Keywords
zirconia - hydrothermal aging - surface roughness - translucency parameter - contrast
ratio
Introduction
Yttrium oxide partially stabilized zirconia (3Y-TZP) or conventional zirconia consists
of 3% mol of yttrium oxide (Y2O3) as a stabilizer which helps in maintaining tetragonal crystal structure at room
temperature. Tetragonal zirconia has high strength and fracture toughness due to tetragonal
to monoclinic transformation around the crack tip after exposed to applied stress
causing crack propagation termination.[1]
[2] However, tetragonal zirconia has been susceptible to low temperature degradation
or hydrothermal aging as it is immersed in water such as saliva. The aging process
is a result of spontaneous transformation of the metastable tetragonal phase to the
monoclinic in the presence of water at low temperatures which are approximately 30°C
up to 400°C.[2]
[3] The progression of hydrothermal aging is relatively a slow process that starts on
zirconia surface causing microcracks formation in the area of transformation. Crack
propagation proceeds because of water penetration into the cracks leading to the intermolecular
bond rupture of at the crack tip. This process results in a remarkable zirconia surface
alteration and decrease in strength.[4]
[5]
[6] The effect of aging occurs obviously in the case of lower yttrium oxide content.
The initial powder consisting of increased amount of yttrium oxide can create the
cubic crystal formation. Since cubic zirconia is a stable phase, it could resist to
hydrothermal aging and has lower ability of transformation.[7]
The increased patients' aesthetic demands are driving the development of dental restorative
materials from conventional zirconia which is apparently opaque to a highly translucent
zirconia. Translucency is considered as an important factor in controlling aesthetics
and plays an important role in decision of material selection.[8] 3Y-TZP becomes more translucent by reducing size and repositioning of alumina grains
resulting in improved light transmission.[7] However, its aesthetic appearance is still far from mimicking natural dentition.
Another way to improve zirconia translucency is increasing the amount of Y2O3 stabilizer that will alter the phase structure from tetragonal to cubic phase. The
amount of Y2O3 stabilizer seems to have an effect on the overall microstructure of zirconia materials.
The common amounts of Y2O3 added to improve translucency are 4 and 5% mol (4Y-TZP and 5Y-TZP).[9] Cubic crystal is the most stable phase structure; therefore, mechanical properties
of cubic-containing zirconia seem to be stable after being hydrothermally aged.[10] Even though cubic-containing zirconia (4Y-TZP and 5Y-TZP) is proved to resist to
hydrothermal aging, some tetragonal crystals presenting in 4Y-TZP and 5Y-TZP are prone
to be transformed to monoclinic phase spontaneously after being immersed in water
in long-term period especially at the zirconia surfaces. Also, there is little evidence
showing that the translucency of cubic-containing zirconia is stable after aging.
Effect of hydrothermal aging usually appears at the zirconia surface so the surface
topography and surface roughness of zirconia with different numbers of cubic crystals
could have changed after being exposed to hydrothermal aging.
Therefore, the aim of this study was to investigate the effect of hydrothermal aging
on translucency parameter (TP), contrast ratio (CR), and surface roughness of various
types of translucent zirconia including noncubic and cubic-containing materials. The
first null hypothesis was that there was no significant effect of hydrothermal aging
on surface roughness and surface topography of all zirconia groups. The second hypothesis
was that hydrothermal aging did not significantly affect translucency of all zirconia
groups.
Material and Methods
Specimen Preparation
Four different types of zirconia were tested in this study ([Table 1]). Fifteen disk-shaped specimens were prepared from each zirconia in a partially
sintered stage using a precision saw (ISOMET 4000, Buehler Ltd., Lake Bluff, Illinois,
United States). All specimens were sintering completely using high temperature furnace
(Zyrcomat 6000 MS, VITA Zahnfabrik H. Rauter GmbH, Bad Sackingen, Germany) according
to manufacturer's instructions. The final dimensions for each test specimen were 15 mm
in diameter and 1 mm in thickness confirmed with a digital calliper (ABSOLUTE Digimatic
Caliper, Mitutoyo Manufacturing Company Ltd, Kawasaki, Japan). Afterward, all specimens
were cleaned with ultrasonic cleaner (Model: 460/M, Elma Schmidbauer GmbH, Singen,
Germany) for 10 minutes before testing. Hydrothermal aging simulation was set up using
an autoclave oven (TOMY ES 215/ES-315, TOMY Kogyo) at 122°C under pressure of 2 atm
for 8 hours.[10] Each specimen was sealed in a sterilization pack and placed in the autoclave. After
aging, all specimens were air-dried for 24 hours before testing.
Table 1
Various types of zirconia and their designated group used in the present study
|
Group
|
Material
|
Amount of cubic (%vol)
|
Lot number
|
Manufacturers
|
|
1
|
3Y-TZP
|
0
|
73830
|
VITA Zahnfabrik. Bad Säckingen, Germany
|
|
2
|
4Y-TZP
|
<30
|
65890
|
VITA Zahnfabrik. Bad Säckingen, Germany
|
|
3
|
5Y-TZP
|
30–50
|
61960
|
VITA Zahnfabrik. Bad Säckingen, Germany
|
|
4
|
5Y-TZP
|
>50
|
ZB6124B
|
Zirkonzahn GmbH, Bruneck, Italy
|
Surface Roughness Testing
Surface topography evaluation and surface roughness measurements were performed in
as-sintered specimens (n = 15/each group) using a noncontact optical 3D surface measurement system (Infinite
Focus SL, Alicona Imaging GmbH, Graz, Austria). The measurements were performed in
both before and after aging condition.
Translucency Measurement
Translucency evaluation was interpreted from the measurement of TP and CR. TP was
calculated by testing the color differences between the same specimens over the black
and white backgrounds, according to the following formula[11]:
TP = [(L*
B
− L*
W
)2 + (a*
B
− a*
W
)2 + (b*
B
− b*
W
)2]1/2
where the subscripts B refers to the color coordinates over the black background and W is the color coordinates over the white background. A higher TP value indicates a
higher translucency.
The CR was calculated from the spectral reflectance of light of the specimen (Y) over black (YB
) and white (YW
) backgrounds, using the following equations:
Y = [(L* + 16)/116]3 × 100
CR = YB
/YW
The CR is 0.0 for a transparent material and 1.0 for a totally opaque material.[11]
Translucency evaluation was done in the same specimens used for surface evaluation.
A spectrophotometer (ColorQuest XE, Hunter Associates Laboratory, Inc., Reston, Virginia,
United States) with a calibration plate and port size of 0.375 inches was used to
record the CIELAB coordinates (L*, a*, and b*) of zirconia disks. The TP and CR measurements were performed in both before and
after aging.
Phase Structure Analysis
Phase identifications of each group before and after aging were done by X-ray diffraction
(Philips X'Pert MPD, Philips, Eindhoven, Netherlands) using Cu-Kα radiation from 0
to 90 degrees (2θ). Scans were performed at 40 kV, 30 mA, step size of 0.05 degrees/step, and a scan
time of 1 s/step. The phase structures were refined as: tetragonal zirconia (t), monoclinic
zirconia (m), cubic zirconia (c) and data was indexed as a library data. The crystalline
phase fraction was reported as the peak position of each crystal structure. X-ray
diffraction analysis (XRD) outputs were mapped with library data to detect the crystal
structure on the surface of materials and volume fraction of monoclinic transformation
was calculated according to studies by Toraya and Yoshimura[12] and Garvie and Nichoson.[13]
Statistical Analysis
Two-way repeated measures ANOVA with Bonferroni test was used to determine the differences
in TP, CR, and surface roughness comparing before and after aging among various types
of zirconia. The statistical analysis was performed using SPSS software, version 24.0
(SPSS, IBM) to detect statistically significant differences (α = 0.05).
Results
Surface topography results, which are based on visual inspection showed that group
1 presented irregular surface in all areas of testing ([Fig. 1A, B]) and followed by group 2 ([Fig. 1C, D]). However, each group revealed no changing on the surface after aging. Two-way repeated
ANOVA results showed significant differences in surface roughness (Ra) values among
the material groups and aging conditions (p < 0.05), while there was no statistically significant interaction between two independent
factors (p = 0.066). Group 1 had a significant difference in surface roughness comparing between
specimens in before aging condition (263.201 ± 15.888 nm) and after aging condition
(325.048 ± 18.047 nm). In addition, group 2 showed significant increase in surface
roughness after aging (from 201.209 ± 17.681 to 228.330 ± 12.248 nm). Groups 3 and
4 presented no alteration on surface roughness ([Fig. 2]).
Fig. 1 Surface topography of all experimental groups in each aging condition (surface area
of 1.25 mm for testing at magnification of 50x). (A) Group 1 before aging; (B) group 1 after aging; (C) group 2 before aging; (D) group 2 after aging; (E) group 3 before aging; (F) group 3 after aging; (G) group 4 before aging; (H) group 4 after aging.
Fig. 2 Surface roughness (Ra) values (mean ± SD) of the different experimental groups. Different
uppercase letters indicate statistically significant among material group (p < 0.05). Different lowercase letters indicate statistically significant comparing
before and after aging within group (p <0.05).
According to translucency evaluation, there was a significant difference in TP values
([Table 2]) based on material difference (p < 0.05). No significant difference based on aging conditions (p = 0.156) and interaction effect (p = 0.274) was detected. Group 1 showed the lowest TP value whether it was aged or
not, significantly. Group 2 showed significantly lower TP value compared with groups
3 and 4 only in aged specimens. Groups 3 and 4 showed no significant difference in
TP values regardless of aging condition (p > 0.05). The CR values are reported in [Table 3]. The results showed statistically significant difference between the material groups
(p < 0.05). The mean CR value of group 1 was significantly the highest value. The CR
values can be ranked as follow: group 4 = 3 <2 <1 regardless of aging condition. After
aging, CR values appeared to be no change statistically comparing within material
group (p = 0.126). Therefore, there was no significant change in translucency within material
group comparing before and after aging.
Table 2
Mean ± SD of translucency parameter (TP) in all groups
|
Group
|
Translucency parameter
|
|
Before aging (n = 15)
|
After aging (n = 15)
|
|
1
|
6.574 ± 0.867Aa
|
6.268 ± 0.821Aa
|
|
2
|
9.794 ± 0.226Ba
|
9.649 ± 0.216Ba
|
|
3
|
10.001 ± 0.319Ba
|
10.068 ± 0.289Ca
|
|
4
|
10.018 ± 0.772Ba
|
10.099 ± 0.253Ca
|
Note: Different uppercase letters in the same column indicate statistically significant
difference (p < 0.05); Different lowercase letters in the same row indicate statistically significant
difference (p < 0.05).
Table 3
Mean ± SD of contrast ratio (CR) in all groups
|
Group
|
Contrast ratio
|
|
Before aging (n = 15)
|
After aging (n = 15)
|
|
1
|
0.838 ± 0.021Aa
|
0.847 ± 0.020Aa
|
|
2
|
0.745 ± 0.005Ba
|
0.746 ± 0.005Ba
|
|
3
|
0.728 ± 0.007Ca
|
0.729 ± 0.006Ca
|
|
4
|
0.726 ± 0.008Ca
|
0.727 ± 0.020Ca
|
Note: Different uppercase letters in the same column indicate statistically significant
difference (p < 0.05); Different lowercase letters in the same row indicate statistically significant
difference (p < 0.05).
The phase structure analysis using XRD showed that no monoclinic phase was detected
in all groups before aging. After aging, the peaks representing monoclinic structure
were seen in groups 1 and 2 ([Figs. 3] and [4]). The amount of monoclinic phase detected in groups 1 and 2 were up to 16.63 and
5.01%, respectively. There was no detected monoclinic phase after aging in groups
3 and 4.
Fig. 3 Phase structure from XRD results for group 1. The black arrow indicates peak of monoclinic
phase, (A) before aging, (B) after aging. XRD, X-ray diffraction.
Fig. 4 Phase structure from XRD results for group 2. The black arrow indicates peak of monoclinic
phase, (A) before aging; (B) after aging. XRD, X-ray diffraction.
Discussion
Conventional zirconia containing 3% mol of yttrium oxide retains tetragonal crystal
and no cubic crystal presented at room temperature. A benefit of having tetragonal
crystal structure is the transformation toughening of zirconia material.[14]
[15] Therefore, 3Y-TZP has superior mechanical properties compared with other dental
ceramics.[16]
[17]
[18] A drawback of retaining tetragonal phase structure at room temperature is opacity
of a material thus it can be used as a high strength substructure of a crown which
has to be veneered with higher aesthetic materials.[19]
[20] Zirconia translucency can be improved by either modifying microstructure or increasing
the amount of Y2O3.[21] Translucent zirconia tested in this study could be varied based upon the amount
of cubic structure. Group 1 was zirconia with no cubic crystal while group 2, 3, and
4 included cubic-containing zirconia with different amount of cubic crystal (30%,
30–50%, and more than 50%, respectively). Since, hydrothermal degradation, a spontaneous
transformation from the metastable tetragonal to the monoclinic structured facilitated
by oral fluid, usually takes place on the zirconia surface, this study aimed to evaluate
surface alteration and translucency change of all experimental group under extreme
in vitro aging conditions. This accelerated hydrothermal aging was performed in an autoclave
at 122°C under constant pressure of 2 atm for 8 hours. It is possible to assume that
1 hour of autoclaving has the same effect as 1 year in clinical services.[6]
Surface roughness evaluation among various types of translucent zirconia and aging
condition showed significant difference; therefore, the first null hypothesis was
rejected. To eliminate the influence of surface polishing protocol, the surface roughness
in this study had been tested on as-sintered specimens. The results showed that hydrothermal
aging affected the Ra in group 1 and 2 due to the progressive spontaneous transformation
of the metastable tetragonal phase to the monoclinic phase in the presence of oral
fluid.[2]
[22] This could be confirmed by XRD results showing detected monoclinic in group 1 and
2 after aging. It is in accordance with some previous studies showing that surface
deterioration occurred in a zirconia material.[23] Groups 3 and 4 retained large amounts of cubic structure, the most stable crystal
structure; at room temperature thus phase transformation would not occur after being
accelerated by hydrothermal aging. A threshold surface roughness below 0.2 µm could
be considered as a smooth surface, in which, no further bacterial accumulation could
be expected.[24]
[25] The roughness threshold detectable by the tongue was more than 0.5 µm.[26] Mean Ra values of aged specimens in group 1 (0.325 µm) and 2 (0.228 µm) were less
than the threshold that can be detected by tongue.
The second null hypothesis was accepted because the optical properties of the cubic
zirconia material were not influenced by hydrothermal aging. There was no significant
effect of aging on translucency of zirconia. Translucency of restorative material
affects esthetic outcome of restorations by mimicking natural appearance. Patients
are concerned more about esthetic outcome so translucency of zirconia material should
be investigated.[27] Residual porosity and zirconia grain size are the main factors controlling light
transmission. Transformation of tetragonal crystal to monoclinic crystal results in
increasing grain size.[22] Therefore, the assumption of hydrothermal aging affecting zirconia translucency
could be possible. However, the transformation occurred only at the surface of material
which was not affecting the overall grain size and translucency which was confirmed
by this study. Zirconia translucency was evaluated by measuring TP and CR. The TP
was determined by calculating the color differences in the same specimen against black
and white backgrounds. A higher TP value indicated a higher translucency. On the contrary,
the CR was calculated from the light spectral reflectance of the same specimen over
a black and a white background, the CR is range from 0.0 and 1.0, in which 0.0 means
a material is transparent material and 1.0 means a material is totally opaque.[11] Group 1 presented less translucent than cubic-containing groups (2, 3, and 4) and
group 2 presented no significant difference in TP values compared with groups 3 and
4. However, the CR in group 2 was lower than groups 3 and 4. These results could be
explained in which TP value was determined based on CIELAB color parameter using L* a* b*data. Nevertheless, a* and b* are hue values which were achieved from color pigments. Each material contained
different amount of color pigments resulting in a different hue value. This might
affect the TP value. This study showed that zirconia with cubic structure had high
translucency since the cubic zirconia is optically isotropic without light scattering
at the grain boundaries.[21]
[23] Thus, zirconia ceramics with a slight variation in cubic structure resulted in a
substantially different translucency. The strategy of increasing the amount of cubic
crystals resulted in higher translucency of zirconia.[28]
[29] A previous study concluded that high translucency monolithic zirconium oxide materials
had TP values in the range of 14.6 to 23.2.[30] TPs of our study showed lower values than the previously mentioned study because
of different surface finishing protocols resulting in differences in smoothness of
the surfaces, leading to decreased light scattering, and higher TP values.[31] All specimens in this study were tested in as-sintered condition; therefore, this
could be a cause of lower translucency.
Phase transformation after aging was confirmed with X-ray diffraction analysis. The
results showed that there was no monoclinic phase presented after hydrothermal aging
in groups 3 and 4. The results indicated that the high amount of cubic crystal caused
a more stable condition of zirconia; therefore, phase transformation did not occur.
The present results were in accordance with a previous study evaluating the aging
stability of 5Y-TZP. The authors reported that cubic-containing zirconia did not show
any transformation after 300 hours of accelerated aging.[6] In our study, group 2 exhibited some monoclinic crystals (approximately 5.01%) after
hydrothermal aging for 8 hours, which was possible because of less amount of yttrium
oxide stabilizer added.[16] For group 1, the presence of monoclinic phase after autoclave aging coincided with
the previous study which evaluated the amount of monoclinic phase on XRD after 8 hours
of aging.[10] They also found that hydrothermal aging did not alter mechanical properties of cubic-containing
zirconia.[10] However, this study found that the hydrothermal aging resulted in surface alteration
of noncubic translucent zirconia and cubic-containing zirconia which consisted of
less than 30% of cubic crystals. Further study should investigate the effect of hydrothermal
aging on surface gloss of highly polished zirconia.
Conclusion
Within the limitations of this in vitro study, the following conclusions were drawn: hydrothermal aging in autoclave for
8 hours caused phase transformation and increasing surface roughness only in group
1 (noncubic) and 2 (cubic less than 30%). However, surface roughness of groups 3 and
4 was not affected by hydrothermal aging. Also, hydrothermal aging did not affect
translucency of all zirconia materials.
