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Influence of filler distribution on the color parameters of experimental resin composites
d e n t a l m a t e r i a l s 2 4 ( 2 0 0 8 ) 67–73
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Influence of filler distribution on the color parameters of experimental resin composites Yong-Kyu Lim a , Yong-Keun Lee b,∗ , Bum-Soon Lim b , Sang-Hoon Rhee b , Hyeong-Cheol Yang b a
Department of Orthodontics, Graduate School of Clinical Dentistry, Korea University, Seoul, Republic of Korea Department of Dental Biomaterials Science and Dental Research Institute, School of Dentistry, Seoul National University, 28 Yeongeon-dong, Jongro-gu, Seoul, Republic of Korea b
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objectives. Simulating the optical properties of natural tooth would be the final goal for
Received 29 May 2006
esthetic restorative materials. Filler distribution in resin composites determines the scat-
Accepted 15 February 2007
tering in composite materials, which in turn would influence the color parameters such as lightness, chroma and hue. The objective of this study was to determine the influence of filler size and amount on the color parameters of experimental resin composites.
Keywords:
Method. Color of 11 experimental resin composites with two different sized fillers (LG:
Resin composite
0.77 m and SG: 0.50 m) in 10–70 wt% was measured by a spectrophotometer. Color coordi-
Filler distribution
nates (CIE L*, a* and b*), chroma and hue angle were determined. Optical constants including
Scattering coefficient
scattering coefficient (S), absorption coefficient (K) and light reflectivity (RI) were calculated.
Color
To determine the influence of the amount of filler on the optical parameters, Pearson cor-
Lightness
relations between the amount of filler (%) and color parameters and optical constants were calculated. Correlations between the optical constants (S, K and RI) and color parameters were calculated (p < 0.05). Results. S value increased as the amount of filler increased. RI value generally increased as the amount of filler increased for LG filler group, and increased up to 40% filler for SG filler group. CIE L* value increased as the amount of filler increased in both of LG and SG filler groups. CIE L* value was highly correlated with S and RI values for both filler groups (r = 0.961–0.974). Conclusion. Lightness was highly correlated with the amount of filler, S and RI values (r = 0.932–0.974). But the correlation coefficients between the amount of filler and chroma/hue were moderate (r = 0.406–0.827); therefore, pigmentation would be tried to simulate the color of resin composites to those of natural tooth. Optical properties of resin composites could be partly simulated to those of teeth by controlling the filler distribution. © 2007 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Knowledge of the optical properties of tooth and an understanding of the origin of these properties are necessary for
∗
the development of biomimetic esthetic restorative materials. Final goals for the optical properties of esthetic restorative materials should be the imitation of those of teeth. Tooth color is determined by the paths of light inside the tooth and absorp-
Corresponding author. Tel.: +82 2 740 8693; fax: +82 2 740 8694. E-mail address: [email protected] (Y.-K. Lee). 0109-5641/$ – see front matter © 2007 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.dental.2007.02.007
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tion along these paths, and the light paths inside the tooth are determined by scattering [1]. Kubelka-Munk’s theoretical diffuse reflectance spectra of enamel and dentin were in good agreement with the observed reflectance in the wavelength range of 400–700 nm [2]. Enamel and dentin are able to collect and distribute light within the tooth, with both enamel prisms and dentinal tubules acting as optical fibers [3]. Dentinal tubules are the predominant cause of scattering in dentin, and hydroxyapatite crystals contribute significantly to the scattering of enamel [4]. Enamel prisms are the most important scatterers but hydroxyapatite crystals are responsible for the back-scattering of enamel [5]. Lightness (CIE L* value) of tooth enamel was significantly correlated with the enamel scattering coefficient [1]. When light passes through a translucent substance such as tooth or esthetic material, it undergoes absorption as well as scattering. Light is scattered at inclusions in a material, and absorption attenuates the light beam [6]. Four phenomena associated with the interaction of a translucent substance with light flux can be described: (1) specular transmission, (2) specular reflection at the surface, (3) diffuse reflection at the surface, and (4) absorption and scattering within a substance [2]. It has been theorized that optical scattering is dependent upon the wavelength of illumination for the case where the size of scattering particle is approximately the wavelength of the illumination [7]. However, when the size of the scattering particle is much greater than the wavelength of the illumination, scattering is inversely proportional to the size of the particle and wavelength has no effect [7]. Optical scattering of esthetic materials has been widely studied because of its significant effects on the color and translucency of these materials [8]. Color and translucency of resin composites are characterized by two wavelengthdependent parameters such as scattering coefficient (S) and absorption coefficient (K) [6]. S, K and light reflectivity (RI) values for resin composites were determined using Kubelka’s equations [9]. RI value indicates the light reflectance of a material of infinite thickness [9], which might indicate the true color of translucent material regardless of the background. At high filler concentration, S value of resin composites could be expected to reach a plateau value [10]. Difference in light transmittance characteristics among esthetic materials and shades will affect their clinical appearance [11]. Influence of optical properties of constituent layers on the color of layered esthetic filling materials was determined, and concluded that CIE L* after layering had a positive correlation with S of the covering layer [12]. CIE L*, a* and b* values of commercial resin composites were correlated with S, K and RI values. Therefore, the size and amount of fillers of resin composites should be controlled for the best tooth color reproduction, considering the refractive indices of filler and resin matrix [13]. Changes in scattering and absorption
properties, after aging, for esthetic restorative materials were closely correlated with changes in color, especially in glass ionomer-based filling materials [14]. Color of resin composites is correlated with S, K, RI, translucency, opalescence and fluorescence. Although there are varied studies on the color of commercial resin composites and several studies on the influence of the optical constants (S, K and RI) on the color of resin composites [6,9,13,14], there have been few studies on the correlation between filler distribution and color for resin composites. Systematic control of filler size and amount would provide basic ideas for the development of biomimetic esthetic restorative materials, which simulate the optical properties of teeth. The null hypothesis of the present study was that the color parameters such as lightness, chroma and hue of resin composites were not correlated with the size and amount of fillers added, and also not with S, K and RI values. The objective of this study was to determine the influence of filler size and amount on the color parameters of experimental resin composites. Two different sized fillers of the same composition were added into the same resin matrix in the ratio of 10–70%, or 10–50% by weight.
2.
Materials and methods
Resin matrix composed of 1:1:1 mixture of BisGMA, UDMA and TEGDMA by weight was used. BisGMA (Batch number: 419-19-04, ESSTECH, Essington, PA, USA), UDMA (Batch number: PB-2451, ESSTECH) and TEGDMA (Batch number: 488-50-06, ESSTECH) were used. For light curing, 0.7% of camphoroquinone (Batch number: JI00703MN, Aldrich, St. Louis, MO, USA), 0.05% of butylated hydroxytoluene (Batch number: 81K0200, Sigma, St. Louis, MO, USA) and 0.35% of (dimethylamino)ethyl methacrylate (Batch number: 00628MU, Aldrich) were added based on weight. Two different sized silanized fillers were added (Table 1). 8235 glass contains 1–10% B2 O3 , 1–10% Al2 O3 , >50% SiO2 , 10–50% BaO, and 1–10% C10 H20 O5 Si by weight. Translucency of LG filler is 40.6% and that of SG filler is 33.5%. Compositions of experimental resin composites are listed in Table 2. Unfilled resin matrix was used as a control. After adding the predetermined fillers into the resin matrix on a hot plate (50 ◦ C), composite material was filled into a metal mold (38 mm in diameter and 1 mm in thickness). The top surface of a specimen was covered with a Mylar strip and made flat by pressing with a glass plate, and the bottom surface consisted of a glass plate covered with a Mylar strip. Light curing was performed with a light curing unit (Type 43, EXAKT Apparatebau, Norderstedt, Germany) by irradiating for 20 min from both sides. Then, the films were removed. Three specimens were made for each material.
Table 1 – Fillers added to experimental resin composites Code LG SG
Name
Size
8235 glass filler 8235 glass filler
Median grain size, d50: 0.77 m, d99: 2.25 m Median grain size, d50: 0.50 m
Batch number SIL 1029 Lab 14763
Manufacturer SCHOTT Glass, Landshut, Germany SCHOTT
d e n t a l m a t e r i a l s 2 4 ( 2 0 0 8 ) 67–73
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Table 2 – Composition of experimental resin composites Code
Filler
Control LG-10 LG-20 LG-30 LG-40 LG-50 LG-70 SG-10 SG-20 SG-30 SG-40 SG-50
Resin matrix without filler 0.77 m glass filler 0.77 m glass filler 0.77 m glass filler 0.77 m glass filler 0.77 m glass filler 0.77 m glass filler 0.50 m glass filler 0.50 m glass filler 0.50 m glass filler 0.50 m glass filler 0.50 m glass filler
Filler content (wt%) 0 10 20 30 40 50 70 10 20 30 40 50
Specimens were stored at 37 ◦ C in distilled water for 7 days before baseline color measurement. After blot drying, color was measured according to the CIELAB color scale [15] in the reflectance mode over a zero calibrating box (CIE L* = 0.0, a* = 0.0, and b* = 0.0), a white background (CIE L* = 94.3, a* = −0.4, and b* = 1.4) and a black background (CIE L* = 0.2, a* = 0.4, and b* = −0.6) using a spectrophotometer (Color-Eye 7000A, GretagMacbeth Instruments Corp., New Windsor, NY, USA). The aperture size was a 10 mm × 7.5 mm oval shape. Illuminating and viewing configurations complied with CIE diffuse/8◦ geometry [15]. Measurements were repeated three times for each specimen. Color coordinates, chroma and hue angle of resin composites were calculated. Chroma was calculated as C∗ab = 1/2
(a∗2 + b∗2 ) , and hue angle was calculated as h◦ = arctan(b*/a*) [15]. Optical constants including scattering coefficient (S), absorption coefficient (K) and light reflectivity (RI) were calculated algebraically from the spectral reflectance data of each material using Kubelka’s equations. Details were described in previous studies [9,13]. In practical formulas, S always occurs together with the thickness X of the specimen, that is, as the product SX, called ‘scattering power’ of the specimen. The absorption coefficient (K) is defined as K = S(a − 1) (mm−1 ) [9]. The light reflectivity (RI) is defined as RI = a − b. To determine the influence of the amount of filler on the optical parameters, Pearson’s correlation coefficient (r) between the amount of filler (%) and color parameters was calculated with linear regression analysis within each filler group (p < 0.05). To determine the influence of optical constants on the optical parameters, Pearson’s correlation coefficient (r) between the optical constants (S, K and RI) and color parameters was calculated with linear regression analysis within each filler group (p < 0.05).
3.
Fig. 1 – Scattering coefficient by the wavelength for LG filler based resin composites.
Absorption coefficients (K) are listed in Table 3. K value generally increased up to 30% filler for the LG filler group, after that decreased, except at 400 nm. K value generally increased up to 30% filler for the SG filler group, after that decreased, except at 400 nm. Light reflectivity (RI) is listed in Table 4. RI value generally increased as the amount of filler increased for the LG filler group, and generally increased up to 40% filler for the SG filler group. Mean S, mean K and mean RI values in the wavelength range of 400–750 nm for the LG filler group are presented in Fig. 3 and in Fig. 4 for the SG filler group. As the amount of filler increased, S and RI values generally increased for both groups. But RI reached a plateau value at 40% LG filler, and S and RI decreased at 50% SG filler. Lightness, chroma and hue angle of LG filler group are presented in Fig. 5 and in Fig. 6 for the SG filler group. Trends in lightness were similar for both fillers. Pearson’s correlation coefficients (r) between the amount of filler (%) and the color parameters are listed in Table 5. CIE L* value increased as the amount of filler increased for both of LG
Results
Scattering coefficient (S) by wavelength for the LG filler group is presented in Fig. 1. S values changed slightly by wavelength, and obviously by the amount of filler, and showed general trends by wavelength. S value by wavelength for SG filler group is presented in Fig. 2. Trends by wavelength were similar to those of the LG filler group. In both filler groups, S value increased as the amount of filler increased.
Fig. 2 – Scattering coefficient by the wavelength for SG filler based resin composites.
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Table 3 – Absorption coefficient (K) of resin composites (mm−1 ) Code Control LG-10 LG-20 LG-30 LG-40 LG-50 LG-70 SG-10 SG-20 SG-30 SG-40 SG-50 a
400–750 nm a
0.10 (0.01) 0.21 (0.01) 0.22 (0.04) 0.21 (0.03) 0.19 (0.03) 0.19 (0.04) 0.19 (0.05) 0.19 (0.01) 0.18 (0.01) 0.21 (0.02) 0.21 (0.04) 0.22 (0.03)
400 nm
500 nm
600 nm
0.12 (0.04) 0.23 (0.03) 0.28 (0.03) 0.28 (0.01) 0.28 (0.01) 0.28 (0.01) 0.35 (0.03) 0.20 (0.01) 0.20 (0.07) 0.28 (0.03) 0.32 (0.01) 0.30 (0.01)
0.11 (0.01) 0.21 (0.02) 0.22 (0.01) 0.21 (0.02) 0.18 (0.01) 0.18 (0.00) 0.19 (0.01) 0.20 (0.01) 0.18 (0.05) 0.21 (0.02) 0.20 (0.02) 0.22 (0.02)
0.09 (0.01) 0.20 (0.02) 0.20 (0.01) 0.19 (0.02) 0.17 (0.01) 0.16 (0.00) 0.16 (0.01) 0.19 (0.01) 0.17 (0.05) 0.21 (0.02) 0.19 (0.03) 0.21 (0.03)
700 nm 0.09 (0.01) 0.20 (0.02) 0.20 (0.01) 0.19 (0.03) 0.17 (0.02) 0.16 (0.00) 0.15 (0.00) 0.19 (0.01) 0.17 (0.05) 0.20 (0.02) 0.17 (0.03) 0.20 (0.02)
Standard deviations are in parentheses.
Fig. 3 – Mean scattering coefficient (S), absorption coefficient (K) and light reflectivity (RI) of experimental resin composites for LG filler based resin composites.
and SG groups (r = 0.932 and 0.839, respectively). CIE a* value decreased (moved towards the green direction) as the amount of filler increased in both LG and SG groups (r = −0.798 and −0.691, respectively). CIE b* value increased (moved towards the yellow direction) as the amount of filler increased to 50%
Fig. 4 – Mean scattering coefficient (S), absorption coefficient (K) and light reflectivity (RI) of experimental resin composites for SG filler based resin composites.
for LG and 40% for SG (r = 0.624 and 0.621, respectively). Trends in chroma generally followed those of CIE b* value. Hue angle of the LG filler group were in the fourth quadrant, which indicated the hue of these resin composites is red and blue in tone. The hue angles of the SG filler group were in the first (SG-10
Table 4 – Light reflectivity (RI) of resin composites Code Control LG-10 LG-20 LG-30 LG-40 LG-50 LG-70 SG-10 SG-20 SG-30 SG-40 SG-50 a
400–750 nm a
0.21 (0.01) 0.21 (0.01) 0.30 (0.03) 0.36 (0.02) 0.44 (0.03) 0.46 (0.04) 0.47 (0.04) 0.19 (0.01) 0.24 (0.01) 0.32 (0.01) 0.41 (0.03) 0.36 (0.02)
Standard deviations are in parentheses.
400 nm
500 nm
600 nm
700 nm
0.16 (0.05) 0.18 (0.02) 0.24 (0.01) 0.29 (0.01) 0.35 (0.03) 0.37 (0.01) 0.36 (0.01) 0.17 (0.04) 0.21 (0.02) 0.26 (0.02) 0.32 (0.04) 0.28 (0.06)
0.20 (0.06) 0.22 (0.03) 0.31 (0.02) 0.36 (0.03) 0.44 (0.04) 0.46 (0.01) 0.47 (0.01) 0.20 (0.04) 0.25 (0.03) 0.33 (0.02) 0.41 (0.06) 0.37 (0.08)
0.22 (0.06) 0.22 (0.03) 0.32 (0.03) 0.37 (0.03) 0.45 (0.05) 0.49 (0.01) 0.49 (0.01) 0.19 (0.04) 0.24 (0.03) 0.33 (0.02) 0.42 (0.07) 0.37 (0.09)
0.21 (0.07) 0.22 (0.03) 0.32 (0.03) 0.37 (0.03) 0.45 (0.05) 0.48 (0.01) 0.48 (0.01) 0.19 (0.04) 0.23 (0.04) 0.32 (0.02) 0.43 (0.08) 0.36 (0.09)
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Table 6 – Correlation coefficient (r) between the scattering coefficient (S), absorption coefficient (K) and light reflectivity (RI) and color parameters Filler
L*
a*
b*
C∗ab
h◦
LG
Sa K RI
0.961b −0.527 0.961
−0.725 0.231 −0.693
0.499 NS 0.522
0.546 NS 0.563
NSc NS NS
SG
S K RI
0.966 0.417 0.974
−0.379 −0.271 −0.360
0.862 0.258 0.823
0.582 NS 0.562
0.699 0.456 0.724
a
b
Fig. 5 – Lightness, chroma and hue angle of LG filler based resin composites.
Constant
c
S indicates scattering coefficient, K indicates absorption coefficient and RI indicates light reflectivity. Mean values between 400 and 750 nm was used. The correlation coefficient (r) was significant at the significance level of 0.05. NS indicates no significant correlation (p > 0.05).
was positively correlated with S and RI values for both filler groups.
4.
Discussion
The hypothesis of the present study was rejected because lightness, chroma and hue of experimental resin composites were significantly correlated with the amount of filler (r = 0.406–0.932, p < 0.05), and lightness was highly correlated with S and RI values (r = 0.961–0.974). Since the correlation coefficients between the amount of filler and chroma/hue of experimental resin composites were not so high and moderate (r = 0.406–0.827), pigmentation would be tried to simulate the color of resin composites to those of natural tooth. S value of human enamel was 0.59(±0.48) mm-1 when measured with 0◦ /0◦ geometry, and 0.60(±0.43) mm−1 with 45◦ /0◦ geometry, when monochromatic light of 560 nm was illuminated [1]. Measuring geometry determines the penetration and scattering depth of light into a substance; therefore, S value was influenced by the measuring geometry. In the present study, the measuring geometry was CIE diffuse/8◦ geometry, which should have influenced the measured value. In the present study, S value of resin matrix at 560 nm was 0.07 mm−1 , that of LG added resin composites was 0.16–0.63 mm−1 , and that of SG added resin composites was 0.12–0.40 mm−1 . Therefore, S values for LG-50 and LG-70 resin composites were similar to that of human enamel, although measuring geometry and the wavelength range of illumination were different. Color coordinates for natural teeth were also measured in the previous study of ten Bosch and Coops
Fig. 6 – Lightness, chroma and hue angle of SG filler based resin composites.Short title: Influence of filler on the color of resin composites.
and -20) and fourth quadrant (others). As the amount of filler increased, yellow hue moved to blue hue through the change of CIE b* value from positive to negative values. S and RI values were highly correlated with the amount of filler for both filler groups (Table 5). However, K value was positive or negative depending on the size of fillers. Pearson’s correlation coefficient (r) between the optical constants and the color parameters are listed in Table 6. CIE L* value was highly correlated with S and RI values for both filler groups (r = 0.961–0.974). CIE a* value was negatively correlated with S and RI values for both filler groups. CIE b* value
Table 5 – Correlation coefficient (r) between the amount of filler (%) and color parameters of resin composites Filler LG SG a b
L* b
0.932 0.839
a*
b*
C∗ab
h◦
Sa
K
RI
−0.798 −0.691
0.624 0.621
0.406 0.406
0.827 0.745
0.913 0.787
−0.620 0.415
0.899 0.793
S indicates scattering coefficient, K indicates absorption coefficient and RI indicates light reflectivity. The correlation coefficient (r) was significant at the significance level of 0.05.
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[1], but the measurement protocols were obviously different from that in the present study. Therefore, direct comparison was impossible. S values for the conventional shades of commercial resin composites were in the range of 0.51–0.66 mm−1 at 600 nm [13], which range of S value could be simulated in the present study. Based on the results of the present study, S value could be mimicked to that of natural enamel. However, deviations in S value by the tooth was very high because the standard deviations were around 80% of mean value referring the data of 0.59(0.48) and 0.60(0.43) mm−1 of the previous study [1]. This diversity of natural teeth might be a challenge when mimicking the optical properties of natural teeth. CIE L* value of natural tooth was correlated with S value (correlation coefficient: r = 0.60) [1]. In the present study, S value was correlated with lightness and chroma; but not with hue in the LG group (Table 6). Anyway, S value was highly positively correlated with CIE L* value (r = 0.961 and 0.966), which indicates that CIE L* value was determined by S value to a great extent. Higher scattering resulted in higher CIE L* value. S value was correlated with the amount of filler for both sizes of fillers (Table 5). The LG filler group showed higher correlation coefficient than that of the SG filler group, which reflected the difference in filler size. However, the influence of filler size on the S value depending on the wavelength was not obvious based on Figs. 1 and 2. The main factor of translucency in dental porcelains is due to multiple scattering of light at scattering centers such as opacifiers [16]. Opalescence is the result of light scattering of the shorter wavelengths of the visible spectrum, and this light scattering is caused by particles smaller than the wavelength of visible light that are dispersed throughout a translucent material of a much lower refractive index [17]. In the present study, only color parameters were analyzed; however, translucency and opalescence of resin composites should be considered when controlling the filler distribution of resin composites to mimic the natural tooth. RI value was highly correlated with S value for both filler groups (r = 0.985 and 0.983, respectively) based on the results of the present study. RI value is defined as the light reflectance of a material of infinite thickness [9]. Since the infinite optical thickness of resin composites was in the range of 2.97–5.90 mm [9], the specimen thickness of 1 mm used in the present study was very thin compared to the infinite optical thickness. However, RI value was highly correlated with lightness of resin composites. Background would influence the reflected color of resin composites in this condition. Further study on the influence of background on the reflected color should be performed. Scattering of body porcelain was found to decrease with increasing wavelength within the visible spectrum, in accordance with the scattering theory for particles not substantially less than the wavelength of the scattered light [18]. However, in the present study, above mentioned phenomena were not observed because the filler sizes were around those of visible light (median particle sizes: 770 and 500 nm, Figs. 1 and 2). Scattering is due to refraction and reflection at the interface between the resin matrix and inclusions such as filler
particles and porosity voids, and S value is greatest when the particle diameter equals the wavelength of the incident light [6]. Based on the study of Grajower et al. [6], S value around 750 nm should have been the highest in LG filler group because the median filler size was 0.77 m, and should also have been the highest around 500 nm in SG filler because the median filler size was 0.50 m. But these trends were not observed (Figs. 1 and 2). These results might be caused partly by the broad filler distribution of both fillers and by small the refractive index constant between resin matrix and filler. The refractive index of 1:1 mixture of BisGMA and TEGDMA was 1.5020 [19], and that of 8235 glass used in the present study was 1.55 based on the manufacturer information. Based on this data, the refractive index constant was 1.55/1.50 (=1.03) assuming that the refractive index of 1:1:1 mixture of BisGMA, UDMA and TEGDMA was similar to that of the BisGMA and TEGDMA mixture. S values for commercial resin composites were calculated [13]. Although the sizes for filler particle varied by the brand and shade of resin composite (5–3000 nm), S value decreased with increasing wavelength in all the resin composites investigated [13]. Compared with the results of the present study, the difference was obvious. Broader filler distributions including micro- or nano-sized filler in commercial resin composites might explain this discrepancy. For samples of polymethylmethacrylate with and without quartz fillers (3.3, 3.9, 9.2 and 15 m), optical scattering by fillers is shown to be linearly related to the amount of the filler [8]. Since the filler sizes were obviously different from those of the present study, direct comparison was impossible. However, as the filler concentration increased, S value also increased in the present study. The reflectance and transmission of thin slabs of tooth enamel was measured between 220 and 700 nm [20]. An absorption peak at 270 nm, which is outside the normal human visible light range (from 380 to 780 nm), was common in all the samples, and it was concluded that the organic component was responsible for most or all of the observed optical absorption. In the present study, the measurement range was from 360 to 750 nm, and K value was the highest around 360 nm. After that, K value decreased gradually. Although it is not clear whether experimental resin composites show peaks in the same wavelength of enamel (270 nm), this peak has little practical impact because the wavelength is outside visible range.
5.
Conclusions
Within the limitations of the present study, lightness was highly correlated with the amount of filler, S and RI values (r = 0.932–0.974). But the correlation coefficients between the amount of filler and chroma/hue of experimental resin composites were moderate (r = 0.406–0.827); therefore, pigment would be added to simulate the color of resin composites to that of natural tooth. Translucency and opalescence of resin composites should be considered when controlling the filler distribution of resin composites.
d e n t a l m a t e r i a l s 2 4 ( 2 0 0 8 ) 67–73
Acknowledgments This study was supported by grant (R01-2006-000-10421-0) from the Basic Research Program of the Korea Science & Engineering Foundation. Resin matrix was kindly provided by ESSTECH (Essington, PA, USA).
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Influence of filler distribution on the color parameters of experimental resin composites Yong-Kyu Lim a , Yong-Keun Lee b,∗ , Bum-Soon Lim b , Sang-Hoon Rhee b , Hyeong-Cheol Yang b a
Department of Orthodontics, Graduate School of Clinical Dentistry, Korea University, Seoul, Republic of Korea Department of Dental Biomaterials Science and Dental Research Institute, School of Dentistry, Seoul National University, 28 Yeongeon-dong, Jongro-gu, Seoul, Republic of Korea b
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objectives. Simulating the optical properties of natural tooth would be the final goal for
Received 29 May 2006
esthetic restorative materials. Filler distribution in resin composites determines the scat-
Accepted 15 February 2007
tering in composite materials, which in turn would influence the color parameters such as lightness, chroma and hue. The objective of this study was to determine the influence of filler size and amount on the color parameters of experimental resin composites.
Keywords:
Method. Color of 11 experimental resin composites with two different sized fillers (LG:
Resin composite
0.77 m and SG: 0.50 m) in 10–70 wt% was measured by a spectrophotometer. Color coordi-
Filler distribution
nates (CIE L*, a* and b*), chroma and hue angle were determined. Optical constants including
Scattering coefficient
scattering coefficient (S), absorption coefficient (K) and light reflectivity (RI) were calculated.
Color
To determine the influence of the amount of filler on the optical parameters, Pearson cor-
Lightness
relations between the amount of filler (%) and color parameters and optical constants were calculated. Correlations between the optical constants (S, K and RI) and color parameters were calculated (p < 0.05). Results. S value increased as the amount of filler increased. RI value generally increased as the amount of filler increased for LG filler group, and increased up to 40% filler for SG filler group. CIE L* value increased as the amount of filler increased in both of LG and SG filler groups. CIE L* value was highly correlated with S and RI values for both filler groups (r = 0.961–0.974). Conclusion. Lightness was highly correlated with the amount of filler, S and RI values (r = 0.932–0.974). But the correlation coefficients between the amount of filler and chroma/hue were moderate (r = 0.406–0.827); therefore, pigmentation would be tried to simulate the color of resin composites to those of natural tooth. Optical properties of resin composites could be partly simulated to those of teeth by controlling the filler distribution. © 2007 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Knowledge of the optical properties of tooth and an understanding of the origin of these properties are necessary for
∗
the development of biomimetic esthetic restorative materials. Final goals for the optical properties of esthetic restorative materials should be the imitation of those of teeth. Tooth color is determined by the paths of light inside the tooth and absorp-
Corresponding author. Tel.: +82 2 740 8693; fax: +82 2 740 8694. E-mail address: [email protected] (Y.-K. Lee). 0109-5641/$ – see front matter © 2007 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.dental.2007.02.007
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tion along these paths, and the light paths inside the tooth are determined by scattering [1]. Kubelka-Munk’s theoretical diffuse reflectance spectra of enamel and dentin were in good agreement with the observed reflectance in the wavelength range of 400–700 nm [2]. Enamel and dentin are able to collect and distribute light within the tooth, with both enamel prisms and dentinal tubules acting as optical fibers [3]. Dentinal tubules are the predominant cause of scattering in dentin, and hydroxyapatite crystals contribute significantly to the scattering of enamel [4]. Enamel prisms are the most important scatterers but hydroxyapatite crystals are responsible for the back-scattering of enamel [5]. Lightness (CIE L* value) of tooth enamel was significantly correlated with the enamel scattering coefficient [1]. When light passes through a translucent substance such as tooth or esthetic material, it undergoes absorption as well as scattering. Light is scattered at inclusions in a material, and absorption attenuates the light beam [6]. Four phenomena associated with the interaction of a translucent substance with light flux can be described: (1) specular transmission, (2) specular reflection at the surface, (3) diffuse reflection at the surface, and (4) absorption and scattering within a substance [2]. It has been theorized that optical scattering is dependent upon the wavelength of illumination for the case where the size of scattering particle is approximately the wavelength of the illumination [7]. However, when the size of the scattering particle is much greater than the wavelength of the illumination, scattering is inversely proportional to the size of the particle and wavelength has no effect [7]. Optical scattering of esthetic materials has been widely studied because of its significant effects on the color and translucency of these materials [8]. Color and translucency of resin composites are characterized by two wavelengthdependent parameters such as scattering coefficient (S) and absorption coefficient (K) [6]. S, K and light reflectivity (RI) values for resin composites were determined using Kubelka’s equations [9]. RI value indicates the light reflectance of a material of infinite thickness [9], which might indicate the true color of translucent material regardless of the background. At high filler concentration, S value of resin composites could be expected to reach a plateau value [10]. Difference in light transmittance characteristics among esthetic materials and shades will affect their clinical appearance [11]. Influence of optical properties of constituent layers on the color of layered esthetic filling materials was determined, and concluded that CIE L* after layering had a positive correlation with S of the covering layer [12]. CIE L*, a* and b* values of commercial resin composites were correlated with S, K and RI values. Therefore, the size and amount of fillers of resin composites should be controlled for the best tooth color reproduction, considering the refractive indices of filler and resin matrix [13]. Changes in scattering and absorption
properties, after aging, for esthetic restorative materials were closely correlated with changes in color, especially in glass ionomer-based filling materials [14]. Color of resin composites is correlated with S, K, RI, translucency, opalescence and fluorescence. Although there are varied studies on the color of commercial resin composites and several studies on the influence of the optical constants (S, K and RI) on the color of resin composites [6,9,13,14], there have been few studies on the correlation between filler distribution and color for resin composites. Systematic control of filler size and amount would provide basic ideas for the development of biomimetic esthetic restorative materials, which simulate the optical properties of teeth. The null hypothesis of the present study was that the color parameters such as lightness, chroma and hue of resin composites were not correlated with the size and amount of fillers added, and also not with S, K and RI values. The objective of this study was to determine the influence of filler size and amount on the color parameters of experimental resin composites. Two different sized fillers of the same composition were added into the same resin matrix in the ratio of 10–70%, or 10–50% by weight.
2.
Materials and methods
Resin matrix composed of 1:1:1 mixture of BisGMA, UDMA and TEGDMA by weight was used. BisGMA (Batch number: 419-19-04, ESSTECH, Essington, PA, USA), UDMA (Batch number: PB-2451, ESSTECH) and TEGDMA (Batch number: 488-50-06, ESSTECH) were used. For light curing, 0.7% of camphoroquinone (Batch number: JI00703MN, Aldrich, St. Louis, MO, USA), 0.05% of butylated hydroxytoluene (Batch number: 81K0200, Sigma, St. Louis, MO, USA) and 0.35% of (dimethylamino)ethyl methacrylate (Batch number: 00628MU, Aldrich) were added based on weight. Two different sized silanized fillers were added (Table 1). 8235 glass contains 1–10% B2 O3 , 1–10% Al2 O3 , >50% SiO2 , 10–50% BaO, and 1–10% C10 H20 O5 Si by weight. Translucency of LG filler is 40.6% and that of SG filler is 33.5%. Compositions of experimental resin composites are listed in Table 2. Unfilled resin matrix was used as a control. After adding the predetermined fillers into the resin matrix on a hot plate (50 ◦ C), composite material was filled into a metal mold (38 mm in diameter and 1 mm in thickness). The top surface of a specimen was covered with a Mylar strip and made flat by pressing with a glass plate, and the bottom surface consisted of a glass plate covered with a Mylar strip. Light curing was performed with a light curing unit (Type 43, EXAKT Apparatebau, Norderstedt, Germany) by irradiating for 20 min from both sides. Then, the films were removed. Three specimens were made for each material.
Table 1 – Fillers added to experimental resin composites Code LG SG
Name
Size
8235 glass filler 8235 glass filler
Median grain size, d50: 0.77 m, d99: 2.25 m Median grain size, d50: 0.50 m
Batch number SIL 1029 Lab 14763
Manufacturer SCHOTT Glass, Landshut, Germany SCHOTT
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Table 2 – Composition of experimental resin composites Code
Filler
Control LG-10 LG-20 LG-30 LG-40 LG-50 LG-70 SG-10 SG-20 SG-30 SG-40 SG-50
Resin matrix without filler 0.77 m glass filler 0.77 m glass filler 0.77 m glass filler 0.77 m glass filler 0.77 m glass filler 0.77 m glass filler 0.50 m glass filler 0.50 m glass filler 0.50 m glass filler 0.50 m glass filler 0.50 m glass filler
Filler content (wt%) 0 10 20 30 40 50 70 10 20 30 40 50
Specimens were stored at 37 ◦ C in distilled water for 7 days before baseline color measurement. After blot drying, color was measured according to the CIELAB color scale [15] in the reflectance mode over a zero calibrating box (CIE L* = 0.0, a* = 0.0, and b* = 0.0), a white background (CIE L* = 94.3, a* = −0.4, and b* = 1.4) and a black background (CIE L* = 0.2, a* = 0.4, and b* = −0.6) using a spectrophotometer (Color-Eye 7000A, GretagMacbeth Instruments Corp., New Windsor, NY, USA). The aperture size was a 10 mm × 7.5 mm oval shape. Illuminating and viewing configurations complied with CIE diffuse/8◦ geometry [15]. Measurements were repeated three times for each specimen. Color coordinates, chroma and hue angle of resin composites were calculated. Chroma was calculated as C∗ab = 1/2
(a∗2 + b∗2 ) , and hue angle was calculated as h◦ = arctan(b*/a*) [15]. Optical constants including scattering coefficient (S), absorption coefficient (K) and light reflectivity (RI) were calculated algebraically from the spectral reflectance data of each material using Kubelka’s equations. Details were described in previous studies [9,13]. In practical formulas, S always occurs together with the thickness X of the specimen, that is, as the product SX, called ‘scattering power’ of the specimen. The absorption coefficient (K) is defined as K = S(a − 1) (mm−1 ) [9]. The light reflectivity (RI) is defined as RI = a − b. To determine the influence of the amount of filler on the optical parameters, Pearson’s correlation coefficient (r) between the amount of filler (%) and color parameters was calculated with linear regression analysis within each filler group (p < 0.05). To determine the influence of optical constants on the optical parameters, Pearson’s correlation coefficient (r) between the optical constants (S, K and RI) and color parameters was calculated with linear regression analysis within each filler group (p < 0.05).
3.
Fig. 1 – Scattering coefficient by the wavelength for LG filler based resin composites.
Absorption coefficients (K) are listed in Table 3. K value generally increased up to 30% filler for the LG filler group, after that decreased, except at 400 nm. K value generally increased up to 30% filler for the SG filler group, after that decreased, except at 400 nm. Light reflectivity (RI) is listed in Table 4. RI value generally increased as the amount of filler increased for the LG filler group, and generally increased up to 40% filler for the SG filler group. Mean S, mean K and mean RI values in the wavelength range of 400–750 nm for the LG filler group are presented in Fig. 3 and in Fig. 4 for the SG filler group. As the amount of filler increased, S and RI values generally increased for both groups. But RI reached a plateau value at 40% LG filler, and S and RI decreased at 50% SG filler. Lightness, chroma and hue angle of LG filler group are presented in Fig. 5 and in Fig. 6 for the SG filler group. Trends in lightness were similar for both fillers. Pearson’s correlation coefficients (r) between the amount of filler (%) and the color parameters are listed in Table 5. CIE L* value increased as the amount of filler increased for both of LG
Results
Scattering coefficient (S) by wavelength for the LG filler group is presented in Fig. 1. S values changed slightly by wavelength, and obviously by the amount of filler, and showed general trends by wavelength. S value by wavelength for SG filler group is presented in Fig. 2. Trends by wavelength were similar to those of the LG filler group. In both filler groups, S value increased as the amount of filler increased.
Fig. 2 – Scattering coefficient by the wavelength for SG filler based resin composites.
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Table 3 – Absorption coefficient (K) of resin composites (mm−1 ) Code Control LG-10 LG-20 LG-30 LG-40 LG-50 LG-70 SG-10 SG-20 SG-30 SG-40 SG-50 a
400–750 nm a
0.10 (0.01) 0.21 (0.01) 0.22 (0.04) 0.21 (0.03) 0.19 (0.03) 0.19 (0.04) 0.19 (0.05) 0.19 (0.01) 0.18 (0.01) 0.21 (0.02) 0.21 (0.04) 0.22 (0.03)
400 nm
500 nm
600 nm
0.12 (0.04) 0.23 (0.03) 0.28 (0.03) 0.28 (0.01) 0.28 (0.01) 0.28 (0.01) 0.35 (0.03) 0.20 (0.01) 0.20 (0.07) 0.28 (0.03) 0.32 (0.01) 0.30 (0.01)
0.11 (0.01) 0.21 (0.02) 0.22 (0.01) 0.21 (0.02) 0.18 (0.01) 0.18 (0.00) 0.19 (0.01) 0.20 (0.01) 0.18 (0.05) 0.21 (0.02) 0.20 (0.02) 0.22 (0.02)
0.09 (0.01) 0.20 (0.02) 0.20 (0.01) 0.19 (0.02) 0.17 (0.01) 0.16 (0.00) 0.16 (0.01) 0.19 (0.01) 0.17 (0.05) 0.21 (0.02) 0.19 (0.03) 0.21 (0.03)
700 nm 0.09 (0.01) 0.20 (0.02) 0.20 (0.01) 0.19 (0.03) 0.17 (0.02) 0.16 (0.00) 0.15 (0.00) 0.19 (0.01) 0.17 (0.05) 0.20 (0.02) 0.17 (0.03) 0.20 (0.02)
Standard deviations are in parentheses.
Fig. 3 – Mean scattering coefficient (S), absorption coefficient (K) and light reflectivity (RI) of experimental resin composites for LG filler based resin composites.
and SG groups (r = 0.932 and 0.839, respectively). CIE a* value decreased (moved towards the green direction) as the amount of filler increased in both LG and SG groups (r = −0.798 and −0.691, respectively). CIE b* value increased (moved towards the yellow direction) as the amount of filler increased to 50%
Fig. 4 – Mean scattering coefficient (S), absorption coefficient (K) and light reflectivity (RI) of experimental resin composites for SG filler based resin composites.
for LG and 40% for SG (r = 0.624 and 0.621, respectively). Trends in chroma generally followed those of CIE b* value. Hue angle of the LG filler group were in the fourth quadrant, which indicated the hue of these resin composites is red and blue in tone. The hue angles of the SG filler group were in the first (SG-10
Table 4 – Light reflectivity (RI) of resin composites Code Control LG-10 LG-20 LG-30 LG-40 LG-50 LG-70 SG-10 SG-20 SG-30 SG-40 SG-50 a
400–750 nm a
0.21 (0.01) 0.21 (0.01) 0.30 (0.03) 0.36 (0.02) 0.44 (0.03) 0.46 (0.04) 0.47 (0.04) 0.19 (0.01) 0.24 (0.01) 0.32 (0.01) 0.41 (0.03) 0.36 (0.02)
Standard deviations are in parentheses.
400 nm
500 nm
600 nm
700 nm
0.16 (0.05) 0.18 (0.02) 0.24 (0.01) 0.29 (0.01) 0.35 (0.03) 0.37 (0.01) 0.36 (0.01) 0.17 (0.04) 0.21 (0.02) 0.26 (0.02) 0.32 (0.04) 0.28 (0.06)
0.20 (0.06) 0.22 (0.03) 0.31 (0.02) 0.36 (0.03) 0.44 (0.04) 0.46 (0.01) 0.47 (0.01) 0.20 (0.04) 0.25 (0.03) 0.33 (0.02) 0.41 (0.06) 0.37 (0.08)
0.22 (0.06) 0.22 (0.03) 0.32 (0.03) 0.37 (0.03) 0.45 (0.05) 0.49 (0.01) 0.49 (0.01) 0.19 (0.04) 0.24 (0.03) 0.33 (0.02) 0.42 (0.07) 0.37 (0.09)
0.21 (0.07) 0.22 (0.03) 0.32 (0.03) 0.37 (0.03) 0.45 (0.05) 0.48 (0.01) 0.48 (0.01) 0.19 (0.04) 0.23 (0.04) 0.32 (0.02) 0.43 (0.08) 0.36 (0.09)
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Table 6 – Correlation coefficient (r) between the scattering coefficient (S), absorption coefficient (K) and light reflectivity (RI) and color parameters Filler
L*
a*
b*
C∗ab
h◦
LG
Sa K RI
0.961b −0.527 0.961
−0.725 0.231 −0.693
0.499 NS 0.522
0.546 NS 0.563
NSc NS NS
SG
S K RI
0.966 0.417 0.974
−0.379 −0.271 −0.360
0.862 0.258 0.823
0.582 NS 0.562
0.699 0.456 0.724
a
b
Fig. 5 – Lightness, chroma and hue angle of LG filler based resin composites.
Constant
c
S indicates scattering coefficient, K indicates absorption coefficient and RI indicates light reflectivity. Mean values between 400 and 750 nm was used. The correlation coefficient (r) was significant at the significance level of 0.05. NS indicates no significant correlation (p > 0.05).
was positively correlated with S and RI values for both filler groups.
4.
Discussion
The hypothesis of the present study was rejected because lightness, chroma and hue of experimental resin composites were significantly correlated with the amount of filler (r = 0.406–0.932, p < 0.05), and lightness was highly correlated with S and RI values (r = 0.961–0.974). Since the correlation coefficients between the amount of filler and chroma/hue of experimental resin composites were not so high and moderate (r = 0.406–0.827), pigmentation would be tried to simulate the color of resin composites to those of natural tooth. S value of human enamel was 0.59(±0.48) mm-1 when measured with 0◦ /0◦ geometry, and 0.60(±0.43) mm−1 with 45◦ /0◦ geometry, when monochromatic light of 560 nm was illuminated [1]. Measuring geometry determines the penetration and scattering depth of light into a substance; therefore, S value was influenced by the measuring geometry. In the present study, the measuring geometry was CIE diffuse/8◦ geometry, which should have influenced the measured value. In the present study, S value of resin matrix at 560 nm was 0.07 mm−1 , that of LG added resin composites was 0.16–0.63 mm−1 , and that of SG added resin composites was 0.12–0.40 mm−1 . Therefore, S values for LG-50 and LG-70 resin composites were similar to that of human enamel, although measuring geometry and the wavelength range of illumination were different. Color coordinates for natural teeth were also measured in the previous study of ten Bosch and Coops
Fig. 6 – Lightness, chroma and hue angle of SG filler based resin composites.Short title: Influence of filler on the color of resin composites.
and -20) and fourth quadrant (others). As the amount of filler increased, yellow hue moved to blue hue through the change of CIE b* value from positive to negative values. S and RI values were highly correlated with the amount of filler for both filler groups (Table 5). However, K value was positive or negative depending on the size of fillers. Pearson’s correlation coefficient (r) between the optical constants and the color parameters are listed in Table 6. CIE L* value was highly correlated with S and RI values for both filler groups (r = 0.961–0.974). CIE a* value was negatively correlated with S and RI values for both filler groups. CIE b* value
Table 5 – Correlation coefficient (r) between the amount of filler (%) and color parameters of resin composites Filler LG SG a b
L* b
0.932 0.839
a*
b*
C∗ab
h◦
Sa
K
RI
−0.798 −0.691
0.624 0.621
0.406 0.406
0.827 0.745
0.913 0.787
−0.620 0.415
0.899 0.793
S indicates scattering coefficient, K indicates absorption coefficient and RI indicates light reflectivity. The correlation coefficient (r) was significant at the significance level of 0.05.
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[1], but the measurement protocols were obviously different from that in the present study. Therefore, direct comparison was impossible. S values for the conventional shades of commercial resin composites were in the range of 0.51–0.66 mm−1 at 600 nm [13], which range of S value could be simulated in the present study. Based on the results of the present study, S value could be mimicked to that of natural enamel. However, deviations in S value by the tooth was very high because the standard deviations were around 80% of mean value referring the data of 0.59(0.48) and 0.60(0.43) mm−1 of the previous study [1]. This diversity of natural teeth might be a challenge when mimicking the optical properties of natural teeth. CIE L* value of natural tooth was correlated with S value (correlation coefficient: r = 0.60) [1]. In the present study, S value was correlated with lightness and chroma; but not with hue in the LG group (Table 6). Anyway, S value was highly positively correlated with CIE L* value (r = 0.961 and 0.966), which indicates that CIE L* value was determined by S value to a great extent. Higher scattering resulted in higher CIE L* value. S value was correlated with the amount of filler for both sizes of fillers (Table 5). The LG filler group showed higher correlation coefficient than that of the SG filler group, which reflected the difference in filler size. However, the influence of filler size on the S value depending on the wavelength was not obvious based on Figs. 1 and 2. The main factor of translucency in dental porcelains is due to multiple scattering of light at scattering centers such as opacifiers [16]. Opalescence is the result of light scattering of the shorter wavelengths of the visible spectrum, and this light scattering is caused by particles smaller than the wavelength of visible light that are dispersed throughout a translucent material of a much lower refractive index [17]. In the present study, only color parameters were analyzed; however, translucency and opalescence of resin composites should be considered when controlling the filler distribution of resin composites to mimic the natural tooth. RI value was highly correlated with S value for both filler groups (r = 0.985 and 0.983, respectively) based on the results of the present study. RI value is defined as the light reflectance of a material of infinite thickness [9]. Since the infinite optical thickness of resin composites was in the range of 2.97–5.90 mm [9], the specimen thickness of 1 mm used in the present study was very thin compared to the infinite optical thickness. However, RI value was highly correlated with lightness of resin composites. Background would influence the reflected color of resin composites in this condition. Further study on the influence of background on the reflected color should be performed. Scattering of body porcelain was found to decrease with increasing wavelength within the visible spectrum, in accordance with the scattering theory for particles not substantially less than the wavelength of the scattered light [18]. However, in the present study, above mentioned phenomena were not observed because the filler sizes were around those of visible light (median particle sizes: 770 and 500 nm, Figs. 1 and 2). Scattering is due to refraction and reflection at the interface between the resin matrix and inclusions such as filler
particles and porosity voids, and S value is greatest when the particle diameter equals the wavelength of the incident light [6]. Based on the study of Grajower et al. [6], S value around 750 nm should have been the highest in LG filler group because the median filler size was 0.77 m, and should also have been the highest around 500 nm in SG filler because the median filler size was 0.50 m. But these trends were not observed (Figs. 1 and 2). These results might be caused partly by the broad filler distribution of both fillers and by small the refractive index constant between resin matrix and filler. The refractive index of 1:1 mixture of BisGMA and TEGDMA was 1.5020 [19], and that of 8235 glass used in the present study was 1.55 based on the manufacturer information. Based on this data, the refractive index constant was 1.55/1.50 (=1.03) assuming that the refractive index of 1:1:1 mixture of BisGMA, UDMA and TEGDMA was similar to that of the BisGMA and TEGDMA mixture. S values for commercial resin composites were calculated [13]. Although the sizes for filler particle varied by the brand and shade of resin composite (5–3000 nm), S value decreased with increasing wavelength in all the resin composites investigated [13]. Compared with the results of the present study, the difference was obvious. Broader filler distributions including micro- or nano-sized filler in commercial resin composites might explain this discrepancy. For samples of polymethylmethacrylate with and without quartz fillers (3.3, 3.9, 9.2 and 15 m), optical scattering by fillers is shown to be linearly related to the amount of the filler [8]. Since the filler sizes were obviously different from those of the present study, direct comparison was impossible. However, as the filler concentration increased, S value also increased in the present study. The reflectance and transmission of thin slabs of tooth enamel was measured between 220 and 700 nm [20]. An absorption peak at 270 nm, which is outside the normal human visible light range (from 380 to 780 nm), was common in all the samples, and it was concluded that the organic component was responsible for most or all of the observed optical absorption. In the present study, the measurement range was from 360 to 750 nm, and K value was the highest around 360 nm. After that, K value decreased gradually. Although it is not clear whether experimental resin composites show peaks in the same wavelength of enamel (270 nm), this peak has little practical impact because the wavelength is outside visible range.
5.
Conclusions
Within the limitations of the present study, lightness was highly correlated with the amount of filler, S and RI values (r = 0.932–0.974). But the correlation coefficients between the amount of filler and chroma/hue of experimental resin composites were moderate (r = 0.406–0.827); therefore, pigment would be added to simulate the color of resin composites to that of natural tooth. Translucency and opalescence of resin composites should be considered when controlling the filler distribution of resin composites.
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Acknowledgments This study was supported by grant (R01-2006-000-10421-0) from the Basic Research Program of the Korea Science & Engineering Foundation. Resin matrix was kindly provided by ESSTECH (Essington, PA, USA).
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