Keywords
dental porcelain - hydrofluoric acid - neutralizing agents - restoration repair -
surface treatment
Introduction
Dental porcelain offers key advantages in its extremely pleasing esthetic appearance
as it can mimic the various colors and shades of natural teeth and its superior biocompatibility.[1]
[2] However, as dental porcelains are brittle, potential technical problems include
the chipping or fracture of the veneering ceramic.[3] Furthermore, it cannot be directly repaired by the same material, due to the high
temperatures involved with the sintering process. The available options for repair
of such fractures range from bonding of a new veneer over the fractured porcelain
to the most commonly used method of bonding composite filling materials to the fractured
surface.[4]
[5]
[6]
The repair process involves hydrofluoric (HF) acid etching of the porcelain surface
followed by the application of silane. These procedures are well known and recommended
for the improved attachment of composite resin to ceramic.[5]
[7]
[8]
[9]
[10]
[11] HF acid etching results in changes in porcelain surface morphology that enhances
micromechanical retention, but HF acid is also known to have hazardous effects in
vivo, as it was found to be a harmful and irritating compound to human soft tissues.[12]
[13]
HF acid is an inorganic acid of hydrogen fluoride in water, known to be hazardous
to human tissues. Unlike other acids, the dissociated fluoride ion produces severe
toxicity. Primary care for patients exposed to HF consists of removal of any contaminated
clothing, thorough flushing of the exposed area with a large amount of water, and
topical or systemic application of calcium gluconate—depending on the severity. The
goal of primary care is to neutralize, precipitate, and eliminate the fluoride ion
and to prevent progressive tissue destruction.[12]
[13]
[14]
Accordingly, many questions have been raised as to the risks and possible toxicity
of using HF acid in the oral cavities. Panah et al[15] recommended the use of rubber dams and neutralizing agents, such as sodium bicarbonate
and calcium carbonate, to protect the tooth surface and oral tissues. However, some
studies reported that using neutralizing agents after HF treatment on ceramic might
reduce bond strength between the adhesive cement/ceramic interface. As a result, precipitation
of HF acid and neutralizing agents could prevent penetration of resin material to
obtain mechanical interlocking on ceramic-etched surface.[16]
[17]
[18]
Interestingly, there have been no studies regarding the use of calcium gluconate and
its effects on bond strength, but very few studies on the effects of other neutralizing
agents were noted. To date, no study has determined the composition of the residual
resulting in products of neutralizing agents and HF acid after surface cleaning.
The aim of this study was divided into two parts: (1) to evaluate the effect of neutralizing
agents such as calcium hydroxide, calcium carbonate, and calcium gluconate on the
shear bond strength of HF-etched dental porcelain, immediately post application and
after simulated aging, and (2) to analyze the surface morphology and determine the
elements present on the porcelain surface after surface treatment using scanning electron
microscope (SEM) and energy-dispersive X-ray spectroscopy (EDX), respectively. The
null hypotheses for this experiment were (1) the neutralizing agents would affect
the shear bond strength of HF-etched porcelain, immediately post application and after
simulated aging, and (2) the porcelain surface treated with neutralizing agents would
demonstrate difference in surface morphology and surface elements.
Subjects and Methods
Shear Bond Strength Testing
The materials used in this study are summarized in [Table 1]. One hundred and twenty specimens were prepared by mixing feldspathic porcelain
powder shade T1 (Noritake, Super Porcelain EX-3, Kuraray Noritake Dental Inc, Aichi,
Japan) with forming liquid. The specimens were subsequently formed using a putty silicone
index (Elite HD, Zhermack, Badia Polesine, Italy) with a diameter of 8.0 mm and a
depth of 6.0 mm. The ceramic slurry was placed into the silicone mold in small incremental
portions by a cement spatula and condensed until the entire mold space was full. Excess
liquid was removed with soft, absorbent paper. The feldspathic porcelain blocks were
removed from the silicone indexes and fired as per the manufacturer's instructions.
Sintering shrinkage was around 25%, making the final specimen 6.0 mm in diameter and
4.5 mm in height. Next, retentive grooves were created at the bottom of the porcelain
specimens with slow-speed diamond discs (Superflex, Edenta, Switzerland) to promote
mechanical retention between the feldspathic porcelain and dental gypsum Type IV.
A polyvinyl chloride (PVC) tube with a diameter of 22.0 mm and a height of 15.0 mm
was then filled with dental gypsum, and the specimen placed to a depth where its margin
was 1.0 mm higher than the edge of the tube. After the gypsum had set, the surface
of the ceramic specimens was polished using a 600-grit silicon carbide abrasive paper
(3M Wetordry abrasive sheet, 3M, Minnesota, United States) and lubricant in an automatic
machine (Nano 2000 grinder-polisher with FEMTO 1000 polishing head, Pace Technologies,
Arizona, United States). A force of 2 kg/cm2 was applied during polishing, and the silicon carbide abrasive papers were set to
rotate at a rate of 100 rotations per minute anticlockwise. During the polishing process,
the specimens were rotated clockwise. Polishing with 600-grit silicon carbide abrasive
paper was carried out for 5 min after which the process was repeated using a 1000-grit
silicon carbide abrasive paper. A new abrasive sheet was used for each specimen. After
polishing, all specimens were ultrasonically cleaned in distilled water for 5 min
to remove any surface residual and then air dried. The specimens were randomly divided
into six groups (n = 10) according to its surface treatments. They were listed as follows:
Table 1
Materials used in this study
|
Materials
|
Manufacturer
|
Compositions
|
|
Abbreviations: Bis-GMA, bisphenyl glycidyl methacrylate; HEMA, hydroxyethyl methacrylate.
|
|
Feldspathic porcelain Lot: DNWHX
|
Noritake, Super Porcelain EX-3, Kuraray Noritake Dental Inc, Aichi, Japan
|
SiO2 (65%), Al2O3 (14%), CaO (<1%), MgO (<1%), K2O (9%), Na2O (9%), Li2O (<1%)
|
|
Hydrofluoric acid Lot: BD68D
|
Ultradent Products Inc, Ultradent Porcelain Etch, Utah, United States
|
9% buffered hydrofluoric acid
|
|
Adper Scotchbond Multipurpose Adhesive Lot N684362
|
3M ESPE, Dental products, St Paul, Minnesota, United States
|
Bis-GMA, HEMA, tertiary amines, photoinitiator
|
|
Calcium hydroxide Lot: K26007147 931
|
Merck KGaA, 64271 Darmstadt, Germany
|
Ca (OH)2 powder
|
|
Calcium carbonate B/No.1601210132
|
UNIVAR, Taren Point NSW 2229, Australia
|
CaCO3 powder
|
|
Calcium gluconate Lot: 592551
|
A.N.B. Laboratories Co, Ltd., Bangkok, Thailand
|
Calcium gluconate (C12H22CaO14) 500 mg in water 10 mL
|
-
Group 1: Served as the control group.
-
Group 2: Etched with 9% HF gel (Ultradent Porcelain Etch, Ultradent Products Inc,
South Jordan, Utah, United States) for 90 s, rinsed with deionized water spray for
60 s, and gently air dried.
-
Group 3: Etched with 9% HF gel for 90 s, application of 0.02 g calcium hydroxide for
60 s, rinsed with distilled water spray for 60 s, and gently air dried.
-
Group 4: Etched with 9% HF gel for 90 s, application of
-
0.02 g calcium carbonate for 60 s, rinsed with distilled water spray for 60 s, and
gently air dried.
-
Group 5: Etched with 9% HF gel for 90 s, application of
-
0.02 g calcium gluconate for 60 s, rinsed with distilled water spray for 60 s, and
gently air dried.
-
Group 6: Etched with 9% HF gel for 90 s, rinsed with distilled water for 1 min, followed
by ultrasonic cleaning in distilled water for 10 min, and then gently air dried.
One-sided tape (ScotchBlue Painter's Tape, 3M, Minnesota, United States) with a thickness
of around 80 μm was cut into a 10 mm × 10 mm size, with a 2.0 mm diameter hole at
the center. The tape was placed on the feldspathic porcelain surface. A small brush
(Applicator tips, Dentsply DeTrey GmbH, Konstanz, Germany) was used to apply bonding
agent (Adper ScotchBond Multi-Purpose Adhesive, 3M ESPE, Dental products, St Paul,
Minnesota, United States) on the entirety of the prepared feldspathic porcelain surface.
Another brush was used to remove excess bonding at the margins of the tape. Next,
a force of approximately 40–50 pounds per square inch was applied to the triple syringe
(Mobile Dental Unit, Thai Dental Products, Bangkok, Thailand) to remove any residual
solvent droplets and confirm water/oil-free air blow. The solvent of the adhesive
was then dried off completely (this was noticed by the absence of moving liquid droplets
and the resulting shiny surface of feldspathic porcelain). With the photopolymerizing
unit placed perpendicularly and at a distance of 1.0 mm from the feldspathic porcelain
specimen, polymerization of the adhesive was performed by light curing for 20 s (Elipar
FreeLight 2 LED Curing Light, 3M ESPE, St. Paul, Minnesota, United States) with the
intensity of 1000 mW/cm2.
A hollow silicone mold with a diameter of 3.0 mm and a depth of 2.0 mm was placed
on the top of the treated feldspathic porcelain specimen to encircle the center hole
of the tape. Next, resin composite shade A3E (Filtek Z350 XT, 3M ESPE, Dental products,
St Paul, Minnesota, United States) was placed until the silicone mold was full; it
was light cured for 20 s. Next, the silicone mold and the tape were carefully removed
and the resin composite light cured for another 20 s. The specimen was then immersed
in 37°C distilled water for 24 h in an incubator (Contherm 160M, Contherm Scientific
Ltd, Korokoro, Lower Hutt, New Zealand) according to ISO/TS 11405. The silicone index
was cleaned with ethyl alcohol and gently air dried between specimens.
Half of the specimens of each group were submitted to shear bond strength testing
by a universal testing machine (EZ-500N, Shimadzu Corporation, Kyoto, Japan). Each
specimen was fixed in the testing machine, and the shearing blade was placed parallel
to the junction between the feldspathic porcelain and resin composite at a distance
of 1.0 mm. The shear load was applied at a 0.5 mm/min crosshead speed until failure.
The shear bond strength (MPa) was calculated by dividing the highest shear bond strength
by the surface area of the resin composite–feldspathic porcelain interface. The adhesive
area of each specimen was measured before testing with a digital caliper (Digital
Vernier Caliper Mitutoyo CD-6 CS, Mitutoyo Co, Japan). After shear bond strength testing,
the debonded surfaces were viewed under a stereomicroscope (Olympus Stereo Microscopes,
SZ61, Japan) at a magnification of ×40 to study the mode of failure. The modes of
failure were divided into one of four categories: (1) adhesive failure at the junction
of feldspathic porcelain and resin composite with no evident resin composite on the
surface of the feldspathic porcelain; (2) cohesive failure within the body of resin
composite, where resin composite was seen covering the entire surface of the feldspathic
porcelain or fracture of the entire layer of resin composite was seen; (3) cohesive
failure within the body of feldspathic porcelain, where feldspathic porcelain was
seen covering the entire surface of resin composite or fracture of the entire layer
of feldspathic porcelain was seen; and (4) mixed failure or a combination of adhesive
and cohesive failure where the feldspathic porcelain surface demonstrated both characteristics
mentioned previously.
The other half of the specimens were thermocycled in water for 5000 cycles alternating
in intervals of 60 s between 5°C and 55°C, with a transfer time of 15 s. After thermocycling,
the specimens’ shear bond strengths were measured following the protocol previously
described.
Surface Analysis after Surface Treatment
Six sintered feldspathic specimens of 8.0 mm in diameter and 6.0 mm in height were
prepared using the same protocol stated previously; each specimen underwent one of
the six surface treatments. They were evaluated by SEM and EDX. The surface of the
ceramic specimen from each surface treatment was analyzed using SEM at a magnification
of ×500 and ×2,000, and their elemental compositions analyzed using EDX spectroscopy.
The data were obtained by an SEM (JSM-5800LV, JEOL, Japan, and ISIS Series 300, Oxford,
England) fitted with an EDX spectrometer. The primary electron energy used varied
from 5 to 20 keV. Test parameters were set to the following: working distance (WD)
= 15 mm, process time = 5 s, live time = 60 s, and dead time = 30%–40%. Three different
areas were selected from each specimen and each area was scanned at five separate
times.
Data Analysis
The shear bond strengths of all groups were analyzed statistically with SPSS 20.0
software for Windows (SPSS Inc, Chicago, Illinois, United States). A normal distribution
of error was found with Kolmogorov–Smirnov test. The bond strength value was further
analyzed by two-way ANOVA. Tukey's HSD test was applied to determine the significant
differences between surface treatment groups at the confidence level of 95%. The surface
morphology and elemental occurrence were analyzed using descriptive statistics.
Results
The means and standard deviations of shear bond strength are reported in [Table 2]. Two-way ANOVA showed significant difference in shear bond strength values between
different surface conditioning methods and simulated aging conditions. No significant
differences were detected among all of the HF-etched porcelain groups (p > 0.05). HF-treated groups revealed significantly higher shear bond strength than
the control group in both thermocycled and nonthermocycled conditions (p < 0.05).
Table 2
The means and standard deviations of shear bond strength
|
Surface treatments
|
Mean ± SD
|
|
No-aging (n = 10)
|
Aging (n = 10)
|
|
The same superscript indicates no significant difference. Abbreviations: SD, standard
deviation; HF, hydrofluoric.
|
|
Group 1: Control
|
7.49 ± 0.98C
|
2.93 ± 1.35D
|
|
Group 2: HF
|
19.44 ± 3.54A
|
12.84 ± 2.87B
|
|
Group 3: HF + calcium hydroxide
|
19.32 ± 2.82A
|
13.31 ± 2.97B
|
|
Group 4: HF + calcium carbonate
|
21.33 ± 2.93A
|
11.83 ± 2.34B
|
|
Group 5: HF + calcium gluconate
|
21.16 ± 3.54A
|
14.07 ± 3.31B
|
|
Group 6: HF + ultrasonic
|
20.69 ± 3.17A
|
13.64 ± 2.70B
|
After stimulated aging, the control group produced the lowest value (2.93 MPa) and
was significantly different from other groups. All aged HF-etched porcelain groups
(groups 2–6) had significantly higher shear bond strength values (11.83–14.07 MPa)
than that of the control group without significant difference between groups.
The shear bond strength of the nonaged control group was significantly higher (7.49
MPa) than the aged control group. However, this control group still presented significantly
lower bond strength than both aged and nonaged HF-etched porcelain groups (groups
2–6). The highest values (19.32–21.33 MPa) were found among nonaged HF-etched porcelain
groups with no significant difference between groups. Significant differences were
found between all thermocycled group and nonthermocycled group receiving the same
treatment.
The control groups demonstrated adhesive failure at the resin/ceramic interface at
both time periods, immediately EDX after surface treatment, and after simulated aging.
On the other hand, failure analysis of all etched groups revealed predominantly cohesive
failure in ceramic and very few mixed failures at both the composite resin and ceramic
surface ([Table 3]).
Table 3
Mode of failure for the different porcelain surface treatments
|
Surface treatments
|
Without thermocycling (n = 10)
|
With thermocycling (n = 10)
|
|
Adhesive
|
Cohesive (composite)
|
Cohesive (ceramic)
|
Mixed
|
Adhesive
|
Cohesive (composite)
|
Cohesive (ceramic)
|
Mixed
|
|
Abbreviation: HF, hydrofluoric.
|
|
Group 1: Control
|
100
|
|
|
|
100
|
|
|
|
|
Group 2: HF
|
|
|
80
|
20
|
|
|
100
|
0
|
|
Group 3: HF + calcium hydroxide
|
|
|
90
|
10
|
|
|
80
|
20
|
|
Group 4: HF + calcium carbonate
|
|
|
100
|
0
|
|
|
90
|
10
|
|
Group 5: HF + calcium gluconate
|
|
|
100
|
0
|
|
|
100
|
0
|
|
Group 6: HF + ultrasonic
|
|
|
90
|
10
|
|
|
100
|
0
|
The SEM images at ×500 and ×2,000 magnification of the samples from each experimental
group after surface treatment are shown in [Figs. 1]
[2]. The nonetched porcelain surface of the control group exhibited a smooth-flat surface
with slight porosity and polishing groove formations ([Figs. 1a]
[2a]).
Fig. 1 The scanning electron microscope images of the porcelain with different surface treatments
at ×500: (a) Control, (b) hydrofluoric, (c) hydrofluoric and calcium hydroxide, (d)
hydrofluoric and calcium carbonate, (e) hydrofluoric and calcium gluconate, and (f)
hydrofluoric and ultrasonic cleaning.
Fig. 2 The scanning electron microscope images of the porcelain with different surface treatments
at ×2000: (a) Control, (b) hydrofluoric, (c) hydrofluoric and calcium hydroxide, (d)
hydrofluoric and calcium carbonate, (e) hydrofluoric and calcium gluconate, and (f)
hydrofluoric and ultrasonic cleaning.
Etching with 9.5% HF acid for 90 s resulted in a morphological honeycomb-like surface
with shallow irregularities, microporosities, and grooves as shown in [Figs. 1b]
[2b]. SEM photographs of experimental Groups 3–6 showed irregular morphology similar
to that of Group 2 ([Fig. 1c-f]). In addition, high magnification of the surface irregularities of all experimental
groups demonstrated no debris or acid-etched residue ([Fig. 2c-f]).
EDX analyses of the treated surfaces of each group are shown in [Fig. 3]. The control group demonstrated peaks in C, O, Na, Mg, Al, Si, K, and Ca spectra
at 0.2774 (Kα), 0.5249 (Kα), 1.0410 (Kα), 1.2536 (Kα), 1.4866 (Kα), 1.7398 (Kα), 3.3129
(Kα), and 3.6905 (Kα) keV, respectively. In addition, HF-etched porcelain showed identical
spectra to the control group. However, F element, with a peak at 0.6768 (Kα) keV,
was not found on any of the specimens.
Fig. 3 The energy-dispersive X-ray spectra of the porcelain with different surface treatments:
(a) Control, (b) hydrofluoric, (c) hydrofluoric and calcium hydroxide, (d) hydrofluoric
and calcium carbonate, (e) hydrofluoric and calcium gluconate, and (f) hydrofluoric
and ultrasonic cleaning.
Discussion
This study investigated the effect of different surface treatments on the shear bond
strength between HF-etched feldspathic porcelain and resin composite at two different
time periods, immediately after surface treatment, and after simulated aging. Immediately
after surface treatment, all five experimental groups showed higher bond strength
values compared to the nonetched control group. Furthermore, there was no significant
difference in shear bond strength between any experimental groups. After simulated
aging, all specimens demonstrated a significant drop in shear bond strength compared
with their nonaged counterparts. Similarly, all aged experimental groups demonstrated
higher bond strength values compared with the control group but with no significant
difference between the individual experimental groups. Therefore, the first hypothesis
was rejected. All HF-etched groups demonstrated similar patterns of irregularities
on the surface micromorphology and identical elements on the specimen surfaces. As
a result, the second hypothesis was also rejected.
HF acid etching and silanization can improve the optimal bond strength of repaired
feldspathic porcelain.[5]
[6]
[9]
[11]
[13]
[19]
[20] Etching porcelain with HF acid selectively dissolves the glassy phase, resulting
in microporosities, thus creating mechanical interlock between resin composite and
ceramic.[19] Silane coupling agent is then used to provide chemical bonding to the ceramic surface.[5]
[19]
[21] The focus of this study is to evaluate the effectiveness of HF acid when used to
etch the porcelain surface to improve the mechanical bond. Thus, the use of chemical
bonding agents such as silane was excluded to isolate the effect of HF acid in increasing
mechanical interlock for investigation in this study.
HF acid is a weak acid when compared to other etching agents. However, it has high
toxicity. When dissolved, the fluoride ion is released and can penetrate tissue. The
deeply penetrated ion can bind to calcium and magnesium ions in bone and blood.[14]
[22] An effective treatment to alleviate HF acid toxicity is to use calcium gluconate.
When dissolved, the calcium ions bind to fluorine to form an insoluble calcium fluoride
salt.[12]
[14]
Generally, neutralizing agents are used to neutralize the pH of a substance and make
it less hazardous. A reaction will result from the acid–base reaction, forming water
and salt. The objective of using neutralizing agent against HF acid is to reduce the
toxicity of fluorine. The calcium-containing compounds used in this research were
calcium hydroxide and calcium carbonate, two materials that can be easily found in
every dental clinic. Calcium gluconate is mainly used to treat HF burns.[22]
Özcan and Volpato[23] proposed that neutralizing agents could prevent the toxic effects of HF acid by
eliminating its residuals within pores on the ceramic surface and by neutralizing
its pH. On the other hand, Canay et al[24] stated that etching ceramic with HF acid would result in fluorosilicate crystalline
precipitates that were insoluble in water. Saavedra et al[18] and Amaral et al[17] reported that using neutralizing agents after HF treatment decreased the bond strength
between adhesive cement and ceramic due to the precipitation on the etched surface.
They suggested that the precipitation was a result of the reaction between HF acid
and neutralizing agents. Bottino et al[16] mentioned that the use of neutralizing agents decreased the surface energy of ceramic
as it created a precipitate at the etched region, thus decreasing both the bonding
capability between resin cement and ceramic and the microtensile bond strength.
In our study, the bond strengths of Groups 2–6 were not different. In contrast to
Bottino et al,[16] their study demonstrated that neutralizing agents decreased the bond strength to
porcelain. They hypothesized that the precipitate from the neutralizing agents interfered
with the bond strength of porcelain.
However, according to the EDX readings of our study, only C, O, Na, Mg, Al, Si, K,
and Ca ions were found in all groups. Fluorine ion was not found in any group. This
might be because the fluorine and calcium ion present in the neutralizing agent were
bound to the fluorine ion on the surface of the sample, which was rinsed away by the
water spray or ultrasonic.
The SEM image showed that HF-etched porcelain, after neutralization with various substances
and cleaned with water spray, demonstrated a clean surface with no acid–base reaction
residue, similar to the group that was ultrasonically cleaned. This was significant
as many studies have reported that ultrasonic cleaning after etching with HF resulted
in maximum cleansing and optimal bond strength.[25]
[26]
[27]
The reason for the disappearance of precipitate may have been from the water spray
cleaning. Steinhauser et al[27] stated that 60 s of water spray cleaning provided comparable microshear bond strength
to cleaning in an ultrasonic bath with distilled water for 5 min. They also reported
that little to no residue was present on the ceramic surface after HF etching and
ultrasonic cleansing. Still, this method may not be suitable for the direct repair
technique.[22]
The results from this study found that the unetched control group mainly demonstrated
adhesive failure, whereas the remaining experimental groups mainly demonstrated either
cohesive or mixed failure. The adhesive failure is mainly correlated with low shear
bond strength as obtained from the unetched specimens. Cohesive failure in porcelain
indicates that the cohesive strength of the porcelain is inferior to the bond of composite
to porcelain. de Melo et al[28] have suggested that porcelain with lower crystalline phase content and higher glassy
phase results in more cohesive failure. Cohesive or mixed failures are often correlated
with high shear bond strength value as obtained from all HF-treated specimens.
This research used unfilled resin with low viscosity as the bonding agent. The unfilled
resin provides better infiltration to the irregularities and improves adaptation between
the resin and ceramic surface which may result in increased bond strength.[10]
[29] Previous studies used resin cement, which had high viscosity compared to unfilled
resin.[16] This may result in less infiltration and poor adaptation and account for the resulting
lower bond strengths of their studies.
Thermocycling is widely used to test the durability of adhesives and simulate intraoral
aging. It significantly decreases the shear bond strength of all experimental groups
compared to their nonaged counterparts. Due to the different coefficients of thermal
expansion of porcelain and resin-based polymer, the series of alternations of compression
and expansion may be responsible for the reduced bond strength at the interface of
two materials.[30]
[31] This study showed all groups to have lower shear bond strength after aging.
Typically, it is recommended that the use of HF should be used in accordance with
the manufacturer's instructions and that a rubber dam be used to protect the patient's
adjacent hard and soft tissues.[22] This study found that the use of neutralizing agents did not negatively affect the
bond strength of porcelain to resin composite. Therefore, it can be considered as
an alternative to reducing the toxicity of HF. Further investigation is needed to
test the effect of HF neutralizing agents on other physical properties (i.e., surface
roughness and flexural strength) of feldspathic porcelain. Moreover, there should
be studies on the effect of neutralizing agents of HF on other ceramic substrates
as well.
Conclusion
Within the limitations of this in vitro study, the following can be concluded:
-
There is no significant difference in shear bond strength between plain etched porcelain,
etched porcelain cleaned ultrasonically, or etched porcelain treated with neutralizing
agents after cleaning with water spray for 60 s, given that unfilled resin is used.
-
The shear bond strength of every group decreases significantly when subjected to 5,000
cycles of thermocycling.
-
Neutralizing agents such as calcium carbonate, calcium hydroxide, and calcium gluconate
can be considered as an alternative to reduce the toxicity of HF.
Financial Support and Sponsorship
The study was supported by the Research Institute of Rangsit University, Thailand
(Project No. 44/2559).
