d e n t a l m a t e r i a l s 3 0 ( 2 0 1 4 ) 993–1004
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Sealing performance of resin cements before and after thermal cycling: Evaluation by optical coherence tomography Alaa Turkistani a,b,c , Alireza Sadr c,∗ , Yasushi Shimada b , Toru Nikaido b , Yasunori Sumi d , Junji Tagami b,c a
Operative Dentistry Division, Conservative Dental Sciences Department, Faculty of Dentistry, King Abdulaziz University, Jeddah, Saudi Arabia b Cariology and Operative Dentistry, Graduate School of Medical and Dental Sciences, Tokyo Medical and Dental University, 1-5-45, Yushima, Bunkyo-ku, Tokyo 113-8549, Japan c Global COE, International Research Center for Molecular Science in Tooth and Bone Diseases, Tokyo Medical and Dental University, 1-5-45, Yushima, Bunkyo-ku, Tokyo 113-8549, Japan d Division of Oral and Dental Surgery, Department of Advanced Medicine, National Center for Geriatrics and Gerontology, National Hospital for Geriatric Medicine, 36-3, Gengo, Morioka, Obu, Aichi 474-8511, Japan
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objectives. Self-adhesive resin cements have been recently introduced; however, there is
Received 25 December 2012
little data available on their long-term performance. In this in vitro study, swept-source
Received in revised form
optical coherence tomography (OCT) at 1310 nm center wavelength was used for monitoring
4 August 2013
adaptation of indirect resin restorations after thermal cycling.
Accepted 21 May 2014
Methods. Resin inlays were luted to class-I cavities of extracted human teeth using three resin cements; Clearfil SA Luting (SA; Kuraray), Bistite II DC or Multibond II (Tokuyama Dental). Each cement was applied with or without pre-coating of dentin by a self-etch adhesive
Keywords:
(Clearfil SE Bond) and a low-viscosity microfilled resin. OCT imaging was performed after
Resin inlay
24 h, after 2000 and after 10,000 thermocycles (n = 5). Selected samples were sectioned for
Resin cement
interfacial observation by confocal laser scanning microscope (CLSM). Floor adaptation (per-
Resin coating
centage) was analyzed by software on 20 B-scans throughout each specimen, and subjected
Adaptation
to statistical analysis by three-way ANOVA test at a significance level of 0.05.
Optical coherence tomography
Results. Resin cement type, resin coating and thermal aging all significantly affected adaptation (p < 0.05). Initially, SA showed the highest adaptation; however, thermal aging significantly affected its sealing. The best results for all the cements were consistently achieved when the resin coating technique was applied where no deterioration of interfacial integrity was observed in the coated groups. CLSM closely confirmed OCT findings in all groups. Significance. OCT could be used for monitoring of composite inlays with several interfacial resin layers. The application of a direct bonding agent in the resin-coating technique improved interfacial sealing and durability of all resin cements. © 2014 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
∗
Corresponding author. Tel.: +81 3 5803 2483; fax: +81 3 5803 0195. E-mail address: [email protected] (A. Sadr). http://dx.doi.org/10.1016/j.dental.2014.05.010 0109-5641/© 2014 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
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Introduction
The aesthetic aspect of dental treatment has become increasingly popular in the recent years, especially with the development of improved materials and adhesive techniques using composite resins. The indirect composite resin restoration technique involves extra-oral fabrication of an inlay and its placement with a resin cement. It has been reported that for large cavities, indirect restorations bear advantages over direct techniques such as improvements in anatomic form, contour, fracture resistance and wear resistance [1]. Furthermore, extra-oral fabrication aids in the relief of residual stresses and ensures that the negative effects of polymerization shrinkage are confined to the thin layer of resin cement [2]. On the other hand, it is believed that the viscous resin cements may not provide dentin bonding comparable to dentin-bonding system (DBS) used for direct composite [3–5]. This may affect the sealing ability of these cements and lead to lower penetration to tooth substrate and hence, lower bonding performances in comparison to DBS. Therefore, a resin coating technique for indirect restorations was introduced in which DBS and a low viscosity microfilled resin are applied to seal dentin surface after preparation, decreasing pulp irritation and postoperative sensitivity and improving bond strength [6–9]. Meanwhile, the effectiveness of this technique for the newly introduced resin cement products (such as self-adhesive resin cements) has not been investigated. The self-adhesive resin cement is proposed to simplify the cementation procedure; it bonds to dentin in one step without the need of conditioning or pre-treatment (priming) of the surface [10,11]. Adhesion tests have been routinely used for laboratory evaluation of these biomaterials. However, the success of a restoration also greatly depends on its sealing ability of the dental tissue in an actual cavity [12]. Different methods are conventionally used to evaluate the marginal integrity and sealing of restorations. The most common method is detecting dye penetration depth under a stereoscopic microscope and/or scanning electron microscope (SEM). However, these methods are considered as destructive methods since they require sample sectioning, and may be subjective. More recently, three-dimensional and in-depth imaging methods have been introduced and utilized for characterization of dental composites [13–18]. Optical coherence tomography (OCT) can provide noninvasive, high resolution cross-sectional images for biologic microstructures and materials based on light backscattering from within the structure. Dental composites and hard tissues are scattering media and therefore can be suitable substrates for OCT imaging [16–24]. Toothrestoration interface under direct resin restorations has been investigated using this technique [18,19,21,25]; however, there are few reports on evaluation of indirect restorations. Thermal cycling procedure has been accepted as an effective means of artificially aging composite restorations to study their interfacial characteristics in the long-term. In this regard, imaging of resin restorations by OCT before and after thermal aging appears to be an attractive research method. Therefore, the aim of this laboratory study was to evaluate the effect of thermal cycling and resin coating technique on the adaptation
of indirect composite inlays luted with resin cements under OCT, and confirmation of OCT findings by cross-sectional confocal laser scanning microscopy (CLSM). The null hypotheses tested were as follows: (1) there was no difference in the interfacial sealing of the composites inlays between different resin cements; (2) the resin coating could not improve the interfacial integrity; and (3) There were no changes in the interfacial integrity of different test groups after thermal aging.
2.
Materials and method
2.1.
Specimen preparation
For this study, thirty extracted human third molars, free of cracks, caries and restorations were selected after the patients’ informed consent, as approved by the Institutional Review Board of Tokyo Medical and Dental University, Human Research Ethics Committee, protocol no. 725. The root structure was removed below the cement-enamel junction and in order to expose a flat dentin substrate; the occlusal thirds were removed by trimming the crowns at right angles to the long axis of the teeth using a model trimmer (Y-230; Yoshida, Tokyo, Japan). Round class I cavities were prepared on the flat occlusal surfaces by using a cylindrical diamond bur attached to a highspeed air turbine under water coolant (carborundum points, 50 m grain size, SHOFU, Kyoto, Japan). Finishing diamond burs were used afterward to have a fine surface finish (SF114, SHOFU, Kyoto, Japan). To maintain cutting efficacy, the bur was replaced every five preparations. The cavity was approximately 4 mm in width and 2 mm in depth. The teeth were then randomly divided into two groups of fifteen teeth each according to the surface treatment. For the first group (control group), dentin surface was kept untreated. In the second group (resin-coated group), the cavity surface was prepared using the self-etching bonding system, Clearfil SE Bond (Kuraray Noritake Dental, Tokyo, Japan) and a low viscosity microfilled resin (Clearfil Protect Liner F, Kuraray Noritake Dental, Tokyo, Japan). According to the manufacturer’s instructions, SE primer was applied first to the cavity for 20 s and gently air dried. Then, SE bond was applied; mildly air dried and light cured for 20 s using a conventional halogen light curing unit (Optilux 501, Kerr, CA, USA; 550 mW/cm2 ). After that, Protect Liner F was placed on the already cured adhesive surface with a brush and light cured for 20 s. The cavities in both groups were then lined (covered) with a separating film (Pechiney Plastic Packaging, Chicago, IL, USA), filled with one increment of composite (Clearfil Majesty Posterior, Kuraray Noritake Dental, Tokyo, Japan), and light cured for 40 s using the light curing unit. After curing, the composite inlays were carefully removed from the cavities and checked for fit. The resin inlays were monitored under OCT prior to cementation and the defective ones were excluded and refabricated. The prepared cavity surfaces in group 1 and the coated surfaces in group 2 were both temporized with a water-setting non-eugenol temporary filling material (Caviton EX, GC, Japan) and stored in an incubator at 37 ◦ C in a humid condition to simulate the clinical situation for indirect composite restorations. After 24 h, the temporary filling material was carefully
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removed with a spoon excavator and surface was wiped with a cotton pellet containing ethanol for 10 s. The coated surfaces were then cleaned for 10 s using 37% phosphoric acid gel, rinsed and dried in order to remove any debris. The fitting surfaces of the resin inlays were treated with 37% phosphoric acid gel for 15 s, rinsed with water and gently air dried. Then, Tokuso Ceramic Primer (Tokuyama Dental) was applied as a silane coupling agent to the surface and air-dried. Specimens from each group were further divided into three subgroups according to the type of luting resin cement used. Table 1 lists the materials used in this study while Fig. 1 shows schematic drawing for the sample preparation. The resin cements used in this study were the dual-cure with self-etching primer Bistite II DC (Tokuyama Dental), the self-adhesive Clearfil SA Luting cement (Kuraray Noritake Dental, Tokyo, Japan) and the MMA-based self-etching chemically-cured Multibond II (Tokuyama Dental). Each of the three cements was applied according to the manufacturer’s instructions.
2.2.
Thermocycling procedure
All specimens were then stored at 37 ◦ C in humid condition for 24 h prior to the initial OCT imaging. Then, all the specimens were thermocycled for 10,000 cycles, which was roughly estimated to represent one year of clinical function approximately [26]. They were fatigued between 5 ◦ C and 55 ◦ C with a dwell time of 30 s in each temperature, and a transfer time of 2 s between baths (Cool Line CL200 and Cool Mate TE200, Yamato Scientific Co., Tokyo, Japan). The specimens were subjected to OCT evaluation to detect any changes in the adaptation of the resin restorations after 2000 cycles and after completing 10,000 cycles.
2.3.
OCT system
A swept-source OCT system (Santec OCT-2000, Santec Co., Komaki, Japan), was used in this study. The spectral bandwidth of the optical source is over 100 nm centered at 1310 nm at a 20 kHz sweep rate. The probe power is within the safety limits defined by American National Standard Institute. The sensitivity of this system and the shot-noise limited sensitivity are 106 and 119 dB, respectively. The axial resolution of the system is 11 m in air, which corresponds to 7 m in tissue assuming a refractive index of approximately 1.5. The lateral resolution depends on the objective lens at the probe and was 17 m in this study. Backscattered light carrying information about the microstructure of the sample is collected, returned to the system, digitized in time scale and then analyzed in the Fourier domain to reveal the depth information of the subject. The system analyzes the frequency components of backscattered light from the sample and creates real-time high resolution 2-D image.
2.4.
holes were drilled on the specimen surface to make sure that specimens were placed at the same orientation as accurately as possible. In order to capture OCT image, the specimen was positioned on a metal stage with a 3–5◦ tilt to avoid peculiar surface reflections. The surface of the specimen was blot dried using air duster to standardize the tooth surface hydration condition [22]. Then, the focus light beam was projected onto the tooth surface at 90◦ and scanned across the cavity in three dimensions using OCT probe. In this manner, 20 serial 2D sections at 200 m interval were obtained. The size of each image was 2000 × 1019 pixels corresponding to 5 mm × 6.6 mm (x, z). For the data analysis purpose, each of the 20 serial 2D sections was digitally analyzed using ImageJ (ver. 1.42q, National Institutes of Health, Bethesda, MD, USA). A custom computer code was developed as a plugin for ImageJ based on a binarization process previously reported [13,21], to facilitate image analysis procedure and distinguish pixel clusters with higher brightness indicating gap or unsealed interface at the cavity floor. The total cavity adaptation (including resin cement and/or coating interface) was calculated as
OCT imaging and analysis
Specimens were subjected to serial 2D scans 24 h after cementation, and after 2000 and 10,000 thermal cycles. To ensure the repeatability of the OCT scans for the same specimen, small
Cavity adaptation%
=
2.5.
1−
gap length at all cross-sections
cavity floor length at all cross-sections
× 100
Confocal laser scanning microscopy (CLSM)
To confirm the presence or absence of gap at tooth-restoration interface, randomly selected specimens after thermal cycling were sectioned with low-speed diamond saw (Isomet, Buehler, Lake Bluff, IL, USA) and then polished using polishing machine (ML-160 A, Maruto, Tokyo, Japan) with silicone carbide (SiC) paper (Sankyo, Saitama, Japan) and diamond pastes with particle size down to 0.25 m. The same interfacial location in a certain OCT cross-sectional slice was observed under CLSM (1LM21H/W, Lasertec Co., Yokohama, Japan) with a He-Ne laser source (632.8 nm) and 0.1 mW maximum output power at magnification levels of 500–1250×.
2.6.
Statistical analysis
For the statistical analysis of the adaptation, the data were statistically analyzed with three-way ANOVA followed by multiple comparisons using t-tests with Bonferroni corrections as post-hoc. The factors were resin cement type, resin coating and thermal cycling. All the statistical procedures were performed at significance level of ˛ = 0.05 with using Statistics package (ver. 16 for windows; SPSS, Chicago, IL, USA).
3.
Results
Representative OCT images from each group after thermal aging and their confirmatory CLSM images with A-scan (SS-OCT signal intensity) profiles plotted against selected areas in the same cross-sections are shown in Figs. 2–4 for BT, SA and MB respectively. There was a considerable loss of
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Table 1 – Materials used in this study. Material (Abbreviation) Manufacturer Lot no. Dentin bonding system Clearfil SE Bond (SE) Kuraray Noritake Dental 011595
Low-viscosity microfilled resin Protect Liner F (PLF) Kuraray Noritake Dental 0074DA Resin cements Bistite II DC (BT) Tokuyama Dental 028012
Clearfil SA Luting (SA) Kuraray Noritake Dental 0141AA
Multibond II (MB) Tokuyama Dental 0780Z1
Indirect resin composite Clearfil Majesty Posterior (MP) Kuraray Noritake Dental 00111A
Procedure
Composition
Primer: MDP, HEMA, hydrophilic dimethacrylate, dl-camphorquinone, N,N-diethanol-p-toluidine, water. Bond: MDP, Bis-GMA, HEMA, hydrophobic dimethacrylate, dl-camphorquinone, N,N-diethanol-p-toluidine, silanated colloidal silica.
Apply the primer for 20 s. Mild air blow. Apply adhesive and air blow gently. Light cure for 10 s.
Bis-GMA, TEGDMA, fluoride-methyl methacrylate, camphorquinone, silanized colloidal silica, pre-polymerized organic filler.
Apply in a thin layer, light cure for 20 s.
Primer 1 (A and B): phosphoric acid monomer, acetone, alcohol, water, initiator. Primer 2: HEMA, acetone, initiator. Resin cement pastes: Paste-A: NPGDMA, Bis-MPEPP, silica-zirconia filler. Paste-B: MAC-10, silica-zirconia filler, benzoylperoxide, photo-initiator. Paste A: Bis-GMA, TEGDEMA, MDP, hydrophobic aromatic dimethacrylate, silanated barium glass filler, silanated colloidal silica, dl-camphorquinone, benzoyl peroxide, initiator. Paste B: Bis-GMA, hydrophobic aromatic dimethacrylate, hydrophobic aliphatic dimethacrylate, silanated barium glass filler, silanated colloidal silica, surface treated sodium fluoride, accelerators, pigments. Primer: phosphoric acid monomer, water, acetone, UDMA, co-activator. liquid: MMA, UDMA, HEMA, MTU-6, borate catalyst. powder: PMMA, co-activator.
Apply primer 1A + 1B, leave for 30 s, air dry, apply primer 2, leave for 20 s, air-dry, place mixed paste A + B, light cure for 20 s.
Silanated glass ceramics, silanted silica filler, surface treated alumina microfiller, Bis-GMA, TEGDMA, hydrophobic aromatic dimethacrylate, dl-camphorquinone.
Apply the cement paste mix to the restoration, place the restoration. Light cure for 2–5 s, and then remove the excess cement. Light cure for 20 s.
Apply primer for 20 s and gently air dry for 10 s. Powder: liquid: 1:3 Mix for 5 s, apply to dentin surface.
Bulk filling and light cure for 40 s.
Abbreviations: MDP: 10-methacryloyloxydecyl dihydrogen phosphate, HEMA: 2-hydroxyethyl methacrylate, Bis-GMA: bisphenol-A diglycidyl ether dimethacrylate, TEGDMA: triethyleneglycol dimethacrylate, MAC-10: methacryloyloxundecane dicarboxylic acid, MMA: methyl methacrylate, PMMA: poly methyl methacrylate, UDMA: urethane dimethacrylate, MTU-6: 6-methacryloxyhexyl 2-thiouracil-5-carboxylate.
signal intensity through the composite inlay as clearly seen in the A-scan profiles in Figs. 2e and f, 3e and f and 4e and f, which were drawn by averaging the OCT signal intensity over an area of 150 m. Despite this attenuation, the peak caused by interfacial gaps was easily detectable in Figs. 2e, 3e and 4e; while in other areas (with no gap), no such peak was seen (Figs. 2f, 3f and 4e and f). A bright area in the OCT image indicates gap due to the presence of optical variation between restorative material, air in the gap and tooth structure leading to light reflection [16]; areas with increased brightness on OCT images were confirmed as gap by CLSM examination in Figs. 2c, 3c and 4c. Resin coating resulted in a layer approximately 100 m in thickness and improved adaptation as
confirmed by the CLSM images in Figs. 2d, 3d and 4d. After thermal cycling, most of the non-coated specimens showed high backscattering from the resin-dentin interface regardless of the type of cement as shown in Figs. 2a, 3a and 4a. In some specimens, the bright area extended throughout the cavity bottom indicating complete loss of seal (Fig. 3a); while in others, the gap was formed only at a part of the specimen (Fig. 4a). On the other hand, most of the specimens in the resin-coated groups showed little or no detectable reflection from the interface (Figs. 2b, 3b and 4b). The mean adaptation percentage of the three different resin cements to dentin with or without resin coating and standard deviation for each group are listed in Table 2 and
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Fig. 1 – Schematic view of study method; resin inlays were cemented in round cavities using a resin cement with or without resin coating, and subjected to OCT observation at baseline and after thermal cycling. CLSM was used for confirmation of OCT findings after cutting the specimens. SE: dentin-bonding system Clearfil SE Bond; PLF: Protect liner F; BT: Bistite II DC; BT-NC: Non-Coated Bistite II DC; BT-C: Coated Bistite II DC; SA: Clearfil SA Luting; SA-NC: Non-Coated SA Luting; SA-C: Coated SA Luting; MB; Multibond II; MB-NC: Non-Coated Multi bond II; MB-C: Coated Multibond II.
presented as bar graphs in Fig. 5. ANOVA test demonstrated a significant effect of resin coating, cement type and thermal cycling on gap formation in the cavity floor (p < 0.05). The interaction between these three factors was also significant
(p < 0.05). The application of resin coating of SE and PLF significantly improved the adaptation of resin inlays to dentin (p < 0.05) regardless the type of cement or sample age. Without resin coating, SA significantly showed better
Fig. 2 – Representative cross-sectional OCT images and signal intensity profiles of BT-NC and BT-C groups after 10,000 thermal cycles and corresponding CLSM images of the same cross-sections. (a) B-scan and binary image of the interface of a resin inlay cemented with BT showing an increase in the signal intensity at the cavity floor. (b) B-scan and its binarization from BT-C group showing an improved adaptation of the resin inlay after resin coating. (c and d) CLSM images from the same sections at 500× and 1250× magnification confirming the OCT findings. The gap under BT-NC specimen in (c) appears to have occurred at the resin cement primer and dentin interface (blank arrow). (e and f) A-scans (SS-OCT signal intensity) plotted over selected areas (indicated by lines) in the same cross-sections. Note the peak in backscatter signal (arrow) in (e) caused by Fresnel reflection due to contrast in refractive index between restorative material and air at the interfacial gap while in (f), no detectable change in signal intensity can be observed when the interface is sealed. In: resin inlay; Ce: resin cement; RC: resin coat; D: dentin.
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Fig. 3 – Images obtained from specimens luted using SA with and without resin coating after 10,000 thermal cycles. (a) B-scan and binary image of the selected interface from a SA-NC sample showing an increase in the signal intensity at the cavity floor. (b) No gap was detected in B-scan and binary image of this SA-C specimen. (c) CLSM images from the same section as in (a) at 500× and 1250× magnification showing gap between SA and dentin in the cavity floor. (d) Confirmatory CLSM image of the same section presented in (b). (e and f) A-scans plotted along the designated lines shown in (a and b). Arrow in (e) indicates the high intensity in backscatter signal caused by air filled gap in the interface. In: resin inlay; Ce: resin cement; RC: resin coat; D: dentin.
sealing compared to BT and MB. However, there was no significant difference in the adaptation between BT and MB (p > 0.05). In the non-coated specimens, thermal cycling regimens caused significant decrease (p < 0.05) in the cavity adaptation percentage of BT and SA only after 2000 cycles.
However, when MB was used as a cement, the adaptation percentage increased non-significantly (p > 0.05). On the other hand, the coated specimens showed no significant (p < 0.05) change in adaptation after thermal cycling with all cement materials.
Table 2 – Cavity adaptation percentage (standard deviation) in each group. Group
Baseline
2,000 Thermocycles
10,000 Thermocycles
Non-coated Bistite II DC (BT-NC) Multi bond II (MB-NC) SA Luting (SA-NC)
72.4 (14.6)aA 68.0 (17.1) aC 85.2 (14.1) bD*
65.5 (16.7) dA 74.5 (15.5) eC 71.3 (20.0) eE
56.5 (17.0) hB 75.0 (15.7) iC 58.5 (20.0) hF
Coated Bistite II DC (BT-C) Multibond II (MB-C) SA Luting (SA-C)
92.3 (7.5)cG 88.8 (8.5) cH 99.4 (2.0)bI*
91.1 (7.05) fG 90.4 (11.7) fH 98.1 (2.3) gI
89.0 (8.0) jG 90.5 (9.0) jH 97.5 (2.8) kI
In each column, values marked by similar lowercase letters are not significantly different. In each row, values marked by similar uppercase letters are not significantly different. (*) indicates no significant difference between coated and non-coated groups (three-way ANOVA multiple comparisons by Bonferroni post-hoc).
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Fig. 4 – SS-OCT 2D images, signal profiles and confirmatory CLSM images of the same cross-sections for selected specimens from MB-NC and MB-C groups after 10,000 thermal cycles. (a) B-scan and binary image of the interface for a MB-NC specimen showing some microgaps at the cavity floor indicated by bright pixels. (b) B-scan and its binarization for MB-C specimen showing good adaptation. (c) CLSM under 500× and 1250× magnification confirm gap locations identified by OCT in (a). (d) CLSM of the same section as in (b) shows good sealing in the resin-coated group. (e) A-scan of two different locations on the same cross-section to show the difference in backscatter signal of areas with (dashed line) and without gap (solid line). The signal from unsealed interface shows sudden increase in the intensity compared to uniform gradual attenuation in case of good sealing. (f) A-scan plotted along the line in cross-section (b). The decrease in signal intensity indicated by blank arrow is caused by low backscattering of light from MB compared to resin composite. In: resin inlay; Ce: resin cement; RC: resin coat; D: dentin.
4.
Discussion
In this study, OCT was used to detect gaps in tooth-restoration interface. OCT is a non-invasive diagnostic imaging technique that can give real time, high resolution images using a safe broadband light source. Nowadays, OCT is being used in various biomedical applications including dentistry. Previous dental studies had showed the ability of OCT to evaluate margins of composite restorations without cutting the sample or using ionizing radiations [21]. OCT is an objective method that allows evaluation of the same section at different times in a long-term study. During the evaluation of tooth-restoration interface, different interfaces were located including dentin-resin coat, resin coat-cement and cement-inlay interfaces in the coated samples and dentin-cement and cement-inlay interfaces in the non-coated samples. Among these, the cement-inlay interface was not included in the process of image analysis in this
study. Occasional gaps at this interface were considered to be internal resin defects. In this study, image analysis was conducted on 2D images to provide data through the whole cavity as presented in Figs. 2–4. As the light propagates through the sample, it passes through different materials, undergoing refraction and partial reflection. The reflection of light as it passes between two media with different refractive indices (Fresnel reflection) result in a peak in the backscatter signal (A-scan) forming a bright area in 2D OCT image. The refractive index of a composite resin is dependent on its composition and can be variable among different materials. According to the Gladstone–Dale relation [20], index of refraction can be related to the ratio and optical constants of the ingredients, which are mainly the resin matrix and the fillers. For example MP has a filler vol% of 82% according to manufacturer, and contains alumina fillers with a high refractive index (n = 1.75), while other composites with lower filler load contain barium glass filler that has a lower refractive index (n = 1.52). Moreover, methacrylate resins
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Fig. 5 – Bar graph representing cavity adaptation percentage and standard deviation of each group at baseline (24 h after cementation), after 2000 thermal cycles and after 10,000 thermal cycles.
Table 3 – Measured refractive indices for materials used in study. Material
n
MP SE + PLF (resin coat) BT SA MB
1.58 1.55 1.50 1.51 1.48
generally have a refractive index of 1.49–1.55 (for, TEGDMA: triethyleneglycol dimethacrylate and Bis-GMA: bisphenol-A diglycidyl ether dimethacrylate, respectively) and PMMA (poly methyl methacrylate) has a refractive index of 1.48, according to various technical reports. Refractive indices of different materials used in this study were measured following the methodology explained in details elsewhere [17,20]. Briefly, a thin slice of each material (approximately 300 m) is prepared and imaged by OCT while placed over a reflective metal stage. The refractive index is then calculated by measuring the ratio of optical path length through the material to the actual thickness of the slice. The results from at least 3 measurements were in the range of 1.48–1.58 as presented in Table 3. These refractive index values are close to those of dentin [20]. In order to confirm the assumption that an increased signal intensity at the interface indicated gap due to the refractive index contrast between the material and a low-refractive index medium such as air (n = 1.0), further investigation was carried out by imaging the specimens after each step of the
inlay placement. Representative OCT images of the specimen after DBS was applied and following placement of the PLF are shown in Fig. 6a and c, that suggest the surface of the applied coating is highly reflective while little additional reflection is rising from the underlying dentin interface showing good initial adaptation of the resin coating to the surface of dentin. The composite inlays were fabricated on the prepared cavity after using a plastic paraffin film separator mold into the cavity. It should be mentioned that replacing the impression step of an indirect technique by this method shortened the fabrication time but could also have yielded a thicker resin cement layer. An OCT image of an inlay placed without any resin cement to check its fit is presented in Fig. 6e. As clearly indicated by the signal intensity profile (Fig. 6f), absence of any intermediate cement layer leads to high light reflection from the border of composite and dentin resulting in the appearance of a double-reflection peak where the distance between two surfaces (i.e. inlay bottom and dentin surface) is wide enough. It was previously reported that since the reflections are resulting from the double refraction at the borders of the defect, the vertical dimension of the target pixels may not indicate the vertical dimension of the gap between two interfaces under this experiment setup, while the horizontal dimension correlated well with the extent of the unsealed portion of the interface [21]. Therefore, the percentage of gap in this study was calculated as the total horizontal length of the target pixels in the selected interface after removing the noise by a median filter. The custom software was used to detect bright clusters indicating increase in the signal intensity in
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Fig. 6 – (a) Cross-sectional OCT image showing prepared cavity after application of DBS (SE). (b) Signal intensity profile along the designated line in (a). Note that it may be difficult to characterize the thin bonding layer (approximately 10 m; which is close to the axial resolution of OCT). (c) OCT image after the application of low viscosity microfilled resin (PLF) to form the resin coat; blank arrow indicates pulp horn. Note that the resin has been applied twice to result in a thicker layer for the purpose of OCT imaging. Corresponding A-scan in (d) indicates good sealing of the resin coat with no increase in backscatter signal intensity. (e) OCT image showing the inlay inserted into a prepared cavity with no cement or resin coat to check for fit. Note the clear reflections from the boundary of the cavity. (f) Double peak in signal intensity profile caused by the boundaries of air-filled space, the inlay (top boundary, first bold arrow) and dentin (lower boundary, second bold arrow). (g) In the left image, previously cured layer of the resin placed over dentin shows a strong reflection from the interface; the intensity peak indicated by bold arrow in (h) confirms the gap, which can not be seen in the right image where the resin cement was adequately pushed against the dentin surface prior to light-curing. The blank arrow in (h) shows signal peak caused by surface reflection from the resin cement due to its contrast in refractive index with air.
restoration interface [13]. This software requires the user to determine an intensity limit to detect the target pixels in the area of interest that includes the resin interface in this study. The target pixels are those with high brightness in a binarized image. To further rule out the possibilities of bias in the detection of gaps, OCT images of the resin cement layer (without inlay) placed over dentin are presented in Fig. 6g and h. A gap between the cement and dentin was simply created by curing the resin cement as a separate layer on a glass slide and then placing the cured cement layer over dentin. In this case, a strong reflection from the interface is evidently indicating the gap. On the other hand, when the resin cement was adequately pushed against the dentin surface and then lightcured, no intensity peak was detected at the interface. Some previous studies have suggested the application of metallic colloids that would highly backscatter the OCT light as a contrast agent applied after placement of the restoration (as in dye penetration tests) [18]. Others have suggested that the metallic particles should be incorporated into the dentin bonding agents [27]. However, the results obtained from a series of research works suggest that such an increased contrast may not be necessary for assessment of a wide range of resin composites investigated under OCT [12,13,16–18,21]. The round cavities were prepared 2 mm in depth [13,18]. It has also been shown that OCT signal attenuation through composites depends on various compositional factors [28]. In the current study, a posterior composite was selected and used to fabricate resin inlays; this composite showed a low attenuation effect and small signal loss through the 2 mm thickness. Nevertheless, bright lines were occasionally observed within composite inlays. These micro defects are thought to be produced during the manipulation of the
highly viscous composite [29]. Such scattering in the superior structures may affect the penetration depth immediately beneath them [30]. Therefore, the fabricated inlays were monitored using OCT before cementation to exclude those with structural voids or defects [31]. The resin coating technique allows for protection and coverage of the prepared dentin immediately after cavity preparation reducing postoperative sensitivity and providing good interfacial adaptation and marginal seal. It was also shown that in a mechanism essentially similar to direct bonding, a reliable hybrid layer is produced [8,32]. Furthermore, resin coating enhanced the bond strength of indirect composite cores to pulpal floor dentin in endodontically treated teeth [33]. The combination of the two-step self-etch adhesive and a low-viscosity resin, which was employed in the current work, could provide the highest bond strength of cement to dentin [32,34]. The resin coating shifted the failure mode from adhesive failure to cohesive failure within the cement [33]. This points out the clinical significance of resin coating on sealing of dentin; as even if the restoration fractures, the dentin remains protected in both vital and non-vital teeth [33]. In this study, the application of resin coating on dentin resulted in a statistically significant increase in the adaptation of the resin cement to dentin (Table 2 or Fig. 5). The additional application of a low-viscosity microfilled resin protects DBS from tearing during removal of temporary restoration. It also enhances the adhesive polymerization through the diffusion of its free radicals that polymerize uncured resin in the oxygen inhibited layer [32,35]. Moreover, the resin composite layer in the coating technique would prevent possibly adverse interactions that have been reported to occur between residual uncured acidic monomers within the self-etch adhesive and the aromatic tertiary amine derived from chemical- and
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dual-cured resin composites. In addition, the low-viscosity microfilled resin with lower filler content combined with a bonding agent with low modulus of elasticity form a stressbreaking resin layer relieving the polymerization stresses of cement and leading to better adaptation of the resin inlays [35]. The association of these factors may have contributed toward the significantly higher adaptation percentage of resin cement to dentin when the surface was coated with DBS and PLF in comparison with non-coated samples. Before thermal cycling, SA-NC showed only scarce unsealed areas indicating good initial seal. MB-NC showed slightly more unsealed areas in the interface compared to BT-NC. However, no statistically significant difference was observed. SA is a self-adhesive resin cement; it is known to adhere to tooth structure without the need of a separate adhesive or etchant. The cement utilizes 10-methacryloxydecyl dihydrogen phosphate (MDP) functional monomer to achieve demineralization and bonding to the tooth surface. MDP is known to have a high chemical bonding potential to hydroxyapatite forming a very stable bond and excellent water resistance confirmed by the low dissolution rate of its calcium salt in water [36–38]. In fact, the acidic monomer is also included in the primer agent of the DBS, which conditions the surface by dissolving the smear layer and demineralizing dentin surface. BT is a dual-cured resin cement that needs pretreatment with two different primers. Its optical adaptation was lower than SA. One reason may be the high filler content and the viscosity of the mixed cement, which may decreased the depth of penetration into the primed dentin. Other factors related to application method should be taken into consideration. Also, residual solvents from primer may create leakage pathway and interfere with monomer polymerization and reduce mechanical properties leading to poor bonding performance. After the specimens were subjected to thermal cycling, SANC and BT-NC showed significant decrease in the adaptation percentage. This may be related to the difference in thermal expansion coefficients between cement material and dentin leading to gap formation or by accelerated hydrolytic degeneration of the cement material [39]. MB is an MMA-based powder-liquid resin cement with a single-bottle self-etching primer. The primer contains phosphoric acid monomer and borate derivative as a surface activator. It had the lowest adaptation performance which may be contributed to the slow rate of its setting chemical polymerization, and hydrophilic nature of the water-based primer [40]. However, MB-NC showed no decrease in adaptation after thermal cycling. The heat during thermal aging may enhance the chemical polymerization of the cement and stimulate completion of its setting reaction. In addition, water uptake by the resin cement may result in expansion of the layer and closure of some microgaps [41,42]. It has been shown that the water sorption by resin containing hydrophilic components is intense in the first days after coming to contact with water, and then gradually plateaus depending on the composition of the resin [43]. In the coated groups, on the other hand, thermal cycling did not significantly influence the restoration adaptation. This should be attributed to the reliability of the direct bonding
system used for resin coating in penetrating into dentin and sealing the interface. SE bond has exhibited good long-term clinical results and high hydrolytic stability; giving it an edge over any of the resin cements used alone in this study. The difference in adaptation among the coated groups is worth attention, since the same coating was used in all groups. The finding was attributed to the defects at resin coat-cement interface, which reflects the differences among the resin cements, such as contraction stresses that develop under the constrained polymerization condition of the resin cement [44]. However, since these defects were predominantly observed in BT-C and MB-C groups, other factors should be considered. During cementation, each cement was applied according to the manufactures’ instructions where primers were applied as well; the application of the water-based primer may interfere with polymerization of the hydrophobic cement and bonding to the resin coat surface. In this context, it is recommended that for cementation of inlays in the resin coating technique, a water-free resin cement system should be applied. In short, the null hypotheses of the study were rejected, as there were significant differences in sealing between resin cements. The use of resin coating technique improved overall interfacial sealing of the resin cements. Thermal aging affected the interfacial integrity depending on the resin cement type and coating.
5.
Conclusion
Within the limitation of this in vitro study, the following can be concluded that OCT is a high-speed imaging technique to study tooth-indirect composite restoration interface without the difficulties of common leakage tests. Treatment of dentin surface with resin coating before cementation improves longterm interfacial sealing of indirect restorations placed with resin cements.
Acknowledgments This research was supported in part by the Global Center of Excellence Program, International Research Center for Molecular Science in Tooth and Bone Diseases at Tokyo Medical and Dental University, partly by grants-in-aid for scientific research no. 24792019 from the Japan Society for the Promotion of Science and partly by King Abdulaziz University.
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Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.intl.elsevierhealth.com/journals/dema
Sealing performance of resin cements before and after thermal cycling: Evaluation by optical coherence tomography Alaa Turkistani a,b,c , Alireza Sadr c,∗ , Yasushi Shimada b , Toru Nikaido b , Yasunori Sumi d , Junji Tagami b,c a
Operative Dentistry Division, Conservative Dental Sciences Department, Faculty of Dentistry, King Abdulaziz University, Jeddah, Saudi Arabia b Cariology and Operative Dentistry, Graduate School of Medical and Dental Sciences, Tokyo Medical and Dental University, 1-5-45, Yushima, Bunkyo-ku, Tokyo 113-8549, Japan c Global COE, International Research Center for Molecular Science in Tooth and Bone Diseases, Tokyo Medical and Dental University, 1-5-45, Yushima, Bunkyo-ku, Tokyo 113-8549, Japan d Division of Oral and Dental Surgery, Department of Advanced Medicine, National Center for Geriatrics and Gerontology, National Hospital for Geriatric Medicine, 36-3, Gengo, Morioka, Obu, Aichi 474-8511, Japan
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objectives. Self-adhesive resin cements have been recently introduced; however, there is
Received 25 December 2012
little data available on their long-term performance. In this in vitro study, swept-source
Received in revised form
optical coherence tomography (OCT) at 1310 nm center wavelength was used for monitoring
4 August 2013
adaptation of indirect resin restorations after thermal cycling.
Accepted 21 May 2014
Methods. Resin inlays were luted to class-I cavities of extracted human teeth using three resin cements; Clearfil SA Luting (SA; Kuraray), Bistite II DC or Multibond II (Tokuyama Dental). Each cement was applied with or without pre-coating of dentin by a self-etch adhesive
Keywords:
(Clearfil SE Bond) and a low-viscosity microfilled resin. OCT imaging was performed after
Resin inlay
24 h, after 2000 and after 10,000 thermocycles (n = 5). Selected samples were sectioned for
Resin cement
interfacial observation by confocal laser scanning microscope (CLSM). Floor adaptation (per-
Resin coating
centage) was analyzed by software on 20 B-scans throughout each specimen, and subjected
Adaptation
to statistical analysis by three-way ANOVA test at a significance level of 0.05.
Optical coherence tomography
Results. Resin cement type, resin coating and thermal aging all significantly affected adaptation (p < 0.05). Initially, SA showed the highest adaptation; however, thermal aging significantly affected its sealing. The best results for all the cements were consistently achieved when the resin coating technique was applied where no deterioration of interfacial integrity was observed in the coated groups. CLSM closely confirmed OCT findings in all groups. Significance. OCT could be used for monitoring of composite inlays with several interfacial resin layers. The application of a direct bonding agent in the resin-coating technique improved interfacial sealing and durability of all resin cements. © 2014 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
∗
Corresponding author. Tel.: +81 3 5803 2483; fax: +81 3 5803 0195. E-mail address: [email protected] (A. Sadr). http://dx.doi.org/10.1016/j.dental.2014.05.010 0109-5641/© 2014 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
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1.
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Introduction
The aesthetic aspect of dental treatment has become increasingly popular in the recent years, especially with the development of improved materials and adhesive techniques using composite resins. The indirect composite resin restoration technique involves extra-oral fabrication of an inlay and its placement with a resin cement. It has been reported that for large cavities, indirect restorations bear advantages over direct techniques such as improvements in anatomic form, contour, fracture resistance and wear resistance [1]. Furthermore, extra-oral fabrication aids in the relief of residual stresses and ensures that the negative effects of polymerization shrinkage are confined to the thin layer of resin cement [2]. On the other hand, it is believed that the viscous resin cements may not provide dentin bonding comparable to dentin-bonding system (DBS) used for direct composite [3–5]. This may affect the sealing ability of these cements and lead to lower penetration to tooth substrate and hence, lower bonding performances in comparison to DBS. Therefore, a resin coating technique for indirect restorations was introduced in which DBS and a low viscosity microfilled resin are applied to seal dentin surface after preparation, decreasing pulp irritation and postoperative sensitivity and improving bond strength [6–9]. Meanwhile, the effectiveness of this technique for the newly introduced resin cement products (such as self-adhesive resin cements) has not been investigated. The self-adhesive resin cement is proposed to simplify the cementation procedure; it bonds to dentin in one step without the need of conditioning or pre-treatment (priming) of the surface [10,11]. Adhesion tests have been routinely used for laboratory evaluation of these biomaterials. However, the success of a restoration also greatly depends on its sealing ability of the dental tissue in an actual cavity [12]. Different methods are conventionally used to evaluate the marginal integrity and sealing of restorations. The most common method is detecting dye penetration depth under a stereoscopic microscope and/or scanning electron microscope (SEM). However, these methods are considered as destructive methods since they require sample sectioning, and may be subjective. More recently, three-dimensional and in-depth imaging methods have been introduced and utilized for characterization of dental composites [13–18]. Optical coherence tomography (OCT) can provide noninvasive, high resolution cross-sectional images for biologic microstructures and materials based on light backscattering from within the structure. Dental composites and hard tissues are scattering media and therefore can be suitable substrates for OCT imaging [16–24]. Toothrestoration interface under direct resin restorations has been investigated using this technique [18,19,21,25]; however, there are few reports on evaluation of indirect restorations. Thermal cycling procedure has been accepted as an effective means of artificially aging composite restorations to study their interfacial characteristics in the long-term. In this regard, imaging of resin restorations by OCT before and after thermal aging appears to be an attractive research method. Therefore, the aim of this laboratory study was to evaluate the effect of thermal cycling and resin coating technique on the adaptation
of indirect composite inlays luted with resin cements under OCT, and confirmation of OCT findings by cross-sectional confocal laser scanning microscopy (CLSM). The null hypotheses tested were as follows: (1) there was no difference in the interfacial sealing of the composites inlays between different resin cements; (2) the resin coating could not improve the interfacial integrity; and (3) There were no changes in the interfacial integrity of different test groups after thermal aging.
2.
Materials and method
2.1.
Specimen preparation
For this study, thirty extracted human third molars, free of cracks, caries and restorations were selected after the patients’ informed consent, as approved by the Institutional Review Board of Tokyo Medical and Dental University, Human Research Ethics Committee, protocol no. 725. The root structure was removed below the cement-enamel junction and in order to expose a flat dentin substrate; the occlusal thirds were removed by trimming the crowns at right angles to the long axis of the teeth using a model trimmer (Y-230; Yoshida, Tokyo, Japan). Round class I cavities were prepared on the flat occlusal surfaces by using a cylindrical diamond bur attached to a highspeed air turbine under water coolant (carborundum points, 50 m grain size, SHOFU, Kyoto, Japan). Finishing diamond burs were used afterward to have a fine surface finish (SF114, SHOFU, Kyoto, Japan). To maintain cutting efficacy, the bur was replaced every five preparations. The cavity was approximately 4 mm in width and 2 mm in depth. The teeth were then randomly divided into two groups of fifteen teeth each according to the surface treatment. For the first group (control group), dentin surface was kept untreated. In the second group (resin-coated group), the cavity surface was prepared using the self-etching bonding system, Clearfil SE Bond (Kuraray Noritake Dental, Tokyo, Japan) and a low viscosity microfilled resin (Clearfil Protect Liner F, Kuraray Noritake Dental, Tokyo, Japan). According to the manufacturer’s instructions, SE primer was applied first to the cavity for 20 s and gently air dried. Then, SE bond was applied; mildly air dried and light cured for 20 s using a conventional halogen light curing unit (Optilux 501, Kerr, CA, USA; 550 mW/cm2 ). After that, Protect Liner F was placed on the already cured adhesive surface with a brush and light cured for 20 s. The cavities in both groups were then lined (covered) with a separating film (Pechiney Plastic Packaging, Chicago, IL, USA), filled with one increment of composite (Clearfil Majesty Posterior, Kuraray Noritake Dental, Tokyo, Japan), and light cured for 40 s using the light curing unit. After curing, the composite inlays were carefully removed from the cavities and checked for fit. The resin inlays were monitored under OCT prior to cementation and the defective ones were excluded and refabricated. The prepared cavity surfaces in group 1 and the coated surfaces in group 2 were both temporized with a water-setting non-eugenol temporary filling material (Caviton EX, GC, Japan) and stored in an incubator at 37 ◦ C in a humid condition to simulate the clinical situation for indirect composite restorations. After 24 h, the temporary filling material was carefully
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removed with a spoon excavator and surface was wiped with a cotton pellet containing ethanol for 10 s. The coated surfaces were then cleaned for 10 s using 37% phosphoric acid gel, rinsed and dried in order to remove any debris. The fitting surfaces of the resin inlays were treated with 37% phosphoric acid gel for 15 s, rinsed with water and gently air dried. Then, Tokuso Ceramic Primer (Tokuyama Dental) was applied as a silane coupling agent to the surface and air-dried. Specimens from each group were further divided into three subgroups according to the type of luting resin cement used. Table 1 lists the materials used in this study while Fig. 1 shows schematic drawing for the sample preparation. The resin cements used in this study were the dual-cure with self-etching primer Bistite II DC (Tokuyama Dental), the self-adhesive Clearfil SA Luting cement (Kuraray Noritake Dental, Tokyo, Japan) and the MMA-based self-etching chemically-cured Multibond II (Tokuyama Dental). Each of the three cements was applied according to the manufacturer’s instructions.
2.2.
Thermocycling procedure
All specimens were then stored at 37 ◦ C in humid condition for 24 h prior to the initial OCT imaging. Then, all the specimens were thermocycled for 10,000 cycles, which was roughly estimated to represent one year of clinical function approximately [26]. They were fatigued between 5 ◦ C and 55 ◦ C with a dwell time of 30 s in each temperature, and a transfer time of 2 s between baths (Cool Line CL200 and Cool Mate TE200, Yamato Scientific Co., Tokyo, Japan). The specimens were subjected to OCT evaluation to detect any changes in the adaptation of the resin restorations after 2000 cycles and after completing 10,000 cycles.
2.3.
OCT system
A swept-source OCT system (Santec OCT-2000, Santec Co., Komaki, Japan), was used in this study. The spectral bandwidth of the optical source is over 100 nm centered at 1310 nm at a 20 kHz sweep rate. The probe power is within the safety limits defined by American National Standard Institute. The sensitivity of this system and the shot-noise limited sensitivity are 106 and 119 dB, respectively. The axial resolution of the system is 11 m in air, which corresponds to 7 m in tissue assuming a refractive index of approximately 1.5. The lateral resolution depends on the objective lens at the probe and was 17 m in this study. Backscattered light carrying information about the microstructure of the sample is collected, returned to the system, digitized in time scale and then analyzed in the Fourier domain to reveal the depth information of the subject. The system analyzes the frequency components of backscattered light from the sample and creates real-time high resolution 2-D image.
2.4.
holes were drilled on the specimen surface to make sure that specimens were placed at the same orientation as accurately as possible. In order to capture OCT image, the specimen was positioned on a metal stage with a 3–5◦ tilt to avoid peculiar surface reflections. The surface of the specimen was blot dried using air duster to standardize the tooth surface hydration condition [22]. Then, the focus light beam was projected onto the tooth surface at 90◦ and scanned across the cavity in three dimensions using OCT probe. In this manner, 20 serial 2D sections at 200 m interval were obtained. The size of each image was 2000 × 1019 pixels corresponding to 5 mm × 6.6 mm (x, z). For the data analysis purpose, each of the 20 serial 2D sections was digitally analyzed using ImageJ (ver. 1.42q, National Institutes of Health, Bethesda, MD, USA). A custom computer code was developed as a plugin for ImageJ based on a binarization process previously reported [13,21], to facilitate image analysis procedure and distinguish pixel clusters with higher brightness indicating gap or unsealed interface at the cavity floor. The total cavity adaptation (including resin cement and/or coating interface) was calculated as
OCT imaging and analysis
Specimens were subjected to serial 2D scans 24 h after cementation, and after 2000 and 10,000 thermal cycles. To ensure the repeatability of the OCT scans for the same specimen, small
Cavity adaptation%
=
2.5.
1−
gap length at all cross-sections
cavity floor length at all cross-sections
× 100
Confocal laser scanning microscopy (CLSM)
To confirm the presence or absence of gap at tooth-restoration interface, randomly selected specimens after thermal cycling were sectioned with low-speed diamond saw (Isomet, Buehler, Lake Bluff, IL, USA) and then polished using polishing machine (ML-160 A, Maruto, Tokyo, Japan) with silicone carbide (SiC) paper (Sankyo, Saitama, Japan) and diamond pastes with particle size down to 0.25 m. The same interfacial location in a certain OCT cross-sectional slice was observed under CLSM (1LM21H/W, Lasertec Co., Yokohama, Japan) with a He-Ne laser source (632.8 nm) and 0.1 mW maximum output power at magnification levels of 500–1250×.
2.6.
Statistical analysis
For the statistical analysis of the adaptation, the data were statistically analyzed with three-way ANOVA followed by multiple comparisons using t-tests with Bonferroni corrections as post-hoc. The factors were resin cement type, resin coating and thermal cycling. All the statistical procedures were performed at significance level of ˛ = 0.05 with using Statistics package (ver. 16 for windows; SPSS, Chicago, IL, USA).
3.
Results
Representative OCT images from each group after thermal aging and their confirmatory CLSM images with A-scan (SS-OCT signal intensity) profiles plotted against selected areas in the same cross-sections are shown in Figs. 2–4 for BT, SA and MB respectively. There was a considerable loss of
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Table 1 – Materials used in this study. Material (Abbreviation) Manufacturer Lot no. Dentin bonding system Clearfil SE Bond (SE) Kuraray Noritake Dental 011595
Low-viscosity microfilled resin Protect Liner F (PLF) Kuraray Noritake Dental 0074DA Resin cements Bistite II DC (BT) Tokuyama Dental 028012
Clearfil SA Luting (SA) Kuraray Noritake Dental 0141AA
Multibond II (MB) Tokuyama Dental 0780Z1
Indirect resin composite Clearfil Majesty Posterior (MP) Kuraray Noritake Dental 00111A
Procedure
Composition
Primer: MDP, HEMA, hydrophilic dimethacrylate, dl-camphorquinone, N,N-diethanol-p-toluidine, water. Bond: MDP, Bis-GMA, HEMA, hydrophobic dimethacrylate, dl-camphorquinone, N,N-diethanol-p-toluidine, silanated colloidal silica.
Apply the primer for 20 s. Mild air blow. Apply adhesive and air blow gently. Light cure for 10 s.
Bis-GMA, TEGDMA, fluoride-methyl methacrylate, camphorquinone, silanized colloidal silica, pre-polymerized organic filler.
Apply in a thin layer, light cure for 20 s.
Primer 1 (A and B): phosphoric acid monomer, acetone, alcohol, water, initiator. Primer 2: HEMA, acetone, initiator. Resin cement pastes: Paste-A: NPGDMA, Bis-MPEPP, silica-zirconia filler. Paste-B: MAC-10, silica-zirconia filler, benzoylperoxide, photo-initiator. Paste A: Bis-GMA, TEGDEMA, MDP, hydrophobic aromatic dimethacrylate, silanated barium glass filler, silanated colloidal silica, dl-camphorquinone, benzoyl peroxide, initiator. Paste B: Bis-GMA, hydrophobic aromatic dimethacrylate, hydrophobic aliphatic dimethacrylate, silanated barium glass filler, silanated colloidal silica, surface treated sodium fluoride, accelerators, pigments. Primer: phosphoric acid monomer, water, acetone, UDMA, co-activator. liquid: MMA, UDMA, HEMA, MTU-6, borate catalyst. powder: PMMA, co-activator.
Apply primer 1A + 1B, leave for 30 s, air dry, apply primer 2, leave for 20 s, air-dry, place mixed paste A + B, light cure for 20 s.
Silanated glass ceramics, silanted silica filler, surface treated alumina microfiller, Bis-GMA, TEGDMA, hydrophobic aromatic dimethacrylate, dl-camphorquinone.
Apply the cement paste mix to the restoration, place the restoration. Light cure for 2–5 s, and then remove the excess cement. Light cure for 20 s.
Apply primer for 20 s and gently air dry for 10 s. Powder: liquid: 1:3 Mix for 5 s, apply to dentin surface.
Bulk filling and light cure for 40 s.
Abbreviations: MDP: 10-methacryloyloxydecyl dihydrogen phosphate, HEMA: 2-hydroxyethyl methacrylate, Bis-GMA: bisphenol-A diglycidyl ether dimethacrylate, TEGDMA: triethyleneglycol dimethacrylate, MAC-10: methacryloyloxundecane dicarboxylic acid, MMA: methyl methacrylate, PMMA: poly methyl methacrylate, UDMA: urethane dimethacrylate, MTU-6: 6-methacryloxyhexyl 2-thiouracil-5-carboxylate.
signal intensity through the composite inlay as clearly seen in the A-scan profiles in Figs. 2e and f, 3e and f and 4e and f, which were drawn by averaging the OCT signal intensity over an area of 150 m. Despite this attenuation, the peak caused by interfacial gaps was easily detectable in Figs. 2e, 3e and 4e; while in other areas (with no gap), no such peak was seen (Figs. 2f, 3f and 4e and f). A bright area in the OCT image indicates gap due to the presence of optical variation between restorative material, air in the gap and tooth structure leading to light reflection [16]; areas with increased brightness on OCT images were confirmed as gap by CLSM examination in Figs. 2c, 3c and 4c. Resin coating resulted in a layer approximately 100 m in thickness and improved adaptation as
confirmed by the CLSM images in Figs. 2d, 3d and 4d. After thermal cycling, most of the non-coated specimens showed high backscattering from the resin-dentin interface regardless of the type of cement as shown in Figs. 2a, 3a and 4a. In some specimens, the bright area extended throughout the cavity bottom indicating complete loss of seal (Fig. 3a); while in others, the gap was formed only at a part of the specimen (Fig. 4a). On the other hand, most of the specimens in the resin-coated groups showed little or no detectable reflection from the interface (Figs. 2b, 3b and 4b). The mean adaptation percentage of the three different resin cements to dentin with or without resin coating and standard deviation for each group are listed in Table 2 and
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Fig. 1 – Schematic view of study method; resin inlays were cemented in round cavities using a resin cement with or without resin coating, and subjected to OCT observation at baseline and after thermal cycling. CLSM was used for confirmation of OCT findings after cutting the specimens. SE: dentin-bonding system Clearfil SE Bond; PLF: Protect liner F; BT: Bistite II DC; BT-NC: Non-Coated Bistite II DC; BT-C: Coated Bistite II DC; SA: Clearfil SA Luting; SA-NC: Non-Coated SA Luting; SA-C: Coated SA Luting; MB; Multibond II; MB-NC: Non-Coated Multi bond II; MB-C: Coated Multibond II.
presented as bar graphs in Fig. 5. ANOVA test demonstrated a significant effect of resin coating, cement type and thermal cycling on gap formation in the cavity floor (p < 0.05). The interaction between these three factors was also significant
(p < 0.05). The application of resin coating of SE and PLF significantly improved the adaptation of resin inlays to dentin (p < 0.05) regardless the type of cement or sample age. Without resin coating, SA significantly showed better
Fig. 2 – Representative cross-sectional OCT images and signal intensity profiles of BT-NC and BT-C groups after 10,000 thermal cycles and corresponding CLSM images of the same cross-sections. (a) B-scan and binary image of the interface of a resin inlay cemented with BT showing an increase in the signal intensity at the cavity floor. (b) B-scan and its binarization from BT-C group showing an improved adaptation of the resin inlay after resin coating. (c and d) CLSM images from the same sections at 500× and 1250× magnification confirming the OCT findings. The gap under BT-NC specimen in (c) appears to have occurred at the resin cement primer and dentin interface (blank arrow). (e and f) A-scans (SS-OCT signal intensity) plotted over selected areas (indicated by lines) in the same cross-sections. Note the peak in backscatter signal (arrow) in (e) caused by Fresnel reflection due to contrast in refractive index between restorative material and air at the interfacial gap while in (f), no detectable change in signal intensity can be observed when the interface is sealed. In: resin inlay; Ce: resin cement; RC: resin coat; D: dentin.
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Fig. 3 – Images obtained from specimens luted using SA with and without resin coating after 10,000 thermal cycles. (a) B-scan and binary image of the selected interface from a SA-NC sample showing an increase in the signal intensity at the cavity floor. (b) No gap was detected in B-scan and binary image of this SA-C specimen. (c) CLSM images from the same section as in (a) at 500× and 1250× magnification showing gap between SA and dentin in the cavity floor. (d) Confirmatory CLSM image of the same section presented in (b). (e and f) A-scans plotted along the designated lines shown in (a and b). Arrow in (e) indicates the high intensity in backscatter signal caused by air filled gap in the interface. In: resin inlay; Ce: resin cement; RC: resin coat; D: dentin.
sealing compared to BT and MB. However, there was no significant difference in the adaptation between BT and MB (p > 0.05). In the non-coated specimens, thermal cycling regimens caused significant decrease (p < 0.05) in the cavity adaptation percentage of BT and SA only after 2000 cycles.
However, when MB was used as a cement, the adaptation percentage increased non-significantly (p > 0.05). On the other hand, the coated specimens showed no significant (p < 0.05) change in adaptation after thermal cycling with all cement materials.
Table 2 – Cavity adaptation percentage (standard deviation) in each group. Group
Baseline
2,000 Thermocycles
10,000 Thermocycles
Non-coated Bistite II DC (BT-NC) Multi bond II (MB-NC) SA Luting (SA-NC)
72.4 (14.6)aA 68.0 (17.1) aC 85.2 (14.1) bD*
65.5 (16.7) dA 74.5 (15.5) eC 71.3 (20.0) eE
56.5 (17.0) hB 75.0 (15.7) iC 58.5 (20.0) hF
Coated Bistite II DC (BT-C) Multibond II (MB-C) SA Luting (SA-C)
92.3 (7.5)cG 88.8 (8.5) cH 99.4 (2.0)bI*
91.1 (7.05) fG 90.4 (11.7) fH 98.1 (2.3) gI
89.0 (8.0) jG 90.5 (9.0) jH 97.5 (2.8) kI
In each column, values marked by similar lowercase letters are not significantly different. In each row, values marked by similar uppercase letters are not significantly different. (*) indicates no significant difference between coated and non-coated groups (three-way ANOVA multiple comparisons by Bonferroni post-hoc).
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Fig. 4 – SS-OCT 2D images, signal profiles and confirmatory CLSM images of the same cross-sections for selected specimens from MB-NC and MB-C groups after 10,000 thermal cycles. (a) B-scan and binary image of the interface for a MB-NC specimen showing some microgaps at the cavity floor indicated by bright pixels. (b) B-scan and its binarization for MB-C specimen showing good adaptation. (c) CLSM under 500× and 1250× magnification confirm gap locations identified by OCT in (a). (d) CLSM of the same section as in (b) shows good sealing in the resin-coated group. (e) A-scan of two different locations on the same cross-section to show the difference in backscatter signal of areas with (dashed line) and without gap (solid line). The signal from unsealed interface shows sudden increase in the intensity compared to uniform gradual attenuation in case of good sealing. (f) A-scan plotted along the line in cross-section (b). The decrease in signal intensity indicated by blank arrow is caused by low backscattering of light from MB compared to resin composite. In: resin inlay; Ce: resin cement; RC: resin coat; D: dentin.
4.
Discussion
In this study, OCT was used to detect gaps in tooth-restoration interface. OCT is a non-invasive diagnostic imaging technique that can give real time, high resolution images using a safe broadband light source. Nowadays, OCT is being used in various biomedical applications including dentistry. Previous dental studies had showed the ability of OCT to evaluate margins of composite restorations without cutting the sample or using ionizing radiations [21]. OCT is an objective method that allows evaluation of the same section at different times in a long-term study. During the evaluation of tooth-restoration interface, different interfaces were located including dentin-resin coat, resin coat-cement and cement-inlay interfaces in the coated samples and dentin-cement and cement-inlay interfaces in the non-coated samples. Among these, the cement-inlay interface was not included in the process of image analysis in this
study. Occasional gaps at this interface were considered to be internal resin defects. In this study, image analysis was conducted on 2D images to provide data through the whole cavity as presented in Figs. 2–4. As the light propagates through the sample, it passes through different materials, undergoing refraction and partial reflection. The reflection of light as it passes between two media with different refractive indices (Fresnel reflection) result in a peak in the backscatter signal (A-scan) forming a bright area in 2D OCT image. The refractive index of a composite resin is dependent on its composition and can be variable among different materials. According to the Gladstone–Dale relation [20], index of refraction can be related to the ratio and optical constants of the ingredients, which are mainly the resin matrix and the fillers. For example MP has a filler vol% of 82% according to manufacturer, and contains alumina fillers with a high refractive index (n = 1.75), while other composites with lower filler load contain barium glass filler that has a lower refractive index (n = 1.52). Moreover, methacrylate resins
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Fig. 5 – Bar graph representing cavity adaptation percentage and standard deviation of each group at baseline (24 h after cementation), after 2000 thermal cycles and after 10,000 thermal cycles.
Table 3 – Measured refractive indices for materials used in study. Material
n
MP SE + PLF (resin coat) BT SA MB
1.58 1.55 1.50 1.51 1.48
generally have a refractive index of 1.49–1.55 (for, TEGDMA: triethyleneglycol dimethacrylate and Bis-GMA: bisphenol-A diglycidyl ether dimethacrylate, respectively) and PMMA (poly methyl methacrylate) has a refractive index of 1.48, according to various technical reports. Refractive indices of different materials used in this study were measured following the methodology explained in details elsewhere [17,20]. Briefly, a thin slice of each material (approximately 300 m) is prepared and imaged by OCT while placed over a reflective metal stage. The refractive index is then calculated by measuring the ratio of optical path length through the material to the actual thickness of the slice. The results from at least 3 measurements were in the range of 1.48–1.58 as presented in Table 3. These refractive index values are close to those of dentin [20]. In order to confirm the assumption that an increased signal intensity at the interface indicated gap due to the refractive index contrast between the material and a low-refractive index medium such as air (n = 1.0), further investigation was carried out by imaging the specimens after each step of the
inlay placement. Representative OCT images of the specimen after DBS was applied and following placement of the PLF are shown in Fig. 6a and c, that suggest the surface of the applied coating is highly reflective while little additional reflection is rising from the underlying dentin interface showing good initial adaptation of the resin coating to the surface of dentin. The composite inlays were fabricated on the prepared cavity after using a plastic paraffin film separator mold into the cavity. It should be mentioned that replacing the impression step of an indirect technique by this method shortened the fabrication time but could also have yielded a thicker resin cement layer. An OCT image of an inlay placed without any resin cement to check its fit is presented in Fig. 6e. As clearly indicated by the signal intensity profile (Fig. 6f), absence of any intermediate cement layer leads to high light reflection from the border of composite and dentin resulting in the appearance of a double-reflection peak where the distance between two surfaces (i.e. inlay bottom and dentin surface) is wide enough. It was previously reported that since the reflections are resulting from the double refraction at the borders of the defect, the vertical dimension of the target pixels may not indicate the vertical dimension of the gap between two interfaces under this experiment setup, while the horizontal dimension correlated well with the extent of the unsealed portion of the interface [21]. Therefore, the percentage of gap in this study was calculated as the total horizontal length of the target pixels in the selected interface after removing the noise by a median filter. The custom software was used to detect bright clusters indicating increase in the signal intensity in
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Fig. 6 – (a) Cross-sectional OCT image showing prepared cavity after application of DBS (SE). (b) Signal intensity profile along the designated line in (a). Note that it may be difficult to characterize the thin bonding layer (approximately 10 m; which is close to the axial resolution of OCT). (c) OCT image after the application of low viscosity microfilled resin (PLF) to form the resin coat; blank arrow indicates pulp horn. Note that the resin has been applied twice to result in a thicker layer for the purpose of OCT imaging. Corresponding A-scan in (d) indicates good sealing of the resin coat with no increase in backscatter signal intensity. (e) OCT image showing the inlay inserted into a prepared cavity with no cement or resin coat to check for fit. Note the clear reflections from the boundary of the cavity. (f) Double peak in signal intensity profile caused by the boundaries of air-filled space, the inlay (top boundary, first bold arrow) and dentin (lower boundary, second bold arrow). (g) In the left image, previously cured layer of the resin placed over dentin shows a strong reflection from the interface; the intensity peak indicated by bold arrow in (h) confirms the gap, which can not be seen in the right image where the resin cement was adequately pushed against the dentin surface prior to light-curing. The blank arrow in (h) shows signal peak caused by surface reflection from the resin cement due to its contrast in refractive index with air.
restoration interface [13]. This software requires the user to determine an intensity limit to detect the target pixels in the area of interest that includes the resin interface in this study. The target pixels are those with high brightness in a binarized image. To further rule out the possibilities of bias in the detection of gaps, OCT images of the resin cement layer (without inlay) placed over dentin are presented in Fig. 6g and h. A gap between the cement and dentin was simply created by curing the resin cement as a separate layer on a glass slide and then placing the cured cement layer over dentin. In this case, a strong reflection from the interface is evidently indicating the gap. On the other hand, when the resin cement was adequately pushed against the dentin surface and then lightcured, no intensity peak was detected at the interface. Some previous studies have suggested the application of metallic colloids that would highly backscatter the OCT light as a contrast agent applied after placement of the restoration (as in dye penetration tests) [18]. Others have suggested that the metallic particles should be incorporated into the dentin bonding agents [27]. However, the results obtained from a series of research works suggest that such an increased contrast may not be necessary for assessment of a wide range of resin composites investigated under OCT [12,13,16–18,21]. The round cavities were prepared 2 mm in depth [13,18]. It has also been shown that OCT signal attenuation through composites depends on various compositional factors [28]. In the current study, a posterior composite was selected and used to fabricate resin inlays; this composite showed a low attenuation effect and small signal loss through the 2 mm thickness. Nevertheless, bright lines were occasionally observed within composite inlays. These micro defects are thought to be produced during the manipulation of the
highly viscous composite [29]. Such scattering in the superior structures may affect the penetration depth immediately beneath them [30]. Therefore, the fabricated inlays were monitored using OCT before cementation to exclude those with structural voids or defects [31]. The resin coating technique allows for protection and coverage of the prepared dentin immediately after cavity preparation reducing postoperative sensitivity and providing good interfacial adaptation and marginal seal. It was also shown that in a mechanism essentially similar to direct bonding, a reliable hybrid layer is produced [8,32]. Furthermore, resin coating enhanced the bond strength of indirect composite cores to pulpal floor dentin in endodontically treated teeth [33]. The combination of the two-step self-etch adhesive and a low-viscosity resin, which was employed in the current work, could provide the highest bond strength of cement to dentin [32,34]. The resin coating shifted the failure mode from adhesive failure to cohesive failure within the cement [33]. This points out the clinical significance of resin coating on sealing of dentin; as even if the restoration fractures, the dentin remains protected in both vital and non-vital teeth [33]. In this study, the application of resin coating on dentin resulted in a statistically significant increase in the adaptation of the resin cement to dentin (Table 2 or Fig. 5). The additional application of a low-viscosity microfilled resin protects DBS from tearing during removal of temporary restoration. It also enhances the adhesive polymerization through the diffusion of its free radicals that polymerize uncured resin in the oxygen inhibited layer [32,35]. Moreover, the resin composite layer in the coating technique would prevent possibly adverse interactions that have been reported to occur between residual uncured acidic monomers within the self-etch adhesive and the aromatic tertiary amine derived from chemical- and
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dual-cured resin composites. In addition, the low-viscosity microfilled resin with lower filler content combined with a bonding agent with low modulus of elasticity form a stressbreaking resin layer relieving the polymerization stresses of cement and leading to better adaptation of the resin inlays [35]. The association of these factors may have contributed toward the significantly higher adaptation percentage of resin cement to dentin when the surface was coated with DBS and PLF in comparison with non-coated samples. Before thermal cycling, SA-NC showed only scarce unsealed areas indicating good initial seal. MB-NC showed slightly more unsealed areas in the interface compared to BT-NC. However, no statistically significant difference was observed. SA is a self-adhesive resin cement; it is known to adhere to tooth structure without the need of a separate adhesive or etchant. The cement utilizes 10-methacryloxydecyl dihydrogen phosphate (MDP) functional monomer to achieve demineralization and bonding to the tooth surface. MDP is known to have a high chemical bonding potential to hydroxyapatite forming a very stable bond and excellent water resistance confirmed by the low dissolution rate of its calcium salt in water [36–38]. In fact, the acidic monomer is also included in the primer agent of the DBS, which conditions the surface by dissolving the smear layer and demineralizing dentin surface. BT is a dual-cured resin cement that needs pretreatment with two different primers. Its optical adaptation was lower than SA. One reason may be the high filler content and the viscosity of the mixed cement, which may decreased the depth of penetration into the primed dentin. Other factors related to application method should be taken into consideration. Also, residual solvents from primer may create leakage pathway and interfere with monomer polymerization and reduce mechanical properties leading to poor bonding performance. After the specimens were subjected to thermal cycling, SANC and BT-NC showed significant decrease in the adaptation percentage. This may be related to the difference in thermal expansion coefficients between cement material and dentin leading to gap formation or by accelerated hydrolytic degeneration of the cement material [39]. MB is an MMA-based powder-liquid resin cement with a single-bottle self-etching primer. The primer contains phosphoric acid monomer and borate derivative as a surface activator. It had the lowest adaptation performance which may be contributed to the slow rate of its setting chemical polymerization, and hydrophilic nature of the water-based primer [40]. However, MB-NC showed no decrease in adaptation after thermal cycling. The heat during thermal aging may enhance the chemical polymerization of the cement and stimulate completion of its setting reaction. In addition, water uptake by the resin cement may result in expansion of the layer and closure of some microgaps [41,42]. It has been shown that the water sorption by resin containing hydrophilic components is intense in the first days after coming to contact with water, and then gradually plateaus depending on the composition of the resin [43]. In the coated groups, on the other hand, thermal cycling did not significantly influence the restoration adaptation. This should be attributed to the reliability of the direct bonding
system used for resin coating in penetrating into dentin and sealing the interface. SE bond has exhibited good long-term clinical results and high hydrolytic stability; giving it an edge over any of the resin cements used alone in this study. The difference in adaptation among the coated groups is worth attention, since the same coating was used in all groups. The finding was attributed to the defects at resin coat-cement interface, which reflects the differences among the resin cements, such as contraction stresses that develop under the constrained polymerization condition of the resin cement [44]. However, since these defects were predominantly observed in BT-C and MB-C groups, other factors should be considered. During cementation, each cement was applied according to the manufactures’ instructions where primers were applied as well; the application of the water-based primer may interfere with polymerization of the hydrophobic cement and bonding to the resin coat surface. In this context, it is recommended that for cementation of inlays in the resin coating technique, a water-free resin cement system should be applied. In short, the null hypotheses of the study were rejected, as there were significant differences in sealing between resin cements. The use of resin coating technique improved overall interfacial sealing of the resin cements. Thermal aging affected the interfacial integrity depending on the resin cement type and coating.
5.
Conclusion
Within the limitation of this in vitro study, the following can be concluded that OCT is a high-speed imaging technique to study tooth-indirect composite restoration interface without the difficulties of common leakage tests. Treatment of dentin surface with resin coating before cementation improves longterm interfacial sealing of indirect restorations placed with resin cements.
Acknowledgments This research was supported in part by the Global Center of Excellence Program, International Research Center for Molecular Science in Tooth and Bone Diseases at Tokyo Medical and Dental University, partly by grants-in-aid for scientific research no. 24792019 from the Japan Society for the Promotion of Science and partly by King Abdulaziz University.
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