Keywords composite resin - temporary - polymerization - flexural strength - color stability
Introduction
Provisional dental restorations are commonly used during the temporization period
between preparing teeth and the placement of final restoration to help maintaining
prepared margins, spaces, periodontal health, masticatory functions, appearance, and
vocal quality for the patient.[1 ]
[2 ] The commonly used provisional materials include polymethyl methacrylate (PMMA) and
bis-acryl resin composites which contain dimethacrylate monomers and fillers.[3 ]
[4 ] These materials can be polymerized chemically which could facilitate the direct
fabrication technique.[3 ] The technique involves the injection of material into the silicone index placed
over the prepared tooth intraorally. The clinicians then allow the material to set
either by light or chemical activations. The use of light activation significantly
enhances physical properties of the material.[5 ] However, the light curing option may not be possible if the transparent silicone
is not available. Thus, the ability to cure chemically is still vitally important
for the materials.
Tertiary amine enables the dissociation of chemical initiator, such as benzoyl peroxide
(BP), to initiate polymerization of the materials. N, N-dimethyl-p-toluidine (DMPT;
molecular weight of 135 g/mol) is the commonly used tertiary amine to enable polymerization
of chemical-activated resin composites or cements.[6 ]
[7 ] The major concern of DMPT is its toxicity. Various studies have reported that DMPT
can be absorbed and distributed to various sites inducing toxic and carcinogenic effects.[8 ]
[9 ]
[10 ] Hence, an alternative activator is needed.
Na-N-tolyglycine glycidyl methacrylate (NTGGMA) consists of methacrylate and amine
groups in the structure ([Fig. 1 ]).[11 ] Hence, the monomer can be polymerized within the polymer network, thus reducing
the risk of leaching of monomers. The monomer also contains carboxylic group which
could potentially help to increase hydrophilic properties and promote the flow of
material on the hydrophilic tooth surface. However, the hydrophilic group of NTGGMA
may increase water sorption to the material that may subsequently reduce the strength
and esthetic properties of the material.[12 ] The materials are required to exhibit sufficient mechanical strength and color stability
to ensure patient satisfaction and function during temporization process.
Fig. 1 Chemical structure of (A ) DMPT (N, N-dimethyl-p-toluidine) and (B ) NTGGMA (Na-N-tolyglycine glycidyl methacrylate).
The aim of this study was therefore to assess the effect of using different amine
activator (NTGGMA vs. DMPT) on the degree of monomer conversion (MC), biaxial flexural
strength (BFS), and color stability of the experimental provisional dental restorations.
Commercial materials were used for comparison. The null hypothesis was that the use
of different amine activators should not detrimentally affect the degree of MC, BFS,
and color stability of the materials.
Materials and Methods
Material Preparation
The experimental composites for provisional restorations were prepared using the power-to-liquid
mass ratio of 2.3:1. Powder phase of the experimental materials contained silanated
borosilicate glass (Esstech Inc.; Essington, Pennsylvania, United States). The liquid
phase contained 70 wt% urethane dimethacrylate (UDMA; lot no. MKCG8230, Sigma-Aldrich,
St. Louis, Missouri, United States), 24 wt% triethyleneglycol dimethacrylate (lot
no. STBF9549V, Sigma-Aldrich), and 5 wt% 2-hydroxyethyl methacrylate (HEMA; lot no.
STBG6525, Sigma-Aldrich). For imitator liquid, 1 wt% BP (lot no. MKCF7091, Sigma-Aldrich)
was added. For activator liquid, 1 wt% of DMPT (lot no. MKBX9809V, Sigma-Aldrich)
or 2 wt% of NTGGMA (lot no. X8630050, Esstech Inc) was added. The experimental composites
containing DMPT and NTGGMA were referred to as D-temp and N-temp, respectively.
The liquid phase was mixed using a magnetic stirrer for 1 hour. The mixed liquid was
left for 24 hours before mixing with powder phase. The powder and liquid phases were
weighed using a four-figure balance. The powder and liquid were hand-mixed using a
plastic spatula (~20 seconds until the paste was mixed homogenously). The mixed initiator
and activator pastes were then loaded into the double-barrel syringe. The syringe
was left in the upright position at room temperature for 24 hours to allow the release
of air bubbles incorporated in the paste.
The experimental materials were mixed and injected using mixing tip with a dispenser
Sulzer Mixpac AG, Switzerland.[13 ] Commercially available provisional dental restorations including PMMA and bis-acryl-based
composites were used as comparisons ([Table 1 ]). The commercial materials were prepared according to the manufacturer’s instruction.
Table 1
The composition of commercial materials
Materials
Composition
Lot no.
Suppliers
Note: The materials are either polymethyl methacrylate-based material (Unifast Trad)
or bis-acryl-based materials (Luxacrown, Luxatemp, and Protemp 4). The ingredients
of Luxacrown and Luxatemp in each catalyst or base pastes were not supplied by suppliers.
Unifast Trad
(UF)
Powder: ethyl-methyl methacrylate monomer, polymethyl methacrylate, barbituric acid
derivative, organic copper compound, pigments
1808271
GC Corporation; Tokyo, Japan
Liquid: methyl methacrylate, N,N-dimethyl-p-toluidine trimethylolpropane, ethylene
glycol dimethacrylate
Luxacrown (LC)
Inorganic and organic filler, matrix of multifunctional acrylates
770658
DMG; Hamburg, Germany
Luxatemp
(LT)
Polymethyl methacrylate, SiO2 , bisphenol A-glycidyl methacrylate, urethane dimethacrylate, other dimethacrylates
802411
DMG; Hamburg, Germany
Protemp 4
(PT)
Base: Ethoxylate bisphenol-A dimethacrylate, silane-treated
amorphous silica, reaction product of 1,6-diisocyanatohexane with 2-[(2-methacryloyl)
ethyl]6-hydroxyhexonate and 2-hydroxyethyl methacrylate
4249947
3M ESPE; St. Paul, MN, USA
Catalyst: ethanol, 2,2’ -[(1-methylethylidene)bis(4,1-phenyleneoxy]bis-diacetate,
benzyl-phenyl-barbituric acid, silane-treated silica
Monomer Conversion
A Fourier-transform infrared spectroscopy (FTIR; Nicolet iS5, Thermo Fisher Scientific,
Massachusetts, United States) equipped with an attenuated total reflection (ATR; ID7,
Thermo Fisher Scientific) was used to determine the MC of the materials (n = 5). The mixed materials were placed in the metal circlip (1 mm in thickness and
10 mm in diameter) on the ATR diamond. The specimens were then covered with an acetate
sheet. The FTIR spectra were recorded initially and after 10 minutes. The spectra
at the region of 700 to 4,000 cm–1 with the resolution of 8 cm were obtained.[14 ] The test was performed at 25 ± 1°C. The degree of MC was calculated using the following
equation:
where B
0 and B
t are the absorbance of the C-O peak (1,320 cm–1 ) above background level at 1,335 cm–1 initially and after time t .15
Biaxial Flexural Strength and Biaxial Flexural Modulus
The materials were loaded into the metal circlip (1 mm in thickness and 10 mm in diameter)
(n = 8). The specimens were covered with an acetate sheet and left at room temperature
for 24 hours to allow the completion of polymerization. The disc specimens were then
removed, trimmed excess, and placed in a tube containing 10 mL of deionized water.
They were incubated at 37°C for 24 hours and 4 weeks. Then, the discs were removed,
blotted dry, and mounted in the ball-on-ring testing jig. BFS test was performed under
the mechanical testing frame (AGSX, Shimadzu, Kyoto, Japan) using 500 N load cell
with the crosshead speed of 1 mm/min. The BFS (Pa) of the specimen was obtained using
the following equation[16 ]:
where F is the load at failure (N), d is the specimen’s thickness (m), r is the radius of circular support (m ), and í is Poison’s ratio (0.3). Additionally, biaxial flexural modulus (BFM) was calculated
using the following equation[17 ]:
where ΔH /ΔWc is the rate of change of load with regards to central deflection or gradient of force
versus displacement curve (N/m), âc is the center deflection junction (0.5024),[18 ] and q is the ratio of support radius to the radius of disc. Additionally, the fracture
surface of tested specimens at 4 weeks was investigated using a scanning electron
microscope (SEM; JSM 7800F, JEOL Ltd., Tokyo, Japan).
Color Stability
The disc specimens (n = 3) were prepared and immersed in 10 mL of deionized water. The specimens were incubated
at 37°C for 3 weeks. The CIELab coordinates of all specimens before and after immersion
were measured using a dental spectrophotometer (Easyshade V; VITA Zahnfabrik, Baden-Württemberg,
Germany). The spectrophotometer was calibrated according to the manufacturer’s instructions
prior to the measurement. The specimens were placed over the opaque white background.
The illumination of the room was 850 lux which was measured by a light meter (LX1330B
Light Meter; Dr. Meter Digital Illuminance, StellarNet Inc.; Florida, United States).[19 ] The spectrophotometer probe tip was positioned perpendicular to the center of the
specimens until the completion of measurement. The color coordinates (CIE L *, a *, b *, C *, and h
o ) were then recorded. The L *, a *, and b * parameters refer to value axis, red-green axis, and yellow-blue axis, respectively.
In addition, C * and h
o were chroma and hue angle.
The measurement for each specimen was performed in triplicate. Color differences or
color changes of the composites after immersion in deionized water was calculated
using the CIEDE2000 (E00 ) formula[20 ]:
where ∆L' , ∆C' , and ∆H' represent the changes in lightness, chroma, and hue, respectively. Furthermore, RT
is a rotation function related to the interaction between chroma and hue differences
in the blue region. Additionally, SL
, SC
, and SH
are weighting functions and KL
, KC
, and KH
are correction terms for experimental conditions.
Statistical Analysis
Values reported in the current study are mean ± standard deviation. The data were
analyzed using Prism 9 (GraphPad Software; San Diego, California, United States).
Normality of the data was tested using the Shapiro–Wilk test. For normally distributed
data (MC, BFS, color stability), one-way analysis of variance followed by post hoc
Tukey multiple comparison was employed. For non-normally distributed data (BFM), Kruskal–Wallis
test followed by Dunn’s multiple comparisons was used. Significance level was set
at p = 0.05. Additionally, power analysis was performed using G*Power version 3.1.9.6
(University in Düsseldorf, Germany) which indicated that the sample used in each test
gave power > 0.95 at α = 0.05.
Results
Degree of Monomer Conversion
The highest MC was obtained from Unifast (UF) (75.6 ± 1.5%) ([Fig. 2 ]). D-temp (57.4 ± 1.3%) exhibited comparable MC to N-temp (59.0 ± 1.3%) (p = 0.8346). The conversion of both D-temp and N-temp were similar to that of Luxacrown
(LC) (60.1 ± 2. 9%) (p = 0.3676, 0.9626). D-temp and N-temp however showed significantly higher MC than
Luxatemp (LT) (48.0 ± 1.6%) (p < 0.01) and Protemp (PT) (48.1 ± 3.4%) (p < 0.01).
Fig. 2 Monomer conversion of Unifast (UF), Protemp (PT), Luxacrown (LC), Luxatemp (LT),
and experimental composites (D-temp and N-temp). Error bars are standard deviation
(SD ) (n = 5). Lines represent p > 0.05.
Biaxial Flexural Strength and Biaxial Flexural Modulus
At 24 hours, the highest and lowest BFS were obtained from LT (214.1 ± 29.7 MPa) and
UF (119.8 ± 13.6 MPa), respectively ([Fig. 3A ]). The BFS of D-temp (164.2 ± 18.1 MPa) was comparable to that of N-temp (168.6 ±
8.9 MPa) (p = 4332). The BFS of both D-temp and N-temp were significantly higher than that of
UF (p = 0.008, 0.002) but comparable to that of PT (185.6 ± 19.0 MPa) (p = 0.2783, 0.5315) and LC (193.5 ± 17.2 MPa) (p = 0.0540, 0.1452). The BFS of D-temp and N-temp was reduced to 121.1 ± 31.2 and 143.2
± 12.8 MPa after immersion in simulated body fluid for 4 weeks. The BFS of D-temp
and N-temp at 4 weeks was significantly lower than that of PT (187.5 ± 26.5 MPa),
LC (173.6.5 ± 23.8 MPa), and LT (184.7 ± 24.0 MPa) (p < 0.05).
Fig. 3 (A ) Biaxial flexural strength (BFS ) and (B ) biaxial flexural modulus (BFM ) of Unifast after immersion in water for 24 hours and 4 weeks. Error bars are standard
deviation (SD ) (n = 8). Same lower-case and upper-case letters denoted p < 0.05 for the strength at 24 hours and 4 weeks, respectively. Stars (*) represent
p < 0.05 for the strength of the same material.
The highest and lowest BFM at 24 hours were obtained from N-temp (4.0 ± 0.2 GPa) and
UF (1.3 ± 0.2 GPa), respectively ([Fig. 3B ]). BFM of N-temp was comparable to that of D-temp (3.0 ± 0.4 GPa) (p = 0.6267) but was significantly higher than that of UF (p < 0.01), PT (2.0 ± 0.2 GPa) (p = 0.0001), and LC (2.6 ± 0.4 GPa) (p = 0.0382). After 4 weeks, the values of D-temp and N-temp were reduced to 2.5 ± 0.7
and 3.5 ± 0.4 GPa, respectively. The highest and lowest observed mean values were
obtained from N-temp (3.5 ± 0.4 GPa) and UF (1.5 ± 0.5 GPa), respectively.
Fracture surfaces of the tested specimens at 4 weeks were examined under SEM ([Fig. 4 ]). PT, LC, and LT showed smooth fractured surfaces. However, multiple voids with
diameter of 20 to 50 μm in the bulk of materials were detected on the fracture surface
of D-temp and N-temp.
Fig. 4 Scanning electron microscope (SEM ) images of fracture surface from Unifast (UF), Protemp (PT), Luxacrown (LC), Luxatemp
(LT), and experimental composites (D-temp and N-temp). The scale bars represent 100
and 10 µm in length. Voids in the core of materials were observed with D-temp and
N-temp (arrows).
Color Stability
The highest and lowest observed color difference (∆E00 ) were obtained from D-temp (2.69 ± 0.66) and UF (0.55 ± 0.17), respectively ([Fig. 5 ]). ∆E00 of D-temp and N-temp (2.46 ± 0.78) were comparable (p = 0.997) but were significantly higher than that of UF (p = 0.0124, 0.0266). The color difference of UF was similar to that of PT (0.91 ± 0.25)
(p = 0.9800), LC (1.16 ± 0.84) (p = 0.8399), and LT (0.98 ± 0.72) (p = 0.9544). Additionally, PT exhibited significantly lower color difference than D-temp
(p = 0.0396).
Fig. 5 Color difference (E00 ) of Unifast (UF), Protemp (PT), Luxacrown (LC), Luxatemp (LT), and experimental composites
(D-temp and N-temp) after immersion in deionized water for 3 weeks. Error bars are
standard deviation (SD ) (n = 3). Stars (*) represent p < 0.05.
Discussion
Currently, one of the main chemical activators used in resin-based materials for provisional
dental restorations is DMPT. However, the major concern of this monomer is its toxic
effects. The aim of this preliminary study was therefore to investigate the effect
of using different amine activators (DMPT or NTGGMA) on degree of MC, BFS, and color
stability of the materials. The hypothesis was accepted as the use of DMPT or NTGGMA
showed no detrimental effect to MC, BFS, and color stability of the materials. It
should be noted that the current study is an in vitro study. Hence, the clinically relevant aspects should be carefully interpreted.
Degree of Monomer Conversion
High degree of MC of provisional restorations may help to ensure adequate physical
and mechanical properties for the restorations.[21 ] Additionally, it was expected that the high conversion could also reduce the risk
of unreacted monomer release that may cause cytotoxic effects.[22 ] It is known that monomer with low glass transition temperature (Tg ) and high flexibility could contribute to high degree of MC of the polymer.[17 ]
[23 ] The highest MC was detected with PMMA-based material (UF) which could be due to
the use of low molecular weight methyl methacrylate monomer (molecular weight = 101
g/mol). However, high MC usually associates with high exothermic reaction which may
affect dentin-pulp complex.[24 ] A study demonstrated that PMMA exhibited increase in temperature during setting
by 4.2 to 11.6°C which was higher than that of bis-acryl composite (2.0–6.6°C).[4 ]
It should be mentioned that the concentration of DMPT (1 wt%) was lower than NTGGMA
(2 wt%). The pilot study showed that using 2 wt% DMPT and 1 wt% NTGGMA enabled suitable
handling characteristics. The MC of D-temp and N-temp were higher than that of two
commercial bis-acryl-based materials (PT and LT). The primary base monomer of D-temp
and N-temp was UDMA. The Tg of UDMA (–38°C)[25 ] was lower than bis-GMA (Tg =–10°C) which was the primary base monomer of bis-acryl-based materials (PT, LT).
However, the actual composition of the monomers of commercial materials was not supplied
from the manufacturers.
Biaxial Flexural Strength and Biaxial Flexural Modulus
The provisional restorations required adequate strength to ensure the survival upon
the repeated chewing forces before the placement of definitive restoration.[3 ] The required flexural strength from 3-point bending test for polymer-based crown
material according to the BS ISO 10477–2018 was 50 MPa.[26 ] The current study employed BFS instead of 3-point bending test as stated in the
standard. It is suggested that BFS test could give similar results to 3-point bending
test but with more reproducibility.[27 ] The results from the current study suggested that flexural strength of the experimental
composites should pass the standard even after aging in water for 4 weeks.
The strength of dimethacrylate-based composites (PT, LT, LC, D-temp, and N-temp) was
higher than of the strength obtained from monomethacrylate-based materials (UF). This
was in accordance with the previous studies.[2 ]
[28 ] The lowest BFS and modulus was obtained from UF which could be due to the lack of
reinforcing fillers in the material. Additionally, the linear structure of monomethacrylate
polymer and the lack of cross-links between polymer chains of UF may result in the
low rigidity and strength.[2 ] The 24-hour flexural strength of commercial materials in the current study was higher
than that reported in the published studies (UF ~64–111 MPa,[28 ]
[29 ]
[30 ] PT ~85–113 MPa,[29 ]
[31 ] LT ~81.7 MPa[5 ]). The possible explanation could be due to the use of different protocol for specimen
preparation. In the current study, the specimens were left undisturbed in the metal
circlip at room temperature for 24 hours prior to immersion. The delay of specimen
removal may therefore allow polymerization reaction to continue, thus increasing the
cross-links polymer network and strength of the materials.[32 ] The highest modulus of elasticity was obtained from N-temp. This may be due to the
increase in filler load of the experimental materials (69.7 wt%) which was higher
than that of commercial materials (30.8–39.3 wt%).[5 ] The high level of fillers may then increase the stiffness of the materials.[33 ] The high stiffness and rigidity of the experimental materials may be considered
suitable for temporizing the long-span fixed dental prostheses.
The fracture surface of PT demonstrated smoother and more homogenous surface compared
with other bis-acryl-based materials.[34 ] This may be due to the lower filler load (~30.8 wt%)[5 ] or smaller filler diameter of PT compared with other materials. The bis-acryl-based
materials and the experimental materials were mixed using the mixing tips which may
decrease the risk of air bubbles incorporation.[35 ] Multiple voids were detected in the fracture surfaces of D-temp and N-temp ([Fig. 4 ]). This could be due to the incorporation of air bubbles during the hand mixing of
powder and liquid to produce initiator/activator pastes. In the future work, the mixed
paste should be stored in a vacuum to help release air bubbles in the materials.
The increase in immersion time enabled materials to absorb water which led to polymer
plasticization, which could reduce the physical/mechanical properties of the materials.[36 ] The use of NTGGMA which contained hydrophilic group (carboxyl group) showed no significance
on the strength. It was expected that the high MC and the incorporation of salinized
glass fillers may help maintain mechanical strength.[2 ] The experimental provisional materials however showed large decrease in strength
after immersion for 4 weeks compared with commercial materials. The possible explanation
could be that the experimental materials contained HEMA. The addition of HEMA was
expected to promote wetting of the materials on the hydrophilic tooth surface. The
hydrophilicity of HEMA may encourage water sorption and reduce strength of the materials.[37 ] However, the strength of the experimental provisional materials after 4-week immersion
were still higher than that required by the ISO standard.
Color Stability
The degree of color changes of provisional materials was associated with various factors
such as chemical properties of the materials, filler size, water sorption, the incorporation
of air bubbles, and degree of cross-liking molecules.[38 ] It was proposed that perceptibility threshold which represent minimum color difference
identifiable by viewer was when ∆E00 = 0.8.[39 ] Additionally, the acceptability threshold (AT) which indicate the level of color
difference that was acceptable by viewer is when ∆E00 = 1.8. Hence, it is expected that the materials should exhibited color difference
within those range. The ∆E00 of all experimental materials (0.6–1.2) were within the range of both perceptibility
threshold and AT. The high MC of experimental composites was expected to reduce color
change of the materials. However, the color differences of both D-temp (2.7) and N-temp
(2.5) were higher than AT level. This could be due to the use of hydrophilic HEMA
polymer which may promote water sorption and affect color stability of the material.[38 ]
[40 ]
Conclusion
Within the limitation of this study, the following conclusions can be drawn:
The use of NTGGMA or DMPT in the current study showed no detrimental effect on MC,
BFS, and color stability of the experimental provisional dental restorations.
The MC and BFS of experimental materials were in the range of those observed with
the commercial composite for provisional restorations. The strength was also higher
than that required by the ISO standard.
The color stability of experimental materials was lower than that of the commercial
provisional materials.
