BONDING ORTHODONTIC RESIN CEMENT TO ZIRCONIUM OXIDE UNDER
ORTHODONTICS LOAD AND THERMOCYCLING EFFECT
106
which may influence the test results [23-25]. Therefore,
we opted for a shear bond test which is the closest to
the clinical reality. We assume that the main reason for
bracket failure is shear. The shear bond strength test
was performed with an orthodontic resin composite
cylinder in order to eliminate tilting moments as much
as possible. However, we understand elastic and plastic
deformation of the cylinder may have some negative
effects on the shear bond strength measured. We are
well aware that in orthodontic treatment brackets
transmit forces to the teeth in all directions, however
unexpected debondings usually occur under shear. The
brackets used in this study were used to apply shear
forces to the bonded cylinders and were attached to
the resin-embedding material (Fig. 3).
Thermocycling is a standard procedure for accelerated
ageing in bond strength tests in vitro. In this study, all
the specimens of the TC group were thermocycled in
a chewing simulator device for 5000 cycles between
5°C and 55°C with a dwell time of 30 seconds with the
mechanical load component of the machine turned
off. At the same time, all the specimens of the non-
TC group were stored in distilled water at 37 ± 1°C.
Extensive water storage and thermal cycling seem to be
important parameters to simulate intraoral conditions
and to stress bonding interfaces. Literature data show
that thermocycling had a much higher impact on the
durability of the resin bond strength to zirconia than
did water storage at a constant temperature alone [26].
Loading the samples represented the clinical reality.
In clinical orthodontic practice, bonding of brackets
and placement of the arch wires can be done in the
same visit, particularly after rebonding of debonded
brackets. Hence, force could be applied to the bracket
within the first hour after bonding. This force could
affect polymerization of the orthodontic adhesive and
subsequently its bond strength.
The force magnitude used for orthodontic tooth
movements varies depending on the type of movement
[16]. In this study, 70 ± 15 g (0.69 ± 0.14 N) was applied.
This force is considered the optimal orthodontic force.
In the clinical situation the average force transmitted to
a bracket during mastication was reported to be 40 to
120 N, the surface area of the bracket is approximately
11.9 mm 2 and therefore it should be able to resist
stress values between 6 and 8 MPa [16], during fixed
orthodontic treatment for clinical success [27]. The
mean bond strength values of brackets bonded
to natural teeth are significantly lower than those
obtained for surfaces other than enamel, especially
when a chemical promoter such as silane is used during
bonding [28]. These results may imply that universal
primer application alone, prior to bonding, would
already enhance the bond strength of the orthodontic
resin composite tested. However, it is known that in vitro
bond strength values are