Introduction
In the last decades, resin-matrix composites have become the most suitable materials in restorative dentistry due to their technological development on the chemical composition and processing. Nowadays, resin-matrix composites are used for indirect and direct restorations in restorative dentistry [
1‐
3]. The chemical composition involving the inorganic fillers and polymeric matrix of the resin-matrix composites is a major factor that determines their optical and mechanical properties [
2,
3]. However, the chemical composition and physical properties of the currently available resin-matrix composites widely vary from manufacturers and types of materials [
4‐
6]. Inorganic fillers are added in the chemical composition of the resin-matrix composites to increase their mechanical properties and to mimicking the optical properties of enamel and dentin [
7‐
9]. On light curing procedures, the inorganic fillers must allow the transmission of visible light required for the activation of the polymerization reaction of the polymeric matrix. Nevertheless, the polymerization can vary depending on the content, type, and size of inorganic fillers considering optimal conditions on the procedure and equipment of light curing procedures [
9‐
11]. A lack of polymerization decreases the strength, elastic modulus, hardness, and wear resistance of the resin-matrix composites leading to the degradation and release of monomers to the surrounding tissues [
12‐
15].
A resin-matrix composite comprises inorganic vitreous fillers dispersed in an organic matrix. The organic matrix involves a cross-linking of dimethacrylate monomers such as bisphenol A-glycidyl dimethacrylate (Bis-GMA), triethylene glycol dimethacrylate (TEGDMA), urethane dimethacrylate (UDMA), and ethoxylated bisphenol A dimethacrylate (Bis-EMA) [
16‐
18]. Such combination of molecules results in an organic matrix that depends on the structure of the monomers and the polymerization reaction. On visible-light polymerization, camphorquinone (CQ) combined with a tertiary amine are incorporated in resin-matrix composites as a photoinitiator system [
19‐
22]. Also, acyl and bisacyl phosphine oxide initiators can be utilized as single-component visible-light alpha-cleavage initiators at wavelengths usually below 450 nm [
20,
23,
24]. In the organic matrix, CQ is stimulated by visible light irradiation in the range between 420 and 490 nm [
15,
25,
26]. Commercially available resin-matrix composites show a weight percentage (wt. %) of inorganic filler content ranging from 40 up to 90 wt. % [
7,
27]. A mixture of different inorganic fillers (i.e., glass ceramics and silica) at different sizes can be found in the chemical composition of current resin-matrix composites [
6,
8,
28]. Nano- and micro-scale particles are combined in the resin-matrix composite microstructure to promote a mechanical reinforcement under chewing loading [
6,
9,
28]. In fact, a high content of nano- and micro-scale particles results in a low organic matrix volume under polymerization. Indeed, inorganic fillers affect the polymerization shrinkage, wear, surface roughness, translucency, opalescence, and fluorescence of resin-matrix composites [
18,
29,
30]. Resin-matrix composites with volume fraction up to 60 wt. % revealed increased values of flexural strength and elastic modulus [
31]. Despite the influence of the inorganic filler content on the mechanical properties of resin-matrix composites, the nature and shape of the inorganic particles must also be considered [
33,
34]. Spherical-shaped inorganic particles allow higher amount of fillers within the resin-matrix composites, and the enhancement of materials' strength comparatively to the materials with irregular filler particles, once the stresses tend to accumulate in the protuberances of the irregular-shaped particles [
32]. Nevertheless, the effects of the content, size, and shape of inorganic fillers on the light transmission through resin-matrix composites are not entirely elucidated in literature.
The polymerization of resin-matrix composites for direct restorations can be achieved by demand using light-curing units (LCU) as a source of light ranging from 360 up to 500 nm [
25,
35,
36]. Currently, light-emitting diodes (LED) are the most typical light source within wavelength ranging from 360 up to 500 nm, light irradiance between 400 and 1765 mW/cm
2, and light exposure from 20 to 60 s [
37‐
39]. The time of light exposure depends on the light irradiance as well as type and thickness of the restorative materials to reach the energy required for the polymerization of the resin-matrix composite. The incident light can be reflected, refracted, absorved, scattered, and/or transmitted towards the material. On the resin-matrix composites’ surfaces, part of the light is reflected, due to the differences in the material refractive indexes [
9]. Scattering also occurs when light passes through the material due to the existence of fillers and defects such pores or cracks. Scattering changes with the wavelength of incident light and is mostly determined by the particle size and by the relationship between their refractive indexes [
9,
40,
41]. The accomplishment of the polymerization of the organic matrix leads to densely crosslinked, glassy polymer networks that provide high values of physical properties such as strength, elastic modulus, hardness, toughness, and wear resistance [
42‐
44]. The optical and mechanical behavior of resin-matrix composites is strongly influenced by the degree of conversion (DC) of the organic matrix [
14,
17]. DC percentage of resin-matrix composites is determined by a comparison of the peak height absorbance intensity of aliphatic carbon–carbon double bond (C = C) with aromatic C = C, before and after irradiation [
18,
45]. Manufacturers recommend resin-matrix composite increments with 2-mm thickness [
46,
47], to achieve adequate DC proportion. The clinical success of direct restorations depends on the light transmission, polymerization, and material properties [
4,
5].
The relationship among light, polymerization, DC, and fillers raised the question on the influence of inorganic fillers content on light transmission through resin-matrix composites. Thus, the purpose in this study was to evaluate the light transmission through five different resin-matrix composites regarding the inorganic filler content. The following hypotheses were established: (i) the percentage of inorganic fillers has a significant influence on the light-curing transmission through the resin-matrix composites; (ii) the increase in the inorganic fillers’ amount induces a high light-curing transmission through the material.
Discussion
The present study focuses on the effect of inorganic particles on the light transmittance through resin-matrix composites by using visible and near-IR spectrophotometry. Thus, the findings acquired in this study revealed the percentage of inorganic fillers influences the light transmittance through the resin-matrix composites. Resin-matrix composites reinforced with 89 wt. % inorganic fillers showed a low light transmittance after polymerization for 10, 20, or 40s. Also, the highest content of fillers resulted in a low ratio of light transmittance on the resin-matrix composites regarding light absence before and light irradiance. On the other hand, flowable resin-matrix composites reinforced with 60 wt. % inorganic fillers revealed the highest light transmittance and consequently light transmittance ratio regarding light absence before and after light irradiance. In this way, the results validate the first hypothesis of the present study. The second hypothesis was rejected on the increase in the fillers’ amount inducing a high light-curing transmission through the material.
In this study, five types of resin-matrix composites were assessed considering the content of inorganic fillers, as seen in Table
1. One group of resin-matrix composites showed a high content of inorganic fillers at 89 wt. %, while another group revealed a low content of inorganic fillers at 60 wt. %. Inorganic fillers were properly identified by scanning electron microscopy coupled to energy dispersive spectroscopy. Resin-matrix composite reinforced with 89 wt. % inorganic fillers was composed of irregular shape glass fillers and spherical amorphous silica (Fig.
2). The dimensions of glass fillers were identified at micro-scale, while silica was detected at submicron- and nano-scale. Considering the type of inorganic fillers, a similar microstructure was noticeable on the resin-matrix composites reinforced with 62.5 or 80 wt. % inorganic fillers composed of ytterbium fluoride or amorphous silica. The mean size of most inorganic fillers was measured at submicron- and micro-scales although nano-scale silica was also enclosed. Furthermore, a similar microstructure was noticeable on the resin-matrix composites reinforced with 60 or 74 wt. % inorganic fillers composed of ytterbium fluoride or amorphous silica at micro-, submicron-, or nano-scale dimensions. The resin-matrix composites were selected considering a similarity of organic components as follows: Bis-GMA, Bis-EMA, TEGDMA, UDMA, HEMA (Table
1). The photoinitiator system, namely, camphorquinone, was analogous for each resin-matrix composite. Thus, the refractive index of monomers and inorganic fillers can affect the light transmittance through resin-matrix composites [
1,
53,
54] that implies the light transmittance is highly dependent on the type of materials [
4,
54,
55].
Low-light transmittance through resin-matrix composites reinforced with a high proportion of inorganic fillers (i.e., 89 wt. %) corroborates with the inorganic particles influencing the light distribution through the resin-matrix composites. Light transmittance increased with the content of inorganic fillers decreased down to 60 wt. %. Such findings are consistent with the literature data which indicates that increased amount of filler promotes lower transmittance values [
1,
56,
57]. That was also noticed by the evaluation of the ratio between the transmittance of the polymerized and non-polymerized specimens since specimens with a high inorganic fillers’ content showed lower ratio values. Nevertheless, a lower ratio was detected for composites containing 62.5 wt. %, that can be explained by a significant proportion of opaque and irregular inorganic fillers, such as ytterbium fluoride [
58]. After polymerization, the optical transmittance values were lower at UV region than the values recorded for the spectra infrared region. Light transmittance in the UV region for non-polymerized and polymerized specimens for 10, 20, or 40 s indicates that light is mainly absorbed in the visible light spectra, while UV light is mainly transmitted through the resin composites. It can be explained by the arrangement of the polymer networks after polymerization, which allows a proper and efficient light pathway [
4,
56]. Camphorquinone consumption, which reduces light absorption, can also be a factor to increase the light transmittance [
53]. Additionally, the exothermic nature of the polymerization chain reaction causes a transient decrease in the refractive index of resin-matrix composites due to a decrease in their density leading to an increase in the light transmittance [
4,
56]. Thus, the decrease of the light transmittance in function of a high filler content can be expected although the nature of the inorganic fillers also plays a key role on the light transmission pathways. For instance, glass ceramics such as silica can provide a high translucency, while other glass ceramics provide a reflection of visible light. The size of inorganic particles also affected the light transmittance ratio since the light transmittance increased when the inorganic particles’ size decreased [
4,
59]. A mixture of different inorganic fillers (i.e., glass ceramics and silica) at different sizes can be found in the chemical composition of current resin-matrix composites. In this way, a balance in the type and size of glass ceramics should be investigated in further studies.
The indirect evaluation of polymerization efficiency and relative DC percentage [
29] can also be predicted by measuring the mechanical properties of the materials including hardness, strength, fracture toughness, or elastic modulus [
60,
61]. Most of resin-matrix composite groups revealed higher light transmittance values after light irradiation for 40 s when compared to the values for 20 s. The results of ratio and light transmittance were consistent with the micro-hardness measurements, indicating a high polymerization of the materials after light curing for 40 s. However, commercially available resin-matrix composites can reveal varying optical behavior under light irradiation depending on several factors including exposure time. As seen in Figs.
4 and
5, the inorganic particles lead to light scattering that interferes in the light transmittance concerning a puzzling function of the filler size, distribution, and the mismatch of refractive indexes between the inorganic fillers and organic matrix [
62,
63]. For instance, resin-matrix composites reinforced with 60 wt. % inorganic fillers were significantly dependent on the light exposure time in the present study. The transmittance ratio was lower under light exposure for 10 s when compared to the light irradiation for 20 or 40 s. Specimens containing 62.5 or 78–80 wt. % fillers showed a similar behavior regarding the ratio, although resin-matrix composites with 62.5 wt. % fillers were more dependent on light exposure time. On the other hand, the resin-matrix composite reinforced with 89 wt. % inorganic fillers was not dependent on the light exposure for 20 or 40 s. Vickers micro-hardness values showed slightly higher values for 20 s when compared to those recorded after polymerization for 40 s, which indicates the resin-matrix composite was properly polymerized for 20 s. The increased filler content of resin-matrix composites tends to improve mechanical properties of resin-matrix composites [
58]. Micro-hardness values recorded for resin-matrix composites containing 89 wt. % and 78–80 wt. % inorganic fillers after polymerization for 20 and 40 s were similar to the results from previous studies [
64]. Also, the elastic modulus and nano-hardness results were consistent with the values provided by the manufacturers. In this study, mechanical behavior of resin-matrix composites was highly dependent on the light exposure time, except on the Vicker’s microhardness for the resin-matrix composites containing 89 wt. % fillers. The highest values of Vicker’s microhardness recorded for the resin-matrix composite containing 89 wt. % fillers were resultant from the high inorganic fillers’ content. The elastic modulus values for resin composites with 89 wt. % fillers were in concordance with the values recorded in previous studies [
31‐
33]. Also, the results showed differences in values recorded after light curing exposure over light curing for 20 or 40 s. The other groups of resin-matrix composites reinforced with 60, 62.5, and 78 wt.% fillers possess a higher portion of organic matrix, which requests a longer exposure of visible light for adequate polymerization. Then, the DC of monomers in the organic matrix of those resin-matrix composites is dependent on the light exposure time to achieve the required mechanical properties. The findings were validated considering the differences in light transmittance through the tested groups of resin-matrix composites with inorganic fillers ranging from 60 up 89 wt.%. Such findings are clinically relevant since an adequate polymerization via chair side light curing units must be accomplished by the clinicians to guarantee proper mechanical properties in the oral cavity environment [
65]. Thus, the relationship among light irradiance, polymerization, DC, and inorganic fillers plays an important role on the physicochemical behavior of the resin-matrix composites. Commercially resin-matrix composites are progressively improved by manufacturers and a wide range of materials are available for restorative dentistry. Then, novel materials must be analyzed by traditional and alternative physicochemical approaches. The present data show that visible and near-IR spectrophotometry could be considered a potential alternative methodology to estimate the DC of resin-matrix composites through the evaluation of the light transmittance behavior. Considering the findings shown in the present study, further studies can be carried out involving a comparison of data with other methods such as Fourier transform infrared (FTIR) or near-infrared (IR).
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