Interaction between photoactivators and adhesive systems used as modeling liquid on the degree of conversion of a composite for bleached teeth

Thiago Jonathan Silva dos SANTOS1, Ana Margarida dos Santos MELO1, Matheus Dantas TERTULINO1, Boniek Castillo Dutra BORGES1, Ademir Oliveira da SILVA2, Maria Cristina dos Santos MEDEIROS1. 1 Department of Dentistry Federal University of Rio Grande do Norte (UFRN) Natal RN Brazil. 2 Institute of Chemistry Federal University of Rio Grande do Norte (UFRN) Natal RN Brazil. doi: 10.14295/bds.2018.v21i3.1558

Interaction between photoactivators and adhesive systems used as modeling liquid on the degree of conversion of a composite for bleached teeth

INTRoDuCTIoN
T he current demand for increasingly clear teeth has promoted the development of composites that enable restorations with optical characteristics which are more similar to natural teeth and have greater longevity [1].For this, proper polymerization of composites is necessary and can be measured by Degree of Conversion (DC) analysis, a method that evaluates the amount of monomers converted to polymer during polymerization [2].
Therefore, the proper choice of light emitting diodes (LEDs) gives better physical properties [3], since it avoids a low DC which increases bacterial formation, monomer release, composite solubility, and staining susceptibility [4]; it also gives better color stability because aesthetic properties of the composite resins are intimately related to the optical light interaction with the restoration [5].
The most popular LEDs currently on the market are second and third generation, denoted monowave and polywave, respectively.However, despite monowave LEDs emitting high light intensity, they do not enable adequate polymerization of some photoinitiators and alternative co-initiators [6][7][8], thereby potentially causing problems in the polymerization reaction.Alternative photoinitiators such as (2,4,6-trimethylbenzoyl)diphenylphosphine oxide (TPO) and bis-alkyl phosphinic oxide (BAPO) have been replacing camphorquinone (CQ) in some composites to reduce the yellowing effect caused by CQ [9].Thus, polywave LEDs were introduced in an attempt to find a proper LED for these alternative co-initiators [10].
Another aspect related to the restorative technique consists in controlling the stickiness of composites provided by the presence of some monomers such as bisphenol-A glycidyl dimethacrylate (Bis-GMA) [11], which makes restoration modeling difficult [12].In an attempt to minimize this effect, clinicians have used some products such as alcohol and adhesive systems on spatulas and brushes during the incremental technique [13,14].Some studies have demonstrated that these modeling liquids did not change physical properties such as cohesive strength [13][14] or even improve the physical stability of these composites [15] when used between layers.However, it has been shown that some adhesives used as modeling liquid have changed the physical properties on the surface of resins in shade A2, such as the degree of conversion and the crosslinking density [16].However, not enough is known about such effects on the surface properties of resins which have clearer shades.
Therefore, this study aimed to analyze the effect of two LEDs on the DC of a composite resin for bleached teeth under the influence of adhesive systems used as modeling liquid.The null hypotheses tested were: 1) the use of an adhesive system as modeling liquid does not change the DC of a composite for bleached teeth; and 2) the use of monowave or polywave LEDs does not change the DC of a composite for bleached teeth submitted to the use of adhesive systems as modeling liquid.

Experimental design and preparation of samples
This study employed a 2 x 3 factorial design, as two LEDs and three groups of adhesive systems used as modeling liquid were analyzed.The analyzed response was the percentage (%) of DC.
A commercial brand of composite resin IPS Empress Direct resin [ED] (Ivoclar, Vivadent, AG, Schaan, Liechtenstein) in BL-L shade was used.The treatments were: 1) Type of adhesive used as modeling liquid (Adper Single Bond 2; the bond component of Adper Scotchbond Multi-Purpose; or no adhesive system was used as modeling liquid in the control group); and 2) Type of LEDs (Bluephase, polywave; and Coltolux, monowave).
A total of 60 samples (n = 10) of ED composite (Ivoclar Vivadent) in the BL-L shade were prepared into circular teflon molds (5 mm diameter x 2 mm height).The composite was inserted in a single increment and adapted to the mold with a Thompson no. 1 spatula (Thompson Dental Products, Houston, TX, USA) under temperature control at 22 °C.Next, the composite surface was modeled using flat composite brush no.1021 (Hot Spot Design, Cugy, Switzerland) and coated with one drop of SB or MP, which were the adhesive systems used as modeling liquid.No adhesive system was used as modeling liquid in the control group (CT).
One drop of the material was applied to the brush head to coat the brush with adhesive Interaction between photoactivators and adhesive systems used as modeling liquid on the degree of conversion of a composite for bleached teeth Santos TJS et al.
systems and excess was removed by stroking each side of the brush onto a glass plate for 3 seconds.Sculpting consisted of three sweeping motions of the adhesive-dampened instrument against each half of the circular composite surface (a total of six motions for each sample) [16].
A microscopic slide was pressed onto the sample and the composite was photoactivated for 20 seconds using polywave Bluephase G2 LED (Ivoclar Vivadent, Liechtenstein) or monowave Coltolux LED (Coltène Whaledent, Altstätten, Switzerland), both with a light intensity of 1200 mW/cm 2 (according to the manufacturer's instructions) and previously measured in a portable radiometer RD-7 (ECEL, Ribeirão Preto, SP, Brazil) in order to ensure adequate power.Then the sample was removed from the mold and the excess edges were removed with a no.15 scalpel blade.The base was scratched with a 1012 diamond drill tip to differentiate it from the surface to be analyzed.The samples were separately placed into properly identified eppendorfs and stored in a light-free amber vessel in conservation at 37 ºC in an incubator for a period of 24 hours until the DC analyzes were performed.
The compositions of the materials used in this study are listed in Table 1.

DegRee of CoNveRsIoN
DC readings were performed using the Fourier transform infrared spectroscopy (FTIR) technique.The samples were inserted on the spectrometer and evaluated on the top surface (which received the adhesive system layer) on a Spectrum100 FTIR spectrometer coupled to an attenuated total reflection device (ATR) (PerkinElmer, Shelton, CT, USA).The DC parameters used 10 scans and wavenumber related to 1/λ, which creates a graph for observing the peaks at 1608 and 1638 cm -1 ; characteristic of the aromatic vinyl bonds of bisphenol and aliphatic bonds of the methacrylate functional group, respectively.Unpolymerized samples of the composite resin used were also evaluated.The DC (%) was calculated using the following equation: Where: R represents the ratio between the absorbance peak at 1638 cm -1 to 1608 cm -1 .[17] sTATIsTICAl ANAlYsIs The data were statistically analyzed by twoway analysis of variance (ANOVA) and Tukey's test (p< 0.05) at a significance level of 5% in ASSISTAT 7.7 beta software.

ResulTs
There was no statistically significant differences between LEDs, neither in the interaction between adhesive system used as modeling liquid and LEDs (p > 0.05).However, there was statistically significant differences among adhesive systems (p < 0.05).However, there was a statistically significant increase in the DC for the two tested adhesive systems concerning the control group for both Bluephase and Coltolux LEDs, with the highest DC values obtained when SB was applied (p < 0.05).Comparisons between groups are listed in Table 2.

Interaction between photoactivators and adhesive systems used as modeling liquid on the degree of conversion of a composite for bleached teeth
Santos TJS et al.
Table 2 -Means (standard deviations) of the degree of conversion (%) according to the kind of LED and adhesive system used as modeling liquid.Means followed by distinct upper case letters denote statistically significant differences between the kinds of LEDs for the same dental adhesive used as modeling liquid (p < 0.05).Means followed by lower case letters denote statistically significant differences between dental adhesive used as modeling liquid for the same kind of LED (p < 0.05).

DIsCussIoN
The first null hypothesis tested in this study was rejected since the use of both tested adhesive systems increased the DC of the composite compared to the control group (Table 2).
It is known that the adhesive systems used have 2-hydroxyethyl methacrylate (HEMA) in their compositions, which is a hydrophilic monomer widely used in commercial adhesive systems and which promotes a higher DC [18], which may have contributed to the increase in DC compared to the control group.In addition, the presence of urethane dimethacrylate monomer (UDMA) in the SB adhesive system can explain the higher value presented in the DC, since this monomer has a flexible structure with less hydrogen bonds than BisGMA, thereby increasing the DC [19][20].On the other hand, the higher amount of BisGMA monomer in the MP composition would explain the lower DC values of this adhesive in comparison to SB, since it has an epoxy central portion and unsaturated acrylic radicals at its extremities, presenting high molecular weight (512 g/mol), high viscosity (500,000-800,000 MPa), low mobility and low DC at ambient temperature [19,21].
In contrast, the second null hypothesis was accepted, since there was no statistically significant difference between the DC values when using second-or third-generation of LEDs (Table 2).
The literature reveals a probable inadequate correlation between the absorption spectrum of alternative photoinitiators and the emission spectrum of photoactivation units.Due to their short emission spectrum, second-generation LEDs may not activate some alternative photoinitiators present in the composite [6,8], consequently compromising the polymerization reaction and decreasing the DC.
Therefore, as the present study has analyzed a composite for bleached teeth in BL-L shade that hypothetically should contain alternative photoinitiators to CQ, a greater effectiveness of the third generation LED was expected.Nevertheless, this fact was not confirmed, leading to the assumption of the presence of CQ in the composition of the composite in question (data not informed by the manufacturer).The unavailability of details about the chemical composition of the products by the manufacturers is a limitation of the study, because to know what kind of photoinitiators were used would help understand their behavior in relation to the type of LED.
Another possibility would be that the ultraviolet LEDs (third generation) accountable for the alternative photoinitiators activation (which have their maximum absorption at the light spectrum of 320-400 nm) [22] should be distributed in the tips of the third generation Bluephase G2 LED (Ivoclar Vivadent, Liechtenstein) in such a way that there is no direct interaction with the alternative photoinitiators present in the tested adhesives during photoactivation [23].Therefore, it is observed that there is a need for information related to the type of photoinitiators present in the composition of the composites by the manufacturers in order to guide the clinician's choice of suitable LED for correct photoactivation of these materials without implications on the longevity of the restorations.

CoNClusIoN
The use of adhesive systems as modeling liquid in the composite insertion and sculpture technique increased the DC on the surface of a composite for bleached teeth, regardless of the light emitting diodes (LEDs) being either monowave or polywave.

Table 1 -
Materials used in this study