Influence of curing light irradiance and ceramic thickness on color stability and translucency of cemented ceramic laminate veneers

Abstract

This in vitro study aimed to evaluate the effects of curing light irradiance and ceramic thickness on the color stability and translucency of cemented laminate veneers after accelerated aging. Eighty ceramic disks were fabricated using shade A3 IPS e.max Press HT specimens of 0.50-mm and 1.00-mm thickness. Specimens of each thickness were randomly divided into four groups, and the A3 shade of Variolink N resin cement was used for bonding in three groups with light irradiances of 700 mW/cm², 900 mW/cm², and 1100 mW/cm². The CIE L*a*b* parameters were measured before and after 5,000 thermal cycles of 5℃ and 55℃. Changes in the color and translucency were measured through spectrophotometry and analyzed using two-way analysis of variance. Results both ceramic thickness and curing light irradiance caused statistically significant differences in color after aging (P < 0.01). The greatest color changes were observed in samples of thickness 0.50 mm that received a light irradiance of 700 mW/cm². Samples with varying thicknesses and curing light irradiances showed statistically significant differences in the extent of decrease in translucency after aging (P < 0.01). The results of this study demonstrated that cemented ceramic laminate veneers tended to become darker and yellower after aging. The light irradiance and ceramic thickness clearly affected the color stability of the veneers. The translucency of the veneers was significantly related to their thickness, and aging clearly decreased the translucency of the veneers.

Introduction

Owing to their outstanding esthetics, consistent longevity, excellent biocompatibility, and minimal invasion of tooth tissue, ceramic laminate veneers have been widely used in esthetic dentistry¹. The IPS e.max Press is a glass ceramic material containing approximately 70% lithium disilicate. Owing to its excellent mechanical and color properties, the IPS e.max Press has become a major component of ceramic laminate veneer manufacturing². However, due to its high translucency, the type and color of the underlying luting agent is an important consideration affecting the aesthetic effect of veneer restoration^3,4. Light-polymerized resin cement has become the most common luting agent used for ceramic veneer cementation because of its low solubility, high bonding strength, and long-term color stability^3,5,6,7,8,9.

Resin cement color changes are relevant to the color stability of cemented laminate veneers, particularly when highly translucent ceramic materials are used6,^10,11. Both external and internal factors can cause color variations in cemented ceramic veneers. The external factors include exposure to ultraviolet radiation, changes in temperature and moisture, and absorption of colored pigments from food^10,12. The internal factors include the composition of the glass–ceramic material and the chemical reactions in the resin cement. For example, the presence of other crystal phases in lithium disilicate glass ceramic can promote water diffusion and increase the solubility of the glass ceramic. In addition, rough surfaces facilitate water penetration, which breaks the silica network, reduces crystallization, and leads to absorption of colored pigments¹³. Hydrolysis of the organic matrix and matrix-filler interface of the resin cements and oxidation of residual unreacted carbon-carbon double bonds (C = C) after incomplete photocuring lead to formation of yellowing compounds^14,15.

The curing of resin cement is affected by the light irradiance, irradiation distance and time, color, thickness and composition of the resin. Despite the translucency of lithium disilicate, the thickness was a significant factor for light attenuation when the resin cements was activated and cured through the ceramic. The thicker the ceramic material, the greater the light attenuation3, and the lower the degree of conversion (DC) of resin cements¹⁶. The light-curing units used in clinical practice also affect the curing effect due to the light attenuation of the service life^17,18. Felipe V et al.¹⁹ assessed the effect of ceramic thickness and light irradiance on DC of resin cements by meta-analysis showed that, ceramic thickness greater than 1 mm significantly reduced the DC. But higher irradiation light-polymerizing unit (3200 ~ 3505 mW/cm²) can improve the DC. These high-power light-curing units have been studied as a solution to increase ceramic thickness and hopefully to actualize shorter clinical light-curing time. However, shortening the time of light activation by using a high-irradiance light-polymerizing unit reduced the polymerization of resin cement^20,21.

The color of cemented laminate veneers in the oral environment will change over time, namely color aging. This phenomenon has been simulated with xenon light^22,23, ultraviolet light²³, coffee storage²⁴, thermal cycling²⁴, and water storage²⁴. The temperature of 5℃ is equivalent to that of typical cola, orange juice, or wine, and a temperature of 55℃ is equivalent to that of coffee¹³.

Color is one of important esthetic parameters in dentistry, and visual judgment is the most frequently used method of evaluating color in dentistry. CIEDE2000 formula is now extensively accepted in dentistry research and outperforms other color difference formulas by a large margin in the evaluation of differences human perceptibility (PT) and acceptability (AT) of color between dental ceramic materials^25,26,27,28. Otherwise, translucency has been considered as one of the primary factors in maintaining esthetics, which is defined as the relative amount of light transmission or diffuse reflectance from a substrate surface through a turbid medium²⁹. The translucency parameter (TP) is used to assess the translucency of esthetic dental restoration materials. It has been reported that aging increased the ceramic surface roughness which results in increasing light scattering and decreasing translucency values²⁹.

The effect of different unattenuated light-curing units (all greater than 1000 mW / cm²) on the polymerization of resin cements was reported in the literature^20,21, while the effect of light attenuation on color stability of cemented laminate veneers has not been reported. The objective of this study was to analyze the effects of light irradiance and ceramic thickness on the color stability and translucency of cemented laminate veneers based on TP and use of accelerated aging with thermal cycling at 5℃ and 55℃, which accurately simulates temperature variations in the mouth. Two null hypotheses were formulated: (1) ceramic laminate veneer thickness and Light irradiance do not influence the color stability of cemented laminate veneers; (2) ceramic laminate veneer thickness and light irradiance do not influence the translucency of cemented laminate veneers.

Materials and methods

The following materials and equipments were used in this study: IPS e.max Press HT A3 ceramic ingots (Ivoclar Vivadent, Schaan, Principality of Liechtenstein), Ceramic oven (Ivoclar Vivadent, Schaan, Principality of Liechtenstein), GC Fuji LINING LC Powder-Liquid Pattern Resin (GC, Tokyo, Japan), digital vernier calipers (Shanggong, Shanghai, China), Variolink N A3 resin cement (Ivoclar Vivadent, Schaan, Principality of Liechtenstein), 10% hydrofluoric acid (Ivoclar Vivadent, Schaan, Principality of Liechtenstein), Bluephase Meter II precise dental radiometer (Ivoclar Vivadent, Schaan, Principality of Liechtenstein), photocuring light (Ivoclar Vivadent, Schaan, Principality of Liechtenstein), standard light source (3nh, Shenzhen, China), VITA EasyShade IV spectrophotometer (VITA Zahnfabrik, Bad Säckingen, Germany), and STST thermal cycling machine (KSUN, Dongguan, China).

A cylindrical stainless steel custom mold was used to create 56 disc-shaped GC resin patterns of 12-mm diameter: 28 had a thickness of 0.60 mm and 28 of 1.10 mm (n = 7). Ceramic specimens of shade HT A3 of IPS e.max Press were fabricating using the lost-wax casting method. They were then ground with 240#–1200# wet grift silicone carbide papers to 0.50-mm and 1.00-mm thicknesses and ultrasonically cleaned in distilled water for 10 min. A digital vernier caliper was used to confirm that the error in the thickness of the specimens was within ± 0.02 mm. After preparing the ceramic specimens, the specimens of each thickness were randomly divided into four groups. One of the four groups for each thickness were randomly selected as the blank control group without resin cement. The other three groups were selected as the experimental group, and one side of each specimen in those groups was etched with 10% hydrofluoric acid for 20 s and then rinsed with running water. The samples were then ultrasonically cleaned for 5 min and dried using oil-free compressed air. A silane coating agent was applied, and the samples were bonded to clean glass slides using Variolink N A3 resin cement and compressed with a 1-kg load for 20 s to achieve a 0.1-mm thick cement layer. Different light irradiances (700 mW/cm², 900 mW/cm², and 1100 mW/cm²) were used for 40 s to cure the specimens in each group (n = 7). Polyethylene films of various thicknesses were placed over the working end of the curing lamp and Bluephase Meter II precise dental radiometer was used to test repeatedly to achieve the desired curing light irradiance. After curing, 600# wet grift silicone carbide papers were used to remove excess resin cement, and the thicknesses of each specimen were measured again and standardized at 0.60 mm and 1.10 mm.

All specimens were placed in a water bath at 37℃ for 24 h, with no light exposure. The shade of each specimen was measured under D65 standard light using a Vita EasysShade IV spectrophotometer under both black and white backgrounds. Artificial accelerated aging. All specimens were placed in a thermal cycling machine for 5,000 cycles at 5℃ and 55℃. The exposure time was 30 s at each temperature at 10-s intervals to simulate half a year of oral use30. The color parameters were measured after 5000 cycles. Figure 1 is the flow diagram of experiment.

Fig. 1 Experimental flow diagram.

The following formulas for ΔE₀₀³¹and TP32were used to calculate the color differences. Two-way analysis of variance (ANOVA) was performed using SPSS 22. The Tukey honestly significant difference (HSD) test was used for post-hoc comparisons (α = 0.01). A paired-sample t-test analysis was used to compare TP before and after accelerated aging (α = 0.01).

where ΔL', ΔC'and, ΔH'are metric differences between the corresponding values of the samples, computed on the basis of uniform color space used in CIEDE2000. RT is a rotation function that explains the interaction between the chroma and hue diferences in the blue region. S_L, S_C, and SH are the weighting functions, whereas K_L, K_C and KHare the correct terms to be adjusted according to the experimental conditions³³. And the subscripts "B" and "W" refer to lightness (L'), chroma(C') and hue (H') of the specimens over the black and the white backgrounds, respectively.

Results

There were obvious changes in the L*, a*, b*, C‘and H’ values of the specimens after accelerated aging (Table 1). In all samples, L* decreased and b* increased. The greatest color difference was 2.16, which was in the group of 0.50-mm thick specimens with a light irradiance of 700 mW/cm²(Table 2). The results of the two-way ANOVA were presented in Table 3. The thickness of the ceramic layer and light irradiance significantly influenced the color differences (P < 0.01). The Tukey HSD test showed significant differences between the group with a light irradiance of 700 mW/cm² and the groups with higher light irradiances (P < 0.001). There was no significant difference between the groups with light irradiances of 900 mW/cm² and 1100 mW/cm² (P = 0.95), without significant interaction between thickness and light irradiance (P = 0.027).

Table 1 B*L*, a* and b* values of specimens with different light radiation intensity results devided into each thickness pre- and post-aging.

Table 2 Color differences of cemented specimens.

Table 3 Two-way analysis of variance of color difference.

The translucency of all specimens decreased after aging (Table 4). Paired t-tests showed statistically significant changes in the translucency of the specimens after aging (P < 0.001). In this study, the light irradiance did not have a statistically significant effect on the translucency of the bonded specimens (P = 0.310). However, when the 0.50 mm specimen was cured by 1100 mW/cm² light irradiance, the change of TP pre- and post-aging were significantly different from those in the control group and the 700 mW/cm² light irradiation intensity group (Table 5). The results of the two-way ANOVA for changes in translucency were listed in Table 6. There was a significant difference in the change in translucency between specimens of varying thicknesses (P = 0.001) but not between specimens with varying light irradiances (P = 0.011). In addition, there was no significant interaction between the specimen thickness and light irradiance (P = 0.018).

Table 4 Translucency parameter (TP) value of specimens with different light radiation intensity results divided into each thickness pre- and post-aging.

Table 5 Change of TP of specimens.

Table 6 Two-way analysis of variance of the translucency parameter.

Discussion

Based on these results, the null hypotheses were rejected. The thickness of the ceramics laminate veneer and intensity of the curing light radiation affect both color stability and translucency of cemented laminate veneers.

For long-term esthetics, it is important for ceramics laminate veneers to maintain stable optical properties after bonding. The color stability of cemented laminate veneers is complex, and the color changes in both resin cement and ceramic material can significantly affect the color stability of the ceramics veneer after bonding^6,34. The color changes of unbonded specimens in this study fell within a clinically unnoticeable range. However, the color changes in the bonded specimens were significantly greater than those in unbonded specimens, especially in 0.50-mm thick bonded specimens. The color differences were noticeable to the naked eye, but clinically acceptable, after artificial accelerated aging. This result supports the conclusions of Turgut et al.’s research, which indicated that the color change in a cemented ceramic laminate veneer has little to do with the color change of the ceramic material itself and is mainly caused by the underlying resin cement¹².

The poor color stability of dual-curing and chemical-curing resin cements is due to the oxidation of aromatic amines^35,36. Thus, luting agents used to cement ceramics laminate veneers are generally light-curing resin materials. Curing of light-curing resin cements is initiated by light, and their color stability depends on complete polymerization8Incomplete curing can cause oxidation of residual unreacted carbon-carbon double bonds, increasing water absorption and solubility, and decreasing bonding strength³⁷. These differences all contribute to lowering of the color stability of the resin cements.

In this study, the color changes of all the bonded specimens exceeded the visual PT after artificial accelerated aging (ΔE00>0.8). The Variolink N A3 (yellow) light-cured resin cement, which mainly contained bisphenol A glycidyl methacrylate (Bis-GMA), urethane dimethacrylate (UDMA), and bisphenol A ethoxylate dimethacrylate, demonstrated good color stability. In this cement system, UDMA is a substitute for triethylene glycol dimethacrylate (TEGDMA), which can lower the absorption of water and improve color stability^4,6. However, Bis-GMA tends to turn yellow on exposure to ultraviolet light and heat^38,39. The cemented specimens in this study tended to become darker and yellower after accelerated aging, which may be related to the color change of Bis-GMA under heating, oxidation of carbon-carbon double bonds, and increased water absorption owing to changes in the resin matrix of the resin cement and silanization processes of the filler particles in the resin^5,6,40.

According to the Beer–Lambert law, when equivalent light rays pass through two objects of the same type of material and thickness, the transmittance through both objects is the same. As the thickness of the ceramic material increases, refraction increases and transmittance decreases; thus, the light reaching the resin cement decreases^16,38,41, which could affect the degree of resin curing. It has been reported an approximately 60% irradiance reduction for 1-mm-thick specimens, 80% reduction for 1.5-mm-thick specimens and 85% reduction for 2-mm-thick specimens^3,16. In this study, cemented ceramics specimens of different thicknesses exhibited significantly different color stabilities. According to multiple studies, the ΔE00visually PT and clinically AT were 0.8 and 1.77, respectively^42,43. The 0.50-mm-thick and 1.00-mm-thick cemented specimens had noticeable color changes (ΔE₀₀ > 0.8). In particular, the Δ E₀₀ of 0.50-mm-thick cemented specimens cured by the light irradiance of 700 W/cm² is 2.16, which is clinically unacceptable. Whereas the 1.00-mm-thick specimens exhibited a better overall color stability, with its ΔE₀₀ both below 1.77. This may be because the added ceramic thickness masked the underlying color changes and effectively cancelled out the color changes in the resin.

Decreasing the light irradiance decreases the degree of curing of the resin^36,41,44 and also affects the color stability of cemented laminate veneers. In this study, the color differences after accelerated aging in specimens cemented with a light irradiance of 700 mW/cm² were clearly more than those cemented with light irradiances of 900 mW/cm² and 1100 mW/cm². In clinical practice, many factors can reduce the curing light irradiance, including an increased distance from the lamp, contaminants on the curing unit, long-term use of the light-curing unit, inappropriate direction of exposure to the curing unit, and inappropriate disinfection methods^16,36. Therefore, to improve curing effectiveness, the light irradiance of the light-curing unit should be checked regularly, and the distance and direction between the lamp and the surface of the resin cement should be carefully optimized.

For superior esthetics, both color and translucency of the ceramics laminate veneer are important. Translucency can give restoratives restorations a vivid, natural appearance⁴¹, and changes in translucency can have obvious effects on the masking ability of the material⁴⁵. In this study, the translucency of all specimens decreased after artificial accelerated aging, which was in accordance with the findings of several previous studies^4,46. Turgut et al. compared the translucency of ceramics laminate veneers cemented with several different brands and shades of luting agents after accelerated ageing. They found that the translucency of 0.50-mm-thick and 1.00-mm-thick cemented ceramic veneers decreased significantly after accelerated aging, but there was no significant difference in the translucency of bonded and nonbonded ceramic veneers. This suggests that the translucency of cemented ceramic veneers is mainly related to the changes in the translucency of the ceramic material itself.

Thickness of the veneer greatly affects its translucency. In this study, the translucency of the 0.50-mm-thick specimens decreased considerably more than that of 1.00-mm-thick specimens. In other words, the effect of aging on the translucency of thicker specimens was less than that on thinner specimens. Decreased translucency improved the masking ability of the ceramic material and helped counteract color changes in the underlying resin cement.

This study still has some limitations. In the present study, one side of the resin cement was completely exposed to the environment, which is not consistent with the clinical environment. The influence of actual oral conditions on the color stability of cemented ceramic veneer is much more complicated. Second, in order to better ensure homogeneity and comparability between specimens, we set the thickness of resin cement to be 0.1 mm, but it’s too much to simulate the clinical conditions. What’s more, Only the effect of attenuation irradiance was simulated, and the high irradiance devices (1400 mW/cm²,3200 mW/cm²) were not used as a comparison. Finally, only one resin cement (Variolink N A3) was used in this study. The type, shade and chemical composition of resin cements can also affect the color stability^47,48. Therefore, we recommend that the effect of light irradiance on the color stability of ceramic veneers bonded by different resin cements need to be studied. However, the results of this in vitro study are clinically relevant as they indicate that monitoring irradiation intensity changes of light-curing units and ensuring adequate light irradiance when adhered with light-cured resin cement are key factors in maintaining the long-term aesthetic effect of veneer restorations.

Conclusions

Within the limitations of this study, the following conclusions can be drawn: (1) cemented ceramic laminate veneers tend to darken and become yellowish after aging, but the color change is clinically acceptable; (2) curing light irradiance and ceramic thickness significantly affect the color stability of cemented ceramic laminate veneers; and (3) the translucency of ceramic laminate veneers is closely related to the thickness of the ceramic material, and the overall translucency of ceramic laminate veneer restorations decreases after aging.

Reference: Omitted