Keywords
dental cement - hardness - sorption - solubility - color
Introduction
The growth of aesthetic procedures in restorative dentistry has increased the use
of ceramic materials for different clinical situations.[1]
[2] Establishing a durable bond between tooth tissue, resin cement, and the ceramic
substrate is critical to the clinical success and longevity of indirect restorations.[3] An important aspect to consider is that the resin cement film around the margins
of the restorations is often located close to the free gingival margin or gingival
sulcus,[4]
[5]
[6] a susceptible area of sorption, hydrolysis, and dynamic fatigue which may lead to
polymer degradation.[7]
Resin-based luting cement is methacrylate monomers with small filler contents and
a photoinitiator.[5]
[6] In general, dental polymers’ networks are not chemically stable and water impermeable.
These monomers have been shown to absorb moisture, which is a critical property due
to the adverse effects on hydrolytic stability.[8] This phenomenon may result in material discoloration, mechanical deterioration,
degradation of filler-matrix interface, polymer swelling, weakening of the polymer
network,[7]
[8] and stress-induced degradation of the resin cement leading to debonding or fracture
of the ceramic restoration.[5]
[8]
Many factors can influence the coefficient of water diffusion between the polymer
network, including filler content and the resin matrix volume.[8]
[9]
[10] Resin cement with a higher percentage of filler content has lower water sorption
and solubility.[8] Although inherent to polymer materials, sorption and solubility can influence the
biocompatibility, mechanical properties, and color stability of resin cements.[11]
The major advantages of light-cured resin cement are color stability, and the working
time when compared with dual-cure and chemically activated systems. The staining of
resin cement may be caused by intrinsic (filler content, material composition, or
type of activation) and extrinsic factors (sorption of media, stains caused by beverages,
and food components).[12] Resin materials composed with smaller fillers showed improved color stability and
gloss retention.[13] Additionally, from a clinical perspective, light-cured resin cements with a lower
concentration of tertiary amine display greater long-term color stability.[4] The extrinsic factors seem to be modulated by the conversion ratio and the physicochemical
characteristics of the resin-based materials such as surface roughness and water sorption.[14]
The performance of resin cement on the luting procedure and their mechanical properties
are important factors to consider for the clinical success of indirect restorations.[6]
[15] Therefore, this study aimed to assess the surface hardness, sorption, solubility,
and color stability of light-cured resin cement. The null hypotheses tested were:
(1) no difference in microhardness between the evaluated resin cement, (2) no difference
in the sorption and solubility between the resin cements tested, and (3) no difference
in color stability would be observed between the tested materials.
Materials and Methods
Preparation of Samples
The sample size used in the present study followed the recommendations of ISO 4049:2009.[16] Three commercially available light-cured resin cements were studied ([Table 1]). Disc-shaped samples of each resin cement were built up by filling a split metal
mold (15 mm in diameter × 1 mm thick).[16]
Table 1
Composition of the studied resin cements
|
Resin cement
|
Composition
|
Manufacturera
batch number
|
|
a3M/ESPE: St. Paul, MN, United States; Ivoclar Vivadent: Schaan, Liechtenstein; FGM:
Santa Catarina, Brazil.
|
|
RelyX Veneer
|
Silane-treated ceramics, 2,2'-ethylenedioxyethyl dimethacrylate, bisphenol A diglycidyl
ether dimethacrylate, silane-treated silica, functionalized dimethacrylate polymer
and water
|
3M/ESPE
1814900518
|
|
Variolink Veneer
|
Urethane dimethacrylate and other methacrylate monomers. Ytterbium trifluoride and
spheroidal mixed oxides
|
Ivoclar Vivadent
X08164
|
|
All Cem Veneer
|
Methacrylic monomers, camphorquinone, coinitiators, stabilizers, pigments, silanized
Barium Aluminum Silicate glass particles and silicon dioxide
|
FGM
120118
|
The mold was filled to excess, and the surface of the sample was covered with a mylar
matrix strip and microscope glass slab placed on the top. Slight digital compression
was applied to the glass for 20 seconds to avoid porosities and minimize possible
voids due to entrapped air on the uncured material inside the mold. The polymerization
of all the samples was performed with a light-emitting diode source (Radii-Cal, Dental
Product SDI, Bayswater, Vitoria, Australia) for 40 seconds on each sample side with
1,200 mW/cm2 irradiance.
The sample was removed from the metal mold, and the excess flash was detached using
silicon carbide papers of decrescent grit (nos. 600, 1,000, and 1,500). The dimensions
of each sample were confirmed using a digital caliper (± 0.01 mm; MDC-25 M, Mitutoyo,
Tokyo, Japan). The samples were then washed in an ultrasonic cleaner (Cristófoli,
Paraná, Brazil) with distilled water for 10 minutes and gently air dried.
Hardness Vickers Number Test
The hardness test was conducted using a digital microhardness tester (ISH-MR150/INSIZE,
São Paulo, Brazil) under a 300-gf load for 15 seconds (n = 10). Three indentations were done in each sample and the mean HVN was then calculated.
Sorption and Solubility
The evaluation of sorption and solubility followed previous studies.[4]
[5]
[9]
[11] Following the hardness test, five samples of each resin cement were selected and
placed in a desiccator containing freshly dried silica gel at 37 ± 2°C. After a 24-hour
period, the samples were maintained at 23 ± 2°C for 2 hours. All the samples were
weighed on an analytical balance (0.01 mg; AUW 220D, Shimadzu Analytical Balance,
Tokyo, Japan).
The samples were repeatedly weighed until a constant mass (m1) was attained. The diameter and thickness of each sample were measured with the digital
caliper at four points. The mean values were used to calculate the volume of the samples
(V) in mm3. The samples were then stored in plastic vials containing distilled water and kept
at 37°C for 7 days. Samples were carefully blotted dried with absorbent paper, weighed,
and the recorded mass was registered (m2). The samples were placed in a desiccator and constantly weighed (daily) until a
consistent mass with variation less than 0.1 mg (m3) was attained. The sorption and the solubility were calculated using the following
equations:[4]
[5]
[9]
Color Stability Evaluation
The color analysis was based on Shiozawa et al (2015)[14] using the CIELAB color space and a digital spectrophotometer (Vita Easy Shade; Wilcos,
Rio de Janeiro, Brazil). Measurements were performed at three different times: (1)
before immersion (baseline), (2) after 1 day, and (3) after 1 week of storage in distilled
water and coffee. The coffee solution was prepared by dissolving 0.51 g of instant
coffee powder in 50 mL of distilled water (Nescafé, Nestle, São Paulo, SP, Brazil).
The same samples used for the hardness test were used for the evaluation of the color
stability after immersion in coffee. The remaining samples, used for sorption and
solubility tests, were used as the control group (immersion in distilled water).
The CIELAB system is composed of three axes: L* (lightness from 0 = black up to 100 = white), a* (from a = green up to +a = red), and b* (from b = blue up to +b = yellow). The following equation calculated the color change (∆E)[12]
[14]:
A higher ∆E value also means a greater color change for each sample. An ∆E ≤ 1 shows a color change undetectable for human eyes. The discoloration is perceptible
but clinically accepted when the mean is between 1 and 3.3 (1 < ∆E < 3.3). For means over 3.3 (∆E > 3.3), the alteration is detected by human eyes and clinically unaccepted.[17]
For the evaluation, samples were positioned over a brown paper as a standard backing.
Two readings in the central area of each sample were done, and the means were considered
for analysis.
Statistical Analysis
Data were analyzed using Statistical Package for the Social Science version 18 (SPSS;
Chicago, Illinois, United States). The Shapiro–Wilk’s test evaluated the presence
of normal distributions. The Mann–Whitney and Kruskal–Wallis’s tests were applied
whenever differences were observed between groups. For all tests, a p < 0.05 was considered statistically significant.
Results
The microhardness results are shown in [Table 2]. The resin cement RLX showed the highest mean values (36.96 VHN) with statistically
significant differences with the other groups ACV (27.40) and VLK (23.23).
Table 2
Mean and standard deviation of Vickers hardness, sorption, and solubility of the studied
resin cements
|
Resin cement
|
VHN
|
Sorption
|
Solubility
|
|
Abbreviations: ACV, All Cem Veneer; RLX, RelyX Veneer; VHN, hardness vickers number;
VLK, Variolink Veneer.
Note: Different superscript letters indicate statistical differences through Mann–Whitney
test.
*Kruskal–Wallis test.
|
|
RLX
|
36.96 (1.15)A
|
23.2 (1.3)A
|
2.40 (0.17)A
|
|
ACV
|
27.40 (0.88)B
|
12.4 (2.5)B
|
0.14 (0.05)B
|
|
VLK
|
23.23 (0.73)B
|
18.1 (2.5)C
|
0.20 (0.10)B
|
|
p-value*
|
<0.001
|
0.003
|
0.006
|
Differences between sorption (p = 0.003) and solubility (p = 0.006) were also observed within materials ([Table 2]). The RLX group showed higher sorption (23.2 µg/mm3) and solubility (2.40 µg/mm3).
For color stability ([Table 3]), the mean ∆E showed a significant difference between water and coffee immersion for all tested
resin cement in both moments of evaluation (p = 0.009), with higher ∆E for the samples immersed in coffee. The VLK resin cement presented statistically
significant differences from the other groups, with higher color changes in coffee
at 1 day (15.14) and after 1 week (23.65).
Table 3
Mean and standard deviation of the color stability for each immersion media and studied
resin cement
|
Resin cement type
|
∆E
1 day
|
p-valueD
|
∆E
1 week
|
p-value*
|
|
Water
|
Coffee
|
Water
|
Coffee
|
|
Abbreviations: ACV, All Cem Veneer; RLX, RelyX Veneer; VLK, Variolink Veneer.
Note: Different superscript letters indicate statistical differences through Mann–Whitney
test. Lowercase letters indicate differences between groups (column) and uppercase letters
indicate differences between evaluation periods for each immersion media (rows).
*Mann–Whitney test
#Kruskal–Wallis test.
|
|
RLX
|
1.69 ± 0.23a,A
|
7.96 ± 0.22a,B
|
0.009
|
2.70 ± 0.15a,A
|
14.50 ± 0.62a,C
|
0.009
|
|
ACV
|
1.62 ± 0.32a,A
|
9.07 ± 0.38a,B
|
0.009
|
2.64 ± 0.35a,A
|
12.46 ± 0.44a,B
|
0.009
|
|
VLK
|
1.00 ± 0.13b,A
|
15.14 ± 0.50b,B
|
0.009
|
2.36 ± 0.33a,A
|
23.65 ± 1.10b,C
|
0.009
|
|
p-value#
|
0.013
|
0.002
|
|
0.254
|
0.002
|
|
Discussion
The physical and mechanical properties investigated were different among the tested
light-curing resin cement. In the present study, all three tested null hypotheses
were rejected. The results could be attributed to the different chemical composition
of the materials, especially their organic matrices.[11]
Surface hardness is defined as resistance to permanent indentation or penetration
of the material surface. This assessment is intuitively related to the wear of dental
materials in the oral environment.[18] The Rely-X Veneer resin cement showed higher VHN with statistically significant
differences from the other materials. The materials higher degree of conversion could
influence the materials VHN results. For resin cement, the percentage of conversion
is around 60%, and incomplete material polymerization results in higher sorption and
solubility values inducing faster resin cement degradation.[6]
[19] For Demarco et al (2012),[20] resin materials with UDMA/TEGDMA-based formulation showed significantly higher wear
resistance. However, the resin cement that presented the highest hardness value in
the present study has Bis-GMA monomers in its composition.
The distilled water is a solvent recommended by the International Organization for
Standardization of polymer-based materials.[16] In the present study, we used this substance as a control medium for the experimental
phases.
Depending on the limit of the cavity preparation (supragingivally), resin cement may
still suffer mechanical brushing action. It is common for the clinician to worry about
how these materials behave in different situations.[9] However, in the study by Nayyer et al (2018),[21] in which the hardness of resinous compounds was evaluated after simulated brushing
cycles, the authors concluded that brushing was not able to compromise the hardness
of the materials.
Different beverages are used to test color stability such as coffee,[14]
[22] black tea,[22]
[23] cola,[22]
[24] and red wine[22]
[23], which are usual drinks in a daily diet with a high potential to stain resin-based
materials. Previous studies have shown that the color change is more significant in
coffee, black tea, and red wine when compared with cola beverages.[24]
[25]
[26] Water can be absorbed in the resin matrix, causing expansion, and plasticizing the
resin. The matrix content and bond strength of the resin matrix-inorganic filler interface
have a more substantial influence on the degree of water sorption, which may create
microcracks where the beverage stain can penetrate cause discoloration on the cement
surface.[14]
In analyzing color stability, there is a difference between the color perceptibility
and acceptability. A perceptibility of color difference value (∆E) of 3.7 is acceptable for dental composite veneer restorations and tooth substance.[27] In the present study, independently of the immersion time, all the resin cement
immersed in the water had an ∆E smaller than the previously reported acceptable threshold. When the samples were
immersed in coffee, regardless of the immersion time, the color differences were higher
than the reported acceptable threshold.
All samples were polymerized by the same LED light source at its maximum power. The
pigmentation of the samples can be influenced by the type of light-curing device used.
Zafa et al[28] evaluated the influence of light-curing units (Quartz Tungsten Halogen [QTH] and
light emitting diode [LED]) on the pigmentation of resin composites. The authors observed
that the materials polymerized with LED showed significant color stability when compared
with QTH.
Even after polymerization, resin materials are not stable,[9] and various components such as unreacted monomers are released from resinous materials
during the first 7 days of immersion in distilled water.[29] Moreover, the saturation period reached for resinous materials may vary within 7
to 60 days.[30] Many studies have reported the influence of different substances on the integrity
of resin materials,[6]
[8]
[9] with the immersion periods and the storage mediums strongly influencing their sorption
and solubility.[9]
[13]
[31]
The sorption and solubility are a diffusion-controlled phenomenon that occurs directly
on the resin matrix[11] and appears to be related to material composition and concentration, size, and type
of fillers.[4] In the present study, the RLX resin cement had the highest sorption and solubility
means (23.2 µm/cm3 and 2.40 µm/cm3, respectively). High sorption is not necessarily correlated to high solubility 8.31
µm/cm3. The water uptake depends on the degree of conversion, monomers mobility, and hydrophilicity.[9]
[13]
Materials with the presence of electropositive metallic ions on their composition
(e.g., barium and zinc) tend to react with water. According to American Dental Association
and ISO specifications, the sorption and solubility of each resin material over a
storage period of 7 days must be smaller than 40 µm/cm3 and 7.5 µm/cm3, respectively. In the present study, the sorption values ranged from 12.4 µm/cm3 to 23.2 µm/cm3, which are accepted by each standard. The solubility values were also under the accepted
values, ranging from 0.14 µm/cm3 to 2.40 µm/ cm3.[3]
[21]
The limitations of the present study are related to the difficulty in simulating oral
conditions. The in vitro setup simulates extreme conditions, such as immersion of samples for 7 consecutive
days in coffee. On the other hand, sorption and solubility testing according to the
ISO standards only evaluates the materials for 1 week of immersion in distilled water.
Conclusion
The light-cured resin cements tested showed good mechanical resistance and showed
good results with respect to sorption and solubility according to ISO 4049:2009 recommendations.
However, resin cements showed low color stability with coffee immersion after 7 days.
The findings suggest that optical properties of these materials could be improved.
