Conversion of Covered Bracket Bonding Adhesive Using Various Curing Lights

Bonding of orthodontic brackets is accomplished through curing of a resin adhesive beneath the bracket by the application of a curing light adjacent to the bracket. The adhesive under the bracket has no direct exposure to the curing light and requires light to be refracted and reflected off of the tooth enamel to be photopolymerized.

Objective: Identify the difference in degree of conversion of bracket bonding resin when exposed to different curing lights versus being occluded by metal.

Methods: An orthodontic bracket adhesive (Transbond XT) was placed in a mold set on flat bovine enamel and covered with a glass slide. Half of the surface area of the bonding adhesive was then covered with a piece of tin foil and a curing light was placed over the glass slide on the uncovered portion of the resin. This procedure was repeated using three types of curing lights: quartz tungsten halogen (QTH), light-emitting diode (LED), and xenon plasma arc (XPA). The exposure times were adjusted so that the total energy of irradiance was equal for all three curing lights. The conversion in the covered and uncovered portions of the adhesive was calculated from near-IR spectra and the differences in conversion statistically analyzed.

Results: The degree of conversion did not statistically significantly vary among the different curing lights when the samples were directly exposed to the light (40±1% conversion). A statistically significant difference was found for the area covered by the metal with the QTH light having a greater decrease in conversion (14.4%) when compared to that produced by either the LED (6.7%) or XPA (4.8%).

Conclusion: The degree of conversion of bracket bonding adhesives under a cover is increased when using LED or XPA curing lights compared with QTH.

Supported by a gift from Septodont