Swept-source optical coherence tomography (SS-OCT) can construct cross-sectional images of internal biological structures. The aim of this study was to evaluate enamel cracks at the cavosurface margin of composite restorations using SS-OCT.
Bowl-shaped cavities were prepared at two locations (mid-coronal and cervical regions) on the enamel surface of 60 bovine teeth. Half of the cavities (30) were treated with phosphoric acid gel. A two-step self-etch adhesive (Clearfil SE Bond) was applied to all cavities and a flowable composite was placed in bulk. After 7 days in water at 37 °C, three-dimensional (3D) images of the specimens were obtained using SS-OCT, and cross-sectional views of the cavosurface margin were examined. Presence and extent of enamel cracks along the cavosurface margin circumference were evaluated using a 5-point scale. The results were statistically compared with Wilcoxon rank sum test with Bonferroni correction.
3D SS-OCT could detect enamel cracks at the cavosurface margin of composite restorations. Cervical regions caused more enamel cracking than mid-coronal regions. Phosphoric acid etching increased the incidence of enamel cracks compared with the preparations without etching.
SS-OCT can be used to detect enamel cracks at the margins of composite restorations noninvasively. Presence and extent of enamel cracks depended on the enamel region and bonding protocol.
SS-OCT can be used to detect enamel cracks at the margins of composite restorations noninvasively. Selective phosphoric acid etching of the enamel significantly increased the incidence of marginal cracks, especially in cervical preparation.
Advancements in adhesive technology have enabled less invasive dental treatment in an invisible repair manner and the indications of direct composite resins have expanded to cases with extensive tooth structure loss, including the proximal contact or cervical areas. However, the resin-based restorative materials shrink during the polymerization process, thereby creating a gap at the resin-tooth interface or a crack in the enamel at the cavosurface margin .
Enamel cracks at the margins of composite restorations were first reported by Fusayama who suggested that this phenomenon was due to polymerization contraction stress. Clinically, the margin with enamel crack appears whitish after adjustment of the cavosurface margin of composite restoration. A micro-crack in the cavosurface enamel can result in post-operative sensitivity and formation of secondary caries .
The magnitude of polymerization contraction stress depends on many factors, including the geometry of the prepared cavity, compositional and curing characteristics of the composite material, and photo-curing strategy. Current evidence suggests that enamel cracks occur at 43%–92% of the circumference of the restoration . Christensen et al. stated that the resin formulation was significantly associated with shrinkage and formation of white lines rather than the design and intensity of the curing light.
Enamel mainly consists of highly mineralized prisms with microstructural anisotropy, while cervical enamel contains randomly oriented hydroxyapatite crystals and atypical enamel prisms . Although this morphological difference in enamel location could potentially affect the occurrence of cracking, limited information is available in this regard in the literature.
Optical coherence tomography (OCT) is an emerging cross-sectional imaging method for observation of internal biological structures. OCT helps in visualizing differences in the optical properties of tissues, which includes the effects of both optical absorption and scattering . The swept-source (SS) type of OCT (SS-OCT) is a highly sensitive and improved variation of the fast Fourier transformation algorithm . In this article, SS-OCT was investigated as a diagnostic tool for enamel cracks at the margins of resin composite restorations. Influence of cavity locations prepared with the tooth structure and effect of selective phosphoric acid etching were evaluated using SS-OCT.
Materials and methods
The SS-OCT system (Yoshida Dental MFG, Tokyo, Japan) used in this experiment is a frequency domain OCT technique that measures the magnitude and time delay of reflected light in order to construct a depth profile. The light source in this system sweeps the nearinfrared spectrum from 1240 nm to 1380 nm, centered at 1310 nm at a 50 kHz rate. Three dimensional (3D) data set is obtained at optical resolution in air of 11 μm in depth and 40 μm in width and length ( Table 1 ).
|Scan axis||Effective imaging depth (in air)||Point||Resolution (in air)|
|Depth (A)||8 mm||1024||11 μm|
|Lateral (B)||10 mm||400||40 μm|
|Axial (C)||10 mm||400||40 μm|
Cavity preparation and assessment of enamel crack using SS-OCT
The materials used in this study and their compositions are listed in Table 2 . Freshly extracted bovine incisors were used in this study. A cavity was prepared on one of the two enamel locations on each specimen; mid-coronal or cervical regions. The round bowl-shaped cavities were 3 mm in diameter and 2 mm in depth, prepared using a carborundum point (#SF22, Shofu, Kyoto, Japan) and super-fine diamond bur (#SF440, Shofu) attached to a rotary grinding instrument under copious water. After the preparation, all the cavities were subjected to SS-OCT observation to check the marginal integrity. Cavities without cavosurface enamel crack forming due to the bur cutting were chosen and employed; as a result 60 cavities (30 in each enamel location) were used in this study.
|K-etchant Gel (Kuraray Noritake Dental Tokyo, Japan)||Phosphoric acid, Colloidal silica, Water, Dyes|
|Clearfil SE Bond (Kuraray Noritake Dental Tokyo, Japan)||Primer: MDP, HEMA,Hydrophilic dimethacrylate,Photoinitiator, Water|
|Adhesive: MDP, Bis-GMA, HEMA, Hydrophobic dimethacrylate, Camphorquinone,Silanated colloidal silica|
|Estelite Flow Quick
(Tokuyama Dental,Tainai, Japan)
|Bis-MPEPP, TEGDMA, UDMA, Silica-zirconia filler, Silica-titania fillers, CQ|
The cavosurface enamel margin of cavities in 15 randomly selected specimens in each enamel location was chosen and treated with 40% phosphoric acid (K-etchant Gel; Kuraray Noritake Dental, Inc.,Tokyo, Japan) for 10 s, while the other 15 specimens were left untreated. All the 60 cavities were then treated with a self-etching primer and bonding system (Clearfil SE Bond, Kuraray Noritake Dental, Tokyo, Japan) according to the manufacturer’s instructions, followed by bulk filling with flowable resin composite (Estelite Flow Quick, Shade A2, Tokuyama Dental,Tainai,Japan), photocured for 20 s using a halogen light curing unit (Optilux 501, Kerr Corporation, Orange, CA, USA) with a power density of 550–600 mW/cm 2 .
The restored surface was then polished using a microengine hand piece with water spray and a silicone point to remove the extruded adhesive and resin from the cavity. In this manner, specimens were prepared in four groups (n = 15) with respect to enamel location (mid-coronal or cervical) and phosphoric acid etching (with or without).
After storage in water at 37 °C for 7 days, a 3D data set of the specimen was obtained using the SS-OCT system ( Fig. 1 ) . The 3D data set included horizontal cross-sectional views of the cavosurface margins up to depth of 1 mm. The extent of enamel cracks along the cavosurface margin circumference was evaluated and scoring was performed using the following 5-point rank scale ( Fig. 2 ):
0 = no enamel crack,
1 = an enamel crack of less than 1/4 of the cavosurface margin circumference,
2 = an enamel crack of ≥ 1/4 and <1/2 of the cavosurface margin circumference,
3 = an enamel crack of ≥ 1/2 and <3/4 of the cavosurface margin circumference, and
4 = an enamel crack of ≥ 3/4 of the cavosurface margin circumference,
The number of cavities for each score was recorded and statistically analysed with a critical value α = 0.05.
Cross-sectioning and laser scanning confocal microscope observation
Five representative cavities with enamel cracks as observed by SS-OCT were selected from each experimental group for confirmatory observations using a laser scanning confocal microscope (3D Laser Scanning Confocal Microscope; Keyence, Osaka, Japan). The specimens were ground and polished using a high-speed rotating device (Automatic Lapping Machine ML-160A, MARUTO INSTRUMENT CO., LTD.) equipped with a diamond point and silicon carbide paper under copious water flow to expose the sagittal section of the cavity. The surface was further polished with diamond paste down to 3 μm under running water. Then, the specimens were viewed under the laser scanning confocal microscope at an optical magnification of × 10.
The results were statistically analysed by using statistical software package (SPSS). Cross-sectional images near the cavity margin were extracted from 3D stereoscopic images constructed from restorations using SS-OCT. Since the distribution of data was not normal, non-parametric tests were performed. Wilcoxon rank sum test with Bonferroni correction was used to determine whether there was any difference between the location and application of enamel etching (α = 0.05).