Effect of a fluoridated etchant on the shear bond strength of a composite resin to enamel

The purpose of this study was to evaluate the effect of a fluoridated etching gel on the shear bond strength of a composite resin to enamel. A total of 75 extracted human permanent molars were used. A flat enamel surface was obtained with 600-grit silicon carbide paper and cleaned with a rubber cup and a water slurry of fine flour of pumice. The teeth were randomly distributed into 5 groups of 15 teeth each and etched as follows: group 1: 37% non-fluoridated phosphoric acid gel (Coe) for 60 seconds (control); group 2: 60% phosphoric acid gel with 0.5% NaF (Orthoprep) for 5 seconds; group 3: Orthoprep for 15 seconds; group 4: Orthoprep for 30 seconds; group 5: Orthoprep for 60 seconds. After etching, rinsing and drying, an unfilled resin (Coe Bond) was thinly applied with a brush and cured for 30 seconds. A nylon ring was placed over the area and filled with a light-cured composite resin (Occlusin). The teeth were thermocyled (100x), mounted in plastic cups and plaster, and sheared with a knife-edged blade in an Instron machine running at a crosshead speed of 1 mm/min. The results in MPa were as follows: group 1: 14.49 +/- 4.43; group 2: 10.81 +/- 3.70; group 3: 13.51 +/- 3.21; group 4: 14.79 +/- 3.76; group 5: 15.47 +/- 4.07. An analysis of variance showed that the results in groups 1, 4 and 5 were not significantly different but that the bond strengths in groups 2 and 3 were significantly lower. Fracture within the enamel occurred in 8 specimens in group 1, 5 in group 2, 6 in group 3, 8 in group 4, and 13 in group 5. In the fluoridated etchant groups the number of specimens showing enamel fracture increased as the application time of the etchant increased.