Keywords
self-etching ceramic primer - lithium meta-silicate ceramic - surface roughness
Introduction
The popularity of indirect restorations made with digital technology has increased
among dental practitioners owing to their reliable long-term clinical performance
due to their excellent mechanical and esthetic properties.[1]
[2] Such indirect restorations are usually fabricated from two categories of materials:
dental ceramics and indirect composites. Dental ceramics can be classified into oxide
ceramics, such as zirconium dioxide (zirconia) and glass-based ceramics. Glass-based
ceramics are mainly composed of glass phase and crystalline phase. They are subdivided
into four types: (1) feldspathic ceramics, (2) leucite-reinforced ceramics, (3) lithium
disilicate ceramics, and (4) zirconia-reinforced lithium silicate (ZLS) ceramics that
contain additional zirconia.[3]
Regardless of the type of the glass-based ceramic, the bond strength of resin-ceramic
is a determinant for the clinical performance of ceramic restorations such as nonretentive
partial ceramic crown.[4] Based on the glass phase and crystalline phase content of glass-based ceramic materials,
resin-ceramic bonding is a two-step procedure; the first step involves the hydrofluoric
acid (HF) etching to dissolve the superficial glass phase in the ceramics, creating
significant topographic changes to enhance micromechanical bonding.[3]
[5] The second step involves the application of a 3-methacryloxypropyltrimethoxysilane
(silane)-based primer to enable chemical adhesion between the primed glass-ceramics
and methacrylate-based materials such as resin-cements or adhesives.[3]
[6] Despite the reliable bond strength achieved by this multistep approach, there is
a risk of technical errors occurring during HF application, deactivating, washing,
and post-etch cleaning. For example, prolonged HF etching can deteriorate the mechanical
properties of glass ceramics due to excessive dissolution of the glass phase, and
inadequate post-etch cleaning can affect the resin-ceramic bond strength due to presence
of residue on the ceramic surface after etching.[7]
[8] In addition, HF is a toxic and hazardous material that can induce immediate nasal
inflammatory responses by inhalation.[9]
Previously, some materials such as titanium tetrafluoride and acidulated phosphate
fluoride were suggested as alternatives to HF for etching lithium disilicate and feldspathic
ceramics; however, none of them possessed an efficacy comparable to that of HF.[10]
[11] Recently, a self-etching ceramic primer (SECP) has been proposed to combine the
effects of HF etching and silane priming, and it offers a safe, less time-consuming,
and less technique-sensitive surface treatment of glass-based ceramic materials.[12]
[13] SECP showed promising results in several in
vitro studies that evaluated the effects of SECP on bonding of lithium disilicate glass
ceramics using shear and microshear bond strength tests.[12]
[14]
[15] However, more studies are required to evaluate its etching efficacy on other types
of ceramics, such as ZLS ceramic materials used for fabrication of full-coverage restorations
such as crowns and partial coverage restorations such as onlays. ZLS ceramic materials
can also be utilized to fabricate ceramic veneers.[5] Therefore, the objective of this study was to assess the etching efficacy of SECP
on two ZLS ceramic materials. The null hypothesis is, there would be no difference
between the surface roughness of ZLS obtained following the surface treatment with
either HF or SECP.
Materials and Methods
Specimen Preparation
Two ZLS ceramics, Celtra Duo (DeguDent GmbH, Hanau-Wolfgang, Germany) and Vita Suprinity
(Vita Zahnfabrik, Bad Säckingen, Germany), were used in this study ([Table 1]). The blocks of each ceramic material were sectioned using a 4-inche diamond cutting
blade (IsoMet Blade, Buehler, Lake Bluff, Illinois, United States) mounted on a low-speed
precision cutting saw (IsoMet 1000 Linear Precision Saw, Buehler) into a total of
36 slices of approximately 2-mm thickness. The ceramic slices were polished under
water coolant with #600 silicon carbide paper disks attached to a grinding machine
(Automata, Jean Wirtz, Germany) performing 200 revolutions per minute. The polished
ceramic slices were subjected to ultrasonic cleaning with distilled water for 10 minutes
and air-dried for 30 seconds. The sintering of Vita Suprinity slices was performed
according to the manufacturer's instructions using VITA V60 i-Line PLUS, Vita Zahnfabrik.[16]
Table 1
The composition of the materials used in the study
|
Group (surface treatment)
|
Celtra Duo
|
|
Monobond Etch and Prime, Ivoclar Vivadent, Schaan, Liechtenstein
|
Ammonium polyflouride, trimethoxypropyl methacrylate, solvents (alcohol and water),
food colorant (fast green)
|
|
Vita Suprinity, Vita Zahnfabrik, Bad
Säckingen, Germany
|
SiO2, Li2O, K2O, P2O5, Al2O3, ZrO2, CeO2, pigments
|
|
Celtra Duo, DeguDent GmbH, Hanau Wolfgang, Germany
|
Lithium silicate with ∼10% ZrO2s
|
Surface Treatment
The specimens of each ceramic material were randomly distributed into three groups
according to the surface treatment applied (n = 12 per group). In group 1 (polished), no surface treatment was performed. In group
2 (SECP), Monobond Etch and Prime, Ivoclar Vivadent, Schaan, Liechtenstein, was applied
with a microbrush onto the top surface of the ceramic specimens, agitated for 20 seconds
with a slight pressure, and allowed to react with the ceramic for 40 seconds. Subsequently,
SECP was thoroughly rinsed with water and air-dried for 10 seconds. Specimens of group
3 (HF) were subjected to 4.5% HF (IPS Ceramic Etching Gel Ivoclar Vivadent) etching.
HF was applied with a small disposable brush onto the top surface of each ceramic
specimen; the treatment duration was 20 seconds for Celtra Duo and 30 seconds for
Vita Suprinity, according to the manufacturer's recommendations. The remaining HF
was removed by intensive spraying with water, followed by air drying. The etched ceramic
specimens were ultrasonically cleaned for 5 minutes to remove any residue.
Surface Topography Evaluation
Half of the specimens (n = 6) from each group were dehydrated in ascending concentrations of ethanol before
gold sputtering using a sputter coater (fine coat ion sputter JFC-1100, JEOL Ltd.,
Tokyo, Japan) for 3 minutes at 30 mA. Subsequently, the gold-sputtered ceramic slices
were fixed onto brass stubs, and the surface topographic alterations were evaluated
using scanning electron microscopy (SEM) (JSM-6610LV, JEOL Ltd.) at a magnification
of 5,000× and 10,000× at a working distance of 8 mm. SEM was operated at 20 kV.
Surface Roughness Evaluation
The surface roughness (Ra) of the other half of the specimens (n = 6) from each group was evaluated using a high-resolution three-dimensional (3D)
noncontact optical profiler (Contour GT-K 3D Optical Microscope, Bruker, Billerica,
Massachusetts, United States). The specimens were vertically scanned at ×5 Michelson
magnification and a field of view of 1 × 1 mm. The scan speed was 1 × , and thresholding
was 4. The software used for the analysis and graphical output was Vision 64 (Bruker,
Billerica). Four scans were obtained and averaged for each specimen.
Statistical Analysis
A two-way analysis of variance (ANOVA) test was utilized to examine the effect of
“surface treatment” and “ceramic material” as well as their interactions on the obtained
surface roughness (Ra). Tukey's multiple comparison test was used to evaluate the
differences between the tested groups. The statistical analyses were performed using
R software version 4.1.2, R Foundation for Statistical Computing, Vienna, Austria.
Results
Surface Topography
Both lithium metasilicate and lithium orthophosphate crystals observed after HF etching
were prominent. In contrast, SECP resulted in milder etching patterns ([Figs. 1B, E] and [2B, E]). Surface microirregularities were less prominent than those created after HF etching;
however, lithium metasilicate and lithium orthophosphate crystals were observed. The
etching patterns obtained after HF etching or SECP application were markedly different.
The etching of both ceramics with either 4.7% HF or SECP resulted in clear topographic
surface alterations compared to the control group, in which a smoother and more homogenous
surface topography was observed ([Figs. 1A, D] and [2A, D]). HF etching resulted in an aggressive etching pattern with deeper and more numerous
microporosities and grooves formed within the ceramic surface.
Fig. 1 Representative scanning electron microscopy (SEM) photomicrographs at 5,000× and
10,000× of Celtra Duo after polishing (A, D) the smooth surface with no etching pattern; self-etching ceramic primer (SECP) surface
treatment (B, E) with a mild etching pattern; hydrofluoric acid (HF) etching (C, F) with an aggressive etching pattern. White circle: metasilicate crystals are larger
in size compared to those in Vita Suprinity ([Fig. 2F]).
Fig. 2 Representative scanning electron microscopy (SEM) photomicrographs of Vita Suprinity
at 5,000× and 10,000 × . Polished (A, D) smooth surface with no etching pattern; self-etching ceramic primer (SECP) surface
treatment (B, E) with a mild etching pattern; hydrofluoric acid (HF) etching (C, F) with an aggressive etching pattern. Yellow circle: metasilicate crystals are smaller
in size compared to those in Celtra Duo ([Fig. 1F]).
Surface Roughness
The mean and standard deviation surface roughness values for the tested groups are
presented in [Table 2]. Two-way ANOVA results ([Table 3]) indicated that only the surface treatment had a significant effect on the obtained
surface roughness (p < 0.001). Both SECP and HF etching surface treatments resulted in a statistically
significant increase in the surface roughness of both ceramic materials, compared
to that of their respective control group specimens (polished). However, the surface
roughness obtained after SECP surface treatment was significantly lower than that
obtained after HF etching. Within each group (polished, SECP, and HF), there was no
statistically significant difference between the surface roughness values of the two
ceramic materials. Representative images for each group illustrate the effects of
each surface treatment on the surface roughness of the ceramic. 3D representative
optical profilometer images ([Fig. 3]) illustrated the variable surface roughness of ceramics subjected to different surface
treatments (polishing, SECP, or HF etching).
Table 2
Mean ± standard deviation surface roughness (Ra) expressed in (μm) of tested groups
|
Group (surface treatment)
|
Celtra Duo
|
Vita Suprinity
|
|
Polished (control)
|
1.021 ± 0.41a
|
0.95 ± 0.2a
|
|
SECP
|
2.28 ± 0.57a
|
2.3 ± 0.65a
|
|
HF
|
3.56 ± 0.79a
|
3.63 ± 1.11a
|
Abbreviations: HF, hydrofluoric acid; SECP, self-etching ceramic primer.
aIndicate statistically significant difference.
Table 3
Two-way analysis of variance (ANOVA) results
|
Variable
|
Df
|
Sum Sq
|
Mean Sq
|
F-value
|
p-Value
|
|
Ceramic
|
1
|
0.01
|
0.000
|
0.000
|
0.882
|
|
Surface treatment
|
2
|
40.83
|
20.41
|
43.89
|
< 0.001[a]
|
|
Ceramic * Surface treatment
|
2
|
0.03
|
0.015
|
0.031
|
0.941
|
|
Residuals
|
30
|
14.11
|
0.47
|
|
Abbreviations: Df, degree of freedom; Sum Sq, sum squares; Mean Sq: mean squares.
a Indicates statistically significant effect.
Fig. 3 Three-dimensional representative optical profilometer images of the two ceramic materials
(Celtra Duo and Vita Suprinity). Self-etching ceramic primer (SECP)-treated group
specimens (B, E) presented significantly higher surface roughness compared to the polished (control)
group (A, D). Hydrofluoric acid (HF)-treated group specimens (C, F) presented the highest surface roughness for both ceramic materials.
Discussion
The surface treatment of indirect ceramic restorations is an indispensable step to
ensure the adequate cementation or repair of such restorations. This study was designed
to evaluate the etching efficacy of SECP, as a new ceramic surface treatment, on two
ZLS ceramic materials. This is clinically relevant because ZLS ceramic materials are
extensively used in modern prosthetic dentistry while dental practitioners might be
lacking essential information regarding the effect of novel surface treatments on
their surface properties, particularly because there is no consensus on the use of
SECP for surface treatment of ZLS ceramics. Both qualitative using SEM at different
magnifications and quantitative using surface profilometer evaluations of the surface
topographic features and surface roughness of ZLS ceramic materials following surface
treatment with SECP or HF were considered. This is believed to provide a multiscale
assessment of the surface changes. ZLS ceramic materials are composed of a glassy
phase, crystalline phase, and small amount of tetragonal zirconia.[17] The crystallization of ZLS ceramic materials can be either full or partial (such
as in Vita Suprinity),[18] which explains the need for sintering to such a ceramic material. The SEM examination
of ceramic surfaces provides a comprehensive qualitative detailed evaluation of the
surface topographic features through high-resolution micrographs at a high magnification[19]
[20]
[21]; however, the use of a surface profilometer can provide a quantitative evaluation
of the surface roughness;[22] thus, statistical analysis can be performed.
There was a statistically significant difference between the surface roughness values
of the two ceramic materials following the surface treatment with SECP or HF. Therefore,
the null hypothesis was rejected. This is in accordance with the SEM evaluation, in
which milder etching patterns and fewer surface topographic changes were noticed in
specimens after SECP, compared with HF etching. The marked difference between the
respective etching patterns and surface roughness of SECP-treated and HF-treated ceramic
materials can be attributed to the extent of reaction with the glassy phase in the
ceramic materials, rather than the acidity, despite the different pH of each treatment.
HF etching is the most reliable surface treatment of glass-based ceramic materials,
such as ZLS.[23] HF etching depends on the chemical reaction between the silicon content in the ceramic
materials and fluoride ions of the HF, which results in the dissolution of the glass
content in the ceramic material.[3] Thus, a significant surface alteration is produced. The application protocol (time)
depends on the composition (glass content) of dental ceramics.[23]
The SECP etching effect is mainly related to its tetrabutylammonium dihydrogen trifluoride
content, which can chemically etch silica-based materials.[24] According to the manufacturer's instructions, SECP should be rubbed onto the ceramic
surface for at least 20 seconds to ensure an intimate contact between the SECP and
the ceramic surface. Recent studies showed that prolonged application or active application
mode (scrubbing) results in more dissolution of the ceramic's glass content, which
creates more significant surface topographic alterations,[14]
[25] even though they are still less distinct compared to those produced by HF etching.
The surface topographic features of both ceramics were similar when they were subjected
to the same surface treatment (HF or SECP). ZLS ceramics are composed of a zirconia-reinforced
matrix phase, in which the major crystalline phase is composed of lithium metasilicate
crystals.[18]
[26]
[27] However, in Celtra Duo, the lithium metasilicate crystals are larger in size compared
to those in Vita Suprinity[27] and can provide a plausible explanation for the different surface topographic features
observed in both ceramics following HF etching or SECP application. However, two-way
ANOVA results showed the effects of the ceramic material on the obtained surface roughness.
This can be explained by their similar glass content. One of the limitations of this
study is that the surface treatments (polishing, SECP, and HF) were applied to a flat
surface specimen, which may be less clinically relevant, as ceramic dental restorations
usually have complex geometry, rather than a flat surface.
Conclusion
The SECP can effectively modify the surface properties of both ZLS ceramic materials,
resulting in a significant increase in the surface roughness of the ceramic. The etching
patterns created after the application of SECP were less aggressive than those produced
by HF etching. The surface topographic features of ZLS ceramic materials can be affected
by both the nature of surface treatment and the composition of the ceramic.
