Indirect versus direct bonding: bond strength and failure location.

Bond strengths and failure locations in direct and indirect bonding of orthodontic brackets with foil-mesh bonding pads were compared in an in vitro study that used extracted human premolars. The direct technique comprised bonding the attachments directly to the premolars with composite resin. The indirect technique comprised bonding the attachments to die-stone models of the teeth with composite resin, making silicone positioners to transfer the attachments from the models to the teeth, and bonding to the teeth with the use of two-part unfilled resin. One part of the unfilled resin was applied to the teeth and the other part to the composite resin that was already bonded to the attachments. Placing the positioners on the teeth brought the two parts together to initiate setting. Inspection after bonding disclosed marginal voids in two thirds of the indirect bonds. Of these, two thirds were then sealed with unfilled resin and one third were left defective. There were no significant differences in strength among direct, void-free indirect, and sealed indirect bonds. Indirect bonds with voids were only half as strong. This seems to indicate that sealing around brackets immediately after positioner removal might be a worthwhile clinical routine. Forty-four percent of the direct bonds fractured predominantly at the bracket-adhesive interface, whereas 72% of the indirect bonds failed mainly at the enamel-resin interface. Grouping the data according to failure location showed no difference in bond strength between those that failed at the enamel and those that failed at the bonding pad. Thus the indirect bonding promised similar bond strength and easier debonding because less resin was left on the teeth.