Evaluation of optical properties of different restorative composite resins

The esthetic demands of patients have encouraged the search for restorative materials increasingly similar to natural teeth. This requires knowledge on the optical properties of dental structures and restorative materials. This study comparatively evaluated the translucency/opacity and fl uorescence of different brands of composite resins. Sixteen brands of composite resins shade A2 were randomly selected; specimens with 1mm thickness and 10mm diameter were fabricated with these resins and evaluated as to the optical properties, scored as high, medium and low translucency/ opacity and fl uorescence. Statistical analysis by the Kendall W test and Friedman test at a signifi cance level of 5% revealed that the resins Opallis D, Four Season D, Concept E, Concept D, and FillMagic presented low translucency. Conversely, the resins Opallis E, Charisma, Tetric Ceram, Four Season E, Natural Look, Z350, and Z250 exhibited medium translucency/opacity, and resins Nova Charisma, Durafi ll, and Supreme XT presented high translucency. With regard to fl uorescence, the resins Charisma, Z350, Supreme XT, and Z250 present low fl uorescence. Medium fl uorescence was observed for the resins Opallis E, Opallis D, Nova Charisma, Durafi ll, Natural Look, TPH Spectrum, Tetric Ceram, Four Season E, and Four Season D. The composite resins Concept E, Concept D, and Fill Magic presented high fl uorescence. Based on these results, it is concluded that there are differences in the degree of translucency/opacity and fl uorescence of different composite resins, which are possibly related to variations in their composition.


Introduction
The esthetics of smile has been increasingly valued.According to Pinlkington (apud CHAIN; RODRIGUES; ANDRIANI, 2002), it has been defi ned as the science to copy or harmonize the professional work with nature, making it an imperceptible art (BARATIERI; MONTEIRO JÚNIOR; ANDRADA, 1995).
With the development of restorative and adhesive dentistry was possible the construction of esthetic restorations with characteristics more similar to natural teeth.For achievement of these perfect restorations, the dental professionals should have current information and skill to offer esthetic treatments, since this is one of the main reasons for search for treatment, which aims at promoting health and self-esteem, beyond to bring social and intellectual acceptance.To achieve the patient expectations is essential have the knowledge of the shape, texture and color of restorations, beyond the understanding the optical properties of dental tissues and inherent properties of esthetic restorative materials.(CHAIN; RODRIGUES; ANDRIANI, 2002;JARDIM et al., 2002;TERRY, 2002;FRANCO, 2007).
The optical properties of an esthetic material include translucency, opacity, fl uorescence and opalescence.Translucency may be defi ned as the property of a substance that permits the passage of light but disperses the light so that objects cannot be seen through the material (POWERS, 2006).Opacity is the property of materials that does not allow the passage of light; i.e. opaque structures have higher intensity of light dispersion (CHAIN; RODRIGUES; ANDRIANI, 2002).Fluorescence is a type of photoluminescence, in which the ultraviolet radiant energy (UV) is absorbed by an object that later emits light energy within the visible spectrum.The natural fl uorescence of dental tissues is an important component that should be reproduced in composite resin restorations; it assigns vitality and luminosity to restorations (CORREIA; OLIVEIRA; SILVA, 2005).Opalescence is assigned by opalizing agents, which are fi ne or extrafi ne particles in charge of light dispersion within the tooth structure.This dispersion varies according to the size and quantity of particles, which consequently infl uences the material refraction index; opalescence is an inherent optical phenomenon of enamel (CHAIN;RODRIGUES;ANDRIANI, 2002;VANINI;MANGANI, 2001).Since enamel contains hydroxyapatite crystals that are smaller than the visible light wavelength, shorter light wavelengths penetrating it suffer dispersion; this assigns the blue aspect of incisal areas without dentine superimposition.Conversely, dispersion hardly occurs in longer light wavelengths, which reach the palatal interface of the tooth and assign the orange aspect to the tooth.Therefore, the enamel acts as a fi lter that absorbs the blue colors and allows the passage of orange colors (WATANABE, 2005).
The concepts for direct restorative esthetic materials are relatively recent.For this reason, there are still doubts during selection and purchase of the ideal material for fabrication of imperceptible esthetic restorations, able to satisfy both the professional and the patient.Therefore, this study aimed to investigate the optical properties of restorative composite resins, for evaluation of translucency, opacity and fl uorescence of different brands of composite resins.

Materials and Methods
The present study was divided into two stages.On the fi rst stage, sixteen brands of composite resins were selected, shade A2, which were commercially available in Santa Maria, Rio Grande do Sul-Brazil, which constituted the study sample (Table 1).On the second stage, tests were performed to evaluate the optical properties of translucency/opacity and fl uorescence, by application of the following methods.

Fabrication of specimens
For fabrication of specimens employed in the present study, a disc-shaped acrylic matrix with 15mm diameter, 1mm thickness and 10mm diameter round central perforation was fabricated, which was positioned on a glass plate for insertion of composite resin into the central perforation in a single increment, with aid of a Tefl on spatula.A polyester strip and another glass plate were placed on the resin and approximately 2kg of fi nger pressure was applied, which allowed better fi t and overfl ow of excess material.The resin was lightcured by three 30-second periods; the fi rst was performed with the glass plate on the specimen, the second was performed soon after removal of the glass plate, and the third was applied on the opposite side of the specimen.This was performed by utilization of a halogen light curing unit (Dabi Atlante) with energy of 400 mW/cm 2 , previously tested by a radiometer.

Storage of specimens
After separation of matrices and specimens, these were identifi ed so as to prevent identifi cation of materials with their brands by the examiners.The specimens were then immersed in labeled individual fl asks containing distilled water and placed in an oven at 37°C for seven days.After this period, tests were performed to evaluate the translucency and fl uorescence.
Evaluation of translucency/opacity was performed by visual comparative analysis, which comprises placement of specimens with 50% of their area on black strips and 50% on white background.For visual analysis of translucency, four independent calibrated examiners employed the following criterion: numerical classifi cation of specimens, from the specimen allowing lower visibility of the black strip to that allowing greater visibility, i.e. from the most opaque to the most translucent, classifying them into high, medium and low translucency.
Analysis of fl uorescence was performed on an image obtained with a digital photo camera in a completely dark environment, in which the specimens were exposed to artifi cial ultraviolent light from a compact fl uorescent lamp (cool-white color 4200K/9W) and had their fl uorescence emitted and recorded.Evaluation of images was performed by the same examiners, following the same methodology, i.e. they classifi ed the specimens into high, medium and low fl uorescence, according to the luminosity emitted by the material, compared to a natural anterior tooth.
Data were organized and statistically analyzed.

Results
For the data obtained, score 0 (zero) was assigned when fl uorescence and translucency were scored as high, score 1 (one) when scored as medium, and 2 (two) when scored as low.Agreement of data was verifi ed by the Kendall W test.
The mean scores obtained by specimens were compared by the non-parametric Friedman test; a multiple comparison test was applied when the Friedman test revealed signifi cant differences.All tests were applied at a signifi cance level of 5%.
The Kendall W test revealed values of 0.872 for fl uorescence and 0.925 for translucency, which revealed high agreement in scores assigned by the examiners.
The Friedman test revealed signifi cant differences between the mean scores of specimens, for both fl uorescence and translucency.
On evaluation of translucency, RC2, RC7, RC11, RC12, and RC13 presented the lowest translucency and RC4, RC8, and RC15 exhibited the highest; the others had intermediate positions, as presented in Table 2 and Graph 1.

Graph 1. Graphic representation of mean translucency of composite resins evaluated
Concerning fl uorescence, RC3, RC14, RC15, and RC16 exhibited the lowest fl uorescence and presented statistically signifi cant difference compared to RC11, RC12, and RC13, which presented the highest fl uorescence.This is evidenced in Table 3 and Graph

Graph 2. Graphic representation of mean fl uorescence of composite resins evaluated
The non-parametric coeffi cient of correlation of Spearman was calculated to verify if there was correlation between translucency and fl uorescence properties, since data were measured on an ordinal scale.The calculated value was signifi cant at the 1% level (rs = 0.400).The coeffi cient obtained indicated association between the two variables, i.e. the higher the value of one of them, the lower would be the other.Most RCs with high fl uorescence tended to present low translucency.

Discussion
Due to patients' demands, the search for esthetic restorations has increased in the last years.Consequently, a new era has been observed in Restorative Dentistry, due to the development of new esthetic materials and increased number of informed patients (HIRATA; AMPESSAN;LIU, 2001;VILLARROEL;HIRATA;SOUSA, 2005).Currently, professionals should not only recover the lost function of the tooth, but also achieve a beautiful outcome, according to the demands of society.This requires correct planning, accomplishment of restorations and precise utilization of the restorative material (BUSATO; HERNANDEZ; MACEDO, 2002).
The great challenge in Restorative Dentistry is to perform restorations similar to the tooth structure, since natural teeth are polychromatic structures, whose crown is performed by superimposition of enamel and dentine, two translucent tissues that play different and complementary roles in the expression of tooth shade (TERRY, 2002).The dentine presents low translucency and high saturation and is the main responsible for the basic hue and saturation of natural teeth.Enamel is signifi cantly more translucent than dentine, attenuating its saturation and acting as an optic fi ber able to transmit light to the underlying dentine (BEHLE, 2001a;2001b;MELO;KANO;ARAÚJO JUNIOR, 2005a;2005b).
Knowledge on the factors that infl uence the optical properties of both tooth structures and esthetic restorative materials is fundamental for achievement of esthetic excellence of direct composite resin restorations.Due to this high demand, the translucency and fl uorescence were investigated.
Translucency is the transmission and diffusion of light through an object, i.e. it is the medium point between complete opa city and complete transparency.It allows similar aspect to both restorative material and natural teeth, and thus the achievement of excellent esthetic outcome (CORREIA; OLIVEIRA; SILVA, 2005; VILLARROEL; HIRATA; SOUSA, 2005;POWERS, 2006).
According to Magne and Belser (2002), the phenomenon of fl uorescence may be created by UV rays of sunlight, which excite the photosensitivity of dentine after penetrating the enamel and reaching the dentine.This phenomenon is caused by dentine, which presents more intense fl uorescence than enamel due to the presence of higher amount of organic pigments that are photosensitive to the UV spectrum (VILLARROEL et al., 2004).
Fluorescent materials and structures are able to absorb light from a wavelength and emit a different light wavelength in response.In natural teeth, fl uorescence is characterized by the absorption of ultraviolent light, followed by emission of visible light at short wavelength.Since the emitted light is combined with refl ected light, there is perceptible increase in the apparent luminosity of the structure (MELO; KANO; ARAÚJO JUNIOR, 2005a;2005b).
Thus, based on results obtained by statistical analysis, it was possible to evaluate the translucency/opacity and fl uorescence of different direct restorative composite resins.
These different translucencies in different resins may be assigned to variations in material composition.Lee (2007) concluded that the size of fi llers and volume fractions should be controlled for the best color reproduction for dental resin composite, since the color of resin composites is highly correlated with scattering and absorption characteristics, which determine the translucency property.
According to Sturdevant et al. (apud VILLARROEL, HIRATA;SOUSA, 2005) the fi llers would be responsible to produce light dispersion in its interior, producing the same phenomenon observed in enamel.Even when these particles present inherent transparency, opacity may be produced by dispersed light, which reaches maximum values when the size of particles have the same dimension as the visible light wavelength.The degree of translucency is infl uenced by the material composition; thus, the translucency of microparticles might be related to the size of fi llers, which allow greater passage of light (JARDIM, 2002;VILLARROEL;HIRATA;SOUSA, 2005).
The basic components of restorative materials do not present fl uorescence; however, this quality is reached by aggregation of higher or smaller amount of fl uorescent components, like rare earth oxides which may assign high, medium or low fl uorescence to the resin (VILLARROEL et al., 2004;UO et al., 2005).
The present results revealed a relationship between the composition of composite resins and their optical properties, as well as correlation between the properties of translucency and fl uorescence.However, it is diffi cult to precisely establish this relationship, since both the scientifi c literature and technical information of materials do not present details on the composition; the manufacturers claim that such information constitutes industrial secret.Thus, the best parameters available so far are comparative studies between different materials and those comparison natural teeth and restorative materials.

Conclusion
Considering the present results, it was concluded that the composite resins evaluated presented different degrees of translucency/opacity and fl uorescence.Possibly, these differences are related to variations in their composition, which may not be clearly demonstrated in the technical information provided to professionals by the manufacturers.The information presented in this study infl uence the clinical practice, since restorations with esthetic involvement depend on correct selection of the restorative material, which demands knowledge on the optical properties of composite resins. 2.

Table 2 . Mean translucency results by application of the Friedman test, at a signifi cance level of 5%
*Same letters indicate mean values that are not signifi cantly different.

Table 3 . Mean fl uorescence results by application of the Friedman test, at a signifi cance level of 5%
*Same letters indicate mean values that are not signifi cantly different.