The study aims to determine the shear bond strength (SBS) values of different flowable composites (Pulpdent® Flows-Rite, 3M™ Filtek Flow, and Heraeus Kulzer® Flow Line) in comparison with a conventional orthodontic adhesive and the bond failure sites of these composites. Eighty extracted human premolars were divided into four groups of 20 teeth each. Brackets were bonded to the teeth in each test group with different composites, according to the manufacturer's instructions. SBS values of these brackets were recorded (in MPa) using a universal testing machine. Adhesive remnant index (ARI) scores were determined after the failure of brackets. Data were analyzed using analysis of variance (ANOVA), Tukey honestly significant difference, and chi-square tests. SBS values of groups 1 (Transbond XT), 2 (Flows-Rite), 3 (Flow), and 4 (Flow Line) were found to be 17.10 ± 2.48 MPa, 6.60 ± 3.2 MPa, 7.75 ± 2.9 MPa, and 8.53 ± 3.50 MPa, respectively. The results of this study demonstrate that the orthodontic adhesive (Transbond XT) had higher SBS values than the flowable composites. Results of ANOVA revealed statistically significant differences among the groups (P < .05). The SBS values were significantly lower in all flowable composite groups than the orthodontic adhesive. ARI scores were significantly different between the orthodontic adhesive and all the flowable groups investigated. The use of flowable composites is not advocated for orthodontic bracket bonding because of significantly lower SBS values achieved.

Buonocore1 introduced the technology that eventually leads to the concept of direct bonding in orthodontics. Ten years later, Newman2 described a technique for acid-etching enamel to enhance the mechanical adhesion of orthodontic brackets to the teeth. Since then several factors that affect the mechanical adhesion of orthodontic brackets to the teeth have been described.

Recently, a wide variety of visible light–cured orthodontic adhesives have become commercially available. Current trends in orthodontic materials research, focusing on this type of bonding system, suggest that these adhesives provide a convenient means of achieving clinically acceptable results.3–10 The advantages of visible light–cured orthodontic adhesives are the high early bond strength,7 minimal extent of oxygen inhibition,4 and the extended working time for optimal bracket placement.8 

Filled dental restorative materials were also used as orthodontic adhesives.9 These materials consist of an organic diacrylate (BIS-GMA), a coupler (silane), and a higher percentage content of inorganic filler (quartz, silica). The inorganic filler makes the material more abrasion resistant, increases the shear bond strength (SBS) values, and decreases the coefficient of thermal expansion to values closer to those of enamel to prevent long-term microleakage.

Ostertag et al11 designed an experimental study to evaluate the influence of adhesive filler concentration on bond strength, keeping the filler particle size constant. The results of that study indicate that there is an increase in shear and torsional bond strength with increasing concentrations of adhesive filler.

Some authors believe that charged particles in the composite resin limit the free flow of adhesive into the enamel pores, inhibiting the formation of resin tags.12–14 Others believe that the liquid phase of the composite is present in sufficient amount to flow into the conditioned enamel porosities and act independently of the charged particles. These workers use this as an explanation for the equal size of resin tags obtained when the composite resin or the sealant is applied directly to the conditioned enamel.15–18 

Flowable resin composites have been recommended for many clinical uses and have been formulated in a variety of compositions and viscosities to meet various uses.19 A plethora of new low-viscosity composite resin materials, or flowable composites, have been marketed during the past three years but little has been published about them.

The aim of this study was to test different flowable composites to determine their SBS values in comparison with a conventional bonding adhesive and the site of bond failure of flowable composites.

Eighty noncarious human premolars, extracted for orthodontic indications, were used in this study. Teeth with hypoplastic areas, cracks, or irregularities of the enamel structure were excluded from the study. The criteria for tooth selection dictated no pretreatment with chemical agents such as alcohol, formalin, and hydrogen peroxide. The extracted teeth were stored in distilled water continuously after extraction. The water was changed weekly to avoid bacterial growth. The sample was randomly divided into four groups of 20 each. Each tooth was mounted vertically in a self-cure acrylic block so that the crown was exposed. The buccal enamel surface of the teeth were cleansed and polished with nonfluoridated pumice and rubber prophylactic cups, washed with water, and dried.

A 37% phosphoric acid gel (3M Dental Products, St Paul, Minn) was used for the acid etching of premolars for 30 seconds. The teeth were rinsed with water for 30 seconds and dried with an oil-free source for 20 seconds. In all etched cases, the frosty white appearance of enamel was present. Standard edgewise premolar stainless steel brackets were bonded to the teeth using the bonding protocols, according to the manufacturer's instructions (product no: EDSTU 418, G&H Wire Company, Greenwood, Ind, US & Canada). The average surface of the orthodontic bracket base used was 14.00 mm2.

After acid etching, the brackets were bonded in the following manner (Figure 1):

  1. Group 1 (Transbond XT): Transbond XT primer was applied to the etched surface in a thin film. Transbond XT (3M Dental Products) adhesive paste was applied to the bracket base, and the bracket was positioned on the tooth and pressed firmly into place. The excess adhesive was removed from around the bracket with a scaler, and the adhesive was light cured from the mesial and distal for 10 seconds each (total time 20 seconds).

  2. Group 2 (Flows-Rite): This product is a highly polishable and contains 68% filled by weight and 47% by volume of flowable composite with the corresponding handling and flow characteristics (Table 1). During this procedure, placing primer on the etched enamel surface was not recommended by the manufacturer (Flows-Rite, Pulpdent® Corporation, Watertown, Mass). Flowable composite paste was applied to the bracket base, and the bracket was positioned on the tooth and light cured for 30 seconds from the incisal edge.

  3. Group 3 (Flow): Flow (Filtek™, 3M Dental Products) is a low-viscosity, visible light–activated, radiopaque flowable composite. 3M Filtek Flow contains BisGMA and TEGDMA resins. The filler in this composite is zirconia/ silica and is loaded 47% by volume (Table 1). According to the manufacturer's instruction, the primer (3M Single Bond Adhesive) was applied to the etched surface in a thin film and light cured for 10 seconds. The bracket was positioned, and the composite was light cured for 20 seconds.

  4. Group 4 (Flow Line): Flow Line (Heraeus Kulzer, Dormagen, Germany) is a medium-viscosity, hybrid composite resin, with the same size filler particles as universal hybrid composites. It consists of a lower percentage of filler, resulting in its flowability. It is 62% filled by weight and 41% by volume (Table 1). According to the manufacturer's instruction, primer (Clearfil SE Bond, Kuraray Co, Ltd, Osaka, Japan) was applied to the etched surface in a thin film and light cured for 10 seconds. The bracket was positioned, and the composite was light cured for 20 seconds.

TABLE 1.

Handling and Flow Characteristics of Composites

Handling and Flow Characteristics of Composites
Handling and Flow Characteristics of Composites
FIGURE 1.

Flowchart for the bonding procedure applied

FIGURE 1.

Flowchart for the bonding procedure applied

Close modal

Debonding procedure

The embedded specimens were secured in a jig attached to the base plate of a universal testing machine (Micro 500, Testometric, Maywood Instruments Limited, Basingstoke Hants, UK). A chisel-edge plunger was mounted in the movable crosshead of the testing machine and positioned so that the leading edge aimed the enamel-composite interface before being brought into contact at a crosshead speed of 0.5 mm per minute. The maximum load necessary to debond the bracket was recorded. The force required to take off the brackets was measured in newtons, and the SBS (1 MPa = 1 N/mm2) was then calculated by dividing the force values by the bracket base area (14 mm2).

Residual adhesive

After debonding, all teeth and brackets were examined under 10× magnification. Any adhesive remaining after bracket removal was assessed with the adhesive remnant index (ARI)6,10 and scored with respect to the amount of resin material adhering to the enamel surface. The ARI scale ranges between 5 and 1, with 5 indicating that no composite remained on the enamel; 4 less than 10% of the composite remained on the tooth; 3 more than 10% but less than 90% remained on the tooth; 2 more than 90% of the composite remained; and 1 all the composite remained on the tooth, along with the impression of the bracket base. The ARI scores were used as a more comprehensive means of defining the sites of bond failure between the enamel, the adhesive, and the bracket base.

Statistical method

Descriptive statistics including the mean, standard deviation, and minimum and maximum values were calculated for each of the four groups of teeth tested. Comparisons of means were made using analysis of variance (ANOVA) and Tukey honestly significant difference (HSD) tests. The chi-square test was used to determine significant differences in the ARI scores among the different groups. All statistical analyses were performed using the SPSS software package (SPSS for Windows, version 10.0.1, SPSS Inc, Chicago, Ill).

Shear bond strength

The descriptive statistics, including the mean, standard deviation, and minimum and maximum values for each of the four groups, are presented in Table 2. Data were analyzed using ANOVA and Tukey HSD tests. The results of this study demonstrate that group 1 had higher SBS values than the flowable composites. Results of ANOVA revealed statistically significant differences in bond strengths among the various groups tested (P < .05). The results of ANOVA and Tukey HSD test are shown in Table 2.

TABLE 2.

Descriptive Statistics of Shear Bond Strength (MPa) of the Four Groups Tested

Descriptive Statistics of Shear Bond Strength (MPa) of the Four Groups Tested
Descriptive Statistics of Shear Bond Strength (MPa) of the Four Groups Tested

Group 1 had a mean value of 17.10 ± 2.48 MPa, the highest of all test groups. Groups 2, 3, and 4 had mean bond strengths of 6.60 ± 3.20 MPa, 7.75 ± 2.90 MPa, 8.53 ± 3.50 MPa, respectively.

Adhesive remnant index

The residual adhesive on the enamel surfaces was evaluated by the ARI scores, and the results are presented in Table 3. The chi-square test indicated that significant differences (χ2 = 77.602, P = .0001) were present among the various groups. In group 1, some or all the composite remained on the bracket. In groups 2–4, the groups where different composites were applied, 90% or more of the composite remained on the tooth (ARI scores 1 and 2).

TABLE 3.

Adhesive Remnant Index (ARI) Scores of the Residual Adhesive on Enamel Surfacea

Adhesive Remnant Index (ARI) Scores of the Residual Adhesive on Enamel Surfacea
Adhesive Remnant Index (ARI) Scores of the Residual Adhesive on Enamel Surfacea

Most bonding studies use commercially available adhesive systems that have different filler particle sizes, viscosities, and concentrations.11 This makes comparisons among the groups difficult because of the increased number of variables involved in the adhesive composition. The aim of the present study was to test different flowable composites, in comparison with a conventional light-cure orthodontic bonding adhesive.

The influence of filler concentration on the viscosity remains a clinically important issue, and the clinical application of specially formulated composites must be addressed.11 The mean SBS values of composites used in this study fell within clinically acceptable ranges. However, flow property is another special feature and is an important clinical consideration that influences both the penetration of the adhesive into the retentive mechanism of the bracket base and the ability of an adhesive to resist bracket drift during direct bonding. Visual and light microscopy evaluation revealed that the composites in this study (particularly, in groups 2, 3, and 4) did not fully penetrate into the retentive mechanism of the bracket bases. However, the clinical acceptability of the flowable composites tested in the present study for direct chair-side bonding of metallic orthodontic brackets appeared to be adequate under their current flow characteristics. Therefore, we believe that the composite had penetrated sufficiently into the retentive pores on the enamel surface.

The mean SBS values of all composites tested were greater than the 5.9 MPa, considered by Reynolds20 to be adequate for routine clinical use. However, for the flowable composites, the 30–40% failure rate was clinically unacceptable (Table 2). The maximum bond strength recommended for successful clinical bonding is estimated to be 7 MPa.21 In another study, shear stresses exerted on attachments during orthodontic treatment were reported to range from 1 to 3 MPa,22 although higher stresses can occur during mastication.23 Studies have shown that metal brackets bonded with composite resins show bond strengths ranging from 2 to 13 MPa.23 However, in the current study, the bond strength for Transbond XT, at 17.10 ± 2.48 MPa, was significantly greater than the bond strengths of the other flowable composites, and this was also greater than that observed in some previous studies,2,24–26 although similar to that found by Sinha et al.27 However, clinical conditions may significantly differ from an in vitro setting. Moreover, heat and humidity conditions of the oral cavity are highly variable. Because of the probable differences between in vivo and in vitro conditions, a direct comparison cannot be made with the findings of the other studies.

Frankenberger et al28 indicated that the flowable composites of thinner viscosity may bond to enamel adequately without the requirement of an intermediate bonding resin. Flows-Rite composite displayed the lowest minimum value in terms of bond strength among all composites, suggesting that application of this material may need a more sensitive technique than the others. In vitro evaluation showed statistically significant differences between the control group and other flowable composites (groups 2, 3, and 4), and our findings were not in accordance with findings of Frankenberger et al.28 Flowable composites are produced mainly for use on etched dentin surfaces. We believe that the manufacturer's recommendation for no primer use on etched enamel surface with Flows-Rite composite makes it unsuitable for bracket bonding. This flowable composite should not be used to replace bonding agents and is not advocated because of the significantly lower SBS values achieved.

The ARI developed by Artun and Bergland29 has been used by many investigators to help standardize bond failure analysis. The ARI may oversimplify the very complex issues of bond failure analysis, but it does allow for statistical analysis and cross-study comparisons. A review of the literature reveals that although many investigators use an ARI system, they often modify the criteria,30 the number system,10,31–34 or both for their project. For the present study, the ARI scores follow the comprehensive criteria used by Bishara et al.6,10,31–34 

Many investigators indicated in SBS studies that metal brackets failed predominantly at the bracket-adhesive interface.11,35,36 When the three flowable composites and one orthodontic adhesive were applied on the etched enamel surface, the failure rate increased and the site of bond failure, statistically significant, shifted toward the bracket-adhesive interface (ARI scores of 1 or 2). The distributions of the ARI scores are shown in Table 3. These findings revealed that the flowable composites did not bond to the bracket base as effectively as did the control group.

Most of the failures of the flowable composites were at the bracket-adhesive interface, but the enamel-adhesive interface was fine. It was believed that possibly a different bracket base may adhere better, or the use of a composite custom base as used in indirect bonding may overcome this weak point. If so, this would make flowable composites unsuitable for direct bonding but possibly suitable for indirect bonding.

In this study, three restorative flowable composites were compared with a conventional orthodontic adhesive (Transbond XT) for in vitro SBS and location of the adhesive failure.

An orthodontic composite resin (Transbond XT) displayed significantly greater SBS values than the other flowable composites (Transbond XT > Flows-Rite = Flow Line = Flow), although the bond strengths for all four composites were clinically acceptable. However, the minimum SBS values of these flowable composites are very low. Clinical conditions may significantly differ from an in vitro setting. Moreover, heat and humidity conditions of the oral cavity are highly variable. Because of the probable differences in in vivo and in vitro conditions, it is not suggested that flowable composites can be applied for orthodontic use.

There were no significant differences in the mean SBS among flowable composites (Flows-Rite, Pulpdent; Flow Line, Heraeus Kulzer; and Filtek Flow, 3M).

Except for the control group (Transbond XT), all the flowable composites tended to display adhesive failure at the bracket-adhesive interface (ARI scores 1 and 2).

We thank Dr Serdar Usumez for his valuable assistance with this project.

1
Buonocore
,
M. C.
Principles of adhesive retention and adhesive restorative materials.
J Am Dent Assoc
1963
.
67
:
382
391
.
2
Newman
,
G. V.
Current status of bonding attachments.
J Clin Orthod
1973
.
7
:
425
449
.
3
Usümez
,
S.
,
M.
Orhan
, and
A.
Usumez
.
Laser etching of enamel for direct bonding with an Er,Cr:YSGG hydrokinetic laser system.
Am J Orthod Dentofacial Orthop
2002
.
122
:
649
656
.
4
Buyukyilmaz
,
T.
,
S.
Usumez
, and
A. I.
Karaman
.
Effect of self etching primers on bond strength? Are they reliable?
Angle Orthod
2003
.
73
:
64
70
.
5
Owens
,
S. E.
and
B. H.
Miller
.
A comparison of shear bond strengths of three visible light-cured orthodontic adhesives.
Angle Orthod
2000
.
70
:
352
356
.
6
Bishara
,
S. E.
,
M. E.
Olsen
,
P.
Damon
, and
J. R.
Jakobsen
.
Evaluation of a new light-cured orthodontic bonding adhesive.
Am J Orthod Dentofacial Orthop
1998
.
114
:
80
87
.
7
Uysal
,
T.
,
F. A.
Basciftci
,
S.
Usumez
,
Z.
Sarı
, and
A.
Buyukerkmen
.
Can previously bleached teeth be bonded safely?
Am J Orthod Dentofacial Orthop
2003
.
123
:
628
632
.
8
Eliades
,
T.
,
G.
Eliades
,
W. A.
Brantley
, and
W. M.
Johnston
.
Polymerization efficiency of chemically cured and visible light cured orthodontic adhesives: degree of cure.
Am J Orthod Dentofacial Orthop
1995
.
108
:
294
301
.
9
Eversoll
,
D. K.
and
R. N.
Moore
.
Bonding orthodontic acrylic resin to enamel.
Am J Orthod Dentofacial Orthop
1988
.
93
:
477
485
.
10
Bishara
,
S. E.
,
V. V.
Gordon
,
L.
Von Wald
, and
M.
Olsen
.
Effect of an acidic primer on shear bond strength of orthodontic brackets.
Am J Orthod Dentofacial Orthop
1998
.
114
:
243
247
.
11
Ostertag
,
A. J.
,
V. B.
Dhuru
,
D. J.
Ferguson
, and
R. A.
Meyer
.
Shear, torsional, and tensile bond strengths of ceramic brackets using three adhesive filler concentrations.
Am J Orthod Dentofacial Orthop
1991
.
100
:
251
258
.
12
Hocevar
,
R. A.
Direct bonding metal brackets with the Concise-enamel system.
J Clin Orthod
1977
.
11
:
473
482
.
13
McLundie
,
A. C.
and
J. G.
Messer
.
Acid-etch incisal restorative materials.
Br Dent J
1975
.
138
:
137
140
.
14
Roberts
,
M. W.
,
J. P.
Moffa
, and
W. A.
Jenkins
.
Clinical evaluation of three acid-etch composite resin systems: two-year report.
J Am Dent Assoc
1978
.
97
:
829
832
.
15
Jorgensen
,
K. D.
and
H.
Shimokobe
.
Adaptation of resinous restorative materials to acid etched enamel surfaces.
Scand J Dent Res
1975
.
83
:
31
36
.
16
Low
,
T.
,
K. W.
Lee
, and
J. A.
von Fraunhofer
.
The adaptation of composite materials to etched enamel surfaces.
J Oral Rehabil
1978
.
5
:
349
355
.
17
Prévost
,
A. P.
,
J. L.
Fuller
, and
L. C.
Peterson
.
Composite and intermediate resin tag formation in acid-etched enamel: a scanning electron microscopy evaluation.
J Prosthet Dent
1984
.
52
:
204
207
.
18
Unterbrink
,
G. L.
and
W. H.
Liebenberg
.
Flowable resin composites as filled adhesives: literature review and clinical recommendations.
Quintessence Int
1999
.
30
:
249
257
.
19
Moon
,
P. C.
,
M. S.
Tabassian
, and
T. E.
Culbreath
.
Flow characteristics and film thickness of flowable resin composites.
Oper Dent
2002
.
27
:
248
253
.
20
Reynolds
,
I.
A review of direct orthodontic bonding.
Br J Orthod
1975
.
2
:
171
178
.
21
Lopez
,
J. I.
Retentive shear bond strengths of various bonding attachment bases.
Am J Orthod
1980
.
77
:
669
678
.
22
Voss
,
A.
,
R.
Hickel
, and
S.
Mölkner
.
In vivo bonding of orthodontic brackets with glass ionomer cement.
Angle Orthod
1993
.
63
:
149
153
.
23
Haydar
,
B.
,
S.
Sarıkaya
, and
Z. C.
Çehreli
.
Comparison of shear bond strength of three bonding agents with metal and ceramic brackets.
Angle Orthod
1999
.
69
:
457
462
.
24
McCourt
,
J. W.
,
R. L.
Cooley
, and
S.
Barnwell
.
Bond strength of light cure fluoride-releasing base-liners as orthodontic bracket adhesives.
Am J Orthod Dentofacial Orthop
1991
.
100
:
47
52
.
25
Meehan
,
P. M.
,
T. F.
Foley
, and
A. H.
Mamandras
.
A comparison of the shear bond strengths of two glass ionomer cements.
Am J Orthod Dentofacial Orthop
1999
.
115
:
125
132
.
26
Lalani
,
N.
,
T. F.
Foley
,
R.
Voth
,
D.
Banting
, and
A. H.
Mamandras
.
Polymerization with the argon laser: curing time and shear bond strength.
Angle Orthod
1999
.
69
:
525
534
.
27
Sinha
,
P. K.
,
R. S.
Nanda
,
M. G.
Duncanson
, and
M. J.
Hosier
.
In vitro evaluation of matrix-bound fluoride releasing orthodontic bonding adhesives.
Am J Orthod Dentofacial Orthop
1997
.
111
:
276
282
.
28
Frankenberger
,
R.
,
M.
Lopes
,
J.
Perdigao
,
W. W.
Ambrose
, and
B. T.
Bosa
.
The use of flowable composites as filled adhesives.
Dent Mater
2002
.
18
:
227
238
.
29
Artun
,
J.
and
S.
Bergland
.
Clinical trails with crystal growth conditioning as an alternative to acid etch enamel pretreatment.
Am J Orthod
1984
.
85
:
333
340
.
30
Lippitz
,
S. J.
,
R. N.
Staley
, and
J. R.
Jakobsen
.
In vitro study of 24-hour and 30-day shear bond strengths of three resin-glass ionomer cements used to bond orthodontic brackets.
Am J Orthod Dentofacial Orthop
1998
.
113
:
620
624
.
31
Bishara
,
S. E.
,
D. E.
Fehr
, and
J. R.
Jakobsen
.
A comparative study of the debonding strengths of different ceramic brackets, enamel conditioners and adhesives.
Am J Orthod Dentofacial Orthop
1993
.
104
:
170
179
.
32
Bishara
,
S. E.
,
L.
Von Wald
,
M.
Olsen
, and
J. F.
Laffoon
.
Effect of time on the shear bond strength of glass ionomer and composites orthodontic adhesives.
Am J Orthod Dentofacial Orthop
1999
.
116
:
616
620
.
33
Bishara
,
S. E.
,
M.
Olsen
, and
L.
Von Wald
.
Evaluation of debonding characteristics of a new collapsible ceramic bracket.
Am J Orthod Dentofacial Orthop
1997
.
112
:
552
559
.
34
Bishara
,
S. E.
,
M.
Olsen
, and
L.
Von Wald
.
Comparisons of shear bond strength of precoated and uncoated brackets.
Am J Orthod Dentofacial Orthop
1997
.
112
:
617
621
.
35
Ødegaard
,
J.
and
D.
Segner
.
Shear bond strength of metal brackets compared with new ceramic bracket.
Am J Orthod Dentofacial Orthop
1988
.
94
:
201
206
.
36
Bradburn
,
G.
and
N.
Pender
.
An in vitro study of the bond strength of two light-cured composites used in the direct bonding of orthodontic brackets to molars.
Am J Orthod Dentofacial Orthop
1992
.
102
:
418
426
.

Author notes

Corresponding author: Tancan Uysal, DDS, Selcuk Universitesi Dis Hekimligi Fakultesi Orthodontics, Konya, Turkey 42079 ([email protected])