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Year : 2022  |  Volume : 25  |  Issue : 5  |  Page : 510-514
Spectrophotometric evaluation of staining of different types of light-cure composite resins after exposure with different light-cure intensities: An in vitro study

1 Department of Conservative Dentistry and Endodontics, M.G.V.'s K.B.H. Dental College and Hospital, Nashik, Maharashtra, India
2 Department of Conservative Dentistry and Endodontics, AECS Maaruti College of Dental Sciences and Research Center, Bengaluru, Karnataka, India

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Date of Submission17-Apr-2022
Date of Decision06-May-2022
Date of Acceptance19-May-2022
Date of Web Publication05-Jul-2022


Context: Relation between the adequate intensity output of curing lights on color stability of composite resin is well accepted.
Aims: To investigate the effect of different light-curing intensities and its relation to color stability of different polymerized composite resin materials using the spectophotometric analysis.
Settings and Design: Comparative in vitro study done on composite resin discs.
Subjects and Methods: A total of 180 discs comprising sixty discs prepared from three different composite resins, namely microhybrid composite resin (Filtek Z100,3M ESPE), nanohybrid composite resin (Filtek Z250 XT, 3M ESPE), and nanofilled composite resin (Filtek Z350 XT,3M ESPE) using three different light curing intensities, viz., 325–425 mW/cm2, 750–850 mW/cm2, and 1000–1100 mW/cm2. Later these discs were stained with 2% methylene blue followed by re-absorption in absolute alcohol for supernatant solution preparation which is used for the spectrophotometric analysis.
Statistical Analysis Used: Spectrophotometric absorption values were analyzed using the one-way ANOVA test for intergroup analysis.
Results: Mean stain absorption was the highest with nanofilled composite resin (Filtek Z350 XT,3M ESPE) after exposure with light-curing intensity of 325–425 mW/cm2 and least with microhybrid composite resin (Filtek Z100, 3M ESPE) after exposure with light-curing intensity of 750–850 mW/cm2 and this difference found was highly significant statistically (P < 0.001).
Conclusions: Microhybrid composite resin (Filtek Z100, 3M ESPE) cured with intensity of 750–850 mW/cm2 showed least stain absorption indication most color stability and esthetic function.

Keywords: Color stability; composite resin; light-curing intensity; spectrophotometry

How to cite this article:
Pawar PA, Gulve MN, Aher GB, Kolhe SJ, Pramaod J. Spectrophotometric evaluation of staining of different types of light-cure composite resins after exposure with different light-cure intensities: An in vitro study. J Conserv Dent 2022;25:510-4

How to cite this URL:
Pawar PA, Gulve MN, Aher GB, Kolhe SJ, Pramaod J. Spectrophotometric evaluation of staining of different types of light-cure composite resins after exposure with different light-cure intensities: An in vitro study. J Conserv Dent [serial online] 2022 [cited 2022 Sep 25];25:510-4. Available from:

   Introduction Top

The influence of adequate intensity output of curing lights which ensures the longevity of restorations and avoid undesirable clinical outcomes is universally accepted.[1] A decrease in the light output of curing units causes a lower degree of monomer conversion affecting properties such as depth of cure (DC), compressive strength, hardness, degree of polymerization or double-bond conversion, and color stability of composite resin.[2] Composite resins with higher percent conversion have greater mechanical properties and better color stability.[3] Therefore, properties of composite resins are indirectly dependent upon optimum absorption wavelength and intensity of light-curing units.[4] Furthermore, color stability of composite resins is dependent on the extrinsic factors and intrinsic factors. The extrinsic factors include food beverages, intensity, duration of polymerization, and other environmental factors.[5] While composition of resin matrix, type of photoinitiator, and percentage of remaining double bonds (C = C) are the various intrinsic factors responsible for color changes in composite resin materials.[6] Hence, the aim of the study was to investigate the effect of the different light-curing intensities and their relation to color stability of different polymerized composite resin materials using spectrophotometric analysis. The results obtained are shown in [Table 1] and [Table 2] as well as [Graph 1].
Table 1: Groups and subgroups according to composite resin materials and light-curing intensities

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Table 2: Comparison of mean stain absorption values of microhybrid, nanohybrid, and nanofilled composite resins after exposure with light-curing intensities of 325-425 mW/cm2, 750-850 mW/cm2 and 1000-1100 mW/cm2

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   Subjects and Methods Top

Total of 180 discs comprising sixty discs prepared from three different composite resins, namely microhybrid composite resin (Filtek Z100), nanohybrid composite resin (Filtek Z250 XT), and nanofilled composite resin (Filtek Z350 XT) with A1 shade. Composite resin discs were prepared using a 10-mm diameter × 2-mm thick Teflon mold placed on a glass plate with a transparent mylar strip. The composite resin was light-cured with one of the light-curing intensities selected for 40s. It is important to emphasize that light tip of the curing unit (Model:-LED-10W, Apoza Enterprise Co. Ltd, New Taipai City, Taiwan) had compatible size with composite resin disc's diameter and kept directly over the Mylar strip to obtain complete polymerization of composite resin. The radiometer (Woodpecker Med.) was used to calibrate the intensity of light-curing unit before preparing each composite resin disc of respective groups.

These prepared 180 composite resin discs were divided into groups as [Table 1].

After polymerization, composite resin discs were removed from mold and placed into empty test tubes in the incubator for 24 h. Later these discs were immersed in 1 ml of 2% Methylene Blue dye solution and placed at 37°C ± 2°C for 24 h. After that, stained composite resin discs were rinsed under distilled water for a minute and were stored in relative humidity for 24 h. Later composite resin discs were individually immersed into new test tubes containing 1 ml of absolute alcohol and placed at 37°C ± 2°C for 24 h. Later, these solutions of absolute alcohol with reabsorbed stain were filtered and centrifuged for 3 min at 4000 revolutions/min to obtain supernatant. This supernatant was used as a solution sample to determine stain absorbance in a spectrophotometer which is based upon CIE L*a*b* system.

   Results Top

The result was analyzed with the one-way ANOVA analysis using IBM SPSS Statistics for Windows, Version 22.0. Armonk, NY, USA: IBM Corp. For the current study, level of significance (α) is 5%. Hence, P value was considered significant if P ≤ 0.05.

Spectrophotometric evaluation was done to measure the stain absorption property of composite resin specimens. Spectrophotometric analysis found maximum spectral wavelength (λmax) absorption of methylene blue stain at 655 nm. Therefore, each sample solution was checked for absorption value of stain at a similar wavelength. Absorption value measured in terms of absolute number of spectrophotometric analysis.

In order to evaluate the statistical difference between nine subgroups, one-way ANOVA test was applied. The results obtained are shown in [Table 1] and [Table 2]. During spectrophotometric analysis, mean stain absorption was the highest with Group IIIA (nanofilled composite resin after exposure with light-curing intensity of 325–425 mW/cm2) and least with Group IB (microhybrid composite resin after exposure with light-curing intensity of 750–850 mW/cm2) and this difference found was highly significant statistically (P < 0.001).

   Discussion Top

Degree of polymerization or double-bond conversion is an important factor affecting esthetic outcome of composite resins. A study done by Prasanna et al.[7] showed that degree of conversion of composite resins at the surface at room temperature was found to be 52.08%, which implies that only 52.08% of methacrylate groups were polymerized to form polymeric carbon-carbon single bonds. This means a significant proportion of methacrylate groups were left unreacted with carbon-carbon double bonds. This increased percentage of unreacted polymer or carbon double bonds increases the viscosity of the system decreasing the availability of reactive special as decreasing physical properties including esthetics, which is known as “Gel effect or TrommsdorfNorrish effect.”[8] It is well-established fact that color stability is dependent on the conversion of monomeric carbon-carbon double bonds into polymeric carbon-carbon single bonds with the help of infrared absorptivity and Raman scattering intensity.[9] While DC is affected by light cure intensity which relates the color stability of composite resins to the light-curing intensities.[10] Therefore, in this study, other intrinsic and extrinsic factors kept similar for reducing bias expect for type of filler component of composite resin and light-curing intensity.

Teflon mold was used in this study for the fabrication of samples as composite resin does not adhere to the Teflon material.[11] Dimensions and method of the preparation of discs were according to a study done by Nitta.[12],[13],[14]

Methylene blue being cationic dye is readily taken up by composite resin discs and easily reabsorbed by solutions with hydroxide groups such as absolute alcohol within time of 357 min.[15] Therefore, for the present study, 2% methylene blue dye was used for staining of composite resin materials and absolute alcohol is used for the uptake of methylene blue dye for spectrophotometric evaluation.[16]

In the present study, curing light intensity range of 325–425 mW/cm2 (Group IA, IIA, and IIIA) showed much worse results than that of other two ranges of curing light intensities namely 750–850 mW/cm2 (Group IB, IIB, and IIIB) and 1000–1100 mW/cm2 (Group IC, IIC, and IIIC). Chandrasekhar et al.[16] have stated that dye penetration indirectly reflects the amount of conversion of the double bonds given that composites containing more than 35% of unconverted C = C bonds are susceptible to discoloration. Therefore, higher the degree of conversion, smaller the amount of residual monomers available to form colored degraded products which influence the quality of photopolymerization.[17] Tarle et al.[18] have shown that light intensities in the nearby ranges of 325–425 mW/cm2 are incapable of converting monomer into the polymer in as higher degree as that of the other two intensities. The degree of conversion of composite resin is influenced by energy density.[19] In the present study, keeping the time of exposure constant, it was observed that energy density decreased for the samples cured with low intensity. Insufficient energy density resulted in less than maximal conversion and the demonstrated differences in the quantity of remaining double bonds are reflected in other polymerization characteristics such as more staining.

The guiding principle that dictates the efficiency of a photopolymerization reaction is how much light energy is absorbed by the photoinitiator during light irradiation. Therefore, light intensity is an important factor in the activation of photoinitiator. However, as reported by Watts[8] photopolymerization is diffusion-controlled reaction after certain point called, “gel point.” Therefore, after critical threshold intensity reached, i.e., intensity required for initiation of photopolymerization reaction in composite resins, further rise in intensity does not necessarily enhance the degree of conversion of monomer into polymer. In addition, high-intensity curing can promote the formation of polymer chains with lower molecular weight and residual monomers. This consequently leads to the partial polymerization of the material, with part of the photoinitiator remaining idle. This small amount of photoinitator in participating composite resins remains inactive, causing a residual yellow in the final color of the composite resin, which may alter the color stability of the composite resins. The reason stated behind this by Malhotra and Mala[20] as high intensity produces a multitude of growth centers during initial irradiation and final high-density cross polymerization as composite resin converts from maneuverable to solid-state. This high intensity and short exposure irradiation in combination accelerate the rate of curing and provides insufficient time for relaxation of internal stress which is translated due to high light density. This ultimately leads to greater shrinkage stresses and poorer interfaces of composite resin.

Considering all these possible reasons, in the present study, the intensity range of 750–850 mW/cm2 (Groups IB, IIB, and IIIB) showed better results in terms of staining susceptibility of composite resin materials than the intensity range of 1000–1100 mW/cm2 (Groups IC, IIC, and IIIC), as correlated to study by Rahiotis et al.[21]

Resin composites with small filler particles such as nanohybrid and nanofilled composite resins assumed to have the best surface finish and gloss thus providing better color stability; however this study revealed that microhybrid composite resin (Groups IA, IB, and IC) showed the least amount of absorbed stain directly indicating more color stability than that of nanohybrid (Groups IIA, IIB, and IIC) and nanofilled composite resins (Groups IIIA, IIIB, and IIIC) after staining. This result was in accordance with the studies done by Mahajan et al.[11] and Kheraif et al.[22] These studies have revealed that the presence of triethylene glycol dimethacrylate (TEGDMA) in the resin matrix has been attributed to be one of the causes for discoloration of the resin matrix. TEGDMA is a diluent monomer added to the bisphenol-A-diglycidyldimethacrylate resin matrix. Due to it's hydrophilicity, composites containing this monomer exhibit increased water sorption. Alberton Da Silva et al.[23] have evaluated water sorption of common monomers used in dental composite resins and stated that it increases in the order as Bisphenol A ethoxylated dimethacrylate (Bis-EMA) (20.10 mg/mm3) < Urethane dimethacrylate (UDMA) (29.46 mg/mm3) < Bisphenol-A-Glycedylmethacrylate (Bis-GMA) (33.49 mg/mm3) <TEGDMA (69.51 mg/mm3). Both nanohybrid and nanofilled composite resins tested in this study contain TEGDMA in their composition which can be attributed to their higher staining capacity than that of microhybrid composite resin which is devoid of TEGDMA and contains mainly UDMA; shown to reduce water uptake and stain susceptibility.

In between nanohybrid (Groups IIA, IIB, and IIC) and nanofilled composite resins (Groups IIIA, IIIB, and IIIC), nanofilled composite resin stained more. Similar results were found by Oliveira et al.,[24] reason being stated as staining capacity might be attributed to the degree of water sorption and hydrophilicity of matrix resin. This extrawater sorption is responsible for expanding and hydrolyzing silane component forming microcraks. Micro-cracks or interfacial gaps between resin matrix and filler of composite resins can absorb stain and able to absorb other fluids with pigments, which results in discoloration as assumed that water acts as a vehicle for stain penetration into the resin matrix. The glass filler particles do not absorb water. Therefore, the greater amount of resin matrix results in greater water sorption. It is reported that composite resins with a lower amount of inorganic fillers presented more color change because the greater resin matrix volume. This ultimately allows greater water sorption. Matrix volume of nanohybrid composite resin is lesser, as it contains more volume of filler particles (67.8%) which does not participate in discoloration than nanofilled composite resin (63.3%). Furthermore, nanohybrid composite resins show more degree of conversion than that of nanofilled composite resins, ultimately leading to a lesser presence of unreacted carbon double bonds, making the composite less susceptible to degradation followed by lesser change in refractive index by a scattering pattern and minimum change in the opacity of the composite resin.[24] This explains more staining susceptibility of nanofilled composite resin than nanohybrid composite resin. However, a study by Demirci et al.[25] stated that nanofilled composite restorations were considered to have a better color match after aging than nanohybrid composite restorations, which could be due to differences between the teeth and operator skills, also it has been stated that the initial gloss of many restoratives was quite good, but in hybrid composite plucking of the larger fillers, caused loss of gloss and color change.

   Conclusions Top

All composite resins, namely microhybrid, nanohybrid, and nanofilled composite resins showed susceptibility to staining after exposure with light cure intensities of 325–425 mW/cm2, 750–850 mW/cm2, and 1000–1100 mW/cm2. Staining results were significantly better when microhybrid, nanohybrid, and nanofilled composite resin exposed with the light-curing intensity of 750–850 mW/cm2 as compared to all other light-curing intensities. Microhybrid, nanohybrid, and nanofilled composite resins had comparable staining results after exposure with light-curing intensities of 325–425 mW/cm2 and 750–850 mW/cm2. Microhybrid composite resin had significantly better staining results as compared to all other composite resins after exposure with light-curing intensity of 1000–1100 mW/cm2.

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Correspondence Address:
Dr. Pawan Anil Pawar
1A, Arpan, Aditya Colony, Ashoka Marg, Nashik - 422 011, Maharashtra
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/jcd.jcd_214_22

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