Introduction

Light-cured resin composites are essential in modern dentistry, providing a versatile option for restorations in both anterior and posterior teeth [1]. Their strong adhesion to dental tissues and natural tooth-colored appearance make them a preferred choice for both patients and dental professionals [2]. A restoration’s durability is largely determined by how effectively monomers transform into polymers, measured by a monomer conversion. Conversion of monomers to polymers is a crucial factor influencing the final physical, mechanical, and biological pro­perties of light-cured composite resins [3]. To determine the degree of conversion, one must compare the proportion of unreacted aliphatic carbon-carbon double bonds in a hardened sample against the overall carbon-carbon bonds. In gene­ral, photoactivated dental composites achieve a degree of conversion between 43% and 75%, largely influenced by factors, such as composite composition, irradiation intensity, and exposure duration [4].

To improve clinical efficiency and mechanical performance, researchers have developed bulk-fill compo­sites that can be applied in a single layer of 4 to 5 mm thickness. These composites ensure effective curing and maintain mechanical properties in layers exceeding 2 mm through several mechanisms: (1) booster photoinitiators, such as benzoyl germanium derivatives, which enhance photocuring activity, leading to a higher polymerization rate and greater depth of cure; (2) poly­merization modulators, including high molecular weight and urethane-based dimethacrylate monomers, which help minimize shrinkage stress; (3) improved flowability for better adaptation; and (4) increased translucency, compared to conventional composites, achieved through fillers of mixed oxide with an optical index corresponding to resin matrix, or incorporation of glass fibers that enhance the permeability of light [5].

The effectiveness of these materials, however, depends not only on their composition but also on a light-curing protocol used. The light-curing unit and irradiation time are crucial for achieving an optimal degree of conversion. A 20 seconds curing time is typically recommended for a 2 mm thick composite layer, a practice that remains widely used in clinical settings [6].

Photopolymerization has been extensively studied over the years, with a focus on reducing the photoactivation time of composite resins. A recent approach involves increasing the irradiance of light-curing units, enabling a 3 seconds exposure to produce results comparable to 40 and 60 seconds activation times. The approach being explored relies on exposure reciprocity, a principle suggesting a reduction in necessary photon dose delivery time through a notable increase in light irradiance or the number of photons per second [7].

In the past, studies often concentrated on resin composites incorporating type II photoinitiators, namely camphorquinone (CQ). More recently, the Stansbury-Bowman team’s research has proposed that based on particular mechanistic characteristics, exposure reciprocity is potentially attainable using type I photoinitiators, such as Ivocerin and Lucerin TPO [8].

Despite these advances, limited data exist comparing the degree of conversion of various bulk-fill composites when cured using a high-irradiance curing light, parti­cularly across different viscosities and filler types. Hence, the present study aimed to evaluate and compare the poly­merization rate of four different bulk-fill composites polymerized with high-intensity curing light under in vitro conditions.

Objectives

The objectives of the study were to assess polymeri­zation rate of bulk-fill composites at the superficial and deep layers as well as to compare the polymerization rate of bulk-fill composites at different time durations of curing using a high-intensity light-curing unit.

The null hypothesis was that there is no significant difference in the degree of conversion when curing at a high intensity at different time intervals on bulk-fill resin composites.

Material and methods

Sample size

Sample size was determined using nMaster software (version 2.0), with four groups to be compared, requiring 45 samples per group, resulting in a total sample size of 180.

The following formula was applied:

n ={2}/{δ²} (σ_b² + {σ²}/{k} (z_{α/2} + z_{β})²,

where σ_b² is between group variance, σ² is within group variance, δ is the effect size, k is the number of repeated measures, a is the significance level, 1-β is the power, and between group variance = 5.85, within group variance = 1.2, effect size = 0.8, number of repeated measurement = 1, power (%) = 80, a error (%) = 5, sided = 2, and required sample size = 180.

Experimental procedure

All samples were prepared in a custom mold with 5 mm height and 3 mm diameter. To create a smooth exterior on the composite resin samples, a mylar strip was positioned on top of them, and a glass plate was gently applied with slight pressure. These samples were grouped based on the type of composite used, and were subjected to a high-intensity curing light of 2300 mW/cm² (Woodpecker iLED Plus Curing Light) at different curing times of 1, 3, and 6 seconds. After curing, the samples were placed in a dark environment until the Fourier transform infrared attenuated total reflectance (FTIR-ATR) analysis was performed with the Bruker ALPHA II Compact FTIR spectrometer.

Four primary groups were established for the samples, and were subdivided into three individual subgroups:

• Group I (n = 45): Beautifil-Bulk Restorative (Shofu Inc., Kyoto, Japan);
• Group II (n = 45): Beautifil-Bulk Flowable (Shofu Inc., Kyoto, Japan);
• Group III (n = 45): Tetric N-Ceram Bulk Fill (Ivoclar Vivadent, Zurich, Switzerland);
• Group IV (n = 45): Tetric N-Flow Bulk Fill (Ivoclar Vivadent, Zurich, Switzerland);
• Subgroup A (n = 15): Samples cured for a duration of 1 second;
• Subgroup B (n = 15): Samples cured for a duration of 3 seconds;
• Subgroup C (n = 15): Samples cured for a duration of 6 seconds.

The groups and subgroups are presented in Table 1.

Analysis of degree of conversion

Following a 24-hour period, FTIR-ATR analysis was performed on the samples. The resulting FTIR spectra from both the surface and internal layers of the hardened samples, along with uncured samples were evaluated to calculate the resin’s degree of conversion.

For the calculation of monomer conversion, the absorbance intensities of aliphatic carbon-carbon bonds (with a peak at 1638 cm^–1) were measured relative to a reference standard, before and after curing, with aromatic carbon-carbon bonds (peak at 1608 cm^–1) used as a reference point, and then compared. The conversion of monomers to polymers was then calculated by subtracting the proportion of carbon-carbon bonds that remained from 100% [9, 10], as follows:

Degree of conversion % =

(1 – cured(area under 1638/area under 1608)/uncured(area under 1638/area under 1608)) × 100

Statistical analysis

Descriptive statistics, such as mean and standard deviation (mean ± SD) or frequency were used to summarize the collected data. Normality was checked using Shapiro-Wilk test. Data were analyzed using ANOVA test for both bottom and top surfaces of all 4 groups as well as between subgroups that tested the degree of conversion between curing a composite for 1, 3, and 6 seconds. Tukey post-hoc test was then performed for groups showing a significant difference. T-test for equality of means was also conducted for groups I, II, III, and IV across the 3 subgroups (A, B and C) to analyze the differences between the superficial and deep surfaces of individual subgroups. A p-value less than 0.05 indicated a statistically significant result. Statistical data were processed using SPSS software version 23.0 (Armonk, New York: IBM Corp).

Results

Comparison between top and bottom surfaces

On comparison of the degree of conversion for groups I, II, III, and IV with subgroups A, B, and C cured at the top and bottom surfaces for 1, 3, and 6 seconds, statistically significant differences for most materials were observed. In Beautifil Bulk Restorative, Tetric N Flow Bulk Fill, and Tetric N Ceram Bulk Fill, the top surfaces showed a higher degree of conversion than the bottom surfaces (Table 2).

Effect of curing times on degree of conversion

Analysis for groups I, II, III, and IV with subgroups A, B, and C cured at the top and bottom surfaces for 1, 3, and 6 seconds showed significant differences between most curing time intervals. A higher degree of conversion was noted in composites that were cured for a duration of 6 seconds followed by 3 seconds, while 1 second curing showed the lowest degree of conversion (Table 3).

Differences among composite materials

For groups I, II, III, and IV with subgroups A, B, and C to compare the degree of conversion at different curing times, Beautifil Bulk Restorative and Beautifil Bulk Flowable demonstrated that 6 seconds curing demonstrated the highest degree of conversion for both the top and bottom surfaces (Table 4).

Discussion

The extent of polymerization, measured by the degree of conversion, significantly influences both the physical and mechanical attributes of dental composites. Degree of conversion is a crucial physical property of composite resins, playing a key role in the comparison and characterization of restorative materials. Composi­tes with a high degree of conversion exhibit superior mechanical strength, elevated mechanical resilience, improved color stability, and enhanced biocompatibility, all of which enhance the durability of restoration [11].

Several features, categorized into extrinsic and intrinsic factors, influence photopolymerization. Extrinsic factors include the light-curing method, light spectrum, positioning of the curing light tip, and light-curing protocols. Whereas intrinsic factors involve the percentage of filler used (percentage and particle size), type of photoinitiator, and the composition and proportion of comonomers [12]. Two key monomer characteristics influence the polymerization rate, i.e., viscosity and flexibility of chemical structure. Research indicates that the degree of conversion differs among monomer systems, increasing in the following order: Bis-GMA < Bis-EMA < UDMA < TEGDMA, with TEGDMA exhibiting the highest and Bis-GMA the lowest degree of conversion [13]. Papadogiannis et al. [14] reported that bulk-fill composites containing UDMA and TEGDMA, without Bis-GMA, demonstrated the highest degree of conversion. This is attributed to the UDMA’s lower viscosity compared to Bis-GMA, which enhances polymerization efficiency.

The principle of exposure reciprocity states that the overall radiant exposure (J/cm²) is determined by multi­plying irradiance (W/cm²) by exposure duration (seconds). This allows an increased intensity of irradiance to be used for a shorter duration of time to produce a high radiant exposure [15]. In this study, a high-intensity light-curing unit (Woodpecker iLED) was used, which operates in two modes: turbo mode (2300-2500 mW/cm²) and normal mode (1000-1200 mW/cm²). The manufacturer recommends curing times of 1 and 3 seconds in turbo mode, while normal mode allows for curing durations of 5, 10, 15, and 20 seconds. This light-curing unit was chosen for its high intensity of 2300-2500 mW/cm², enabling rapid curing of the composite layer. The physical pro­perties of light-cured composites can vary depending on the distance from the irradiated surface. To minimize potential interference in this study, the distance between the light guide tip and the samples was standardized by placing the light guide tip directly on a mylar strip, which was positioned over the composite [16].

Our study evaluated and compared the degree of conversion of four bulk-fill resin composites, including Beautifil Bulk Restorative, Beautifil Bulk Flowable, Tetric N Ceram Bulk Fill, and Tetric N Flow Bulk Fill. These materials were cured at both the top and bottom surfaces for 1, 3, and 6 seconds. The results suggests that for Beautifil Bulk Restorative and Tetric N Ceram Bulk, a longer exposure time improve the degree of monomer conversion in these composites, with the top surfaces having a higher value than the bottom surfaces. Their higher filler content and lower translucency likely need more light to achieve optimal polymerization at greater depths. In contrast, Beautifil Bulk Flowable showed similar values between the top and bottom surfaces. This could be due to its lower filler content and higher translucency, which allow enough light to pass through even with shorter curing times, leading to less variation among exposure durations.

The differences between shorter (1 second) and longer (6 seconds) curing intervals highlight that the exposure time is crucial for conversion, even with high-irradiance curing protocols. While the concept of exposure reciprocity suggests that higher irradiance can offset shorter exposure times, this study indicates that this may not apply equally across all composite formulations. Materials that are opaquer or have a higher filler concentration seem less responsive to very short curing times, which could compromise polymerization at the bottom surface.

Overall, these findings show that even with improvements in photoinitiator technology and light-curing devices, proper polymerization of bulk-fill composites, especially at deeper levels, still relies on sufficient exposure time. While ultrafast curing might be clinically convenient, it may not consistently achieve optimal conversion for all materials. Therefore, it is important to maintain prolonged or manufacturer-recommended curing durations to ensure the mechanical stability and longevity of restorations.

In a 2024 study, Ribeiro et al. [17] examined the use of a 3-second, high-intensity curing light unit on bulk-fill composites. They reported that the 3-second, high-irradiance protocol yielded no significant difference in the degree of conversion values for two of the composites when compared to the lower-irradiance standard mode, while the remaining composites exhibited a difference. Our study also demonstrated that monomer conversion was greater on the superficial surface compared to the deep surface for all of the tested resin-based composites.

Similarly, a 2024 study by Sampaio et al. [18] eva­luated the polymerization rate of bulk-fill composites at varied curing times using a high-intensity curing light. The study reported a decrease in the degree of conversion from the top to the bottom surfaces across all tested materials.

The necessity for alternative photoinitiators that can be activated by the violet spectrum has been previously emphasized in research, showing the need for improved polymerization efficiency and depth of cure in resin-based composites [19]. The violet spectrum has limited penetration depth in resin-based composites because most of the violet light is absorbed in the upper layers, reducing the amount of light that reaches deeper regions and potentially affecting polymerization at greater depths [20]. Another reason for the reduced effectiveness of violet light is its interaction with inorganic filler particles in resin-based composites. According to the Rayleigh scattering law, shorter wavelengths, such as violet light, are more prone to scattering when they encounter filler particles. As a result, the violet spectrum experiences greater light attenuation compared to the blue spectrum, further limiting its penetration depth [21].

The characteristics of resin composites are significantly influenced by both the photoinitiator type and filler content, even when they are cured with equivalent radiant exposure achieved through adjustments in irradiance and/or exposure time. While the principle of exposure reciprocity suggests a predictable outcome based on the time and intensity, practical findings occasionally reveal that equivalent material transformations and curing depths result from varying application procedures.

The monomer conversion achieved in composites is significantly modulated by the monomer matrix composition, owing to the inherent disparities in monomeric viscosity, reactivity, and polymerization kinetics [13]. Research demonstrates that UDMA-based resin composites tend to exhibit superior final polymerization levels compared to those formulated with Bis-GMA.

This can be attributed to UDMA’s relatively high molecular weight (470.0 g/mol vs. 510.6 g/mol for Bis-GMA), its higher concentration of double bonds (4.25 mol/kg vs. 3.90 mol/kg for Bis-GMA), and its significantly lower viscosity (23.1 Pa·s vs. 1200 Pa·s for Bis-GMA). These properties allow UDMA-based composites to undergo more efficient polymerization [13]. To achieve greater polymerization rates, construct robust, and tightly bound polymer structures, Bis-GMA is frequently combined with UDMA or TEGDMA in a copolymerization process. In our study, it was observed that group II (Beautifil Bulk Flowable) exhibited a greater degree of conversion, followed by group IV (Tetric N Flow Bulk Fill), indicating that the formulation and composition of these materials contribute to improved polymerization efficiency.

In the study by Feng and Suh [22], it was concluded that the relationship described by the exposure reciprocity law is reliable when the Bis-GMA/TEGDMA ratio is 60/40 or more, measured by mass. However, when the ratio falls below 40/60 mass %, the law no longer applies, suggesting that the composition of the monomer matrix significantly influences the polymerization behavior and light-curing efficiency. Beun et al. [23] proposed that the substantial surface interaction between fillers and resin results in a significant localized thickening of a material. It was observed that CQ-based composites with fillers, when exposed to highest irradiance, experience a notable rise in conversion, implying that the presence of fillers may facilitate more effective polymerization with strong light sources.

It was seen that both groups II and IV showed a higher polymerization rate than groups I and III, suggesting that the low-viscosity bulk-fill composites achieve an elevated degree of conversion compared to high-viscosity counterparts. Low-viscosity bulk-fill resin-based composites demonstrate enhanced polymerization capacity in deeper restorations compared to regular-viscosity composites. This is attributed to the greater monomer mobility in the less viscous matrix, allowing for improved polymerization. In contrast, the higher filler content in regular-viscosity composites impedes light penetration, potentially limiting the depth of cure [24].

The null hypothesis stating that there is no significant difference in the degree of conversion when curing at a high intensity at different time intervals on bulk-fill resin composites was rejected, as in most of the group comparisons a significant difference in degree of conversion was observed. This change was primarily seen between the 1 and 6 seconds subgroups of all the composites. This shows that by using a shorter curing duration of 6 seconds with a high irradiance of around 2300 mW/cm², satisfactory levels of degree of conversion in bulk-fill composites can be achieved.

The limitations of this study include the potential for high irradiance to increase polymerization shrinkage in bulk-fill composites; however, the effects of high irradiance on polymerization shrinkage were not eva­luated and should be investigated in future studies. Additionally, the degree of conversion achieved with high irradiance (2300 mW/cm² for 3-6 seconds) was not compared to that obtained using standard irradiance (800 mW/cm² for 20 seconds), limiting conclusions regarding the comparative effectiveness of ultrafast curing protocols with standard protocols. The influence of different shades of bulk-fill composites on the degree of conversion was also not evaluated, as variations in shade may affect light penetration. Future studies should address these aspects to provide a more comprehensive understanding of factors influencing polymerization in bulk-fill composites.

Conclusions

The top surfaces of all tested composites showed higher degrees of conversion than the bottom surfaces. The bulk-fill composites cured for 6 seconds at high irra­diance achieved greater conversion than those cured for 1 second. Low-viscosity composites exhibited higher conversion values than high-viscosity types. These findings indicate that high-intensity curing lights can achieve adequate polymerization within 1-6 seconds, supporting their potential for efficient clinical use.

Disclosures

Author contributions: Conceptualization: K.K.L., A.S.H.; Methodology: K.K.L., H.S.K.; Investigation and data collection: K.K.L., H.S.K.; Formal analysis: K.K.L., H.S.K.; Data curation: K.K.L.; Writing of original draft: K.K.L., H.S.K.; Writing – review and editing: A.S.H., K.K.L., H.S.K.; Supervision: A.S.H.; Project administration: A.S.H. All authors have read and agreed to the published version of the manuscript.

Funding: This research received no external funding.

Institutional Review Board statement: Not applicable.

Informed consent statement: Not applicable. This in vitro study was based exclusively on previously published literature and did not involve human participants, primary data collection, or direct patient contact.

Data availability statement: All the data generated or analyzed in this study are included in this manuscript. The data generated in this study may be requested from the corresponding author.

Acknowledgments: None.

Conflicts of interest: The authors declare no conflicts of interest.

AI use statement: Portions of this manuscript were assisted by OpenAI’s ChatGPT (version 4), utilized for language editing and structural refinement. The authors reviewed and verified all content to ensure accuracy and integrity. The final manuscript was approved by all authors, who take full responsibility for its content.

References