Abstract
This study investigated the effect of a modified photoactivation protocol using two simultaneous light-curing units on the shear bond strength (SBS) of brackets to enamel. Metal brackets were bonded to bovine incisors using the resin-based orthodontic cement Transbond XT (3M Unitek). Four photoactivation protocols of the orthodontic cement were tested (n = 15): photoactivation for 10 s on each proximal face of the bracket at a time; Simultaneous: photoactivation for 10 s on both proximal faces of the bracket at the same time; One side-20s: photoactivation for 20 s at one proximal face of the bracket only; and One side-10s: photoactivation for 10 s only at one proximal face of the bracket. SBS was tested immediately or after 1000 thermal cycles. Adhesive remnant index (ARI) was classified. Data were subjected to two-way ANOVA and Student-Newman-Keuls' test (α = 0.05). Pooled means ± standard deviations for SBS to enamel (MPa) were: 10.2 ± 4.2 (CONTROL), 9.7 ± 4.5 (Simultaneous), 5.6 ± 3.1 (One side-20s), and 4.6 ± 1.9 (One side-10s). Pooled SBS data for immediate and thermal cycled groups were 6.3 ± 2.6 and 8.8 ± 5.2. A predominance of ARI scores 1-2 and 0-1 was observed for the immediate and thermally cycled groups, respectively. In conclusion, simultaneous photoactivation of the orthodontic cement using two light-curing units, one positioned at each proximal face of the bracket, yielded similar bonding ability compared to the conventional light-curing method. Photoactivation of the orthodontic cement at one proximal face of the bracket only is not recommended, irrespective of the light-curing time used.
Highlights
The development of adhesive materials has significantly facilitated the clinical practice in orthodontics [1,2]
No statistically significant differences were observed among the photoactivation protocols in the storage times immediate (p=0.589) and thermally cycled (p=0.481)
The higher bond strength observed for the protocols that involved photoactivation in both proximal faces of the bracket is explained by a higher exposure of the orthodontic cement to light during these photoactivation methods
Summary
The development of adhesive materials has significantly facilitated the clinical practice in orthodontics [1,2]. Polymerization of the material used for bracket fixation depends on the access to curing light; orthodontic devices in general block the direct passage of the light electromagnetic spectrum For this reason, multiple light exposures of the bracket-enamel interfaces are required to deliver a minimum radiant exposure (energy dose, J/ cm2) for appropriate curing of the orthodontic cement [3]. The development of new photoactivation materials and/or protocols able to reduce the time to light exposure without interfering in the bond strength is pertinent. This is relevant considering that finding the optimal tooth position at which to place and bond a bracket is already time-consuming in the clinical practice [6]
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