Light Emitting Diode Curing Units in Orthodontic Bonding

Objectives: This LED bond strength study (1) evaluates the effect of increasing exposure times on the shear bond strength (SBS), (2) compares SBS with the light source maintained at 1 and 10mm from the composite/bracket base. Materials and Methods: Bovine incisors, 120 randomly divided into 8 groups, had stainless steel maxillary incisor brackets (3M Unitek, Monrovia, CA) bonded (Transbond XT, 3M Unitek) to the facial surfaces. The bond of each specimen was tested to failure using an Instron Universal Testing Machine (Instron Corp, Canton, MA). Each fracture site was examined to evaluate the mode of failure using the ARI index (Artun, Bergland 1984). Results: Mean SBS increased for each increasing time period in a logarithmic curve. Two-way ANOVA revealed no significant interaction between time and distance with respect to SBS. There also were no significant differences in SBS between the 1mm and 10 mm distances. There were significant (p < 0.001) differences between exposure times. Post-hoc tests revealed that SBS at 5 second exposures were significantly less than at 20 second (p < 0 .001) and 40 second (p < 0.001) exposures. In addition, SBS after 10 seconds of exposure were significantly less than those after 40 seconds (p =0.004). The chi-square test revealed no significant differences in the frequencies of ARI scores in relation to distance. The frequencies of ARI scores were significantly different in relation to exposure time (p < 0.001), with significantly more adhesive failures than cohesive failures. Significant differences in frequencies of ARI scores were seen when comparing the 5 second cure time to other time periods. Conclusions: SBS increased with increasing time periods in a curvilinear fashion, with no difference between the distances evaluated from source to specimen.