- Email: [email protected]
Light transmission on dental resin composites
d e n t a l m a t e r i a l s 2 4 ( 2 0 0 8 ) 571–576
available at www.sciencedirect.com
journal homepage: www.intl.elsevierhealth.com/journals/dema
Light transmission on dental resin composites G.B. dos Santos a,b , R.V. Monte Alto a,b , H.R. Sampaio Filho a , E.M. da Silva b , C.E. Fellows c,∗ a
Faculdade de Odontologia, Universidade do Estado do Rio de Janeiro, Boulevard 28 de Setembro, 157, Vila Isabel, Rio de Janeiro 20.0000, Brazil b Departamento de Odontologia Restauradora, Faculdade de Odontologia, Universidade Federal Fluminense, ˜ Paulo, 30, Centro, Niteroi ´ 24110-040, Brazil R. Sao c Laboratorio ´ de Espectroscopia e Laser, Instituto de F´ısica, Universidade Federal Fluminense, Av. Gen. Milton Tavares de Souza, ´ 24210-340, RJ, Brazil s/n, Campus da Praia Vermelha, Boa Viagem, Niteroi
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objectives. The purposes of this study was: (1) to examine the light transmittance character-
Received 20 November 2006
istics of two light-cured resin composites, for different thickness, (2) to correlate the light
Accepted 6 June 2007
transmittance through the resin composites and the filler contents, and (3) to determine the penetration depth of the light as a function of the wavelength. Methods. Two resin composites (Filtek Z250, shade A2 and Filtek Supreme XT, shade A2E)
Keywords:
were used. Specimens of six different thicknesses (0.15, 0.25, 0.30, 0.36, 0.47 and 0.75 mm)
Resin composite
were prepared (n = 3). The transmittance at wavelengths from 400 to 800 nm was measured
Penetration depth
using a UV–visible spectrophotometer, before and after light polymerization.
Curing efficiency
Results and significance. Significant differences were found in the wavelength dependence
Depth of cure and light transmission
of transmittance between the two materials, and between the unpolymerized and polymerized stages of each resin composite. At lower wavelengths, the light transmittance of the Filtek Supreme XT resin composite was lower than the Filtek Z250. At the higher wavelengths, however, Filtek Supreme XT presented higher light transmittance. For both resin composites, the penetration depth was higher after polymerization. However, Filtek Supreme XT showed a higher gain in transmittance at the 0.15 mm thickness. The difference in light transmittance characteristics of the resin composites may affect their depth of polymerization. © 2007 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Since resin composite was introduced in the clinical practice [1], many efforts have been made to improve the clinical behavior of this esthetic restorative material. One of the most important steps in this field was the introduction of visible light-cured resin composites [2]. The first light source used to start the polymerization reaction of dental resin composites was ultraviolet. However, this light source, produced by
∗
plasma arcs, has shown a lower light penetration, limiting the depth of cure. In the last few years, quartz–tungsten–halogen lamps have been used, showing more satisfactory results [3,4]. Today, as a function of its esthetic features and physical properties, this restorative dental material is widely used in restorative dentistry. Dental resin composite’s mechanical properties are directly influenced by the degree of conversion [5]. From this point of view, after light curing, activation is desirable for this restorative material, in order to attain the
Corresponding author. Tel.: +55 21 2629 5800; fax: +55 21 2629 5887. E-mail address: [email protected] (C.E. Fellows). 0109-5641/$ – see front matter © 2007 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.dental.2007.06.015
572
d e n t a l m a t e r i a l s 2 4 ( 2 0 0 8 ) 571–576
highest mechanical properties, to convert all of its monomer into polymer. However, it has not been observed up to now and the cure rate is not higher than 61% on the surface directly illuminated by the light source [6], and always presents a reduction of this cure rate as a function of the depth [7]. During the photoactivation process, the light that passes through the resin composite is absorbed and scattered. Thus, the light intensity is attenuated and its effectiveness is reduced as the depth increases [8,9]. Although the depth of cure depends on the light irradiance [10], exposure time [11,12] and other several factors, such as material composition [13], resin composite shades [14] and translucency [15,16], the most important limiting factor for depth of cure is light scattering and this is maximized when the filler particle size is close to half of the wavelength emitted by the light source [17]. Others optical phenomena are important to understand light transmission through a resin composite layer. A considerable part of the irradiation light that illuminates the resin composite surface is reflected. The other part that penetrates has the function of exciting the photosensitizer to start the polymerization process as deeply as possible. But the extent of this light’s depth depends on the absorption coefficients of the resin composite’s component parts [18]. Therefore, the study of the depth of penetration of light in resin composites is of fundamental importance, the depth of penetration being the inverse of the absorption coefficient [19]. If the Naperian absorption coefficient, 1/˛, is used, the depth of penetration (˛) is the distance at which the radiant power, P , decreases to 1/e of its incident value, P0 , being e, the Naperian number. The aim of the present study was to investigate the light transmittance of a hybrid and a nanofilled resin composite. The analysis was conducted in wavelength spectra from 400 to 800 nm, before and after resin composite light polymerization. The research hypothesis was that the size of filler particles would influence the light transmittance in the resin composites.
2.
Material and methods
The resin composites used in this work were Filtek Z250, Shade A2, and Filtek Filtek Supreme XT, Shade A2E (3M-ESPE, St. Paul, Minnesota, USA). These resin composites had the same polymeric matrix differing only in filler content and filler size (Table 1). The light source used for resin composite photoactivation was an Optilux 501/Demetron-Kerr
(Demetron Research, Danbury, CT), used in normal mode with 850 mW/cm2 irradiance, for 40 s for each sample. Stainless steel matrixes (8 mm in internal diameter and heights of 0.15, 0.25, 0.30, 0.36, 0.47 and 0.75 mm) were placed over a microscope glass slide and slightly overfilled with the resin composites (n = 3). After that, the resin composite surfaces were covered with another microscope glass slide. The light transmission through the resin composites was measured, for each thickness, using a UV–visible HP 8452A Diode Array Spectrophotometer (Palo Alto, California) in a selected light spectrum varying from 400 to 800 nm wavelength. This spectrophotometer was chosen due to the fact that light exposure of the sample is only for a short period of time, preventing the monomer conversion of the resin composite during the measurements. Light transmission measurement was first performed in an unfilled matrix positioned between two microscope slides, in order to obtain a calibration parameter for the measurements (blank measurement). The blank measurement was used to correct the intensities in the transmission spectra. Then, after the blank measurement, light transmission analysis was performed in the polymerized and unpolymerized resin composites. With the transmittance values for each resin composite, for each thickness and for each wavelength, two types of analysis were performed. The first analysis was, considering only the thinnest sample t (0.15 mm) and some previously chosen wavelengths (400, 450, 468, 500, 600, 700 and 800 nm), the rate of percentage transmittance r between polymerized and unpolymerized resin composites was calculated, for each wavelength. The 468 nm wavelength was chosen considering the absorption spectra of the camphoroquinone, used as photosensitizer in both resin composites. Then, this rate r can be considered, for a fixed thickness t, as the gain in light transmission for a particular resin composite after polymerization, being in some way a measurement of its translucence after polymerization. The second analysis was to determine the light penetration depth for each resin composite. For this the following procedure was performed: percentage transmittance values for wavelengths varying from 400 up to 800 nm, with 50 nm interval, for each sample, were picked out and plotted as a function of the thickness. These values were then fitted to a first order exponential decay of the form
P = P0 e−(t/˛)
(1)
Table 1 – Restorative materials used in this studya Composites
Lot no.
Composition
Filtek Z250
6PR
Filler: zirconia/silica with size of 0.6 m. The filler loading is 60% by volume Organic matrix: bis-GMA, bis-EMA, TEGDMA and UDMA
Filtek supreme XT
6BW
Filler: non-agglomerated nanosilica of 20 nm size filler and agglomerated zirconia/s´ılica nanocluster with size of 5–20 nm. The filler loading is 78.5% by weight Organic matrix: bis-GMA, bis-EMA, TEGDMA and UDMA
a
According data furnished by the manufacturer (3M, St. Paul, Minnesota, USA).
d e n t a l m a t e r i a l s 2 4 ( 2 0 0 8 ) 571–576
Fig. 1 – Transmittance spectrum of Filtek Z250 resin composite, as-explained in the text, for unpolymerized and polymerized samples.
573
Fig. 3 – Percentage transmittance rate r, as-explained in the text, for a fixed thickness t = 0.15 mm for both resin composites.
where ˛ is the depth of penetration of light, P the radiant power at a distance t for the wavelength , and P0 is the incident radiant power for the wavelength and e the Naperian number. The ˛ values, obtained from the fitting procedure, were then plotted against the wave number and the behavior of the depth of penetration of light, for each resin composite, could be analyzed.
3.
Results
The spectra obtained can be seen in Figs. 1 and 2. For Filtek Z250 and Filtek Supreme XT, respectively, the percentage of transmitted light at each sample thickness can be observed as a function of the incident wavelength. In Fig. 3, the rate of percentage transmittance r between polymerized and unpolymerized resin composites, for a fixed thickness t = 0.15 mm, is Fig. 4 – First order exponential plot for Filtek Supreme XT resin composite for = 500 nm at different thickness, considering unpolymerized and polymerized samples.
Fig. 2 – Transmittance spectrum of Filtek Supreme XT resin composite, as-explained in the text. It should be noted that the scale for the polymerized samples is twice the scale for the unpolymerized ones.
plotted as a function of the selected wavelengths. From this plot it can be seen that for the Filtek Z250 resin composite there is no significant change in the rate r as a function of the wavelength, and the rate for this resin composite remains quite constant at around 1.5. Otherwise, for the Filtek Supreme XT resin composite the change in r is remarkable, varying from 5.5 to 2.2 with increasing wavelength. In order to better understand how the penetration depth of the light is calculated, Fig. 4 shows an example for one resin composite (Filtek Supreme XT) at a fixed wavelength ( = 500 nm). For the six different sample thicknesses the transmittances were plotted and the experimentally determined points fitted to a first order exponential decay, as discussed in Section 2 and described by Eq. (1). From the fit, a ˛ value corresponding to the penetration depth for the corresponding wavelength and resin composite was then obtained. The
574
d e n t a l m a t e r i a l s 2 4 ( 2 0 0 8 ) 571–576
ite where irradiance of 221 mW/cm2 is reached at 0.09 mm, in the unpolymerized condition, changing to 0.14 mm when polymerized.
4.
Fig. 5 – Penetration depth for Filtek Z250 resin composite unpolymerized and polymerized, as a function of the incident wavelength.
results of this procedure are shown in Figs. 5 and 6, for Filtek Z250 resin composite and Filtek Supreme XT resin composite, respectively. For both resin composites it can be seen that the penetration depth increases with increasing wavelength. However, for Filtek Z250 resin composite, below 500 nm the differences in penetration depth between polymerized and unpolymerized samples are more accentuated. Otherwise, for Filtek Supreme XT resin composite, the difference in penetration depth between polymerized and unpolymerized samples grows up from 400 to 800 nm. The concept of penetration depth can be understood by a simple example: for the Filtek Z250 resin composite, irradiance from incident light of 600 mW/cm2 (for = 500 nm) reaching a depth of 0.15 mm, falls to 1/3 the initial value, i.e. 221 mW/cm2 , when unpolymerized. Once polymerized, the same irradiance will be found at a depth of 0.16 mm. This behavior is more accentuated for the Filtek Supreme XT resin compos-
Fig. 6 – Penetration depth for Filtek Supreme XT resin composite unpolymerized and polymerized, as a function of the incident wavelength.
Discussion
During the last few decades, resin composites were more commonly classified according to their filler particle size as hybrid (8–30 m), microhybrid (0.6–3.6 m), and microfilled (around 0.04 m) [20]. In the last few years, however, with the introduction of nanotechnology in dentistry [21], a new class of resin composite, so called nanocomposite, is available to practitioners. In this field, some in vitro studies have shown a good performance for nanofilled resin composites [8,9,22,23]. Few studies, however, have focused on the light transmission of this new class of restorative material [24]. Some previous studies have shown that factors such as filler and polymeric matrix refractive index, monomer type, filler type and filler content can influence the light transmittance of resin composites [24–26]. Since in the present study both resin composites have the same polymeric matrix (BisGMA, BisEMA, UDMA and TEGDMA), and the same filler particle types, the discussion of the results was based only on the influence of filler particles size. From Figs. 1 and 2, it can be seen that polymerized specimens showed a higher light transmittance than unpolymerized ones. This behavior can be explained by two points of view. First, after polymerization, the resin composites underwent a vitrification process that leads to a glass stage. In this stage, the air trapped in micro-voids frozen into the polymer network would allow an optical transmission gain [27]. Secondly, the ordination of the polymer network, after polymerization, would allow the light to pass more easily through the bulk specimens. This effect, known as photobleaching is linked to the polymerization processes, called frontal photopolymerization (FPP) and has already been studied by Cabral et al. [28] and Warren et al. [29]. This behavior has already been observed, for Z100 resin composite [30], a material that has the same filler content as Filtek Z250, a resin composite used in the present study. In that study, the authors showed that the light absorption coefficient (˛ ) decreases upon curing. In addition, these authors also showed that, at wavelengths between 400 and 700 nm, the light absorption coefficient of the Z100 resin composite decreases as the wavelength increases. This can explain the increase in light transmittance from 400 to 800, irrespective of resin composite thickness, observed in the present research (Figs. 1 and 2). The concept that filler particles with mean size close to half of the wavelength produces a high light scattering is well understood [31]. Massoti et al. [24] have shown that light transmittance at wavelengths of 400 and 560 nm was higher for Filtek Supreme XT resin composite dentin shade, which has a particle size of 5–20 nm, than for the translucent shade, which has particles of 75 nm. According to these authors, at shorter wavelengths the microfilled resin composites cause greater light scattering, while macrofilled resin composite causes this effect at longer wavelengths. In addition, according to the Rayleigh scattering Eq. (2), it is clear that the mean size of filler particles has a crucial effect
d e n t a l m a t e r i a l s 2 4 ( 2 0 0 8 ) 571–576
on light transmittance in resin composites
T = e − 2303d
3Vp r3 (np /nm − 1) 44
references
(2)
where d is the thickness of the sample, Vp the volume fraction of the particles, r the particle radius, np the refractive indices of the particles, nm the refractive indices of the polymeric matrix and is the light wavelength. This kind of scattering may influence the depth of cure of the resin composite [32]. From the analysis of Figs. 5 and 6, it can be verified that in wavelengths higher than 600 nm, Filtek Supreme XT resin composite presented greater light penetration depth than Filtek Z250. This finding can be based on the fact that the filler size of Filtek Supreme XT (20 nm) is far from half of the 600 nm wavelength, so light scattering was reduced in the range between 600 and 800 nm. This behavior is sustained by the aforementioned discussion [24]. When comparing the light penetration depth in both unpolymerized and polymerized samples, interesting features can be noted. The first one concerns the Filtek Z250 resin composite. In Fig. 5, it can be observed that the penetration depth, for wavelengths higher than 500 nm, has almost the same value under the experimental error for both states of the resin composite. However, for wavelengths lower than 500 nm, the variation in the penetration depth from unpolymerized to polymerized state is considerable, changing from around 0.17 to 0.24 mm at 400 nm. This variation can be attributed to the camphorquinone absorption peak (e.g. at about 467.5 nm). In fact, Chen et al. [30] showed that the light absorption coefficient decreases upon curing, especially at wavelengths between 440 and 500 nm (1.06 cm−1 for unpolymerized and 0.68 cm−1 for polymerized samples). On the other hand, Fig. 6 shows that, except in the range between 400 and 450 nm, Filtek Supreme XT’s polymerized samples present an increase in the penetration depth of the light as a function of crescent wavelength. The fact that the behavior related to the absorption peak of camphorquinone found for Filtek Z250 was not the same for Filtek Supreme XT can be explained by the masking produced by the higher light scattering between 400 and 500 nm [33].
5.
575
Conclusion
The results obtained in the present study lead the authors to accept the research hypothesis. The resin composites analyzed presented an improvement in the light transmission after polymerization. However, the nanocomposite showed a higher gain in transmittance for a fixed thickness of the sample than the hybrid. This finding was related to the filler particle size of the nanocomposite. With regard to depth penetration, the hybrid resin composite showed a deeper light penetration in the lower wavelength region, from 400 to 500 nm, that close to the absorption peak of camforquinone, the photosensitizer used in the resin composites analyzed in this study.
[1] Bowen RL. Properties of a silica-reinforced polymer for dental restoration. J Am Dent Assoc 1963;66:57–64. [2] Dart EC, Cantwell JB, Traynor JR, Jaworzyn JF, Nemeck J. Method of repairing teeth using a composition which is curable by irradiation with visible light. US patent 4089763; 1976. [3] Swartz ML, Phillips RW, Rhodes B. Visible light-activated resins—depth of cure. J Am Dent Assoc 1983;106:634–7. [4] Tirtha R, Fan PL, Dennison JB, Powers JM. In vitro depth of cure of photoactived composites. J Dent Res 1982;61:1184– 7. [5] Ferracane JL. Correlation between hardness and degree of conversion during the setting reaction of unfilled dental restorative resins. Dent Mater 1985;1:11–4. [6] Silikas N, Eliades G, Watts DC. Light intensity effects on resin-composite degree of conversion and shrinkage strain. Dent Mater 2000;16:292–6. [7] Pianelli C, Devaux J, Bebelman S, Leloup G. The micro-Raman spectroscopy, a useful tool to determine the degree of conversion of light-actived composite resins. J Biomed Mater Res 1999;48:675–81. [8] Vargas MA, Cobb DS, Schmit JL. Polymerization of composite resins: argon laser vs conventional light. Operative Dent 1998;23:87–93. [9] Watts DC, Amer O, Combe EC. Characteristics of visible-light-activated composite systems. Br Dent J 1984;156:209–15. [10] Rueggeberg FA, Erge JW, Mettenburg DJ. Polymerization depth of contemporary light-curing units using microhardness. J Esthet Dent 2000;12:340–9. [11] Halvorson RH, Erickson RL, Davidson CL. Energy dependent polymerization of resin-based composite. Dent Mater 2002;18:463–9. [12] Koran P, Kurschner R. Effect of sequencial versus continuous irradiation of light-cured resin composite on shrinkage, viscosity, adhesion and degree of polymerization. Am J Dent 1998;10:17–22. [13] Atmadja G, Bryant RW. Some factors influencing the depth of cure of visible light-activated composite resins. Aust Dent J 1990;35:213–8. [14] Tanoue N, Koishi Y, Matsumura H, Atsuta M. Curing depth of different shades of a photo-actived prosthetic composite material. J Oral Rehabil 2001;28:618–23. [15] Ferracane JL, Aday P, Matsumoto H, Marker VA. Relationship between shade and depth of cure for light-activated dental composites resins. Dent Mater 1986;2:80–4. [16] Leloup G, Holvoet PE, Bebelman S, Devaux J. Raman scattering determination of depth of cure of light activated composites: influence of different clinically relevant parameters. J Oral Rehabil 2002;29:510–5. [17] Ruyter IE, Oysaed H. Conversion in different dephs of ultraviolet and visible light activated composite materials. Acta Odontol Scand 1982;40:179–92. [18] Davidson CL, Gee AJde. Light-curing units, polymerization, and clinical implications. J Adhes Dent 2000;2:167–73. [19] McNaught AD, Wilkinson A. IUPAC compendium of chemical terminology. In: International union of pure and applied chemistry. 2nd ed. Williston: Blackwell Science; 1997. [20] Willems G, Lambrechts P, Braem M, Cellis JP, Vanherle G. A classification of dental composites according to their morphological and mechanical characteristics. Dent Mater 1992;8:310–9. [21] Mitra SB, Wu D, Holmes BN. An application of nanotechnology in advanced dental materials. J Am Dent Assoc 2003;134:1382–90.
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[22] Turssi CP, Ferracane JL, Serra MC. Abrasive wear of resin composites as related to finishing and polishing procedures. Dent Mater 2005;21:641–8. [23] Beun S, Glorieux T, Devaux J, Vreven J, Leloup G. Characterization of nanofilled compared to universal and microfilled composites. Dent Mater 2007;23:51–9. ´ ˜ EN, Spohr AM. UV–vis [24] Masotti AS, Onofrio AB, Conceic¸ao spectrophotometric direct transmittance analysis of composite resins. Dent Mater 2007;23:724–30. ¨ ¨ [25] Emami N, Sjodahl M, Soderholm K-JM. How filler properties, filler fraction, sample thickness and light source affect light attenuation in particulate filled resin composites. Dent Mater 2005;21:721–30. [26] Campbell PM, Johnst WM, O”Brien WJ. Light scattering and gloss of an experimental quartz-filled composite. J Dent Res 1986;65:892–4. [27] Vrentas JS, Duda JL. A free volume interpretation of the influence of the glass transition on diffusion in amorphous polymers. J Appl Polym Sci 1978;22:2325–39.
[28] Cabral JT, Hudson SD, Harrison C, Douglas JF. Frontal photopolymerization for microfluidic applications. Langmuir 2004;20:10020–9. [29] Warren JA, Cabral JT, Douglas JF. Solution of a field theory model of frontal photopolymerization. Phys Rev E 2005;72:021801. [30] Chen YC, Ferracane JL, Prahl SA. A pilot study of a simple photon migration model for predicting depth of cure in dental composite. Dent Mater 2005;21:1075–86. [31] Ruyter IE, Oysaed H, Yearn JA. Factors affecting cure of visible light activated composites. Int Dent J 1985;35:218–25. [32] Santos GB, Medeiros IS, Fellows CE, Muench A, Braga RR. Composite depth of cure obtained with QTH and LED units assessed by microhardness and micro-Raman spectroscopy. Oper Dent 2007;32:79–83. [33] Watts DC, Cash AJ. Analysis of optical transmission by 400–500 nm visible light into aesthetic dental biomaterials. J Dent 1994;22:112–7.
available at www.sciencedirect.com
journal homepage: www.intl.elsevierhealth.com/journals/dema
Light transmission on dental resin composites G.B. dos Santos a,b , R.V. Monte Alto a,b , H.R. Sampaio Filho a , E.M. da Silva b , C.E. Fellows c,∗ a
Faculdade de Odontologia, Universidade do Estado do Rio de Janeiro, Boulevard 28 de Setembro, 157, Vila Isabel, Rio de Janeiro 20.0000, Brazil b Departamento de Odontologia Restauradora, Faculdade de Odontologia, Universidade Federal Fluminense, ˜ Paulo, 30, Centro, Niteroi ´ 24110-040, Brazil R. Sao c Laboratorio ´ de Espectroscopia e Laser, Instituto de F´ısica, Universidade Federal Fluminense, Av. Gen. Milton Tavares de Souza, ´ 24210-340, RJ, Brazil s/n, Campus da Praia Vermelha, Boa Viagem, Niteroi
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objectives. The purposes of this study was: (1) to examine the light transmittance character-
Received 20 November 2006
istics of two light-cured resin composites, for different thickness, (2) to correlate the light
Accepted 6 June 2007
transmittance through the resin composites and the filler contents, and (3) to determine the penetration depth of the light as a function of the wavelength. Methods. Two resin composites (Filtek Z250, shade A2 and Filtek Supreme XT, shade A2E)
Keywords:
were used. Specimens of six different thicknesses (0.15, 0.25, 0.30, 0.36, 0.47 and 0.75 mm)
Resin composite
were prepared (n = 3). The transmittance at wavelengths from 400 to 800 nm was measured
Penetration depth
using a UV–visible spectrophotometer, before and after light polymerization.
Curing efficiency
Results and significance. Significant differences were found in the wavelength dependence
Depth of cure and light transmission
of transmittance between the two materials, and between the unpolymerized and polymerized stages of each resin composite. At lower wavelengths, the light transmittance of the Filtek Supreme XT resin composite was lower than the Filtek Z250. At the higher wavelengths, however, Filtek Supreme XT presented higher light transmittance. For both resin composites, the penetration depth was higher after polymerization. However, Filtek Supreme XT showed a higher gain in transmittance at the 0.15 mm thickness. The difference in light transmittance characteristics of the resin composites may affect their depth of polymerization. © 2007 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Since resin composite was introduced in the clinical practice [1], many efforts have been made to improve the clinical behavior of this esthetic restorative material. One of the most important steps in this field was the introduction of visible light-cured resin composites [2]. The first light source used to start the polymerization reaction of dental resin composites was ultraviolet. However, this light source, produced by
∗
plasma arcs, has shown a lower light penetration, limiting the depth of cure. In the last few years, quartz–tungsten–halogen lamps have been used, showing more satisfactory results [3,4]. Today, as a function of its esthetic features and physical properties, this restorative dental material is widely used in restorative dentistry. Dental resin composite’s mechanical properties are directly influenced by the degree of conversion [5]. From this point of view, after light curing, activation is desirable for this restorative material, in order to attain the
Corresponding author. Tel.: +55 21 2629 5800; fax: +55 21 2629 5887. E-mail address: [email protected] (C.E. Fellows). 0109-5641/$ – see front matter © 2007 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.dental.2007.06.015
572
d e n t a l m a t e r i a l s 2 4 ( 2 0 0 8 ) 571–576
highest mechanical properties, to convert all of its monomer into polymer. However, it has not been observed up to now and the cure rate is not higher than 61% on the surface directly illuminated by the light source [6], and always presents a reduction of this cure rate as a function of the depth [7]. During the photoactivation process, the light that passes through the resin composite is absorbed and scattered. Thus, the light intensity is attenuated and its effectiveness is reduced as the depth increases [8,9]. Although the depth of cure depends on the light irradiance [10], exposure time [11,12] and other several factors, such as material composition [13], resin composite shades [14] and translucency [15,16], the most important limiting factor for depth of cure is light scattering and this is maximized when the filler particle size is close to half of the wavelength emitted by the light source [17]. Others optical phenomena are important to understand light transmission through a resin composite layer. A considerable part of the irradiation light that illuminates the resin composite surface is reflected. The other part that penetrates has the function of exciting the photosensitizer to start the polymerization process as deeply as possible. But the extent of this light’s depth depends on the absorption coefficients of the resin composite’s component parts [18]. Therefore, the study of the depth of penetration of light in resin composites is of fundamental importance, the depth of penetration being the inverse of the absorption coefficient [19]. If the Naperian absorption coefficient, 1/˛, is used, the depth of penetration (˛) is the distance at which the radiant power, P , decreases to 1/e of its incident value, P0 , being e, the Naperian number. The aim of the present study was to investigate the light transmittance of a hybrid and a nanofilled resin composite. The analysis was conducted in wavelength spectra from 400 to 800 nm, before and after resin composite light polymerization. The research hypothesis was that the size of filler particles would influence the light transmittance in the resin composites.
2.
Material and methods
The resin composites used in this work were Filtek Z250, Shade A2, and Filtek Filtek Supreme XT, Shade A2E (3M-ESPE, St. Paul, Minnesota, USA). These resin composites had the same polymeric matrix differing only in filler content and filler size (Table 1). The light source used for resin composite photoactivation was an Optilux 501/Demetron-Kerr
(Demetron Research, Danbury, CT), used in normal mode with 850 mW/cm2 irradiance, for 40 s for each sample. Stainless steel matrixes (8 mm in internal diameter and heights of 0.15, 0.25, 0.30, 0.36, 0.47 and 0.75 mm) were placed over a microscope glass slide and slightly overfilled with the resin composites (n = 3). After that, the resin composite surfaces were covered with another microscope glass slide. The light transmission through the resin composites was measured, for each thickness, using a UV–visible HP 8452A Diode Array Spectrophotometer (Palo Alto, California) in a selected light spectrum varying from 400 to 800 nm wavelength. This spectrophotometer was chosen due to the fact that light exposure of the sample is only for a short period of time, preventing the monomer conversion of the resin composite during the measurements. Light transmission measurement was first performed in an unfilled matrix positioned between two microscope slides, in order to obtain a calibration parameter for the measurements (blank measurement). The blank measurement was used to correct the intensities in the transmission spectra. Then, after the blank measurement, light transmission analysis was performed in the polymerized and unpolymerized resin composites. With the transmittance values for each resin composite, for each thickness and for each wavelength, two types of analysis were performed. The first analysis was, considering only the thinnest sample t (0.15 mm) and some previously chosen wavelengths (400, 450, 468, 500, 600, 700 and 800 nm), the rate of percentage transmittance r between polymerized and unpolymerized resin composites was calculated, for each wavelength. The 468 nm wavelength was chosen considering the absorption spectra of the camphoroquinone, used as photosensitizer in both resin composites. Then, this rate r can be considered, for a fixed thickness t, as the gain in light transmission for a particular resin composite after polymerization, being in some way a measurement of its translucence after polymerization. The second analysis was to determine the light penetration depth for each resin composite. For this the following procedure was performed: percentage transmittance values for wavelengths varying from 400 up to 800 nm, with 50 nm interval, for each sample, were picked out and plotted as a function of the thickness. These values were then fitted to a first order exponential decay of the form
P = P0 e−(t/˛)
(1)
Table 1 – Restorative materials used in this studya Composites
Lot no.
Composition
Filtek Z250
6PR
Filler: zirconia/silica with size of 0.6 m. The filler loading is 60% by volume Organic matrix: bis-GMA, bis-EMA, TEGDMA and UDMA
Filtek supreme XT
6BW
Filler: non-agglomerated nanosilica of 20 nm size filler and agglomerated zirconia/s´ılica nanocluster with size of 5–20 nm. The filler loading is 78.5% by weight Organic matrix: bis-GMA, bis-EMA, TEGDMA and UDMA
a
According data furnished by the manufacturer (3M, St. Paul, Minnesota, USA).
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Fig. 1 – Transmittance spectrum of Filtek Z250 resin composite, as-explained in the text, for unpolymerized and polymerized samples.
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Fig. 3 – Percentage transmittance rate r, as-explained in the text, for a fixed thickness t = 0.15 mm for both resin composites.
where ˛ is the depth of penetration of light, P the radiant power at a distance t for the wavelength , and P0 is the incident radiant power for the wavelength and e the Naperian number. The ˛ values, obtained from the fitting procedure, were then plotted against the wave number and the behavior of the depth of penetration of light, for each resin composite, could be analyzed.
3.
Results
The spectra obtained can be seen in Figs. 1 and 2. For Filtek Z250 and Filtek Supreme XT, respectively, the percentage of transmitted light at each sample thickness can be observed as a function of the incident wavelength. In Fig. 3, the rate of percentage transmittance r between polymerized and unpolymerized resin composites, for a fixed thickness t = 0.15 mm, is Fig. 4 – First order exponential plot for Filtek Supreme XT resin composite for = 500 nm at different thickness, considering unpolymerized and polymerized samples.
Fig. 2 – Transmittance spectrum of Filtek Supreme XT resin composite, as-explained in the text. It should be noted that the scale for the polymerized samples is twice the scale for the unpolymerized ones.
plotted as a function of the selected wavelengths. From this plot it can be seen that for the Filtek Z250 resin composite there is no significant change in the rate r as a function of the wavelength, and the rate for this resin composite remains quite constant at around 1.5. Otherwise, for the Filtek Supreme XT resin composite the change in r is remarkable, varying from 5.5 to 2.2 with increasing wavelength. In order to better understand how the penetration depth of the light is calculated, Fig. 4 shows an example for one resin composite (Filtek Supreme XT) at a fixed wavelength ( = 500 nm). For the six different sample thicknesses the transmittances were plotted and the experimentally determined points fitted to a first order exponential decay, as discussed in Section 2 and described by Eq. (1). From the fit, a ˛ value corresponding to the penetration depth for the corresponding wavelength and resin composite was then obtained. The
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ite where irradiance of 221 mW/cm2 is reached at 0.09 mm, in the unpolymerized condition, changing to 0.14 mm when polymerized.
4.
Fig. 5 – Penetration depth for Filtek Z250 resin composite unpolymerized and polymerized, as a function of the incident wavelength.
results of this procedure are shown in Figs. 5 and 6, for Filtek Z250 resin composite and Filtek Supreme XT resin composite, respectively. For both resin composites it can be seen that the penetration depth increases with increasing wavelength. However, for Filtek Z250 resin composite, below 500 nm the differences in penetration depth between polymerized and unpolymerized samples are more accentuated. Otherwise, for Filtek Supreme XT resin composite, the difference in penetration depth between polymerized and unpolymerized samples grows up from 400 to 800 nm. The concept of penetration depth can be understood by a simple example: for the Filtek Z250 resin composite, irradiance from incident light of 600 mW/cm2 (for = 500 nm) reaching a depth of 0.15 mm, falls to 1/3 the initial value, i.e. 221 mW/cm2 , when unpolymerized. Once polymerized, the same irradiance will be found at a depth of 0.16 mm. This behavior is more accentuated for the Filtek Supreme XT resin compos-
Fig. 6 – Penetration depth for Filtek Supreme XT resin composite unpolymerized and polymerized, as a function of the incident wavelength.
Discussion
During the last few decades, resin composites were more commonly classified according to their filler particle size as hybrid (8–30 m), microhybrid (0.6–3.6 m), and microfilled (around 0.04 m) [20]. In the last few years, however, with the introduction of nanotechnology in dentistry [21], a new class of resin composite, so called nanocomposite, is available to practitioners. In this field, some in vitro studies have shown a good performance for nanofilled resin composites [8,9,22,23]. Few studies, however, have focused on the light transmission of this new class of restorative material [24]. Some previous studies have shown that factors such as filler and polymeric matrix refractive index, monomer type, filler type and filler content can influence the light transmittance of resin composites [24–26]. Since in the present study both resin composites have the same polymeric matrix (BisGMA, BisEMA, UDMA and TEGDMA), and the same filler particle types, the discussion of the results was based only on the influence of filler particles size. From Figs. 1 and 2, it can be seen that polymerized specimens showed a higher light transmittance than unpolymerized ones. This behavior can be explained by two points of view. First, after polymerization, the resin composites underwent a vitrification process that leads to a glass stage. In this stage, the air trapped in micro-voids frozen into the polymer network would allow an optical transmission gain [27]. Secondly, the ordination of the polymer network, after polymerization, would allow the light to pass more easily through the bulk specimens. This effect, known as photobleaching is linked to the polymerization processes, called frontal photopolymerization (FPP) and has already been studied by Cabral et al. [28] and Warren et al. [29]. This behavior has already been observed, for Z100 resin composite [30], a material that has the same filler content as Filtek Z250, a resin composite used in the present study. In that study, the authors showed that the light absorption coefficient (˛ ) decreases upon curing. In addition, these authors also showed that, at wavelengths between 400 and 700 nm, the light absorption coefficient of the Z100 resin composite decreases as the wavelength increases. This can explain the increase in light transmittance from 400 to 800, irrespective of resin composite thickness, observed in the present research (Figs. 1 and 2). The concept that filler particles with mean size close to half of the wavelength produces a high light scattering is well understood [31]. Massoti et al. [24] have shown that light transmittance at wavelengths of 400 and 560 nm was higher for Filtek Supreme XT resin composite dentin shade, which has a particle size of 5–20 nm, than for the translucent shade, which has particles of 75 nm. According to these authors, at shorter wavelengths the microfilled resin composites cause greater light scattering, while macrofilled resin composite causes this effect at longer wavelengths. In addition, according to the Rayleigh scattering Eq. (2), it is clear that the mean size of filler particles has a crucial effect
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on light transmittance in resin composites
T = e − 2303d
3Vp r3 (np /nm − 1) 44
references
(2)
where d is the thickness of the sample, Vp the volume fraction of the particles, r the particle radius, np the refractive indices of the particles, nm the refractive indices of the polymeric matrix and is the light wavelength. This kind of scattering may influence the depth of cure of the resin composite [32]. From the analysis of Figs. 5 and 6, it can be verified that in wavelengths higher than 600 nm, Filtek Supreme XT resin composite presented greater light penetration depth than Filtek Z250. This finding can be based on the fact that the filler size of Filtek Supreme XT (20 nm) is far from half of the 600 nm wavelength, so light scattering was reduced in the range between 600 and 800 nm. This behavior is sustained by the aforementioned discussion [24]. When comparing the light penetration depth in both unpolymerized and polymerized samples, interesting features can be noted. The first one concerns the Filtek Z250 resin composite. In Fig. 5, it can be observed that the penetration depth, for wavelengths higher than 500 nm, has almost the same value under the experimental error for both states of the resin composite. However, for wavelengths lower than 500 nm, the variation in the penetration depth from unpolymerized to polymerized state is considerable, changing from around 0.17 to 0.24 mm at 400 nm. This variation can be attributed to the camphorquinone absorption peak (e.g. at about 467.5 nm). In fact, Chen et al. [30] showed that the light absorption coefficient decreases upon curing, especially at wavelengths between 440 and 500 nm (1.06 cm−1 for unpolymerized and 0.68 cm−1 for polymerized samples). On the other hand, Fig. 6 shows that, except in the range between 400 and 450 nm, Filtek Supreme XT’s polymerized samples present an increase in the penetration depth of the light as a function of crescent wavelength. The fact that the behavior related to the absorption peak of camphorquinone found for Filtek Z250 was not the same for Filtek Supreme XT can be explained by the masking produced by the higher light scattering between 400 and 500 nm [33].
5.
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Conclusion
The results obtained in the present study lead the authors to accept the research hypothesis. The resin composites analyzed presented an improvement in the light transmission after polymerization. However, the nanocomposite showed a higher gain in transmittance for a fixed thickness of the sample than the hybrid. This finding was related to the filler particle size of the nanocomposite. With regard to depth penetration, the hybrid resin composite showed a deeper light penetration in the lower wavelength region, from 400 to 500 nm, that close to the absorption peak of camforquinone, the photosensitizer used in the resin composites analyzed in this study.
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