Bracket bond strength dependence on light power density

Abstract

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. Stainless steel brackets were bonded to bovine incisors using light-cured adhesive. Six groups of 20 incisors each were exposed to 4s of halogen light with different PD increasing from 500 to 3000 mW/cm(2) in steps of 500 mW/cm(2). Two more groups were exposed to a PD of 3000 mW/cm(2) for 6s (n=15) and 8s (n=19), thus simulating non-available PD of 4500 and 6000 mW/cm(2) for 4s. A halogen lamp with a PD of 1000 mW/cm(2) was used for 40s in the control group (n=15). After storage for 24h at 37 degrees C in water, SBS and adhesive remnant index (ARI) were recorded. 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/cm(2). 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/cm(2). ARI scores showed that higher PD was associated with better adhesion at the bracket/adhesive interface. 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.