Keywords zirconia - 5Y-PSZ - surface roughness - biaxial flexural strength
Introduction
Zirconia is an alternative material in restorative dentistry due to its superior mechanical
properties, biocompatibility, and esthetic.[1 ]
[2 ] Zirconia is a polymorphic metastable material consisting of three phases, namely,
monoclinic phase (m -ZrO2 ), tetragonal phase (t -ZrO2 ), and cubic phase (c -ZrO2 ). Due to the unstable tetragonal phase, 3 mol% yttrium was typically added to form
3 mol% yttria-tetragonal zirconia polycrystal (3Y-TZP). However, the 3Y-TZP mainly
contains the tetragonal phase, which can be transformed into a monoclinic phase under
stress application conditions, called phase transformation toughening. The phase transformation
phenomenon in 3Y-TZP is associated with a volumetric expansion (3–5%) of the tetragonal
phase to the monoclinic phase, which induces compressive stresses that eliminate crack
propagation.[3 ]
Despite the exceptional mechanical properties of 3Y-TZP, the major drawback is its
low translucency. Consequently, highly translucent monolithic zirconia has been modified
by increasing the yttria content to 5 mol% of yttrium oxide to form the cubic phase
up to 53% and decreasing the content of the alumina (Al2 O3 ) dopant, known as 5 mol% yttria-partially stabilized zirconia or 5Y-PSZ.[4 ] The component of 5Y-PSZ is formed with a mixed cubic/tetragonal content.[5 ] Cubic grains are larger than tetragonal grains, contributing to a lower grain boundary
between the zirconia crystals and leading to higher translucency. Since the fraction
of the cubic phase increases, the amount of the tetragonal phase is reduced. Therefore,
the mechanical strength of 5Y-PSZ is also decreased to almost half of that of 3Y-TZP.[6 ]
5Y-PSZ is commonly used for dental crowns and partial-fixed prostheses. After the
restoration cementation, adjustment is another crucial step to create a proper contour
and occlusion. The smooth surface of the restoration prevents antagonist wear and
plaque deposition that could compromise the longevity of the restoration.[7 ] In clinical procedures, glazing and polishing are commonly performed to achieve
a smooth restoration surface. Several studies reported that the surface roughness
of zirconia restoration from glazing and polishing was comparable.[8 ]
[9 ]
[10 ] However, the glazed surface is commonly worn off after use over time; it can exhibit
significantly higher surface roughness than the polished surface.[11 ]
[12 ] Therefore, the polishing procedure is more practical and applicable in clinical
situations.
Due to the high mechanical strengths and hardness of zirconia, zirconia polishing
systems have been widely introduced to the market, which improves the diamond particle
coating in the grinding burs.[13 ] Coarse-grit and fine-grit diamond burs are used for polishing zirconia.[14 ] Polishing the 3Y-TZP following the company guideline, from a coarse finishing diamond
bur to a fine silicone-impregnated diamond bur, had the lowest surface roughness.[15 ] A stone-grinding bur could also be used to grind and polish zirconia. Previous studies
reported that grinding 3Y-TZP with a stone grinding bur without water coolant exhibited
lower surface roughness and phase transformation.[14 ]
[16 ] Furthermore, various polishing systems such as diamond rotary instruments, stone
grinding burs, silicone rubber disks, and silicon carbide or aluminum oxide–coated
abrasive disks showed similar surface roughness and did not exhibit phase transformation.[9 ]
[17 ]
Furthermore, the parameters during clinical adjustment, such as heat, pressure, and
force, could alter the surface structure, generate a stress-induced transformation
of zirconia, and affect the mechanical properties.[13 ]
[18 ] There are limited studies on the influence of grinding and polishing with different
grinding burs on the 5Y-PSZ. Therefore, this study aimed to evaluate the effect of
two grinding and polishing protocols on surface roughness and biaxial flexural strength
of 5Y-PSZ. The null hypothesis was that finishing polishing with two grinding and
polishing protocols would not change the surface roughness and flexural strength of
5Y-PSZ.
Materials and Methods
Specimen Preparations
All materials used in this study are presented in [Table 1 ]. Two commercial 5Y-PSZ products, Lava Esthetic (3M ESPE, St Paul, MN, United States)
and Cercon xt (DeguDent GmbH, Hanau-Wolfgang, Germany), were used to fabricate the
specimens.
Table 1
Materials, grinding, and polishing burs used in the study.
Products
Particle size
Batch number
Manufacturer
Lava Esthetic
5mol% yttrium oxide
REF 69321 LOT 7312198
3M ESPE, Minnesota, United States
Cercon xt
Yttrium oxide 9%
Hafnium oxide <3% aluminum oxide and silicon oxide <1%
REF 53 6611 1114 Batch code LOT 18037374
DeguDent GmbH, Hanau-Wolfgang, Germany
Protocol I
EVE DIASYNT PLUS HP
Organic synthetic stone diamond
particle sized 150–200 μm
REF 7784
EVE Ernst Vetter GmbH, Pforzheim, Germany
EVE DIACERA RA
Twist medium
Diamond particle sized 15–20 μm
REF 7684
LOT 433138
EVE Ernst Vetter GmbH, Pforzheim, Germany
EVE DIACERA RA Twist fine
Diamond particle sized 4–8 μm
REF 7784
LOT430937
EVE Ernst Vetter GmbH, Pforzheim, Germany
Protocol II
Jota Z850.FG
Diamond particle sized 38–45 μm
LOT 569103
Jota, Rüthi, Switzerland
Jota ZIR Gloss Chairside Set medium
Diamond particle sized 15–20 μm
LOT 807227
Jota, Rüthi, Switzerland
Jota ZIR Gloss Chairside Set fine
Diamond particle sized 4–8 μm
LOT 618343
Jota, Rüthi, Switzerland
The specimen was designed using a computer-aided design/computer-aided manufacturing
(CAD/CAM) system with Autodesk Fusion 360 program (Autodesk Inc., San Francisco, CA,
United States). Partially sintered yttrium-stabilized zirconia blanks from each brand
were milled into disk-shaped specimens measuring 18 mm in diameter and 1.2 mm in height
for the control groups. For the testing groups, disk-shaped specimens of the same
dimensions as the control group were prepared, with an additional design featuring
a simulated high-contact area measuring 3.6 mm in diameter and 0.6 mm in height at
the center of each disk. All specimens were fabricated by a five-axis milling machine
(inLab MC X5, Dentsply Sirona, Bensheim, Germany). The specimens were sintered in
a sintering furnace (inFire HTC speed, Dentsply Sirona, Bensheim, Germany) with a
heating and cooling time rate of 20°C per minute until reaching 800°C. Then, the temperature
was continuously increased to 1,500°C to be sintered-holding time of 2 hours according
to the manufacturer's instruction. Volumetric shrinkage compensation was at 20% after
sintering.
The sintered specimens were polished with 320-, 600-, 1,200-, and 2,000-grit silicon
carbide papers (TOA CO. LTD., Osaka, Japan) on a polishing machine (NANO2000, PACE
Technologies, Tucson, AZ, United States) at a speed of 300 rpm for 15 seconds with
continuous water irrigation. The specimen dimensions were verified with a digital
vernier caliper (Mitutoyo, Kanagawa, Japan). The final dimensions of all specimens
were 15 ± 0.2 mm in diameter and 1 ± 0.2 mm in height, following the ISO 6872:2015
specimen preparation guideline for biaxial flexural strength tests for ceramic materials.[19 ] Moreover, the simulated high-contact area of the tested groups was verified to be
3 ± 0.2 mm in diameter and 0.5 ± 0.2 mm in height at the center of each disk. The
dimensions of the testing and control groups are presented in [Fig. 1 ].
Fig. 1 The dimensions of the testing and control groups.
Grinding and Polishing Protocols
Sixty-six specimens were divided into six groups, i.e., LC and CC (unpolished), LE
and CE (protocol I), and LJ and CJ (protocol II), following the grinding and polishing
protocols in [Table 2 ]. A custom-made polishing machine was used to control the speed, direction, and application
force during the procedures. The grinding and polishing burs were mounted on the machine,
ground, and polished the specimen in a continuously sweeping motion within a 5-mm
distance at a rate of 1 mm/s with an applied force of 0.98 N[20 ]
[21 ] and 2 N[22 ]
[23 ] on the grinding and polishing burs, respectively.
Table 2
Grinding and polishing protocols.
Protocol
Group
Grinding and polishing bur
Speed (rpm)
Cycle
Water coolant
Applied force (N)
I
EVE Diasynt Plus (LE and CE)
EVE DIASYNT PLUS HP
10,000
40
No
0.98
EVE DIACERA RA W14DCmf
10,000
40
No
2
EVE DIACERA RA W14DC
10,000
40
No
2
II
Jota (LJ and CJ)
Jota Z850.FG.018
160,000
40
Yes
0.98
Jota ZIR9861M.RA.040
10,000
40
No
2
Jota ZIR9863F.RA.140
10,000
40
No
2
The sequence of grinding and polishing procedures with a diamond stone bur and a superfine
diamond bur is shown in [Fig. 2 ]. The group with diamond stone bur (Protocol I) was performed with a low-speed handpiece
(NSK Nakanishi Inc., Tochigi, Japan). In the grinding procedure of fine diamond bur
(Protocol II), a high-speed handpiece (NSK Nakanishi Inc.) was used with copious water
coolant irrigation. The polishing was subsequently performed using medium and fine
diamond-impregnated silicone burs with a low-speed handpiece. The burs were renewed
every four specimens. The specimens were rinsed with distilled water, ultrasonically
cleaned for 3 minutes, and air dried before surface roughness evaluation. Surface
roughness values were measured after each step as baseline (Ra0), after grinding (Ra1),
after polishing (Ra2), and after high-gloss polishing (Ra3).
Fig. 2 Flowchart of the experimental procedures.
After surface roughness evaluation, the specimen of all groups was randomly selected
and stored in a desiccator for 24 hours before undergoing evaluation of microscopic
surface topography by using scanning electron microscopy (SEM) and phase analysis
by using X-ray diffractometry (XRD).
Surface Roughness Evaluation
The surface roughness values of all specimens were evaluated from a contact-type profilometer
(Talyscan 150, Taylor Hobson Ltd., Leicester, England) with a 5-μm diamond stylus
tip. The specimens were fixed on the fixation stand and the stylus was adjusted to
be in the position with a contact area of 2 × 2 mm2 on a specimen surface. A constant load of 5 N with a speed of 1,500 μm/s was applied.
The measurements were performed with five parallel lines before calculating the mean
surface roughness value.
Biaxial Flexural Strength Test
The biaxial flexural strength test was performed using a universal testing machine
(Servo Hydraulic system Model 8872, Instron, Buckinghamshire, England). Each specimen
was placed with the polished side facing down toward the support balls (tension side),
with three steel balls of 3.2-mm diameter positioned 120 degrees apart from each other,
forming a 12.0-mm diameter circular platform. A loading force was applied with a 1.0-mm
tip positioned at the center of the specimen with a crosshead speed of 0.5 mm/min.
The maximum fracture load of each specimen was recorded in newtons and calculated
to the biaxial flexural strength in megapascals.[19 ]
Scanning Electron Microscopy Analysis
The microscopic surface topography was evaluated using a scanning electron microscope
(TM3000 Tabletop microscope, Hitachi-High Technologies, Tokyo, Japan). A randomly
selected specimen in each group (n = 1) was mounted on a metallic cylinder and sputtered with a thin coat of Au-Pt using
a fine coater (JFC-1200, Jeol, Tokyo, Japan). The SEM was operated at 20 kV with 3,000x
and 10,000x magnifications.
X-Ray Diffraction Analysis
A quantitative analysis of phase transformation was performed using X-ray diffraction
(XRD) analysis (Bruker D8, Cambridge, MA, United States) and the DIFFRAC.EVA analysis
program version 2 (Bruker AXS GmbH, Karlsruhe, Germany). Specimens were randomly selected
in each group (n = 2) to determine the composition of zirconium oxide crystals and the phase transformation
by a peak intensity ratio from the XRD patterns. Structural studies of crystal phase
transformation were performed using XRD. Diffraction patterns were obtained using
Cu-Kα radiation (λ = 1.5406 Å) in the range of 20 to 40 degrees of 2θ with a step
size of 0.01 and a step duration of 50.165 seconds. The peaks were refined using the
pattern-decomposition and profile fitting functions of the HighScore Plus software
(Malvern Panalytical, Worcestershire, United Kingdom). The phase structure determination
and refinement were performed using Rietveld refinement with TOPAS V3.0 software (Bruker,
Karlsruhe, Germany).
Statistical Analysis
All statistical tests were analyzed by using an IBM SPSS software version 28.0 for
Mac (IBM Corp., Armonk, New York, United States). The results of statistical analyses
with p -value less than 0.05 were interpreted as statistically significant. Normal distribution
was determined using the Shapiro–Wilk test. Homogeneity of variances was done using
Levene's test. Difference of surface roughness after each step and biaxial flexural
strength between groups was determined using one-way analysis of variance (ANOVA),
followed by Bonferroni post hoc analysis. Difference of surface roughness among four
different time points within group was evaluated using one-way repeated measure ANOVA
followed by Bonferroni post hoc analysis.
Results
Surface Roughness Evaluation
The statistical analysis revealed that all Ra groups followed a normal distribution,
as determined by the Shapiro–Wilk test. The mean of Ra and p -values of both 5Y-PSZ at baseline (Ra0), after grinding (Ra1), after polishing (Ra2),
and after high-gloss polishing (Ra3) are shown in [Table 3 ]. All groups revealed a similar pattern of surface roughness values and in the Ra0
there was no significant difference. The roughness values were significantly increased
in Ra1 (p < 0.05) and mainly decreased in Ra2 and Ra3 (p < 0.05) in all groups. However, the Ra3 values were still significantly higher than
the Ra0 (p < 0.05) and only Ra3 of the CJ group was comparable to the baseline (p > 0.05).
Table 3
Means and standard deviations of surface roughness of the 5Y-PSZ at baseline (Ra0),
after grinding (Ra1), after polishing (Ra2), and after high-gloss polishing (Ra3)
(μm)
Groups
Mean surface roughness (SD; μm)
Zirconia brands
Burs
Baseline (Ra0)
Grinding (Ra1)
Polishing (Ra2)
High-gloss polishing (Ra3)
p -value
Lava Esthetic
EVE (LE)
0.12(0.02)aA
1.62(0.33)bA
0.21(0.06)cAB
0.18(0.03)cA
< 0.001
Jota (LJ)
0.12(0.03)aA
0.74(0.12)bB
0.13(0.06)aAB
0.14(0.04)aAB
< 0.001
Cercon xt
EVE (CE)
0.12(0.02)aA
0.38(0.16)bC
0.21(0.04)cAC
0.18(0.03)cA
0.001
Jota (CJ)
0.12(0.04)aA
0.38(0.07)bC
0.18(0.06)cAB
0.13(0.03)acB
< 0.001
p -value
0.966
< 0.001
0.023
0.004
Note: Different superscript lowercase letters indicate statistically significant difference
among four different steps in the same row, analyzed by one-way repeated analysis
of variance (ANOVA) followed by Bonferroni post hoc analysis (p < 0.05).
Different superscript uppercase letters indicate statistically significant difference
between groups in the same column, analyzed by one-way analysis of variance (ANOVA),
followed by Bonferroni post hoc analysis (p < 0.05).
When comparing different polishing protocols within the same material, the LJ group
had significantly lower roughness values than the LE group in Ra1, Ra2, and Ra3 (p < 0.05). On the other hand, there were no significant differences after the grinding
and polishing steps between the CE and CJ groups (p > 0.05) in Ra1 and Ra2. However, Ra3 of the CJ group was significantly lower than
that of the CE group (p < 0.05).
Biaxial Flexural Strength
The mean and standard deviation of the biaxial flexural strength values are presented
in [Table 4 ]. In both zirconia products, the control groups had the highest flexural strength,
that is, LC and CC. After grinding and polishing, the LE group was the only group
with a significantly reduced flexural strength compared with the control groups (p < 0.05). There was no difference in the flexural strength among the Cercon xt groups
(p > 0.05).
Table 4
Means and standard deviations of the biaxial flexural strength of the 5Y-PSZ (MPa)
Groups
Mean ± SD (n = 60)
Biaxial flexural strength (MPa)
LC
508.16 ± 34.01a
LE
423.77 ± 77.96b
LJ
486.69 ± 70.58a,b
CC
532.37 ± 15.73a
CE
513.82 ± 21.14a
CJ
516.40 ± 37.18a
Note: Different superscript lowercase letters indicate statistically significant difference
among three different groups in the same material, analyzed by one-way analysis of
variance (ANOVA) followed by Bonferroni post hoc analysis (p < 0.05).
Scanning Electron Microscope Analysis
The microscopic surface topographies of all the tested groups at 3,000X and 10,000X
magnification are shown in [Figs. 3 ] and [4 ]. The LC and CC groups represent the control groups and appeared to have a smooth
surface. The LE and CE groups showed rougher surfaces and scratches compared with
the LJ and CJ groups.
Fig. 3 The scanning electron microscopy (SEM) images showed the surface topography of Lava
Esthetic specimens (LC, LE, LJ) at 3,000X and 10,000X magnifications.
Fig. 4 The scanning electron microscopy (SEM) images showed the surface topography of Cercon
xt specimens (CC, CE, CJ) at 3,000X and 10,000X magnifications.
X-Ray Diffraction Analysis
The XRD analysis is presented in [Table 5 ], with the corresponding XRD diffractogram shown in [Fig. 5 ]. The LC and CC control groups exhibited comparable amounts of zirconia phases, with
the tetragonal phase dominating at 69.82 and 64.92%, respectively, followed by the
cubic and monoclinic phases. Following the grinding and polishing procedures, the
LE and LJ groups showed an increase in the tetragonal phase compared with the LC group,
while the cubic phase markedly decreased. This trend was also observed in the CC,
CE, and CJ groups, with the tetragonal phase predominant, followed by the cubic and
monoclinic phases. The percentage of the tetragonal phase in the CE and CJ groups
increased to 91.32 and 93.31%, respectively, while the cubic phase decreased to 5.91
and 7.60%, respectively. However, the monoclinic phase remained comparably consistent
across all groups.
Fig. 5 Representative X-ray diffractograms of Lava Esthetic specimens with the LC, LE, and
LJ groups (left column) and Cercon xt specimens in CC, CE, and CJ group (right column).
The diffractograms showed the view of peak from the 30 to 35°C and 50 to 62°C in all
groups.
Table 5
Composition of zirconia phase from XRD analysis
Groups
Tetragonal phase (t- ZrO2 )
Monoclinic phase (m- ZrO2 )
Cubic phase (c- ZrO2 )
LC
69.82
0.75
29.43
LE
92.74
1.13
6.13
LJ
89.79
1.00
9.21
CC
64.92
0.73
34.35
CE
91.32
1.08
7.60
CJ
93.31
0.78
5.91
Discussion
This study aimed to evaluate the effect of two grinding and polishing protocols on
the surface roughness and biaxial flexural strength of 5Y-PSZ. The results revealed
that both protocols led to changes in surface roughness and biaxial flexural strength
of 5Y-PSZ. Therefore, the null hypothesis was rejected. After the zirconia restoration
is fabricated, the grinding and polishing procedures, that is, occlusal adjustment,
are inevitable in clinical situations. These procedures may generate heat and induce
stress on the surface of zirconia, potentially leading to a transformation toughening
phenomenon and iatrogenic damage to the restoration. However, previous studies revealed
that the biaxial flexural strength was unaffected.[24 ]
[25 ]
Two commercial 5Y-PSZ were chosen because of the improvement of its translucency by
altering the grain size of zirconia. In addition, the fraction of the tetragonal and
cubic phases is also altered, as the amount of cubic phase increases, while the tetragonal
phase decreases. Consequently, the phase transformation toughening is unlikely to
occur in 5Y-PSZ.[4 ]
Currently, various zirconia polishing kits are available in the market, categorized
based on bur speeds (slow and high speed) and different diamond particle sizes. The
protruded platform on the tested specimen was designed to simulate a high-contact
point of the restoration requiring adjustment in clinical situations. To maintain
the properties of 5Y-PSZ and minimize the deterioration of the material, the selection
of grinding and polishing burs for the adjustment of zirconia must be considered.
The sequential polishing step should be applied to acquire an effective polished surface.[26 ] Diamond abrasive is commonly used as a primary component of zirconia polishing bur
to improve the grinding efficiency due to the highest Mohs hardness scale. Therefore,
two types of zirconia adjustment kits, slow-speed diamond burs (protocol I) and a
high-speed superfine diamond bur followed by slow-speed diamond burs, were selected
for evaluation (protocol II).
Many studies have investigated the effect of polishing procedures on zirconia with
different polishing kits. However, only a few studies controlled the grinding and
polishing protocols with custom polishing machines.[9 ]
[15 ]
[27 ] To minimize operator fatigue and errors during the procedures, in this study, the
grinding and polishing protocols followed the manufacturer's instructions. The process
progressed from large to small grain size of the burs in each step, utilizing a custom
polishing machine to control direction, speed, number of polishing strokes, and coolant.
The application force for grinding burs was regulated to 0.98 N to reflect the typical
grinding force used by general dentists.[20 ]
[21 ] Moreover, the polishing burs were set to 2 N, which is considered the appropriate
force for zirconia polishing.[22 ]
[23 ]
The surface roughness of the restoration plays an important role in bacterial adhesion
and restoration durability.[28 ] In this study, Ra0 was controlled as the baseline surface roughness value, comparable
to the surface roughness value of the glazed surface of the ceramic restoration,[29 ] simulating the glazed surface of the restoration before starting the clinical adjustment
procedure. The mean surface roughness of the materials was significantly increased
with the rough grinding burs and started to decrease with the polishing and high-gloss
polishing burs. This was corresponded with the SEM images that exhibited smoother
and uniform surfaces after the high-gloss polishing. The roughness values after the
final high-gloss polishing were within the clinically acceptable range of less than
0.20 μm, which could be susceptible to plaque accumulation.[30 ]
Despite being the same 5Y-PSZ zirconia, containing 5%mol yttrium oxide, there was
a significant difference after grinding, which could be attributed to the product
compositions. Cercon xt contained hafnium oxide, aluminum oxide, and silicon oxide.
In contrast, Lava Esthetic did not contain hafnium oxide. The incorporation of metal
oxides in the composition provided the potential for enhancing physical and mechanical
properties. An increase in hafnium concentration correlated with a reduction in porosity
between the gain boundaries within the zirconia material.[31 ]
[32 ] Moreover, the resistibility of the material to withstand the abrasive particles
of the grinding burs also affected the roughness values. It was suggested that the
surface roughness highly depended on the microstructure of the ceramics, such as the
types, configurations, and grain size.[33 ]
Variations in the type, shape, size, density, and binding material of the diamond
particle in the grinding burs also demonstrated different roughness values.[27 ] When comparing the two grinding and polishing protocols, the surface roughness after
polishing with protocol I was significantly higher than that with protocol II. This
could be explained by the slow speed and the large diamond particle size of the grinding
bur that created a rougher surface of the material. However, this was not observed
in Cercon xt, which could be due to the compositions of the material. Thus, it could
be suggested that protocol II might be preferred for Cercon xt.
Despite the superior smoothness of the glazed surface, the longevity of the glaze
was not well established after use in clinical situations.[12 ] Chairside adjustments normally removed the glaze layer from the restoration, leaving
a rougher surface. The grinding procedure also resulted in an increased surface roughness
value, as indicated by the increase in Ra1. Therefore, it is recommended to perform
the polishing sequence step-by-step, starting with medium to fine diamond polishing
burs after grinding to achieve the smoothest surface.[33 ]
The mean flexural strength values obtained in this study were 423 to 532 MPa, which
were nearly 460 to 630 MPa according to the value of the manufacturer's information
and previous studies.[4 ]
[34 ] The flexural strength of both 5Y-PSZ in the control group exhibited the highest
value. On the contrary, the groups that had undergone grinding and polishing procedures,
that is, the LE, LJ, CE, and CJ groups, revealed decreased mean flexural strength
values, which correlated with Iseri et al's study.[19 ] That study utilized low- and high-speed coarse grinding burs for grinding the 5Y-PSZ
and observed an increase in the surface temperature of 5Y-PSZ. The results revealed
decreased flexural strength values due to higher stress compression occurring on the
high-temperature surface with both speeds of grinding burs. Moreover, a previous study
also reported the effect of chairside adjustment using a coarse-grit 150-μm diamond
bur without coolant. The flexural strength values significantly decreased compared
with the unpolished 5Y-PSZ.[35 ] To explain this phenomenon, the authors speculated that the decrease in material
strength was caused by surface defects such as microcracks and flaws on the tension
surface. These defects were created during the grinding procedure and may not be completely
removed even after high-gloss polishing. These factors could increase the origin of
failure in the biaxial flexural strength, as observed in the results of this study.
The stabilized zirconia cannot undergo the t-m phase transformation due to the presence
of the cubic phase, which prevents it from being affected by stress induced by environmental
factors such as grinding, polishing, and sandblasting.[36 ]
[37 ] In clinical settings, occlusal adjustment could generate heat and force, potentially
inducing phase transformation.[38 ] This change in the crystallographic microstructure of zirconia compromises the mechanical
properties and longevity of the restoration.[39 ] The XRD result showed a reduction in the cubic phase and an increase in the tetragonal
phase, indicating a reversal transformation.[40 ] This transformation was attributed to the internal structure of zirconia, affected
by the martensitic transformation due to thermomechanical effects. However, there
was no evidence supporting this phenomenon affecting the properties of the material.[41 ] The percentage of the monoclinic phase in the XRD showed a slight increase compared
with the control, which was not related to the value of the biaxial flexural strength,
similar to the results of Hatanaka et al's study.[42 ]
Moreover, a previous study revealed that the grinding and polishing procedures may
have also induced the rhombohedral phase fraction, which was a new phase formed under
stress application from the cubic phase (c -ZrO2 ) and tetragonal phase (t -ZrO2 ) in 5Y-PSZ.[43 ] However, there was no consensus on whether this phase represents the true rhombohedral
phase or a distorted form of the tetragonal and/or cubic phases.[43 ]
[44 ] From the XRD results, the rhombohedral phase was not detected, as only a minimal
amount of this phase was present in the testing specimens. Furthermore, it is interesting
to observe the translucency of 5Y-PSZ after grinding and polishing, which occurs due
to the change from the cubic phase to the tetragonal phase.
The limitation of the study was the use of only two products of monolithic 5Y-PSZ.
Nowadays, there are multiple brands of zirconia, including the multilayer type, and
more grinding and polishing systems are available in the market, which need to be
evaluated. Moreover, the limited number of specimens in the biaxial flexural strength
test was insufficient for the required Weibull statistical analysis according to the
ISO standard.[45 ] Further studies with larger sample sizes are needed for accurate test determination.
Finally, an in-depth study of the zirconia structure needs to be conducted to provide
detailed information on the material surface.
Conclusion
As a result of this study, it is advised that the grinding and polishing protocol
adhere to the manufacturer's instructions step by step, culminating in the final high-gloss
polishing bur, to achieve the lowest surface roughness. Additionally, it was observed
that the flexural strength of Lava Esthetic was affected by grinding and polishing
following protocol I.
