Color Stability and Degree of Cure of Direct Composite Restoratives After Accelerated Aging
Purpose: This study evaluated the color changes and amount of remaining C=C bonds (%RDB) in three dental composites after hydrothermal- and photoaging. Methods and Materials: The materials tested were Estelite Σ, Filtek Supreme and Tetric Ceram. Specimens were fabricated from each material and subjected to L* a* b* colorimetry and FTIR spectroscopy before and after aging. Statistical evaluation of the ΔL,* Δa,* Δb,* ΔE and %ΔRDB data was performed by one-way ANOVA and Tukey's test. The %RDB data before and after aging were statistically analyzed using two-way ANOVA and Student-Newman-Keuls test. In all cases an α=0.05 significance level was used. Results: No statistically significant differences were found in ΔL*, Δa*, ΔE and %ΔRDB among the materials tested. Tetric Ceram demonstrated a significant difference in Δb*. All the materials showed visually perceptible (ΔE>1) but clinically acceptable values (ΔE<3.3). Within each material group, statistically significant differences in %RDB were noticed before and after aging (p<0.05). Filtek Supreme presented the lowest %RDB before aging, with Tetric Ceram presenting the lowest %RDB after aging (p<0.05). The %ΔRDB mean values were statistically significantly different among all the groups tested. No correlation was found between ΔE and %ΔRDB.SUMMARY
INTRODUCTION
Currently available composites provide satisfactory strength and high esthetic appearance. However, despite recent improvements, color stability of light-cured composites after long-term intraoral exposure remains a concern.1 Various studies have shown that the color stability of composites can be influenced by several extrinsic factors, such as intensity and duration of the polymerization,2–3 exposure to environmental factors, including ambient and UV radiation,4 heat, water5 or food colorants.6–8 Moreover, color stability is also affected by intrinsic factors, such as the composition of the resin matrix, filler loading and particle size distribution,9 type of photoinitiator10 and percentage of remaining C=C bonds (%RDB).11 The latter is considered a fundamental parameter influencing a variety of mechanical, chemical and biological properties.12–14
To simulate testing of the long-term intraoral color stability of composites, several in vitro methods have been developed, including storage in water and in dark and dry conditions,2 exposure to visible and UV radiation or to several food staining solutions.15–16
This study evaluated the color stability and amount of %RDB of three direct composite restorative materials after artificial aging.
The study hypothesis was that the color parameters of the materials tested would remain within a clinically acceptable level after aging, whereas, significant differences exist in %RDB.
METHODS AND MATERIALS
Two light-cured microhybrid restorative resin composites (Estelite Σand Tetric Ceram) and one nanofilled composite (Filtek Supreme) were tested. All the materials were of the A2 shade (Table 1).
Specimen Preparation
Metal rings (1.2 mm in height and 8.5 mm in diameter) were filled with each composite paste, pressed between two microscopic glass slides covered with transparent polystyrene matrix strips to remove excess material and light-cured for 40 seconds under standard conditions (850 mW/cm2, 500–400 nm) using a halogen curing unit (Trilight, 3M ESPE, Seefeld, Germany). Five specimens were prepared from each material.
Aging
The specimens were subjected to hydrothermal-cycling (2000X, 5°/55°C, 3 c/minute), then photoaged in a weathering apparatus (Suntest CPS+, Atlas, Chicago, IL, USA) operated under the following conditions: 800-300 nm wavelength, 765 W/m2 irradiance, 66 MJ/m2 daily radiant exposure, 37°C chamber temperature and 98 hour exposure time.
Colorimetry
The color of the specimens before and after aging was measured with a portable colorimeter (Dr Lange Microcolor Data Station, Braive Instruments, Liege, Belgium). Specimen orientation against the colorimeter head was standardized by a custom-made specimen holder. The measurements were performed according to the CIE L*a*b* system and the mean L*, a* and b* values for each material calculated. The total color change (ΔE) obtained was calculated for each specimen using the equation: ΔE =[(ΔL*)2+(Δa*)2+(Δb*)2]1/2, where ΔL*, Δa* and Δb* are differences in the respective values before and after aging. Color changes were considered visually perceptible when ΔE>1 and clinically acceptable when ΔE<3.3.15–17
Degree of Cure
The degree of cure of the composite specimens before and after aging was evaluated by FTIR spectroscopy, employing a micro-MIR accessory (Perkin Elmer Corp, Norwalk, CT, USA) attached to an FTIR spectrometer (Spectrum GX, Perkin Elmer Corp). Spectra were acquired under the following conditions: 4000-400 cm−1 range, 4 cm−1 resolution, 60° para-edge KRS-5 mini-crystal (10×5×1 mm) of seven internal reflections, 50 scans coaddition at 23±1°C and ~1 μm sampling depth at 1000 cm−1. Three specimens were analyzed per composite. Spectra of the unset pastes were used as a reference. Quantitative measurements of the percentage amount of remaining C=C bonds (%RDB) were performed based on the two-frequency technique. The aliphatic C=C vibrations (1638 cm−1) were used as the analytical frequency; whereas, the aromatic C…C vibrations (1605 cm−1) were used as the reference frequency. The %RDB was calculated according to the equation:
where AN is the net peak absorbance heights of the polymer (p) and monomer (m) peaks at the corresponding wave numbers.
Statistical Analysis
One-way ANOVA and Tukey's test were used to determine the statistically significant differences in ΔL*, Δa*, Δb*, ΔE and %ΔRDB among the materials tested. Two-way ANOVA and Student-Newman-Keuls multiple comparison tests were used to assess the mean differences in %RDB before and after aging using the type of material and aging status as independent variables. In all cases, an α=0.05 level of statistically significant differences was selected. Pearson Product Moment Correlation was used to access the correlation between ΔE and %ΔRDB. Statistical analysis was performed using SigmaStat software (Jandel, St Rafael, CA, USA).
RESULTS
Colorimetry
The results of the color measurements are presented in Table 2. No statistically significant differences were found in ΔL*, Δa* and ΔΕamong the materials tested (p<0.05). The Δb* values in Tetric Ceram demonstrated a statistically significant difference from the rest. Filtek Supreme exhibited a yellow shift (Δb*=+1.06, Δa*=0), Tetric Ceram a green-yellow shift (Δa*=–1.6, Δb*=0.42) and Estelite Σa blue-green shift (Δb*=–2.68, Δa*=–0.5), with the two latter materials also showing an increase in lightness (ΔL*).
Degree of Cure
Figures 1 through 3 show representative FTIR spectra of unset, set and aged specimens. The results of the %RDB measurements are given in Table 3. Two-way ANOVA revealed a statistically significant interaction between the materials and aging (p<0.001). The Student-Newman-Keuls multiple comparison test doc-values manifested statistically significant differences among all the materials (p<0.05). No correlation was found between %ΔRDB and ΔE (r=0.367).
Citation: Operative Dentistry 32, 4; 10.2341/06-127
Citation: Operative Dentistry 32, 4; 10.2341/06-127
Citation: Operative Dentistry 32, 4; 10.2341/06-127
DISCUSSION
According to the results of this study, both parts of the hypothesis were verified. All the materials showed clinically acceptable but visually perceptible ΔE values (1<ΔE<3.3) and a statistically significant reduction in %RDB.
However, there is a controversy regarding the clinically perceptible values of color change (ΔE). Reported values range from 0.4 to 3.7.18–19 But, in most studies, clinically acceptable values are considered to comply with ΔE=3.3.4–51620–22
Many studies have been performed on the color stability of resin composites employing various experimental protocols. In this study, hydrothermal-cycling was combined with photoaging as a means of accelerated aging. Since water plays a major role in monomer leaching, molecular substitution, degradation of the interfacial filler-matrix silane layer and filler particles23 that affect the color parameters of the restorations,16 water and temperature conditioning of the samples was included as a sequential treatment to photoaging. Nevertheless, this experimental design does not allow for the drawing of conclusions on the specific effects of hydrothermal-cycling or photoaging on color change.
In general, hydrothermal-cycling results in color changes of resin composites within clinically acceptable limits (ΔE<3.3).1 As photoaging is considered more aggressive than water storage in color stability testing,3 it follows that the effect of hydrothermal-cycling in this study is essentially within the clinically acceptable ΔE limits as well. Accelerated photoaging has been shown to provoke a significant increase in Δb* in direct and indirect light-cured composite restorations.318
In the current study, Filtek Supreme exhibited a yellow shift, Tetric Ceram a green-yellow shift and Estelite Σa blue-green shift, with the two latter materials showing an increase in lightness. Residual camphorquinone (photoinitiator) has been considered to be responsible for the yellow color shifts over time.4 Furthermore, tertiary aromatic amines (accelerators) are known to form photoreactive by-products that tend to cause yellow to red/brown discoloration under the influence of light or heat.324 Therefore, differences in the concentration and structure of the photoinitiators and reducing amines may explain the wide spectrum of Δb*and Δa* values obtained among the materials tested.
Apart from catalyst systems, the resin composition, type and size of fillers influence the color stability of tooth-colored restorative materials.25 Partial substitution of TEGDMA for urethane dimethacrylate (UDMA) comonomer in BisGMA/TEGDMA resin matrix has been shown to reduce water uptake and stain susceptibility. Water uptake in BisGMA-based resins has been found to increase proportionally to TEGDMA concentration.26 This leads to a change in stain susceptibility, which is expressed primarily by changes in L* values rather than a* and b* values.8 However, according to Dietschi and others and Vichi and others,59 differences in the filler-resin ratio and, consequently, variations in water sorption, could not explain the high ΔE values of composite materials alone, since filler size is also implicated in color stability, with a the greater size being more susceptible to water aging discoloration. A possible explanation might be the direct relationship between color perception and scattering, because hydrolytic degradation of the particle-resin matrix interface can modify the way that light is scattered by the particles.5
According to the results of this study, Estelite Σ, a BisGMA/TEGDMA resin composite, showed the highest mean ΔE value but with no statistically significant difference from Tetric Ceram, a BisGMA/UDMA/TEGDMA resin. Possibly, the increased filler volume fraction of Estelite Σ(Table 1) may counterbalance the documented increased water uptake of its resin matrix compared to Tetric Ceram. The excessive blue shift of Estelite Σmay be attributed to the increased translucent scattering in the material, which is designed to provide a chameleon effect that matches the performance of enamel or incisal shades (negative b* and low L* values).25
Filtek Supreme, containing DUDMA and BisPMA monomers, both of which are less hydrophilic than BisGMA and TEGDMA,26 and nanomer and nanocluster fillers, showed the least susceptibility to discoloration, although it exhibited the lowest filler volume content. However, an interesting finding was that Filtek Supreme manifested the lowest %RDB before aging and the lowest %ΔRDB after aging. As the color stability of composite materials is influenced by monomer conversion,11 the highest conversion of Filtek Supreme before aging may result in a more stable network with fewer changes after aging and therefore improved color stability.
It has been shown that composites containing more than 35% of unconverted C=C bonds tend to be highly susceptible to discoloration.12 In the current study, the %RDB values obtained before aging ranged from 44.4% to 55.8%, which is in agreement with previous findings.27 Photoaging of the composites induced a significant amount of post-setting conversion (6.4% to 16.4%), which is inversely related to the %RDB values before aging, apparently due to reduced mobility of the residual C=C bonds in materials with a high initial conversion (steric hindrance). Since photoaging was performed immediately after hydrothermal-cycling (an approximate 11-hour delay), it seems reasonable to expect that free radicals were available to initiate a post-setting conversion.28 However, the extent to which the residual C=C bonds, after photoaging, are converted to an additional crosslinked network or, in the presence of atmospheric oxygen and UV radiation, are oxidized; promoting production of yellow compounds is not known.29
CONCLUSIONS
Color changes were observed after composite aging, leading to visually perceptible (ΔE>1) but clinically acceptable results for all the materials tested (ΔE<3.3). No statistically significant differences were found in color parameters (ΔL*, Δa*, ΔE) among the materials tested, except for Tetric Ceram in Δb*.
Filtek Supreme showed the lowest %RDB before aging and Tetric Ceram showed the lowest %RDB after aging (p<0.05). The %ΔRDB values were statistically significantly different among all the materials tested.
No correlation was found between %ΔRDB and ΔE.
Representative FTIR spectra of unset, set and aged specimens of Estelite Σ.
Representative FTIR spectra of unset, set and aged specimens of Tetric Ceram.
Representative FTIR spectra of unset, set and aged specimens of Filtek Supreme.
Contributor Notes
Aspasia Sarafianou, DDS, PhD, Department of Prosthodontics, School of Dentistry, National and Kapodistrian University of Athens, Athens, Greece
Soultana Iosifidou, BS, Physics, Department of Biomaterials, School of Dentistry, National and Kapodistrian University of Athens, Athens, Greece
Triantafillos Papadopoulos, associate professor, Department of Biomaterials, School of Dentistry, National and Kapodistrian University of Athens, Athens, Greece
George Eliades, professor, Department of Biomaterials, School of Dentistry, National and Kapodistrian University of Athens, Athens, Greece