Yoshikawa et al.

Asian Pac J Dent 2015; 15: 13-19

Environmental observation of enamel crack and resin-tooth cavity
gap formation
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 of Medicalband Dental Sciences, Tokyo Medical and Dental University (TMDU), Department of Dentistry,c
Toranomon Hospital, Tokyo, Japan, and Department of Restorative Dentistry, School of Dentistry,
University of Washington, Seattle, WA, USA
Purpose: The aim of this study was to observe marginal sealing and resin composite adaptation to cavity walls
using the dye penetration test and environmental scanning electron microscope.
Materials and Methods: Cylindrical cavities were prepared on cervical regions of teeth. Teeth were restoredwith Clearfil Liner Bond 2V adhesive and filled with Clearfil Photo Bright or Palfique Estelite resin composites.
These resin composites were cured with a conventional light-curing method. After thermal cycling, thespecimens were subjected to the dye penetration test to evaluate marginal sealing and resin composites
adaptation to the cavity walls. These resin-tooth interfaces were then observed using environmental scanningelectron microscope.
Results: Enamel crack formation was observed in all specimens. Gap formation between the resin composite
and cavity walls was observed for both resin composites.
Conclusion: Environmental scanning electron microscope showed high enamel crack formation for both resin
composites after light curing. Enamel cracks were located at 25-100 µm from the resin-enamel interface on
environmental scanning electron microscope observations. Enamel cracks were observed in the enamel surface
to the dentin-enamel junction. Environmental scanning electron microscope showed cavity wall gap formation in
the dentin-adhesive interface or resin-adhesive interface.
(Asian Pac J Dent 2015; 15: 13-19.)
Key Words: adaptation, dye penetration test, environmental SEM, polymerization, resin composite

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

Teeth withabrasions and erosion cannot be treated for conventional Black’s cavities with metal inlays; they can only berestored with resin composites using a direct bonding technique.
However, resin composite polymerization results in volumetric shrinkage, also known as polymerization
shrinkage, and the ensuing stress created results in greater gap formation between the resin and cavity wall.1-4
These marginal gaps and subsequent microleakage may cause marginal staining, postoperative sensitivity,5,6 andsecondary caries.

In addition, gap formation between the restoration and cavity wall may result in pain on

biting and adhesion failure after repeated occlusal loading.

Light-cured resin composites are widely used inclinical practice because of their esthetic advantages, ease of use, good bonding to tooth structures, and excellent
mechanical properties.
The polymerization reaction for light-cured composites is rapid, which results in the development of higher
stresses in cured materials than in self-activated materials.7

Furthermore, the maximum interfacial stressgenerated at cavity walls filled with light-cured composites is twice compared with that generated with
self-cured composites.8

Therefore, gap formation between the resin and cavity wall because this stress isgreater when light-cured materials are used than when self-cured materials are used.

In addition, when thebond strength exceeds these polymerization shrinkage stresses, a crack is initiated in the tooth structure,generally in the enamel,4,9-12 resulting in the direct contact of the restoration with the oral cavity.

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Asian Pac J Dent 2015; 15: 13-19

It is often difficult to observe real enamel cracks without damage because the enamel prisms are veryfragile.13

Observations with scanning electron microscope (SEM) require absolutely dry specimens.

Therefore, enamel cracks are usually observed with a stereomicroscope10,14,15 and replica-SEM.16,17

However,stereomicroscopic observations and impressions to create a replica also require dry specimen surfaces.
dry conditions promote enamel crack formation.

These

In addition, the observation of the resin-dentin interface ismore difficult than that of the resin-enamel interface.
Because dentin comprises a significant amount of water, it is more sensitive to environmental conditions
and shrinks under dry conditions.

Cryo-SEM observations do not require dry conditions.

However,cryo-SEM observations require complicated handling and time-consuming specimen preparation.18
Alternatively, environmental SEM observations can be used to observe liquid and hydrated specimens, such as
native plants and living insects.

In these cases, to maintain water in its liquid phase, a minimum water vapor

pressure of 609 Pa is required at 0°C, and this pressure can be arbitrarily chosen for the required indication.
Therefore, environmental SEM observations require a lower pressure compared with conventional SEM
observations, at least in the terms of the pressure required to observe distilled water.

However, environmental

SEM equipment is very expensive and the observation technique is time consuming.

In comparison, the dyepenetration test is an easy, rapid technique to observe resin composite adaptation to the cavity wall.

We

observed resin-tooth interfaces by environmental SEM and the dye penetration test.

Materials and Methods
Specimen preparation
The materials, components, manufacturers, batch numbers, and bonding procedures used in the present
study are listed in Table 1.
Table 1.

Study materials

Material/Manufacturer

Componentsa

Batch No.

Bonding Instructionb

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

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

00147Ba, b, c, d, e, f

Primer B: HEMA , dimethacrylates,
accelerator, water

00145B

Bond A: MDP, HEMA,
00238A
Bis-GMA, dimethacrylates, photoinitiator,
microfiller, others
Clearfil Photo Brightsilanated colloidal silica, prepolymerized
00044C(Kuraray Noritake Dental Co.
organic filler containing colloidal silica,
Ltd.)
Bis-GMA, dimethacrylates, photoinitiator,
shade (US)othersFiller load: 82 wt%
Palfique Estelite
silica-zirconia filler,
15597(Tokuyama Dental Co. Ltd.,
Bis-GMA, TEGDMA,Tokyo, 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 a gentle stream of air, (d)apply adhesive, (e) gently blow air to dry adhesive, (f) light-cure for 20 s

Erupted intact human molars were used after extraction and collected under our protocol No. 725, which

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Asian Pac J Dent 2015; 15: 13-19

was approved by the appropriate institutional review board.

Cylindrical cavities with 2/3 enamel and 1/3 dentin

margins, 2-mm depth, 3-mm diameter, and C-factor of 3.7 were prepared on the buccal or lingual cervicalregions of each molar using a diamond point (# B12, GC Corp., Tokyo, Japan) under copious air-water
irrigation.
Each of the 14 cavities was treated with the adhesive Clearfil Liner Bond 2V (Table 1).

After thisadhesive was cured, the cavities were bulk-filled with Clearfil Photo Bright resin composite or Palfique Estelite
resin composite.

The resin composites were then polymerized by the conventional light-curing method (6002mW/cm for 60 s).

An experimental quartz-tungsten halogen light-curing unit (GC Corp., Tokyo, Japan) thatwas connected to a slide regulator was used.

This light-curing unit had a control system for lamp voltage andadjustable light intensity, which was measured using a curing radiometer (model 100, Demetron Research,
Danbury, CT, USA).
37°C.

After light curing, the specimens were stored in the dark for 24 h in water maintained at

The restorations were finished and polished using wet 600-grit SiC paper.

The specimens were

thermocycled for 500 cycles between 5°C and 55°C, with a 30 s dwell time.
Marginal seal and cavity wall adaptation
We used the dye penetration test to determine the degree of adaptation to the cavity margins and walls.
This dye penetration test was performed by placing a 1.0% acid red propylene glycol solution (Caries Detector,
Kuraray Noritake Dental Inc., Tokyo, Japan) at the margin of the restoration for 5 s, followed by rinsing withwater and gentle blow-drying.
magnification).

The extent of dye penetration was observed with a stereomicroscope (20×A photographic record of each specimen was acquired at this stage.

The specimens were then longitudinally cut in half using a diamond saw microtome (Leitz 1600 Saw
Microtome, Ernst Leitz, Wetzlar, Germany) under running water, the dye was reapplied to the cavity walls, andimages were acquired to observe gaps.

In these images, the extent (length) of dye penetration along the cavity

margins and walls was measured using a digitizer (KD4300 model, Graphtec Co., Tokyo, Japan).
marginal dye penetration in the enamel were considered to be enamel cracks.

The areas of

The degree of marginal enamelcrack formation was then determined as the ratio of the margin stained with the dye divided by the total length ofthe enamel cavity margin, which was converted to a percentage.

The degree of dentin marginal leakage wasdetermined as the extent (length) of dye penetration as a percentage of the total length of the dentin cavity
margin.

The degree of marginal defect (marginal enamel crack formation and dentin marginal leakage) was

determined as the length of dye penetration as a percentage of the total length of the cavity margin.
penetration along the cavity walls was calculated as a percentage of the total cavity wall length.
referred to as the cavity wall-resin gap.

DyeThis area was

The dye penetration test scores were compared using the Mann-

Whitney U test.
Finally, the resin-tooth interfaces were observed using an environmental SEM (Quanta 200 FEG, FEI Co.,
Hillsboro, OR, USA) under conditions of saturated water vapor pressure of 600-650 Pa.

We also assessed therelationship between dye penetration test scores and environmental SEM observations.

Results
Table 2 shows the results for marginal defect, enamel crack formation, marginal leakage, and cavity
wall-resin gap formation.

Photographic and environmental SEM observations for the sample specimens are15

Yoshikawa et al.

shown in Figs. 1-4.

Asian Pac J Dent 2015; 15: 13-19

Palfique Estelite showed significantly less dentin marginal leakage compared with Clearfil

Photo Bright.
Table 2.

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

Dentinmarginal
leakage

Dentincavity-wall
gap formation

Palfique Estelite
Marginal Enamel
defectmarginal
crack

DentinDentinmarginal
cavity-wallleakage gap formation

600 mW/cm2 36.0 (20.2) 61.9 (41.5) 42.1 (29.3) a 40.3 (11.0)
54.6 (10.0) 86.7 (13.5) 4.8 (12.8)a 36.2 (13.9)
60saIntergroup data designed with same superscript small letters for each resin composite are significantly different (p<0.05).

Fig. 1

Fig. 1cFig. 21a1bEnvironmental SEM observation of enamel cracks on the margins around Palfique Estelite resin composite restorations
1a (500×), 1b (1,000×)1c
2
Environmental SEM observation of enamel cracks on the margins around Palfique Estelite resin composite restorations(2,000×)
Environmental SEM observation of enamel cracks on the cut surface around Palfique Estelite resin composite
restorations (500×).

Enamel crack formation was observed in all specimens.

In all groups, enamel crack formation appeared asa white margin on a surface that represented cracks in the enamel surrounding the resin composite restorations;

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Asian Pac J Dent 2015; 15: 13-19

environmental SEM showed that these cracks were located at 25-100 µm from the resin-enamel interface (Figs.
1a, 1b, 1c, 2).

Fig. 3

Enamel cracks were observed in the enamel surface to the dentin-enamel junction (Fig. 2).

3aEnvironmental SEM observations of cavity wall-resin gap formation in a tooth restored with Palfique Estelite
3a (500×), 3b (1,000×)3b

Figures 3a and 3b shows an environmental SEM image of a cutting surface obtained after curing Palfique
Estelite using the conventional curing method.

This shows the environmental SEM image of this gap formation

at the line angle between the cavity wall and floor.

The gap formation was observed in the dentin-adhesiveinterface.

Fig. 44a4bEnvironmental SEM observations of cavity wall-resin gap formation in a tooth restored with Clearfil Photo Bright4a (500×), 4b (1,000×)Figures 4a and 4b show an environmental SEM image of a cutting surface obtained after curing Palfique
Estelite using the conventional curing method.
between the cavity wall and floor.

This shows environmental SEM images at the line angle

The gap formation was observed in the resin-bonding interface.

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Asian Pac J Dent 2015; 15: 13-19

Discussion
In the present study, the conventional light-curing method resulted in poor dentin marginal sealing anddentinal cavity wall adaptation with both Clearfil Photo Bright and Palfique Estelite resin composites.
The curing pattern of light-cured composite resins has several disadvantages that may compromise theability to achieve an excellent seal along the cavity wall, such as the direction and speed of polymerization
shrinkage, depth of curing, and polymerization shrinkage stress.

A previous report19 indicated that at a certain

light intensity, the amount of activated starter radicals was optimal to form cross-linked, long-chain molecules.
A higher concentration of radicals stopped a reaction earlier than short-chain molecules did, suggesting thatcuring with a high-intensity light was more likely to result in marginal gaps and poor adaptation of resin
composite to the cavity wall.
Polymerization shrinkage that occurs after gelation or curing results in the build-up of large stresses in aresin composite.20

Conversely, a decreased rate of surface hardness development because of a prolonged gel

state and the accompanying absence of dye penetration suggest that this particular protocol results in an
increased material flow, which provides for stress relief despite the high elastic modulus and photosensitivity of
the resin composite examined.10
We also observed enamel crack formation in all of our specimens.
with a modulus of elasticity greater than that of dentin.

Enamel is a highly mineralized tissue,

Enamel prisms are very fragile; the tensile strength ofenamel is under 10 MPa when pulled vertical to these enamel prisms13
exhibits disruption, even with a decreased power density of 100 mW/cm.

10

The enamel cavosurface reportedly
Therefore, enamel crack formation

was observed in all specimens, when the cavities were prepared with butt-joint margins.

Further research is

required to determine measures for preventing enamel crack initiation and propagation.
Dye penetration test is useful for both in vivo and in vitro studies to detect marginal defect and cavity wall
gap formation of resin composite restorations when using Caries Detector.
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; a Grant-in-Aid No. 22592115, No. 25462950 from the Japan Society for the
Promotion; and the Global Center of Excellence (GCOE) Program, the International Research Center for Molecular Sciencein Tooth and Bone Diseases.

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Correspondence to:Dr. Takako Yoshikawa
Cariology and Operative Dentistry, Department 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 June 24, 2015.
Copyright ©2015 by the Asian Pacific Journal of Dentistry.
Online ISSN 2185-3487, Print ISSN 2185-3479

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