Shear bond strength of orthodontic brackets bonded with light-emitting diode curing units at various polymerization times

Light emitting diode (LED) light-curing units have recently been used to polymerize resin-based orthodontic adhesives, and preliminary studies indicate their use has been successful. The purpose of this study was to evaluate the relationship between the shear bond strength of orthodontic brackets bonded to enamel and the duration of photopolymerization with LEDs and conventional quartz-tungsten-halogen light-curing units. Three LED light-curing units (GC e-light, GC America, Alsip, Ill; Elipar FreeLight, 3M ESPE Dental Products, St Paul, Minn; and UltraLume LED 2, Ultradent Products, South Jordan, Utah) and 1 halogen-based light-curing unit (Ortholux XT, 3M Unitek, Monrovia, Calif) were evaluated. Two hundred forty metal orthodontic brackets were bonded to extracted molars. Specimens were divided into 12 groups of 20 teeth each. Each group was cured with a different light-curing unit for 40, 20, or 10 seconds. The specimens were stored in water at 37° C for 24 hours and then subjected to a shear force with a universal testing machine until bracket failure. Two-way ANOVA detected significantly weaker mean shear bond strength with the GC e-light at 10 and 40 seconds ( P< .001) and higher mean shear bond strength for the UltraLume LED 2 at 40 seconds ( P< .001). All experimental groups had laboratory mean shear bond strengths greater than 8 MPa, even with a 10-second cure.