Influence of light dose on bond strength of orthodontic light-cured adhesives

Although the polymerization reaction in light-cured orthodontic adhesive continues for some time after light irradiation, it is unclear whether insufficiently irradiated adhesive develops sufficient bond strength. This in vitro study examined the maturation of bond strength after exposure of a variety of light doses. Large metal brackets were bonded to the enamel of 288 bovine mandibular incisors by irradiation at two light intensities (200 and 400 mW/cm(2)) and for three exposure times (3, 5, and 10 seconds) using three orthodontic adhesives (TB, OP, and BOB). Shear bond strengths and adhesive remnant indices (ARIs) were determined immediately (T1) and 24 hours after bonding (T2; n = 8 in each group). Comparisons were made using the Kruskal-Wallis H-test, the Bonferroni-corrected Mann-Whitney U-test, and the Yates-corrected chi-square test. Bond strengths of the adhesives that showed maturation at low light intensity (200 mW/cm(2)) increased by 1.4- to 2.0-fold in 24 hours. An increase in exposure time increased bond strength more than did an increase in light intensity for most orthodontic adhesives. With an exposure time of 3 seconds at 200 mW/cm(2), the ARI scores of TB and OP differed significantly between T1 and T2. Thus, the most acceptable procedure when applying low-dose light intensity to a bracket before the placement of a wire is to increase the exposure time and/or wait for sufficient maturation of bond strength.