Objectives:To assess the shear bond strength of orthodontics brackets cured with a LED unit for 20 seconds vs 40 seconds , and compared to a 40 seconds halogen cured batch.LED units are more and more used by orthodontists because of their ergonomic design and their reliability.They are shown as able to bond an orthodontic bracket under a 10 second flash ,and only few studies are available on this topic. Methods:We used 48 human bicuspids recently extracted and bonded with metal brackets through a light-cured composite(Transbond XT-3M).Teeth were all pumiced and etched for 30 seconds with a 37% phosphoric acid. The specimens were divided in 3 groups: Group 1:LED(Elipar-3M-600mW/cm2-tip:7.5mm) 20" Group 2:LED 40" Group 3(control):Halogen(Optilux 501-850mW/cm2-tip:7,5mm)40" After curing,teeth were stored in water at 37°C for 24 hours and then tested in shear with an Instron 8561 at a crosshead speed of 0,5 mm/min until the bracket debonded.Statistical analysis of the data was carried out by a two-sample Wilcoxon rank-sum(Mann-Whitney) tests. Results:Bonding forces increased with time exposure(p= 0,0037)when using LED(means:6,39 to 10,06 MPa).Both LED cured samples produced highly significant(p=0,0006) lower bond strength values than halogen cured group(mean=14,71MPa) Conclusion:Our study points out that LED devices are less efficient than conventional halogen bulbs for orthodontic bonding.However,LED are clinically adequate according to the mean bond failure strength value registered with a 40" curing time.

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