2.1

Materials and sample preparation

Thirty extracted bovine incisors were selected for this study. Box formed cavities with round internal line angles 4 mm in diameter and 2 mm in depth were prepared on the flattened labial surfaces using high speed hand piece and round diamond bur (Shofu, Kyoto, Japan). The specimens were randomly distributed among six groups based on the flowable composites used for filling ( n = 5). After applying a dental adhesive (Clearfil SE Bond; Kuraray Medical, Japan) according to the manufacturer’s instructions, each composite resin was injected into the cavity. The composite materials used and their compositions are listed in Table 1 .

2.2

SS-OCT system and tomographic procedure

The SS-OCT system used in the present study (IVS-2000, Santec, Komaki, Japan) was a frequency domain OCT with 100 nm bandwidth, centered at 1310 nm spectra at 20 kHz scan rate. The sensitivity of the current OCT system is 106 dB, whilst the shot-noise limited sensitivity is 119 dB. The system incorporated a hand-held probe with a power less than 20 mW. The axial resolution of this system is 11 μm in air, equivalent to approximately 7 μm in oral hard tissue structures and resin composites (assuming a refractive index of around n = 1.5), and the lateral resolution is about 17 μm .

The laser light beam is projected from the probe onto the sample, cross-sectionally scanning the selected area. Backscattered light is collected and digitized in time scale and analyzed in the Fourier domain to reveal the depth-resolved reflectivity profile (A-scan) at each location. The combination of a series of A-scans along a scan path creates a B-scan. B-scans contain the signal intensity value (dB) of each point on the x and z axes. By converting the B-scan raw data into gray-scale, a 2D cross-sectional image can be created. This system is capable of 3D acquisition of data from the structure by serial B-scans over an area.

All the composites were carefully filled into the cavities at room temperature by the same operator following the same placement technique. During injection of the composite into the cavity, the nozzle tip was constantly held away from the dispensed composite. After placing of each composite material into the prepared cavity and irradiation with a halogen light curing unit (Optilux 501, Kerr, Orange, CA, USA) according to the instructions in Table 1 . The specimens were then cross-sectionally scanned with SS-OCT. 3D scans were performed over a volume of interest of 5 mm × 5 mm × 5 mm centered at the cavity obtained in the form of 300 serial sagittal B-scans at a resolution of 760 × 430 pixels ( x , z ).

2.3

Void evaluation

2.3.1

Void volume ( V _V )

Fig. 1 demonstrates the appearance of voids in an OCT tomogram. The dimensions of each void were calculated by measuring the distance between the peaks resulting from refraction of light at the top and bottom of each void on the OCT signals from the corresponding A-scan. The volume of each void was calculated, and V _V for each restoration was obtained as the sum of void volumes at each restoration in μm ³ ( Fig. 1 ).

(a) The appearance of a void in a composite restoration on an OCT tomogram. E, enamel; D, dentin; C, composite; S, surface. (b) The OCT A-scan corresponding to the OCT scan line in (a). Three notable peaks are present. The first peak correspond to the light interaction at the composite surface, the second peak (UB) corresponds to the light interaction while entering the void and third peak (LB) corresponds to the light interaction while leaving the void and entering the composite. The distance between UB and LB corresponds to the diameter of the void. (c) Schematic calculation method for V _V and V _F . In the equation, n is the number of voids, r _{1,2, …, n} is radius of each void. Note that there is a dark area with low signal intensity underneath the void, indicating signal loss due to multiple refractions at the void boundaries.

2.3.2

Void frequency ( V _F )

The total number of independent voids within each restoration was counted in the OCT 3D-scans obtained from each specimen.

2.3.3

Statistical analysis

V _V and V _F values were statistically analyzed using Kruskal–Wallis ANOVA followed by Mann–Whitney post-hoc test ( α = 0.05). The correlation between the void parameters of the materials was evaluated using the Pearson’s correlation test. All statistical analysis was performed using SPSS (ver. 16, SPSS, Chicago, IL, USA).

2.4

Micro-CT analysis

One of the specimens subjected to OCT scanning was selected and mounted on the computer-controlled turntable of a micro-CT system (InspeXio SMX-100CT; Shimadzu, Kyoto, Japan) so that the X-ray beam was almost perpendicular to the tooth axis and cavity walls. A 0.5-mm-thick aluminum metal sheet was used as a filter for beam-hardening correction . X-rays were generated at 90 kV with 100 μA current. Ten-frame averaging was also applied in the acquisition phase to improve the signal-to-noise ratio. The tooth was rotated 360° within an integration time of 400 s. Data were acquired as TIFF files for reconstruction of the 3D image of the coronal part with 1024 × 1024-pixel resolution and 12-μm isotropic voxel size. A 3D imaging software (TRI/3D-BON; Ratoc, Tokyo, Japan) was used to extract the composite and retain voids based on the CT values contrast between the composite structure and air entrapped in each void. The volume of each void as well as V _F and V _V parameters were then determined by the software volume calculation function. The micro-CT volume data was then compared to corresponding OCT results by paired t -test and Pearson’s correlation using SPSS ( α = 0.05).

2.5

SEM observation

One specimen from each group was randomly selected and cut across the restoration using a low-speed diamond saw (Isomet, Buehler, Lake Bluff, IL, USA) to obtain a 2 mm thick slice. The slices were polished using 2000 grit SiC paper (Sankyo Rikagaku, Saitama, Japan) followed by a series of diamond pastes with particle sizes from 6 to 1 μm. The slices were gold sputtered and filler particles in the resin matrix of each composite were observed under SEM (JSM 5310 LV, JEOL, Tokyo, Japan) at 2000× magnification.

2.6

Flowability

An equal amount (0.15 g) of each composite was dispensed on a micro-glass slide while the glass was placed on a digital scale. The glass was then kept up-right for 30 s, and the displacement of each material was qualitatively observed on photographs taken from each slice using a digital camera (D90, Nikon, Tokyo, Japan).

2.1

Materials and sample preparation

Thirty extracted bovine incisors were selected for this study. Box formed cavities with round internal line angles 4 mm in diameter and 2 mm in depth were prepared on the flattened labial surfaces using high speed hand piece and round diamond bur (Shofu, Kyoto, Japan). The specimens were randomly distributed among six groups based on the flowable composites used for filling ( n = 5). After applying a dental adhesive (Clearfil SE Bond; Kuraray Medical, Japan) according to the manufacturer’s instructions, each composite resin was injected into the cavity. The composite materials used and their compositions are listed in Table 1 .

Table 1

List of materials utilized in this study.

Material	Composition	Instruction for light curing by halogen unit
Clearfil Majesty LV: MJ (Kuraray Medical) LOT:00306B	TEGDMA, hydrophobic aromatic dimethacrylate photoinitiator Barium glass, colloidal silica 81 wt.%, 62 vol.%	Up to 2 mm bulk-fill Light cure 20 s (>300 mW/mm 2 )
MI FIL: ML (GC) LOT:1009223	UDMA, Bis-MEPP, TEGDMA photoinitiator Silicon dioxide, strontium glass 69 wt.%, 50 vol.%	Up to 2 mm bulk-fill Light cure 20 s (>700 mW/mm 2 )
MI FLOW: MW (GC) LOT:1104111	UDMA, Bis-MEPP, TEGDMA photoinitiator Silicon dioxide, Strontium glass 65 wt.%, 50 vol.%	Up to 2 mm bulk-fill Light cure 20 s (>700 mW/mm 2 )
BEAUTIFIL Flow Plus F00: BF (Shofu) LOT: 061012	Bis-GMA, TEGDMA photoinitiator Fluoroboroaluminosilicate S-PRG 67.3 wt.%, 47 vol.%	Up to 2 mm bulk-fill Light cure 20 s (intensity not specified)
PALFIQUE Estelite Low Flow: EL (Tokuyama) LOT: 670	Bis-GMA, TEGDMA, Bis-MPEPP photoinitiator Silica–zirconia 71 wt.%, 53 vol.%	Up to 2 mm bulk-fill Light cure 30 s (intensity not specified)
Surefil SDR flow: SF (DENTSPLY) LOT:1103111	EBPADMA, TEGDMA, modified UDMA photoinitiator, BHT UV stabilize, TiO 2 , FeO pigment Ba–Al–F–Si glass, Sr–Al–F–Si glass 68 wt.%, 44 vol.%	Up to 4 mm bulk-fill Light cure 20 s (>550 mW/mm 2 )

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