Keywords
CAD/CAM materials - glass ceramic - lithium disilicate - polishing bur - zirconia
Introduction
Due to advancing digital technologies and material evolution, computer-aided design/computer-aided
manufacturing (CAD/CAM) ceramic materials have increased in popularity in restorative
and prosthodontic dentistry. These materials allow for chairside production and reduce
clinical chair-time while maintaining the precision and esthetics of the conventional
ceramic materials.[1] Similar to the conventional ceramics, the CAD/CAM ceramic materials can be classified
into three types according to the phase present in their chemical composition: glass-matrix
ceramic, polycrystalline ceramic and resin-matrix ceramic.[2] Lithium disilicate glass ceramic and stabilized zirconia is the most widely used
glass-matrix and polycrystalline ceramic, respectively. To combine the advantages
of esthetics and translucency in lithium disilicate glass ceramics and good mechanical
properties in zirconia, a zirconia-reinforced lithium silicate (ZLS) has been introduced.
ZLS contains zirconium dioxide dissolved in a lithium disilicate-based glass matrix.[3]
Clinically, grinding and polishing procedures are important steps for dental ceramic
materials, especially for occlusal adjustment.[4] Appropriate polishing prevents crack propagation and subsequent biomechanical failure,
reduces biofilm accumulation,[5] prevents excessive wear of opposing and adjacent teeth,[6]
[7] and enhances esthetics properties such as surface gloss and translucency.[8]
[9] Currently, a wide variety of commercial dental ceramic polishing kits are available.
Porcelain polishing kits, which mainly consist of silica-carbide abrasive, have been
used for porcelain and glass ceramic polishing. In contrast, zirconia requires a specific
zirconia polishing kit that consists of diamond particles as the main abrasive due
to their toughness, and some polishing systems also include a diamond polishing paste
for the final polishing step.[6]
[10] To avoid having to use more than one polishing kit in the clinic, some manufacturers
have introduced a multipurpose ceramic polishing kit for polishing all types of ceramic
materials, such as Eve Diacera[11] and ZiLMaster.[12] Although EVE Diacera was originally developed for polishing zirconia,[13] the manufacturer subsequently claimed that it could also be used for silica-based
ceramic because a high concentration of diamond fillers is incorporated. Thus, using
EVE Diacera in polishing silica-based ceramic might be easier due to its lower surface
hardness than zirconia-based ceramic materials. However, there is a lack of studies
confirming this assumption.
The effect of different polishing systems on the surface properties of the ceramic
materials has been investigated. Previous in vitro studies determined the effect of different polishing systems on the surface roughness
of various ceramic materials by matching between porcelain or zirconia polishing kits
and its specified glass ceramic or zirconia material,[8]
[10]
[14] and cross-use between zirconia or porcelain polishing kits and glass ceramic, ZLS,
or zirconia.[4]
[15]
[16] However, the findings varied depending on the polishing systems and ceramic materials.
In addition to surface roughness, hardness testing is necessary for ceramic materials.[17] This is because the hardness value reflects the ease of milling and marginal chipping,
which are important factors for dental ceramic use.[18] However, there is little information on the surface roughness and hardness of CAD/CAM
ceramic materials after polishing with a multipurpose polishing kit at different polishing
durations. Also, lack of studies has been conducted to determine the changes of polishing
bur weight after the CAD/CAM ceramic materials reach an optimal surface roughness
level.
Therefore, the primary objective of this study was to investigate the effect of the
multipurpose polishing kit on the surface roughness of three CAD/CAM ceramic materials;
lithium disilicate glass-matrix ceramic, zirconia, and ZLS at different polishing
durations. The secondary hypotheses were to determine the effects of the multipurpose
polishing kit on the hardness of the three materials, and on the changes of polishing
bur weight. The primary null hypothesis was that the surface roughness changes of
the three CAD/CAM ceramic materials would not be different after polishing with the
multipurpose polishing kit. The secondary null hypotheses were that the hardness changes
of all the three materials and the polishing bur weight would not be different after
final polishing process.
Materials and Methods
[Table 1] presents the three CAD/CAM ceramic materials used in the present in vitro study: lithium disilicate glass ceramic (IPS e.max CAD, Ivoclar Vivadent, Schaan,
Lichtenstein), translucent zirconia (VITA YZ, VITA Zahnfabrik H. Rauter GmbH & Co.),
and ZLS ceramic (Celtra Duo, Dentsply Sirona), all of which were shade A3. Twelve
rectangular (7 × 5 × 4 mm) specimens of each ceramic material were prepared. The IPS
e.max CAD and Celtra Duo ceramic blocks were prepared into the predetermined dimensions
using a low-speed saw (Isomet 1000; Buehler), and the VITA YZ XT blank was prepared
using the CAD/CAM technique with a 20% enlarged dimension of 6.5 × 9 × 5 mm to compensate
for sintering shrinkage. The IPS e.max CAD and VITA YZ specimens were sintered in
a CEREC SpeedFire furnace (Dentsply Sirona) and a VITA Zyrcomat 6000 MS (VITA Zahnfabrik),
respectively, per the manufacturers' recommendation.[19]
[20] The Celtra Duo specimens were prepared by sintering.[21] A digital vernier caliper (Digimatic; Mitutoyo) was used to measure the specimens'
dimensions. The ceramic specimens underwent ultrasonic cleaning (Bransonic model 5210;
Branson) in distilled water for 10 minutes and dried with absorbent paper. They were
fixed with clear resin in a polyvinyl chloride pipe as illustrated in [Fig. 1].
Table 1
Materials and instruments used in this study
|
Material type
|
Brand
|
Manufacturer
|
Main compositions
|
|
CAD/CAM ceramic materials
|
|
Lithium disilicate glass ceramic
|
IPS e.max CAD (LT) shade A3
|
Ivoclar Vivadent
|
SiO2 (57–80%), Li2O (11–19%), K2O (0–13%), P2O5 (0–11%), ZrO2 (0–8%), ZnO (0–8%), Al2O3 (0–5%), MgO (0–5%), Coloring oxides (0–8%)
|
|
Translucent zirconia
|
VITA YZ (XT)
|
VITA Zahnfabrik
H. Rauter GmbH & Co. KG
|
ZrO2 (86–91%), Y2O3 (8–10%), HfO2 (1–3%), Al2O3 (0–1%), Pigments (0–1%)
|
|
Zirconia-reinforced lithium disilicate
|
Celtra Duo (LT) shade A3
|
Dentsply Sirona
|
SiO2 (58%), Li2O (15%), P2O5 (5%), ZrO2 (10.1%), Al2O3 (1.9%), CeO2 (2%), Tb2O3 (1%)
|
|
Polishing instruments
|
|
Fine diamond grinding:
300,000 rpm polishing speed
|
Fine diamond bur 881F 014 (1.4 × 8 mm dimension)
|
Hager & Meisinger GmbH
|
Diamond grit size (27–76 μm)
|
|
Multipurpose polishing kit
1. Coarse polishing:
10,000 rpm polishing speed
2. Fine polishing:
10,000 rpm polishing speed
|
EVE DIACERA HP
1. H2DCmf
(4 × 13 mm dimension)
2. H2DC
(4 × 13 mm dimension)
|
EVE Ernst Vetter GmbH
|
1. Diamond impregnated (25–35 µm) in polyurea
2. Diamond impregnated (3–6 µm) in polyurea
|
Abbreviations: CAD/CAM, computer-aided design/computer-aided manufacturing; rpm, revolutions
per minute.
Fig. 1 Ceramic specimen embedded in resin.
The sample size was calculated by a software program (G*Power, version 3.1.9.2; Heinrich-Heine-Universität
Düsseldorf) using the F-test and analysis of variance (ANOVA)-fixed effects, omnibus, one-way. The data from
our pilot findings in the three CAD/CAM ceramic materials (n = 3 per group) demonstrated that the surface roughness changes (ΔRa in µm) between
grinding and complete fine polishing of IPS e.max CAD, VITA YZ, and Celtra Duo were
0.981, 1.267, and 1.148, respectively, and the average standard deviation (SD) was
0.150. Giving an alpha value of 0.05, a power of 0.80, and the effect size of 0.782,
a sample size of 10 for each group was calculated. Two additional specimens were included
in each material for scanning electron microscope (SEM) analysis.
The grinding and polishing instruments are described in [Table 1]. To ensure standardization of the applied force, a custom pressure control device
was used ([Fig. 2]). The device consists of an electronic control panel, pressure gauge, direction
control joystick, load cell, and handpiece connector. The load cell is a transducer
that converts the polishing force into a measurable electrical output as a pressure
gauge. The direction control joystick controls the polishing bur on the vertical and
horizontal axis to provide the desired polishing force. A slow-speed handpiece (NSK
Nakanishi Inc.) was used to polish the specimens in a forward-backward direction.
Fig. 2 A custom polishing device for controlling the applied force.
To simulate clinical gross contouring, each ceramic specimen was ground with a fine
diamond bur (Meisinger, Hager & Meisinger GmbH) for 15 seconds using a high-speed
handpiece (KaVo Dental GmbH) mounted on the custom pressure control device. A 1-N
force with a grinding speed of 1 mm/s velocity and 200,000 revolutions per minute
(rpm) was applied on each specimen using a gentle stroking forward-backward motion.
A new bur was used after grinding five specimens to maintain a consistent amount of
diamond grit.
The polishing process was performed by using a two-step ceramic multipurpose polishing
kit (EVE Diacera; EVE Ernst Vetter GmbH) that consisted of a coarse polishing bur
(EVE Diacera H2DCmf, green rubber) and a fine polishing bur (EVE Diacera H2DC, pink
rubber). The polishing force was controlled at 1 N,[22] and the 10,000 rpm polishing speed was used per the manufacturer's recommendation.[11] The sweeping motion was performed in the same direction as the grinding process
for 30 seconds, then rotated 90 degrees and swept perpendicularly to the previous
direction for another 30 seconds. The polishing duration consisted of 60 seconds for
coarse polishing and 60 seconds for fine polishing, which was obtained from our pilot
result that revealed the surface roughness value plateaued after 60 seconds of polishing.
A new polishing bur was used after polishing five specimens. Each polishing procedure
was paused every 15 seconds for the surface roughness measurement ([Fig. 3]). The weight of the coarse and fine polishing burs (n = 6) was measured at baseline and after polishing five specimens using a digital
analytical balance (Radwag AS220/C/2).
Fig. 3 Schematic diagram of the grinding and polishing steps, surface roughness (Ra) measurement,
and scanning electron microscope (SEM) analysis.
The surface roughness value (Ra) was analyzed using a noncontact optical profilometer
(Alicona InfiniteFocusSL) at 50× magnification with a laser to assist in focusing
and controlling the same measurement position. Prior to each Ra measurement, the specimen
underwent ultrasonic cleaning in distilled water for 5 minutes and air-dried. For
each specimen, five areas of 0.4 × 0.4 mm2 were measured, one at the center and the others 1-mm away from the center in four
directions. The measurement direction was set perpendicular to the grinding direction,
providing a 4-mm evaluation length according to the International Organization for
Standardization standard.[23] The mean Ra was calculated from the average value of the five areas. The mean Ra
of the three ceramic materials was calculated after grinding and every 15 seconds
of coarse and fine polishing until 60 seconds of polishing was achieved.
The complete polishing Ra of the specimens was compared with the lab as-received specimens
and human enamel Ra. Five specimens that were prepared in the laboratory were high-gloss
polished by a laboratory technician, and their surface roughness was measured. Extracted
human teeth were collected and the experimental protocol was approved by the Human
Research Ethics Committee of the Faculty (HREC-DCU 2021-067). Thirty extracted human
maxillary and mandibular teeth, consisting of 10 incisors, 10 premolars, and 10 molars
without visible dental caries or restoration, were collected and stored in 0.1% thymol
solution after extraction. The teeth were sectioned mesiodistally and embedded in
a resin block exposing the buccal and lingual surfaces for surface roughness measurement,
and the mean Ra value was calculated. The specimen surface was examined using a SEM
(JSM-6400; JEOL). Three specimens from each ceramic material were randomly selected
and were gold-coated in a vacuum prior to the examination.
A Vickers hardness tester (FM-810, Future-Tech Corp) with a load of 100 g for 15 seconds
was used to determine the effect of the polishing procedures on the surface hardness
of the materials. The hardness was measured at prior to polishing (lab as-received
specimens) and after the final polishing process (n = 4 per group). Each sample received five measurements on a polished surface in a
linear pattern, and the mean Vickers hardness was calculated.
The data were analyzed using a statistical software program (IBM SPSS Statistics,
v28.0; IBM Corp) at α = 0.05. The normality of the data was determined using the Shapiro–Wilk test, and
parametric statistics were adopted. Two-way repeated-measures ANOVA was used to determine
the effect of material types and polishing duration on surface roughness. Because
there was an interaction between two independent variables, one-way ANOVA and Tukey
post hoc comparison test were used to determine the surface roughness differences
between material types after each grinding and polishing duration. One-way repeated-measures
ANOVA and Tukey post hoc comparison test were used to determine the surface roughness
change in each ceramic material after grinding and polishing. Moreover, one-way ANOVA
and Tukey post hoc comparison test were used to compare the surface roughness change
(ΔRa), Vickers hardness, and bur weight at baseline, and after polishing of three
ceramic materials. Changes in Vickers hardness were determined using the dependent
t-test, and the bur weight after polishing was compared to the initial value using
a one-sample t-test.
Results
After grinding, the Ra values (mean ± SD) in the VITA YZ and Celtra Duo groups were
significantly higher those in the of IPS e.max CAD group ([Table 2]). However, the Ra value in the VITA YZ and Celtra Duo groups were reduced more than
the IPS e.max CAD group after 15-second coarse polishing. The surface roughness of
all ceramics gradually decreased after coarse and fine polishing. However, the VITA
YZ groups' surface roughness was relatively stable from 30-second fine polishing onwards.
At all polishing durations, the Ra values of the three ceramic materials were not
significantly different. The final Ra value after complete fine polishing were not
significantly different from that of enamel (mean Ra ± SD = 0.573 ± 0.167) and the
lab as-received specimens.
Table 2
Surface roughness (Ra in µm) (mean ± standard deviation) of CAD/CAM ceramic materials
and after grinding and polishing and lab as-received specimens (n = 10)
|
Material types
|
Lab as-received specimens
|
Grinding
|
Coarse polishing
|
Fine polishing
|
|
15 s
|
30 s
|
45 s
|
60 s
|
15 s
|
30 s
|
45 s
|
60 s
|
|
IPS e.max CAD
|
0.698 (±0.084)
|
1.440 (±0.136)aB
|
1.121 (±0.201)b
|
1.095 (±0.164)b
|
0.969 (±0.122)c
|
0.936 (±0.131)c
|
0.864 (±0.141)c
|
0.756 (±0.065)d
|
0.701 (±0.089)de
|
0.647 (±0.110)e
|
|
VITA YZ
|
0.668 (±0.072)
|
1.838 (±0.205)aA
|
1.318 (±0.230)b
|
1.149 (±0.232)bc
|
1.093 (±0.206)c
|
0.912 (±0.299)d
|
0.803 (±0.151)d
|
0.658 (±0.131)e
|
0.635 (±0.143)e
|
0.595 (±0.132)e
|
|
Celtra Duo
|
0.637 (±0.100)
|
1.663 (±0.113)aA
|
1.256 (±0.121)b
|
1.185 (±0.141)bc
|
1.124 (±0.124)c
|
1.018 (±0.159)d
|
0.768 (±0.140)e
|
0.698 (±0.095)f
|
0.647 (±0.122)fg
|
0.593 (±0.088)g
|
Abbreviation: CAD/CAM, computer-aided design/computer-aided manufacturing.
Note: Different lowercase letters indicate significant difference in rows (p < 0.05). Different uppercase letters indicate significant difference in columns (p < 0.05).
After complete 60-second coarse polishing, the greatest Ra reduction was seen in the
VITA YZ group, followed by the Celtra Duo and IPS e.max CAD groups ([Table 3]). The amount of Ra reduction was not significantly different between the three ceramic
materials after continuing from the coarse to complete fine polishing. In the ceramic
materials, coarse polishing resulted in a greater Ra reduction compared with fine
polishing. After complete fine polishing, the Vickers hardness of the ceramic materials
was similar to that of the lab as-received specimens. The Vickers hardness in the
VITA YZ group was highest, followed by the Celtra Duo and IPS e.max CAD, respectively.
Table 3
Changes in the surface roughness (ΔRa in µm) and Vickers hardness (mean ± standard
deviation) of the CAD/CAM ceramic materials after polishing
|
Ceramic types
|
Surface roughness reduction (ΔRa, µm)
|
Vickers hardness
|
|
Grind-coarse
|
Coarse-fine
|
Grind-fine
|
Lab as-received specimens
|
After complete polishing
|
|
IPS e.max CAD
|
0.504 (±0.162)B
|
0.289 (±0.151)A
|
0.793 (±0.207)B
|
557.9 (±30.7)A
|
554.7 (±4.2)A
|
|
VITA YZ
|
0.926 (±0.350)A
|
0.317 (±0.210)A
|
1.243 (±0.256)A
|
1235.9 (±20.3)B
|
1191.6 (±20.1)B
|
|
Celtra Duo
|
0.645 (±0.173)B
|
0.425 (±0.143)A
|
1.070 (±0.128)A
|
640.7 (±35.6)C
|
632.4 (±5.1)C
|
Abbreviation: CAD/CAM, computer-aided design/computer-aided manufacturing.
Note: Different uppercase letters indicate significant difference in columns (p < 0.05).
After polishing five specimens, the weight of the coarse polishing burs was significantly
decreased from the baseline value. However, the weight of the fine polishing burs
was not ([Table 4]). The amount of weight loss in the coarse bur was the greatest after polishing VITA
YZ, followed by Celtra Duo and IPS e.max CAD.
Table 4
The weight of coarse and fine polishing burs (g) (mean ± standard deviation) after
5-specimen polishing (n = 6 per group)
|
Material types
|
Bur weight (g)
|
|
Coarse polishing
|
Fine polishing
|
|
(Initial weight = 1.445 g)
|
(Initial weight = 1.432 g)
|
|
IPS e.max CAD
|
1.433 (±0.004)A
[a]
|
1.427 (±0.001)A
|
|
VITA YZ
|
1.406 (±0.004)B
[a]
|
1.424 (±0.006)A
|
|
Celtra Duo
|
1.429 (±0.013)A
[a]
|
1.428 (±0.004)A
|
Note: Different uppercase letters indicate significant difference in columns (p < 0.05).
a Significant reduction from the initial weight (p < 0.05).
The SEM analyses at 300 × , 1,000 × , and 10,000× magnification demonstrated a wavy-like
pattern on the CAD/CAM ceramic materials' surfaces after grinding, with a bead-like
wavy pattern on the VITA YZ and a scaly pattern on the Celtra Duo surfaces ([Fig. 4]). The surface morphology of the materials became progressively smoother from coarse
to fine polishing, which was a similar pattern to those of the lab as-received specimens
([Fig. 5]).
Fig. 4 Scanning electron microscope (SEM) of the ground specimens at 300 × , 1,000 × , and
10,000× magnification. IPS e.max CAD and Celtra Duo demonstrated grooves with a scale-like
pattern; however, VITA YZ had grooves with a bead-like pattern. At 10,000× magnification,
IPS e.max CAD presented a combination of large and small crystal grains with a scale-like
glass matrix (upper-left corner), and VITA YZ and Celtra Duo had a more homogenous
grain size.
Fig. 5 Scanning electron microscope (SEM) of the ground and polished ceramic specimens at
5,000× magnification. Each ceramic demonstrated shallower grooves after coarse polishing
and a smoother surface after fine polishing, compared with the lab as-received specimens.
The VITA YZ specimen presented a greater roughness reduction after coarse polishing
than the other ceramics.
Discussion
The present study evaluated the effect of the multipurpose polishing kit on the surface
roughness of three CAD/CAM ceramic materials; lithium disilicate glass-matrix ceramic,
zirconia, and ZLS at different polishing durations. We found that the surface roughness
of the three CAD/CAM ceramic materials was significantly reduced after polishing with
the multipurpose ceramic polishing kit. The surface roughness at all polishing durations
was similar among the materials regardless of material hardness, except after grinding
where the surface roughness of VITA YZ specimens was highest, followed by the Celtra
Duo and IPS e.max CAD. After complete fine polishing, the greatest reductions in surface
roughness and a coarse polishing bur weight were shown in VITA YZ, followed by Celtra
Duo and IPS e.max CAD. However, the hardness changes of all materials could not be
detected. Based on these results, the null hypothesis was partially rejected.
Types of ceramic material and polishing systems are major factors that affects the
material's surface roughness.[4]
[5]
[8]
[15]
[24] As demonstrated in previous studies,[14]
[24] the optimal surface roughness of a ceramic material should be at the level of the
lab as-received specimen and opposing enamel. Previous in vitro studies used the polishing kits that were specific to porcelain/glass ceramic or
zirconia, to polish the ceramic materials. In contrast, the present study used EVE
Diacera as the multipurpose ceramic polishing system, and the findings revealed that
surface roughness of the CAD/CAM ceramic materials after complete polishing was similar
to that of enamel and the lab as-received specimens. Smoothening the ceramic surface
close to that of enamel decreases the enamel wear of the opposing tooth.[6]
Our findings were supported by the study by Matzinger et al which found that the chairside
and labside polishing had similar effectiveness in reducing surface roughness of three
CAD/CAM materials, comprising IPS e.max CAD, Celtra Duo, and VITA Suprinity which
is a ZLS material.[16] A study by Jum'ah et al revealed that polishing the 3Y-translucent zirconia with
the two-step EVE Diacera Twist for 90 seconds could reduce surface roughness of the
ground material to the similar level of the material undergone glazing.[10] Vichi et al reported that the surface roughness of VITA Suprinity after polishing
with Suprinity polishing kits for 60 seconds was significantly lower than that of
IPS e.max CAD after polishing with the Optrafine polishing kits. However, the study
used specific type of polishing kit for the indicated CAD/CAM ceramic materials.[8] In the present study, the three CAD/CAM ceramic materials had similar surface roughness
values after complete polishing; however, their changes across the polishing durations
demonstrated different patterns. As demonstrated by Vichi et al, these results may
be due to different material's microstructures,[8] and it is expected that the surface roughness would reduce more when the grain size
of a polishing bur is larger than that of the ceramic crystalline structure. The crystalline
structure of IPS e.max CAD lithium disilicate glass ceramic is needle-like with an
800-nm diameter and 5,000-nm length, while VITA YZ zirconia has an oval-shape with
a 815-nm diameter. Celtra Duo, a ZLS material, contains a mixture of shorter needle-like
500 to 700 nm crystals, of which the diameters are smaller than those of VITA YZ.
In contrast, the grain size of the coarse and fine polishing bur is approximately
2,500 to 3,500 nm and 300 to 600 nm, respectively. Due to the larger grain size of
the coarse polishing bur compared with VITA YZ and Celtra Duo, the two materials demonstrated
a greater surface roughness reduction compared with IPS e.max CAD after coarse polishing.
The VITA YZ had greater surface roughness reduction than Celtra Duo due to its more
homogenous crystal structure. In contrast, IPS e.max CAD presented the least surface
roughness reduction among the three materials after complete polishing which could
be due to a relatively larger and heterogeneous crystalline structure compared with
the polishing bur's grain size. Thus, the efficacy of the multipurpose ceramic polishing
kit is rather depended on the polishing bur's grain size in relative with the morphology
and size of the ceramic's crystalline structure.
In the present study, the surface roughness was measured after every 15 seconds of
polishing, and the total 60-second duration was chosen for coarse and fine polishing
because that was when the surface roughness of the ceramic materials reached the plateau
level. It was found that the surface roughness of VITA YZ did not reduce beyond 30-second
fine polishing, which might be because the surface roughness of VITA YZ was already
substantially reduced after coarse polishing. In accordance with our finding, Huh
et al found no surface roughness difference between the zirconia undergoing 60- and
120-second polishing durations,[13] which might be because the surface roughness had already reached the plateau level
after 60-second of polishing. Vichi et al also reported lower surface roughness value
of VITA Suprinity and IPS e.max CAD after 60-second compared with 30-second polishing.
Therefore, a polishing duration plays important role in achieving clinically acceptable
surface roughness level.
In contrast to surface roughness, the hardness of all the three CAD/CAM ceramics did
not change after complete fine polishing. The hardness value of VITA YZ was highest,
followed by Celtra Duo and IPS e.max CAD, according to the degree of material toughness.
Higher Vickers hardness value reflects milling difficulty, being less prone to marginal
chipping, and less permanent deformation of the material surface.[17]
[18]
[25] Our findings were consistent with those of Mavriqi et al who found that the Vickers
hardness of IPS e.max CAD was lower than that of ZLS ceramic, including Celtra Duo
and VITA Suprinity. This is likely because zirconia, which has a finer-grain structure,
would be more resistant to permanent deformation compared with ZLS and glass-based
ceramic materials.[25] Our findings suggest that the degree of increased surface roughness did not rely
on the hardness of the ceramic materials, and the two properties were not correlated.
In addition, the multipurpose polishing kit smoothened the ceramic materials without
altering the hardness property of the CAD/CAM ceramic materials.
After five-specimen polishing, the coarse polishing bur demonstrated the greatest
weight loss after VITA YZ polishing, compared with Celtra Duo and IPS e.max CAD. These
results were consistent with the surface roughness reduction in that the more the
surface roughness reduction, the greater bur abrasion. In contrast, the amount of
weight loss in the fine polishing bur and surface roughness reduction in ceramics
were not related. This might be because the diamond abrasives of the fine polishing
bur are homogenous and well-embedded in a polyurea core due to their smaller size
than the coarse polishing bur. Celtra Duo, a ZLS material, combines the advantage
of surface roughness reduction after using the multipurpose ceramic polishing kit
as compared to VITA YZ, but similar to IPS e.max CAD in terms of lower polishing bur's
abrasion.
Possible confounding variables that could affect surface roughness of ceramic materials
were controlled in this study, including the ceramic color and translucency, applied
pressure, and polishing device speed.[15] The materials used in this study were A3 color and their translucency was relatively
similar. Previous studies found that the translucency of IPS e.max CAD–LT is closed
to Celtra Duo–LT,[9] which is closed to VITA YZ–XT.[26] For the polishing procedures in several in vitro studies, grinding and polishing was performed by a calibrated operator using finger
pressure,[4]
[8]
[15]
[24] with an applied force ranging from 0.4 to 2 N.[8]
[15]
[24] However, a custom device is still needed to standardize the applied pressure.[24] Thus, we fabricated a custom polishing machine to standardize the grinding/polishing
speed and the applied force exerted on the materials. A 1-N force was chosen as the
polishing pressure because it was reported in a previous study,[22] and was also the average value obtained from our pilot result that evaluated the
polishing pressure applied by the 20 prosthodontists.
The present study has some limitations. Only one system of the multipurpose ceramic
polishing kit and one brand for each type of CAD/CAM ceramic materials were used as
a representative, which limited a generalizability of the finding. Thus, our results
may be differed from what was achieved clinically because the applied force and polishing
duration could be inconsistent among dentists. In addition, surface roughness change
can further be affected by the oral environment. Further studies should explore other
properties of the CAD/CAM ceramic materials, such as wear of the material and its
opposing tooth to comprehensively evaluate the clinical performance of the material.
Conclusion
After using the multipurpose polishing kit, surface roughness of the CAD/CAM ceramic
materials was reduced to the clinically acceptable level compared with the enamel
and the lab as-received specimens. Surface roughness and the weight of coarse polishing
bur reductions were greatest in VITA YZ, followed by Celtra Duo and IPS e.max CAD.
However, the material hardness did not change after final polishing process.
