Yoshikawa et al.

Asian Pac J Dent 2019; 19: 59-64

Effect of light curing method and thermal cycling on resin
composite adaptation to the cavity wall
Takako Yoshikawa, DDS, PhD (1), Alireza Sadr, DDS, PhD (1,2), and Junji Tagami,
DDS, PhD (1)(1) Department of Cariology and Operative Dentistry, Division of Oral Health Sciences, Graduate School of
Medical and Dental Sciences, Tokyo Medical and Dental University, Tokyo, Japan, and(2) Department of Restorative Dentistry, School of Dentistry, University of Washington, Seattle, WA, USA

Purpose: This study evaluated the effects of the light curing methods and thermal cycling on adaptation to thecavity wall of different type of resin composite using a dye penetration test.
Materials and Methods: Cylindrical cavities with one-half enamel and one-half dentin margins were preparedon the labial cervical region of bovine incisors. Cavities were restored using Clearfil tri-S Bond ND Quick andfilled with Clearfil AP-X or Clearfil Photo Bright composite. The resin composites were cured using theconventional or the slow-start curing method. Half of specimens were thermocycled.
Results: Clearfil AP-X showed significantly greater cavity-wall gap formation than that of Clearfil Photo Brightwith both the conventional curing method and slow-start curing method with thermal cycling at 500 cycles (p <0.05). The slow-start curing method showed significantly improved resin composite adaptation to the cavity wallcompared with the conventional curing method for thermocycled Clearfil Photo Bright specimens (p < 0.05).
Thermal cycling at 500 cycles significantly decreased cavity-wall gap formation compared with 0 cycle for
Clearfil Photo Bright resin composites (p < 0.05). Clearfil Photo Bright using the slow-start curing method withthermal cycling at 500 cycles showed least cavity-wall gap formation.
Conclusion: Light-cured composite, increased contrast ratio during polymerization with thermal cycling at 500
cycles, improved adaptation to the cavity wall using the slow-start curing method. The slow-start curing method
facilitated the high reduction for residual stress of composite that had increased contrast ratio.
(Asian Pac J Dent 2019; 19: 59-64.)
Key Words: adaptation, composite, dye penetration, polymerization, residual stress, slow-start curing method

Introduction
Resin composite polymerization results in volumetric shrinkage, and the resultant stress leads to the formation ofgaps between the resin and cavity surfaces [1,2]. Such marginal gaps and the subsequent micro-leakage may
cause marginal staining, postoperative sensitivity [3,4], and secondary caries. Light-cured resin composites arewidely used in clinical practice. However, the polymerization reaction of light-cured composites is faster thanthat of self-cured composites, which has led to the development of higher stresses during setting thanself-activated materials [5]. Therefore, the maximum stress to the interface generated at the cavity wall inlight-cured resin composite restorations is twice that of self-cured composite [6]. This stress has been shown tolead to formation of greater gaps between the resin and cavity surfaces than self-cured resin composite.
Moreover, contraction of resin composite polymerization leads to a cuspal deflection [7-9], a high-stressed toothcomposite structure [10] and residual shrinkage stresses in a resin composite restoration and restored tooth[11,12].
There are several ways to overcome the curing stresses generated by light-cured, bulk-filled resin composites.
One technique involves of using a flowable resin composite as a lining material [13,14]. The low polymerizationstress [15] contributes to improved adaptation to the cavity wall [13,14,16]. However, the poor mechanicalproperties of flowable resin composite [16,17] decrease the bond strength to the dentin wall [18]. Many
clinicians use the incremental filling technique [19]. This technique was thought to decrease the curing stresswhich occurs at the tooth-resin interface when a cavity is bulk-filled with light-cured resin composites. However,in a theoretical study using finite element analysis, it was reported that the incremental filling technique

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produced higher polymerization shrinkage effects at the restoration-enamel interface compared with bulk filling[20]. Previously, it was confirmed that incremental filling could not improve the bond strength to the floor of abox-like cavity.
Another study found that, increasing the curing velocity of light-cured resin composite decreased theadaptation to the cavity wall when a resin with a different composition was used [21]. Clearly, thepolymerization rate has a significant effect on the development of stress. With argon ion laser output, the deep
internal hardness of cured resin composites increase as the laser intensity increases, but the maximum internal
hardness is reduced with the intense light intensity [22].
The use of an intense light source may lead to more frequent marginal and wall gap formation [2,23]. A
slow-start light curing method with an initial low-intensity light followed by a high-intensity light has been used
to cure resin composite with decreased stresses. This method was found to produce excellent marginal sealing
and cavity adaptation [2,24-29]. Previous studies have reported that an initial light intensity of 270 mW/cm2 for
10 s, followed by a 5 s interval, and then a light intensity of 600 mW/cm2 for 50 s hardened the resin composite
base faster than at the surface adjacent to the light source [2,30]. This procedure, known as the slow-start curingmethod [26,27], allowed most polymerization contraction to occur during the initial flowable stage of resin
composite polymerization [2].
Bonding durability is an important factor for a long-lasting bonded restoration clinically. One of thedegradation simulated techniques is the thermal cycling test. It was demonstrated that increased destruction
occurs to bonds between tooth substrates and resin composite restorations after thermal stress [31,32]. High
number of thermal cycling decrease resin composite adaptation to the cavity wall [33]. It is hypothesized that theslow-start curing method will improve cavity wall adaptation of different components of resin composites,
thermal cycling decrease cavity wall adaptation of resin composite.

Materials and Methods
The materials, components, manufacturers, batch numbers, and bonding procedures used in this study are listed
in Table 1. An experimental quartz-tungsten halogen light-curing unit (GC, Tokyo, Japan) that was connected toa slide regulator (Type SD-135, Matsunaga MFG, Yokohama, Japan) was used. This light-curing unit had acontrol system for lamp voltage and adjustable light intensity, which was measured using a curing radiometer(model 100, Sybron Kerr, Milwaukee, WI, USA).
Cylindrical cavities with 1/2 enamel and 1/2 dentin margins, 2 mm depth, 3 mm diameter, and a C-factor of
3.7 were prepared on the labial cervical region of extracted intact erupted bovine incisors using a diamond point(# B12, GC) under copious water spray. Each of the 40 cavities was treated with the adhesive Clearfil tri-S BondND Quick (Kuraray Noritake Dental, Tokyo, Japan). After this adhesive was cured (600 mW/cm2 for 10 s) withlight guide contacted at the cavity margin, the cavities were bulk-filled with Clearfil AP-X (shade A3; Kuraray
Noritake Dental) resin composite or Clearfil Photo Bright (shade US; Kuraray Noritake Dental) resin composite.
The resin composites were light-cured using the conventional curing method (600 mW/cm2 for 40 s) or theslow-start curing method (270 mW/cm2 for 10 s + 5 s interval + 600 mW/cm2 for 30 s). The tip of the light guideplaced at the resin composite surface.
The specimens were stored in water maintained at 37˚C in the dark for 24 h. Then, the restorations were
finished and polished using wet 600-grit SiC paper, and half of specimens were thermocycled between 5˚C and60

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55˚C for 500 cycles [34] with a 30 s dwell time. The specimens were then longitudinally cut in half using a
diamond saw microtome (MC-110, Maruto, Tokyo, Japan) under running water. The dye penetration test wasused to determine the degree of adaptation to the cavity walls. This test was performed by placing a 1.0% acidred propylene glycol solution (Caries Detector, Kuraray Noritake Dental) at the cavity wall/restoration for 5 s,followed by rinsing with water and gentle blow-drying. The extent of dye penetration was observed with amesoscope (20× magnification), and a photographic image of each specimen was acquired.
Table 1 Study materials
Componentsa
Batch No.
microfiller, Bis-GMA, MDP, HEMA, dimethacrylates, ethanol,
8N0022
photoinitiator, water, sodium fluoride, others
prepolymerized organic filler, silanated silica filler, silanated colloidal silica,
450006silanated silica glass filler, Bis-GMA, TEGDMA, urethane tetramethacrylate,hydrophilic aliphatic dimethacrylate, photoinitiator, catalysts, accelerators,
pigments, others, Filler load: 82.0 wt%
Clearfil AP-X,
silanated barium glass filler, silanated silica filler, silanated colloidal silica,
6J0038shade: A3
Bis-GMA, TEGDMA, photoinitiator, catalyst, accelerator, pigments, others
Filler load: 84.5 wt%aAbbreviations: Bis-GMA, bisphenol A-glycidyl methacrylate; MDP, 10-methacryloyloxydecyl dihydrogen phosphate;
HEMA, 2-hydroxyethylmethacrylate; TEGDMA, triethyleneglycol dimethacrylatebBonding instruction / Procedures: a, apply adhesive; b, dry with gently air-blowing (5 s); c, light-cure (10 s)Material
Clearfil tri-S Bond ND
QuickbClearfil Photo Brightshade: US

In these images, the length of dye penetration along the cavity walls was measured using a digitizer (KD4300
model, Graphtec, Tokyo, Japan). Dye penetration along the cavity walls was calculated as a percentage of thetotal cavity wall length. This area was referred to as the cavity wall-resin gap. The dye penetration test scores
were compared using the Mann-Whitney U test at a significant level of 5%.

Results
The results for cavity-wall gap formation are shown in Table 2. Clearfil AP-X showed significantly greater
cavity-wall gap formation than that of Clearfil Photo Bright with both the conventional curing method andslow-start curing method with thermal cycling at 500 cycles (p < 0.05). The slow-start curing method showedsignificantly improved resin composite adaptation to the cavity wall compared with the conventional curingmethod for thermocycled Clearfil Photo Bright specimens (p < 0.05). Thermal cycling at 500 cycles significantly
decreased cavity-wall gap formation compared with 0 cycle for Clearfil Photo Bright resin composites (p < 0.05).
Clearfil Photo Bright using the slow-start curing method with thermal cycling at 500 cycles showed least
cavity-wall gap formation.
Table 2 Cavity-wall gap formation
Light-curing method
Non-thermal cycling
500-thermal cycling
Material
Clearfil AP-X
Clearfil Photo Bright
Clearfil AP-X
Clearfil Photo Bright
600 mW/cm2
54.9 (21.6) A
36.8 (6.3) B
47.5 (16.2) C
17.6 (12.0) a, A, B, C
2ABC
270 / 600 mW/cm
56.2 (15.7)
37.9 (10.6)
33.4 (6.2)
1.4 (3.1) a, A, B, C
% means with standard deviations in parentheses. Intergroup data with the same superscripted lower-case letters for eachlight curing method are significantly different (p < 0.05). Intergroup data with the same superscripted upper-case letters foreach thermal cycling number and each resin composite are significantly different (p < 0.05).

Discussion
Clearfil AP-X showed significantly greater cavity-wall gap formation than Clearfil Photo Bright with both

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conventional and slow-start curing methods with thermal cycling at 500 cycles. In a micro-computed
tomography three-dimensional (µCT-3D) visualization analysis showed that the amount of polymerization
shrinkage of Clearfil AP-X restoration was much larger than that of Clearfil Photo Bright restoration in a cavity
with adhesive [35]. A significant correlation has been reported among polymerization shrinkage, flow, and gap
formation [36]. Marginal integrity is inversely correlated with Young’s modulus of the resin composite [37], andthe marginal quality can be improved by the selection of resin composite with modulus of elasticity close to thatof dentin [38]. However, flexural modulus of elasticity is not considered to be a significant determinant of gap
formation [36]. Peutzfeldt et al. have attributed this to the fact that flexural modulus is determinant later after thegaps were formed [36]. The filler content of Clearfil AP-X is higher than that of Clearfil Photo Bright resin
composite. Higher filler content decreased the flow and increased the modulus of elasticity [16,39] and stiffness.
Therefore, large polymerization shrinkage, low flow and high modulus of elasticity of Clearfil AP-X [39] caused
greater cavity-wall gap formation compared with Clearfil Photo Bright.
The slow-start curing method significantly improved resin composite adaptation to the cavity wall comparedwith the conventional curing method for thermocycled Clearfil Photo Bright specimens. Polymerization
shrinkage that occurs after gelation or curing results in the buildup of large stresses in a resin composite [40].
Therefore, a decreased rate of development of surface hardness due to prolonged gel state and an accompanying
absence of dye penetration suggest that this protocol results in increased material flow, which provides stressrelief, despite its high elastic modulus and the photosensitivity of the resin composite [41]. Yodhikawa et al.
reported on µCT-3D visualization analysis. In that study the slow-start curing method reduced the volume ofpolymerization shrinkage to half of that produced by the conventional curing method for both Clearfil AP-X andClearfil Photo Bright resin composite restorations in the cavities with adhesive [35]. Moreover, it was reported
that the time required for complete polymerization shrinkage after the slow-start curing method was less thanhalf that required after the conventional curing method [2]. The slow-start curing method resulted in a lower
microhardness at the top surface of Clearfil Photo Bright resin composite compared with the conventional curingmethod up to 60 s from the start of curing [2]. The viscosity of resin composite until 40 s from the start of curing
was lower when initial low-intensity curing was used followed by high-intensity curing was used than when only
high-intensity curing [42]. The use of a light intensity lower than the maximum resulted in a significant decreasein post-gel contraction without significantly affecting the degree of conversion [42,43]. This irradiation allowed
for most of the resin composite polymerization shrinkage to occur during the initial flowable stage [2].
A high correlation between dye penetration test using caries detector and environmental scanning electron
microscope observation of composite-cavity wall gap has been reported [29]. The slow-start curing method
resulted in significantly decreased dye penetration, namely, better adaptation of Clearfil Photo Bright to thecavity wall compared with the conventional curing method. Previous studies have shown that, when Clearfil
Photo Bright resin composite is light-cured using the slow-start curing method, the resin hardens faster at thebottom surface than at the top surface [2,30]. Light transmission through light-cured resin composites is strongly
affected by the opacity and shade of the resin, and opacity changes during polymerization. The opacity of theresin composite is indicated by the mismatched refractive index between the matrix and filler [44] or the contrast
ratio [45]. Optimizing the mismatched filler/resin refractive index increases the curing depth [44]. The contrast
ratio decreases as the transparency of the resin composite increases. The ratio is equal to 1 for a completely
opaque material and ranges between 0 and 1 for a translucent material [45]. Most resin composite materials tend

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to show a decrease in the contrast ratio during polymerization. However, the contrast ratio of Clearfil Photo
Bright increases during polymerization (increased opacity), where as that of Clearfil AP-X decreases during
polymerization (increased transparency) [35]. Therefore, the delay in hardening of Clearfil Photo Bright,
particularly at the top surface [2,30], may decrease curing stresses and allow more time for relief.
The rate of curing for resin directly adjacent to the cavity wall may be enhanced by free radicals that are
already present in the bonding resin. An initial low-intensity light may enhance the polymerization rate at thislocation, rather than at the resin surface. The process of polymerization is then completed by high-intensity
radiation, which allows for more uniform curing throughout the bulk of the resin composite. Therefore, most of
the polymerization shrinkage occurs during the initial flowable stage of the resin composite polymerization. This
allow the resin to flow freely and presents it from pulling away from the cavity walls [2,24-29,35].
Thermal cycling at 500 cycles significantly decreased cavity-wall gap formation for Clearfil Photo Brightresin composites. Clearfil Photo Bright using the slow-start curing method with thermalcycling at 500 cycles
showed least cavity-wall gap formation. The conventional curing method causes more internal stress in resin
composite restoration than the soft-start curing method [25]. The slow-start curing method allows for more
uniform curing throughout the bulk of the resin composite [2,30], especially for increased contrast ratio (Clearfil
Photo Bright) during polymerization [35]. Therefore, the slow-start curing method seems to reduce residual
stress especially for Clearfil Photo Bright resin composite restoration. It is thought that residual stress of resin
composite is released during thermal cycling. When residual stress is high, high resin composite gap formation
to the cavity wall occurs during thermal cycling. When residual stress is low, high resin composite adaptation to
the cavity wall occurs because of accelerated adhesive polymerization with thermal cycling at 500 cycles. It
appeared that Clearfil Photo Bright using the slow-start curing method with thermal cycling at 500 cycles
showed the highest cavity wall adaptation.
Acknowledgment
This work was supported by Grant-in-Aid for Scientific Research (C) No. 25462950 and 16K11543 from the Japan Society for the
Promotion of Science.
Conflict of Interest
This study was funded by Kuraray Noritake Dental Inc.

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Correspondence to:Dr. Takako Yoshikawa
Department of Cariology and Operative Dentistry, Division of Oral Health Sciences,
Graduate School of Medical and Dental Sciences, Tokyo Medical and Dental University (TMDU)
1-5-45, Yushima, Bunkyo-ku, Tokyo 113-8549, Japan
Fax: +81-3-5803-0195 E-mail: yoshikawa.ope@tmd.ac.jp
Accepted September 21, 2019.
Copyright ©2019 by the Asian Pacific Journal of Dentistry.
Online ISSN 2185-3487, Print ISSN 2185-3479

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