An In Vitro Study to Evaluate the Effect of Artificial Aging on Translucency, Contrast Ratio, and Color of Zirconia Dental Ceramic at Different Sintering Levels
by
, , , , ,
Mallika Shetty
1, 
Saurabh Jain
2,*,
Tushar Milind Wankhede
3,
Mohammed E. Sayed
2,4
,
Zahid Mohammed
1,
Sanath Shetty
1,
Mohammed Hussain Dafer Al Wadei
5,
Saeed M. Alqahtani
6,
Ahlam Abdulsalam Ahmed Othman
7,
Mashael Adullah Alnijaiban
8,
Alhanouf K. Alnajdi
9,
Tariq Ibrahim Akkam
10,
Saad Saleh AlResayes
11,
Abdulkarim Hussain Alshehri
2 and
Fawzia Ibraheem Shaabi
2 1
Department of Prosthodontics, Yenepoya Dental College, Mangaluru 575018, India
2
Department of Prosthetic Dental Sciences, College of Dentistry, Jazan University, Jazan 45142, Saudi Arabia
3
Mahalaxmi Dental Laser and Implant Center, Kolhapur 416003, India
4
Rutgers School of Dental Medicine, Rutgers University, Newark, NJ 07103, USA
5
Department of Restorative Dental Science, College of Dentistry, King Khalid University, Abha 62529, Saudi Arabia
6
Department of Prosthetic Dentistry, College of Dentistry, King Khalid University, Abha 62529, Saudi Arabia
7
Department of Fixed prosthodontics, Faculty of Dentistry, Sana’a University, Sana’a 421302, Yemen
8
Ministry of Health, Hospital Administrations, Bisha 67714, Saudi Arabia
9
College of Dentistry, Jazan University, Jazan 45142, Saudi Arabia
10
Chief Resident Prosthodontics Program, Rayyan Hospital, Riyadh 14212, Saudi Arabia
11
Department of Prosthetic Dental Sciences, College of Dentistry, King Saud University, Riyadh 11545, Saudi Arabia
*
Author to whom correspondence should be addressed.
Coatings 2022, 12(5), 642; https://doi.org/10.3390/coatings12050642
Submission received: 3 April 2022
/
Revised: 3 May 2022
/
Accepted: 5 May 2022
/
Published: 8 May 2022
(This article belongs to the Special Issue Surface Properties of Dental Materials and Instruments)
Abstract
:Increasing demands for aesthetically pleasing dental restorations have promoted the use of materials that display superior optical properties. Zirconia-based all-ceramic systems have good optical properties, thus providing the desired esthetics for dental restorations. The altered oral conditions impact the physical properties of these materials. Multiple studies have been conducted to evaluate the effect of aging on the mechanical properties of computerized-aided design and computerized-aided manufacturing (CAD/CAM)-based zirconia; however, there is a scarcity of literature discussing the effect of aging on change in translucency, contrast ratio, and color. Therefore, this research aimed to evaluate the effect of accelerated artificial aging on translucency parameter (TP), contrast ratio (CR), and color of CAD/CAM zirconia at different sintering temperatures. Twenty-eight rectangular-shaped specimens were obtained by CAD/CAM milling of zirconia blank. Sintering of the samples was carried out at four different temperatures 1350 °C, 1400 °C, 1450 °C, and 1500 °C, respectively. Thermocycling of the samples was performed in a thermocycler. TP, CR, and color evaluation of all the samples was done again using a Spectrophotometer. Data acquired were statistically evaluated by one-way ANOVA and Post Hoc test. The highest amount of change in TP was recorded for specimens sintered at 1400 °C (1.86), whereas the least change was attributed to 1350 °C (0.51). The highest change in CR was observed in specimens sintered at 1500 °C (0.0971), and the least change in CR was observed at 1450 °C (0.0086). The highest ΔE values were recorded for 1400 °C against both white (1.86) and black background (2.17), whereas the least change was attributed to 1350 °C against white (0.86) and against a black background (1.41). The changes in TP were significant, but CR did not show any significant change. ΔE values for all the experimental groups were significant, but were below the clinically perceptible range. An increase in sintering temperature increased the translucency.
Keywords:
zirconia; ceramic; color stability; esthetics; spectrophotometry; translucency; contrast ratio; sintering; aging1. Introduction
With the advent of zirconia, as a form of restorative material along with computerized-aided design and computerized-aided manufacturing (CAD/CAM) technology, the applicative horizons for prosthetic rehabilitation in different clinical scenarios have widened.
Zirconia-based all-ceramic restorations have adequate optical and superior mechanical properties compared with conventional feldspathic porcelain crowns (PJCs) [1,2,3,4,5,6]. Veneering ceramic is not required for all-ceramic zirconia-based posterior restorations; thus, less tooth preparation is required for such restorations, chances of chipping of ceramic are minimized, and no heat treatment is needed, which may deteriorate the mechanical properties of zirconia [7].
The heating rate, sintering temperature and duration, source of stabilizing oxides, and heating source, of ceramic materials can strongly affect the final mechanical properties of zirconia restorations [8]. However, the literature lacks substantial information on the influence of the ceramic processing parameters on the optical properties of full-contour zirconia materials.
An aesthetically pleasing restoration should fulfill all the criteria concerning shape, size, surface texture, and most importantly, shade and translucency. The higher level of translucency provided by the zirconia-based all-ceramic system which lacks metal substructure, proves advantageous in providing better control over the aesthetics [9,10]. Understanding the mechanism of translucency parameters is important for achieving the desired esthetics but many researchers neglect this parameter.
Translucency is the relative amount of passage of light through an object [11]. The natural looks and liveliness of restorations depends upon the translucency of dental materials used. Besides imitating the color, it is vital to imitate the translucency of the natural teeth to attain favorable esthetic outcomes [12].
The color and appearance of teeth is a complex phenomenon that includes many factors [13,14,15]. Natural teeth are not of a uniform structure and are characterized by different colors and grades of translucency from the cervical to the incisal part.
Relation of incident light wavelength and number and size of dental ceramic particles will govern the translucency of the material [16,17,18]. Due to varying optical densities, mixed white light, which falls on the ceramics surface, gets reflected and refracted (pass-through) proportionately. Restoration will be transparent if more light is refracted (passes through) and lesser amount is reflected. The higher the TP Value of the material, the higher its translucency will be. Translucency (TP value) will be near zero for the opaque materials [19,20].
Translucency is affected by several factors like grain size, grain distribution, processing method, type of cement and their thickness, saliva, dietary products, sintering temperature, number of firing cycles, zirconia-based all-ceramic system brand, and thickness. Although many agree that translucency is a crucial component for ceramic restorations, limited quantitative literature is available.
Multiple studies have been conducted to assess the effect of aging on the mechanical properties of CAD/CAM-based zirconia; however, there is a scarcity of literature discussing the effect of aging on change in translucency, contrast ratio, and color. The altered conditions and the inter-individual changes in the composition of saliva may impact the optical properties of the material.
Zirconia based all-ceramic systems have superior mechanical properties (Biaxial strength, Fracture toughness, Hardness, and Modulus of elasticity) when compared to glass-ceramics, which are the material of choice in esthetic zones due to high translucency [3,4,21]. Glass matrix comprising dispersed crystalline minerals (feldspar, silica, alumina) are the principal constituent of glass-ceramics (feldspathic, leucite-reinforced glass-ceramics lithium disilicate glass-ceramics) [1]. Mechanical properties (as stated by the manufacturer) of Zolid HT plus used in the current study are: Bending Strength: 1100 ± 150 MPa, Modulus of Elasticity: ≥200, Vickers Hardness: 1300 ± 200, Coefficient of thermal expansion 25–500 °C: 10.4 ± 0.5, Chemical Solubility: <100.
Contrast ratio and color difference against the black and white background describes the material’s translucency [22]. CR can be calculated according to the following formula: CR = Yb/Yw, as the ratio of luminous reflectance of light through the test specimen against a black background (Yb, low reflectance) to that against a white background (Yw, high reflectance). Fully transparent materials have a CR of 0, while opaque materials have a CR of 1 [16].
Lughi and Sergo [23] defined low temperature degradation or ageing as spontaneous tetragonal to monoclinic phase (t-m) transformation occurring overtime at low temperature, when the t-m transformation is not triggered by the stresses produced at the tip of an advancing crack. Phase transformation toughening (PTT) is not possible after being transformed to monoclinic phase. Aging is reported to cause an increase in surface roughness due to the loss of small surface zirconia particles [23,24,25].
Previous studies have evaluated the effect of aging on the mechanical properties and usability of the material. The current literature lacks the relevant information related to the effect of aging on aesthetic and translucency changes. Therefore, this study aimed to evaluate the effect of accelerated artificial aging on translucency parameter (TP), contrast ratio (CR), and color of CAD/CAM zirconia at different sintering temperatures. Desirable optical properties for zirconia-based all-ceramic systems include higher contrast ratio, high translucency, and minimal change in color after aging. The null hypothesis tested is that there will be no statistically significant difference in change in the translucency, contrast ratio, and color of zirconia sintered at different temperatures, after artificial aging.
2. Materials and Methods
2.1. Materials
A total of twenty-eight rectangular-shaped zirconia specimens of the uniform dimension of 10 mm breadth × 12 mm length × 1 mm thickness were fabricated using CAD/CAM subtractive technique. The commercial name and details of the material used in the study are listed in Table 1.
2.2. Specimen Preparation
A total of 28 zirconia specimens were fabricated. A minimum sample size of 28 (7 in each group) was found to be appropriate for an alpha of 0.05, power of 80%, and effect size of 0.80. G*Power software (version 3.1.9.4, 2019, Heinrich Heine University Düsseldorf, Düsseldorf, Germany) was used to determine the sample size assuming a four-group comparison.
Based on different sintering temperatures, the specimens were divided into four experimental groups consisting of seven specimens each. Temperatures used for sintering were 1350 °C, 1400 °C, 1450 °C and 1500 °C. The ideal sintering temperature recommended by the manufacturer was 1450 °C, which was kept as the control. The heating rate was 8 °C, and the holding time was about 2 h.
2.3. Designing and Subtractive Manufacturing
A stereolithography (STL) file of the proposed specimen was virtually designed using TinkerCAD software (software tool for 3D designing, version 1.3, Autodesk, Inc., San Rafael, CA, USA) with 10 mm breadth, 12 mm length, and 1 mm thickness, thereby reproducing a rectangle shape (Figure 1).
The STL file was sent to the CAM software (Ceramill mind software. Version 3.0, Engine build 7783, Think vision, Herrschaftswiesen, Koblach, Austria).
To compensate for the shrinkage, oversized samples were planned, according to the manufacturer’s recommendations (ranging from 24.5–25%) [26]. This design was nested onto a 16 mm Ceramill zirconia blank (Figure 2). Dry milling of this sample design was done using a milling machine (AmannGirbachCeramill motion2, manufactured-2014, Serial No. AAB75621, Herrschaftswiesen, Koblach, Austria) (Figure 3).
Once the milling was completed, the collected zirconia samples were de-dusted. The sides having the mark of sprue attachments were polished using abrasives. The samples were then placed on the zirconia holding tray and placed inside the sintering furnace (Zircom, KDF U.S., INC., Signal Hill, CA, USA). Four different sintering temperatures were implemented with a difference of 50 °C amongst each temperature. A total sintering cycle of 6 h and 10 min was kept constant for all the samples. The rate of rising and fall in temperature with a holding period was specific for each temperature. The procedure was repeated for other groups, respectively. Samples were cleaned ultrasonically with 80% ethanol solution (Figure 4).
2.4. Segregation of Samples
Total of twenty-eight specimens were segregated as Group A (1350 °C), Group B (1400 °C), Group C (1450 °C), and Group D (1500 °C) according to different sintering temperatures they were subjected to.
2.5. Recording of Initial Colorimetric Data in Yxy and CIELAB (“L*a*b*”) Color Space Values
All the specimens were evaluated for the Yxy (The Y value in the Yxy color space represents the illuminance, where x is the value of hue and y is the value of chroma [27]) and CIELAB (“L*a*b*”) color space (three values: L* for perceptual lightness, and a* and b* for the four unique colors of human vision: red, green, blue, and yellow) using an integrating-sphere reflective light-recording spectrophotometer (i1PRO SPECTROPHOTOMETER—X-Rite, Grand Rapids, MI, USA). Specimens were kept at the center of the measuring port using 45°/0° geometry with CIE illuminant D50 and 2° observer function against a standard reference of white (L = 94.00, a = −1.60, b = 0.20) and black background (L = 22.8, a = 0.4, b = 0.4) to calculate the required coordinates. The spectrophotometer was calibrated according to the manual to maintain the standardization of the procedure. The device was placed centrally onto the specimen such that it was in complete contact with the specimen surface. Measurements were taken against a white and a black background three successive times, and mean values were recorded. This accounted for the initial readings (T0) before subjecting the samples to accelerated artificial ageing.
2.6. Thermocycling
To imitate the oral environment, thermocycling of zirconia samples was done. The samples were first kept in a Petri dish containing artificial salivary substitute for seven days and then exposed to 5000 thermal cycles between 5 °C and 55 °C water temperature with a dwell time of 30 s per bath in the thermocycler (Model 1100, SD Mechatronik, Bayern, Germany). After thermocycling, the specimens were again segregated according to the different sintering temperatures.
2.7. Recording of Final Colorimetric Data in Yxy and CIELAB (“L*a*b*”) Color Space Values
Final Yxy and CIELAB (“L*a*b*”) color space values (T1) were recorded after aging was completed using the same protocol for initial testing. Measurements were taken against a white and a black background three successive times, and the mean values of each specimen were recorded.
2.8. Calculation of Translucency Parameter
The numerical values Translucency parameter was calculated according to Formula (1) [19,27,28,29].
where L represents lightness of the color, a represents its position between red and green, b represents its position between yellow and blue, the subscript (b) refers to the color coordinate against a black background, while the subscript (w) refers to the color coordinate against white background [10]. Change in TP (ΔTP) was calculated by subtracting the TP values obtained before and after artificial aging.
TP = [(L*b − L*w)2 + (a*b − a*w)2 + (b*b − b*w)2]1/2
2.9. Calculation of Contrast Ratio
CR was calculated according to Formula (2).
CR = Yb/Yw
It is the ratio of luminous reflectance of light through the test specimen against a black background (Yb, low reflectance) to that against a white background (Yw, high reflectance). CR of 0 represents totally transparent materials, while CR of 1 represents totally opaque materials [23]. The change in contrast ratio (ΔCR) was calculated by subtracting the CR values obtained before and after artificial aging.
2.10. Calculation of Change in Color
Change in color ΔE was calculated by Formula (3) [30].
where subscript 2 represents final color coordinated values and 1 represents initial color coordinated values recorded over a white background.
ΔE*ab = [(L*2 − L*1)2 + (a*2 − a*1)2 + (b*2 − b*1)2]1/2
2.11. Data Analysis
The collected data were tabulated in a Microsoft Excel spreadsheet (version 1910, 2019, Microsoft Inc., Redmond, WA, USA), SPSS version 24.0 (IBM SPSS Statistics for Windows, Version 24.0., 2016, IBM Corp., Armonk, NY, USA) was used for analyzing the data statistically. One-way ANOVA was conducted to check for the significant difference of translucency between the test groups. Tukey’s post hoc test was performed to check for intergroup significance of translucency parameter and contrast ratio.
3. Results
The mean of color, translucency parameter (TP), contrast ratio (CR), and intergroup comparison between four groups before and after artificial aging is presented in Table 2 and Table 3, respectively. The results showed a significance in the p-value between the groups. The amount of change in the translucency after subjecting to artificial aging was also significant.
When ΔE values were compared pre-aging and post-aging, there was a gradual rise in the change as temperature is increased from 1350 °C to 1400 °C. The highest amount of change was recorded for 1400 °C against both white and black backgrounds compared to other groups. 1400 °C showed 1.86 (against white backing) and 2.17 (against black backing), followed by 1450 °C (1.81—against white & 2.06—against black), which was the control group. The least change was attributed to 1350 °C followed by 1500 °C (1.79—against white& 1.71—against black). There was a significant change in the ΔE values after accelerated artificial aging in all the groups, but the change was considered perceptibly insignificant.
Table 4 shows the ΔE and L coordinate of the L*a*b* system showing a statistically significant difference against a white background at 1350 °C, while a* and b* coordinates did not show any significant changes. The L*, a*, and ΔE showed significant differences against the black background. There was no significant change observed in the contrast ratio and translucency parameter. Scheme 1 shows paired comparison at time 0 (initial) and time 1 (final) at temperature 1350 °C
Table 5 shows the ΔE, L, and a* coordinate of L*a*b* system showing statistically significant differences against a white background at 1400 °C while b* coordinate did not show any significant changes. The L*, b*, and ΔE showed significant differences against the black background. A statistically significant change was observed in contrast ratio and translucency parameter in this group. Scheme 2 shows paired comparison at time 0 and time 1 at a temperature of 1400 °C.
Table 6 shows the ΔE, L, and b* coordinate of L*a*b* system, showing a statistically significant difference against a white background at 1450 °C, while a* coordinates did not show any significant changes. The L*, a* b*, and ΔE showed significant differences against the black background. No significant change was observed in this group for the contrast ratio, but the translucency parameter showed a significant difference. Scheme 3 shows paired comparison at time 0 and time 1 at a temperature of 1450 °C.
Table 7 shows the ΔE and L coordinates of the L*a*b* system showing statistically significant differences against the white background at 1500 °C, while a* and b* coordinates did not show any significant changes. The L*, b*, and ΔE showed significant differences against the black background. There was a significant change observed in the contrast ratio. No significant change was observed in the translucency parameter. Scheme 4 shows paired comparison at time 0 and time 1 at a temperature of 1500 °C.
4. Discussion
This research investigated the effects of accelerated artificial ageing on translucency parameter (TP), contrast ratio (CR) and color of CAD/CAM zirconia at different sintering temperatures. A significant difference in TP and color was observed when the four groups were compared. Therefore, the null hypothesis can be rejected; however, CR did not show significant change, so the null hypothesis for CR was accepted. The extent of these changes varied according to sintering temperatures.
Optimizing the esthetics in zirconia-based restorations is necessary to foretell all-ceramic’s optical behavior. Wang et al. [16] evaluated the Translucency parameter of some dental ceramics and found some degree of translucency in zirconia ceramics, which were less sensitive to thickness than glass-ceramics. The material used in this study was Ceramill Zolid HT+, which has combined mechanical and excellent aesthetic characteristics with good translucency. At 1100 MPa, the strength lies in the same range as Zolid; however, the aesthetics well surpass that of Zolid, which is its predecessor. This was the reason for selecting Zolid HT+ for the study.
The property of translucency depends on the size of particles, the volume of crystals, chemical components, and sintered densities, and they affect the quantity of light that is absorbed, reflected, or transmitted [31]. When the light passes, smaller crystals have higher transmittance with less refraction and less absorption. The type of material also has a positive influence on the translucency along with the thickness. A decrease in thickness will increase the translucency and the amount of pre-sintering, the grain size of the material determines the translucency. The monochromatic structure of the slab-shaped ceramic specimens of 1 mm thickness, used in the present study differs from the tooth and ceramic restoration; but it is the commonest shape of specimen employed in previous in vitro studies [31]. The specimens were fabricated with a standard dimension of 10 × 12 × 1 mm3.
Opacity is increased due to the scattering of light caused by the increased density of particles. Antagonistically, opacity can decrease due to less scattering of light caused by greater particle materials which results in a reduction of crystals per unit volume [26].
The polycrystalline nature of zirconia can be the main cause of such an outcome. When a light beam passes through a material, crystals within the microstructure of the substance interfere with the light passage, deflecting the light beam and leading to increased scattering. This increased scattering and decreased light penetration lead to the less translucent appearance of a more crystalline structure.
Grain size affects the optical properties of monolithic zirconia [32]. There are two theories for this, (a) larger the grain size, the fewer will be the grain boundaries, thus the higher the translucency [33,34,35,36], (b) for tetragonal zirconia, larger grain size is correlated to a reduction in translucency [37,38,39] attributed to the birefringent nature of tetragonal zirconia crystal. Higher sintering temperatures are reported to increase the translucency due to an increase in grain size, and an increase in density [33]. The grain size of Ceramill Zolid is reported to be 0.088 ± 0.004 micrometers [40].
In the present study, CIE L*a* b* analysis was performed, and ΔE (color difference) was calculated. With a rise in the temperature by every 50 °C, the ΔE increased progressively in the pre-accelerated aging phase (T0). However, the overall color change obtained after comparing the post accelerated aging (T1) ΔE values were within the perceptible limits. All the ΔE values obtained were below the clinically perceptible range (ΔE < 3.3) [41]. ΔE < 3.3 is considered clinically imperceptible, ΔE between 3.0–5.0 is considered clinically acceptable, and ΔE > 5.0 is considered clinically unacceptable [31].
The present study showed the highest amount of changes in the 1400 °C (1.86-against white and 2.17—against the black background) followed by 1450 °C (1.81—against white, 2.06—against black). The 1500 °C interestingly showed a decrease in the amount of change following a rise in the temperature from 1450 °C to 1500 °C. This is in accordance with a study conducted by Jiang et al. [33] and Gómez S et al. [42], where the authors stated that the transmittance increases, porosity decreases, and density increases with an increase in sintering temperature at 1500 °C, where the relative density increased from 50% to 99% and null porosity at sintering temperature 1500 °C. As the temperature increases, there is reduction in defects, and porosities while the zirconia crystal becomes denser [33]. An increase in temperature increases translucency. Therefore, TP can be changed according to the requirements, within the permissible limits recommended by the manufacturer.
An increase in translucency and a negative change in color may be attributed to oxidation, which would have taken place at such high temperatures as sintering was accomplished without an argon atmosphere. This reasoning is in accordance with a study by Zhang et al. [43].
In the present study, when the effects of accelerated aging on color change of monolithic zirconia were investigated using an artificial accelerated aging process by thermocycling at 5 °C to 55 °C for 5000 cycles, the amount of color change in group B (1400 °C) was still within the limit of clinical acceptability, it surpassed the level of perceptibility. It showed the largest color shift among the four groups. The ability of dental zirconia to withstand the oral environment changes and maintain its properties remains an issue of concern. Compared to the more recent fully stabilized cubic zirconia, the lower translucency anticipated from partially stabilized tetragonal zirconia is not the only point of concern. Phase transformation does have a significant impact on the microstructure, affecting the translucency. Another concern in the dental literature regarding the partially stabilized tetragonal zirconia is its hydrothermal aging susceptibility and the ability of this zirconia to maintain its properties over time when serving in tough oral conditions [44]. The reason can also be attributed to the structural difference and differences in the chemical composition of the material. Zolid HT+ is partially stabilized zirconia, which means it has less concentration of yttria as a stabilizing oxide [41]. The probable reasons that can be attributed to the amount of change seen with accelerated artificial aging at different sintering temperatures can be summarized as follows:
- Less yttria content in partially stabilized zirconia with non-prevalent cubic lattice phase.
- Non-isotropic and birefringent nature of cubic crystals makes for an unfavorable passage light pathway through Zolid HT (PSZ) compared to FSZ.
- Spontaneous tetragonal to monoclinic transformation can be a possible cause for the material’s change after artificial aging. This is in accordance to Alraheam et al. [41]
The TP showed a significant increase with an increase in the sintering temperature, which is in agreement with studies by Sabet et al. [45], but the aging has a significant impact on TP values. There was an increase in TP from 1350–1400 °C, followed by a decrease in TP as the temperature increased to 1450 °C and 1500 °C. This can be conclusive that translucency parameter decreases after artificial ageing for higher temperatures. Aging usually affects the surface of zirconia specimens, and the surface only contains a greater percentage of zirconia grain volume. Therefore, changes in microstructure caused by aging, such as; the release of small zirconia grains, surface uplifts, grains pull-out, and surface roughening; have a pronounced effect. These findings were in accordance with Lughi et al. [23], but in contrast with those of Alamledin et al. [46]. In the present study, accelerated aging did not have a significant impact on the contrast ratio. The changes in CR were statistically significant, but clinically imperceptible, as they were below the human eye threshold (0.06 to 0.07). This finding is in accordance with Walczak et al. [46].
5. Conclusions
Within the limitations of this study, the following conclusions can be drawn:
- After the samples were subjected to different sintering temperatures, it was observed that the delta E values changed.
- The color difference did not occur when stored in artificial salivary substitute for the specified time according to the standardized protocols.
- Thermocycling affects the translucency of samples; thus showing a significant difference in the translucency among the groups, and a difference was observed when pre- and post-thermocycling results were evaluated.
- Even though the changes observed were statistically significant, they were considered well within the clinically perceptible value and, therefore, clinically imperceptible.
- An increase in sintering temperature causes a significant increase in the TP, which reduces with aging. At the time of fabrication of the prosthesis, sintering temperatures can be changed according to the requirement, within the limit, to achieve the best clinical outcome.
Author Contributions
Conceptualization, M.S., S.J., S.S. and M.E.S.; methodology, T.M.W., Z.M., M.H.D.A.W., S.M.A., A.K.A., S.S.A. and F.I.S.; software, A.A.A.O., M.H.D.A.W., T.M.W., M.A.A., T.I.A., A.H.A. and F.I.S.; validation, S.J., M.E.S., Z.M., S.S., A.A.A.O., M.A.A., T.I.A. and F.I.S.; formal analysis, M.S., S.S.A. and S.J.; investigation, M.S., T.M.W., Z.M., M.A.A., A.K.A., T.I.A. and S.S.A.; resources, M.E.S., M.H.D.A.W., S.M.A., M.A.A., A.K.A., T.I.A. and S.S.A.; data curation, M.S., S.J., S.M.A., A.A.A.O. and A.H.A.; writing—original draft preparation, M.S., T.M.W., S.J. and Z.M.; writing—review and editing, M.S., T.M.W., M.E.S., S.J., Z.M., S.S., M.H.D.A.W., S.M.A., A.A.A.O., M.A.A., A.K.A., T.I.A., S.S.A., A.H.A. and F.I.S.; visualization, S.J., S.S., A.A.A.O., A.K.A., S.S.A., A.H.A. and F.I.S.; supervision, M.S., M.E.S., S.J. and S.S.; project administration, T.M.W., Z.M., M.H.D.A.W., S.M.A., A.H.A. and F.I.S. All authors have read and agreed to the published version of the manuscript.
Funding
This research received no external funding.
Institutional Review Board Statement
Not applicable.
Informed Consent Statement
Not applicable.
Data Availability Statement
The data that support the findings of this study are available from the corresponding author upon reasonable request.
Conflicts of Interest
The authors declare no conflict of interest.
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Figure 1.
Sample design (10 × 12 × 1 mm3) made in TinkerCAD software.
Figure 2.
Design nested onto 16 mm Ceramill zirconia blanks.
Figure 3.
Dry milling of the design was done using a milling machine.
Figure 4.
Specimens prepared by milling.
Scheme 1.
Paired comparison at time 0 and time 1—Temperature 1350 °C.
Scheme 2.
Paired comparison at time 0 and time 1—Temperature 1400 °C.
Scheme 3.
Paired comparison of at time 0 and time 1—Temperature 1450 °C.
Scheme 4.
Paired comparison at time 0 and time 1—Temperature 1500 °C.
Table 1.
Commercial name and detail of the material used in the study.
| Material Trade Name | Manufacturer | Chemical Composition |
|---|---|---|
| Ceramill Zolid HT+ | AmannGirbachCeramill® CAD/CAM zirconia blank (16 mm) Amann Girrbach AG, Herrschaftswiesen, Koblach, Austria |
|
Table 2.
Mean of color, translucency parameter (TP), contrast ratio (CR), and intergroup comparison between four groups before artificial aging.
Table 2.
Mean of color, translucency parameter (TP), contrast ratio (CR), and intergroup comparison between four groups before artificial aging.
| N | Mean | Std. Deviation | 95% Confidence Interval for Mean | Significance | |||
|---|---|---|---|---|---|---|---|
| Lower Bound | Upper Bound | ||||||
| LWT0 | 1350 °C | 7 | 95.529 | 0.7931 | 94.795 | 96.262 | F = 2597.805 p-value < 0.001 Significant |
| 1400 °C | 7 | 85.471 | 0.3498 | 85.148 | 85.795 | ||
| 1450 °C | 7 | 80.471 | 0.3498 | 80.148 | 80.795 | ||
| 1500 °C | 7 | 73.186 | 0.2734 | 72.933 | 73.439 | ||
| Total | 28 | 83.664 | 8.2862 | 80.451 | 86.877 | ||
| aWT0 | 1350 °C | 7 | −0.214 | 0.2795 | −0.473 | 0.044 | F = 185.204 p-value < 0.001 Significant |
| 1400 °C | 7 | −0.843 | 0.0535 | −0.892 | −0.793 | ||
| 1450 °C | 7 | −1.443 | 0.0535 | −1.492 | −1.393 | ||
| 1500 °C | 7 | −2.043 | 0.0976 | −2.133 | −1.953 | ||
| Total | 28 | −1.136 | 0.7077 | −1.410 | −0.861 | ||
| bWT0 | 1350 °C | 7 | 1.4429 | 0.78921 | 0.7130 | 2.1728 | F = 203.577 p-value < 0.001 Significant |
| 1400 °C | 7 | −0.8714 | 0.13801 | −0.9991 | −0.7438 | ||
| 1450 °C | 7 | −1.8100 | 0.14387 | −1.9431 | −1.6769 | ||
| 1500 °C | 7 | −3.8714 | 0.07559 | −3.9413 | −3.8015 | ||
| Total | 28 | −1.2775 | 1.98187 | −2.0460 | −0.5090 | ||
| LBT0 | 1350 °C | 7 | 94.614 | 0.8533 | 93.825 | 95.403 | F = 2668.384 p-value < 0.001 Significant |
| 1400 °C | 7 | 81.043 | 0.5127 | 80.569 | 81.517 | ||
| 1450 °C | 7 | 74.043 | 0.5127 | 73.569 | 74.517 | ||
| 1500 °C | 7 | 67.614 | 0.3891 | 67.254 | 67.974 | ||
| Total | 28 | 79.329 | 10.2211 | 75.365 | 83.292 | ||
| aBT0 | 1350 °C | 7 | −0.457 | 0.2225 | −0.663 | −0.251 | F = 242.951 p-value < 0.001 Significant |
| 1400 °C | 7 | −1.300 | 0.0000 | −1.300 | −1.300 | ||
| 1450 °C | 7 | −1.729 | 0.0756 | −1.798 | −1.659 | ||
| 1500 °C | 7 | −2.271 | 0.1113 | −2.374 | −2.169 | ||
| Total | 28 | −1.439 | 0.6866 | −1.706 | −1.173 | ||
| bBT0 | 1350 °C | 7 | 0.957 | 0.7829 | 0.233 | 1.681 | F = 305.702 p-value < 0.001 Significant |
| 1400 °C | 7 | −2.343 | 0.1272 | −2.461 | −2.225 | ||
| 1450 °C | 7 | −3.357 | 0.1397 | −3.486 | −3.228 | ||
| 1500 °C | 7 | −5.457 | 0.0787 | −5.530 | −5.384 | ||
| Total | 28 | −2.550 | 2.3886 | −3.476 | −1.624 | ||
| CR T0 | 1350 °C | 7 | 1.0186 | 0.00378 | 1.0151 | 1.0221 | F = 1577.545 p-value < 0.001 Significant |
| 1400 °C | 7 | 1.1500 | 0.00000 | 1.1500 | 1.1500 | ||
| 1450 °C | 7 | 1.2243 | 0.00976 | 1.2153 | 1.2333 | ||
| 1500 °C | 7 | 1.2114 | 0.00690 | 1.2050 | 1.2178 | ||
| Total | 28 | 1.1511 | 0.08319 | 1.1188 | 1.1833 | ||
| TP T0 | 1350 °C | 7 | 1.0741 | 0.1371 | 0.9473 | 1.2010 | F = 1198.764 p-value < 0.001 Significant |
| 1400 °C | 7 | 4.6896 | 0.2313 | 4.475 | 4.9035 | ||
| 1450 °C | 7 | 6.6222 | 0.2271 | 6.412 | 6.832 | ||
| 1500 °C | 7 | 5.797 | 0.1259 | 5.681 | 5.914 | ||
| Total | 28 | 4.5459 | 2.1645 | 3.706 | 5.385 | ||
W: white background, B: black background, T0: Initial reading, CR: contrast ratio, TP: Translucency parameter.
Table 3.
Mean of color, translucency parameter (TP), contrast ratio (CR), and intergroup comparison between four groups after artificial ageing.
Table 3.
Mean of color, translucency parameter (TP), contrast ratio (CR), and intergroup comparison between four groups after artificial ageing.
| N | Mean | Std. Deviation | 95% Confidence Interval for Mean | Significance | |||
|---|---|---|---|---|---|---|---|
| Lower Bound | Upper Bound | ||||||
| LWT1 | 1350 °C | 7 | 93.043 | 0.9484 | 92.166 | 93.920 | F = 1963.333 p-value < 0.001 Significant |
| 1400 °C | 7 | 83.457 | 0.1512 | 83.317 | 83.597 | ||
| 1450 °C | 7 | 77.486 | 0.3671 | 77.146 | 77.825 | ||
| 1500 °C | 7 | 70.943 | 0.4467 | 70.530 | 71.356 | ||
| Total | 28 | 81.232 | 8.2954 | 78.016 | 84.449 | ||
| aWT1 | 1350 °C | 7 | −0.257 | 0.0535 | −0.307 | −0.208 | F = 492.617 p-value < 0.001 Significant |
| 1400 °C | 7 | −0.686 | 0.0690 | −0.750 | −0.622 | ||
| 1450 °C | 6 | −1.367 | 0.0816 | −1.452 | −1.281 | ||
| 1500 °C | 7 | −1.986 | 0.1345 | −2.110 | −1.861 | ||
| Total | 27 | −1.063 | 0.6862 | −1.334 | −0.792 | ||
| bWT1 | 1350 °C | 7 | 1.257 | 0.2637 | 1.013 | 1.501 | F = 10.041 p-value < 0.001 Significant |
| 1400 °C | 7 | −0.714 | 0.0900 | −0.797 | −0.631 | ||
| 1450 °C | 7 | −1.671 | 0.1604 | −1.820 | −1.523 | ||
| 1500 °C | 7 | −2.171 | 2.5078 | −4.491 | 0.148 | ||
| Total | 28 | −0.825 | 1.7898 | −1.519 | −0.131 | ||
| Delta EW | 1350 °C | 7 | 0.8857 | 0.3436 | 0.5678 | 1.2035 | F = 21.827 p-value < 0.001 Significant |
| 1400 °C | 7 | 1.857 | 0.1511 | 1.7173 | 1.9969 | ||
| 1450 °C | 7 | 1.814 | 0.1772 | 1.6503 | 1.9782 | ||
| 1500 °C | 7 | 1.785 | 0.3287 | 1.4816 | 2.0897 | ||
| Total | 28 | 1.585 | 0.4820 | 1.3987 | 1.7726 | ||
| LBT1 | 1350 °C | 7 | 91.657 | 0.3645 | 91.320 | 91.994 | F = 6565.305 p-value < 0.001 Significant |
| 1400 °C | 7 | 77.014 | 0.1574 | 76.869 | 77.160 | ||
| 1450 °C | 7 | 71.586 | 0.1773 | 71.422 | 71.750 | ||
| 1500 °C | 7 | 67.000 | 0.5477 | 66.493 | 67.507 | ||
| Total | 28 | 76.814 | 9.4497 | 73.150 | 80.478 | ||
| aBT1 | 1350 °C | 7 | −0.229 | 0.2812 | −0.489 | 0.031 | F = 190.015 p-value < 0.001 Significant |
| 1400 °C | 7 | −1.300 | 0.0000 | −1.300 | −1.300 | ||
| 1450 °C | 7 | −1.300 | 0.0000 | −1.300 | −1.300 | ||
| 1500 °C | 7 | −2.214 | 0.1345 | −2.339 | −2.090 | ||
| Total | 28 | −1.261 | 0.7310 | −1.544 | −0.977 | ||
| bBT1 | 1350 °C | 7 | 0.714 | 0.1773 | 0.550 | 0.878 | F = 475.442 p-value < 0.001 Significant |
| 1400 °C | 7 | −1.686 | 0.1215 | −1.798 | −1.573 | ||
| 1450 °C | 7 | −2.186 | 0.1574 | −2.331 | −2.040 | ||
| 1500 °C | 7 | −4.729 | 0.4716 | −5.165 | −4.292 | ||
| Total | 28 | −1.971 | 1.9847 | −2.741 | −1.202 | ||
| Delta EB | 1350 °C | 7 | 1.4142 | 0.2035 | 1.2260 | 1.6025 | F = 14.701 p-value < 0.001 Significant |
| 1400 °C | 7 | 2.1714 | 0.1799 | 2.005 | 2.337 | ||
| 1450 °C | 7 | 2.0571 | 0.15118 | 1.9173 | 2.1969 | ||
| 1500 °C | 7 | 1.7142 | 0.3579 | 1.3832 | 2.0452 | ||
| Total | 28 | 1.8392 | 0.3764 | 1.6933 | 1.9852 | ||
| CR T1 | 1350 °C | 7 | 1.0357 | 0.02878 | 1.0091 | 1.0623 | F = 90.685 p-value < 0.001 Significant |
| 1400 °C | 7 | 1.2200 | 0.00816 | 1.2124 | 1.2276 | ||
| 1450 °C | 7 | 1.2157 | 0.00976 | 1.2067 | 1.2247 | ||
| 1500 °C | 7 | 1.1143 | 0.03780 | 1.0793 | 1.1492 | ||
| Total | 28 | 1.1464 | 0.08143 | 1.1149 | 1.1780 | ||
| TP T1 | 1350 °C | 7 | 1.5851 | 0.9292 | 0.72574 | 2.444 | F = 26.884 p-value < 0.001 Significant |
| 1400 °C | 7 | 6.5458 | 0.2334 | 6.329 | 6.761 | ||
| 1450 °C | 7 | 5.9597 | 0.3458 | 5.5968 | 6.3226 | ||
| 1500 °C | 7 | 5.009 | 1.9562 | 3.2000 | 6.8184 | ||
| Total | 28 | 4.7311 | 2.2446 | 3.8431 | 5.6190 | ||
W: white background, B: black background, T1: Final reading, CR: contrast ratio, TP: Translucency parameter.
Table 4.
Paired sample statistics (1350 °C).
| Mean | N | Std. Deviation | Mean Difference | ||
|---|---|---|---|---|---|
| Pair 1 | LWT0 | 95.529 | 7 | 0.7931 | 2.4857 |
| LwT1 | 93.043 | 7 | 0.9484 | p-value = 0.000 * | |
| Pair 2 | awT0 | −0.214 | 7 | 0.2795 | 0.0429 |
| aWT1 | −0.257 | 7 | 0.0535 | p-value = 0.723 | |
| Pair 3 | bWT0 | 1.4429 | 7 | 0.78921 | 0.1857 |
| bWT1 | 1.257 | 7 | 0.2637 | p-value = 0.544 | |
| Pair 4 | ΔEW T0 | 1.300 | 7 | 0.4282 | 0.8857 |
| ΔEW T1 | 0.414 | 7 | 0.1069 | p-value = 0.000 * | |
| Pair 5 | LBT0 | 94.614 | 7 | 0.8533 | 2.9571 |
| LBT1 | 91.657 | 7 | 0.3645 | p-value = 0.000 * | |
| Pair 6 | aBT0 | −0.457 | 7 | 0.2225 | −0.2286 |
| aBT1 | −0.229 | 7 | 0.2812 | p-value = 0.000 * | |
| Pair 7 | bBT0 | 0.957 | 7 | 0.7829 | 0.2429 |
| bBT1 | 0.714 | 7 | 0.1773 | p-value = 0.406 | |
| Pair 8 | ΔEB T0 | 2.457 | 7 | 0.1988 | 1.4143 |
| ΔEB T1 | 1.043 | 7 | 0.1618 | p-value = 0.000 * | |
| Pair 9 | CR T0 | 1.0186 | 7 | 0.00378 | −0.0171 |
| CR T1 | 1.0357 | 7 | 0.02878 | p-value = 0.158 | |
| Pair 10 | TP T0 | 1.074 | 7 | 0.13711 | −0.5109 |
| TP T1 | 1.5851 | 7 | 0.9292 | p-value = 0.212 |
W: white background, B: black background, T0: Initial reading, T1: Final reading, CR: contrast ratio, TP: Translucency parameter, *: p-value significant at p < 0.05.
Table 5.
Paired sample statistics (1400 °C).
| Mean | N | Std. Deviation | Mean Difference | ||
|---|---|---|---|---|---|
| Pair 1 | LWT0 | 85.471 | 7 | 0.3498 | 2.0143 |
| LwT1 | 83.457 | 7 | 0.1512 | p-value = 0.000 | |
| Pair 2 | awT0 | −0.843 | 7 | 0.0535 | −0.1571 |
| aWT1 | −0.686 | 7 | 0.0690 | p-value = 0.005 | |
| Pair 3 | bWT0 | −0.8714 | 7 | 0.13801 | −0.15714 |
| bWT1 | −0.714 | 7 | 0.0900 | p-value = 0.062 | |
| Pair 4 | ΔEW T0 | 7.329 | 7 | 0.1976 | 1.8571 |
| ΔEW T1 | 5.471 | 7 | 0.0951 | p-value = 0.000 * | |
| Pair 5 | LBT0 | 81.043 | 7 | 0.5127 | 4.0286 |
| LBT1 | 77.014 | 7 | 0.1574 | p-value = 0.000 * | |
| Pair 6 | aBT0 | −1.300 | 7 | 0.0000 | - |
| aBT1 | −1.300 | 7 | 0.0000 | - | |
| Pair 7 | bBT0 | −2.343 | 7 | 0.1272 | −0.6571 |
| bBT1 | −1.686 | 7 | 0.1215 | p-value = 0.000 * | |
| Pair 8 | ΔEB T0 | 10.657 | 7 | 0.2820 | 2.1714 |
| ΔEB T1 | 8.486 | 7 | 0.2268 | p-value = 0.000 * | |
| Pair 9 | CR T0 | 1.1500 | 7 | 0.00000 | −0.07000 |
| CR T1 | 1.2200 | 7 | 0.00816 | p-value = 0.000 * | |
| Pair 10 | TP T0 | 4.689 | 7 | 0.23131 | −1.8561 |
| TP T1 | 6.545 | 7 | 0.23348 | p-value = 0.000 * |
W: white background, B: black background, T0: Initial reading, T1: Final reading, CR: contrast ratio, TP: Translucency parameter, *: p-value significant at p < 0.05.
Table 6.
Paired sample statistics (1450 °C).
| Mean | N | Std. Deviation | Mean Difference | ||
|---|---|---|---|---|---|
| Pair 1 | LWT0 | 80.471 | 7 | 0.3498 | 2.9857 |
| LwT1 | 77.486 | 7 | 0.3671 | p-value = 0.000 * | |
| Pair 2 | awT0 | −1.450 | 6 | 0.0548 | −0.0833 |
| aWT1 | −1.367 | 6 | 0.0816 | p-value = 0.093 | |
| Pair 3 | bWT0 | −1.8100 | 7 | 0.14387 | −0.13857 |
| bWT1 | −1.671 | 7 | 0.1604 | p-value = 0.054 | |
| Pair 4 | ΔEW T0 | 12.329 | 7 | 0.1976 | 1.8143 |
| ΔEW T1 | 10.514 | 7 | 0.2116 | p-value = 0.000 * | |
| Pair 5 | LBT0 | 74.043 | 7 | 0.5127 | 2.4571 |
| LBT1 | 71.586 | 7 | 0.1773 | p-value = 0.000 | |
| Pair 6 | aBT0 | −1.729 | 7 | 0.0756 | −0.4286 |
| aBT1 | −1.300 | 7 | 0.0000 | p-value = 0.000 * | |
| Pair 7 | bBT0 | −3.357 | 7 | 0.1397 | −1.1714 |
| bBT1 | −2.186 | 7 | 0.1574 | p-value = 0.000 * | |
| Pair 8 | ΔEB T0 | 15.171 | 7 | 0.2628 | 2.0571 |
| ΔEB T1 | 13.114 | 7 | 0.3716 | p-value = 0.000 * | |
| Pair 9 | CR T0 | 1.2243 | 7 | 0.00976 | 0.00857 |
| CR T1 | 1.2157 | 7 | 0.00976 | p-value = 0.200 | |
| Pair 10 | TP T0 | 6.596 | 6 | 0.2376 | 0.6370 |
| TP T1 | 5.959 | 6 | 0.3458 | p-value = 0.012 |
W: white background, B: black background, T0: Initial reading, T1: Final reading, CR: contrast ratio, TP: Translucency parameter, *: p-value significant at p < 0.05.
Table 7.
Paired sample statistics (1500 °C).
| Mean | N | Std. Deviation | Mean Difference | ||
|---|---|---|---|---|---|
| Pair 1 | LWT0 | 73.186 | 7 | 0.2734 | 2.2429 |
| LwT1 | 70.943 | 7 | 0.4467 | p-value = 0.000 * | |
| Pair 2 | awT0 | −2.043 | 7 | 0.0976 | −0.0571 |
| aWT1 | −1.986 | 7 | 0.1345 | p-value = 0.356 | |
| Pair 3 | bWT0 | −3.8714 | 7 | 0.07559 | −1.700 |
| bWT1 | −2.171 | 7 | 2.5078 | p-value = 0.120 | |
| Pair 4 | ΔEW T0 | 16.357 | 7 | 0.1813 | 1.7857 |
| ΔEW T1 | 14.571 | 7 | 0.2289 | p-value = 0.000 * | |
| Pair 5 | LBT0 | 67.614 | 7 | 0.3891 | 0.6143 |
| LBT1 | 67.000 | 7 | 0.5477 | p-value = 0.056 | |
| Pair 6 | aBT0 | −2.271 | 7 | 0.1113 | −0.0571 |
| aBT1 | −2.214 | 7 | 0.1345 | p-value = 0.324 | |
| Pair 7 | bBT0 | −5.457 | 7 | 0.0787 | −0.7286 |
| bBT1 | −4.729 | 7 | 0.4716 | p-value = 0.000 * | |
| Pair 8 | ΔEB T0 | 20.714 | 7 | 0.2673 | 1.7143 |
| ΔEB T1 | 19.000 | 7 | 0.3215 | p-value = 0.000 * | |
| Pair 9 | CR T0 | 1.2114 | 7 | 0.00690 | 0.09714 |
| CR T1 | 1.1143 | 7 | 0.03780 | p-value = 0.000 * | |
| Pair 10 | TP T0 | 5.797 | 7 | 0.12594 | 0.7884 |
| TP T1 | 5.009 | 7 | 1.956 | p-value = 0.280 |
W: white background, B: black background, T0: Initial reading, T1: Final reading, CR: contrast ratio, TP: Translucency parameter, *: p-value significant at p < 0.05.
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MDPI and ACS Style
Shetty, M.; Jain, S.; Wankhede, T.M.; Sayed, M.E.; Mohammed, Z.; Shetty, S.; Al Wadei, M.H.D.; Alqahtani, S.M.; Othman, A.A.A.; Alnijaiban, M.A.; et al. An In Vitro Study to Evaluate the Effect of Artificial Aging on Translucency, Contrast Ratio, and Color of Zirconia Dental Ceramic at Different Sintering Levels. Coatings 2022, 12, 642. https://doi.org/10.3390/coatings12050642
AMA Style
Shetty M, Jain S, Wankhede TM, Sayed ME, Mohammed Z, Shetty S, Al Wadei MHD, Alqahtani SM, Othman AAA, Alnijaiban MA, et al. An In Vitro Study to Evaluate the Effect of Artificial Aging on Translucency, Contrast Ratio, and Color of Zirconia Dental Ceramic at Different Sintering Levels. Coatings. 2022; 12(5):642. https://doi.org/10.3390/coatings12050642
Chicago/Turabian StyleShetty, Mallika, Saurabh Jain, Tushar Milind Wankhede, Mohammed E. Sayed, Zahid Mohammed, Sanath Shetty, Mohammed Hussain Dafer Al Wadei, Saeed M. Alqahtani, Ahlam Abdulsalam Ahmed Othman, Mashael Adullah Alnijaiban, and et al. 2022. "An In Vitro Study to Evaluate the Effect of Artificial Aging on Translucency, Contrast Ratio, and Color of Zirconia Dental Ceramic at Different Sintering Levels" Coatings 12, no. 5: 642. https://doi.org/10.3390/coatings12050642
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