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Micro-tensile bond strength of a self-etching primer bonding system tofluorosed enamelPrimali R. Jayasooriya, BDS,a Kamal A. Wettasinghe, BDS, MS, FDSRCS,b Miwako
Ogata, DDS, PhD,a Toru Nikaido, DDS, PhD,a and Junji Tagami, DDS, PhDaa

Cariology and Operative Dentistry, Department of Restorative Sciences,
Graduate School, Tokyo Medical and Dental University, Tokyo, Japan, andb
Department of Restorative Dentistry, Faculty of Dentistry, University of
Peradeniya, Peradeniya, Sri-Lanka
Purpose: The aim of the study was to evaluate micro-tensile bond strength and the conditioning patterns ofa self-etching primer bonding system to un-etched (with extended priming) and etched ground fluorosed
enamel.
Materials and Methods: Extracted premolars were classified according to severity of fluorosis using
Thylstrup and Fejerskov index (TFI: 0, normal; 1-3, mild fluorosis; and 4-6, moderate fluorosis) into threegroups, which were again divided into four sub groups to receive different enamel conditioning. The
ground enamel surfaces of the teeth in first and second sub groups were primed with a self-etching primer(SE primer, Clearfil SE Bond) for 60 and 90 s respectively prior to application of the adhesive. The teeth
in the remaining sub groups were etched with 37% phosphoric acid (H3PO4) for 60 or 90 s prior toapplication of the self-etching primer bonding system according to the manufacturer’s instructions. All theteeth were then restored with resin composite, stored for 24 hours in water and prepared for micro-tensile
bond strengths. The data were analyzed with analysis of variance (ANOVA) and Fisher’s PLSD test
(p<0.05).
Results: Significantly higher micro-tensile bond strengths were observed when the self-etching primer
bonding system was applied to etched ground fluorosed enamel (p<0.05) compared to un-etched enamelwith extended priming. The bond strengths were significantly stronger in normal and mildly fluorosed
enamel compared to moderately fluorosed enamel when etched for 60 s (p<0.05). Predominantly Type IIetching pattern was observed in H3PO4 conditioned fluorosed enamel while relatively shallow etchingpattern was seen in un-etched fluorosed enamel with extended priming.
Conclusion: Severity of fluorosis, etching time and pattern influence the bond strength of the self-etchingprimer bonding system to fluorosed enamel. (Int Chin J Dent 2002; 2: 107-115.)
Clinical Significance: Before application of self-etching primer bonding systems, H3PO4 conditioning isnecessary to obtain high bond strength for fluorosed enamel.
Key words: bond strength, enamel fluorosis, self-etching primer.

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Enamel fluorosis is characterized by surface hypermineralyzation and sub surface porosity. Clinically,
moderate/severe fluorosis appears as irregular opacities with staining and pitting resulting in loss of surface
enamel.1 Therefore, these patients usually require tooth colored restorations such as composite/porcelain
veneers.2
Compared to normal enamel, fluorosed enamel is difficult to etch, as fluoroapatite is more resistant toacid dissolution than hydroxyapatite.3 Moreover, the fluoroapatite content of the fluorosed enamel varies
with severity.4

Therefore, previous investigators5,6 recommended extended enamel conditioning withphosphoric acid when bonding fluorosed enamel. In contrast, Ng’ang’a et al.7 and Ateyah et al.8 reported
no significant differences in the bond strengths in fluorosed and normal enamel at different etching times.
Therefore, these controversies make it difficult to determine the most suitable etching time with phosphoricacid for varying degrees of fluorosis.
Self-etching primer bonding systems are effective for bonding resin composite to dentin.9,10 However,controversies exist as to use of these bonding systems in enamel.11-14 Moreover, these primers have shown
less etching ability in enamel compared to phosphoric acid because of their high pH.13 Therefore, with
self-etching primer-bonding systems extended priming or additional phosphoric acid etching may benecessary to obtain adequate bonding to fluorosed enamel.

However, there are no reports on bondstrengths or conditioning patterns produced by self-etching primers on fluorosed enamel.
The aim of this study therefore, was to evaluate micro-tensile bond strength ( µ-TBS) and theconditioning patterns of a self-etching primer bonding system to fluorosed enamel of varying severity using
primer of the bonding system or 37% phosphoric acid at different etching and priming times.

MATERIALS AND METHODS
The materials used in this study are listed in Table 1 with their abbreviation code. Thirty non-carious
premolars extracted for orthodontic reasons from patients in areas endemic for fluorosis in Sri-Lanka wereused for the study. The teeth were obtained from patients who consented verbally to use their teeth forresearch and educational purposes. The extracted teeth were cleaned and stored in water in a refrigerator at
4˚C. Prior to use, the teeth were classified according to the severity, using Thylstrup and Fejerskov index1(TFI: 0, normal; 1-3, mild fluorosis; and 4-6, moderate fluorosis) into three groups. Teeth with severe
fluorosis (TFI: 7-9) were not available for the present study. Eight teeth from each group were randomly
divided into four subgroups to receive different enamel conditioning as described below in preparation forµ-TBS.
Specimen preparation for µ-TBS
Both buccal and lingual surfaces of each tooth were used for the bond strength measurement as previous
studies have shown that there are no significant differences in the fluoride concentration between buccal
and lingual aspects of the same tooth.7 In order to standardize the enamel reduction, 0.5 mm grooves were108

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prepared and superficial enamel was removed initially using a superfine diamond bur (SF #145, Shofu Inc.,
Kyoto, Japan) on a high-speed hand piece followed by grinding with #600 grit SiC paper under water
coolant. The teeth were then ultrasonically cleaned in distilled water for 5 minutes prior to the bonding
procedure.
Table 1. Materials used.
Product

Batch number

Components

Self-etching primer bonding system
Clearfil SE Bond

Primer (SE primer) 003A

MDP, HEMA, Hydrophilic dimethacrylates,
Photo initiator, Water

Bond (SE bond)

MDP, Bis-GMA, HEMA, Photo initiator, Water

Conditioner
K-etchant

184

37% Phosphoric acid gel

0482

Silanated barium glass, Silica, Colloidal silica,

Restorative material
Clearfil AP-X

Bis-GMA, TEGDMA, Photo initiator

All materials were manufactured by Kuraray Medical, Tokyo, Japan.
Bis-GMA: bisphenol A-diglycidyl methacrylate, HEMA: 2-hydroxyethyl methacrylate, MDP: 10-methacryloyloxydecyl
dihydrogen phosphate, TEGDMA: triethyleneglycol dimethacrylate.

The buccal and lingual enamel surfaces of teeth in first and second sub-groups were primed with SE
primer for 60 and 90 s and gently dried. The adhesive of SE Bond was then applied and light cured for 20 s.
Thereafter, both surfaces were built up with the resin composite to a height of 5 mm in three increments,
light curing each increment for 20 s, at 400 mW/cm2 (New Light VL-II, GC Corp., Tokyo, Japan).
The teeth in the remaining sub groups were etched with 37% phosphoric acid (H3PO4, K-etchant) for 60
and 90 s, rinsed with water for 20 s, and dried for 20 s. Thereafter, SE Bond was applied according to the
manufacturer’s instructions as follows; SE primer was applied for 20 s and gently dried. Then the adhesive
was applied and light cured for 20 s. The resin composite was bonded in a manner similar to the primed
groups. Thereafter, all the specimens were stored in water at 37˚C for 24 hours.
The following day, each tooth was sectioned perpendicular to the bonded interface to obtain three to four
0.7 mm thick slabs. Then each slab was longitudinally sectioned at the center to separate the buccal andlingual surfaces. Thereafter, each section was trimmed with a superfine diamond bur (v16ff, GC Corp.) to
obtain an hourglass shape so that the narrowest portion at the adhesive interface has a surface area of
1.0±0.2 mm2 in preparation for µ-TBS test.15 Each specimen was attached to a Bencore-Multi T testing
apparatus (Danville Engineering Co., San Ramon, CA, USA) with Zap-It cyano-acrylate adhesive (Dental

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Ventures of America, Corona, CA, USA) and placed in a universal testing machine (EZ test, Shimadzu
Corp., Kyoto, Japan) for µ-TBS testing at a crosshead speed of 1 mm/minute (Fig. 1). The data wereanalyzed with ANOVA and Fisher’s PLSD test at 5% level of significance.

12

3
Composite

Enamel conditioninga) 60/90 s primeb) 60/90 s PA-etch + 20 s
prime

Composite

EnamelSectioning andtrimming

Dentin5

4Fig. 1. Schematic illustration of specimen preparation. 1) ground buccal and palatal enamel,2) enamel conditioning, 3) restoration with resin composite, 4) specimen tested withµ-TBS, and 5) assembly for tensile test.

The fracture modes
The fracture patterns after µ-TBS testing were observed visually, and were classified into two groups asfollows; A, cohesive failure in fluorosed enamel, and B, adhesive failure at the interface between bonding
resin and enamel.
Specimen preparation for microscopic observation of conditioning patterns of fluorosed enamel
Two teeth from each group were used to observe the effect of extended priming and conditioning with
37% H3PO4 on ground fluorosed enamel. Ground palatal and buccal enamel was sectioned with a diamondsaw (Isomet, Buheler Co., Lake Bluff, IL, USA) to obtain eight to ten 1 mm thick slabs from each tooth.

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Alternate slabs from one tooth were either primed or etched for 60 s while the alternate slabs of the othertooth were primed or etched for 90 s. Each primed slab was then rinsed with water and acetone and dried
while etched slabs were rinsed with water only. All the slabs were mounted on stubs and gold coated forobservation under a scanning electron microscope (SEM, JSM-5310LV, JEOL, Tokyo, Japan).

RESULTSThe µ -TBS of the self-etching primer bonding system to fluorosed enamel is summarized in Table 2.
Extended priming did not significantly improve the bond strengths of un-etched fluorosed enamel (p>0.05).
However, phosphoric acid etching prior to application of the self-etching primer bonding system resulted insignificantly higher bond strengths in fluorosed enamel (p<0.05). The bond strengths were significantly
stronger in normal and mildly fluorosed enamel compared to moderately fluorosed enamel when etched for
60 s (p<0.05). The bond strengths of moderately fluorosed enamel significantly improved to a levelcomparable to other groups when etched for 90 s (p<0.05). Therefore, 60 s phosphoric acid etching was
sufficient to improve the bond strengths of the self-etching primer bonding system to mildly fluorosed
enamel while moderately fluorosed enamel required longer etching time (90 s) to improve the bondstrength.
Table 2. The micro-tensile bond strength results.
Surface conditioning
Bond strength (MPa)NormalMild fluorosis

Moderate fluorosis

Mean

SD

Category

MeanSD

Category

MeanSD

Category

20 s prime (Control)

16.3

4.3

a,b

12.4

5.6

c

12.0

2.5

c

60 s prime

15.9

3.2

b

13.8

3.8

b,c

11.6

2.5

c

90 s prime

16.6

6.0

a,b

13.5

6.9

b,c

12.1

3.8

c

60 s etch + 20 s prime

20.1

6.6

a

19.7

4.0

a

15.5

4.5

b

90 s etch + 20 s prime

19.1

4.2

a

18.8

7.3

a

18.4

3.5

a

SD: standard deviation. Sample size: 12-16. Classification of fluorosis according to the Thylstrup and Fejerskov index
1(TFI: 0, normal; 1-3, mild fluorosis; and 4-6, moderate fluorosis). Identical letters among the surface conditioning groups
represents no statistically significant difference (p>0.05).

Visual observation of fracture modes revealed cohesive fractures involving enamel (A) in 5-10% of thespecimens in each group. The remaining 90-95% of the specimens showed a mixed fracture pattern (B).
There were no significant differences in fracture patterns observed between groups. A relatively shallow
etching pattern was seen in un-etched fluorosed enamel with extended priming (Fig. 2). Moreover, scratchmarks created during tooth preparation were also observed (Fig. 2, arrow). All three etching patterns
previously described for normal enamel with phosphoric acid conditioning16 was also observed with111

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fluorosed enamel. Namely, Type I pattern in which hollowing of the prism centers were observed withrelatively intact prism peripheries (Fig. 3a); Type II pattern where prism peripheries appeared to be
removed (Fig. 3b); and a Type III pattern with generalized roughening which did not confirm to any prism
structure (not shown).

Fig. 2. Scanning electron micrograph of the etchedenamel surface with extended priming (90 s)
on ground fluorosed enamel.
10 µm

a

5 µm

b

5 µm

Fig. 3. Scanning electron micrographs of the etching patterns of ground mild fluorosed enamel
following H3PO4 conditioning for 60 s; a) Type I and III etching patterns, and b) Type IIetching pattern.

DISCUSSION
Though there are controversies regarding the bonding performance of self-etching primer bonding
systems in normal uncut enamel, there is sufficient evidence to indicate that these bonding systems provide
bond strengths comparable to bonding systems that use phosphoric acid in ground/cut enamel.12,13
However, the pilot study revealed low bond strengths of ground fluorosed enamel when the self-etchingprimer was applied according to the manufacturer’s instructions (results-control). Therefore, the effect ofextended priming or prior enamel conditioning with phosphoric acid on the bond strength of theself-etching primer bonding system to ground fluorosed enamel was evaluated in the present study. The

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conditioning time of 60 and 90 s with phosphoric acid was chosen according to the previous studies by
Al-Sugair3 and Ateyah’s8 recommendations.
Results revealed significantly higher µ-TBS of Clearfil SE Bond when the self-etching primer bonding
system was applied to etched ground fluorosed enamel (p<0.05) compared to un-etched enamel withextended priming. SEM observation of the conditioned surfaces revealed relatively shallow etch patternwith extended priming. Therefore, insufficient etching-abilities of the self-etching primer may result inlack of resin tag formation resulting in low bond strengths.17
No statistically significant differences were observed in the µ-TBS of teeth with the normal and mildgroups etched for 60 s with H3PO4. This is similar to the previous reports by N’gang’a et al.7 and Ateyah etal.8 However, teeth with the moderate group etched for 60 s resulted in significantly lower bond strengths,
though it improved sufficiently to levels comparable to other groups when etched for 90 s. Al-Sugair et al.3
reported the presence of a subsurface insoluble organic network in fluorosed teeth (TFI: 4-5) when etchedfor 45 s. They also reported the reappearance of the typical etching patterns (Type I, II, and III) after
treating the enamel with H3PO4 for 90 s. This insoluble organic network may have contributed to thesignificantly low bond strengths reported for the moderate group etched for 60 s in our study and thereappearance of the typical etch patterns for the improvement of the bond strengths when etched for 90 s.
However, in contrast, Ateyah et al.8 reported similar bond strengths for all groups of fluorosis in young
patients. They attributed this similarity to the loss of hypermineralyzed surface layer because of using
ground enamel as bonding substrate. Moreover, they also reported that bonds tend to fail predominantly
cohesively in enamel when the fluorosis was severe. This difference in the fracture pattern was notobserved in our study as unequal stress distribution resulting in cohesive bond failures in enamel can be
avoided with the µ-TBS test.15 The bond strengths of non-fluorosed enamel reported in this study are
relatively lower than the previous studies, which used the same bonding system.13,14 Operator variability18
may have influenced this difference. In addition, the fact that these teeth were also collected from same
areas as fluorosed teeth with high fluoride content in water may have contributed to relatively low enamelbond strengths. However, previous studies have indicated that enamel bond strengths in the range of 17-20
MPa as sufficient to ensure retention of composite resin restorations.19 Moreover, there were no significant
differences in the bond strengths with extended priming (90 s) compared to etched groups in non-fluorosed
ground enamel. Therefore, non-fluorosed ground enamel does not need prior conditioning with H 3PO4.
However, fluorosed enamel need prior enamel conditioning as fluoroapatite is more resistant to aciddissolution than hydroxyapatite of non-fluorosed teeth. As the self-etching primer has a pH of two20
compared to 0.6 of H3PO4 even extended priming is not sufficient to obtain adequate etching of the acidresistant fluorosed enamel. Therefore, according to the results of this study bonding systems that utilize
phosphoric acid etching may be more suitable for bonding fluorosed enamel.

CONCLUSIONS
Phosphoric acid etching prior to application of the self-etching primer bonding system resulted in higher

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enamel bond strengths in ground fluorosed teeth. Accordingly, teeth with mild fluorosis can be bonded
using a similar etching time as non-fluorosed teeth, while moderate fluorosis require longer etching time toobtain adequate bond strengths. Moreover, there are differences in the conditioning patterns obtained withthe self-etching primer and H3PO4. It is therefore concluded that severity of fluorosis, etching time andpattern influence the bond strengths of the self-etching primer bonding system to fluorosed enamel.

REFERENCES
1. Thylstrup A, Fejerskov O. Clinical appearance of dental fluorosis in permanent teeth in relation to histologic changes.
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20. Nakatsuka K. Characteristics of Clearfil Mega Bond. In: Momoi Y, Akimoto N, Kohno A, Modern trends in adhesive
dentistry. Proceedings of the Adhesive Dentistry Forum ’99; 1999 Feb 20; Yokohama, Japan. Osaka: Kuraray Co.,
1999. p. 142-51.

Reprint request to:
Dr. Toru Nikaido
Cariology and Operative Dentistry, Department of Restorative Sciences
Graduate School, Tokyo Medical and Dental University
1-5-45, Yushima, Bunkyo-ku, Tokyo 113-8549, Japan
FAX: +81-3-5803-0195
E-mail: nikaido.ope@tmd.ac.jp
Received on August 20, 2002. Revised on September 17, 2002. Accepted on September 26, 2002.
Copyright ©2002 by the Editorial Council of the International Chinese Journal of Dentistry.

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