Highlights
- •
Sealing performance was measured in five adhesives by optical coherence tomography.
- •
Sealing and bond strength performance within individual specimens were correlated.
- •
Interfacial defects increased after thermal aging.
- •
Increased interfacial defects tended to decrease the bond strength.
- •
Two- and three-dimensional images were useful in assessing bonding performance.
Abstract
Objectives
To evaluate the sealing ability and the microtensile bond strength (MTBS) of different adhesive systems bonded to dentin in class I cavities.
Methods
Round tapered dentin cavities (3-mm diameter, 1.5-mm height) prepared in extracted human molars were restored using composite resin (Clearfil Majesty Posterior) with two-step etch-and-rinse adhesive system (Adper Single Bond 2: ASB2), two-step self-etch adhesive (Clearfil SE Bond: CSEB), all-in-one adhesives (G-Bond Plus: GBP; Tri-S Bond Plus: TSBP), or no adhesive (Control), or bonded using low-shrinkage composite with its proper adhesive (Filtek Silorane, Silorane Adhesive System: FSS). After 24-h water storage or 10,000 cycles of thermal stress, the specimens were immersed into a contrast agent. Two and three-dimensional images were obtained using optical coherence tomography (OCT). The mean percentage of high brightness (HB%) at the interfacial zone in cross-sectional images was calculated as an indicator of contrast agent or gap at the interface. The specimens were then sectioned into beams and the MTBS measured.
Results
The HB% (ASB2 = TSBP = CSEB < FSS = GBP) and MTBS (CSEB = ASB2, CSEB > TSBP = GBP = FSS, ASB2 > FSS) differed significantly among the adhesives. After aging, HB% increased for GBP and FSS specimens, and the MTBS decreased for FSS specimens (ANOVA, Tukey’s post hoc, p < 0.05). The HB% and MTBS were significantly and negatively correlated ( p = 0.002). Confocal laser scanning and scanning electron micrographs confirmed contrast agent infiltration within the gap.
Significance
There was a significant correlation between sealing performance and bond strength of the adhesives in the whole cavity. After aging, the two-step systems showed equal or superior performance to the all-in-one and Silorane systems.
1
Introduction
Restorative dentistry has evolved in recent years with improvements in resin composite and adhesive formulations. However, polymerization shrinkage of methacrylate-based resin composites during curing may result in contraction stress and a loss of marginal adaptation . The main challenge for dental adhesives is to provide an equally effective bond to both the tooth tissue and resin composite. Inadequate marginal sealing of the restoration can lead to microleakage , postoperative sensitivity , and debonding , which ultimately reduce the longevity of the restoration.
The bonding performance of restorative systems can be evaluated by various parameters, including marginal adaptation, bond strength, and the interaction with the tooth substrate . Marginal adaptation tests often require multiple sectioning of the samples, followed by immersion into a staining solution, and surface polishing before observation using light microscopy, or scanning electron microscopy (SEM), or ultrathin sectioning for observation using transmission electron microscopy (TEM) . These procedures are time-consuming and are limited to in vitro studies. Microtensile bond strength tests are also commonly used to evaluate the strength of resin-tooth tissue bonds ; however, the clinical significance of this test over time is unknown .
Optical coherence tomography (OCT) has been proposed as a new non-destructive method of producing high-resolution, cross-sectional images of the internal biological structures at the micron scale . Recently, this technology has been applied in dentistry to characterize caries , assess gaps between the composite-tooth interface in two-dimensional (2D) and three-dimensional (3D) images , and evaluate voids and internal defects in restorations . While several studies have evaluated the marginal adaptation of the restorations using OCT , few studies have investigated the association between OCT findings and the bond strength .
Based on these concerns, in the current study, we evaluate the sealing ability and the bond strength of different restorative systems in class I cavities after 24 h or 10,000 thermocycles using OCT. Silver nitrate solution was used as an infiltrating agent to enhance the contrast in OCT images . The null hypotheses were that there was no difference in sealing ability between all adhesives tested, and there was no relationship between the sealing ability and bond strength.
2
Materials and methods
2.1
Specimen preparation
Thirty-six extracted intact human third molars were used according to the guidelines set by the Ethics Committee of Tokyo Medical and Dental University (Protocol number 725). The occlusal one-third and root of each tooth were cut with a diamond saw (Isomet, Buehler Ltd., Lake Bluff, IL, USA) underwater. The exposed coronal flat dentin surface was polished with 600-grit silicon carbide paper under running water to ensure that the enamel isles were completely removed. Standardized class I cavities (3 mm diameter × 1.5 mm deep) with rounded margins located in the occlusal dentin, tapered walls, and angled at approximately 130° were created using a flat-end, tapered cylinder diamond bur (custom-made FG#3132, KG Sorensen, Cotia, SP, Brazil) and finished with a fine diamond bur (FG #3132F, KG Sorensen). The bur was attached to a high-speed air turbine hand piece, and the cavities were prepared under water coolant.
The cavities were randomly assigned to six groups ( n = 6 per group) according to the material used: two-step, etch-and-rinse adhesive using Adper Single Bond 2 (ASB2; 3M ESPE, St. Paul, MN, USA); two-step, self-etch adhesive using Clearfil SE Bond (CSEB; Kuraray Noritake Dental, Tokyo, Japan); all-in-one adhesive using G-Bond Plus (GBP; GC Corp., Tokyo, Japan); all-in-one adhesive using Tri-S Bond Plus (TSBP, Kuraray Noritake Dental); and no adhesive (Control). The cavities were then restored with resin composite; Clearfil Majesty Posterior (Kuraray Noritake Dental). The last group was restored with a low-shrink composite and its silorane-based adhesive system; Filtek Silorane Adhesive System (FSS; 3M ESPE). The specimens were prepared according to each manufacturer’s instructions ( Table 1 ) and cured using a halogen light curing unit (Optilux 501, Kerr, CA, USA; 600 mW/cm 2 intensity). After polymerization, all the specimens were slightly polished once more with 2000-grit silicon carbide paper to remove the excess of resin composite and standardize the occlusal surface. The specimens were stored in water at 37 °C for 24 h.
| Material | Brand (Lot#) | Manufacturer | Composition | Application Instruction |
|---|---|---|---|---|
| Two-step, etch-and-rinse adhesive | Adper Single Bond 2; Lot N263397BR |
3 M ESPE, St. Paul, MN, USA | Etchant: 35% phosphoric acid (Scotchbond Etchant); Bond: Bis-GMA, HEMA, water, ethanol, UDMA, bisphenol A glycerolate, polyalkenoic silica nanoparticles treated with acid copolymer, dimethacrylate | Apply etchant for 15 s. Rinse for 10 s. Blot excess water. Apply adhesive for 15 s. Gently air-dry for 5 s. Light cure for 10 s. |
| Two-step, self-etch adhesive | Clearfil SE Bond Primer: Lot 01115A Bond: Lot 01671A |
Kuraray Noritake Dental Inc., Tokyo, Japan | Primer: MDP, HEMA, dimethacrylate hydrophilic, camphorquinone, N,N-diethanol p-toluidine, water; Bond: MDP, Bis-GMA, HEMA, dimethacrylate hydrophobic, camphorquinone, N,N-diethanol p-toluidine, silanated colloidal silica | Apply the primer onto the surface for 20 s. Air-dry and apply the adhesive. Air-dry and light cure for 10 s. |
| All-in-one, self-etch adhesive | G-Bond Plus or Gaenial Bond; Lot 1110051 |
GC Company, Tokyo, Japan | Phosphoric acid ester monomer, 4-MET, dimethacrylate, water, acetone, nano-silica filler, photoinitiator | Apply one-bottle agent for 10 s. Aggressively air-dry for 10 s and light cure for 10 s. |
| All-in-one, self-etch adhesive | Clearfil Tri-S Bond Plus or SE ONE; Lot 00002A |
Kuraray Noritake Dental Inc., Tokyo, Japan | MDP, Bis-GMA, HEMA, ethanol, sodium fluoride, hydrophilic aliphatic dimethacrylate, hydrophobic aliphatic methacrylate, colloidal silica, dl-camphorquinone, accelerators, initiators, water. | Apply bond and leave for 10 s. Gently air-dry for 10 s and light cure for 10 s. |
| Resin composite | Clearfil Majesty Posterior Shade A2; Lot 0114B |
Kuraray Noritake Dental Inc., Tokyo, Japan | Bis-GMA, Silanated barium glass powder, silanated colloidal silica, TEGDMA, hydrophobic aromatic dimethacrylate | Bulk filling. Light cure for 20 s. |
| Low shrink composite and silorane-based adhesive system | Filtek Silorane Shade A2; Lot 359005 Silorane Adhesive System; Primer: Lot N407720 Bond: Lot N418250 |
3M ESPE, St. Paul, MN, USA | Primer: Bis-GMA, HEMA, phosphoric acid-methacryloxy-hexylesters, ethanol, water, silane treated silica, 1,6-hexanediol dimethacrylate, copolymer of acrylic and itaconic acid, ethyl methacrylate, phosphine oxide, dl-camphorquinone, ethyl 4-dimethyl aminobenzoate, methyl alcohol. Bond: Substituted dimethacrylate, silane treated silica, TEGDMA, phosphoric acid methacryloxy-hexylesters, 1,6-hexanediol dimethacrylate, dl-camphorquinone | Prime for 15 s, air-dry, and light cure for 10 s. Shake the bond bottle down, bond it, air-dry, and light cure for 10 s. Place the restorative composite and light cure twice for 20 s. |
2.2
Thermocycling procedure
Half of the specimens in each group were randomly selected to undergo thermocycling ( n = 3/per material). The specimens were fatigued with 10,000 thermocycles between 5 °C and 55 °C at a dwell time of 30 s per temperature and a transfer time of 4 s between baths (K178-08 Tokyo Giken, Tokyo, Japan).
2.3
Contrast agent
After 24 h or 10,000 thermocycles, all the specimens were coated with two layers of nail varnish, except at the 1 mm region surrounding the restoration, and immersed into 50% ammoniacal silver nitrate solution for 24 h. Thereafter, they were rinsed thoroughly under running tap water and exposed to photodeveloping solution for 6 h under fluorescent light to reduce the penetrating ammoniacal silver nitrate into metallic silver grains.
2.4
OCT system
The OCT (Santec OCT-2000, Santec Co., Komaki, Japan) used in this study is a swept-source OCT. The low-coherence near-infrared light source used in this system is centered at 1310 nm and a 20-kHz sweep rate. The unit includes a hand-held probe emitting less than 5 mW of power, which is within the safety limits defined by the American National Standards Institute.
The laser emanating from the probe is projected onto the sample, across the area of interest. Backscattered light carrying information about the microstructure of the sample is collected, returned to the system, digitized in a time scale, and then analyzed in the Fourier domain to reveal the depth information of the subject. The system has an axial resolution of 11 μm in air, and 7 μm in tissue, assuming an approximately 1.5 refractive index. A 2001 × 1019-pixel 2D cross-sectional image (4 mm × 4 mm) can be created by converting the raw data into a grey scale digital image. A 500 × 500 × 600-pixel 3D image (4 mm × 4 mm × 4 mm) can be obtained using a series of 2D images within 4 s including data acquisition and processing time.
2.5
2D and 3D tomography imaging
After 24 h or 10,000 thermocycles and silver infiltration, the 2D and 3D images were generated using OCT. During the scan, the OCT probe was set at a fixed distance over the restoration surface, and the scanning beam was positioned 90° to the restoration occlusal plane. Cross-sectional 2D images of each restoration were taken at every 500-μm intervals by moving the sample through the laser beam in a mesio-distal direction; 3D scans were recorded at the same area of interest. Five cross-sectional images were obtained from each restoration, totaling 30 images per material.
In the presence of air within a defect at the restoration-tooth interface, as the light passes between these two mediums with different refractive indices, part of the light reflected from the interface is visualized as bright areas on the OCT image. Using a silver infiltration contrast agent, the reflection from the metallic agent resulted in stronger reflectivity signal from OCT . In order to evaluate the sealing ability of different adhesives, OCT raw data were imported into a digital image analysis software (NIH ImageJ 1.60, Scion; Frederick, MD, USA) , and a median filter was applied to decrease the background noise . A plugin for ImageJ with an algorithm to determine signal threshold was used for the image analysis . As shown in Fig. 1 , a standard area surrounding the entire length of the restoration interface, excluding the specimen surface, was selected for analysis The region was approximately 70-pixels wide, and the brightest pixels and surrounding area were placed at the center. The pixel values on each scan line (corresponding to 2 pixels in width) within the selected area were ranked by the software plugin. The pixels that ranked higher than 90% of the pixels on the same line (i.e. the top 10%) were selected. Among them, the pixels with intensity values equal to or greater than the summed background noise and median values were designated as target pixels and all other pixels were designated as null . The percentage distribution of brighter pixels with significantly higher signal intensity compared to surrounding pixels at the interfacial area was automatically calculated by the plugin on the cross-sectional OCT 2D image ( Figs. 1 and 2 ), as high brightness (HB%) parameter. Five cross-sectional images were obtained of each restored sample in 500-μm intervals, and the mean percent high brightness (HB%) per sample was calculated ( Fig. 2 a). Moreover, based on the contrast produced by the silver infiltration, it was possible to selectively mark and visualize the brighter areas in the 3D images (Avizo 6.2 imaging software, Visualization Sciences Group, Burlington, MA, USA) ( Fig. 2 c and d).
2.6
Microtensile bond strength (MTBS) measurement and fracture analysis
After obtaining the OCT images, the specimens were serially sectioned using a low-speed diamond saw (Isomet, Buehler Ltd.) under water coolant to produce parallelepiped sticks (0.6 mm wide × 0.6 mm thick × 2.5 mm long) with the long axis perpendicular to the cavity floor. The ends of the sticks were carefully fixed with cyanocrylate glue (Model Repair II Blue, Sankin Industry Co., Tokyo, Japan) to a jig in a universal testing machine (EZ Test, Shimadzu, Kyoto, Japan). The MTBS was determined by subjecting the specimens to a tensile force at a crosshead speed of 1 mm/min. After MTBS testing, the fractured surfaces were mounted on brass stubs, gold-coated, and examined by SEM and Energy Dispersive X-ray Spectroscopy (EDS) (JSM5600, JEOL Ltd., Tokyo, Japan). Initial energy spectra analyses were performed to determine the elemental composition of the entire area. Additionally, select surface areas were mapped for elements including silver, calcium, and silicon. The failure mode of each beam was determined and classified in consensus by three experienced researchers as follows: cohesive failure in composite resin (CR); failure between composite resin and adhesive (RA); failure between adhesive and dentin (AD); mixed failure of composite resin, adhesive, and dentin (RAD); mixed failure of adhesive, hybrid layer, and dentin (AHD); cohesive failure within adhesive (AA), cohesive failure in hybrid layer (HL); and cohesive failure in dentin (D).
2.7
Confocal laser scanning microscope (CLSM)
Ten additional specimens were restored ( n = 2/per group) and immersed into the contrast agent for 24 h or 10,000 thermocycling as previously described in this study. Middle cross-sectional OCT images were recorded, and the specimens were reduced up to the same cross-section by polishing with silicon carbide paper (600–2000-grit), followed by diamond pastes with particle sizes down to 0.25 μm under running water. Finally, the polished specimens were observed with CLSM (1LM21H/W, Lasertec Co., Yokohama, Japan) at a 1250× magnification. The presence of an interfacial space with more than 1 μm in height was defined as a gap under CLSM.
2.8
Statistical analysis
The mean HB% and MTBS values were analyzed by one-way analysis of variance (ANOVA) and Tukey’s post hoc test. Correlation between HB% and MTBS data was determined using Pearson’s correlation test followed by linear regression analysis. All statistical analyses were performed using Statistical Package for Social Sciences software (SPSS for Windows, Version 16.0, SPSS, IL, USA) with the significance defined as α = 0.05.
2
Materials and methods
2.1
Specimen preparation
Thirty-six extracted intact human third molars were used according to the guidelines set by the Ethics Committee of Tokyo Medical and Dental University (Protocol number 725). The occlusal one-third and root of each tooth were cut with a diamond saw (Isomet, Buehler Ltd., Lake Bluff, IL, USA) underwater. The exposed coronal flat dentin surface was polished with 600-grit silicon carbide paper under running water to ensure that the enamel isles were completely removed. Standardized class I cavities (3 mm diameter × 1.5 mm deep) with rounded margins located in the occlusal dentin, tapered walls, and angled at approximately 130° were created using a flat-end, tapered cylinder diamond bur (custom-made FG#3132, KG Sorensen, Cotia, SP, Brazil) and finished with a fine diamond bur (FG #3132F, KG Sorensen). The bur was attached to a high-speed air turbine hand piece, and the cavities were prepared under water coolant.
The cavities were randomly assigned to six groups ( n = 6 per group) according to the material used: two-step, etch-and-rinse adhesive using Adper Single Bond 2 (ASB2; 3M ESPE, St. Paul, MN, USA); two-step, self-etch adhesive using Clearfil SE Bond (CSEB; Kuraray Noritake Dental, Tokyo, Japan); all-in-one adhesive using G-Bond Plus (GBP; GC Corp., Tokyo, Japan); all-in-one adhesive using Tri-S Bond Plus (TSBP, Kuraray Noritake Dental); and no adhesive (Control). The cavities were then restored with resin composite; Clearfil Majesty Posterior (Kuraray Noritake Dental). The last group was restored with a low-shrink composite and its silorane-based adhesive system; Filtek Silorane Adhesive System (FSS; 3M ESPE). The specimens were prepared according to each manufacturer’s instructions ( Table 1 ) and cured using a halogen light curing unit (Optilux 501, Kerr, CA, USA; 600 mW/cm 2 intensity). After polymerization, all the specimens were slightly polished once more with 2000-grit silicon carbide paper to remove the excess of resin composite and standardize the occlusal surface. The specimens were stored in water at 37 °C for 24 h.
