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Table of Contents   
ORIGINAL ARTICLE  
Year : 2011  |  Volume : 14  |  Issue : 4  |  Page : 391-394
Spectrophotometric and colorimetric evaluation of staining of the light cured composite after exposure with different intensities of light curing units


Department of conservative Dentistry and Endodontics, Mamata Dental College, Khammam, Andhra Pradesh, India

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Date of Submission15-Jul-2010
Date of Decision23-Aug-2010
Date of Acceptance05-Sep-2011
Date of Web Publication5-Nov-2011
 

   Abstract 

Aim/Objective : To understand the importance of intensity of light in polymerizing light cured composites and its relation to color stability.
Materials and Methods : Forty specimens of composite disc with 3mm diameter and 1.5 mm thick were divided into two groups of 20 samples each. Group1: Twenty samples were cured with a light curing unit of380mw/cm 2 . Group2: Twenty samples cured with a light curing unit of 680mw/cm 2 . These polymerized samples were immersed in methylene blue dye for 24hoursand later washed and immersed in absolute alcohol for 24 hours. The amount of color released into absolute alcohol was assessed by spectrophotometric and colorimetric analysis.
Results : Results were analyzed for spectrophotometric and colorimetric values by using the Mann-Whitney test. The group cured with low intensity light stained more compared to the group cured with a normal intensity of light.
Conclusions : Intensity of light plays a crucial role in staining of the polymerized light cured composite. The intensity of the curing unit has to be maintained in acceptable limits to achieve good clinical results.

Keywords: Irradiance; light intensity; spectrometry; staining of composite

How to cite this article:
Chandrasekhar V, Reddy L P, Prakash T J, Rao G A, Pradeep M. Spectrophotometric and colorimetric evaluation of staining of the light cured composite after exposure with different intensities of light curing units. J Conserv Dent 2011;14:391-4

How to cite this URL:
Chandrasekhar V, Reddy L P, Prakash T J, Rao G A, Pradeep M. Spectrophotometric and colorimetric evaluation of staining of the light cured composite after exposure with different intensities of light curing units. J Conserv Dent [serial online] 2011 [cited 2019 Apr 18];14:391-4. Available from: http://www.jcd.org.in/text.asp?2011/14/4/391/87208

   Introduction Top


Composite resin restorations have begun to constitute a significant portion of dentist's routine practice due to the patient's increasing demand for an esthetic appearance. Currently available dental composites provide satisfactory strength and high esthetic appearance. Proper color match of a composite restoration with the adjacent teeth is important not only at the initial stage of the restoration, but also over a longer period of time. However, despite the recent developments, color stability of light cured composites after long term intraoral exposure remains a concern. Various extrinsic and intrinsic factors [1] have been cited as a reason. Extrinsic factors include intensity and duration of polymerization, [2] exposure to environmental factors including ambient and Ultraviolet radiation, [3] heat [4] water or food colorants. [5] Intrinsic factors include composition of resin matrix, [6] type of photo initiator [7] and percentage of remaining double bonds (C=C). The latter is considered a fundamental parameter influencing a variety of mechanical, chemical and biological properties. [8]

A lot of research has been undertaken on various light curing units and methods to cure the composite (light intensity, depth of cure, area of cure, irradiance of the light curing gun);however, the lack of maintenance of the light curing unit and constant measurement of the Logarithmic total exposure [9] can play a deleterious effect on the esthetic light cured restorations.

The purpose of the study was to investigate the effect of intensity of the light curing unit and its relation to color stability of the polymerized composite material. Spectrophotometric and Calorimetric analysis were done to measure the dye absorption.


   Materials and Methods Top


Forty specimens of composite resin disc (Filtek Z250 IvoclarVivadent) with 3mm diameter and 1.5 mm thick were prepared by using spherical polyethylene moulds, and the samples were divided into two groups of 20 samples each. Two light curing units were taken from general dentists and their intensities were measured by using a radiometer. (IvoclarVivadent). The first group was cured with the light curing unit which had an intensity of 380mW/cm 2 and the second group was cured with the light curing unit with an intensity of 680mW/cm 2 . Mylar strips were used on the surfaces and polymerization was done according to manufacturer guidelines (20 samples for each group). After polymerization, the composite resin samples were removed from moulds, placed in an empty test tube and each specimen was placed in an incubator (37±2°C) for 24 hours. Later they were individually immersed in 1 ml of 2% methylene blue solution and placed at 37°C ± 2°C. After 24 hours, the specimens were rinsed under running distilled water for 1 minute and stored at 37°C ± 2°C at relative humidity for 24 hours. The specimens were immersed in new test tubes containing 1 ml of absolute alcohol for 24 hours. The solutions were filtered and centrifuged for 3 minutes at 4,000 revolutions per minute, and the supernatant was used to determine dye absorbance in a spectrophotometer and calorimeter at 590 nm.


   Results Top


Results were analyzed statistically for spectrophotometer and calorimetric values by using the Mann-Whitney test (Graph 1). Spectrometric analysis showed that the composite cured with a low intensity light had higher dye absorption (P value 0.009); and, the results for calorimetric analysis were statistically insignificant (P value 0.171). Sensitivity of spectrophotometer is greater than colorimeter in detecting wavelength of colour. Hence, the results obtained with spectrophotometer are more reliable than the colorimeter.


   Discussion Top


The irradiance of the light source, the exposure time and light transmission of composite are significant variables that affect the hardness or conversion profile. It is suggested that the depth of cure corresponds to the minimum amount of energy required to initiate polymerization. [9] For a resin composite, the properties depend among other things, such as, the degree of conversion of the monomer. A high degree of conversion gives hardness and strength to the material, [10] as well as color stability [11] . Thus, a reduction in remaining double bonds to the lowest possible level is normally considered a desirable feature of a polymerization system.



Staining susceptibility of resin composite might be attributed to their degree of water sorption and hydrophilicity of matrix resin, that is, if the resin composite can absorb water it can also absorb other fluids like Methylene blue dye, tea and coffee. [5] The glass filler particles will not absorb water into the bulk of the material;however, can adsorb water onto the surface. Extra water sorption may decrease the life of the resin composites by expanding and plasticizing the resin component, hydrolyzing the silane and leading to microcrack formation. Therefore, the micro-cracks or the interfacial gaps at the interface between filler and matrix allow stain penetration and discoloration. [5] As a consequence of the high water sorption and solubility of restorative resins, studies [12] have shown that composite restorations may have decreased mechanical properties and reduced longevity . The present study measures the dye penetration and indirectly reflects the amount of conversion of the double bonds as it is given that composites containing more than 35% of unconverted C=C bonds are susceptible to discoloration. [13]

In the present study methylene blue was used, as it is the most commonly used dye for microleakage studies and the size of this dye is 120Ε or 1.2nm 2 which is less than that of bacteriae. This dye dissolves as monomer and dimer in aqueous environment. [14] and is readily taken up by the composite discs by sorption phenomenon.

Two composite light curing (LC) units were used for this study; these LC units were taken from a general dentist who was using them on a regular basis. Justifying our use, various studies [1],[15],[16] have concluded that there is no difference in curing found between composite resins light-cured by Light Emitting Diode or halogen light-curing units.To prevent the bias regarding the surface finishing and polishing, we used Mylar strips on the surface of the disc so that a smooth surface is achieved. For analyzing the dye absorbance, the discs were placed in methylene blue and then in absolute alcohol by which there was a release of methylene blue absorbed by the composite discs into the alcohol. [16] Alcohol uptake of the dye may be due to the hydroxide groups that are suitable for cationic dye removal like methylene blue. [17]

Since instrument measurements eliminate the subjective interpretation of visual-color comparison, spectrophotometers and colorimeters [18] have been used to measure color changes in dental materials. The colorimeter and spectrophotometer works on the basis of Beer-Lambert's law, which states that the absorption of light transmitted through a medium is directly proportional to the concentration of the medium. Colorimeter is an instrument for psychophysical analysis, provides measurements that correlate with human eye-brain perception. It consists of sensor and simple data processor and has a set of illuminant and observer combination. It isolates the broad band of wavelengths using a tri stimulus absorption filter.

Spectrophotometer is an instrument for physical analysis, and provides wavelength-by-wavelength spectral analysis of the reflecting and/ or transmitting properties of objects without interpretation by human. It consists of a sensor plus data processor or computer with software. It has many available illuminant/observer combinations. It isolates a narrow band of wavelengths using prism, grating or interference filter. Dye penetration method was used since the uncured resin matrix is more susceptible to dye absorption and also release of dye when placed in absolute alcohol. The degree of conversion of a given resin composite is influenced by the energy density. [19] Insufficient energy density results in less than maximal conversion, [12] and the demonstrated differences in quantity of remaining double bonds are reflected in other polymerization characteristics.

Energy density (J/cm 2 ) is equal to the product of light intensity and the irradiation time [12],[20] . The peak wavelength among light curing units varies from about 450-490 nm, and the irradiance ranges from 400-800mW/cm΂. A 2mm thick resin composite restoration requires a radiant exposure of 8 J/cm 2 (400mW/cm 2 x20s=8000mWs/cm΂). [16] In the present study keeping the time of exposure constant, it was observed that the energy density decreased for the samples cured with low intensity.

Loupes et al., reported that longer curing time is required to generate sufficient energy density to cure 2mm increments of composite resins in lights with low output intensity. However,in another study [20] it was revealed that there is only a modest increase in the depth of cure (<25%), even upon doubling the exposure time.

Decrease in line voltage of 6% shows a corresponding reduction in output of about 25%. A remarkable difference in intensity (40-50mW/cm΂) was also observed when sterilization sleeves or jackets were worn on the tip of the light curing unit. [21] Vivek Hegde et al., [20] reported that only 10% LED machines and 2% quartz-tungsten-halogen machines used by practitioners were in good condition. Most of the curing lights examined had low output intensity ranging from 200-400 mW/cm΂. Most of the curing unit tips had resin composite remnants on them.Barghi N reported 30% of curing units had power densities < 199mW/cm΂ and were considered inadequate for curing the composite resin.

In coordination with studies, [9] the results suggest that there is a greater staining of the composite cured with low intensity light, which might be related to unpolymerized organic matrix. It must be considered that esthetic restorations especially on anterior teeth look pleasing initially; however, in due course of time in the oral environment, are deemed to stain resulting in failure of restoration, and lead to an early replacement of the restoration causing a negative psychological effect and financial loss to the patient. When discussing the clinical applications of these results, it must be considered that the oral environment differs in several ways from in vitro conditions. Factors such as the variety of food, thermal and mechanical stresses, saliva and their interactions may intensify discoloration in vivo. [22]

A few limitations exists in this study like the effect of saliva which was not taken into consideration, and also the effect of alcohol, which softens [23] and effect the surface integrity of composite resin while measuring the dye uptake was not considered.Before concluding, we would recommend further evaluation with a larger group of light curing units with low intensity.


   Conclusions Top


The role of different intensities of light on the color stability of the composite resins was determined. The specimens cured with low intensity light had a higher dye absorption, signifying the role of light intensity on the physical properties of composite resins like color stability.

 
   References Top

1.Yazici AR, Celik C, Dayangac B, Ozgunaltay G. The effect of curing Units and staining solutions on the color stability of resin composites. Oper Dent 2007;32:616-22.  Back to cited text no. 1
    
2.Hosoya Y. Five-year color changes of light-cured resin composites: Influence of light-curing times. Dent Mater 1999;15:268-74.  Back to cited text no. 2
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3.Kolbeck C, Rosentritt M, Reinhold L, Handel G. Discoloration of facing and restorative composites by UV irradiation and staining food. Dent Mater 2006;22:63-8.  Back to cited text no. 3
    
4.Ferracane JL, Moser JB, Greener EH. Ultraviolet light-induced yellowing of dental restorative resins. J Prosthet Dent 1985;54:483-7.  Back to cited text no. 4
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Correspondence Address:
L Pramod Reddy
Department of Conservative Dentistry and Endodontics, Mamata Dental College, Khammam, Andhra Pradesh
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0972-0707.87208

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