|Year : 2021 | Volume
| Issue : 4 | Page : 500-504
|A comparative evaluation of colour stability of different resin cements and its influence on the final shade of All-Ceramic Restorations: An in-vitro Study
Suraj D Khalap1, Pooja P Wadkar2, Ramandeep Dugal3, Pallavi Madanshetty3, Abhishek Gupta1
1 Department of Prosthodontics, Rajaram Dental Clinic, Mumbai, India
2 Department of Periodontics, D. Y. Patil University, School of Dentistry, Nerul, Navi Mumbai, India
3 Department of Prosthodontics & Implantology, M. A. Rangoonwala College of Dental Sciences & Research Centre, Pune, Maharashtra, India
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|Date of Submission||12-Apr-2021|
|Date of Decision||23-Feb-2022|
|Date of Acceptance||03-Mar-2022|
|Date of Web Publication||18-May-2022|
| Abstract|| |
Introduction: Colour changes of the luting material can become clinically visible affecting the aesthetic appearance of thin ceramic veneers. Therefore, unfortunately, the long-term success of veneers is tied to the colour stability of the luting agent used to cement them. Aim: To compare the colour stability of different resin cements and its influence on the final shade of overlying ceramic using two different thicknesses of ceramic. Material and Methodology: Sixty disc-shaped specimens of high translucency (HT) A2 shade of two thicknesses (0.5 mm & 1 mm) were prepared from lithium disilicate glass-ceramic. The discs of each thickness were further divided into three groups randomly depending upon the resin cements used. Group A – light cure resin cement – Variolink N LC by Ivoclar Vivadent in clear shade, Group B – base paste of dual-cure resin cement – Variolink N by Ivoclar Vivadent in transparent shade and Group C dual-cure resin cement – Clearfil esthetic cement EX by Kuraray in clear shade were used to compare their colour stability. A spectrophotometer was used for the colour measurements of the specimens before and after accelerated ageing. The colour stability was determined by colour differences (ΔE) using the coordinates L*, a* and b* in the pre and post accelerated ageing. Results: On comparing the three resin cements, Group A showed the lowest ΔE, whereas the highest ΔE was observed in Group B. This finding was constant for discs of both thicknesses. Conclusion: Even though statistically significant results were observed between the groups, they were not visibly differentiable.
Keywords: Accelerated ageing, ceramic, colour stability, resin cements
|How to cite this article:|
Khalap SD, Wadkar PP, Dugal R, Madanshetty P, Gupta A. A comparative evaluation of colour stability of different resin cements and its influence on the final shade of All-Ceramic Restorations: An in-vitro Study. Indian J Dent Res 2021;32:500-4
|How to cite this URL:|
Khalap SD, Wadkar PP, Dugal R, Madanshetty P, Gupta A. A comparative evaluation of colour stability of different resin cements and its influence on the final shade of All-Ceramic Restorations: An in-vitro Study. Indian J Dent Res [serial online] 2021 [cited 2022 Jun 29];32:500-4. Available from: https://www.ijdr.in/text.asp?2021/32/4/500/345420
| Introduction|| |
One of the major challenges in aesthetic dentistry is to achieve the perfect optical properties of natural teeth with artificial materials. Ceramic veneers represent the most conservative approach to satisfy the patient's restorative needs and esthetic desires. Lithium disilicate is the material of choice as it is the strongest glass-ceramic and is also highly translucent, which minimizes the internal scattering of the light as it passes through the material.
Due to the inherent brittle nature of glass ceramics, adhesive cementation is used to improve fracture resistance by penetrating flaws and irregularities on internal surfaces, minimizing crack propagation and allowing a more effective stress transfer from the restorative to the supporting tooth structure. Resin cements with different polymerization methods are available for luting the restorations, each having their own specific advantages and disadvantages.
Unfortunately, resin cements are reported to go through intrinsic and extrinsic discolouration over a period of time. Colour changes of the luting material can become clinically visible, affecting the aesthetic appearance of thin ceramic veneers. Therefore, the long-term success of ceramic veneers is tied to the colour stability of the resin cement used.
Many studies have been performed to determine the colour stability of resin cements. Nevertheless, most of these studies have concentrated on the resin cement itself rather than its influence on the overall colour of the final all-ceramic restoration., Thickness of the ceramic restoration was also not considered. With developments in new formulations and polymerization techniques, clinical longevity and colour stability of resin cements has improved. Therefore, the aim of this in vitro study was to examine different light and dual-cure resin cements cured under two different thicknesses of lithium disilicate ceramic veneers and to determine potential discolouration of the luted ceramic veneers after accelerated ageing.
| Materials and Methods|| |
A total of sixty disc-shaped specimens of high translucency (HT) A2 shade (30 each of 0.5 mm & 1 mm thickness) were prepared from lithium disilicate glass-ceramic (IPS e.max Press; Ivoclar Vivadent, Schaan, Liechtenstein) according to the manufacturer's directions. The specimens were prepared by burning out a 0.5- or 1-mm thickness of wax with a diameter of 6 mm. The specimens were heat-pressed at 920°C and finished flat on grinder/polisher with wet #400 to #1200 grit silicon carbide paper and ultrasonically cleaned in distilled water for 10 min [Figure 1]. Specimens were then coated on one side with a layer of neutral-shade glaze and fired at 765°C. Finally, they were ultrasonically cleaned for 10 min before cementation. The ceramic discs of both thicknesses were randomly divided into three groups of ten each depending upon the resin cement used.
The study included three resin cements. Light cure resin cement – Variolink N LC by Ivoclar Vivadent in clear shade (Group A), Base paste of dual-cure resin cement – Variolink N by Ivoclar Vivadent in transparent shade (as recommended by the manufacturer) (Group B) and Dual cure resin cement – Clearfil esthetic cement EX by Kuraray in clear shade (Group C). [Figure 2]
Two metal moulds were fabricated with 0.7 mm × 6 mm and 1.2 mm × 6 mm inner dimensions to ensure a 0.2 mm uniform thickness of the resin cement [Figure 2]. The open end of the mould facilitated light-curing by direct contact with the sample and easy removal of the sample from the mould.
All the ceramic specimens were embedded in condensation silicone putty with the surface to be treated (unglazed surface) exposed. For groups A and B, the surfaces were etched for 20 s with 9.5% hydrofluoric acid gel (IPS ceramic etching gel, Ivoclar-Vivadent, Schaan, Liechtenstein). Hydrofluoric acid gel carefully washed off the specimens using a water jet, and the specimens were then cleaned in an ultrasonic bath for 2 min. All specimens were then air-dried and observed for the absence of white residues. A silane coupling agent (Monobond-S; Ivoclar-Vivadent, Schaan, Liechtenstein) was applied with a brush and after 60 s dried with a strong stream of air. For group C, Clearfil Ceramic Primer (Kuraray Noritake Dental Inc, Kurashiki, Okayama) which is a one-step, self-etch, dual-cure adhesive was applied on the flat surfaces of the specimens.
The respective resin cement for each group was dispensed in the metal mould, and the pre-treated surface of the ceramic disc was placed over it ensuring that the surface of the ceramic disc is flushed with the metal mould. Each surface was light-cured for 20 s using an LED light-curing unit (StealthMax, Equinox Medical Technologies B. V., Zeist, Holland). Then, the sample was carefully removed from the mould. Excess cement was removed with a hand scaler.
The initial colour measurements (baseline) were determined by a spectrophotometer (Vita Easy Shade guide, Vita Zahnfabrik, Postfach, Bad Säckingen) after calibration. The colour evaluation was made according to the CIELAB colour space. After the initial colour measurements, the samples were subjected to accelerated ageing using xenon light according to ISO standards. Q-SUN B02 Xenon Test Chamber (Q-LAB Corporation, Ohio, USA) was used to apply 0.55 W/m2/nm of xenon light filtered through borate borosilicate glass at 340 nm. The cycles of the weathering procedure were 40-min light, 20 min light and specimen spray, followed by 60 min light and 60 min dark with specimen and back spray. Back panel temperature was maintained at 70 ± 3°C in the light cycle and at 38 ± 2°C in the dark cycle. Dry bulb temperature was maintained at 47 ± 3°C in the light cycle and 38 ± 2°C in the dark cycle. Relative humidity was 50% ± 5% in light cycle and 95% ± 5% in dark cycle. A total of 150 kJ/m2 radiant energy was applied for 65 h. A new spectrophotometric evaluation was made under the same initial conditions, following the accelerated ageing. After the L*, a*, and b* values were obtained (pre and post ageing), the colour difference (ΔE) was derived by using the following formula: -
ΔE = [(L*1-L*2)2 + (a*1-a*2)2 + (b*1-b*2)2]1/2; or ΔE = [(ΔL*)2 + (Δa*)2 + (Δb*)2]1/2
The obtained data were compiled systematically. A master table was prepared, and the data set was subdivided and distributed meaningfully and presented as individual tables. Statistical analysis was done with SPSS (version 17) USA. Data comparison was done by applying specific statistical tests to find out the statistical significance of the results. As the data was a continuous type, parametric tests were used for analysis. Mean and Standard Deviation (SD) were calculated. Statistical tests employed for the obtained data in our study were as follows:
- Paired sample t-test was used for comparison.
- One way Analysis of Variance (ANOVA) test was used for multiple group comparisons followed by Tukey's post hoc for group-wise comparisons.
| Results|| |
The mean values of ΔL*, Δa*, Δb* and ΔE are shown in [Table 1]. The intra-group comparison of ΔE is shown in [Table 2], and the inter-group comparison is shown in [Table 3]. P value <0.05 was considered to be statistically significant in the current study.
For the 0.5 mm ceramic disc groups, statistically significant changes were observed in ΔE in inter-group (except for Group B) as well as intra-group comparison. Group A showed the lowest ΔE, whereas the highest ΔE was observed in Group B. For the 1 mm ceramic disc groups, statistically significant changes were observed in ΔE in inter-group comparison. The intra-group comparison with 0.5 mm ceramic disc showed nonsignificant results in Group B, whereas statistically significant results were observed in Group A and Group C. Group A showed the lowest ΔE, whereas highest ΔE was observed in Group B.
| Discussion|| |
Ferrari et al. reported that the thickness of enamel of 114 sectioned anterior teeth was 0.3 to 0.5 mm at the gingival third, 0.6 to 1 mm at the middle third and 1.0 to 2.1 mm at the incisal third. Thus, for the present study, 0.5 mm and 1.0 mm thick specimens were prepared to imitate the varying thickness of ceramic veneers.
Light cure resin cements are recommended mostly for anterior restorations due to better colour stability., Hence, Variolink N LC cement was included in the study. Dual cure resin cements have higher bond strength than light-cured resin cements, but the conventional dual-cure resin cements are not colour stable because they contain benzoyl peroxide/amine components. New dual-cure resin cements like Clearfil esthetic cement EX come without a traditional benzoyl peroxide/amine redox initiator system which could be more colour stable over time and hence was included in this study. The manufacturer of Variolink N cement recommends using only the base paste of the dual-cure cement for luting laminate veneers. Hence, it was included in the current study to compare it with other cements dedicated for aesthetic restorations.
Several studies,, have researched the optical effects of resin cements, but they tested the luting agents with thicknesses that were not clinically compatible with the film thickness observed below the ceramic veneers. In the present study, a luting agent of 0.2 mm thickness was bonded to the lithium disilicate ceramic discs to reproduce the clinical condition and to avoid overestimating results regarding the effect of colour changes of the underlying cement.
Many studies have used accelerated ageing procedures, combining UV light exposure with cycles of humidity and light to better simulate the oral environment., But Kilinc et al. reported that the colour stability of resin material needs to be evaluated under the xenon light to closely replicate the oral environment. Therefore, in this study, accelerated ageing was carried out in a xenon testing chamber.
A spectrophotometer can measure colour changes much smaller than those detectable by the human eye. Therefore, the clinical relevance of colour change recorded by the spectrophotometer must be evaluated. Vichi et al. used three different ranges for distinguishing colour differences: ΔE values lower than 1.0 were considered undetectable by the human eye, values between 1.0 and 3.3 were considered visible by skilled operators but clinically acceptable, and ΔE values greater than 3.3 were considerably appreciable also by non-skilled persons and for that reason clinically not acceptable. Chang et al. reported the gold standard threshold of 2.0, which was considered a perceptible colour change able to determine the optical effect of resin cements. This shows the literature is not in agreement with respect to the limit for the human eye to appreciate differences in colour, considering that this limit differs from individual to individual (as it is a combination of eye characteristics and skill from the operator). Hence, the following pattern was used in the current study. When the ΔE value is 0, the colour difference is described as 'perfect'; a value of 0.5 to 1.5 units is 'very good'; 1 to 2 is 'good'; 2 to 3.5 is 'clinically perceptible'; and >3.5 is 'unacceptable';
[Table 1] shows the mean of ΔL*, Δa* and Δb* values (pre and post accelerated ageing) of the samples. The negative values of ΔL* (measure darkness) for all specimens in the present study are consistent with the literature suggesting that resin-based materials tend to darken after accelerated ageing. Group B samples darkened maximally and group A samples minimally. a* coordinate values decreased, whereas the b* coordinate values increased for all groups after ageing. The smallest variations were found in a* coordinates and the greatest in the b* coordinates, with the highest positive value indicating yellowing of a material over time. This could be related to an increased amount of camphorquinone in the resin cements formulation. Another explanation for the tendency of yellowing could be the exposition of Bis-GMA–based material to ultraviolet light and heat.
As per the colour change values (ΔE) illustrated by [Table 2], group B and C discoloured more compared to group A. Statistically significant differences in ΔE were observed in groups with 0.5 mm disc and 1 mm disc. Also, the maximum colour change was recorded through the ceramic veneer of thickness 0.5 mm when bonded to Variolink N base cement with ΔE value of 2.69, but this amount is below this study's accepted level of a noticeable colour change (ΔE > 3.5). Also, it is important to note that with an increase in thickness of the ceramic disc, the overall colour change was reduced, hence indicating that the ceramic did mask most of the actual colour change of the resin material.
Colour change in the samples in this study after the UV aging process may be caused by the discolouration of ceramics or cements beneath the ceramics. Colour changes in resin cements have been related to chemical alterations in the initiator system, activators, and the resin itself. Degradation of residual amines and oxidation of residual unreacted carbon–carbon double bonds culminate in the formation of yellowing compounds., As reported by the respective manufacturers, Variolink N contains dimethacrylates; Variolink N base contains bis-GMA, urethane dimethacrylate and triethylene glycol dimethacrylate (TEGDMA); Clearfil Esthetic Resin Cement Ex also contains bis-GMA, urethane dimethacrylate and TEGDMA. As these materials age, the water sorption characteristics of the resin monomers may contribute to differences in the degree of colour stability. TEGDMA is the monomer responsible for higher rates of water sorption in resin-based materials due to its hydrophilic ether linkages. Therefore, it could be argued that as both the dual-cure cements contain TEGDMA in their composition, they showed more water uptake and related colour changes.
| Conclusion|| |
Although all three resin cements used in this study affected the final colour of the ceramic discs, Variolink N base cement showed maximum discolouration. It can also be concluded that with an increase in the thickness of the ceramic discs the overall discolouration of the specimens was reduced. However, as the discolouration effects were less than 3.5 ΔE units they were considered clinically acceptable.
Limitations of the study
- Due to the difficulty in the adaptation of dietary, habitual and salivary variations of the human subjects, the samples were stored in water.
- Though this colour study has used accelerated ageing procedures combining the xenon light exposure with cycles of humidity and light in order to better simulate the oral environment, this protocol however still cannot reliably simulate the clinical situation.
- Even though accelerated ageing is considered as a harsh environment for its testing conditions, it also serves as a low threshold for material safety determination considering the exclusion of external factors such as absorption of dietary colourants and plaque accumulation.
- Photo ageing procedure can be used to compare different materials with each other but cannot be correlated with a clinical time frame.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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Dr. Suraj D Khalap
14, 10th Floor, Soham Building, Taikalwadi, Matunga West, Mumbai - 400 016, Maharashtra
Source of Support: None, Conflict of Interest: None
[Figure 1], [Figure 2]
[Table 1], [Table 2], [Table 3]
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