Effect of Aging on the Microstructure and Optical Properties of Translucent ZrO 2 Ceramics.

OBJECTIVES: The development and placement of translucent zirconia ceramics on the dental materials market is in full swing. This research aimed to investigate how aging protocols affect the microstructure, color parameters and translucency of a new-generation monolithic zirconia ceramic.
MATERIAL AND METHODS: Translucent zirconia ceramics KATANA-Zirconia STML with different surface treatments (as sintered - control, glazed, polished) was tested using two aging protocols (hydrothermal degradation in autoclave at 134 °C and 2 bars for three hours, chemical degradation in four-percent acetic acid at 80 °C for 16 hours) in order to examine phase composition using X-ray diffraction analysis and ΔE, ΔL and ΔC color parameters through spectrophotometry. The translucency parameter (TP) was calculated using parameters L*, a* and b* on a black and white surface.
RESULTS: Regardless of the surface treatment, aging protocols did not cause a tetragonal-to-monoclinic phase transformation, although hydrothermal degradation in the autoclave transformed the hybrid tetragonal-cubic structure of all specimens to a tetragonal one. All polished and glazed specimens during chemical degradation demonstrated a significant color change ΔE. Lightness ΔL significantly changed in polished specimens aged in the autoclave. In all specimens, ΔC underwent a change manifested through statistically insignificant yellowing. None of the aging protocols altered the translucency of specimens.
CONCLUSIONS: Aging, regardless of the final surface treatment, did not manifest a monoclinic phase in the specimens. A tetragonal-cubic microstructure dominates. Unlike polishing, glazing the surface of translucent zirconia ceramics contributed to minor changes in color, lightness and chromaticity. The translucency of translucent zirconia ceramics remains stable regardless of aging and surface treatment.

Introduction

Owing to its excellent mechanical properties, zirconia ceramics (partially stabilized zirconia) is widely employed in dental medicine (). Due to its highly opaque character, it has been used as core material in bilayer systems, meaning that the final shape and appearance of a restoration have been attained by veneering a ceramic with better optical properties (, ). Two main problems arise in the clinical application of these materials. The first problem is the chipping of veneering porcelains. The cohesive fracture is caused by a strain exerted on the more fragile material due to a discord in the coefficient of thermal expansion between the two materials (core and veneering) (). Another problem is the “aging” phenomenon, i.e. low temperature degradation (LTD) (-). Aging is described as a spontaneous transformation of a tetragonal crystal structure into a monoclinic one in a moist medium at a room temperature (-). LTD causes the expansion of grains’ volume (4-5%), and consequently strains grain boundaries, which results in an emergence of micro-cracks in the material (-). On the other hand, the expansion of the volume of grains can reduce crack propagation in the bulk of the material by reducing or closing the crack; this phenomenon is called transformation toughening (-) and it is the cause for high values of the strength of the material and toughness (, , ). Spontaneous phase transformation of zirconia ceramics is prevented by adding different percentages of stabilizers, such as yttrium, calcium, magnesium or cerium oxide (, -). Zirconia ceramics of the first (core in bilayer systems) (, , ) and second generation (older monolithic ceramics) (-, , ) contain approximately 2-3 mol% of yttrium oxide (Y2O3) as the structure stabilizer. In spite of the addition of a stabilizer, exposing first-generation and second-generation zirconia ceramics to hydrothermal degradation in an autoclave always results in tetragonal-to-monoclinic transformation on the surface, which is exposed to moist environment and temperature shifts, in a similar way as the exposed side of a restoration in the oral cavity (-). The loading of the material during chewing causes the emergence of new and expansion of existing micro-cracks, thus triggering the advancement of tetragonal-to-monoclinic transformation from the surface into the depth, which may eventually result in restoration fracture (-). The expansion of volume causes monoclinic grains to rise above the surface of the materials, thereby increasing the roughness of the materials and the consequential wear of antagonists in the chewing process ().

Translucency is the main esthetic reason in choosing materials for fixed prosthetic restorations (). Insufficient translucency of first-generation and second-generation zirconia ceramics has limited their application in the esthetically more demanding anterior part of dental arch. Zhang et al. reported an increased manifestation (50%) of a cubic crystal structure in translucent zirconia ceramics with 5 mol% of yttrium oxide (fully stabilized zirconia) (). A cubic crystal lattice is resistant to transformation, which should eliminate the problem of aging. Dispersion on the grains of a cubic crystal lattice is abated, while transmission through material is amplified, which fosters translucency and the possibility of imitating dental hard tissues (). According to Kolakarnprasert et al., grain size differs in different translucent materials (). They reported that KATANA-Zirconia UTML had the largest grain size (4.05 ± 0.85 µm) and the highest cubic phase percentage, followed by Katana STML (2.81 ± 0.17 µm) and finally Katana ML (0.63 ± 0.3 µm), which also displayed the lowest share of cubic phase in its structure (). Due to the aforementioned problems with first-generation and second-generation zirconia ceramics, a need arose for a new, third-generation zirconia ceramic which would eliminate those problems. By exposing materials to aging protocols, one may predict their performance in the oral cavity over a longer period (-, , ).

The aim of this research was to evaluate the effect of hydrothermal and chemical aging protocols on microstructure, color parameters, i.e. color stability and translucency of monolithic translucent multilayered zirconia ceramic. The following hypotheses have been tested:

Hydrothermal degradation in an autoclave and chemical degradation in a corrosive medium do not affect the microstructure of specimens.

Hydrothermal degradation in an autoclave and chemical degradation in a corrosive medium bring about color changes expressed by parameters CIE ΔE, CIE ΔL and CIE ΔC, which are within clinically acceptable values over a longer period of application.

Hydrothermal degradation in an autoclave and chemical degradation in a corrosive medium do not affect the translucency of monolithic translucent zirconia ceramic.

The microstructure and translucency of monolithic translucent zirconia ceramic correlate.

Material and methods

Specimen Preparation

Monolithic translucent zirconia ceramics KATANA-Zirconia Super Translucent Multi Layered - STML (Kuraray Noritake Dental Inc., Tokyo, Japan), shade A2 (Table 1) was used in the study. Overall, 18 specimens (11mm x 11mm x 1.5mm with ± 5% tolerance) were fabricated using the CAD/CAM technique in a milling machine (Zenotec Easy Wieland, Pforzheim, Germany) and sintered in a furnace (Wieland, Pforzheim, Germany) according to the manufacturer’s instructions (2 hours at 1550 °C). The heating rate of 10 °C/min was employed until the sintering temperature was reached. The same dynamics was repeated in cooling protocol after the final sintering. Depending on the surface treatment, the specimens were divided into two groups. Eight specimens were glazed following the manufacturer’s instructions (G1-G8), while eight other specimens were polished (P1-P8) with diamond rubbers for porcelain (Ceragloss, EDENTA AG, Hauptstrasse 7, Switzerland) and cooled with water. Each group was further divided into two subgroups, according to the aging protocol applied. The remaining two specimens received no surface treatment and acted as control specimens in the research (C1, C2).

Table 1

Overview of the most important compounds in zirconia ceramics KATANA-Zirconia STML.

MATERIAL	COMPOUND SHARE (%)
KATANA-Zirconia STML	ZrO2	~ 86
	Y2O3	~ 11.5-12
	HfO2	~ 2-2.5

Null Measurements and Analyses

An X-ray diffraction analysis (Philips PW 1820 diffractometer, Philips Co., Amsterdam, Netherlands) was performed on two polished samples (P1, P5) so as to attain the initial crystal structure, which was assumed to be identical in all specimens. Two ceramic tiles (P1, P5) were subjected to CuK-alpha radiation in the space between 10 ° and 70 ° of the 2theta angle, with a step size of 0.02 ° and step time of 1 s/step.

The initial values of color parameters L*, a* and b* on all specimens, including the control ones, were calculated with an X-rite Exact spectrophotometer (Grand Rapids, Michigan, USA). The instrument was calibrated and all the settings were adjusted before measuring (measurement mode 0, light source D50, standard observer 2 °). The aperture size was 2 mm. Five measurements were conducted for each specimen on a black surface and a white surface. Changes in color and lightness as well as the chromaticity of specimens were expressed through CIE ∆E, CIE ∆L and CIE ∆C values, which were calculated using the CIE∆E2000 formula. Lower ΔE values denote a minor colorimetric difference between two points representing colors in the color coordinate system. ΔE values under 3.5 are considered to be clinically acceptable in the conditions of the oral cavity (). The translucency parameter (TP) was extracted from L*, a* and b* parameters on black and white surfaces using the following formula:

where w and b denote L*, a* and b* values measured on both a white surface and a black surface.

Experimental Protocols

All glazed and polished specimens were divided into four subgroups (G1-G4, G5-G8, P1-P4, P5-P8) according to the type of experimental protocol they were subjected to (Figure 1). Four glazed specimens (G1-G4) and four polished specimens (P1-P4) were sterilized in an autoclave (SKO7, Faro, Italy) for a period of three hours, at a temperature of 134 °C and under 313-318 kPa of pressure (six 30-minute cycles). Distilled water was used for sterilization. Four glazed (G5-G8) and four polished specimens (P5-P8) were placed in a 1000-mL glass measuring flask. A 2.49-pH corrosive medium (four-percent acetic acid (CH3COOH)) was added and the specimens were submerged in the medium for 16 hours at a temperature of 80 °C (ISO 6872).

Figure 1

Graphic division of groups of specimens.

Control specimens (C1, C2) were not subjected to experimental protocols, but were kept at a room temperature of 20 °C and under a standard pressure of 1 atm throughout the duration of the research.

After the aging protocols, the same measurements and analyses from the beginning of the research were repeated on all specimens and one specimen from each subgroup (G1, G5, P1 and P5) was subjected to an X-ray diffraction analysis.

Statistical Analysis

The obtained results are displayed in figures and tables. In order to analyze the statistical significance of CIE ∆E, CIE ∆L and CIE ∆C values as well as the significance of the difference in TP values before and after experimental protocols, the Tukey test, with a 95% confidence interval, was employed along with the One-way ANOVA comparison procedure for detecting differences between groups with regard to the examined parameters. All P values below 0.05 were deemed to be statistically significant. The XLSTAT software was used in the analysis.

Results

The results of a XRD analysis conducted on polished specimens from both subgroups (P1, P5) before aging protocols (Figure 2) have revealed the diffraction maximum of tetragonal zirconia (position at about 30°2Theta) and cubic phase peaks. Phase shares are expressed as percentages. The share of the tetragonal phase was 62.7%, while the share of the cubic phase was 37.3%. Phase composition analyses of glazed (G1) and polished (P1) specimens exposed to hydrothermal degradation in an autoclave revealed that the cubic phase fully transitioned into the tetragonal one (Figure 3). The phase composition of both specimens exposed to chemical degradation in a corrosive medium (G5, P5) did not significantly change (Figure 4).

Figure 2

X-ray diffractometry of polished specimens P1 and P5 before aging.

Figure 3

X-ray diffractometry after hydrothermal degradation in an autoclave for glazed specimen G1 (2A) and polished specimen P1 (2B).

Figure 4

X-ray diffractometry after chemical degradation in a corrosive medium for glazed specimen G5 (3A) and polished specimen P5 (3B).

∆E, ∆L and ∆C values were calculated using the CIE∆E2000 formula and are presented as value intervals (arithmetical mean ± standard deviation (AM ± SD)) for all specimens (Table 2). Hydrothermal degradation resulted in a major color change in polished specimens (P1-P4) (ΔEavg=2.75) and a minor color change in glazed specimens (G1-G4) (ΔEavg=1.64), but the latter change was significantly minor in comparison with other subgroups (G5-G8, P1-P4, P5-P8) (Tables 2 and 3). The chemical degradation of polished specimens (P5-P8) led to an average color change of ΔEavg=2.65, while the same protocol caused a somewhat greater color change of ΔEavg=2.87 in glazed specimens (G5-G8) (Tables 2 and 3). The change of color of ΔEavg=2.15 occurred in control specimens which, from a statistical point of view, did not differ significantly from the specimens in other subgroups (Tables 2 and 3). In the analysis of obtained color changes, a somewhat more pronounced difference in the component defining the lightness of the specimens was observed. In polished specimens (P1-P4), hydrothermal degradation brought about an average lightness change of ΔLavg=-1.16, which was statistically much greater than in other subgroups (P5-P8, G1-G4, G5-G8) (Tables 2 and 3). Differences in chromaticity (variations between the values of a* and b* coordinates) were less pronounced than differences in lightness (L*). Such deviations primarily occurred along the b* coordinate (value increase). No statistically significant differences in chromaticity ΔC among the subgroups were recorded (Tables 2 and 3). Figures 5, 6 and 7 graphically show all resulting deviations in the coloration of specimens subjected to experimental protocols, along with deviations in lightness and chromaticity. A significant color change ΔE in specimens G5-G8, P1-P4 and P5-P8 is visible in the graphs. Parameters ΔL and ΔC underwent a more significant change in subgroup P1-P4, although the change in parameter ΔC has not been statistically significant.

Table 2

Display of value intervals for ΔE, ΔL and ΔC with their average values of AMs (∆Eavg, ∆Lavg, ∆Cavg) of glazed (G1-G8), polished (P1-P8) and control (C1, C2) specimens (Tukey test with a 95% confidence interval).

SPECIMEN	∆EAM ± SD	∆Eavg(average value of arithmetical means)	∆LAM ± SD	∆Lavg(average value of arithmetical means)	∆CAM ± SD	∆Cavg(average value of arithmetical means)
G1	2.057 ± 0.791	G1-G4: 1.64	0.556 ± 1.043	G1-G4: 0.47	0.891 ± 1.368	G1-G4: 0.28
G2	1.526 ± 0.876		-0.088 ± 1.156		-0.025 ± 1.516
G3	2.057 ± 0.826		1.236 ± 1.090		-0.597 ± 1.429
G4	0.799 ± 0.826		0.039 ± 1.090		0.654 ± 1.429
G5	3.400 ± 0.826	G5-G8: 2.87	1.047 ± 1.090	G5-G8: 0.65	0.293 ± 1.429	G5-G8: 0.73
G6	1.809 ± 0.826		1.288 ± 1.090		0.493 ± 1.429
G7	2.526 ± 0.826		0.354 ± 1.090		0.524 ± 1.429
G8	3.762 ± 0.826		-0.104 ± 1.090		0.937 ± 1.429
P1	1.905 ± 0.791	P1-P4: 2.75	-1.272 ± 1.043	P1-P4: -1.16	1.045 ± 1.368	P1-P4: 1.25
P2	2.226 ± 0.876		-1.200 ± 1.156		0.398 ± 1.516
P3	3.282 ± 0.826		-1.125 ± 1.090		1.223 ± 1.429
P4	3.638 ± 0.826		-1.043 ±1.090		1.618 ± 1.429
P5	2.823 ± 0.826	P5-P8: 2.65	1.732 ± 1.090	P5-P8: 0.27	-0.039 ± 1.429	P5-P8: 0.43
P6	1.774 ± 0.826		1.317 ± 1.090		0.337 ± 1.429
P7	2.303 ± 0.826		-0.772 ± 1.090		0.573 ± 1.429
P8	3.686 ± 0.826		-1.215 ± 1.090		0.851 ± 1.011
C1	1.831 ± 0.826	C1,C2: 2.15	0.305 ± 1.090	C1,C2: 0.03	-0.394 ± 1.429	C1,C2: -0.35
C2	2.478 ± 0.826		-0.253 ± 1.090		-0.302 ± 1.429

Table 3

Statistical analysis of ΔE, ∆L and ∆C parameters among specimen subgroups including a display of significance (One-way ANOVA comparison, P<0.05). G – glazed specimens, P – polished specimens, H – hydrothermal aging, C – chemical aging, C – control specimens.

COMBINATIONS BETWEEN SUBGROUPS	P (∆E)	P (∆L)	P (∆C)
G-H vs G-C	0.002	0.697	0.66
G-H vs P-H	0.007	0.036	0.197
G-H vs P-C	0.03	0.725	0.787
G-H vs C	0.163	0.44	0.347
G-C vs G-H	0.002	0.697	0.66
G-C vs P-H	0.779	0.0005	0.483
G-C vs P-C	0.65	0.572	0.855
G-C vs C	0.153	0.408	0.321
P-H vs G-H	0.007	0.036	0.197
P-H vs G-C	0.779	0.0005	0.499
P-H vs P-C	0.847	0.024	0.344
P-H vs C	0.261	0.065	0.05
P-C vs G-H	0.029	0.725	0.787
P-C vs G-C	0.653	0.572	0.855
P-C vs P-H	0.847	0.024	0.339
P-C vs C	0.297	0.794	0.261
C vs G-H	0.163	0.44	0.347
C vs G-C	0.153	0.408	0.321
C vs P-H	0.261	0.065	0.05
C vs P-C	0.297	0.794	0.261

Figure 5

Graphic display of CIE ΔE interval values (AM ± SD) after aging protocols on glazed and polished specimens.

Figure 6

Graphic display of CIE ΔL interval values (AM ± SD) after aging protocols on glazed and polished specimens.

Figure 7

Graphic display of CIE ΔC interval values (AM ± SD) after aging protocols on glazed and polished specimens.

The value intervals (AM ± SD) for the translucency parameter (TP) of all specimens before (TP0) and after (TP1) experimental protocols displayed a homogeneity in results without statistical significance (Table 4). Hydrothermal degradation in an autoclave and chemical degradation in a corrosive medium did not have a statistically significant effect on the translucency parameter in tested subgroups, although a decrease in TP (ΔTP) was greater in polished (ΔTPavg from -0.96 to -0.5) than in glazed specimens (ΔTPavg from -0.42 to -0.07) (Table 4).

Table 4

Value intervals for translucency parameter before (TP0) and after (TP1) aging protocols with average change of arithmetical means (∆TPavg) of glazed (G1-G8), polished (P1-P8) and control (C1, C2) specimens (Tukey test with a 95% confidence interval, P<0.05).

SPECIMEN	TP0 (AM ± SD)	TP1 (AM ± SD)	∆TPavg (average change of arithmetical means)	P
G1	7.63 ± 0.666	7.617 ± 0.666	-0.07	1.000
G2	7.784 ± 0.738	7.774 ± 0.738
G3	7.392 ± 0.695	7.142 ± 0.695
G4	6.849 ± 0.695	6.84 ± 0.695
G5	7.842 ± 0.695	7.12 ± 0.695	-0.42	1.000
G6	7.477 ± 0.695	7.25 ± 0.695
G7	7.45 ± 0.695	7.116 ± 0.695
G8	7.365 ± 0.695	6.912 ± 0.695
P1	7.391 ± 0.675	7.254 ± 0.675	-0.93	0.29
P2	8.044 ± 0.748	6.661 ± 0.748
P3	8.296 ± 0.705	7.239 ± 0.705
P4	7.837 ± 0.705	6.683 ± 0.705
P5	8.007 ± 0.705	7.317 ± 0.705	-0.5	0.403
P6	7.501 ± 0.705	6.896 ± 0.705
P7	7.483 ± 0.705	7.736 ± 0.705
P8	8.021 ± 0.705	7.031 ± 0.705
C1	8.027 ± 0.705	7.981 ± 0.705	-0.16	1.000
C2	7.651 ± 0.705	7.374 ± 0.705

Discussion

Long-term conditions inside the oral cavity are simulated by subjecting samples to hydrothermal degradation in an autoclave, while chemical degradation in a corrosive medium (ISO 6872:2015) is a way of testing the resistance of materials to chemicals (, ). Acetic acid, applied in accordance with the standard, has a pH value (pH 2.49) very similar to the values inside the plaque and in the oral cavity when acidic food and drinks are consumed (, ). Hydrothermal degradation in an autoclave effectuated a change in the microstructure of both specimen groups (G1-G8, P1-P8), i.e. grains in the cubic crystal lattice were fully transformed into a tetragonal crystal lattice (Figure 3), but a tetragonal-to-monoclinic transformation did not occur. Likewise, Muñoz et al. did not prove the occurrence of a monoclinic phase, but their study of monolithic zirconia with a tetragonal-cubic microstructure found that hydrothermal degradation triggered an increase in the share of the cubic phase (). Kolakarnprasert et al. also did not prove the presence of a monoclinic phase in the material when subjecting specimens to a temperature of 120 °C over a 12-hour period (). The microstructure of specimens subjected to corrosive degradation did not change, i.e. the ratio between cubic and tetragonal phase remained almost unchanged from the start of the present research (Figure 4). These results led to conclusions that the first hypothesis of this research is partially accepted. According to Kolakarnprasert, translucent zirconia ceramics owes its stable tetragonal-cubic structure to its share of yttrium oxide – Y2O3 (approximately 5 mol %), which is larger than in previous generations of the same material (2-3 mol %) (). A combined tetragonal-cubic structure affects the mechanical and optical properties of the material. According to Sulaiman et al. (-) and Zhang et al. (, ), augmented dimensions of cubic crystals make materials with such a structure more fragile, but also more translucent, as dispersion is abated, while transmission though material is amplified vis-à-vis the previous generations. Yttrium oxide has an impact on mechanical properties; its share has been increased, while the share of aluminum oxide crystals, which contributed to superior mechanical properties of previous generations, has been reduced (-).

Colorimetry is a scientific discipline dealing with color and surface measurement as well as the precise presentation of those measurements in various color spaces employed for coloring delineation (, ). At the moment, the most commonly utilized color space is the three-dimensional CIE LAB, which, apart from high-quality visualization, it also enables easier definition of color deviations between two shades (-). In order to measure color differences on specimens before and after aging protocols as well as intergroup differences, a reflex spectrophotometer was employed. This device is used in graphic industry and would normally not be suitable for intraoral color determination in dental medicine. However, taking into account the fact that this was an in-vitro study in which disc-shaped specimens were employed, a device for measuring color parameters on flat surfaces was deemed to be the most suitable choice as it provided objective results and comparable numerical values, while the effect of the human factor, i.e. subjective perception, was eliminated (-). In this research, the color change, expressed as the average value of arithmetical means ΔEavg, in glazed specimens exposed to hydrothermal degradation in an autoclave (G1-G4) exhibited clinically more acceptable values (ΔEavg=1.64), which has been confirmed in prior studies (, , , ). The mean values ΔE in the subgroup subjected to chemical degradation (G5-G8) in a corrosive medium (the average value of arithmetical means being ΔEavg=2.87) were significantly higher than in subgroup exposed to hydrothermal degradation (Tables 2 and 3). Due to the stability of phase composition, i.e. the absence of a monoclinic phase, this color change may be attributed to the dissolving of glaze in an aggressive medium, which in turn affects the perception of optical properties (-). An amorphous material, glaze, is not resistant to aggressive external influences nor is it chemically stable (-). In an aggressive medium, surface ions in the glaze perish, which leads to irregularities (secondary porosity) that augment roughness – a finding that corresponds to the findings of other authors (-). It is important to emphasize that long-term color changes on restorations in the oral cavity would not be noticeable, but the following question arises: what would have happened to the material if, in addition to permanent moisture as well as changing temperature and pH values, it had been also exposed to discoloration-inducing beverages and oral hygiene product? Haralur et al. () offered the evidence that the greatest color change occurs after a thermo-cycling process as well as immersion in coffee, tea and chlorhexidine in monolithic zirconia, as opposed to lithium-disilicate ceramics and bilayer zirconia systems. Kurt et al. () have proven the direct connection between color changes on monolithic zirconia and the aging process that the material is subjected to; this phenomenon is explained by the direct exposure of monolithic surface of zirconia to a watery milieu, i.e. saliva, which stimulates the LTD of crystals on the surface of the materials, while the consequential expansion of grains in the monoclinic phase results in micro-cracks and an increased roughness of the surface, which has repercussions on the extent of the color change. Lithium-disilicate ceramics demonstrate greater color stability, i.e. their ΔE change is smaller than in monolithic zirconia (). In this research, polished specimens demonstrated ΔE color differences that were within the scope of clinically acceptable values, but were also significantly greater than in glazed specimens aged in an autoclave (average values of arithmetical means ΔEavg in polished specimens P1-P4: ΔEavg=2.75; P5-P8: ΔEavg=2.65) (Tables 2 and 3). There were no statistically significant differences in ΔE values among glazed specimens subjected to chemical degradation (G5-G8) and between both subgroups of polished specimens (P1-P4, P5-P8), i.e. the ensuing color changes were similar (Table 3). The impact of hydrothermal degradation in an autoclave on glazed specimens (G1-G4) was deflated by a protective coating on the surface (glaze), which, unlike in specimens without a protective coating, inhibited the penetration of water into the material, i.e. the degradation of the material. This assertion is confirmed by Palla et al., who have affirmed that the infiltration of water through the surface of a non-glazed glass ceramic causes the structure to disintegrate (). Camposilvan et al. have suggested to glazie all zirconia restorations in order to inhibit the impact of the hydrothermal aging protocol on the surface of the material (). Previous studies have also shown that the quality of the final surface of the restoration has an impact on color stability, i.e. glazed and highly polished surfaces manifest better color stability in the long run (). In the current study, greater changes ΔL and ΔC were observed in polished specimens subjected to hydrothermal degradation (P1-P4), followed by glazed specimens subjected to chemical degradation (G5-G8) (Tables 2 and 3). An increase of chromaticity along the b* co-ordinate in all subgroups, in varying degrees, indicated light yellowing of specimens after aging protocols, which did not occur in control specimens not subjected to experimental protocols (Table 3). A decrease in the lightness of polished specimens aged in an autoclave (P1-P4),which was statistically significant when compared to other subgroups (P5-P8, G1-G4, G5-G8), resulted in greater color saturation, i.e. greater ΔC change after aging in an autoclave (ΔLavg=-1.16, ΔCavg=1.25), although this values were not statistically significant compared with other subgroups (Table 3). These data are in line with a study by Ledić (), in which a decrease in the lightness of lithium-disilicate ceramic specimens consequently led to an increase in color saturation. Glazed specimen G8 and polished specimens P4 and P8 manifested mean ΔE values marginally greater than those that are clinically acceptable (, , , ) (Table 2).

When discussing optical parameters of the tested material, it has to be taken into account that the material is multilayered – it contains four layers that mutually differ in lightness and color saturation, i.e. the degree of transmission through the material. The material was layered to obtain a faithful reproduction of optical properties in a natural tooth (from a zone with less lightness and more saturation – corresponding to the cervical region – to a zone with more lightness and less saturation of color, in this case: A shade in the classical shade guide). Ueda et al. () have pointed out that light transmits through all four layers in multilayered monolithic zirconia differently, meaning that the layers have diverging optical properties. This fact may be of significance for these materials and would make the second hypothesis acceptable as long-term color changes on the material in the conditions of the oral cavity would be less discernible, especially if the restoration were to be glazed and the impact of the milieu in the oral cavity thus diminished. This part of the research could be supplemented and more insight into the color stability of this material could be acquired by immersing specimens into discoloration-inducing solutions of varying temperature and pH values while also conducting the experimental protocols here described; this is a subject for potential future research.

Speaking of optical properties of the material, one of the most important properties for expressing similarity with dental hard tissue and the factor affecting the choice of materials in fixed prosthodontic therapy is translucency (). In the results of this study, the translucency values, as expressed through the translucency parameter (TP), were over 6.8, but under 8.3 at the start of the study, with specimen thickness being 1.5mm ± 5%. At the end of the study, the translucency values were over 6.6., but under 8; however, these differences were not statistically significant (Table 4). The translucency parameter did not change significantly following the aging protocol, which affirms the third hypothesis. Other studies confirm similar findings (, ). Kurt et al. () demonstrated the translucency parameter stability of a Zirkonzahn Prettau [ZZ] monolithic zirconia ceramics, which was unaffected by either the final surface treatment or aging protocols. In a study by Abdelbary et al. (), the TP value was much greater (TP=16.12 ± 0.75) in 0.5mm thick InCoris TZI monolithic zirconia ceramic specimens, whereas the TP value of 1.2mm thick specimens was 9.25 ± 1.45. In that study, TP values of 0.5mm thick specimens significantly decreased after hydrothermal degradation in an autoclave at a temperature of 134 °C and under the pressure of 2 bar over a five-hour period (from 16.12 ± 0.75 to 12.56 ± 1.36), unlike the TP values of 0.8mm (from 13.67 ± 1.21 to 13.24 ± 2.42), 1mm (from 11.49 ± 0.95 to 11.08 ± 0.33) and 1.2mm (from 9.25 ± 1.45 to 9.74 ± 1.31) thick specimens, which were not significantly impacted by aging in the autoclave (). The thickness of the specimens in this study was 1.5mm ± 5%, hence the TP values found here are comparable to the TP values found by Abdelbary et al. for corresponding material thickness. Results of other studies correlate with the results of this study, thus confirming the connection between specimen thickness and TP value (-). This phenomenon can be explained in the following manner: the thicker the specimen/restoration, the greater the amount of absorbed and reflected light and the weaker the transmission through the material. This is caused by a greater number of grains (, ) and a larger grain boundary surface () that the light encounters. When a specimen is thinner, transmission is stronger (-). However, it is important to stress the need of minimal material thickness that ensures an adequate degree of translucency and resistance to fractures (, ). Although changes in the translucency parameter before and after aging protocols were not of significance in any specimen subgroup, a somewhat greater mean change in translucency parameter ΔTP was found in polished samples (P1-P8) (Table 4). This finding can be explained by the fact that a layer of glaze added to the base material in specimens G1-G8 had an insulating effect and minimized the impact of experimental aging protocols on the material (, ). A study by Lawson et al. confirmed the stability of the TP in the material tested in the present study – Katana STML (). The results of that study did not manifest a change in the TP of the Katana STML zirconia ceramic after two sintering programs – classical sintering, following the manufacturer’s instructions, and high-speed sintering, lasting 18-30 minutes (). Two other zirconia ceramics, Prettau Anterior and Zpex Smile, manifested a decrease in the TP value after sintering programs (). The changes in the TP value of zirconia ceramics can be explained with a post-aging manifestation of the monoclinic phase, which leads to a formation of micro-cracks that account for porosity in the structure and result in dispersion, thus reducing translucency (-). The results of a study by Ledić et al. demonstrated that the TP values of glass ceramics varying in color and manufactured using different techniques did not change significantly after being immersed in a 4-percent acetic acid for 16 hours at 80°C (). TP values in this research confirm the fourth hypothesis: the presence of a cubic structure resulted in translucency that is more optimized than in the previous generations of zirconia ceramics, which is line with the results of the research study by Camposilvan et al. (). While testing the translucency, microstructure and mechanical properties of second-generation and third-generation zirconia ceramics, Camposilvan et al. detected structural stability, the presence of a cubic phase and translucency that was superior in the third generation of zirconia ceramics; the translucency was described as “medium”, not high, and inferior to the translucency of glass ceramics. They also listed the inferior mechanical properties, which were similar to those in glass ceramics (). This research could be supplemented by investigations into mechanical properties of the tested material as well as surface state after aging protocols (, ).

Conclusions

Within the limitations of this in-vitro study, it can be concluded that experimental aging protocols do not cause a tetragonal-to-monoclinic phase transformation, i.e. aging of materials. A fully tetragonal and a hybrid tetragonal-cubic microstructure dominate. Experimental aging protocols resulted in a certain level of color change, expressed through CIE ΔE, CIE ΔL and CIE ΔC parameters; the change was more pronounced in polished specimens. Unlike polishing, glazing specimens generally decreased the value of color changes CIE ΔE, CIE ΔL and CIE ΔC. Light yellowing was observed in specimens subjected to aging protocols, unlike the control specimens, which did not undergo testing of any kind. These changes can be deemed clinically acceptable over a longer period of application in the conditions of the oral cavity.

Experimental aging protocols had no significant impact on translucency of third-generation monolithic zirconia ceramics, which can be explained by the presence of cubic microstructure and the absence of monoclinic phase.