Influence of Light Intensity on Bond Strength of Self-etch Adhesives

Objectives: This study evaluated the influence of light intensity on surface free energy and bond strength of self-etch adhesives.
Methods: The systems used were: Scotchbond Universal (3M ESPE), Clearfil tri-S Bond (Kuraray Noritake Dental), and G-Bond Plus (GC). Bovine mandibular incisors were wet ground to expose the labial dentin with #600-grid SiC paper. The adhesives were applied according to manufacturers’ instructions. One set of samples was light irradiated at the power densities of 0 (no irradiation), 200, 400 or 600 mW/cm² for 10 sec. The other set was light irradiated with different combinations of light intensity and irradiation time to apply a constant amount of light energy of 4,000 mJ/m². Three irradiation conditions (light intensity. irradiation time) were chosen: 100 mW/cm² for 40 sec; 200 mW/cm² for 20 sec; 400 mW/cm² for 10 sec. Afer light irradiation of the adhesive, resin composite was condensed into a mold and polymerized. Ten samples per test group were stored for 24 h, and then shear bond strength was measured. The surface-free energies of the adhesive surface was determined by measuring the contact angles of three test liquids. Data were analyzed by two-way ANOVA and Tukey’s test (α=0.05).
Results: When the irradiation time was fixed, 400 and 600 mW/cm² groups had higher bond strength (15.3-24.8 MPa) than 0 and 200 mW/cm² groups (25.3-30.3 MPa).; on the other hand, when the total light energy was fixed, 100 mW/cm² resulted in lower bond strength (18.3-20.8 MPa) than 200 and 400 mW/cm² groups (23.0-30.3 MPa). In both sets, the surface-free energies of the adhesive treated dentin surfaces decreased with an increase in the light intensity of the curing unit.
Conclusions: These data suggest that higher light intensities are essential to obtain dentin bond strength, through their effect on the polymerization of the adhesives.