Effects of surface conditioning on the shear bond strength of orthodontic brackets bonded to temporary polycarbonate crowns

An increase in the number of adults seeking orthodontic treatment has given rise to new problems for orthodontists, one of which is the need to bond orthodontic brackets to teeth restored with temporary crowns. Many prefabricated temporary crowns are composed of polycarbonates or thermoplastic polymers; bonding to these surfaces is complex because of the composition, surface integrity, and resistance of the crowns. The bond must be sufficient to resist breakage from the forces of orthodontic biomechanics and oral functions including mastication. The purpose of this study was to test, in vitro, the effect of different surface treatments on the shear bond strength of metal and ceramic orthodontic brackets bonded to temporary polycarbonate crowns.

Eighty polycarbonate crowns for the maxillary right central incisor were evenly divided into 4 groups, and the facial surfaces were subjected to one of the following conditions: group A (control): no treatment; group B: the surface was sandblasted with 50 mum aluminum oxide particles; group C: the glazed surface was removed with a diamond bur; and group D: the surface was etched with 9.6% hydrofluoric acid. Precoated Victory metal brackets (3M Unitek, Monrovia, Calif) were bonded to the facial surface of half (n = 10) of the polycarbonate crowns in each group, and precoated Clarity ceramic brackets (3M Unitek) were bonded to the facial surface of the other half (n = 10). Each was debonded with a shear load in a universal testing machine at a crosshead speed of 0.254 mm per minute, and the adhesive remnant index (ARI) was used to analyze the sites of bond failure. Analysis of variance (ANOVA), post-hoc t test with the Bonferroni adjustment, Student t test, and chi-square test with the Yates correction were used for statistical analysis.

There was a significant difference between group B (sandblasting) and all other ceramic and metal groups. In the metal groups, there was a slight difference between group C (diamond bur) and group A (control). According to the ARI, sandblasting was the only surface treatment to significantly affect the adhesion of metal and ceramic brackets to polycarbonate crowns. There was no statistically significant difference between the metal and ceramic brackets in group B (sandblasting). There was a statistically significant difference between metal and ceramic brackets in each group, with the exception of group C (diamond bur), which was just below statistical significance.

Metal and ceramic orthodontic brackets bonded to temporary polycarbonate crowns will most likely not withstand the forces of orthodontic biomechanics. However, sandblasting polycarbonate crowns consistently increased the shear bond strength of metal and ceramic brackets. A diamond bur effectively roughens the surface of a polycarbonate crown but with no gain in bond strength. Likewise, etching the surface of polycarbonate crowns with 9.6% hydrofluoric acid is ineffective. Ceramic brackets bonded to sandblasted polycarbonate crowns produced the highest mean shear bond strength (2.87 MPa), although this value is far below bond strengths with natural tooth surfaces.