Adhesion of bonded orthodontic attachments to dental amalgam: In vitro study

Bonding orthodontic attachments to molars is difficult in the presence of extensive buccal amalgam restorations. The purposes of this study were (1) to examine different amalgam surface preparations, (2) to examine properties of adhesive cements to amalgam, (3) to determine the most shear-resistant bonding technique and (4) to discuss whether these shear bond strengths were of adequate magnitude to be of clinical acceptability. The sample consisted of 108 standardized amalgam cylinders divided into 9 groups of 12 based on surface treatment technique and resin type. SPEED brackets (Strite Industries, Cambridge, Ontario) were bonded to amalgam surfaces that were either polished, sandblasted with 50 microm aluminium oxide, or chemically corroded. Adhesives used were Phase II (Reliance Orthodontic Products Inc, Itasca, Ill), Panavia EX (J Morita USA Inc, Tustin, Calif), or C & B Metabond (Parkell, Farmingdale, NY). After thermocycling from 10 degrees C to 50 degrees C 10,000 times, all samples were tested for shear bond strength with the Universal Testing Machine (Instron Corporation, Canton, Mass). The results show significantly higher bond strengths for all of the resin systems when sandblasting of the amalgam surface is used (P <.0001). Only Panavia EX bonded strongly to polished samples, suggesting the presence of a chemical bond. Laboratory acceptable bond strengths to amalgam are possible. The surface characteristics of the amalgam appear to be more influential in the strength of the bond than does the nature of the resin.