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ORIGINAL ARTICLE
Ahead of print publication  

The effect of in vitro aging on the color stability of cubic and tetragonal zirconia materials


1 Prosthodontics Department, Riyadh Elm University, Riyadh, Saudi Arabia
2 Department of Prosthodontics, College of Dentistry, Prince Sattam Bin Abdulaziz University, Al-Kharj 11942, Saudi Arabia

Date of Submission21-May-2020
Date of Decision25-May-2020
Date of Acceptance25-May-2020
Date of Web Publication07-Aug-2020

Correspondence Address:
Maha S Mezied,
Prosthodontic Department, College of Dentistry, Riyadh Elm University, Riyadh
Saudi Arabia
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/sjos.SJOralSci_26_20

  Abstract 


Aim: The aim of this study is to evaluate the effect of artificial-accelerated aging (AAA) on the color stability of three types of monolithic cubic Zirconia materials of the third generation compared with tetragonal Zirconia materials of the first generation.
Materials and Methods: A four groups of 10 disc-shaped specimens (10 mm × 1.2 mm) were made from the following CAD/CAM Zirconia blocks: first-generation Zirconia (Ceramill Zi LT) as a control, and three brands of third-generation Zirconia (Ceramill Zolid FX UT, Katana UTML, and Cercon XT) as the experimental groups. Ten discs from each group were subjected to the color measurement using a Spectrophotometer (Hunterlab, EasyMatch QC. Ver 4.90). Then, same discs were subjected to AAA for 3500 cycles. The data were analyzed with the one-way analysis of variance (ANOVA) and Tukey's post hoc test. Data analyses were evaluated at a statistically significance level of P < 0.05.
Results: Statistically significant differences were detected in the L*, a*, and b* values among the four groups both before and after AAA. One-way ANOVA was used, which showed statistically significant differences among the groups (P < 0.001). The Ceramill ZI LT showed the greatest change in color (ΔE = 2.74 ± 0.23), followed by Katana UTML (ΔE = 1.91 ± 0.23), Ceramill Zolid FX UT (ΔE = 1.52 ± 0.43), and least change in color was seen in Cercon XT (ΔE = 1.44 ± 0.25). Tukey's post hoc test showed a statistically significantly difference among the test materials, except between Ceramill Zolid Fx UT and Cercon XT, where there was no significant difference seen in ΔE.
Conclusion: The AAA protocols used in this study resulted in a significant effect in the (ΔE) of the four-tested Zirconia materials. The Ceramill ZI LT (First Generation) showed the greatest change in color and the least change in color was observed with Cercon XT (third generation). The color changes in all four tested Zirconia, though statistically significant was not a clinically perceivable

Keywords: All ceramic, artificial-accelerated aging, color stability, zirconia



How to cite this URL:
Mezied MS, Alqahtani FS. The effect of in vitro aging on the color stability of cubic and tetragonal zirconia materials. Saudi J Oral Sci [Epub ahead of print] [cited 2020 Sep 23]. Available from: http://www.saudijos.org/preprintarticle.asp?id=291608




  Introduction Top


Translucency and color reproduction of natural teeth would be one of the main goals for esthetic dental restorations. All ceramic restorations were the materials of choice because they provide excellent esthetic results, are biologically very well tolerated, and provide for less plaque accumulation.[1] However, these are not the only essential criteria but the good mechanical properties of a dental ceramic, enabling its use for fixed dental prosthesis, are also important.[2]

Tetragonal, partially stabilized Zirconia was developed over a decade ago, owing to its opaque nature, it lacked favorable esthetic properties and limited its application to restoration frameworks that needed to be veneered with a more esthetically pleasing glass-ceramic.[3] This came with its own drawbacks as the veneering ceramic and Zirconia had different coefficients of thermal expansion. This, coupled with the multistep fabrication process, created residual tensile stresses which rendered a significant decrease in bond strength, resulting in the chipping of the layered ceramic.[4]

Monolithic Zirconia restorations come with the several advantages such as the reduction in fabrication process which enhances the cost and time efficiency, elimination of a veneering layer of glass ceramic thus avoiding chipping, improved mechanical properties which enables the practice of minimally invasive dentistry.[5],[6] The main drawback experienced with such types of restorations is their comparably lower esthetic property to other ceramic systems.[7]

The color stability of a restoration throughout its functional lifetime is as important as the mechanical properties inherent in that material. The longevity and quality of restorations depend partly on the color stability of a restorative material over time.[8],[9] Therefore, restorative materials should possess good color stability to be able to retain its color from the extrinsic factors that occur due to plaque accumulation, stains from solutions, surface roughness, and chemical degradation.[10],[11]

However, studies have shown that, when the Zirconia surface is in contact with water or water steam, it can suffer a slow, unprompted, and gradual phase transformation. This phenomenon is termed low-temperature degradation.[12] These oral ecological factors may affect the physical and chemical framework of restorative materials.

In vitro study conducted by Alp et al.[13] where the externally shaded of new-generation translucent Zirconia had a significant color change after coffee artificial-accelerated aging (AAA) than the preshaded translucent Zirconia in monolithic or veneered specimens. However, color changes in both materials were within clinically acceptable limits.

The aim of thisin vitro study is to evaluate the effect of AAA on the color stability of three types of monolithic cubic Zirconia materials of the third generation compared with tetragonal Zirconia materials of the first generation.


  Materials and Methods Top


Material

Three cubic Zirconia (third generation) brands of 5-mol%-Yttria-stabilized tetragonal Zirconia polycrystals (5Y-TZP) and a Tetragonal Zirconia ( first generation) brand of 3-mol%-Yttria-stabilized tetragonal Zirconia polycrystals (3Y-TZP) were selected and used in this study, as shown in [Table 1]. Forty disc-shaped specimens and 10 discs from each ceramic brand were prepared from partial-sintered Zirconia blocks according to the manufacturer's instructions.
Table 1: Materials used in the study

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Methods

Preparation of the disc specimens

Disc-shaped specimens were designed using the 3D software (Dental Wings, Canada) with a diameter of 10 mm and a thickness of 1.2 ± 0.05 mm, according to ISO 6872, and a Stereolithography (STL) file was generated.

Partially sintered blocks of each ceramic brand were used [Figure 1], and the STL file was uploaded to the CAD/CAM milling machine (Ceramill Motion 2, 5-axis, Amann Girrbach, Koblach, Austria) to prepare 10 disc-shaped specimens (1.2 ± 0.05 mm in thickness and 10 mm in diameter) with machine compensation for the sintering shrinkage of 20%–25% according to the manufacturer's instructions. A digital caliper (Electronic Digital Caliper, Shan, China) was used to check the thickness and the diameter after sintering.
Figure 1: Study sample groups: A group is Ceramill Zi LT, B group is Ceramill Zolid FX UT, C group is Katana UTML, D group is Cercon XT

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Color measurement before thermal cycling aging

The forty specimens, ten from the first generation (Ceramill Zi LT) and ten from each third generation (Cubic Zirconia) (Katana UTML, Ceramill Zolid FX UT, Cercon XT), were subjected to color measurements, which was performed using a Spectrophotometer (Hunterlab, EasyMatch QC. Ver 4.90). The procedure of color measurement was carried out in a viewing booth with a white background under D65 illumination standards, stipulated by the International Organization for Standardization (ISO) number ISO 7491. The spectrophotometer head was oriented to record the measurements in the middle of each specimen. Three readings were taken per specimen, and the average of these three readings was used as the L * a* b* values for the corresponding specimen [Figure 2].
Figure 2: Spectrophotometer

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Thermocycling aging (low-temperature degradation)

The forty specimens, ten from the control (Ceramill Zi LT) and ten from each third generation (Cubic Zirconia) (Katana UTML, Ceramill Zolid FX UT, Cercon XT), were placed on the thermocycling machine (Thermocycler 1100 SD Mechatronik GmbH, Germany) and were subjected to 3500 cycles, which represents 1 year of usage in the oral cavity.[14] The process alternated between two water baths, one cold at 5°C and the other warm at 55°C. Each cycle lasted 60 s: 20 s in 5°C bath, 10 s transfer time to the 55°C bath, 20 s in 55°C bath, and 10 s transfer time back to the 5°C bath [Figure 3].
Figure 3: Thermocycling machine

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Color measurement after thermal cycling aging

The forty specimens which underwent AAA were subjected to the color measurements following the same criteria as described above before the aging process.

The CIE color difference (ΔE), per type of ceramic was calculated according to the following equation:

ΔE = [(L2* − L1*) 2+ (a2* − a1*) 2+ (b2* − b1*) 2]1/2

Where

L1* value represents the brightness of the specimen before AAA

L2* value represents the brightness of the specimen after AAA

a1* value represents red-green axis (x-axis) of the specimen before AAA

a2* value represents red-green axis (x-axis) of the specimen after AAA

b1* value represents yellow-blue axis (z-axis) of the specimen before AAA

b2* value represents yellow-blue axis (z-axis) of the specimen after AAA.


  Statistical Analysis Top


The mean and standard deviation of the L*, a*, and b* values of Zirconia restorations specimens before and after AAA were calculated. The one-way analysis of variance (ANOVA) and Tukey's post hoc test were used to compare the color characteristics among the four different materials used. The level of significance for all tests was set at P < 0.05.


  Results Top


Before AAA, one-way ANOVA revealed a statistically significant difference between the groups (P < 0.05). Then, Tukey's post hoc test [Table 2] was performed and the L* value presented a statistical difference in the mean among the groups, but there was no significant difference between the Katana UTML and the Ceramill Zolid FX UT groups (P = 0.181). However, regarding a* value, there was a statistically significant difference in the mean among the groups, but there was no significant difference between the Katana UTML and the Ceramill Zolid FX UT groups (P = 0.840). With regard to the b* value, there was a significant difference when compared to the other.
Table 2: Values of the Tukey's post hoc test for the L*, a*, and b* values before artificial-accelerated aging

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After AAA, one-way ANOVA was used and revealed a statistically significant difference between the groups (P < 0.05). Then, Tukey's post hoc test [Table 3] was performed, and it was observed that the L* value was statistically different in the mean among the groups. However, regarding a* value, there was a significant difference in the mean among the groups, but there was no significant difference between the Katana UTML and the Ceramill Zolid FX UT groups (P = 0.962). With regard to the b* value, there was a significantly different when compared to the other.
Table 3: Values of the Tukey's post hoc test for the L*, a*, and b* values after artificial-accelerated aging

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One-way ANOVA was used and showed statistically significant differences among groups (P < 0.001). The Ceramill ZI LT showed the greatest change in color (ΔE = 2.74 ± 0.23), followed by Katana UTML (ΔE = 1.91 ± 0.23), Ceramill Zolid FX UT (ΔE = 1.52 ± 0.43), and least change in color was Cercon XT (ΔE = 1.44 ± 0.25) [Figure 4]. The Tukey's post hoc test showed that the Ceramill Zi LT had significantly greater color change when compared to the other three materials (P < 0.05).
Figure 4: Comparison of ΔE among groups

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  Discussion Top


Based on the results of this experiment, the null hypothesis that no effect of AAA on color stability among the three types of cubic Zirconia (third generation) compared to the tetragonal Zirconia ( first generation) was rejected. One-way ANOVA was used and showed statistically significant differences among the groups (P < 0.001). The Ceramill Zi LT showed the greatest change in color (ΔE = 2.74 ± 0.23), followed by Katana UTML (ΔE = 1.91 ± 0.23), Ceramill Zolid FX UT (ΔE = 1.52 ± 0.43), and least change in color was Cercon XT (ΔE = 1.44 ± 0.25).

It is essential to evaluate the visual color difference thresholds to guide the selection of esthetic dental materials which, in turn, has a significant impact on the esthetic outcome of the restoration.[15],[16] There are several factors which affect the color stability of Zirconia, such as ceramic glaze, polishing, duration and method of sintering, and color application.[17],[18],[19] During sintering when temperature-time is increased, the particles condense together and porosity on the boundaries are decreased, thus forming a well-organized crystalline structure that allows light reflection.[17],[20] This reflective variance may be the main factor affecting ΔE.

The proposed reason for the low ΔE of third-generation Zirconia was that it had a significant increase in the yttrium content, which increased the amount of cubic phase by approximately 50% and the proportion of alumina was lower. Furthermore, increasing the sintering temperature of the cubic Zirconia played a major role in increasing its optical properties.[21] In addition, low color changes were also attributed to minimum the surface degradation caused by the thermal cycles.[22]

Regarding the aging effect on color stability, our findings are in agreement with a previous study conducted by Alp et al.[13] where the externally shaded of new-generation translucent Zirconia had a significant color change after coffee AAA than the preshaded translucent Zirconia in monolithic or veneered specimens. However, color changes in both materials were within the clinically acceptable limits. In another study conducted by Derafshi et al.,[23] color measurements of dental direct cube x2 (third generation) compared to VITA VMK 95 as feldspathic porcelain were measured following immersion in chlorhexidine mouthwash, Listerine®, and distilled water. The results showed there was a statistically significant change in ΔE between the three solutions (all P < 0.001). The mean ΔE values were the highest in chlorhexidine, followed by Listerine®, and distilled water.

In contrast, the study results are not in agreement with Papageorgiou-Kyrana et al.[22] where they evaluated the effect of AAA on the color stability of monolithic Zirconia. Discs were prepared from Zirconia (BruxZir) either preshaded or characterized. The results showed that no statistically significant ΔE (P > 0.05) was observed among the different groups after thermocycling. All ΔE values were below the limit of the 3.7 that an untrained observer can perceive. In another study by Sulaiman et al.,[24] they demonstrated that fully stabilized Zirconia absorbs more color than partially stabilized due to microstructure changes and grain boundary dimensions.


  Conclusion Top


Within the limitations of thisin vitro study, the following can be concluded:

  1. The color change was significantly higher in the first-generation Zirconia than third-generation Zirconia used in the current study
  2. The color changes in all four tested Zirconia, was not a clinically perceivable change, wherein ΔE was <3.3.


Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4]
 
 
    Tables

  [Table 1], [Table 2], [Table 3]



 

 
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Introduction
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