Light-curing of Orthodontic Adhesive by Transillumination Through Dentine and Enamel

Objectives: This study investigated bonding properties of metallic bracket to enamel when light-curing adhesive was cured by transillumination through the tooth, or from the sides of the bracket (conventional technique). Light transmission through slices of dentine and enamel, intact premolars and incisors was measured and plotted against the degree of cure (DC%) of the adhesive and the debonding force (DF).
Methods: Discs of human dentine and enamel (1mm thick) were used to build a dentine-enamel system of total thickness of 3mm and 4mm for determining light transmission through the system. DC% of the adhesive (Transbond XT) was measured through the dentine-enamel system with curing times of 20s, 40s and 60s (n=5). Intact premolars and incisors were used to validate the simulated transillumination curing method by measuring the irradiance power through the tooth and by bonding metal bracket to the tooth enamel. Adhesive was cured by transillumination or from sides of the bracket. DF of the bracket was measured (n=6 incisors, n=10 premolars).
Results: The mean values of transmitted light irradiances were 91.0mW/cm² (3mm), 22.2mW/cm² (4mm), 6.2mW/cm² (incisors) and 37.6mW/cm² (premolars) and there were no statistical difference with the 4mm specimen and the incisors (P>0.05). In all groups, except with premolars, the DC% didn’t increase significantly when the curing time was increased from 40s to 60s (P<0.005).

DF for incisors were 78.0N (conventional technique), 114.4N (transillumination technique) and for premolars 77.3N (conventional technique) and 75.1N (transillumination technique). ANOVA showed significant difference of DF with the incisors (P<0.005), whereas with premolars, the difference wasn’t significant.
Conclusions: Based on the results of this study, it can be concluded that the irradiation power of light curing results in adequate DC% of adhesive and bonding of bracket to the enamel when the light is directed from the side of the bracket or through the tooth.