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School of Dentistry, The University of Birmingham, St Chads Queensway Birmingham B4 6NN, U.K.
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Abstract |
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 Top Abstract IntroductionObjectives of the Present...Materials and MethodsResultsDiscussionConclusionsReferences |
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Two types of orthodontic adhesives were used. Transbond XT and Right-On. Three different
curing times were evaluated with the APC brackets in order to find the best. Adhesive remnants
on the
enamel surface following debond were evaluated using the adhesive Remnant Index (Artun
and Bergland, 1984
).
Bond strengths ranged from 11.00 to 22.08 MPa. For both types of brackets Transbond produced a significant increase in bond strength compared to Right-On. The Dyna-Lock/Right-On combination produced the poorest results. APC brackets cured for 40s had similar bond strengths to uncoated brackets fixed by means of Transbond. Overall, 79 per cent of specimens had less than half the tooth surface covered with adhesive following debond. Significantly more adhesive remained on tooth surfaces following debond of the Straight-Wire/Right-On group than any other bracket/adhesive combination.
Bond strengths were higher with light-cured Transbond than with chemically-cured Right-On. When Transbond is used in association with APC brackets a 40-second cure time is recommended.
Key words: Adhesive Precoated Brackets, Bond strength, Bracket base
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Introduction |
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TopAbstractIntroductionObjectives of the Present...Materials and MethodsResultsDiscussionConclusionsReferences |
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) and development of modern
adhesive
materials has lead to the widespread use of bonded attachments in fixed appliances. Several
factors influence the strength of the bond obtained, including the nature of the enamel surface,
enamel conditioning procedures, the type of adhesive used, and the shape and design of the
bracket base.
Composite resins are the most popular orthodontic adhesives. Early self-polymerizing materials
were two-paste systems, one paste containing an activator and the other an initiator. Two-paste
systems were time-consuming and multiple mixes were often required for a full mouth
`bond-up'. Another disadvantage was that air bubbles incorporated into the
adhesive during mixing could have adverse effects if entrapped under bracket bases (Mitchell, 1994), No-mix systems were therefore developed to improve the handling
properties of orthodontic adhesives.
Light-cured composite resin systems provide clinicians with virtually unlimited working time,
thus allowing more accurate bracket placement and easier removal of excess adhesive before
setting is initiated. There are, however, the potential disadvantages of increased bonding time and
the possibility of incomplete polymerization under metallic brackets due to insufficient exposure
to the curing light. This may result in reduced bond strength (Sargisson et al.,
1995). However, other reports that have examined this variable have produced
conflicting results (Wang and Meng, 1992; Pearson, 1995).
Three main bracket base designs are commercially available: mesh, integral bases with undercut
channels, and micro-etched. Mesh bases are generally made by the lamination of a fine mesh to a
foil. The bracket body and the base are assembled with a light weld tack, after which a paste
containing brazing alloy is applied to the joint (Matasa, 1996). The first
integral
bases available were Dyna-Lock 3M Unitek, P.O. Box 1, Bradford, BD5 9UY brackets in which
retention is provided by horizontal undercut channels open at the mesial and distal extremities,
with a V-grooved pattern running vertically on the surface of the base.
Adhesive Precoated Brackets (APC) (3M Unitek, P.O. Box 1, Bradford, BD5 9UY) are available
as both metallic and ceramic types. The precoated composite used is a version of Transbond (3M
Unitek, P.O. Box 1, Bradford, BD5 9UY), modified to give increased viscosity (Bergstrand,
1996,
personal communication). Cooper et al. (1992)
listed the following advantages of APC over conventional light-cured systems:
In addition, improved control of both the bracket and composite resin-associated with the use of
APC is claimed to improve bond strength and thereby reduce clinical failure rate (3M
Unitek
product literature, 1997; F. Bergstrand, 1996, personal
communication.)
Only a few studies have evaluated the bond strength of APC brackets. Bearn et al. (1995) compared the ex vivo shear bond strength of metallic
APC
brackets with that of identical brackets bonded with Transbond and found no significant
differences between the two.
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Objectives of the Present Study |
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TopAbstractIntroductionObjectives of the Present...Materials and MethodsResultsDiscussionConclusionsReferences |
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Materials and Methods |
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TopAbstractIntroductionObjectives of the Present...Materials and MethodsResultsDiscussionConclusionsReferences |
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). The study design required specimens to be divided into seven groups, with 10 in each group. All of the tubes containing collected teeth were mixed together thoroughly and 10 specimens were allocated to each group by a process of physical randomization.
Using a hand-held fret-saw, each tooth was sectioned about 1 mm below the cemento-enamel junction.
The crowns were then mounted in cold cure acrylic contained within brass cylinders, so that the
buccal surfaces were parallel to and projected slightly above the rim of the cylinder as described
by Bin Abdullah and Rock (1996).
Enamel preparation and bracket bonding was standardized as follows:
The appropriate primer was then applied to the etched enamel surface and bracket base, and the
tooth was lightly blown with air to ensure that only a thin layer of primer remained. APC
brackets were already precoated with adhesive, and therefore were applied directly to the etched
and primed enamel surfaces. For the other two bracket types the appropriate adhesive was loaded
onto the bracket base which was then placed on the LA point as in the clinical situation (Andrews,
1976). Brackets were seated with firm pressure to minimize the thickness of the
resin film and a probe was used to remove excess resin from around the bracket before it had set.
Light curing of Transbond was achieved with an Ortholux XT light unit 3M Unitek, P. I. Box 1,
Bradford, BD5 9UY, U.K. directed for 10 seconds mesially and then 10 seconds distally at each
bracket. Three groups of APC brackets were evaluated with 10, 20, and 40 light curing.
Following bonding, all specimens were stored in distilled water in darkness for 24 hours.
Bond Strength Testing
During testing, a brass cylinder with its embedded tooth was assembled in a special jig
manufactured to fit the lower crosshead of the Instron machine (Extra, model 1185). A
looped 0.018 x 0.025-inch stainless steel wire was attached to the fixed upper
crosshead and passed beneath the bracket wings (Fox et al., 1994;
Figure 2). The jig allowed the brass cylinder to be adjusted so that the
shear
forces were at right
angles to the long axis of the bracket body. Specimens were mounted so that the direction of
force application was occlusogingival. During testing, the lower crosshead was moved down at
5mm/min., and measurements were read on a scale of 5:1.
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Statistical Analysis
Data relating to bond strengths for various bracket/ adhesive combinations were analysed using
two-way ANOVA, followed by paired t-tests to investigate differences between groups. The
effect of curing time on APC bond strength was studied by means of one-way ANOVA and
Tukey's pairwise comparisons.
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Results |
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TopAbstractIntroductionObjectives of the Present...Materials and MethodsResultsDiscussionConclusionsReferences |
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The results of Adhesive Remnant Index scoring for the various groups are shown as Table 2. The most frequent ARI score for most groups was 1, with the exception of the Straight-Wire/Right-On combination which had a majority of ARI 2 scores. None of the specimens tested scored 3.
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Discussion |
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TopAbstractIntroductionObjectives of the Present...Materials and MethodsResultsDiscussionConclusionsReferences |
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).
The method for specimen preparation and testing followed the guidelines recommended by Fox et
al. (1994). Care was taken to adhere to the protocol and so minimize
intra-group variation. This is evident by the low standard deviations for stress values in Table 1
and gives assurance that the number of specimens per group was adequate. Although Fox et
al. recommended a minimum of 20 specimens per group, their survey of 66 studies revealed
10 to be the group size most often employed.
Results of shear bond strength testing are presented in units of Newtons (N) for load and
MegaPascals (MPa) for stress to allow for comparison with other studies. The conversion of
Newtons to MegaPascals (N/mm2) was made by dividing load by the bracket base
area of 13.47 mm2 for Dyna-Lock brackets and 13.38 mm2 for
Straight-Wire brackets. This was calculated by measuring 10 brackets with a
digital calliper and taking the mean. Shear bond strengths ranged from 11.00 to 22.32 MPa. This
is larger than the 6-MPa recommended by Reynolds (1975) and Whitlock
et al. (1994) as adequate for orthodontic purposes. The
Dyna-Lock/Transbond
combination produced the highest bond strength, while Dyna-Lock/Right-On gave the lowest.
It is apparent that for both types of brackets light-cured Transbond consistently produced
significantly higher bond strengths than the chemically-cured adhesive Right-On. The data
therefore does not support the findings of Sargisson et al. (1995)
who
found no significant differences between the ex vivo shear bond strengths of Transbond
and Right-On.
Transbond performed equally well with the two types of bracket bases. Right-On adhesive,
however, gave significantly inferior results with Dyna-Lock brackets, in agreement with the work
of Ferguson et al. (1984) who suggested that the poor
performance
of
Dyna-Lock brackets with Right-On adhesive was due to incomplete blending of the two
dissimilar phases of Right-On in association with undercut-case brackets.
A 40-second light curing time significantly increased bond strength with APC brackets. This is in
agreement with the findings of Wang and Meng (1992) who reported
higher
bond
strengths with Transbond XT when light curing was increased from 20 to 40 seconds. In the
present study no significant differences were detected between the bond strengths of the APC-10-
and 20-second groups, respectively. After a 40-second cure APC brackets produced similar bond
strengths to Dyna-Lock brackets when adhesive was applied by the operator. This finding agrees
with the conclusions of Bearn et al. (1995).
The Adhesive Remnant Index (Artun and Bergland, 1984) provides an
easy
method of evaluating adhesive remnants following debond. Almost 80 per cent of all specimens
had less than half the tooth surface covered with adhesive. The Straight-Wire/Right-On group,
however, had a preponderance of 2 scores more than 5 per cent tooth coverage. It is difficult to
explain why this occurred as the combination of a mesh base with a lightly filled composite resin
should provide a strong base/adhesive interlock so that the majority of the adhesive would come
away with the bracket.
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Conclusions |
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TopAbstractIntroductionObjectives of the Present...Materials and MethodsResultsDiscussionConclusionsReferences |
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Results of ex vivo studies should be interpreted with care. A prospective randomized clinical trial is currently under way to test the clinical performance of the materials used in this study using the same bonding technique. It is hoped that this will also give some indication of the clinical applicability of ex vivo bond strength testing.
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Acknowledgments |
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References |
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TopAbstractIntroductionObjectives of the Present...Materials and MethodsResultsDiscussionConclusionsReferences |
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Artun, J. and Bergland, S. (1984) Clinical trials with crystal growth conditioning as an alternative to acid-etch enamel pretreatment,American Journal of Orthodontics , 85,333 –340.[Medline]
Bearn, D. R., Aird, J. C. and McCabe, J. F. (1995) Ex vivo bond strength of adhesive precoated metallic and ceramic brackets,British Journal of Orthodontics , 22,233 –236.[Abstract]
Bin Abdullah, M. S. and Rock, W. P. (1996) The effect of etch time and debond interval upon the shear bond strength of metallic orthodontic brackets,British Journal of Orthodontics , 23,121 –124.[Abstract]
Buonocore, M. G. (1955)
A simple method of increasing the adhesion of acrylic filling materials to enamel surfaces,Journal of Dental Research
, 34,849
–853.
Cooper, R. B., Goss, M. and Hamula, W. (1992) Direct bonding with light-cured adhesive precoated brackets,Journal of Clinical Orthodontics , 26,477 –479.
Ferguson, J. W., Read, M. J. F. and Watts, D. C. (1984) Bond strength of an integral bracket-base combination: an in vitro study,European Journal of Orthodontics , ,267 –276.
Fox, N. A., McCabe, J. F. and Bucklet, J. G. (1994) A critique of bond strength testing in orthodontics,British Journal of Orthodontics , 21,33 –43.[Abstract]
Matasa, C. (1996) For whom the bells toll: milling, casting, or injection molding?The Orthodontic Materials Insider , 9, No. 1.
Mitchell, L. (1994) Orthodontic bonding adhesives,British Journal of Orthodontics , 21,79 –82.[Medline]
Pearson, A. (1995) Optimal light curing of adhesive precoated brackets,Journal of Clinical Orthodontics , 2, , 583 –585.
Reynolds, I. R. (1975) A review of direct orthodontic bonding,British Journal of Orthodontics , 2,171 –178.
Sargison, A. E., McCabe, J. F. and Gordon, P. H. (1995) An ex vivo study of self-, light-, and dual-cured composites for orthodontic bonding,British Journal of Orthodontics , 22,319 –323.[Abstract]
Tavas, M. A. and Watts, D. C. (1979) Bonding of orthodontic brackets by trans-illumination of a light activated composite: an in-vitro study,British Journal of Orthodontics , 6,207 –208.[Medline]
3M Unitek product literature (1997) Ref. 16-871-1
Wang, W. N. and Meng, C. L. (1992) A study of bond strength between light- and self-cured orthodontic resin,American Journal of Orthodontics and Dentofacial Orthopedics , 101,350 –354.[Medline]
Whitlock, B. O., Eick, J. D., Ackerman, R. J., Glaros, A. G. and Chappell, R. P. (1994) Shear strength of ceramic brackets bonded to porcelain,American Journal of Orthotontics and Dentofacial Orthopedics , 106,358 –364.
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| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
