Yoshikawa et al.

Asian Pac J Dent 2014; 14: 13-18

Effects of light curing method on resin composite adaptation to
the enamel and dentin cavity wall
Takako Yoshikawa, DDS, PhD,a Makoto Morigami, DDS, PhD,b Alireza Sadr, DDS,
PhD,c and Junji Tagami, DDS, PhDaa

Cariology and Operative Dentistry, Department of Oral Health Sciences, Graduate School, Tokyob
Medical and Dental University (TMDU), Department of Dentistry, Toranomon Hospital, Tokyo, andcInternational Exchange Center, Tokyo Medical and Dental University (TMDU), Tokyo, Japan
Purpose: This study aimed to evaluate the effects of light-curing method on the marginal seal and resin
composite adaptation to the cavity wall, and the formation of enamel cracks around resin composite restorations.
Materials and Methods: Cylindrical cavities were prepared on the buccal or lingual cervical region of humanmolars. The teeth were restored with Clearfil Liner Bond 2 V and filled with Clearfil Photo Bright or Palfique
Estelite resin composite. The resins were cured with the conventional light-curing method (600 mW/cm2 for 60s) or the slow-start curing method (270 mW/cm2 for 10 s+5-s interval+600 mW/cm2 for 50 s). After thermal
cycling, the specimens were subjected to a dye penetration test with a caries detector.
Results: Light curing with the slow-start curing method resulted in a complete dentin marginal seal with both
Clearfil Photo Bright and Palfique Estelite resin composites. This method also resulted in significantly better
cavity wall adaptation of both resin composites compared with that achieved by the conventional method(p<0.05). The conventional method resulted in a poor dentin marginal seal and poor wall adaptation of bothresin composites. Enamel crack formation was observed in all specimens.
Conclusion: The slow-start curing method improved the marginal sealing and resin composite adaptation to thecavity wall. However, enamel crack formation could not be avoided, even with the slow-start curing method.
The dye penetration test using a caries detector can be used to evaluate resin composite adaptation to the cavity
wall both in vitro and in vivo.
(Asian Pac J Dent 2014; 14: 13-18.)
Key Words: cavity wall adaptation, dye penetration test, marginal sealing, resin composite

Introduction
Adhesive dentistry is an important aspect of dental practice, particularly because of recent increases in casesof abrasion and erosion attributed to an aging society, eating disorders, and reflux esophagitis.

Teeth withabrasion and erosion cannot be treated using metal inlays placed for conventional Black’s cavities; such teeth
can be restored only using resin composites with a direct bonding technique.
However, resin composite polymerization results in volumetric shrinkage, and the stress created leads togreater gap formation between the resin and cavity wall.1-4

Such marginal gaps and subsequent microleakage

5

may cause marginal staining, postoperative sensitivity, and secondary caries.

In addition, cavity-wall gap

formation may lead to pain on biting and failure of adhesion after repeated occlusal loading.

Thepolymerization reaction of light-cured composites is rapid, which leads to the development of higher stresses in
the cured materials compared with that in self-activated materials.6

Furthermore, the maximum interfacial

stress generated at the wall of cavities filled with light-cured composites is twice that generated with self-cured
composites.7

Therefore, this stress lead to greater gap formation between the resin and cavity walls whenlight-cured materials are used compared with that when self-cured materials are used.1

On the other hand,when the bond strength exceeds polymerization shrinkage stress, a crack is initiated in the tooth structure,
usually the enamel,1,8-10 leading to direct communication with the oral cavity.
There are several ways to overcome the curing stresses generated by light-cured, bulk-filled resin
composites.

One technique to decrease curing stresses is to use a flowable resin composite as a lining

11,12

The low shrinkage stress contributes to improved resin composite adaptation to the cavity wall.13

material.

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Asian Pac J Dent 2014; 14: 13-18

However, the low mechanical property of flowable composites materials decreased the bond strength to thedentin wall.14,15
Furthermore, an incremental filling technique for the insertion of resin composite is widely used by many
clinicians.16

This technique is thought to decrease the curing stress at the tooth-resin interface that occurs whena cavity is bulk-filled with light-cured resin composites.

However, Versluis et al.17 performed a theoretical

study using the finite element analysis method and reported that incremental filling techniques caused increased
polymerization shrinkage effects at the restoration-enamel interface compared with the bulk filling techniques.
In addition Yoshikawa et al.18 demonstrated that incremental filling was unable to increase the bond strength tothe cavity floor of a box-like cavity.
Alternatively, increasing the velocity of light-cured resin composite decreased in composite adaptation to
the cavity wall when a resin composite of a different composition was used.19
amount of tensile force exerted by the hardening resin.
effect on strain development.

Composite flow decreased theTherefore, the polymerization rate has a significant

The internal hardness of cured resins increased with argon ion laser output alongwith increasing intensity, but the maximum hardness was not always increased.20
source may lead to more frequent marginal and wall gap formation.

3,4,21,22

The use of an intense light

When a composite was cured withan initial low-intensity light followed by high-intensity light, excellent marginal sealing and cavity adaptation
were achieved.3,4,21-23

Previous work has shown that when a composite was light cured with an initial lightintensity of 270 mW/cm2 for 10 s, followed by a light intensity of 600 mW/cm2 for 50 s after a 5-s interval, it
helped in decreasing the curing stress in the resin composite.3,4
curing method.

This method has been termed the slow-start4

The purpose of this study was to evaluate the effects of light-curing method on the marginal seal and resin
composite adaptation to the cavity wall, and the formation of enamel cracks around resin composite restorations.

Materials and Methods
Specimen preparation
The materials, components, manufacturers, batch numbers, and bonding procedures used in this study arelisted in Table 1.

Erupted intact human molar after extraction were employed in this study.

collected under protocol no. 725 approved by the appropriate institutional review board.

The teeth were

Cylindrical cavities

with 2/3 enamel and 1/3 dentin margins, 2 mm depth, and 3 mm diameter, with a C-factor of 3.7, were preparedon the buccal or lingual cervical regions of each molar using a diamond point (# B12, GC Corp., Tokyo, Japan)
under copious air-water irrigation.
Each of the 20 cavities was treated with an adhesive, Clearfil Liner Bond 2 V (Table 1).

After curing of

the adhesive, the cavities were bulk filled with Clearfil Photo Bright resin composite or Palfique Estelite resin
composite.

The resin composites were then polymerized using the conventional light curing method; 600

2mW/cm for 60 s or the slow-start curing method; 270 mW/cm2 for 10 s+5-s interval+600 mW/cm2 for 50 s.
The light curing unit used was an experimental quartz-tungsten halogen light curing unit (GC Corp., Tokyo,
Japan) connected to a slide regulator.
intensity was adjustable.

This light curing unit has a control system for lamp voltage, and the light

The light intensity was measured using a curing radiometer (Curing Radiometer,model 100, Sybron Kerr Corp., Milwaukee, WI, USA).
dark for 24 h in 37°C water.

After light curing, the specimens were stored in the

The resin composite restorations were finished with wet 600-grit SiC paper.

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Asian Pac J Dent 2014; 14: 13-18

Specimens were thermocycled for 500 cycles between 5°C and 55°C with a 30 s dwell time.
Table 1.

Study materials

Material/Manufacturer

Componentsa

Batch No.

Bonding Instructionb

Clearfil Liner Bond 2V(Kuraray Noritake Dental Inc.,Tokyo, Japan)

Primer A: MDP, HEMA, dimethacrylates,
photoinitiator, water, others

00002Aa, b, c, d, e, f

Primer B: HEMA , dimethacrylates,
accelerator, water

00002A

Bond A: MDP, HEMA ,
00003A
Bis-GMA, dimethacrylates, photoinitiator,
microfiller, others
Clearfil Photo Brightsilanated colloidal silica, prepolymerized
0036(Kuraray Noritake Dental Inc.)
organic filler containing colloidal silica,
shade (US)Bis-GMA, dimethacrylates, photoinitiator,
othersFiller load: 82 wt%
Palfique Estelite
silica-zirconia filler,
15597(Tokuyama Dental Corp., Tokyo,
Bis-GMA, TEGDMA,Japan)
photo initiatorshade (A3)Filler load: 82 wt%aAbbreviations: MDP, 10-methacryloyloxydecyl dihydrogen phosphate; HEMA, 2-hydroxyethylmethacrylate; Bis-GMA,
bisphenol A diglycidylmethacrylate; TEGDMA, triethyleneglycol dimethacrylateb
Procedures: (a) mix equal volumes of primers A and B, (b) apply primer for 30 s, (c) dry with gentle air-blowing, (d) apply
adhesive, (e) gently blow air, (f) light-cure for 20 s.

Evaluation of marginal sealing and cavity wall adaptation
To determine the degree of adaptation to the cavity margins and walls, a dye penetration test was performedby placing 1.0% acid red propylene glycol solution (Caries Detector, Kuraray Noritake Dental Inc., Tokyo,
Japan) at the margins of the restorations for 5 s.

The solution was then rinsed with water and gently blown dry.

The degree of dye penetration was observed using a stereomicroscope at 20× magnification.
record of each specimen was obtained at this stage.

A photographic

Then, the specimens were longitudinally cut in half with adiamond saw microtome (Leitz 1600 Saw Microtome, Ernst Leitz, Wetzlar, Germany) under running water, andthe dye reapplied to the cavity walls and observed to determine gaps and photographed.

From the photographs,

the length of dye penetration along the cavity margins and cavity walls was measured using a digitizer (KD4300
model, Graphtec Co., Tokyo, Japan).
enamel cracks.

Areas of marginal dye penetration in the enamel were considered to be

Then, the degree of enamel crack was determined as the ratio of the margin stained with thedye divided by the total length of the enamel cavity margin and converted to a percentage.

The degree of

dentin marginal leakage was determined as the length of dye penetration as a percentage of the total length of thedentin cavity margin.
wall length.

Dye penetration along the cavity walls was calculated as a percentage of the total cavity

This area was referred to as the cavity-wall gap.

Dye penetration scores were compared andanalyzed using Mann-Whitney U and Kruskal-Wallis tests.

Results
The results for enamel crack formation, marginal leakage, and cavity-wall gap formation are shown in
Table 2.

Light curing using the slow-start curing method showed a complete dentin marginal seal with both

Clearfil Photo Bright and Palfique Estelite resin composites.

This method also resulted in significantly better

cavity wall adaptation compared with the conventional method with both resin composites (p<0.05).

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Asian Pac J Dent 2014; 14: 13-18

Furthermore, the slow-start curing method resulted in a significantly better dentin marginal seal compared withthe conventional method with Clearfil Photo Bright (p<0.05).

The conventional method resulted in a poor

dentin marginal seal and poor wall adaptation with both resin composites.
Enamel crack formation was observed in all specimens.

There were no significant differences in the

degree of enamel crack formation between the two light-curing methods (p>0.05).
Table 2.

Enamel crack formation, dentin marginal leakage, and cavity-wall gap formation [%: mean (SD)]
Clearfil Photo Bright

Palfique EsteliteLight curing method

Enamelcrack

Dentinmarginal
leakage

Dentincavity-wall
gap formation

Enamelcrack

Dentinmarginal
leakage

Dentincavity-wall
gap formation

600 mW/cm2 60 s

46.7 (39.6)

45.9 (26.3) A, a44.9 (7.7) B

81.3 (12.3)

12.3 (17.0) a

41.1 (7.6) C

270 mW/cm2 10 s
+ 5-s (interval)
61.2 (22.1)
0A
11.5 (6.5) B
85.4 (17.7)
016.3 (9.7) C+ 600 mW/cm2 50 saIntragroup data with same superscript capital letters for each light curing method are significantly different (p<0.05).
bIntergroup data designated with same small letters for each resin composite are significantly different (p<0.05).

Discussion
In this study, the conventional light curing method resulted in poor marginal integrity and cavity wall
adaptation, whereas the slow-start light curing method resulted in a complete dentin marginal seal with for both
Clearfil Photo Bright and Palfique Estelite resin composites.

The slow-start method also showed significantly

better cavity wall adaptation compared with the conventional method with both resin composites.
A previous report indicated that at a certain light intensity, the amount of activated starter radicals is
optimal to form cross-linked long-chain molecules.23
reaction stop with short-chain molecules.

A higher concentration of radicals leads to an earlier

This suggested that curing with a high-intensity light is more likely

to lead to marginal gaps and poor adaptation of resin composite to the cavity walls.
decreased rate of surface hardness

3-6

On the other hand, thedevelopment due to the prolongation of the gel state and the accompanying

absence of dye penetration suggests that this particular protocol may result in increased material flow, providing
stress relief despite the high elastic modulus and photosensitivity of the resin composite examined.9

Previous

work has shown that when a composite was light cured with the slow-start curing method, the resin composite
hardened earlier at the cavity base than at the surface.3,4In addition, this method allowed most polymerization

contraction to be completed during the initial flowable stage of resin composite polymerization.

Moreover, thismethod apparently decreases curing stresses by delaying the hardening of the resin composite and permits moretime for relief of the stress induced by polymerization contraction.3,4

Furthermore, the rate of cure of the resin

directly adjacent to the cavity wall may be enhanced by the free radicals that already exist in the bonding resin.
The initial low light intensity was thought to boost the polymerization rate at this location rather than at thesurface of the resin.

The process of polymerization is then completed by the high-intensity radiation, allowing

for a more uniform rate of cure throughout the bulk of the resin composite.

The radiation allows most of thepolymerization contraction to be completed during the initial flowable stage of the resin composite, enabling theresin to flow toward the cavity walls.3
The slow-start curing method resulted in a significantly better dentin marginal seal compared with the16Yoshikawa et al.

Asian Pac J Dent 2014; 14: 13-18

conventional method for Clearfil Photo Bright resin.

Light transmission through the light-cured resin

composite is strongly affected by the opacity of the resin composite.
the resin composite is cured.

24

This opacity is different before and after

Almost all resin composite materials have the property of increasingtransparency during polymerization.25

The opacity of Clearfil Photo Bright increases during polymerization,

whereas that of Palfique Estelite slightly decreases during polymerization.25

Therefore, with Clearfil Photo

Bright, the delay in the hardening of the resin composite, particularly at the top surface,26 may apparently
decrease curing stresses and permit more time for relief of the stress induced by polymerization contraction.27
This allows most of the polymerization contraction to be completed during the initial flowable stage ofpolymerization, enabling the resin to flow within itself and preventing it from pulling away from the marginalcavity walls.3,4The dye penetration test using a caries detector can be used to evaluate resin composite

adaptation to the cavity wall both in vitro and in vivo.
Enamel crack formation was observed in all specimens.

There were no significant differences between theslow-start method and the conventional method in terms of the degree of enamel crack formation.
highly mineralized tissue and has a modulus of elasticity higher than that of dentin.
fragile.

Enamel is a

Enamel prisms are very

The tensile strength of enamel is 7.4 MPa when pulled vertical to the enamel prisms.28

The enamelcavosurface reportedly exhibits disruption even when a decreased power density of 100 mW/cm2 is used.9
Therefore, enamel crack formation could not be avoided, even with the slow-start curing method, when thecavities were prepared with butt-joint margins.

Further research is required to prevent enamel crack initiationand propagation.
Acknowledgments
This work was supported by a Grant-in-Aid for Scientific Research No. 12307043 from the Ministry of Education,
Science, Sports, Culture and Technology, Japan; by a Grant-in-Aid No. 22592115, No. 25462950 from the Japan Society for
the Promotion; and by Global Center of Excellence (GCOE) Program; the International Research Center for Molecular
Science in Tooth and Bone Diseases.

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Correspondence to:Dr. Takako Yoshikawa
Cariology and Operative Dentistry, Department of Oral Health Sciences, Graduate School,
Tokyo Medical and Dental University (TMDU)
1-5-45, Yushima, Bunkyo-ku, Tokyo 113-8510, Japan
Fax: +81-3-5803-0195 E-mail: yoshikawa.ope@tmd.ac.jp
Accepted May 17, 2014.
Copyright ©2014 by the Asian Pacific Journal of Dentistry.
Online ISSN 2185-3487, Print ISSN 2185-3479

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