- Article
Objectives: The distance between a light-curing tip and an orthodontic adhesive may alter the energy reaching the resin and affect polymerization-related mechanical performance. This study evaluated the effect of 0-, 5-, and 10-mm LED curing distances on resin microhardness, orthodontic bracket shear bond strength, and adhesive remnant index (ARI). Methods: Orthodontic resin was photoactivated for 10 s at 0, 5, or 10 mm. Vickers microhardness was measured in 90 independent resin specimens (30/group; three indentations/specimen) using a 5-N load for 5 s. In a separate tooth-based experiment, 45 extracted permanent human teeth received conventional metallic brackets. After 24 h of water storage, shear bond strength was tested at 1 mm/min and ARI was assessed microscopically. Results: Microhardness decreased from 50.01 ± 11.47 HV at 0 mm to 30.24 ± 7.49 HV at 5 mm and 25.03 ± 8.37 HV at 10 mm (p < 0.001). The difference between 5 and 10 mm was not statistically significant (p = 0.081). Shear bond strength decreased from 4.76 ± 1.31 MPa to 2.46 ± 0.92 MPa and 0.95 ± 0.69 MPa, respectively (p < 0.001). High ARI scores predominated, and the four-category ARI analysis did not detect a significant difference among groups (p = 0.270). Conclusions: Increasing LED curing-tip distance was associated with lower immediate microhardness and bracket bond strength. Microhardness was significantly lower at 5 and 10 mm than at 0 mm, whereas bond strength decreased significantly across all three distances. These outcomes were obtained in separate experimental systems and were interpreted as complementary rather than mechanistically linked. Clinically, the curing tip should be positioned as close and perpendicular to the bracket as access permits, while recognizing that these in vitro findings do not predict long-term bracket survival.
13 November 2026






