Micro-CT and SS-OCT may be useful non-destructive methods for evaluating internal adaptation.
The microleakge measured by micro-CT was lower than that of SS-OCT, however the two measurements were relatively high-correlated.
When adhesion depends mostly on the dentin surface, a two-step self-etch adhesive system should be considered for long-term longevity.
Swept-source optical coherence tomography (SS-OCT) and micro-CT can be useful non-destructive methods for evaluating internal adaptation. There is no comparative study evaluating the two methods in the assessment of internal adaptation in composite restoration.
The purpose of this study was to compare internal adaptation measurements of SS-OCT and micro-CT. Two cylindrical cavities were created on the labial surface of twelve bovine incisors. The 24 cavities were randomly assigned to four groups of dentin adhesives: (1) three-step etch-and-rinse adhesive, (2) two-step etch-and-rinse adhesive, (3) two-step self-etch adhesive, and (4) one-step self-etch adhesive. After application, the cavities were filled with resin composite. All restorations underwent a thermocycling challenge, and then, eight SS-OCT images were taken using a Santec OCT-2000™ (Santec Co., Komaki, Japan). The internal adaptation was also evaluated using micro-CT (Skyscan, Aartselaar, Belgium). The image analysis was used to calculate the percentage of defective spot (%DS) and compare the results. The groups were compared using one-way ANOVA with Duncan analysis at the 95% significance level. The SS-OCT and micro-CT measurements were compared with a paired t -test, and the relationship was analyzed using a Pearson correlation test at the 95% significance level.
The %DS results showed that Group 3 ≤ Group 4 < Group 1 ≤ Group 2 on both SS-OCT and micro-CT images. The %DSs on micro-CT were lower than SS-OCT ( p < 0.05) and the Pearson correlation coefficient between SS-OCT and micro-CT was r = 0.787 ( p < 0.05).
Polymerization shrinkage of the resin composite can create open margins at the outer interface of the restoration and can also affect the integrity of internal adaptation between the resin composite and the tooth substrate . Marginal microgaps can be identified by inspection from the outside. However, detecting internal defective interfaces caused by polymerization shrinkage requires other techniques. To find the microgaps inside a tooth, dye and tracer penetration methods have been used . Tracers, such as methylene blue, rhodamine, erythrosin and silver nitrate, can be infiltrated into teeth. These methods are very simple and definite, but the restorations must be cut for examination . In addition, once the teeth are sectioned, they cannot be evaluated again. Another drawback of this method is that some degree of dentin staining should be differentiated from actual microleakage between the cavity and restoration .
Current dentin bonding systems can be divided into two categories: etch-and-rinse system and self-etch system. Etch-and-rinse adhesives may be three-step or two-step system. Self-etch adhesives can be two-step or one-step system. Three-step etch-and-rinse adhesives are generally considered the gold standard when they are compared with other systems. Although two and three steps etch-and-rinse adhesives show similar strength of dentin bonding immediately following application, long-term bond strength was found to be different between the two adhesives . As two-step etch-and-rinse adhesive has combined hydrophilic components of primer and hydrophobic components of bonding agent, it makes its hybrid layer semipermeable . This characteristic influences long-term bond strength negatively. Two-step self-etch adhesives can have a problem with enamel bonding. If two-step self-etch system with mild or moderate acidity (pH > 1) is applied, there cannot be the same extent of demineralization obtained with the use of phosphoric acid . One-step self-etching adhesives are known to have several shortcomings; a reduced immediate bond strength, lower long-term effectiveness, phase-separation, enhanced water-sorption and increased nanoleakage . For their different features, these dentin bonding systems may not show the same extent of internal adaptation.
Optical coherence tomography (OCT) was introduced as a non-invasive cross-sectional imaging method for biological systems . It provides real time visualization of the restoration or tissue without the need of X-ray radiation exposure. After a laser source is projected over a restoration, the backscattered light is transformed into a signal intensity that can be shown as an image. OCT is based on the coherent properties of light. The OCT signal is based on the interference of light reflections from the reference mirror and backscattered light from the tissue. It shows cross-sectional images based on the depth-resolved optical reflective property. OCT is a non-invasive method that can be used for different purposes such as detecting dental caries or cracks . A specific type of OCT is a swept-source optical coherence tomography (SS-OCT). SS-OCT has a better image resolution and scanning speed using a wavelength-tuned laser as the light source . It should be noted that the imaging depth of OCT is limited to less than 3 mm of tissue.
The OCT system has been utilized to investigate the cavity floor for the evaluation of internal adaptation. On the OCT image, interfacial microgap is observed as bright spot or line with high signal intensity . The change in the signal intensity at the interface appears as a white cluster on the image. When light passes the interface between two media with different refractive indices, a portion of light is reflected. This is known as the Fresnel phenomenon and depends on the incidence angle and refractive index ( n ). The refractive index of air is 1.0 ( n ) and that of a tooth or resin composite is 1.5–1.6 ( n ) . If there is a microgap formed by incomplete adhesion, air or water may exist at the interface. When light transverses the air at the interface, a portion of light is reflected and the OCT system shows a higher signal intensity. If the microgap is filled with another medium, such as water 1.33 ( n ), the reflection would not be as strong as that of the air.
Microfocus X-ray computed tomography (micro-CT) is another useful method to evaluate the internal adaptation of restorations . Recently, Kwon and Park proposed a method in which silver nitrate was penetrated from the pulp space through the dentinal tubules, and the amount of silver nitrate penetration was assessed by micro-CT . The authors reported it as a new measuring method for evaluating the internal adaptation without any destruction. In another study, the internal adaptation of dentin-composite was analyzed using micro-CT, and the correlation of internal adaptation with polymerization shrinkage was evaluated . Due to the penetrating ability of X-rays, micro-CT enables the evaluation of dental hard tissue irrespective of its depth.
Micro-CT and SS-OCT can be useful non-destructive methods for evaluation of internal adaptation. There are currently no comparative studies evaluating the two methods in the assessment of internal adaptation in composite restoration. The purpose of this study was to compare internal adaptation from SS-OCT and micro-CT. For this purpose, bovine cavities were restored with resin composite using different adhesive systems, and the internal adaptation was evaluated using SS-OCT and micro-CT.
The null hypotheses tested in the study were the following:
There is no difference in internal adaptation among the restorations in which different dentin adhesive systems are used
There is no difference between the internal adaptations measured by SS-OCT and those measured by micro-CT.
Materials and methods
This study used twelve extracted bovine mandibular incisors. The labial surface of each tooth was flattened with a trimmer and 600-grit sandpaper. Two round cylinder-shaped class I cavities (3 mm diameter, 2 mm depth) were made on the labial surface of each tooth. First, a flat-end tapered diamond burr attached to a high-speed air-turbine handpiece and a water coolant was used. Then, a cylinder-shaped stone point was used to create a standardized cavity.
The cavities were randomly assigned to four groups, each with six cavities. Two cavities in the same tooth were restored using the same adhesive system. The four groups were used to test four different materials as the dentin adhesive: Group 1, three-step etch-and-rinse adhesive (Scotch Bond Multipurpose, 3 M, MN, USA); Group 2, two-step etch-and-rinse adhesive (Single bond 2, 3 M, MN, USA); Group 3, two-step self-etch adhesive (Clearfil Megabond, known as Clearfil SE Bond outside Japan, Kuraray Noritake Dental, Tokyo, Japan); and Group 4, one-step self-etch adhesive (Clearfil SE One, Kuraray Noritake Dental, Tokyo, Japan). Each adhesive system was used according to the manufacturer’s instructions. The adhesives were applied to the cavity surfaces, dried with gentle air and irradiated with a halogen light curing unit (Optilux 501, Kerr, CA, USA; 550 mW/cm 2 ) for 10 s. The cavities were filled with the resin composite (Estelite Sigma Quick, Tokuyama Dental, Tokyo, Japan) using the bulk-filling technique and were light-cured for 40 s. The restorations were stored in 100% humid conditions at room temperature.
All of the restorations underwent thermocycling ( n = 6 per group). A CS-4.8 chewing simulator (SD Mechatronik, Feldkirchen-Westerham, Germany) was used. The restoration underwent 100,000 thermocycles between 5 °C and 55 °C, with a dwelling time of 30 s in each temperature and a transferring time of 10 s.
Swept-source optical coherence tomography (SS-OCT)
The SS-OCT used in this study is the Santec OCT-2000™ (Santec Co., Komaki, Japan). It is a frequency domain OCT system integrating a high-speed frequency sweeping external cavity. The laser probe power is less than 20 mW. The light source in the system sweeps the wavelengths from 1260 nm to 1360 nm at a rate of 20 kHz. The axial resolution of the OCT system is 11 μm in air, which is equivalent to 7 μm in a biological structure. A hand-held scanning probe connected to the SS-OCT was placed over the occlusal surface of the restorations and was oriented at a right angle to the occlusal surface of the restoration. The first SS-OCT image of a restoration was taken 100 μm from the mesial end of a cavity. Eight OCT images were taken for each restoration at intervals of 400 μm along the section of the restoration.
Silver nitrate infiltration and micro-CT imaging
All of the teeth were cut at the Cemetoenamel junction (CEJ) to expose the pulp chamber. The restorations were soaked in 17% ethylenediamine tetraacetic acid (EDTA) for 5 min to remove the smear layer in the pulp chamber. The teeth were then rinsed with distilled water. The teeth were immersed in a 25% silver nitrate solution and were placed under 3.75 kPa pressure that was applied from the apical toward the pulpal side for 3 days. This step was performed to facilitate silver nitrate infiltration beneath the cavity floor. The restorations were then rinsed thoroughly with distilled water and stored in saline.
Eight cross-section images of micro-CT were taken for each restoration, and the images were extended from the mesial end of the cavity to its distal end at 400 μm pitch intervals. A high-resolution micro-CT (Model 1076, Skyscan, Aartselaar, Belgium) was used to obtain the images. The imaging settings were the following: acceleration voltage: 100 kV, beam current: 100 μA, Al filter: 0.5 mm, resolution: 18 μm and rotation: 360° in 0.5° steps. Two-dimensional sagittal images were obtained from each restoration. Each tooth was mounted on a special template that was exclusively designed for it. This template minimized the changes in the position of the restoration during repeated processes. The 2D images were analyzed using image analysis software (ImageJ™ ver. 1.45) ( Fig. 1 ).
Preliminary study for negative control
The cavities of the same dimension were prepared on labial surface of bovine teeth as described before ( n = 4). Without priming or adhesive bonding, the same resin composite was filled and light cured. SS-OCT and micro-CT images were taken for negative control group.
SS-OCT images analysis
The SS-OCT raw data were imported into the ImageJ™ program. The presence of air, which indicates a microgap within a defect, is visualized as bright areas in the SS-OCT images. It is known that when there is a microgap between two media of different refractive indices, the reflections of light at the interface will be dissimilar. The high signal intensity at the resin-dentin interface created bright spots or a line ( Fig. 2 ). To measure the size of the bright spots, the image was subjected to a median filter to reduce the noise and then cropped to the area including the cavity floor. The cropped image of the cavity floor underwent a binarization process to determine which brightness level of pixels should be included. The image binarization process changes a grayscale image into a binary black-and-white image as previously described by Bakhsh et al. . The length of each bright spot was calculated after processing to binary image. To verify the leakage length of the floor, the function of plot profile in ImageJ™ was used. It could show signal intensity plot for rectangular selections. Bright spot or cluster was presented as high intensity value ( Fig. 2 lower window). A part of cavity floor where signal intensity was above the threshold was thought to be imperfective margin.
The percentage of defective spots on the cavity floor (%DS) equals the sum of the bright spots or the cluster length/the length of the floor cavity × 100.
Micro-CT images analysis
Eight images were obtained for each restoration at a 400 μm pitch, which was the same interval of the SS-OCT. The same evaluation method was used as that of the SS-OCT. The silver nitrate penetration into the microgap between the tooth and restorative material was identified by peaks of the graph on the plot profile function ( Fig. 3 lower window). Using ImageJ™ program for each restoration, the leakage spot or area was identified and measured as previously described for SS-OCT. All the leakage spots were summed per sample. The length of the leakage spots relative to the whole cavity floor was calculated as percentage.
Lower window shows signal intensity plot for rectangular selection
The statistical analysis was conducted by PASW statistics 18 software (SPSS for windows: SPSS Inc., Chicago, IL, USA). Groups were compared using one-way ANOVA with Duncan’s analysis at the 95% significance level. The SS-OCT %DSs before thermocycling was compared with the ones after thermocycling. The %DSs of SS-OCT and micro-CT were also compared with a paired t-test and the relationship was analyzed using a Pearson correlation test at the 95% significance level.