Abstract
Objectives
The purposes of this study were: (1) to determine the light transmission characteristics (straight-line and diffusion transmission) of bulk-filled and layered resin composites, and (2) to evaluate the effect of layering filling on translucency and color appearance of resin composites.
Methods
Three light-cured resin composites (EsteliteΣ, Tokuyama Dental; Beautifil II, Shofu and Clearfil Majesty, Kuraray Medical) in A2 and OA2 shades were used in this study. 2 mm-thick resin composite discs were prepared by two methods; bulk filling technique (2 mm filling at one time), layering technique (two layers each 1 mm thickness). The transmitted light intensity of each resin composite disc was measured using a goniophotometer, and diffusion factor (DF), an indicator for diffuse transmission property, and peak gain (G0) for straight-line transmission were calculated from the light distribution graphs. Color was measured according to the CIELAB color scale on a spectrophotometer, and the translucency parameter (TP) and color difference (Δ E * ) between bulk-filled and layered specimens were calculated. The data were statistically analyzed using three-way ANOVA, and Dunnett’s T3 and t -test for post-hoc test.
Results
Three-way ANOVA revealed that the layered specimens had significantly lower G0 values and higher DF values than the bulk-filled specimens, and significantly reduced TP values. The color differences (Δ E * ) ranged from 1.07 to 1.85 between the bulk-filled and layered specimens.
Significance
Resin composite placed in layers exhibited reduced straight-line light transmission and increased diffusion transmission compared with bulk-filled resin composite. The layering technique affected the translucency and color rendition of resin composite.
1
Introduction
With the increasing requirement for good esthetics and a minimal intervention approach in anterior and posterior restorations, direct resin composite restorations enjoy great popularity due to their superior adhesion to tooth substrates, excellent esthetics, acceptable longevity and relatively low cost. The color and optical properties of resin composite are determined by many factors such as resin matrix composition, filler composition and content, pigment and other additives . When light illuminates resin composite, it scatters at the surface of the filler particles and diffuses in multiple directions. Light then emerges as diffuse transmission with wide spread and as straight-line transmission with no change of direction. Transmitted light through resin composite scatters and reflects on the cavity walls and a quantity of light is transmitted again through the resin composite. The straight-line transmission characteristics would be dependent upon the effect of color reflection from background environment, while the diffuse transmission characteristics would be dependent upon the effect of color reflection from surrounding environment. Additionally, the diffuse transmission characteristics would affect masking of background color . Azzopardi et al. calculated direct transmittance and diffuse transmittance values of resin composites to evaluate the effect of the resin matrix composition on their translucency . In many other studies, the translucency of resin composites has been evaluated using the translucency parameter (TP) , which refers to the color difference between a uniform thickness of material over a black and a white background. It has been demonstrated that the TP value corresponds directly to the common visual assessments of translucency , and is regarded as an indicator for masking ability . Recently, Arimoto et al. calculated and evaluated both TP value and the light transmission properties (straight-line and diffuse transmission) of resin composites from the distribution graph of transmitted light intensity measured using a goniophotometer, and found that TP value has a significant correlation with the diffuse light transmission of resin composite, however this was not the case with straight-line transmission. The two-dimensional goniophotometer provides to analyze intensity distribution of transmitted light by illuminating the material, which also can guide optical information of the material owed itself. Thus, both the diffuse and direct transmission can be measured by goniophotometer. Clinically, for successful color matching of resin composite with the adjacent tooth structure, it is necessary to make use of the background and surrounding colors, with masking of the background color. Therefore, it is important to know the light transmission characteristics and also the masking ability of resin composite for predicting the final color appearance of the resin composite restoration.
Layering techniques are often used in the clinic in order to perform proper color matching of the resin composite restoration, and to overcome the limited curing depth of light-cured resin composite and to minimize the effect of the contraction stress on adhesion to the cavity walls. Some studies have described the effect of constituent layers on the resultant color of layered resin composites, achieving optical contact using an optical fluid (refractive index = 1.5) between two cured composite layers . Optical fluid between the specimens can guard against errors in perceived colors that might be caused by a variable air interface and eliminate an optical effect at join of the specimens. However, when resin composites are incrementally built-up in clinic, a composite–composite join is formed, in which the resin matrix of the overlying resin composite co-polymerizes with the oxygen-inhibited uncured layer on the surface of the cured underlying resin composite . This join might be affected optical properties of layered resin composite. Some studies have investigated the bond strength of the composite–composite join formed during incremental filling . However, there have been few studies on the effect of this join on the optical properties of layered resin composite. Therefore, the purpose of this study was to determine the light transmission characteristics (straight-line and diffusion transmissions) of resin composites when placed using either a layering or a bulk filling technique with a goniophotometer and also to investigate their translucency (translucency parameter) and color appearances, and to evaluate the effect of the composite–composite join on the optical properties. The null hypotheses tested were that the composite–composite join formed by layering technique does not affect the optical properties of resin composite.
2
Materials and methods
2.1
Specimen preparation
Three light-cured resin composites (EsteliteΣ, Tokuyama Dental Corp., Tokyo, Japan; Beautifil II, Shofu Dental Corp., Kyoto, Japan and Clearfil Majesty, Kuraray Medical Inc., Tokyo, Japan) in universal shade A2 and opaque shade OA2 were used in this study ( Table 1 ). Five discs, 2.0 mm thick, of each resin composite material were prepared using bulk and layering filling techniques.
| Materials | Composition | Manufacturer | Batch number |
|---|---|---|---|
| Estelite Σ | Filler: 82 wt% (71 vol%) silica–zirconia spherical of 0.1–0.3 μm (average 0.2 μm) | Tokuyama Dental Corp., Tokyo, Japan | 006067 |
| Base resin: Bis-GMA, TEGDMA | |||
| Beautifil II | Filler: 83.3 wt% (68.6 vol%) Multi-functional glass and S-PRG filler based on fluoroboraluminosilicate glass particle size range: 0.01–4.0 μm (mean 0.8 μm) Base resin: Bis-GMA, TEGDMA | Shofu Dental Corp., Kyoto, Japan | 040718 |
| Clearfil Majesty | Filler: 78 wt% (66 vol%) Silanated barium glass filler, pre-polymerized organic filler | Kuraray Medical, Tokyo, Japan | 0028AA |
| Micro filler (glass filler): mean 1.5 μm | |||
| Nano filler: mean 20 nm | |||
| Base resin: Bis-GMA, TEGDMA, hydrophobic aromatic dimethacrylate |
2.1.1
Bulk filled specimens
Resin composite discs (6 mm in diameter) were made and covered with celluloid strips on glass plates, which were separated by spacers, 2.0 mm thick. After curing with a light-curing unit (Optilux 500, Demetron, Danbury, CT, USA, 600 mW/cm 2 ) for 60 s each from the top and bottom sides, the strips and glass plates were removed.
2.1.2
Layered specimens
Underlying discs of resin composite were made and covered with celluloid strips on glass plates, which were separated by spacers of 1.0 mm thick. After light irradiation for 30 s each from the top and bottom sides, the top side glass plate with the strip was removed and the spacers were replaced with 2.0 mm thick spacers. Then the overlying resin composite was placed and light-cured for 60 s from the top side with the glass plate and strip.
All the specimens were stored at 37 °C in 100% relative humidity for 24 h before measurement.
2.2
Measurements of light transmission characteristics by goniophotometer
For each specimen, the two-dimensional distribution of transmitted light intensity (incidence angle: 0°, measurement range: −90° to +90°) was measured using a goniophotometer (Model GP-200, Murakami Color Research laboratory, Tokyo, Japan) ( Fig. 1 ) under regulated conditions (sensitivity: 950; volume: 460). The lights are converged onto the pin hole through the condenser lens, and converted into parallel beams through the collimator lens. These lights reach the specimen plane through the beam iris. The lights reflected or transmitted from the specimen plane are fed to receptor via a telescope lens and receiving iris. The reflectance, transmittance and diffuse reflectance of specimen can be obtained by measuring the light intensity. Using the distribution graph, the straight-line light transmittance was calculated from the peak gain at 0° angle (G0), and the transmitted light diffusion property was calculated as the diffusion factor (DF)using the following calculation formula:
Diffusion factor ( % ) = { ( B 70 ° + B 20 ° ) / 2 } B 5 ° × 100
2.3
Measurements of color parameter by spectrophotometer
The reflectance color of resin composite over white and black backgrounds was measured using a dental spectrophotometer (Crystaleye, Olympus, Tokyo, Japan). The spectrophotometer uses 7 LEDs (light emitting diode) as an illumination source with 45°/0°-geometry . The resin composite discs prepared using the different filling techniques, were placed in a mold with a round-shape hole (2 mm in depth and 6 mm in diameter), which was set on a jaw model in the check box shielded from external light. After calibration and positioning with a contact cap placed on the head, the spectrophotometer captured the specimen image of resin composite for a duration time of 0.2 s, from which the spectral data of the resin composite specimen was acquired. The reflectance values, from 400 to700 nm, with 1-nm intervals for each pixel, were transferred from the spectrophotometer to a computer (Endavor NJ 2000; EPSON, Nagano, Japan), and the color was analyzed using the CIELAB (Commission Internationale del’ Eclarirage) color coordinates L * , a * and b * .
2.3.1
Calculation of the translucency parameter (TP)
The translucency parameter (TP) was obtained by calculating the color difference of the specimen over a black and white backing. The formula is:
TP = [ ( L B − L W ) 2 + ( a B − a W ) 2 + ( b B − b W ) 2 ] 1 / 2
subscript B refers to the color coordinates over a black background and the subscript W refers to those over a white background.
2.3.2
Calculation of color difference (Δ E * )
Color difference (Δ E * ) between bulk and layering specimen was calculated from the CIEL * , a * and b * values of the resin composites measured over the white background.
Δ E = [ ( L Layer − L Bulk ) 2 + ( a Layer − a Bulk ) 2 + ( b Layer − b Bulk ) 2 ] 1 / 2
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