Download Original Manuscript Aged Composite

The effect of surface pre-treatment on bond strength of
orthodontic brackets to aged composite restorations using a
universal adhesive
Introduction:
Back in the days when Angle first introduced his orthodontic appliance system, the
edgewise brackets were soldered to bands which were custom made for each tooth.
1
The entire process was extremely time consuming. The advent of direct bonding
of the orthodontic bracket to the tooth surface has been declared the most
significant advance in the second half of the 20th century.2 For the first time it was
possible to position the orthodontic bracket with precision on the desired location
on the tooth. Ever since the introduction of the acid etch method to dentistry by
Bounocore 3, there has been an ongoing effort to enhance tooth surface preparation
prior to the bonding of composite resins. Many studies have focused on the
replacement of the conventional acid etching method with other surface treatment
procedures including air abrasion 4, 5 and laser treatment 6, 7 while others have been
focusing on improvement of the original method. The time necessary for the
application of the 3 step bonding procedure has been a particular field of interest.
Many generations of bonding agents were developed, each aiming for a reduction
in clinical chair time and addressing the limitations of its predecessors. A fairly
successful development was the self-etching primer (SEP), which eliminated the
need for a separate etching and rinsing.8 The SEPs were assessed and quickly
gained wide-spread acceptance in restorative dentistry.8 The substitution of the
conventional etch and rinse bonding agents with SEPs in orthodontic bracket
bonding to enamel surfaces has been the subject of several studies.9-11 The results
in these studies are contradictory, and while some authors recommended the use of
the SEPs, others reported an inferior performance regarding bond strength.10
Disregarding the label on the SEPs, a few studies investigated the bond strength
obtained by these systems after enamel etching and demonstrated an improvement
in performance.12, 13 The most recent universal or multi-mode bonding systems can
be used in a range of clinical situations.
14, 15
This means that they can be applied
with or without a separate etching step without alteration of their properties or
performance.16
While bonding to enamel surfaces is continuously improved through the evolution
of adhesives, a new challenge has presented itself; bonding to composite surfaces.
Composite resins have replaced the conventional amalgam restorations especially
in the buccal surface of anterior teeth. Therefore many patients undergoing
orthodontic treatment will require bracket attachment to a composite restoration.
Both the etch/rinse method and the SEPs have been demonstrated as being inferior
in providing bond strength with composite resins
17, 18
Authors have investigated
the effect of adding adjunctive treatment including sandblasting, HF etching, bur
roughening and laser treatment in the shear bond strength of adhesives to
composite surfaces. 17-19 Eliasson et. al demonstrated that the addition of a silane
layer can significantly improve bonding to composite resin restorations.20 The
recent universal bonding systems contain silane agents and can therefore provide
better bond strengths compared to conventional adhesives, furthermore they have
to potential to eliminate the need for an additional stage of silane application prior
to bracket bonding. Furthermore the presence of MDP in the chemical composition
of universal bonding agents may provide an additional enhancement in the
chemical adhesion to stainless steel brackets.
In the present study we aim to investigate the bond strength provided by a
universal adhesive (Scotchbond™) to aged composite surfaces. We will compare
the shear bond strength of this procedure with conventional means of bonding to
composite resins. Moreover, we will assess the addition of laser (Er: YAG)
treatment prior to bonding to demonstrate any potential improvements in the bond
strength on orthodontic brackets.
Materials and methods:
Preparing the composite discs
A total of 144 composite resin discs, 6mm in diameter with a thickness of 3mm
will be made from a nano-hybrid resin composite (Filtek Z250, 3M ESPE, St Paul,
Minnesota, USA). For the purpose of obtaining a flat smooth surface, a glass layer
will be placed upon the composite before curing with an intensity of 950 mW/cm2
for 20 seconds.
Initial thermocycling
To simulate the effect of intraoral aging on resin composite restorations, we will be
subjecting the discs to a thermocycling procedure. Following the preparation of the
discs, they will be placed in distilled water at 37 °C for a day. Subsequently
thermocycling will be carried out at 6000 cycles between 5 °C and 55 °C with a
dwelling time of 30 seconds between each cycle.
Treatment of the composite surfaces
The 144 composite discs will be divided randomly to 6 groups (n=24) each
receiving a different method of preparation. The treatment groups will be as
follows:
Group A:
Treatment: Etching with 37% phosphoric acid, rinsing with water and
drying with air. Application of a silane agent.
Bonding agent: Transbond™ XT primer
Group B:
Treatment: Sandblasted with 50µm Al2O3 particles at a pressure of 3.5 to
4.5 bar using a micro-etcher for 7 seconds at a distance of 10mm.
Application of a silane agent.
Bonding agent: Transbond™ XT primer
Group C:
Treatment: Treated using an Er:YAG laser working at a wavelength of
2940 nm and a power of 75mJ-1.1W with an angle of 90 degrees.
Application of a silane agent.
Bonding agent: Transbond™ XT primer
Group D:
Treatment: No surface treatment.
Bonding agent: Scotchbond™ Universal Adhesive
Group E:
Treatment: Etching with 37% phosphoric acid, rinsing with water and
drying with air.
Bonding agent: Scotchbond™ Universal Adhesive
Group F:
Treatment: Treated using an Er:YAG laser working at a wavelength of
2940 nm and a power of 75mJ-1.1W with an angle of 90 degrees.
Application of a silane agent.
Bonding agent: Scotchbond™ Universal Adhesive
Bonding of the orthodontic brackets
All 144 discs will be bonded using right upper incisor brackets (Mini Master
Series; American Orthodontics, Sheboygan, Wisconsin, USA) with a bracket base
area of 10.88 mm2. The light-cure adhesive system which will be used for bonding
of the brackets will be Transbond™ XT (3M Unitek, Monrovia, California, USA).
Before application of the adhesive a thin layer of the same bonding agent used
during surface treatment will be added to the bracket base.
Second thermocycling:
To recreate the oral environment and fatigue the resin composite the bonded
samples will be stored in 37° C distilled water for a day and then subjected to 3000
cycles between 5 °C and 55 °C with a dwelling time of 30 seconds between each
cycle.
Shear bond strength testing
For the shear bond strength testing each sample will be placed in a universal
testing machine in a manner that ensures parallel positioning of the bracket base
with the loading rod. Shear load will be applied to the composite bracket interface
at a speed of 0.5 mm/min until fracture. The results will be recorded in Newtons
and subsequently converted to Megapascals. Half of the brackets in each group
(n=12) will be debonded at 30 minutes post-bonding and before the second
thermocycling to measure the initial bond strength which is significant clinically as
it represents the initial insertion of the archwire.
Adhesive remnant index and SEM evaluation
Following the debonding of the brackets from the composite discs, they will be
subjected to microscopic evaluation (×40) to be categorized according to the
adhesive remnant index (ARI) system based on the amount of adhesive material
remaining on the composite discs. One sample from each group will be selected
randomly and sent for SEM evaluation (×1000).
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