Difference in the color stability of direct and indirect resin composites.

UNLABELLED: Indirect resin composites are generally regarded to have better color stability than direct resin composites since they possess higher conversion degree.
OBJECTIVE: The present study aimed at comparing the changes in color (ΔE) and color coordinates (ΔL, Δa and Δb) of one direct (Estelite Sigma: 16 shades) and 2 indirect resin composites (BelleGlass NG: 16 shades; Sinfony: 26 shades) after thermocycling.
MATERIAL AND METHODS: Resins were packed into a mold and light cured; post-curing was performed on indirect resins. Changes in color and color coordinates of 1-mm-thick specimens were determined after 5,000 cycles of thermocycling on a spectrophotometer.
RESULTS: ΔE values were in the range of 0.3 to 1.2 units for direct resins, and 0.3 to 1.5 units for indirect resins, which were clinically acceptable (ΔE<3.3). Based on t-test, ΔE values were not significantly different by the type of resins (p>0.05), while ΔL, Δa and Δb values were significantly different by the type of resins (p<0.05). For indirect resins, ΔE values were influenced by the brand, shade group and shade designation based on three-way ANOVA (p<0.05).
CONCLUSION: Direct and indirect resin composites showed similar color stability after 5,000 cycles of thermocycling; however, their changes in the color coordinates were different.

INTRODUCTION

Resin-based indirect restorative systems were developed as possible alternatives to metallic or ceramic-based restoratives[2]. However, concerns as to wear resistance, color stability, expansion/contraction and sensitivity of these restorations still remain[11]. Apparent disadvantage of resin-based systems stems from the fact that all resins are inherently more porous than ceramics; therefore, there is a greater propensity for these materials to stain or discolor over time and to accumulate plaque[2,24]. Clinical trial investigating the color stability of indirect resin composites showed that even after 1-2 years, verifiable and visible changes in color occurred[24].

Color of resin composites is determined not only by more macroscopic phenomena such as matrix and filler composition and filler content, but also by relatively minor pigments additions and potentially by initiator system and filler coupling agent[15]. Color changes of resin composites have been attributed to a wide variety of possible causes, such as chemical degradation, oxidation of the unreacted carbon double bonds, stain accumulation, dehydration, water sorption, leakage, poor bonding and surface roughness[6,9,12-14,23,24]. Color changes of resin composites are due to exogenous or endogenous reasons[6,9,24]. Among them, more important for the entire color stability of a light-curing material is the internal color changes, which mainly depend on the photoinitiator system as well as on the applied form and time-span of polymerization[13,14]. Color stability of resin composites under various physicochemical conditions improved when materials showed low water sorption, high filler-resin ratio, reduced particle size and hardness, and an optimal filler-matrix coupling system[6].

There are arguments on the threshold color difference levels based on instrumental color measurements that can be visually perceivable or clinically acceptable[8,25]. However, the clinically acceptable value for color difference in dental materials is assumed to be ∆E≤3.3[25]. Based on previous color stability studies of indirect resin composite, all tested indirect resin composite facing materials showed sufficient color stability (∆E<3.3) after UV-irradiation and storage in red wine[17]. However, unacceptable color changes (∆E>3.3) were detected in indirect resin composites after accelerated aging for 300 h[7] and 383 h[28]. It has also reported that unacceptable (∆E>3.3) discoloration was observed when indirect resin composite facings were aged in tea, coffee, mouthrinse and UV irradiation[27]. Therefore, it remains essential to improve the color stability of these materials.

Although there have been a number studies on the properties of indirect resin composites[7,16,17,27], there is limited information on the difference in the color stability based on contemporary direct and indirect resin composites by the type and also by the brand and shade group of indirect resin composites. Besides, indirect resin composites are generally regarded to have better color stability than direct resin composites since they possess higher conversion degree. The purpose of this study was to determine the color stability of one brand of varied shades of direct resin composites and two brands of indirect resin composite after 5,000 cycles of thermocycling. The null hypotheses assumed in the present study were that (1) the color stability of direct and indirect resin composites is be different; (2) the color change of resin composites after aging is not acceptable (∆E>3.3); (3) the color changes of indirect resin composites after thermocycling are not influenced by the brand, shade group and shade designation in each shade group.

MATERIAL AND METHODS

One brand of direct (Estelite Sigma: ES; Tokuyama, Tokyo, Japan) and two brands of indirect resin composites (BelleGlass NG: BG; Kerr, Orange, CA, USA; Sinfony: SF; 3M ESPE, St. Paul, MN, USA) were investigated (Figure 1). Since the designations for the shade groups in both of the indirect resin composites were similar (Enamel groups of BG material and SF material, Opaceous dentin group of BG material and Dentin group of SF material, and Translucent dentin group of BG material and Transparent opal group of SF material), three corresponding shade groups in two indirect resin composites were regarded as the same shade group, namely EN, DT and TL.

Figure 1

Shades and shade groups of resin composites investigated

Resin composites were packed into a polytetrafluoroethylene mold (12 mm in diameter and 1 mm in thickness) on a polyethylene terephthalate film. After packing the resin composite, another film was laid on the top of the specimen, and pressed with a 49 N load for 1 min to produce a uniform thickness. Specimens were light cured for 40 s in 3 overlapping areas from one side with a light-curing unit (Spectrum 800; Dentsply/Caulk, Milford, DE, USA) with an intensity setting of 400 mW/cm2. The output of the curing light was checked with a radiometer (SDS; Kerr). After curing, specimens were removed from the mold and both films were removed.

Then, secondary curing was applied to the indirect resin composites. For BG material, specimens were post-cured in a proprietary curing chamber (BelleGlass HP Curing Unit; Kerr) following the manufacturer’s instructions. For SF material, specimens were post-cured in a proprietary curing chamber (Visio Beta Vario Light Curing Unit; 3M ESPE) following the manufacturer’s instructions. Five specimens were made for each shade. After curing, baseline color measurements were made after storage in 37ºC distilled water for 24 h. After then, thermocycling was performed for 5,000 cycles in distilled water at 5°C and 55°C with a dwell time of 15 s.

Color was measured before and after thermocycling according to the CIELAB color scale relative to the standard illuminant D65 over a white tile (CIE L*=94.3, a =-0.1 and b*=-0.4) on a reflection spectrophotometer (Color-Eye 7000A; GretagMacbeth, New Windsor, NY, USA) after blot dry. An ultraviolet (UV) filter was positioned to 100% UV included condition with the specular component excluded (SCE) geometry. The aperture size was 3 × 8 mm, and illuminating and viewing configuration was CIE diffuse/8-degree geometry[4]. Measurements were repeated 3 times for each specimen and averaged.

Color change (∆E) after thermocycling was calculated as ∆E=[(∆L)2+(∆a)2+(∆b)2]½. Changes in color coordinates [CIE L* (∆L), a* (∆a) and b* (∆b)] were calculated as ‘the value after thermocycling - the value before thermocycling’. CIE L* is a measure of the lightness, CIE a* is a measure of redness (positive direction) or greenness (negative direction) and CIE b* is a measure of yellowness (positive direction) or blueness (negative direction) of an object.

∆E, ∆L, ∆a and ∆b values by the type of resin composites were analyzed by t-tests. ∆E, ∆L, ∆a and ∆b values by the shade group of resin composites were analyzed by one-way analysis of variance (ANOVA) and Scheffe’s post hoc test (SPSS 12.0; SPSS, Chicago, IL, USA; α=0.05). ∆E, ∆L, ∆a and ∆b values of indirect resin composites were analyzed by three-way ANOVA with the fixed factors of the brand (2 brands), shade group (3 groups) and shade designation in each shade group.

RESULTS

∆E, ∆L, ∆a and ∆b values by the shade are presented in Figures 2-5. ∆E values were in the range of 0.3 to 1.2 (mean: 0.8±0.2), 0.3 to 0.9 (mean: 0.5±0.2) and 0.4 to 1.5 units (mean: 0.8±0.4) for ES, BG and SF material, respectively (Figure 2). ∆L values were in the range of -0.3 to 0.3 (mean: 0.0±0.2), -0.7 to 0.0 (mean: -0.4±0.3) and -0.5 to 0.9 units (mean: 0.5±0.5) for ES, BG and SF material, respectively (Figure 3). ∆a values were in the range of 0.0 to 0.6 (mean: 0.2±0.1), 0.0 to 0.1 (mean: 0.0±0.0) and 0.0 to 0.3 units (mean: 0.1±0.1) for ES, BG and SF material, respectively (Figure 4). ∆b values were in the range of -1.2 to -0.4 (mean: -0.7±0.3), -0.6 to 0.1 (mean: -0.2±0.3) and -0.9 to 0.2 units (mean: -0.3±0.3) for ES, BG and SF material, respectively (Figure 5).

Figure 2

Color changes after 5,000 cycles of thermocycling

Figure 5

Changes in CIE b* after 5,000 cycles of thermocycling

Figure 3

Changes in CIE L* after 5,000 cycles of thermocycling

Figure 4

Changes in CIE a* after 5,000 cycles of thermocycling

Differences in ∆E, ∆L, ∆a and ∆b values by the type and shade group of resin composites are presented in Table 1. Based on t-test, indirect and direct resin composites showed no significant difference in ∆E values (p>0.05); however, they showed significant differences in ∆L, ∆a and ∆b values (p<0.05). Based on Scheffe’s post hoc test, ∆E values of indirect resin composites were influenced by the shade group: EN (mean ∆E=0.6)=TL (mean ∆E=0.6)