P < 0.05).
doubtful in a short application time, and 4-MET-Ca salts were unstable.31 For the other hand, this is probably due to the low concentration of the functional resin monomers incorporated in the commercial adhesives, since XRD can only detect relatively large amounts (3–5 wt.%) of a material.25 It is speculated that the typical peaks of these salts may become more visible if pure functional monomers are used for examination.32 In addition, HEMA, which is frequently added to adhesives is also contained in S3 Bond. This water-soluble monomer with hydroxyl group did not prevent the adsorption of 10-MDP onto HAp, yet reduced the demineralisation rate of HAp, and inhibited the formation of 10-MDP-Ca salts.33 It is possible that the ‘HEMA-free/poor’ adhesives containing functional resin monomers may possess easily detected chemical bonding. Microtensile bond strengths were critical aspect of mechanical properties of dentine bonding. Sodium hypochlorite is widely used in endodontics as a root canal irrigant. Deproteinisation of dentine by NaOCl is generally believed to exert negative effects on bond strength because of degradation of the organic matrix and destruction of the stereo structure of fibrillar collagen.34,35 However, in the present study, the immediate bond strength between NaOCltreated dentine and self-etching adhesives with functional resin monomers (4-MET in G-Bond and 10-MDP in S3 Bond) did not decrease. The pH of the self-etching adhesives investigated was 1–3, which is acidic enough to etch dentine and enhance adhesive penetration into dentine micropores to increase micro-mechanical interlocking.36 It should be remembered that the resultant micro-mechanical interlocking is a prerequisite to achieve good bonding. However, the potential benefit of additional chemical interaction should be concerned too.13 Based on the ‘Adhesion-Decalcification’ concept,11,12 chemical interactions between the functional resin monomers and apatite may overcome the detrimental effects of NaOCl on MTBS. According to a previous study,37 when dentine is pretreated with hydroxyapatite paste, G-Bond shows better bonding efficiency than the blank control. Ratios of hydroxyapatite on the surface of dentine treated with NaOCl and hydroxyapatite paste were higher than those in sound dentine, which indicates enhanced opportunities for chemical interaction. The chemical bonding efficiency of the functional resin monomers contributed to their adhesive performance. However, various functional monomers showed different contributions to bond strengths. In this study, adhesives with functional resin monomers (10-MDP) maintained the both the immediate and aged bond strengths of the specimen deproteinised by NaOCl. Nevertheless, adhesives containing 4-MET
as functional monomer only maintained the immediate bond strengths of NaOCl-treated teeth. Such ability was undetected after 10,000 thermal cycles. This may be due to the unstable properties of 4-MET-Ca salts compared with the 10-MDP-Ca salts formed by the interaction between adhesive functional monomers and apatite of dentine.31 Within the limits of the present study, it may be concluded self-etch adhesives containing 10-MDP as functional monomers maintain immediate and aged bond strengths after 10,000 thermal cycles, when bonding to NaOCl-treated dentine. While the 4-MET have a positive effect on the bonding effectiveness of self-etch agents in the initial stage when bonded to deproteinised dentine. This finding may help clinicians achieve satisfactory bonding efficiency when treating deproteinised dentine substrates. In the present study, TF-XRD could not adequately detect low amounts of functional resin monomer–Ca salts to verify the appearance of chemical bonding because of sensitivity limitations. High-resolution XRD may provide more information on the interaction surfaces of dentine and adhesives. Since NaOCl is mostly used in root canal treatment as an antibacterial irrigant and for removal of the organic components of the endodontic smear layer, the bonding efficiency of self-etching adhesives on NaOCl-treated root dentine has to be investigated in future work.
Conflict of interest The authors declare that they have no conflict of interest.
Acknowledgments This work was financially supported by grants from National Natural Science Foundation of China (No. 81070852), National Natural Science Foundation of Hubei Province (No. 2011CDB468) and the Fundamental Research Funds for the Central Universities.
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Please cite this article in press as: Zhou L, et al. Effect of chemical interaction on the bonding strengths of self-etching adhesives to deproteinised dentine. Journal of Dentistry (2015), http://dx.doi.org/10.1016/j.jdent.2015.05.010