Micro tensile bond strength and SEM evaluation of orthodontic adhesives

Objectives: To evaluate the micro-tensile bond strength (MTBS) of four commonly used orthodontic adhesives to enamel and the differences in adhesion mechanism and mode of failure. Methods: 16 freshly extracted human first lower molars (1-6 months) stored initially in 0.5% Chloramine T. The gelatine capsule method was used to prepare specimens for MTBS testing of (Right-On [R]: paste-liquid contact adhesive, TP Orthodontics, USA; Concise [C]: paste-paste, 3M Unitek, USA; Transbond [T]: light activated paste, 3M Unitek, USA; Fuji ORTHO [F]: liquid-powder GC corporation Tokyo, Japan, a self cured resin modified glass ionomer). The method introduced by Sano et al (1994) was used. Each test group contained 20 hour-glass shape slabs of approximately 0.60 mm thickness and 1mm width along the bonding interface. Debonded surfaces of two specimens from each group were partially demineralised by being immersed in Hydrochloric acid for 20 seconds, and then prepared for SEM, to asses the differences in resin-enamel interfaces among the materials used. Results: The micro-tensile bond strengths to enamel varied among materials used, C was significantly stronger than T, R, and F. Values of MTBS (MPa mean and sd) were: C 36.8 ± 15.4, T 23.2 ± 8.3, R 24.5 ± 6.2, and F 22.8 ± 11.5 (p<0.05, ANOVA and Tukey test). SEM observations showed a poorly developed hybrid layer with enamel for Fuji ORTHO and significantly more highly developed hybrid layer for Concise and Right-On. Conclusion: F (GIC) provided good adhesion to enamel surface and produced a comparable bond strength to other composite materials.