Introduction
Modern composite materials for direct restorations offer excellent esthetics and reasonable mechanical properties (1). However, the durability of composite restorations is reduced by their susceptibility to development of secondary caries (2). The issue of secondary caries is being addressed by investigations of bioactive composites with the capability to combat cariogenic bacteria or release remineralizing ions (3). Various experimental bioactive formulations have been investigated with promising results (3-7).
Among different ion-releasing compounds which are being investigated as bioactive fillers in experimental materials, bioactive glass (BG) offers multiple potential benefits, such as sealing of the interfacial gap with hydroxyapatite precipitate (5), remineralization of demineralized dental hard tissues (8), reduction of dentin hypersensitivity and postoperative sensitivity (9) and antibacterial action (10). The properties of BG such as the balance between stability and solubility in an aqueous environment are determined by the BG composition (11). Various types of BG have been incorporated into dental materials, e.g. Nb-modified BG (12), fluoride-containing BG (13), BG S53P4 (14), and BG 45S5 (15). The “classical” BG 45S5 used in this study resembled the original BG formulation which is marketed under the commercial name of “Bioglass” and has a widespread use in orthopedics (16). The BG 45S5 comprises (in wt%): 45% SiO2, 25% Na2O, 25% CaO, and 5% P2O5 (17). Preliminary studies on dental composites filled with BG45S5 have shown favorable properties such as the degree of conversion (18), depth of cure (19), the capability to precipitate hydroxyapatite (20), water sorption and solubility (21).
The setting of composite materials exerts a potentially harmful effect on the dental pulp due to the increase in temperature during light-curing (22). Two main sources of heat are irradiation from the light-curing unit and exothermic polymerization of the resinous component (23). The use of contemporary high-irradiance curing units (1000 mW/cm2 and more) contributes to temperature increase through both the direct heating effect and by accelerating the polymerization reaction (24). However, curing of dental composites using high-irradiance curing units has been considered safe for dental pulp if curing times recommended by manufacturers are followed (25).
The aim of this study was to investigate the real-time temperature rise during light-curing of five experimental composite materials containing different fractions (0-40 wt %) of BG 45S5 and compare it to the temperature rise of three commercial composites. The null hypotheses were that: (I) the fraction of BG does not affect the temperature rise during light-curing of experimental composites; (II) the fraction of BG does not affect the time at which temperature reaches the maximum value; (III) the temperature rise in experimental bioactive composites does not differ from that in commercial reference materials.
Materials and methods
Composite materials
The composition of five experimental composite materials according to preliminary studies (18-20) is detailed in Table 1. Resins and photoinitiator system were mixed in a dark room using a magnetic stirrer for 48 hours. Blending of the resulting photoactivated resin with fillers was performed in a mixing device (Speed Mixer TM DAC 150 FVZ, Hauschild & Co. KG, Hamm, Germany) at 2700 rpm during five minutes (26). The experimental composite pastes were then kept for 12 h in vacuum to remove air inclusion.
Bioactive glass: SiO2 45%, Na2O 25%, CaO 25%, P2O5 5%, particle size (d50/d99 [µm]): 4.0/13.0, silanization: none, product name/manufacturer: G018-144/Schott, Germany.
Barium-fillers (Ba): SiO2 55.0%, BaO 25.0%, B2O3 10.0%, Al2O3 10.0%, particle size (d50/d99 [µm]): 1.0/4.0, silanization 3.2 wt%, product name/manufacturer: GM27884/Schott, Germany.
Silica-fillers (Si): SiO2 ≥ 99.8%, primary particle size: 12 nm, silanization 4-6 wt%, product name/manufacturer: Aerosil DT/Evonik Degussa, Germany.
Bis-GMA: Bisphenol A glycidyl methacrylate, Esstech, PA, USA; TEGDMA: tri-ethylene glycol dimethacrylate, Esstech; CQ: camphorquinone, Aldrich, WI, USA; 4E: ethyl-4- (dimethylamino) benzoate, Aldrich.
Apart from the experimental composites, three commercial composites (flowable, nano- and micro-hybrid) were used as a reference (Table 2). Information on the composition of commercial materials was obtained from manufacturer provided datasheets and references (27, 28).
Bis-EMA: ethoxylated bisphenol A dimethacrylate, UDMA: urethane dimethacrylate
Real-time temperature monitoring
To measure the temperature rise during light-curing, cylindrical composite specimens of 6 mm in diameter and 2 mm thick were prepared in black Teflon molds. The uncured composite material was cast into molds and covered from both sides with 0.05 mm thick polyethylene terephthalate (PET) film. The composite-filled molds were covered from both sides with 1 mm thick glass plates and pressed between two flat metal slabs to remove excess material. The curing unit was centered above the specimen at the distance of 1 mm from its surface. A custom-made t-type thermocouple was positioned at the opposite side of the specimen below the PET film. Preliminary measurements showed that placing the PET film between the composite specimen and thermocouple had no significant effect on real-time temperature curves, rather adding only slightly to the random measurement error. The detailed description of the experimental setup is given in the reference (23). Six specimens were prepared for each composite material (n=6).
During the light-curing for 30 s by means of a blue-violet light emitting diode (LED) curing unit (Bluephase G2, Ivoclar-Vivadent, Schaan, Liechtenstein, irradiance of 1200 mW/cm2), the temperature of the specimens was monitored in real-time (20 s-1) using a custom-made computer program prepared in LabVIEW 2011 (National Instruments, Austin, Texas, USA). Temperature monitoring was continued after the curing has been completed in order to observe the specimen cooling. Approximately 160 seconds after the start of the curing, temperature returned to the baseline and the specimen was illuminated again for 30 seconds. This procedure was performed in order to discern the heating effect of the curing unit from the polymerization exotherm. Such an approach assumes that polymerization has been completed during the first illumination, which was demonstrated in preliminary experiments. The environmental temperature recorded using a t-type thermocouple as the baseline temperature before initiating the real-time temperature measurement was 21±1 °C.
Statistical analysis
No significant deviations from a normal distribution and an acceptable homogeneity of variances were confirmed by the Shapiro Wilk and Levene’s test, respectively. The one-way ANOVA with Tukey post-hoc adjustment was used to compare mean values of temperature rise among all composites, as well as times of the temperature peak among the experimental composites. A Pearson correlation analysis was performed to correlate the BG fraction with the reaction exotherm and time of the temperature peak. Statistical analysis was performed in SPSS 20 (IBM, Armonk, NY, USA) with α=0.05.
Results
A representative plot of the temperature increase as a function of time is presented in Figure 1. The temperature rise during the first illumination represents the total temperature rise during light-curing (T1), while the temperature rise during the second illumination reflects the heating effect of the curing unit (T2). The difference between T1 and T2 represents the temperature rise due to the polymerization exotherm (Texotherm).
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Mean values of T1, T2, and Texotherm are presented in Figure 2. The temperature rise due to the curing unit heating (T2) was similar for all composites. T1 values for BG-20 and BG-40 were significantly lower than those for composites with lower BG fractions. The Texotherm values were statistically more discriminative regarding the BG fraction and showed a gradual decline as the BG fraction increased. The flowable commercial reference (TEF) showed T1 and Texotherm values similar to those of BG-20, while T1 and Texotherm for the nano- and micro-hybrid commercial references (TEC and Gradia) were lower than in experimental composites.
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Figure 3 shows temperature curves during the first illumination for experimental composites (a) and commercial references (b). In experimental composites, the temperature peak became lower and shifted towards later times as the BG fraction increased (Figure 3a). In commercial composites, the temperature peak was observed only in TEF, while TEC and Gradia showed a gradual temperature increase until the end of the illumination (Figure 3b). The time at which temperature maximum in experimental composites was reached is summarized in Figure 4. Statistically significant influence of the BG fraction on the time of temperature peak was observed.
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Figure 5 plots temperature rise and time of temperature peak as a function of the BG fraction. High correlations with BG fraction were identified for the time of the temperature peak and temperature rise (R2 values of 0.98 and 0.94, respectively).
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Discussion
This study assessed the real-time temperature rise during light-curing of experimental composites filled with 0-40 wt% of BG 45S5 and total filler loading of 70 wt%. Three commercial composites (flowable, nano- and micro-hybrid) were used as references.
The composition of experimental composites followed a previous study which demonstrated their capability to precipitate hydroxyapatite (20). The potential for hydroxyapatite precipitation was increased with higher BG fractions. However, increasing the fraction of water-soluble BG fillers is expected to impair mechanical properties (29). Thus composites with various BG fractions were investigated since the optimal composition which would provide a balance between bioactivity and mechanical stability has not yet been determined (21).
The thickness of the specimens used in this study was 2 mm in order to simulate the layer thickness which is common in the clinical use of conventional composites. Since the amount of heat released depends on the composite amount (22), the dimensions of composite specimens were chosen to represent the volume of the composite which would be used in a large increment for filling an extensive preparation in posterior teeth. The curing time of 30 seconds exceeds the commonly recommended curing times, which usually range between 10 and 20 seconds for a high-irradiance curing unit with an output of 1200 mW/cm2. Such a long curing time was used in order to gain a better insight into temperature curves during polymerization and capture the temperature peaks for all experimental materials (Figure 3).
The approach of two consecutive illuminations (Figure 1) was adopted according to a previous study which demonstrated its usefulness for separating thermal contributions of curing unit and reaction exotherm (23). The statistical analysis showed that temperature differences among composites were mostly attributable to the reaction exotherm, while the heating effect of the curing unit was comparable in all composites due to their similar thermal capacity (Figure 2). Thus the tendency of the temperature values reached during light-curing (T1) to decline as the BG fraction increased practically reflects a similar pattern observed for Texotherm. The effect of various BG fractions produced more statistically significant differences for Texotherm compared to T1 due to lower data variability of the former. In any case, it is apparent that the BG fraction had an effect on the temperature rise during polymerization. Since all of the experimental composites had similar resin volume fractions (48-52%) and thus comparable amounts of C=C bonds available for conversion into single bonds, the observed effect could be explained by the tendency of the degree of conversion to decrease with the increasing BG filler fraction. This effect was identified in a preliminary study (18) and is hypothesized to originate from the inhibition of free radical polymerization by the oxides on the surface of BG particles (30).
Although excessive thermal stimuli are known to be harmful to dental pulp, no concrete critical threshold temperature value has been defined which would separate reversible from irreversible pulpal damage. Some studies on the pulpal capability to survive a thermal insult suggested the intrapulpal temperature increase of 5.5 and 11 °C (31, 32) as possible threshold values for distinguishing the irreversible from reversible pulpal damage. The validity of both of these values has been questioned in the dental literature (33) since the former study is considered methodologically flawed and in the latter the measurements were made on intact teeth and may not represent conditions encountered clinically. Therefore, as to date, the reference temperature value which would indicate irreversible pulpal damage remains unknown. Even if such a threshold value was defined, it would be very difficult to translate the temperature changes measured in composite specimens in vitro to the clinically relevant intrapulpal temperature since the influence of many factors needs to be taken into account. For example, all of the following factors play a role: remaining denting thickness at the pulpal wall, amount of the dentin and enamel surrounding the cavity, dentine water content which governs its thermal buffering capability, dentine thermal conductivity, the volume of pulpal tissue and pulpal blood flow. Since it is impossible to measure all these factors individually, the intrapulpal temperature rise occurring during the composite polymerization in a clinical setting cannot be identified. Thus, the only way to assess the potential of experimental composites to thermally damage dental pulp is through the comparison of their temperature rise to that produced by long-standing and clinically successful commercial composites (23). Since the established commercial composites are considered safe for dental pulp if handled properly (34), the temperature increase that occurred during their light-curing in vitro was used as a reference value for comparison with experimental materials.
Nano- and micro-hybrid commercial composites demonstrated similar values of temperature rise during light-curing which was significantly lower than that in the flowable composite (Figure 2). This difference can be attributed to a higher fraction of the resinous component in the flowable composite (Table 2), resulting in more C=C bonds available for the exothermic polymerization reaction. This explanation is supported by Texotherm values being twice higher in TEF compared to TEC and Gradia. Experimental composites BG-20 and BG-40 showed values of temperature rise during light-curing similar to those of the flowable reference composite, while the composites with lower BG fraction reached temperature values up to 1.8 °C higher. Although the difference was statistically significant, its clinical importance is questionable. It should be noted that temperature ranges of 6.6-14.1 °C (35) and 5.8-14.0 °C (36) were reported in studies with similar experimental setups as in the present study, while much higher temperature rise (up to 43 °C) was measured in a study using infrared scanning system (37). Thus, within the context of the literature data, the temperature rise during light-curing produced by the experimental BG-containing composites was within the range of commercial composites.
The real-time plots of temperature rise during light-curing provide information on the dynamics of temperature buildup. In experimental composites, different shapes of temperature curves were identified (Figure 3a); the temperature peaks showed a tendency to widen and shift to later times as the BG fraction increased, suggesting a slower and more gradual temperature buildup in composites with higher BG fractions. To quantify this behavior, the time at which the temperature peaks occurred was determined and presented in Figure 4. The time of temperature peaks prolonged significantly as a function of higher BG fractions. The flowable commercial composite (TEF) showed a temperature peak similar to that observed in experimental composites, unlike the nano- and micro-hybrid commercial composites (TEC and Gradia) which showed no distinct temperature peak during the light-curing but rather a gradual temperature increase throughout the curing period (Figure 3b). This difference is attributable to TEC and Gradia presenting with the lowest Texotherm of all the materials which resulted in the polymerization exotherm being masked by the more pronounced heating effect of the curing unit. Hence, a separate temperature peak was not distinguishable in these materials.
Since temperature rise due to the polymerization exotherm (Texotherm) and time needed to reach the temperature peak appeared to be systematically influenced by the BG fraction (Figure 2 and 4{ label needed for fig[@id='f4'] }), these parameters were analyzed using Pearson correlation analysis (Figure 5). The finding that both the Texotherm and the time of the temperature peak were strongly correlated to the BG fraction suggests a possible impact of BG fillers on composite polymerization. The negative correlation between the BG fraction and Texotherm could be related to the aforementioned effect of BG to affect the degree of conversion. The correlation of the time of temperature peak and BG fraction implies that composites with higher BG fractions not only released less heat but were also slower in reaching the maximum temperature values, providing more time for heat dissipation by dental hard tissues and pulpal tissue. These findings led to the rejection of the first and the second null hypotheses. However, the third null hypothesis was partially rejected since the temperature rise during curing of BG-20 and BG-40 was statistically similar to that of TEF.
Conclusions
Experimental composites with high BG fractions (20 and 40 wt. %) showed the temperature rise during polymerization similar to that of the commercial reference composite. A slightly higher temperature rise was measured in experimental composites with lower BG fractions (0-10 wt %); however, the obtained temperature values were within the range of commercial composites reported in previous studies. Thus the amount of heat released by experimental composites can be considered tolerable by dental pulp. Increasing the BG fraction in experimental composites decreased the temperature rise during curing and increased the time of temperature peak in a linear manner, suggesting a systematical effect of BG fraction on polymerization kinetics.
