INTRODUCTION

Laminate veneers have long been the ultimate esthetic treatment for anterior teeth.1 Due to improvements in bonding systems and resin cements, more long-lasting, reliable results are expected.2 These treatments can provide patients with satisfying dental restorations. Unfortunately, the long-term success of porcelain laminates is tied to the color stability of resin composites used to cement them. Resins with different polymerization methods are available for luting indirect esthetic restorations, each having specific advantages and disadvantages.

The color stability and optical properties of resin composites have long been studied.3–4 Color change and loss of shade match with surrounding tooth structure is a major reason for replacement of esthetic restorations. 5 The color stability of composites has been studied by artificial aging in different conditions and for different durations.6–9 Additionally, immersion in different colored drinks has been tested.10–11 Discoloration can occur by oxidation and results from water exchange within the polymer matrix and its interaction with unreacted polymer sites and unused initiator or accelerator.12

With developments in polymerization techniques and composite materials, clinical longevity and color stability are expected to improve.13 Hekimoglu and others investigated the effects of accelerating aging on an auto-polymerized glass cement (Dyract Cem) and light- and dual-polymerized resin cements (Enforce and Twinlook). They found the color stability to be acceptable;14 however, changes in the color and contrast ratio of newer resin materials were not clarified. The results of studies on the color assessment of self-, light- and dual-polymerizable resin cements and the effects of aging on the opacity of these materials are conflicting. The current study evaluated the change in optical properties of four resin cements used in bonding porcelain laminate veneers after subjecting them to accelerated aging.

METHODS AND MATERIALS

In the current experimental in vitro study, the color parameters of four resin cements (A3 Shade) were evaluated. Table 1 shows the characteristics of the tested cements.

Table 1 Resin Cements Used in This Study

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Forty feldspathic porcelain disks (0.7 mm thick and 18 mm in diameter) were prepared (Ceramco powder universal A3 Shade, Dentsply, York, PA, USA) to simulate veneers. The disks were divided into four groups of 10 specimens. In each group, one of the cements was applied according to the manufacturer's instructions. The porcelain disk was placed at the bottom of a ring mold that had the same diameter as the disks and a thickness of 1 mm. The prepared cements were then applied on the disk according to the manufacturer's instructions. A mylar sheet and a glass slab were used to cover the ring. Since the thickness of the ring was 1.0 mm and the porcelain disk was 0.7 mm, the thickness of the cement was 0.3 mm. To spread the cements evenly, a 2.5 kilogram weight was applied for 20 seconds. After removing the weight, the auto-polymerizing group was set aside for 10 minutes to ensure sufficient polymerization. The light- and dual-polymerizing resins were polymerized with a light-curing unit (Optilux 501; Kerr, Orange, CA, USA) with an intensity setting of 300 mw/cm², which was verified by a Demetron radiometer (Sybron, New Port Beach, CA, USA). The tip of the light-curing unit was applied on four overlapping parts of the specimens for 60 seconds each to improve polymerization efficiency. The ring was then removed and the specimens that were unexposed to light were transferred to the spectrophotometer in distilled water. A spectrophotometer (ColorEye 7000 A, GretagMacbeth, New Windsor, NY, USA) was used in this experiment, since the spectrophotometer is a reference color measuring instrument used to determine color parameters.15 It was set for specular component excluded (SCE) measurements. Evaluation of the baseline color properties was done on a black and white background in the CIE L*a*b*color space. L*, a* and b* were recorded when the baseline color properties were evaluated. Most of the color studies in dentistry use the CIELAB color space and the color difference formula to measure color change. The total color difference between two objects is calculated as: ΔE = (ΔL*²+Δa*²+Δb*²)^1/2, where L* stands for lightness, a* represents green-red (−a= green, +a= red) and b* denotes blue-yellow (−b= blue, +b= yellow).

The saturation enables the differentiation of pale and strong colors and hue distinguishes the differentiation of color families, for example, red, green, blue.

A positive ΔL* indicates that the specimens become lighter, while a negative ΔL* indicates that the specimens become darker. A positive ΔC indicates greater saturation and a negative ΔC indicates less saturation. Since the hue angle (h) of the specimens tested was about 90°, representing yellow, a positive hue indicated that the color became greener and a negative hue indicated that the color became more red. Opacity/translucency was assessed by obtaining the contrast ratio (CR) of the lightness or Y value of the specimens against both a black (Yb) and white (Yw) background, with CR= Yb/Yw2.

The pre-weathering color coordinates of each specimen are represented by l₁, a*₁ and b*₁ and the post-weathering coordinates are represented by L*₂, a*₂ and b*₂.

The accelerated aging process was performed according to the modification proposed by Lee and Powers6 and Khokhar and others7 for ISO 7491,16 using the Xenotest Alpha chamber (Heraeus Kulzer, Hanau, Germany). The accelerated aging process simulated the effects of long-term exposure to environmental conditions through an accelerated weathering process that involved ultraviolet light exposure, temperature and humidity changes. Accelerated aging has been adopted since 1978 to test the color stability of dental resins.691417 An aging device (Xenon weather–Ometer, Atlas Material Testing Technology, Chicago, IL, USA) was used to subject samples to both visible and UV light and 100% relative humidity at 37°C. The manufacturer of the weathering instrument has estimated that 300 hours of aging is equivalent to one year of clinical service.18 Khokhar and others stated that the greatest amount of color change occurred in the first 100 hours of accelerated aging.7 In the current study, 100 hours of accelerated aging was found to be satisfactory for demonstrating the early color change of the resin cement.

The specimens were exposed to a xenon lamp of 5,000°K–7,000°K and 150,000 Lux, with a filter that shifted the spectral energy distribution of the xenon lamp into the visible light domain. The exposure time was 100 hours at 37°C and 100% humidity. After aging, the color properties of each group were measured on a black and white background. The evaluations contained the following parameters: L* represents the lightness of the color (L*=0 is black and L*=100 is white), a* represents the position between red and green (negative values indicate green, while positive values indicate red) and b* shows the position between yellow and blue (negative values indicate blue and positive values indicate yellow). Parameter equations are as follows:

Statistical analysis of the data was performed using SPSS software, version 11.0 (SPSS Inc, Chicago, IL, USA). The paired t-test was used to compare the color parameters in each group before and after aging and one-way analysis of variance was used to compare the groups. The statistical significance was set at the 0.05 level.

RESULTS

Means and standard deviations of the changes observed in the color parameters of each group are summarized in Table 2. The ΔE of each group, before and after aging, was not significantly different (p>.05). Also, one-way analysis of variance showed no significant difference in ΔE among the studied groups (p=0.71).

Table 2 Mean (standard deviation) Values of Difference in Color Parameters

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Before aging, the difference in contrast ratio (CR) of the specimens was significant (p=0.00). The Tukey post-hoc test revealed significant differences between Variolink Veneer and dual-polymerized Variolink (p=0.02), Variolink Veneer and Multilink (p<0.00), light-polymerized Variolink and dual-polymerized Variolink and also light polymerized Variolink and Multilink (p<0.00). After aging, CR was significantly different (p=0.01). The post-hoc Tukey test showed significant differences between Variolink Veneer and dual-polymerized Variolink (p=0.02) and also between light-polymerized Variolink and dual-polymerized Variolink (p=0.02) (Figure 1). The ΔCR was significantly different among the groups (p<0.00). The Tukey post-hoc test showed that these differences existed between Variolink Veneer and Multilink (p=0.04), light-polymerized Variolink and Multilink (p=0.02) and dual-polymerized Variolink and Multilink (p<0.00).

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Other parameters were not significantly different in the tested groups.

DISCUSSION

Accelerated aging in the Xenotest chamber has become a method of simulation of oral conditions for a relatively long service time. Ultraviolet light, temperature and humidity are known to cause oxidation of the amine, which is necessary for initiating the polymerization process. Aromatic tertiary amines used in self-cured resins are more likely to oxidize than aliphatic amines used in light-cured materials; therefore, light-cure cements are expected to have more color stability.2–3 In the current study, when using the CIE L*a*b* color space as the system of color assessment, the ΔE of the cements (whether light-, self- or dual-activated) were not different. While some researchers agree that a ΔE ranging between 2.2 and 4.4 can be considered clinically acceptable,13 others believe it should be ≤3.3.2141719–20 The ΔE calculated for the studied cements in the current study was acceptable (less than 3.3); however, the auto polymerizing resin cement seemed to be a short distance from the borderline of 3.3.

Koishi and others found that, after immersion in 37°C distilled water for 24 weeks, the color change of a dual-cure cement was significantly lower than that of a self-cure resin.21 As a dual-cured resin cement, the base paste of Variolink II contains both aliphatic and aromatic tertiary amines, and the catalyst paste contains benzoyl peroxide. The light-cured specimens of Variolink were made of base paste only, while the dual-cured resins were prepared by mixing base and catalyst, so that the aromatic tertiary amine remained intact in the light-cured Variolink groups. The inclusion of two kinds of amine in both materials can explain why the ΔE of the different groups did not significantly differ in the current study, which is in agreement with the findings of Berrong and others.22 The other light-polymerizing resin, Variolink Veneer, claims to be amine reduced, thus, it is expected to be more color stable. However, its ΔE was in the range of other cements (p>0.05).

Light-cured resin cements should receive enough light energy to polymerize sufficiently. Rasetto demonstrated that a minimum exposure time of 60 seconds from a conventional halogen light, or 10–15 seconds from a high intensity light, is necessary to polymerize Variolink II, regardless of the composition of the esthetic veneer.1

An overlapping method was used in the current study to let the light energy cover the whole area of the specimens. Sufficient curing of the specimens can somewhat explain the relative color stability of the groups. Apart from ΔE, another important factor among the optical properties of esthetic restorations is the covering factor, or opacity. Opacity, on one hand, can give the cement the ability to cover underlying tooth discolorations, while it may render the restoration less lively. After aging, the cements showed a different behavior with regard to their covering ability. Multilink showed more opacity than the other groups. The exact reason for the change in contrast ratio and the different behavior of the various cements still needs to be studied; however, it is believed that stress cracks within the polymer matrix and partial debonding of the filler from the resin as a result of hydrolysis, may increase opacity and alter the appearance.12 In the current study, all of the groups became more opaque; however, auto polymerized Multilink was the most affected [ΔCR=0.05 (0.02)].

A study by Lee and others indicated that commercial composites, claiming to have similar Vita shades, have significant color differences and different opacity levels. 6 The opacity of resin cements is as important as the hue. Accelerated aging may have caused some degradation of the resin cements, and the opacity observed may have resulted from this disintegration. Since the specimen size for the spectrophotometer used in the current study had to be 18–20 mm, it was not possible to use real tooth specimens. With digital photography, it may be possible to conduct precise color assessments of tooth-cement-veneer assemblies and evaluate the color properties in situ.

CONCLUSIONS

The esthetic performance of porcelain laminates can be affected by the color of the resin cements used to bond them to teeth. The color of resin cements can change during service. The current results indicate that resin cements, whether light-, dual- or auto-polymerized, have acceptable ΔE after aging, but their opacity increases. Auto-polymerizing resin cements presented significantly more opacity after aging.