Color stability of nanocrystallite-treated and silicate-treated fillers of calcium phosphate composite resin: An in vitro study Wen Cheng Chen, PhDa and Hui Yu Wu, MSb College of Engineering, Feng Chia University, Taichung, Taiwan; College of Dental Medicine, Kaohsiung Medical University, Kaohsiung, Taiwan Statement of problem. Some composite resins fail esthetically because the color differences to adjacent tooth structure are more than the limits of color tolerance (values are higher than 3.3 DE units). Purpose. The purpose of this study was to assess the effect on color stability of surface-modified dicalcium phosphate anhydrous particles treated with nanocrystallites and silane coupling agents used as fillers for dental composite resin. Material and methods. Specimens were divided into 4 groups of fillers on unmodified (without modification) and on modified surfaces (silanization, nanocrystallites, and nanocrystallites with silica). These groups served as reinforcements and had 2 mass ratios of fillers (fillerþresin) at 30% and 50%. The color differences were measured from day 1 after thermocycling procedures and for different specimen-treated procedures (drying, immersion, and thermocycling) at 1 to 16 days after 24 hours of immersion (n¼5). ANOVA test was used to analyze the differences. The Student t test was used to evaluate the significant group comparisons, and a 3-way ANOVA was used to determine differences and interactions with the filler amount data (a¼.05). Results. Specimens with lower amounts of silica-treated fillers exhibited more variations in color than specimens with larger amounts of fillers. The main color variation was observed within the dried specimens after 24 hours of immersion. The color difference stabilized within 8 days. Fewer changes in the DE values were noted in the groups of filler surfaces with nanocrystallites than in groups without nanocrystallized treatment after the 1 day to 16 days of aging. Conclusions. Color difference was significantly reduced when the fillers reached a certain proportion, which further indicated that fillers with nanocrystal treatment could stabilize color variations within perceptible color tolerances (2.0 DE units) after immersion and thermocycling. (J Prosthet Dent 2014;111:416-424)
Clinical Implications After nanocrystal treatment, the surface of the calcium phosphate composite resin has a better color stability and the treatment also counteracts the color differences in it caused by silanization treatment, therefore, providing better indicators in esthetic and restorative procedures. Accurate color matching and color stability between tooth-colored restorative materials and the natural tooth are important in dental restorations and in enhancing the quality of life of patients.1-4 Materials made of visible
light-polymerized composite resins have been widely used in dental restorations. Several systems, such as ShadeEye NCC (Shofu Inc), VITA EasyShade (VITA Zahnfabrik), and Shadepilot (DeguDent GmbH), that involve visual and
instrument inspections have been developed to match tooth color.5,6 These systems help reduce the color difference between the composite resin and target tooth. However, long-term color change in the material is an
Supported by grant No. NSC99-2314-B037-051-MY3 from the National Science Council of the Executive Yuan, Taiwan and grand No. 102-EG-32-09-16-102 from the Institute of Kaohsiung Medical Device Special Zone in Southern Taiwan Science Park. a Chief, Advanced Medical Devices and Composites Laboratory; and Associate Professor, Department of Fiber and Composite Materials, College of Engineering, Feng Chia University. b Research Assistant, Department of Fiber and Composite Materials, College of Engineering, Feng Chia University.
The Journal of Prosthetic Dentistry
Chen and Wu
May 2014 important issue for esthetic dentistry. Although studies on the color stability of composite resin restorations are important,7 studies that involve longterm color stability of reinforced resin are lacking. Long-term color stability is crucial for the esthetic requirements and endurance of prostheses; any color changes should be within the perceptible color tolerances to create esthetically pleasing long-term prostheses.8-10 In vivo studies have demonstrated that a perceptible color difference falls in the range of 1 to 2 DE units, with a difference of less than 1 unit being imperceptible.11 The significance of color changes by assessing visually detected values is called the perceptibility threshold (general value of approximately 2.0 DE units).12 Other studies have found that DE values more than 3.3 units indicate the possibility of a mismatch during visual color measurement.13 DE values more than 6.8 units are considered unacceptable (the acceptability threshold).14-17 Festuccia et al18 indicated that the color changes in composite resin restorations are multifactorial and are associated with the intrinsic discoloration and extrinsic staining of the materials. The intrinsic factors involve the chemical properties caused by the degrees of cross-linking, interfaces between the dispersive (reinforcement) and continuous (matrix) phases, and decay rates for chemical bonding caused by dental caries. The extrinsic factors are related to personal life and hygiene habits, such as the frequency of drinking staining solutions (coffee, tea, cola, red wine, and whisky), intake of food and drugs, smoking, and mastication of betel nut.19 The structures of visible lightpolymerized cross-linked composite resins are related to the mechanical properties and color reflection behavior of their inorganic fillers, which have a significant effect on interface characteristics. Calcium phosphates (CaP) have been used in composite resin materials to prevent caries and in resin matrices. It has been demonstrated in in vitro
Chen and Wu
417
Table I. Nomenclature of groups with different processing procedures for fillers in composite resins Filler Surface Modifications
Group Abbreviation
Filler surface without modification
Without modification
Filler surface modification with silica
With silica
Filler surface modification with nanocrystallites
With nanocrystal
Filler surface modification with nanocrystallites plus silane coupling agent
With nanocrystal/silica
studies that they remineralize carious enamel and dentin lesions.20-22 The CaP fillers function similarly to the remineralizing agents used in toothpaste and can deliver supersaturated ions to the surface, thus promoting the restoration of demineralized areas, particularly in a relatively acidic environment.23-25 Restorative materials that contain CaPs as reinforcements in a resin matrix have been referred to as “smart composite resins.”26,27 Several studies have shown that composite resins are susceptible to color changes when exposed to staining solutions.28,29 This phenomenon depends on the water absorption capability of the composite resins and the hydrophilic or hydrophobic properties of the fillers.30,31 The phase changes or the remineralizing ability of the fillers also result in noticeable color differences.32,33 The reinforced strength and remineralization potential of such composite resins with nanocrystallites and silica on the modified filler surfaces of dicalcium phosphate anhydrous (DCPA) have been studied.27,34-36 Although filler surfaces modified with silica hinder interfacial interactions and have better flexibility, filler surfaces modified with nanocrystallites and silica films demonstrate superior remineralization.37-39 The present study investigated the color stability of specimens at various times before and after immersion and thermocycling. The null hypothesis tested was that the use of polymerized resins with filler surface treatments and after immersion and thermocycling has no effect on the color difference of 30 and 50 wt% CaPbased composite resins.
MATERIAL AND METHODS Specimen preparations The specimens were divided into 4 groups of fillers on unmodified and modified surfaces (Table I). These fillers had 2 mass ratios (fillerþresin) at 30 and 50 wt% in composite resins. The color differences of the specimens were measured after different filler treatments (drying, immersion, and thermocycling) from 1 day to 16 days. A power analysis was not conducted before the study, and an arbitrary sample size was used, based on those of previous studies for testing with the device in the CIELAB color space.2-4 A sample of 5 was selected to determine color matching differences having statistical significances at an alpha level of .05.
Treatment of nanocrystallite film Varied surface modifications were conducted on the particle surfaces of dicalcium phosphate anhydrous (DCPA) (CaHPO4, Alfa Aesar GmbH & Co KG) powder with a particle size of approximately 1 to 3 mm and a purity of 98%. The procedures and characterizations of nanocrystal treatments on the DCPA surfaces have been referenced in the literature.27,34,35 A total of 5 g of DCPA powder was mixed in 40 mL of saturated solution (2.0 molar ratio of Ca/P) with a stabilized pH value of 4.5 to 5.0 for 20 minutes at 26 C. After a solid film of the nanocrystallites had been deposited on the surfaces, the powders were vacuum filtered, washed twice with deionized water, rinsed with 99.8% alcohol,
418
Volume 111 Issue 5 and then dried in an oven. The results of transmission electron microscopy showed that the ring patterns of dicalcium phosphate dihydrate (DCPD) were formed and predominated in the precipitation phase. This spotted ring phenomenon indicated that large quantities of DCPD nanocrystal clustered mainly on the DCPA fillers.27
Silane modification A solution of 100 mL cyclohexane solvent with 4 (v/v%) (3mercaptopropyl) trimethoxysilane and 2 (v/v%) n-propylamine (Alfa Aesar GmbH & Co. KG) was used as the silane coupling agent of silica. Thereafter, 5 g of DCPA powder was added to the colloidal solution with rapid agitation for 30 minutes at room temperature before raising the temperature to 60 C. The solvent was further removed by drying the specimens in a vacuum system.
Composite resins The resin matrix was formulated mainly with bisphenol-A diglycidyl methacrylate and triethylene glycol dimethacrylate monomers. The free radical polymerization initiation system in light-polymerized composite resins is a light activator camphoroquinone with an accelerator of dimethylaminoethyl methacrylate and a photostabilizer of butylated hydroxytoluene. All chemicals used were obtained from Sigma-Aldrich Inc. The composition of the resin matrix was 48.98 wt% bisphenol-A diglycidyl methacrylate, 48.98 wt% triethylene glycol dimethacrylate, 1 wt% camphoroquinone, 1 wt% dimethylaminoethyl methacrylate, and 0.05 wt% butylated hydroxytoluene. To form a composite resin, the organic matrix and inorganic fillers of the DCPA (30 and 50 wt%) with untreated and treated surfaces were prepared in a dark room. Composite resins were mixed with a
magnetic stirrer until the colloid formed. The details of the control group and the groups of experimentally modified filler surfaces are shown in Table I. The composite resin was then loaded into a 10-mL syringe for injection. The syringe was covered in aluminum foil to prevent a possible reaction with room light. The prepared composite resins were used within 24 hours of preparation. After the composite resin had been injected into the cylindrical metal mold with a 6-mmwide and 3-mm-deep opening for polymerization validation, the polymerizing procedures were divided into different steps. Both the left and right sides of the demolded specimens were polymerized for 40 seconds by using a lightpolymerized machine (Demetron Optilux 401). Light intensity was maintained between 580 and 600 mW/cm2. The optical head was facing the specimens at a working distance of 1.5 mm. The in vitro measuring methodology was divided into 3 groups. The dried
1 Differences in CIELAB values of varying filler amounts in surface-modified and unmodified groups at different processing times (1, 2, 4, 8, and 16 days; n ¼ 5).
The Journal of Prosthetic Dentistry
Chen and Wu
May 2014 specimen group was tested immediately after polymerizing. The methodology for the wet specimen group involved immersing the polymerized dried specimen into deionized water at 37 C for 24 hours. The specimens in the third group were subsequently subjected to thermocycling (600 cycles of 5 C to 55 C for 2 min/cycle) (Long Wha Enterprise). The immersion ratio was set at 1 g of specimen to 10 mL of distilled deionized water. The color stability was tested after processing at different aging times from 1 to 16 days.
419 and the minimum L* was 0, which represents blackness. The positive values of a* were magenta, and the negative values were green. The positive values of b* were yellow, and the negative values were blue. The values of the a* and b* axes have no specific numerical limits. The total color difference, that is, the DE value, was calculated by using the following equation11:
DE¼(DL*2þDa*2þDb*2)1/2 The DE is a single value that considers the differences between the L*, a*, and b* values of the specimen and the
standard specimen.15-17,40 After the different specimens were prepared, the specimens were tested at different aging times (1, 2, 4, 8, and 16 days). The DE was set as the baseline for the first measurement. Five cylindrical specimens were prepared and analyzed in each group (n¼5).
Statistical analyses The statistical analyses, which were selected to determine the effects of different factors, were primarily performed with 1-way ANOVA (a¼.05).
Spectrometric measurement Color was measured at the central part of the specimen, and the data were converted into the International Commission on Illumination (CIE) (L*, a*, b*) color space (CIELAB) values by using a computer-controlled spectrophotometer (Shadepilot; DeguDent). The color-measuring instrument was a spectrophotometer that quantitatively measures the reflection properties of a specimen with a measured width area of a whole tooth. The exported data were the specimen color information with average colorimetric data. A smooth, homogenous white tile (standard white) was used as the standard specimen. All measurements were made by the same person according to the instructions. Based on the results of a prior study, a 10-minute warm-up was conducted to ensure the stability of the machine. The validity of the colormeasurement instrument configuration was tested with an integrating sphere by comparing the data from measurement of the standard white and Munsell color tabs to ensure repeatability. In all color measurements, the operating location and room brightness were controlled, and spectral reflectance was obtained from measurements that ranged from 410 to 680 nm (a chargecoupled device measured the data in the visible range from 400 to 700 nm). The CIELAB color scale is composed of the 3 coordinates of a uniform color scale. The maximum L* was 100, which represents a perfect reflecting diffuser,
Chen and Wu
2 Light images of color differences with varying filler amounts in surface-modified and unmodified groups at different aging periods of up to 16 days.
420
Volume 111 Issue 5 The Student t test was used to evaluate the significant group comparisons among different groups in the properties of the CIELAB and DE values by using JMP 9.0 software (SAS Institute Inc). A 3-way ANOVA for different testing groups, measured conditions, and processing times was used to determine significant differences and interactions with the filler amount data (a¼.05).
RESULTS The influence of varied filler amounts before and after 24-hour immersion and at different times up to 16 days on the color stability of composite resins was investigated by using the CIE L*a*b* color-measuring system (Fig. 1). The light images of the color changes are shown in Figure 2. The statistical data of all of the individual variables were divided into 3 variance tables (Tables II, III, and IV), and the interactions of the variables are shown in Tables V and VI. After analyzing the CIE a* and b* values, the main analyzed terms were subdivided into 3 categories (Tables II and III list the filler amount, filler surface conditions, specimens, and the varied processes). Significant differences in 50 and 30 wt% fillers were found in the values of L*, a*, and b* (Pdried
a*
2
12.2
immersion¼dried
b*
2
3.9
.029
L*
2
1.2
Dried >thermocycling¼immersion
.321
a
None
a
None
a*
2
1.7
.191
b*
2
8.2
dried
L*
2
14.2
dried
a*
2
37.3
dried¼immersion
b*
2
15.3
thermocycling
Superscript letter indicates that testing groups were not significantly different (P>.05).
randomly selected observers (technicians, dental assistants, dentists, and non-clinical researchers)12 as a color difference (50% perceptibility) of 2.6 and 1.9 DE units. The general color difference of 50% acceptability was caused by higher DE differences (below 3.3 units).10-17 Furthermore, the determined color difference value, in which 50% of all observers preferred to remark the restoration because of unacceptable color matching of DE, was above 3.3 and 4.2 DE units. A value above 6.8 DE units was considered to indicate a poor match.12-17 For composite resins
Chen and Wu
with 50 wt% fillers, a clinically insignificant change in DE values was noted for most aging periods (below 3.3 DE units), although the aging tests in this study may not have been long enough to measure long-term color stability. However, the results showed that the plateau was reached within 8 days, and fewer changes in DE values were noted in the groups of filler surfaces with nanocrystallites than in groups without nanocrystal treatment after 1 day and up to 16 days of aging. Nasim et al7 conducted color stability measurements on various
commercially available composite resins and reported that all tested composite resins showed color differences after a period of between 7 and 30 days. The tendency of the colors to change was not identical with the results in this study. The color difference exhibited by the 4 groups of 30 wt% fillers was significantly larger than the acceptable value (above 3.3 DE units). The major color difference was observed in the early stages (day 2 to day 8) in this study. Numerous researchers investigated the influence of water on color stability28-31 and concluded that composite resin
422
Volume 111 Issue 5
Table IV. One-way ANOVA for comparison of CIELAB values with different filler amounts with none and modified surfaces under different processing times (1, 2, 4, 8, and 16 days) 30% Filler Groups Without modification
With silica
With nanocrystal
With nanocrystal/silica
a
50% Filler df
F
P
Groups
df
F
P
L*
4
2.2
.084a
L*
4
1.3
.282a
a*
4
1.6
.182a
a*
4
2.8
.039
a
b*
4
.8
.542a
L*
4
.5
.715a
Without modification
b*
4
.8
.509
L*
4
2.7
.045
a*
4
.7
.617a
a*
4
.4
.783a
b*
4
4.5
.004
b*
4
.8
.565a
L*
4
5.7
Clinical Implications After nanocrystal treatment, the surface of the calcium phosphate composite resin has a better color stability and the treatment also counteracts the color differences in it caused by silanization treatment, therefore, providing better indicators in esthetic and restorative procedures. Accurate color matching and color stability between tooth-colored restorative materials and the natural tooth are important in dental restorations and in enhancing the quality of life of patients.1-4 Materials made of visible
light-polymerized composite resins have been widely used in dental restorations. Several systems, such as ShadeEye NCC (Shofu Inc), VITA EasyShade (VITA Zahnfabrik), and Shadepilot (DeguDent GmbH), that involve visual and
instrument inspections have been developed to match tooth color.5,6 These systems help reduce the color difference between the composite resin and target tooth. However, long-term color change in the material is an
Supported by grant No. NSC99-2314-B037-051-MY3 from the National Science Council of the Executive Yuan, Taiwan and grand No. 102-EG-32-09-16-102 from the Institute of Kaohsiung Medical Device Special Zone in Southern Taiwan Science Park. a Chief, Advanced Medical Devices and Composites Laboratory; and Associate Professor, Department of Fiber and Composite Materials, College of Engineering, Feng Chia University. b Research Assistant, Department of Fiber and Composite Materials, College of Engineering, Feng Chia University.
The Journal of Prosthetic Dentistry
Chen and Wu
May 2014 important issue for esthetic dentistry. Although studies on the color stability of composite resin restorations are important,7 studies that involve longterm color stability of reinforced resin are lacking. Long-term color stability is crucial for the esthetic requirements and endurance of prostheses; any color changes should be within the perceptible color tolerances to create esthetically pleasing long-term prostheses.8-10 In vivo studies have demonstrated that a perceptible color difference falls in the range of 1 to 2 DE units, with a difference of less than 1 unit being imperceptible.11 The significance of color changes by assessing visually detected values is called the perceptibility threshold (general value of approximately 2.0 DE units).12 Other studies have found that DE values more than 3.3 units indicate the possibility of a mismatch during visual color measurement.13 DE values more than 6.8 units are considered unacceptable (the acceptability threshold).14-17 Festuccia et al18 indicated that the color changes in composite resin restorations are multifactorial and are associated with the intrinsic discoloration and extrinsic staining of the materials. The intrinsic factors involve the chemical properties caused by the degrees of cross-linking, interfaces between the dispersive (reinforcement) and continuous (matrix) phases, and decay rates for chemical bonding caused by dental caries. The extrinsic factors are related to personal life and hygiene habits, such as the frequency of drinking staining solutions (coffee, tea, cola, red wine, and whisky), intake of food and drugs, smoking, and mastication of betel nut.19 The structures of visible lightpolymerized cross-linked composite resins are related to the mechanical properties and color reflection behavior of their inorganic fillers, which have a significant effect on interface characteristics. Calcium phosphates (CaP) have been used in composite resin materials to prevent caries and in resin matrices. It has been demonstrated in in vitro
Chen and Wu
417
Table I. Nomenclature of groups with different processing procedures for fillers in composite resins Filler Surface Modifications
Group Abbreviation
Filler surface without modification
Without modification
Filler surface modification with silica
With silica
Filler surface modification with nanocrystallites
With nanocrystal
Filler surface modification with nanocrystallites plus silane coupling agent
With nanocrystal/silica
studies that they remineralize carious enamel and dentin lesions.20-22 The CaP fillers function similarly to the remineralizing agents used in toothpaste and can deliver supersaturated ions to the surface, thus promoting the restoration of demineralized areas, particularly in a relatively acidic environment.23-25 Restorative materials that contain CaPs as reinforcements in a resin matrix have been referred to as “smart composite resins.”26,27 Several studies have shown that composite resins are susceptible to color changes when exposed to staining solutions.28,29 This phenomenon depends on the water absorption capability of the composite resins and the hydrophilic or hydrophobic properties of the fillers.30,31 The phase changes or the remineralizing ability of the fillers also result in noticeable color differences.32,33 The reinforced strength and remineralization potential of such composite resins with nanocrystallites and silica on the modified filler surfaces of dicalcium phosphate anhydrous (DCPA) have been studied.27,34-36 Although filler surfaces modified with silica hinder interfacial interactions and have better flexibility, filler surfaces modified with nanocrystallites and silica films demonstrate superior remineralization.37-39 The present study investigated the color stability of specimens at various times before and after immersion and thermocycling. The null hypothesis tested was that the use of polymerized resins with filler surface treatments and after immersion and thermocycling has no effect on the color difference of 30 and 50 wt% CaPbased composite resins.
MATERIAL AND METHODS Specimen preparations The specimens were divided into 4 groups of fillers on unmodified and modified surfaces (Table I). These fillers had 2 mass ratios (fillerþresin) at 30 and 50 wt% in composite resins. The color differences of the specimens were measured after different filler treatments (drying, immersion, and thermocycling) from 1 day to 16 days. A power analysis was not conducted before the study, and an arbitrary sample size was used, based on those of previous studies for testing with the device in the CIELAB color space.2-4 A sample of 5 was selected to determine color matching differences having statistical significances at an alpha level of .05.
Treatment of nanocrystallite film Varied surface modifications were conducted on the particle surfaces of dicalcium phosphate anhydrous (DCPA) (CaHPO4, Alfa Aesar GmbH & Co KG) powder with a particle size of approximately 1 to 3 mm and a purity of 98%. The procedures and characterizations of nanocrystal treatments on the DCPA surfaces have been referenced in the literature.27,34,35 A total of 5 g of DCPA powder was mixed in 40 mL of saturated solution (2.0 molar ratio of Ca/P) with a stabilized pH value of 4.5 to 5.0 for 20 minutes at 26 C. After a solid film of the nanocrystallites had been deposited on the surfaces, the powders were vacuum filtered, washed twice with deionized water, rinsed with 99.8% alcohol,
418
Volume 111 Issue 5 and then dried in an oven. The results of transmission electron microscopy showed that the ring patterns of dicalcium phosphate dihydrate (DCPD) were formed and predominated in the precipitation phase. This spotted ring phenomenon indicated that large quantities of DCPD nanocrystal clustered mainly on the DCPA fillers.27
Silane modification A solution of 100 mL cyclohexane solvent with 4 (v/v%) (3mercaptopropyl) trimethoxysilane and 2 (v/v%) n-propylamine (Alfa Aesar GmbH & Co. KG) was used as the silane coupling agent of silica. Thereafter, 5 g of DCPA powder was added to the colloidal solution with rapid agitation for 30 minutes at room temperature before raising the temperature to 60 C. The solvent was further removed by drying the specimens in a vacuum system.
Composite resins The resin matrix was formulated mainly with bisphenol-A diglycidyl methacrylate and triethylene glycol dimethacrylate monomers. The free radical polymerization initiation system in light-polymerized composite resins is a light activator camphoroquinone with an accelerator of dimethylaminoethyl methacrylate and a photostabilizer of butylated hydroxytoluene. All chemicals used were obtained from Sigma-Aldrich Inc. The composition of the resin matrix was 48.98 wt% bisphenol-A diglycidyl methacrylate, 48.98 wt% triethylene glycol dimethacrylate, 1 wt% camphoroquinone, 1 wt% dimethylaminoethyl methacrylate, and 0.05 wt% butylated hydroxytoluene. To form a composite resin, the organic matrix and inorganic fillers of the DCPA (30 and 50 wt%) with untreated and treated surfaces were prepared in a dark room. Composite resins were mixed with a
magnetic stirrer until the colloid formed. The details of the control group and the groups of experimentally modified filler surfaces are shown in Table I. The composite resin was then loaded into a 10-mL syringe for injection. The syringe was covered in aluminum foil to prevent a possible reaction with room light. The prepared composite resins were used within 24 hours of preparation. After the composite resin had been injected into the cylindrical metal mold with a 6-mmwide and 3-mm-deep opening for polymerization validation, the polymerizing procedures were divided into different steps. Both the left and right sides of the demolded specimens were polymerized for 40 seconds by using a lightpolymerized machine (Demetron Optilux 401). Light intensity was maintained between 580 and 600 mW/cm2. The optical head was facing the specimens at a working distance of 1.5 mm. The in vitro measuring methodology was divided into 3 groups. The dried
1 Differences in CIELAB values of varying filler amounts in surface-modified and unmodified groups at different processing times (1, 2, 4, 8, and 16 days; n ¼ 5).
The Journal of Prosthetic Dentistry
Chen and Wu
May 2014 specimen group was tested immediately after polymerizing. The methodology for the wet specimen group involved immersing the polymerized dried specimen into deionized water at 37 C for 24 hours. The specimens in the third group were subsequently subjected to thermocycling (600 cycles of 5 C to 55 C for 2 min/cycle) (Long Wha Enterprise). The immersion ratio was set at 1 g of specimen to 10 mL of distilled deionized water. The color stability was tested after processing at different aging times from 1 to 16 days.
419 and the minimum L* was 0, which represents blackness. The positive values of a* were magenta, and the negative values were green. The positive values of b* were yellow, and the negative values were blue. The values of the a* and b* axes have no specific numerical limits. The total color difference, that is, the DE value, was calculated by using the following equation11:
DE¼(DL*2þDa*2þDb*2)1/2 The DE is a single value that considers the differences between the L*, a*, and b* values of the specimen and the
standard specimen.15-17,40 After the different specimens were prepared, the specimens were tested at different aging times (1, 2, 4, 8, and 16 days). The DE was set as the baseline for the first measurement. Five cylindrical specimens were prepared and analyzed in each group (n¼5).
Statistical analyses The statistical analyses, which were selected to determine the effects of different factors, were primarily performed with 1-way ANOVA (a¼.05).
Spectrometric measurement Color was measured at the central part of the specimen, and the data were converted into the International Commission on Illumination (CIE) (L*, a*, b*) color space (CIELAB) values by using a computer-controlled spectrophotometer (Shadepilot; DeguDent). The color-measuring instrument was a spectrophotometer that quantitatively measures the reflection properties of a specimen with a measured width area of a whole tooth. The exported data were the specimen color information with average colorimetric data. A smooth, homogenous white tile (standard white) was used as the standard specimen. All measurements were made by the same person according to the instructions. Based on the results of a prior study, a 10-minute warm-up was conducted to ensure the stability of the machine. The validity of the colormeasurement instrument configuration was tested with an integrating sphere by comparing the data from measurement of the standard white and Munsell color tabs to ensure repeatability. In all color measurements, the operating location and room brightness were controlled, and spectral reflectance was obtained from measurements that ranged from 410 to 680 nm (a chargecoupled device measured the data in the visible range from 400 to 700 nm). The CIELAB color scale is composed of the 3 coordinates of a uniform color scale. The maximum L* was 100, which represents a perfect reflecting diffuser,
Chen and Wu
2 Light images of color differences with varying filler amounts in surface-modified and unmodified groups at different aging periods of up to 16 days.
420
Volume 111 Issue 5 The Student t test was used to evaluate the significant group comparisons among different groups in the properties of the CIELAB and DE values by using JMP 9.0 software (SAS Institute Inc). A 3-way ANOVA for different testing groups, measured conditions, and processing times was used to determine significant differences and interactions with the filler amount data (a¼.05).
RESULTS The influence of varied filler amounts before and after 24-hour immersion and at different times up to 16 days on the color stability of composite resins was investigated by using the CIE L*a*b* color-measuring system (Fig. 1). The light images of the color changes are shown in Figure 2. The statistical data of all of the individual variables were divided into 3 variance tables (Tables II, III, and IV), and the interactions of the variables are shown in Tables V and VI. After analyzing the CIE a* and b* values, the main analyzed terms were subdivided into 3 categories (Tables II and III list the filler amount, filler surface conditions, specimens, and the varied processes). Significant differences in 50 and 30 wt% fillers were found in the values of L*, a*, and b* (Pdried
a*
2
12.2
immersion¼dried
b*
2
3.9
.029
L*
2
1.2
Dried >thermocycling¼immersion
.321
a
None
a
None
a*
2
1.7
.191
b*
2
8.2
dried
L*
2
14.2
dried
a*
2
37.3
dried¼immersion
b*
2
15.3
thermocycling
Superscript letter indicates that testing groups were not significantly different (P>.05).
randomly selected observers (technicians, dental assistants, dentists, and non-clinical researchers)12 as a color difference (50% perceptibility) of 2.6 and 1.9 DE units. The general color difference of 50% acceptability was caused by higher DE differences (below 3.3 units).10-17 Furthermore, the determined color difference value, in which 50% of all observers preferred to remark the restoration because of unacceptable color matching of DE, was above 3.3 and 4.2 DE units. A value above 6.8 DE units was considered to indicate a poor match.12-17 For composite resins
Chen and Wu
with 50 wt% fillers, a clinically insignificant change in DE values was noted for most aging periods (below 3.3 DE units), although the aging tests in this study may not have been long enough to measure long-term color stability. However, the results showed that the plateau was reached within 8 days, and fewer changes in DE values were noted in the groups of filler surfaces with nanocrystallites than in groups without nanocrystal treatment after 1 day and up to 16 days of aging. Nasim et al7 conducted color stability measurements on various
commercially available composite resins and reported that all tested composite resins showed color differences after a period of between 7 and 30 days. The tendency of the colors to change was not identical with the results in this study. The color difference exhibited by the 4 groups of 30 wt% fillers was significantly larger than the acceptable value (above 3.3 DE units). The major color difference was observed in the early stages (day 2 to day 8) in this study. Numerous researchers investigated the influence of water on color stability28-31 and concluded that composite resin
422
Volume 111 Issue 5
Table IV. One-way ANOVA for comparison of CIELAB values with different filler amounts with none and modified surfaces under different processing times (1, 2, 4, 8, and 16 days) 30% Filler Groups Without modification
With silica
With nanocrystal
With nanocrystal/silica
a
50% Filler df
F
P
Groups
df
F
P
L*
4
2.2
.084a
L*
4
1.3
.282a
a*
4
1.6
.182a
a*
4
2.8
.039
a
b*
4
.8
.542a
L*
4
.5
.715a
Without modification
b*
4
.8
.509
L*
4
2.7
.045
a*
4
.7
.617a
a*
4
.4
.783a
b*
4
4.5
.004
b*
4
.8
.565a
L*
4
5.7
