Direct bonding of orthodontic brackets.

The direct adhesive bonding of orthodontic brackets to enamel was investigated. For a number of resin systems, all based on a bisphenol-GMA type of resin, commercially available or synthesized, the adhesive bonding to the enamel as well as to the bracket was studied. The maximum average bond strength to enamel was 121 Kg. per square centimeter; the maximum adhesion to the bracket material was 53 Kg. per square centimeter. Consequently, the attachment of the adhesive to the bracket material is the bottleneck of the direct-bonding procedure. Special attention has been given to preconditioning procedures to enhance the bonding between adhesive and bracket. The best results were obtained when the bracket base was roughened by means of a small amount of resin powder. Because all adhesives studied show large standard deviations of the bond strength, criteria of reliability are discussed. They show that future approaches should aim not only for higher average bonds strengths but also for a smaller standard deviation. The setting process, one of the factors influencing reliability, has been considered in some detail. Results of different mixing ratios as well as the influence of the amount of catalyst are presented.