Bracket bond strength dependence on light power density

Objectives In order to reduce curing time for bracket bonding with light-cured composites, manufacturers increase the power density (PD) of light sources. The present study aims at investigating the relationship between PD and shear bond strength (SBS) at short exposure time. Methods Stainless steel brackets were bonded to bovine incisors using light-cured adhesive. Six groups of 20 incisors each were exposed to 4 s of halogen light with different PD increasing from 500 to 3000 mW/cm2 in steps of 500 mW/cm2. Two more groups were exposed to a PD of 3000 mW/cm2 for 6 s (n = 15) and 8 s (n = 19), thus simulating non-available PD of 4500 and 6000 mW/cm2 for 4 s. A halogen lamp with a PD of 1000 mW/cm2 was used for 40 s in the control group (n = 15). After storage for 24 h at 37 °C in water, SBS and adhesive remnant index (ARI) were recorded. Results SBS was significantly different among groups (ANOVA, p < 0.001). SBS comparable to the control group could only be achieved with a PD of at least 3000 mW/cm2. An exponential model described the relationship between SBS and PD. The best-fit curve based on this model reached 85% of the highest possible SBS at approximately 4000 mW/cm2. ARI scores showed that higher PD was associated with better adhesion at the bracket/adhesive interface. Conclusions Our findings show the SBS dependence on PD, and thus provide a valuable tool for the development of light-curing systems. An exponential model suggests that SBS enters a region of saturation and cannot be improved significantly by further increasing PD.