Shear bond strength of orthodontic buccal tubes to porcelain

Bonding of molar tubes is becoming more popular in orthodontics. Occasionally, these bonding are done on posterior porcelain crowns or bridges. The purpose of this study was to evaluate the shear bond strength of buccal tubes on feldspathic porcelain crowns with two different methods.

Forty porcelain right molar crowns were fabricated for this study. The crowns were randomly divided into two groups. In group 1, the crowns were etched with 9.6% hydrofluoric acid, silane coupling agent applied, coated with bonding primer and bonded with Transbond XT (3M Unitek, Monrovia, Calif). In group 2, the crowns were etched with phosphoric acid 37%, silane coupling agent applied, coated with bonding primer and bonded with Transbond XT. All the crowns were stored for 24 hours at 37°C and thermo-cycled before the shear bond test. The analysis of variance (ANOVA) was used to determine whether significant difference were present between the groups.

The results of the analysis of variance (F = 0.23) indicated the shear bond strength of group 1 (3.57 ± 0.87 MPa) was not significantly different (P > 0.05) from group 2 (3.46 ± 0.65 Mpa). Fisher's exact test for the adhesive remnant index (ARI) revealed significant difference between both groups (P < 0.05). Eighty percent of group 1 buccal tubes failed at buccal tube/resin interface and eighty percent of group 2 mostly failed at porcelain/resin interface.

Etching with phosphoric acid with the use of silane coupling agent would be safer and should make it easier for clinicians to clean the adhesive on the porcelain surface after debonding.