Indirect Bonding Method: in vitro Comparison of the Shear Bond Strength between Metallic Orthodontic Brackets and Different Porcelain Surface Preparations
Background : Bond strength at the metal-ceramic interface of auto-polymerizing resins used in orthodontic indirect bonding has not yet been evaluated and a literature-based clinical protocol is lacking.  Goals : 1) To compare shear bond strength (SBS) between metal brackets and differently treated porcelain surfaces; 2) To suggest efficient and predictable chairside approaches. Materials and methods : Ninety leucite discs (6 groups; n=15/group) were prepared following 6 combinations of mechanical (+/- bur roughening) and chemical (hydrofluoric acid, primer, silane) treatments. Metal brackets with custom composite resin Transbond XT (3M Unitek, Monrovia, California) bases were bonded with the adhesive resin system Sondhi A+B Rapid Set (3M Unitek, Monrovia, California). Samples were stored (H2O/24hrs), thermocycled (500 cycles) and tested (Instron, Norwood, Massachusetts). Maximum SBS and adhesive remnant index (ARI) scores were collected for each sample. ANOVAs were performed on ranks since data was not normally distributed, and then adjusted with the post-hoc Tukey method. A Kruskall-Wallis, a Mann Whitney U pairwise comparison and a Weibull analysis were also performed.  Results : SBS medians of groups ranged from 17.0 MPa (- bur + hydrofluoric acid) to 26.7MPa (- bur + hydrofluoric acid + silane). Bur roughening did not affect bond strength. The chemical preparation of (- bur + primer + silane) showed statistically significant higher SBS than (- bur + hydrofluoric acid) preparation (p<0,05), while having similar SBS to the popular recommended protocol (- bur + hydrofluoric acid + silane). ARI scores were statistically significant lower in group (- bur + hydrofluoric acid) with p<0,05, while all other 5 groups were not different from each other. Percentage of porcelain damage in these 5 groups were very high at 80-100%. Conclusion : All the tested surface preparations combinations provided clinically adequate adhesion for orthodontic tooth movement. A silane and primer combination for porcelain surface preparation is clinically attractive as it is safe and simple and provides great adhesion for orthodontic tooth movement. It is wise to warn patients that there is a risk of porcelain fractures when debonding brackets. If one prefers to avoid porcelain fracture at all cost, one may treat porcelain with hydrofluoric acid only.
Sondhi Porcelain Ceramic Surface preparation Indirect bonding Metal bracket Orthodontic