The Influence of Etching Times and Fluoride Acid Gels on the Bonding of Orthodontic Brackets

Abstract

This study evaluated the influence of etching times and the addition of fluoride to a phosphoric acid gel on the tensile bond strength of orthodontic brackets bonded to human enamel using a 40% phosphoric acid gel containing no fluoride, 0.44% sodium fluoride and 0.8% stannous fluoride. The percent cohesive failure after debonding was also studied. In addition, the enamel surface was evaluated for reaction products and etching morphology by scanning electron microscopy. Such a reduction in etching times and the addition of fluorides into the etchants should result in less damage to the tooth and a decrease in enamel decalcification.

For the tensile bond strength part of the study, 189 noncarious and nonrestored human premolar teeth were divided into three groups corresponding to the three different etching gels. Each group was subdivided into three other groups which consisted of the three etching times, 15, 30 and 60 seconds. There were a total of 21 samples in each of the nine groups.

After etching with the appropriate gels and etching times, brackets were bonded to the buccal surfaces of the teeth with Concise orthodontic resin. The samples were thermocycled and stored in 37°C until testing. Seven days after bonding, the samples were tested in tension in an Instron testing machine. After debonding, the percent cohesive failure was evaluated under the microscope at 40X.

All groups were compared using the two way analysis of variance. There was no significant difference in tensile bond strength between the three etchants. However, etching with the NaF etchant for 15 seconds produced significantly different results from the other NaF groups. However, there was no significant difference related to etching times for the other two etchants. The highest tensile bond strength (71.7 ± 2 3.0 kg/cm2 ) was recorded with the H3PO4 gel when etched for 60 seconds and the lowest tensile bond strength (48.9 ± 13.6 kg/cm2) was recorded with the NaF etchant when etched for 15 seconds.

There was no significant difference in the percent cohesive failure between the three etchants. There was a significant difference in the NaF group when the etching times were changed. Etching with the NaF group for 15 seconds showed a significant difference when compared with the other NaF groups. However, the other two etchants showed no significant difference between the different etching times. Cohesive failure for all groups occurred mainly at the mesh-resin interface. The highest percent cohesive failure (95.7 ± 14.8%) occurred in the SnF2 group when etched for 30 seconds. The lowest percent cohesive failure (70.9 ± 40.6%) occurred in the NaF group when etched for 15 seconds.

In the second part of this study nine human maxillary premolar teeth were etched with the three different gels at the three time intervals and were evaluated under the SEM at magnifications of 1500X and 5000X for reaction products and etching morphology.

Fluorides incorporated into the etching gels should reduce enamel decalcification, and decreasing the etching times also reduces enamel loss. Results of the present study suggest that incorporating of these fluorides in the etchants, along with decreasing the etching times, warrants clinical evaluation.