Polymerization behavior of bulk-fill and hybrid-type resin composites using different light curing mode

Abstract

Purpose: This study aimed to investigate if the hardness ratio of the bulk-fill-type resin composite is higher than that of the hybrid-type resin composite, to determine when the radiant exposure was the same, and to investigate if the polymerization of resin composites at the top and bottom surfaces and the hardness ratio is affected by differences in irradiance light sources. Methods: The light-curing unit used was the LED G-Light Prima-II Plus (blue + violet LED) light-curing unit. The light-cured resin composites were hybrid-type Clearfil AP-X and bulk-fill flowable-type Gracefil BulkFlo. Composite specimens with 2-mm-thick were polymerized in Teflon molds using radiant exposure of 24 J/cm2. The light-curing methods were 1,100 mW/cm2 for 22 s and 600 mW/cm2 for 40 s. Just after light-curing, the Knoop hardness was measured at the top and bottom surfaces of each specimen using a hardness tester, and the hardness ratio was calculated. Results: The bulk-fill flowable type resin composite, Gracefil BulkFlo, showed a lower hardness ratio than the hybrid-type resin composite, Clearfil AP-X. When radiant exposure was the same, 600 mW/cm2 irradiance light created more uniform polymerization of the resin composite than 1,100 mW/cm2 irradiance light for both bulk-fill and hybrid-type resin composites. Conclusion: It was suggested that curing with an irradiance of 600 mW/cm2 for 40 s created more uniform polymerization for both materials.

Introduction

It has been reported an intense light source cause more frequent marginal and wall gap formation [1,2,3]. Furthermore, high irradiance of up to 2,000 mW/cm² results in heat generation, which may damage the pulp [4,5,6]. Moreover, it was reported that the homogenous polymerization of resin composites improved resin cavity adaptation [2]. Thus, the polymerization rate has a significant effect on strain development. It has been reported that the maximum flexural strength and modulus of light-cured resin composites are obtained by intermediate irradiance at the same radiant exposure [7].

However, the polymerization depth of hybrid-type resin composites is approximately 2 mm [8]; so, an incremental filling technique, which is widely used by many clinicians, was introduced [9,10]. It is believed to reduce the curing stress at the tooth resin interface that occurs when a cavity is bulk-filled with light-cured resin composites. However, it was reported in a theoretical study using finite element analysis methods that incremental filling techniques could produce higher polymerization shrinkage stress at the restoration-enamel interface than bulk filling techniques [11]. Moreover, it was demonstrated that incremental filling was unable to improve the bond strengths in a cavity floor of box-like cavities compared with bulk filling methods [12]. Thus, bulk-fill resin composites, which were introduced for placement as a single increment up to 4-5 mm depth, were developed [13].

The soft-start, pulse activation, and slow-start light-curing methods have been shown in decreasing curing stress and improve cavity wall adaptation [2,3,14,15,16,17,18,19]. It was reported that when a composite was light-cured via the slow-start curing method, the light-cured composite hardened faster at the cavity base than at the surface [2,12], reduced the volumetric shrinkage of composite restoration in the cavity [19], and improved adaptation to the cavity [2,3,17,18,19]. This indicates that homogenous polymerization of resin composites helped improve resin composite adaptation to the cavity wall [2].

Therefore, the measurement of resin composite hardening is critical for the assessment of resin composite adaptation to the cavity wall. Resin microhardness is an indicator of the degree of conversion [20,21], and Knoop hardness (KHN) has been found to highly correlate with infrared spectroscopy assessment [21]. The hardness ratio is calculated as the ratio of KHN of the bottom surface/KHN of the top surface [22]. Radiant exposure is an important factor in the degree of conversion of the light-cured resin composite. Radiant exposure (J/cm²) is calculated as irradiance (W/cm²) multiplied by irradiation time (s). It has been reported that similar conversion and material properties were obtained with similar radiant

Table 1 Study materials

Material	Components	Batch No.	Manufacturer
Clearfil AP-X (AP) shade: A3	silanated barium glass filler, silanated silica filler, silanated colloidal silica, Bis-GMA, TEGDMA, photoinitiator, catalyst, accelerator, pigments, camphorquinone, additional photo initiator, others, Filler load: 84.5 wt%	CHO132	Kuraray Noritake Dental
Gracefil BulkFlo (BF) shade: U	Bis-MEPP, dimethacrylate, barium glass filler, silicon dioxide filler, colorant, camphorquinone, additional photo initiator, Filler load: 70 wt%	2103121	GC

Bis-GMA, bisphenol A-glycidyl methacrylate; TEGDMA, triethyleneglycol dimethacrylate

exposure [23,24,25]. However, close examinations have shown that the irradiance and irradiation time independently influence the degree of conversion and mechanical properties [7,26,27].

This study aimed to test the hypothesis that the hardness ratio of the bulk-fill-type resin composite is higher than that of the hybrid-type resin composite and to test the hypothesis that when the radiant exposure is the same, the polymerization of resin composites at the top and bottom surfaces and the hardness ratio is not affected by differences in irradiance light.

Materials and Methods

The materials, components, manufacturers, and batch numbers used in this study are listed inTable 1. The light-curing unit used was the LED (blue + violet LED) light-curing unit (G-Light Prima-II Plus, GC Corp., Tokyo, Japan) connected to a slide regulator. The irradiance of the LED light-curing unit was measured using a radiometer (L.E.D. Radiometer, Demetron/Kerr, Orange, CA, USA).

Two types of resin composites were polymerized using the following two light-curing methods; (1) 1,100 mW/cm² (light tip-resin distance: 0 mm) for 22 s; (2) 600 mW/cm² (light tip-resin distance: 6 mm) for 40 s. The radiant exposure for both light-curing methods was 24 J/cm². Hybrid-type Clearfil AP-X (AP, shade A3, Kuraray Noritake Dental Inc., Tokyo, Japan) and bulk-fill-flowable-type Gracefil BulkFlo (BF, shade U, GC Corp.) resin composites were placed in a Teflon mold (6 mm diameter and 2 mm deep) with 27-μm-thick polyethylene strips at the bottom surface. The composite was covered with 27-μm-thick polyethylene strips and slide glass to prevent the formation of an oxygen-inhibited layer. Then, the resin composite was polymerized using the two curing methods with 24 J/cm² as described above. Immediately after the completion of the light-curing process, Knoop hardness measurements were taken from the top and bottom surfaces using a load of 100 g and a dwell time of 15 s (Hardness Tester, model MVK-E, Akashi, Tokyo, Japan). The microhardness was measured for the 24 resin specimen surfaces, after which the hardness ratio [22] was calculated. All experiments were performed at room temperature (23 ± 2˚C) and 50 ± 10% humidity. Knoop hardness results ( n = 6) and hardness ratios were compared and analyzed using the Bonferroni/Dunn test at a significance level of 5% using StatView 5.0 software (SAS Institute Inc., Cary, NC, USA).

Results

Knoop hardness results for the top and bottom surfaces of resin specimens and their statistical comparisons are shown inTable 2. Hardness ratio results and their statistical comparisons are shown inTable 3.

Immediately after light-curing, the KHN for the bottom surfaces of the AP and BF resin composites were significantly lower than that for their top surfaces when cured using 1,100 mW/cm² for 22 s ( P < 0.05) There were no significant differences between the KHN for the top and bottom surfaces of AP and BF when cured using 600 mW/cm² for 40 s ( P > 0.05). BF exhibited a hardness ratio significantly lower than that of AP when cured using both 1,100 mW/cm² for 22 and 600 mW/cm² for 40 s ( P < 0.05). AP with 600 mW/cm² for 40 s showed the highest hardness ratio in all groups ( P < 0.05). BF with 1,100 mW/cm² for 22 s resulted in the lowest hardness ratio in all groups ( P < 0.05). For AP and BF, exposure to 1,100 mW/cm² for 22 s resulted in a hardness ratio significantly lower than that produced by exposure to 600 mW/cm² for 40 s ( P < 0.05).

Discussion

The KHN at the top surface of AP when cured with 1,100 mW/cm² for 22 s was significantly higher than that produced when cured with 600 mW/cm² for 40 s. The Knoop hardness at the bottom surfaces of resin composites was significantly lower than that of the top surfaces for AP and BF resin composites when cured using 1,100 mW/cm² for 22 s. Light-cured composites are usually polymerized at the resin surface near the light source. Therefore, the microhardness at the top surface of the resin composite is typically significantly higher than that at the bottom surface [8,22,28].

Table 2 Knoop hardness testing results

		Hardness: KHN mean (SD)
Curing mode/ Material		1,100 mW/cm2 22 s	600 mW/cm2 40 s
AP	Top	55.7 (1.3)a	52.0 (1.8)
	Bottom	48.8 (1.3)a	52.6 (1.8)
BF	Top	28.9 (0.5)b	29.7 (1.1)
	Bottom	23.3 (1.1)b	28.7 (0.9)

Intragroup data designated with same superscript lowercase letters for each top and bottom Knoop hardness are significantly different ( P < 0.05).

Table 3 Hardness ratio of resin composites

	Hardness ratio: Mean (SD)
Curing mode/ Material	1,100 mW/cm2 22 s	600 mW/cm2 40 s
AP	0.88 (0.02)A	1.01 (0.01)A,B
BF	0.81 (0.03)A,B	0.86 (0.01)B

Intergroup data designated with same superscript uppercase letters for each hardness ratio are significantly different ( P < 0.05).

BF showed a lower hardness ratio than that of AP using both 1.100 mW/cm² for 22 s and 600 mW/cm² for 40. A hardness ratio near 1 suggests a similar level of polymerization at the top and bottom surfaces of the resin composite. It is typically assumed that there is uniform polymerization in the resin composite. It was believed that the bulk-fill flowable resin composite depth of cure was higher than that of the hybrid-type resin composite. However, the result of this study supported the finding that the bulk-fill resin composite, Tetric Bulk-fill, showed significantly lower KHN at the bottom surface than at the top surface of 2-mm thick specimen [29]. Moreover, it was reported that three bulk-fill resin composites showed significantly lower KHN at the 4-mm bottom than at the 2-mm bottom [29]. These findings suggest that the polymerization of some bulk-fill resin composites at the bottom is inferior to that of the hybrid-type resin composite even, with the 2-mm-thick resin specimen.

Moreover, the incremental filling technique using the hybrid-type resin composite, Filtek Supreme Ultra Universal Restorative, produced better internal adaptation than the three bulk-fill resin composites [30]. The bulk-fill resin composite demonstrated similar marginal gap formation to conventional hybrid-type resin composite restorations [31].

AP and BF resin composites, when cured via exposure to 1,100 mW/cm² for 22 s, resulted in a significantly lower hardness ratio than when cured with 600 mW/cm² for 40 s. The radiant exposure was 24 J/cm². Exposure to 600 mW/cm² for 40 s resulted in a hardness ratio near 1 for AP and BF resin composites. It was demonstrated that when the irradiance is increased, the degree of conversion decreases linearly, and different curing methods may result in polymer networks with different cross-link densities [7]. Moreover, it was reported that irradiation durations shorter than 10 seconds increase the risk of insufficient polymerization at the bottom surface of a resin composite increment, even when using a high-power LED lamp [32]. A higher concentration of radicals stops the cross-linking reaction earlier and leads to the formation of short-chain molecules [15]. That is why a high irradiance light of 1,100 mW/cm² leads to the low polymerization of the resin composite at the bottom surface, even if the 1,100 mW/cm² irradiance was the regular irradiance of this LED light-curing unit. For AP and BF, there were no significant differences between the Knoop hardness for the top and bottom surfaces using 600 mW/cm² for 40 s. This result supports the previous finding that optimal irradiance leads to maximum hardness [33] and mechanical properties; i.e., the flexural strength and modulus of the resin body [7]. Moreover, the use of bottom/top ratios for both hardness and conversion resulted in a linear relationship that is independent of the filler size or filler loading [34].

The polymerization of bulk-fill flowable-type resin composite, Gracefil BulkFlo, was inferior to that of the hybrid-type resin composite, Clearfil AP-X. It was suggested that curing with an irradiance of 600 mW/cm² for 40 s created more uniform polymerization of the resin composites than curing with an irradiance of 1,100 mW/cm² for 22 s for both the hybrid-type and bulk-fill flowable-type resin composites.

Author Contributions

TY contributed to conceptualization, data curation, formal analysis, methodology and writing – original draft. AS contributed to validation and writing – review and editing. YS contributed to validation and writing – review and editing. All authors read and approved the final version of the manuscript.

Conflicts of Interest

This study was funded by Kuraray Noritake Dental Inc.

Data Availability Statement

All data generated or analyzed during the current study are available from the corresponding author on reasonable request.

Acknowledgments

This work was supported by Grants-in-Aid for Scientific Research (C) No. 25462950 and No. 16K11543 from the Japan Society for the Promotion of Science.

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