Influence of radiant exposure values from two third generation LED curing units on polymerization profile and microhardness of orthodontic composite under ceramic and metallic brackets

ABSTRACT Introduction: Third generation of LED light curing units might be used in short exposure periods for orthodontic brackets bonding. Objective: This study evaluated the effect of the different radiant exposure (RE) values: Manufacturers’ instructions (MI), ½ MI, 1/4 MI and Turbo mode. Two third-generation LED curing units were used: VALO® and Bluephase 20i® . The degree of conversion (DC) and Vickers hardness (VHN) of an orthodontic composite (OC) (Transbond XT) under metallic (MB) or ceramic brackets (CB) were measured. Methods: OC was applied to the bracket base, which was then placed over an attenuated total reflectance (ATR) table coupled to an infrared light spectroscope, or to a glass surface for the VHN analysis. The specimens were light-cured and DC values were calculated. The VHN was obtained in a microhardness tester. The data were analyzed with 2-way ANOVA followed by Tukey’s post-hoc test (pre-set α=0.05). Linear regression analysis evaluated the relationship between RE values and dependent variables. Results: CB allowed higher DC and VHN values than MB (p< 0.001). No significant difference was noted among groups when CB were used. For MB, MI groups showed the highest DC and VHN values. A significant, but weak relationship was found between delivered RE values and dependent variables. Conclusions: The decrease in RE values from third generation LED CU did not jeopardize the DC values when CB were used, but can compromise DC and VHN values when MB are used.

Infl uence of radiant exposure values from two third generation LED curing units on polymerization profi le and microhardness of orthodontic composite under ceramic and metallic brackets

INTRODUCTION
The treatment success with fixed orthodontic appliance depends substantially on the accurate bracket bonding to enamel surface. The "adhesive" dentistry became viable after the introduction of the enamel etching with phosphoric acid, by Buonocore, 1 and the release of resin composites.
In Orthodontics, this advance allowed predictable direct bonding of brackets to enamel surface. 2 Different types of orthodontic composites (OC) have been used in clinical practice. Some are light-cured materials, others are self-cured resins, and there are also dual-cured OCs, which have both photoinitiators and self-curing components in their composition. 3 Although these products have shown acceptable mechanical proprieties, 4 the use of light-curing units (LCU) is required regardless of the OC type, to ensure that brackets are bonded without wasting chairtime, once photo-activated polymerization is considerably faster than self-cured polymerization. 5,6 Indeed, in order to provide optimal degree of conversion (DC) and mechanical properties of OCs, the radiant emittance values must be considerably high. 7 In this regard, recently, third generation light-emitting diode (LEDs) curing units have become available for dental practitioners. 8 Also known as multi-peak LCUs, these LED devices are capable of emitting light with varying wavelength ranging from 390 nm to 490 nm. 8,9 Arana AFM, Justus B, Dávila-Sánchez A, Sugahara MO, Coelho U, Farago PV, Arrais C -Influence of radiant exposure values from two third generation LED curing units on polymerization profile and microhardness of orthodontic composite under ceramic and metallic brackets 5 The polymerization efficiency of OCs depends on the radiant emittance values, exposure time, and the light source. In general, the physical and mechanical properties of resin-based materials are closely related to the DC. 10,11 In addition, poor monomer conversion results in monomer leaching and the release of plasticizers and polymerization initiators. 12,13 Such an issue is a matter of concern as monomer leaching from poorly polymerized resin-based composites has been associated with metabolic diseases, problems in gene expression, 14 and also problems in immune responses. 15 Despite the advances in adhesion and LED technology, the currently used bonding protocol for metallic and ceramic brackets still remains a time-consuming procedure, once clinicians usually avoid short exposure intervals. Longer chairtime also increases the chance of bonding failures due to contamination, mainly in posterior and lower teeth. 16 In this regard, some in vitro studies have evaluated the influence of LCU types and shorter exposure periods of LCU on monomer conversion of OCs. 17 Although most studies properly addressed this issue and observed the influence of exposure period and LCU type, the differences in products and curing protocols among studies resulted in controversial findings. In addition, none of these studies evaluated the influence of both metallic and   Table 1.

ANALYSIS OF THE DEGREE OF CONVERSION
Therefore, the RE values delivered to the specimens ranged from 6 J/cm 2 to 22.9 J/cm 2 when metallic brackets were used, and from 2.85 J/cm 2 to 11.4 J/cm 2 when ceramic brackets were used (Table 1). In addition, in an attempt to simulate the clinical scenario where LCU tip is placed on the mesial and distal portion of the metallic bracket, the LCU tip was placed in two directions, so light was delivered for half exposure period in each side. Conversely, when ceramic brackets were used, the LCU tip was placed directly against the ceramic bracket. Seven specimens were evaluated for each experimental group (n=7).   For VHN, 80 metallic and ceramic brackets were used. Prior to OC placement, both brackets had the rough back-surface smoothed to remove any retention, so the OC could be removed and the bracket could be reused. The OC was applied to the bracket according to MI, and the set orthodontic bracket/composite layer was placed on a glass surface. The LCU tip was placed on the bracket and the specimens were exposed to the LED CUs at varying exposure intervals, as previously describe.

LED LCU
Hardness was immediately evaluated with microhardness indenter (Microhardness tester-Shimadzu Corporation, Kyoto, Japan). VHN analysis was performed as reported by Garcia-Contreras et al. 19 A diamond indenter was applied to the OC surface at 5 N or 50 Kgf, and a 15-s indentation interval was used. Five indentations were obtained on each corner, resulting in a total of 20 indentations in each specimen (Fig 1).

STATISTICAL ANALYSIS
Because light beam profiles of the evaluated LCUs were not similar, no comparison between results of each tested LCU was made. Therefore, the DC, VHN and Rpmax (%/s) values were evaluated using 2-way ANOVA ("exposure mode" and "bracket type" as independent variables) within each LCU, followed by Tukey's post-hoc test at a pre-set alpha of 5%.    are shown in Table 4. Two-way ANOVA detected statistical significance for the interaction between independent variables "exposure mode" and "bracket type", regardless of LCU (p<0.001).
When the OC layer having metallic bracket was exposed to light emitted from VALO, MI group exhibited the highest VHN values, which were significantly higher than those of the other groups (p<0.001). No significant difference in VHN values was observed between 1/2-MI and Turbo groups, which in turn

DISCUSSION
In  (Figs 3 and 4). Therefore, the second conditions. In addition, the profile of polymerization kinetics in groups exposed to Plasma or Turbo modes at short exposure periods were similar to those observed in the control groups, corroborating the exposure reciprocity law previously observed in most photo-activated resin-based composites. 24 In this study, DC and VHN values were measured approximately 7 min after light exposure to LED CUs. Therefore, once polymerization of resin-based composites may continue for over 24 hours, further increase in DC values is expected. However, it should be emphasized that evaluating initial monomer conversion and hardness of orthodontic composites is crucial for the success of orthodontic treatment, as these products are subjected to tension soon after they are exposed to curing light. Thus, OCs should achieve optimal monomer conversion and mechanical properties within the first minutes after exposure to light emitted from LED CUs. 25 In addition, the current results were based on one commercially available photo-activated composite with camphorquinone as the main photoinitiator. As a consequence, the results should not be extrapolated to products with other photoinitiators. The current results cannot predict the actual influence of these exposure modes on bond strength and long-term consequences of bonding to enamel surface. Further investigation is required to address these issues.