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 Table of Contents  
Year : 2023  |  Volume : 23  |  Issue : 1  |  Page : 57-64

Effect of sintering procedures on optical properties, chemical composition, and grain size of monolithic zirconia ceramic at different thicknesses after hydrothermal aging: An in vitro study

1 Department of Prosthodontics, Faculty of Dentistry, Erciyes University, Kayseri, Turkey
2 Department of Textile Engineering, Erciyes University, Kayseri, Turkey

Date of Submission15-Jul-2022
Date of Decision09-Oct-2022
Date of Acceptance21-Nov-2022
Date of Web Publication29-Dec-2022

Correspondence Address:
Haydar Albayrak
Department of Prosthodontics, Faculty of Dentistry, Erciyes University, Melikgazi, Kayseri
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/jips.jips_345_22

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Aim: The purpose of the present study was to examine the optical properties, chemical composition, and grain size (GS) of monolithic zirconia (MZ) ceramic at different thicknesses sintered using different procedures after hydrothermal aging.
Settings and Design: An in vitro study.
Materials and Methods: Forty MZ discs (0.5-mm thickness [Group-0.5] and 1-mm thickness [Group-1]; 12 mm diameter) were milled and divided according to standard (Group-ST) and speed (Group-SP) sintering procedures. All specimens were hydrothermally aged at 134°C after sintering. Translucency (TP), opalescence (OP), and fluorescence (ΔEab*-FL) parameters were calculated using the color coordinates (L*, a*, b*, respectively) of the discs. The chemical composition and the GS of the specimens were characterized using X-ray fluorescence spectroscopy and a scanning electron microscopy, respectively.
Statistical Analysis Used: TPs and ΔEab*-FLs were analyzed using independent samples t-tests and Mann–Whitney U-tests while a two-way analysis of variance (ANOVA) was used for OPs.
Results: Group-1 showed significantly lower TP than Group-0.5 (P < 0.001) but a significantly higher OP (P = 0.014). Group-SP showed significantly higher OP (P = 0.00003) and ΔEab*-FL (P = 0.0026) values than Group-ST without considering the thickness. Group-SP (0.29 ± 0.119 μm) had a smaller GS than Group-ST (0.306 ± 0.142 μm). Compared to Group-ST, Group-SP had a lower percentage of Y2O3 and a higher percentage of Al2O3.
Conclusion: The effect of the sintering procedure on TP and OP of MZ was not perceived by the naked eye. The speed sintering procedure may increase Δ E*ab-FL of MZ to higher values than natural teeth when compared with standard sintering. The speed sintering may cause minor changes in GS and the chemical composition of MZ.

Keywords: 3 Mol% yttria-stabilized tetragonal zirconia polycrystal, fluorescence parameter, grain size, sintering procedure

How to cite this article:
Albayrak H, Eraslan R, Aydinlioglu &. Effect of sintering procedures on optical properties, chemical composition, and grain size of monolithic zirconia ceramic at different thicknesses after hydrothermal aging: An in vitro study. J Indian Prosthodont Soc 2023;23:57-64

How to cite this URL:
Albayrak H, Eraslan R, Aydinlioglu &. Effect of sintering procedures on optical properties, chemical composition, and grain size of monolithic zirconia ceramic at different thicknesses after hydrothermal aging: An in vitro study. J Indian Prosthodont Soc [serial online] 2023 [cited 2023 Feb 6];23:57-64. Available from: https://www.j-ips.org/text.asp?2023/23/1/57/365949

  Introduction Top

Monolithic zirconia (MZ) restorations have become widely preferred in prosthetic dentistry because of advantages including favorable fracture strength, biocompatibility, production simplicity, reduced tooth preparation, reduced fracture risk of veneering porcelain, and acceptable esthetic appearance.[1] MZ crown restorations with different thicknesses may be needed depending on intraoral requirements.[2] Therefore, evaluating the optical properties of MZ restorations of different thicknesses is crucial.

A dental material should be similar optical properties to the intact dental tissue to succeed in esthetic restorations.[3] Translucency (TP), opalescence, and fluorescence are the most critical optical properties for controlling the esthetic outcome of dental restorations.[3] The TP is a reliable method used in many studies to assess the transparency of dental materials.[4],[5],[6],[7],[8],[9] TP of dental materials is calculated as a color difference against a black and white backgrounds.[2],[10],[11] The opalescence parameter (OP) has usually been used to estimate the opalescence of dental materials. The OP of a restorative materials is calculated from the difference in the chromas determined under reflected light and transmitted light or a black-and-white background.[3] Natural teeth appear whiter and lighter under daylight while they have a bluish-white fluorescence under ultraviolet (UV) light.[3],[12] The fluorescence property of a dental material can be determined by the color difference (fluorescence parameter [ΔEab*-FL]) in the presence and absence of UV light using a spectrophotometer.[12]

MZ restorations are commonly fabricated with a soft milling process of a partially sintered zirconia block. The milled MZ restorations must be sintered to gain final density and final strength. There are slow, standard, speed and high-speed sintering procedures with the parameters (final temperature, dwell time, heating and cooling rate) varying according to the manufacturer.[13] Previous studies reported that the speed sintering procedure increased,[4] decreased.[5],[9] or did not.[6],[7],[8] affect the TP values compared to standard sintering among the studies that examined 3 Mol% Yttria-Stabilized Tetragonal Zirconia Polycrystals (3Y-TZPs). It has also been reported that the OP value of the standard sintering procedure was similar to speed sintering[7],[8] but lower than high-speed sintering[14] procedure considering the study including 3Y-TZP. Therefore, the effect of the sintering procedure on the TP and OP is still controversial. Although the fluorescence property of MZ has been investigated,[15],[16] no study has yet been found that investigated the effect of the sintering procedure on the fluorescence property of MZ. There is a lacuna of research on the fluorescence of MZ in this area. Moreover, the effect of the sintering procedure on the optical properties of MZ may be related to the change in grain size (GS)[5],[8],[9] and chemical composition.[13]

MZ restorations are exposed to oral fluids, which leads to hydrothermal aging[17] and may affect the optical properties of MZ restoration.[18] Therefore, MZ specimens should be subject to hydrothermal aging to predict the long-term results of the restorations. Only one[5] of the previous studies[4],[5],[6],[7],[8],[9],[14] examining the effect of the sintering procedure on the optical properties of MZ (3Y-TZP) includes hydrothermal aging. The present study aimed to assess the impact of sintering procedures and thicknesses, on the optical properties, chemical composition, and GS of MZ after hydrothermal aging. The null hypotheses were that no differences were found between the TP, OP, or ΔEab*-FL values of tested thicknesses (first) and sintering procedures (second) after the aging.

  Materials and Methods Top

The total sample size was 40 (n = 10). Power analysis was done using G Power 3.1.9 software (Franz Faul University, Kiel, Germany), according to the effect size (f) was 1.33, and power was 0.80 (two tails).[19]

Fabrication of zirconia discs

A zirconia ceramic brand was tested in the present study [Table 1]. Disk-shaped (diameter = 12 mm) solid models were created with two different thicknesses (0.5 mm and 1 mm) in a three-dimensional software program (SolidWorks 2018; Dassault System SolidWorks Corp, Vélizy-Villacoublay, France) and saved in the standard triangle language (STL) file format. To compensate for volume shrinkage of the presintered zirconia block, the STL files were resized by a computer-aided manufacturing software program (DWOS; Dental Wings, Montréal, Canada) according to the magnification factor of the corresponding block. Forty specimens (n = 40) were milled from presintered blanks on a milling machine (Yenadent D40; Yenadent, ZenoTec, Istanbul, Turkey) using the resized files. New burs were attached to the holder of the milling machine after the production of every 20 specimens was completed. The specimens were divided into two groups according to thicknesses: Group-0.5 (0.5 mm, n = 20) and Group-1 (1 mm, n = 20). Each group was divided into two subgroups according to the standard (Group-ST) sintering and speed (Group-SP) sintering procedures [Table 2]. The final dimensions of all discs were gauged (293-821-30, Mitutoyo Micrometer, Kanagawa, Japan) with an accuracy of ± 0.01 mm after sintering. Any discs with unsuitable dimensions were excluded from the present study, and new discs were produced.
Table 1: Main properties of zirconia block

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Table 2: Details of standard and speed sintering procedures

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Hydrothermal aging

After sintering, each specimen was placed in a sterilization pack labeled with brand, sintering procedure, and specimen number. All specimens were hydrothermally aged in an autoclave[16] (Lina, W&H, Brusaporto, Italy) at 134°C under two bars for 5 h[17] and then were cleaned ultrasonically in distilled water at 40°C for 15 min (Sonorex, Bandelin, Berlin, Germany). All specimens were dried with compressed air.

Determination of optical parameters

The Commission Internationale de l'Eclairage (CIE) color coordinates (L*, a*, b*) of the specimens were measured over black (L*=94.171, a*=−0.65, and b*=2.19) and white (L*=0.21, a*=0.04, and b*=−0.22) backgrounds using a spectrophotometer (Lovibond RT 400, Tintometer Inc., Florida USA). Measurements were performed with 10°standard observer angle and D65 standard illuminant.[4] The specular component was excluded. The spectrophotometer had an 8-mm diameter aperture. The average value of the three measurements for each specimen was recorded. The spectrophotometer was calibrated as stated by the user guides before measuring per specimen.

The TP was calculated as the difference in L*, a*, b * measurements on white (LW*, aW*, bW*) and black (LB*, aB*, bB*) backgrounds using the following formula:[3]

TP= [(LB*-LW*) 2+ (aB*-aW*) 2+ (bB*-bW*) 2]-1/2.

The difference in chromaticity of the specimens over the white (aW*, bW*) and the black (aB*, bB*) backgrounds was used to assess the opalescence using the following formula.[3]

OP= [(aB*-aW*) 2+ (b *B-b*W) 2]-1/2.

The color coordinates of each specimen were determined by a spectrophotometer (CM-3600d, Konica Minolta, Osaka, Japan), including (UV 100%) or excluding (UV 0%) the UV component. Measurements were carried out against a white background (l = 95.08, a=-0.52, b = 0.84) using a standard illuminant D65 and 10° observer function.[4] Fluorescence was defined as color difference ΔEab*-FL in the presence and absence of the UV component. It was calculated using the following formula: ΔEab*-FL= [(L100*-L0*) 2+ (a100*-a0*) 2+ (b100*-b0*) 2]-1/2.[12] The color coordinates of the UV included (100%) and excluded (0%) conditions are shown respectively with the subscripts 100 and 0 in the equation.

Analysis of chemical composition

As previously described, one specimen (1 mm × 12 mm × 12 mm) for each sintering group was produced. Silicon-carbide papers with 400, 600, 800, 1200, 1600, and 2000-grit (Struers, Cleveland, United States) were used for polishing the specimens. The chemical composition (wt %) of one specimen from each sintering group was defined using a wavelength dispersive X-ray fluorescence (WD-XRF; PANalytical AXIOS Advanced, Malvern, United Kingdom) spectrometer operated at 60 kV and 50 mA.

Grain size measurement

After WD-XRF analysis, the same specimens were thermally etched (30 min at 1450°C; Programat S1 Ivoclar, Schaan, Liechtenstein) to reveal the grain boundaries. The microstructures of the same specimens were inspected using field emission scanning electron microscopy (FE-SEM; Zeiss Gemını 500, Oberkochen, Germany) at magnifications of 20000× and 30000× after each specimen was coated with gold/palladium. The average GS of each specimen was determined (at least 200 grains at ten different locations) on FE-SEM micrographs using software (Image J, National Institutes of Health) according to the linear intercept method.[20]

Shapiro–Wilk test is used for testing the normality of optical data. The TP values did not show normal distribution according to sintering procedures only. The sintering variable was analyzed with a Mann–Whitney U-test, while the thickness factor was analyzed with an independent samples t-test for the TP. A two-way ANOVA was used analysis of OP, where the two factors were the sintering procedure and thickness. The homogeneity of variances was assessed using Levene's test. Independent sample tests were used for post hoc comparisons. The ΔEab*-FL did not show normal distribution according to the thickness factor. ΔEab*-FL data were analyzed using an independent samples t-test for the sintering procedure and a Mann–Whitney U-test for the thickness factor. All statistical analyses (α = 0.05) were carried out using a software program (SPSS, Version 20, Chicago, IL, USA).

  Results Top

Descriptive statistics of TP, OP, and ΔEab*-FL values are presented in [Table 3], [Table 4], [Table 5], respectively. The effect of the sintering procedure on the TP was not statistically significant (P = 0.433) without considering the thickness. The effect of the thickness on the TP was statistically significant (P < 0.001). Reducing the thickness of a specimen from 1 mm to 0.5 mm caused a statistically significant increase in all TP values [Figure 1].
Figure 1: Boxplot of translucency values

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Table 3: The translucency values: Mean±standard deviation (median, interquartile range) (minimum-maximum)

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Table 4: The opalescence parameter values: Mean±standard deviation

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Table 5: The fluorescence parameter values: Mean±standard deviation (median, interquartile range) (minimum-maximum)

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OP values were statistically significantly affected by thickness and sintering procedure [Table 4] and [Table 6]. Speed sintered specimens showed higher OP values than standard sintered specimens, while 1-mm-thick specimens showed higher OP values than 0.5-mm-thick specimens [Figure 2].
Figure 2: Boxplot of opalescence parameter values

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Table 6: The outcome of two-way analysis of variance for opalescence parameter values

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ΔEab*-FL values were statistically significantly affected by the sintering procedure but not by thickness [Figure 3]. Speed-sintered specimens showed a higher ΔEab*-FL value than standard-sintered specimens [Table 5].
Figure 3: Boxplot of fluorescence parameter values

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The FE-SEM micrographs are presented in [Figure 4]. The grain boundaries were obviously observable in standard and speed sintering. The pores and voids were not observed at the grain boundaries in all specimens. The mean GS and chemical composition (wt%) of specimens are shown in [Table 7].
Figure 4: FE-SEM micrographs of standard and speed sintered specimens; (a and c) Indicate standard sintered specimens, and (b and d) indicate speed sintered specimens. Images (a and b) were obtained at magnification of ×20000; (c and d) were ×30000

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Table 7: Chemical analysis (weight %) and mean grain size of the different sintered specimens

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  Discussion Top

The first null hypothesis was rejected for the TP because the TPs of all specimens with 0.5-mm thickness were significantly higher than those of 1 mm specimens [Table 3]. The present study found an inverse relationship between the TP and thickness. This result is compatible with many studies investigating the optical properties of 3Y-TZP.[2],[10],[11] Perceptibility and acceptability thresholds of TP were 1.33 and 4.46 CIELab units, respectively.[19] In the present study, the effect of the thickness (4.26 units) on the TP is clinically perceptible by human observers while it is within acceptable limits [Table 3].

MZ restorations replace the enamel layer of the natural tooth structure.[21] Due to this, the TP level of MZ restorations should be similar to the natural tooth's enamel.[21] Yu et al. reported the TP of 1-mm thick enamel was 18.7.[22] In the present study, the TP of all specimens with 0.5-mm thickness was in the range of 14.03–18.38 and was significantly higher than those of 1 mm specimens [Table 3]. According to the results, it can be considered that the TP of 0.5-mm-thick specimens was more similar to the natural tooth's enamel than 1-mm-thick specimens.

The second null hypothesis was accepted for the TP because the effect of the sintering procedure on the TP of MZ was not statistically significant (P = 0.445) without considering the thickness [Table 3]. This result is consistent with some previous studies,[6],[7],[8] but not others.[4],[5],[9] The shorter (speed and high speed) sintering procedures were defined with changes in heating rate, final temperature, dwell time at the final temperature, and cooling rate.[13] It has been reported that the increase of at least 70°C in the final sintering temperature[23] or a cooling rate of at least 45°C/min[24] causes a statistically significant increase in the TP, but the heating rate[25] did not. A significant increase in the TP values was shown when the final temperature of at least 100°C and dwell time of at least 60 min were increased together.[26] In the present study, the speed sintering procedure had a 15°C higher final temperature, 35°C/min higher cooling rate, and a 90-min shorter sintering time than the standard [Table 2]. The final temperature and cooling rate differences between the two sintering procedures in the present study were lower than the values reported in previous studies.[23],[24] The shortening of the dwell time may have prevented the statistical difference. This result supports that dwell time significantly affects the TP.

The TP values of the standard sintered specimens were significantly higher (P = 0.002344) than the speed specimens within the 1 mm group only [Table 3]. This result is in agreement with most of the studies that compared standard and speed sintering procedures of 3Y-TZP specimens.[4],[6],[9] However, this TP difference between the sintering procedures was lower than 1.33 CIELab Units[19] in the present study and the reported studies.[4],[6],[9] Therefore, the effect of the sintering procedure on the TP cannot be clinically perceived by the human eye.

It has been shown that the average GS of 3Y-TZP specimens varies between 0.2 and 0.8 μm.[27] The mean GS calculated in the present study [Table 7] was consistent with the previous studies.[5],[27] Considering the study including 3Y-TZP, speed sintering procedures usually had smaller GS than standard sintering procedures[5],[8],[9] with the clinically imperceptible decrease in TP values.[8],[9] These results are also consistent with the present study. The reduction in GS of the speed procedure can be explained by decreased Y2O3[13] and increased Al2O3[28] concentration [Table 7] in the present study.

The first and second null hypotheses were rejected for the OP because the main effect of the thickness and sintering procedure on the OP was statistically significant [Table 6]. The significant increase in OP values as the thickness increases is consistent with some previous studies.[29],[30] Speed sintering group had a significantly higher (P = 0.000125) OP value than the standard sintering group in the 0.5 mm thickness [Table 4]. However, the effect of sintering on the OP was not statistically significant in the 1 mm thickness (P = 0.265). These results support that the main effect of sintering originates in the 0.5 mm group.

The OP values in the present study were weaker than the enamel-dentin complex (4.8) and the enamel layer of the natural tooth (7.4).[31] The weakest opalescence in the present study may be explained by the small[15] GS [Table 7]. Considering the study including 3Y-TZP, the OP values of standard sintering group were lower than in some previous studies.[8],[14],[25] High OP values in the previous studies[8],[14],[25] may be related to an increased amount of oxide materials such as ZrO2, Y2O3,[3],[32] SnO2, V2O5,[32] or usage of zirconia blanks with a high chromatic shade.[3]

The first null hypothesis was accepted for Δ E*ab-FL because the thickness did not cause a significant change in the ΔE*ab-FL values. The mean ΔE*ab-FL value of the speed sintered group was greater than that of the standard sintered group with or without considering the thickness [Table 5]. Therefore, the second null hypothesis was rejected for ΔE*ab-FL. The increase in the Al2O3 percentage [Table 7] may explain the statistical difference.

The dentin layer of human teeth has greater fluorescence than the enamel layer, and most of the fluorescence of human teeth arises from the dentin.[33] Lee et al. reported that Δ E*ab-FL value of human dentin was 0.73 ± 0.04.[34] ΔE*ab-FL values of the standard sintered group in the present study are closer to human dentin than the speed sintered group. Clinicians should consider that the speed sintering procedure may increase the fluorescence of MZ. Therefore, standard sintering may be beneficial in terms of achieving an esthetic result much similar to natural teeth.

The use of nonparametric tests for the analysis of TP and ΔE*ab-FL values is a main limitation of the present study. The authors of the present study think that if there were a different thickness group (between 0.5 mm and 1 mm) for the TP and a different sintering group for ΔE*ab-FL, all values would show normal distribution. Additional limitations of the present study include that only one brand of zirconia with one shade and Y2O3 ratio was studied. The results may not be suitable for other commercial zirconia blocks with different shades, Y2O3 ratios, and manufacturers. In addition, chewing forces, different surface treatments, and the underlying tooth, and the cement's color could be simulated in further studies. Therefore, future studies, including high-speed sintering, at least three thickness groups, and multi-materials with different Y2O3 ratios, would be beneficial to clarify the effects of the sintering procedure on the optical properties (especially fluorescence) of MZ. Further research is advised to determine the optimal sintering procedure that achieves the tooth-like fluorescence property without deteriorating the TP and opalescence properties of MZ. It is a noteworthy issue that the result of the present study requires supporting clinical investigations before final clinical proposals.

  Conclusions Top

Within the limitations of the present study, the following conclusions were drawn:

  • The effect of the sintering procedure on TP and OP of MZ was not perceived by the naked eye
  • The speed sintering procedure may increase ΔE*ab-FL of MZ to higher values than natural teeth when compared with standard sintering
  • The speed sintering may cause minor changes in the chemical composition and GS of MZ.

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Conflicts of interest

There are no conflicts of interest.

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

  [Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6], [Table 7]


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