Light-cured composite resin materials have been widely used in everyday clinical practice. Composite materials present several advantages, such as ease of handling, satisfactory physical and mechanical properties, and most importantly, excellent esthetic appearance. However, light-cured resin-based materials must be exposed to a sufficient amount of blue light energy to achieve satisfactory conversion of resin monomers into polymers (1).
Degree of polymerization of composite materials depends not only on their chemical composition but also on the properties of the light-curing unit. Improper curing of the composite materials would decrease their physical properties, leading to marginal leakage, secondary caries, higher wear, and poor esthetic appearance of the composite restoration (2, 3).
In order to enhance clinical success of composite resin restorations, dental manufacturers have focused on the development of new light curing units, as well as the improvement of composite material composition. The introduction of nanotechnology enabled the development of composite resins with higher filler content, decreased filler size, as well as, enhanced composition in methacrylate-base organic matrix (4, 5). Changing the monomer structure of composite resins also led low-shrinkage and tooth-colored silorane-based resins, composed of siloxane and oxirane molecules (5-9). Due to the siloxane components, silorane materials have lower water sorption and solubility than conventional methacrylate-based composite materials (8, 9). The oxirane components provide lower polymerization shrinkage and higher strength through a cationic ring-opening mechanism and cationic polymerization of the composite resin (8, 10).
Another variable, which is important for durability of the light-curing composite resins is their limited depth of cure. In general, only increments up to 2 mm thick should be placed to ensure adequate light transmittance and composite resin polymerization. Recently, some bulk-fill based composite materials with low polymerization shrinkage and higher dissipation of induced energy, which increases the depth of cure, have been introduced to the market (11). Studies have shown that some bulk-fill materials can be cured adequately at depths up to 5 mm due to their increased light transmittance (12, 13). Controversially, other studies have shown significantly less depth of cure for the bulk-fill composite resins than claimed by manufacturers (14, 15).
However, regardless of the composite resin used, inadequate light-curing, especially in the deepest area of the composite restoration, remains an issue. In cavity preparation with deep undercut areas, it is impossible to place the tip of the light guide directly on the top of the light-cured composite resin (16-18). In such cases, enamel and dentin would attenuate intensity of the light delivered to the resin based composite material, depending on their optical properties like light transmittance and light diffusion (16-19).
Light transmittance through enamel and dentin is not well described in the literature so far. When the light irradiation is applied parallel to the dentine tubules the light is in that case scattered mainly from the dentine tubules while at the same time scattering pattern of obliterated dentine tubules will not be different from the scattering pattern of the regular structured dentine (20, 21). Some published studies described the effects of light irradiation through enamel on light-activated restorative materials and reported that the light-attenuation effect of enamel significantly diminished the depth of cure and hardness of the cured resin restoration (17, 19, 22, 23). However, it is still not clear if light-attenuation by enamel and enamel/dentin tissues affect the mechanical properties and degree of conversion of composite resin.
The degree of conversion of the resin monomer formulations is one of the most significant variables evaluated for assessing mechanical properties of polymerized composite resin materials. The degree of conversion of composite resin can be determined by direct and indirect methods. Direct methods for assessment the quality of polymerization of composite material are usually determinated using Fourier transform infrared spectroscopy (FTIR) or Raman Spectroscopy. Indirect methods for assessment of quality of polymerization of composite materials are Knoop and Vickers hardness (24, 25).
Therefore, the aim of this study was to evaluate the depth of cure of different composite resins cured directly and transdentally through enamel and enamel/dentin tissues. The null hypothesis assessed was that light-attenuation by dental tissues does not decrease the depth of cure or mechanical properties of light-cured composite resins used in this study.
MATERIALS AND METHODS
In this in vitro study, five different composite resins were evaluated: two flowable [SureFil SDR (SDR), Filtek Supreme Plus Flowable (FSP),], two low-shrinkage [Aelite LS (ALS), Filtek LS (FLS)], and one microhybrid [TPH 3 Micro Matrix Restorative (TPH)]. The composition of composite resins used in this study was presented in Table 1, according to the manufacturer’s information.
Enamel and enamel/dentin discs preparation
An intact freshly extract, non-carious, non-restored human third molar was selected after the donors’ informed consent was obtained under a protocol approved by the institutional review board of the University of Southern California. The tooth was scaled, cleaned, stored in 0.5% chloramine solution at 4◦ C to prevent bacteria growth and used within three months after extraction.
The tooth was sectioned in mesio-distal direction, parallel to its long axis using a diamond saw (Isomet 1000, Buehler Ltd., Lake Bluff, IL, USA) under distilled water-cooling to obtain a buccal and a lingual tooth slab. Both slabs were then further trimmed with a fine diamond bur in a high-speed hand piece under water cooling to obtain discs with a final diameter of 4 mm each. The discs were then polished using a waterproof 600-grit silicon carbide paper under running water to create a disc of enamel with a final thickness of 1 mm and another disc of enamel and dentin with a final thickness of 2 mm. Both discs were immersed in 0.5 M ethylenediaminetetraacetic acid (EDTA) solution for 2 min for cleaning and removal of the smear layer. The discs were then thoroughly rinsed with distilled water for 60 s and then stored in the same solution to avoid dehydration.
Depth of Cure by Vickers Microhardness
Opaque standardized polytetrafluoroethylene molds, with 2- and 4-mm thickness and an internal diameter of 2 mm, were used to fabricate the composite specimens. The molds were then placed on the top of a glass slide; the internal portion of the mold was filled in bulk with each composite resin and covered with another glass slide with a pressure of 1 kg for 30 sec. The composite resin specimen was then light-cured directly, either through a disc of enamel with 1 mm of thickness, or through a disc of enamel/dentin with 2 mm of thickness. All specimens were light-cured for 40 sec with irradiance of 800mW/cm2 (Elipar 2500, 3M ESPE, St Paul, MN, USA), keeping the tip of the light-curing unit in contact with the glass slide or the dental tissue disc (Figure 1).
After polymerization, each specimen was removed from the mold and stored in distilled water for 24 h at 37o C in a dark container. Subsequently, microhardness measurements were taken from the top and bottom surfaces of each specimen using a Vickers microhardness tester (Buehler MicroMet, Buehler Ltd., Lake Bluff, IL, USA). The micro indenter was pressed to the composite specimen using a load of 100 g for 15 sec. Vickers microhardness was measured at four points of each surface (top and bottom) of the specimen to minimize measurement errors within a specimen.
The Vickers hardness values were analyzed using a statistical software package (SPSS 17, SPSS Inc., Chicago, IL, USA). The data was first analyzed for normality with the Shapiro-Wilk test. Since all groups showed a normal distribution, the differences in the microhardness between the composite resins, light-curing procedure (direct, transdental through enamel or enamel and dentin) and depth of cure (top and bottom surfaces) were statistically analyzed using 3-way ANOVA. To isolate statistical significance among the groups, the data was submitted to Tukey HSD post-hoc test at a confidence level of 95%.
Results of the VH measurements for all composite resin, light-curing mode and surfaces were presented in Table 2. The VH of the top surface of the composite resins revealed that the highest values of microhardness were observed with the low shrinkage hybrid resin (ALS), regardless of the curing procedure (directly, through enamel or enamel/dentin discs). Both flowable composite resins, SDR and FSP, show the lowest values of VH on the top surface, regardless of the light-curing modes. No statistically significant difference was observed between two flowable composites, regardless of the light-curing procedure except when curing through 4 mm sample (P>0.05). At the bottom surface of the 4-mm thick sample, the lowest VH values were observed for the groups SDR, FSP and FLS for all curing procedures. The highest values at bottom surface of the 4 mm thick sample were observed for the composites ALS and THP.
For the composite SDR, the VH on the top surfaces was not affected by the curing procedure (Figure 2). There was no statistically significant difference between the VH values of the surfaces evaluated (top and 4-mm bottom) when the light was applied directly on the composite resin (P>0.05). However, the interposition of a disc of 2-mm enamel and dentin significantly reduced VH on the bottom surface of specimens with 2-mm of thickness. There were statistically significant difference between the VH of bottom surfaces of 2- and 4-mm SDR specimens when transdental polymerization was used (P<0.05).
For the composite FSP, the curing procedure did not affect the VH top surface (Figure 3). There was, however, a significant reduction in the VH at the bottom surface of specimens with 4-mm, even when the light was applied directly on the composite resin (P<0.05). The presence of dental tissue discs significantly reduced the VH on the bottom surfaces, regardless of the specimen thickness.
For the composite ALS, the curing procedures significantly reduced the VH on top and both bottom surfaces of the composite resin (Figure 4). There is a significant reduction on the microhardness at the bottom surfaces (2- and 4-mm) regardless of the light-curing procedure.
For the composite FLS, the presence of a disc of 1 mm of enamel or 2 mm of enamel and dentin affect the top and bottom (2- and 4-mm) surfaces VH (P>0.05) (Figure 5). The lowest values of VH were observed at the bottom surface with 4 mm of thickness for when polymerized through the enamel and dentin disc (9.82 VH).
For the composite TPH, the curing mode did not affect the top surface polymerization (Figure 6). There was a significant reduction on the microhardness at the bottom surface (4-mm) even when the light was applied directly on the resin specimen. The presence of a dental tissue disc significantly reduced the VH on the bottom surfaces of specimens with 2- and 4-mm thickness (P<0.05).
There are many published studies analyzing transmission of the blue light through different composite materials, however, there is a lack of data about transmission of the blue light through hard tooth structure, dentin and enamel, and how that will impact curing of composite materials (26, 27). Manufactures try to optimize and improve light transmittance through composite resin by changing and modifying organic matrix chemistry and morphological properties of fillers (26). As the dental composites are heterogeneous substances, the passing light is scattered at the resin-filler interface due to the differences in the refractive indices of the individual compounds (28, 29).
Uusitalo et al. (20) conducted a study where authors tested light transmittance through differently treated dentin and enamel surface. They concluded that light transmission through dentin was less than light transmission through enamel. Further, they found that the light is transmitted better through wet dentin and enamel than through dry substrate. Also, the exposed dentin tubules enhanced light transmission through the dentin surface (20). In our study, light irradiation through dental tissue significantly reduced the depth of cure of all composite resins evaluated, including the low-shrinkage and bulk-fill composites. Therefore, the null hypothesis stating that light-attenuation by dental tissues does not decrease the mechanical properties or the depth of cure of composite resin was rejected.
A standardized test for depth of cure, ISO 4049 test, is mandatory for manufacturers to certify their resin-based composites and to set its curing time and increment thickness (3, 30). This test advocates scraping off the unset materials immediately after irradiation and measuring the length of the remaining specimen, which is then divided by two (12, 31).
Surface hardness tests (Knoop or Vickers) have been used most often to characterize the depth of cure and mechanical properties of visible light-cured resin based composite materials (31, 32). Uhl et al (33) showed that the degree of polymerization of composite materials could be better evaluated with Knoop or Vickers hardness than with depth of cure tests using a penetrometer. However, data obtained from different studies is difficult to compare mainly due to the different molding methods used. Black molds produce shorter depths of cure than a stainless-steel molds (34). White molds, generally made of Teflon or other translucent material, may allow more of the curing light to pass through the mold than through the composite resin (5, 35). Consequently, this may result in exaggerated depths of cure. Human tooth molds have also been used but they have varied size and have not been compared with a 4 mm diameter stainless steel mold, as specified in the ISO test to determine the depths of cure (12, 17, 36, 37).
Variations in the depth of cure between different composite materials may be ascribed initially to light scattering at particle interfaces and light absorbance by photoinitiators and pigments present in light-cured resin-based material. Both, light scattering, and light absorbance reduces the light penetration through the composite resin sample and therefore a reduced degree of conversion or degree of cure (12, 38, 39). Ilie et al (40) tested the correlation between the Vicker Hardness and filler loading in composite materials and concluded that increased filler loading reduces the volume of resin matrix for polymerization and intrinsically increases hardness (12, 38). Furthermore, the study stated that satisfactory degree of conversion might be due to the refractive index matching between the resin and filler, which enhances light transmission through the composite resin sample (40). Reduction in refractive index differences between resin and filler improved the degree of conversion and increased depth of cure as well as color shade matching (12, 28, 41). Uhl et al (37, 42) explained in their study that the influence of co-initiators in a composite on the Knoop hardness was less important in the depth of a composite. This was explained by the fact that the light transmittance in dental composites is higher for longer wavelengths than for shorter wavelengths. Therefore, it can be concluded that a high percentage of the shorter wavelengths is absorbed near the surface of the composite and cannot excite co-initiators in a deeper portion of the composite filling. Another study compared light penetration in bulk filled flowable and packable composites in comparison to regular flowable and conventional composites. The authors confirmed that amount of light transmitted through composite was dependent on the amount of scattered and absorbed light (13).
Light transmittance in the dental composite materials was shown to decrease with increased filler content and for irregular filler shape, which is due to the increase of specific surface between fillers and resin (13, 39, 43-45). A further important fact in light transmission through composite is the treatment of fillers. Silane-coated fillers were shown to enhance, while uncoated fillers were shown to decrease light transmission (36). In our study FSP composite was used which has silane treated ceramic (52-60%) and silane treated silica (<11%) as stated in the manufacturer instructions. Also, FLS, low-shrinkage composite, has silane treated quartz, 60-70% wt. according to the manufacturer. However, in this study neither FLS nor FSP has shown superior Vicker’s micro-hardness in comparison to other tested flowable or low-shrinkage composite.
The curing light in this study was halogen-curing unit with broad spectral emission. Arikawa et al (19) discussed in their study that amount of light transmitted through 1 mm thick enamel was only about 35% of the original light in the overall wavelength from 400-600 nm. Furthermore, the light transmission of dental enamel and dentin increased with increasing wavelength (17-20). In their study Arikawa et al (19) used an enamel filter made of a mixture of composite materials, while in our study natural enamel and enamel/dentin were used. In this recent study, the enamel and enamel/dentin samples were collected from the same tooth to avoid any difference in enamel and dentin composition with taking samples from different tooth. The enamel and dentin samples were also maintained in distilled water and used immediately to avoid any dehydration, which can have influence on the final results of light transmission. Regardless of composite materials used, higher Vickers microhardness values were observed when they were cured through enamel disc than enamel/dentin disc. Arikawa et al (17) showed in their study that dentin had strong light diffusion characteristics which would encourage the light-attenuating effect of the hard tissue. It is well established that light intensity drops with distance regardless of curing unit or composite materials used (46, 47). It was confirmed that the light intensity used to irradiate the composite material through 0.5 mm of enamel is almost half of the direct irradiation, while the light intensity attenuation through 1.5 mm enamel was almost 80% of the original light intensity (48).
From the table 2 it is visible that only for SDR bulk fill flowable composite material there is no difference between VH on the top (25.72) and on the bottom – 2 mm (25.46) and bottom – 4 mm (25.00) when using direct curing protocol. For all other materials there is a drop in the VH value when compared the top and bottom 2 and 4 mm in case of direct curing protocol. The highest drop was noticed in case of ALS low shrinkage hybrid composite: 75.97 on the top, 44.33 on 2 mm bottom and 36,35 on 4 mm bottom when direct curing protocol was used.
In clinical setting, when composite material needs to be cured through tooth structure, special care should be employed to minimize the reduction of restoration’s mechanical properties. Prolonging irradiation time could compensate the light-attenuating effect of enamel and dentin. However, prolonging irradiation time also can raise the temperature in the composite resin itself, hard dental tissues and the pulp (49). Further studies are needed to explore alternatives for adequate polymerization of composite resins under hard dental tissues and restorative materials, as well as, to evaluate the response of pulpal cell to prolonged light irradiation.
The results of this study showed that light-curing attenuation of dental structures negatively affect the micro-hardness of all composite resins at the bottom surface. Bulk fill flowable composite SDR and flowable FSP composite have lower VH on the top surface. Flowable (SDR and FSP) and micro-hybrid (TPH) composite resins can be properly polymerized by direct light irradiation on increments with a thickness up to 2 mm. To achieve better clinical results for Bulk fill composite polymerization it can be suggested to decrease incremental thickness and/or to extend the curing time.