Interlayer formation and its effect on debonding polycrystalline alumina orthodontic brackets

During the debonding of ceramic orthodontic brackets, there is a risk of causing fractures, cracks, or flaking of enamel or of the bracket itself. Preliminary work on the resin-enamel interface under bonded brackets with the indirect (modified Thomas) or the thermal-cured indirect bonding techniques revealed an interlayer of unfilled resin formed between the filled resin and the enamel surface. The direct bonding technique, on the other hand, showed no such layer. This study was designed to determine the effect of the interlayer on conventional debonding techniques for polycrystalline ceramic orthodontic brackets. Variables examined were bracket failure or fracture (BF), amount of remnant adhesive (ARI), and enamel damage. Brackets were bonded to 90 fresh bovine teeth. These were divided into three groups of 30 each, based on three methods of bonding, i.e., direct, indirect (modified Thomas), and an indirect technique that used a thermal-cured resin. Each bonding group was further divided into three groups of 10 each, based on the type of debonding technique used, i.e., lift off, delamination, and twisting. Brackets bonded by the indirect (modified Thomas, BF x̄ = 0.27, ARI x̄ = 0.93) and the indirect technique that used a thermal-cured resin (BF x̄ = 0.03, ARI x̄ = 0.43) resulted in an overall significantly lower failure (p < 0.01) and ARI score on debonding (p < 0.0001) compared with those bonded by the direct technique (BF x̄ = 1.03, ARI x̄ = 1.97). Specimens evaluated under the stereomicroscope revealed that the brackets bonded with the indirect techniques debonded at the filled-unfilled resin interface or within the interlayer of unfilled resin. Specimens bonded with the direct technique showed failures primarily at the resin-bracket interface. No enamel damage was observed in any of the groups evaluated under a stereomicroscope and a scanning electron microscope. Interlayer formation in the indirect bonding techniques significantly affects the debonding of polycrystalline orthodontic brackets by reducing bracket failure and causing no enamel damage. (AM J ORTHOD DENTOFAC ORTHOP 1995;108:455-63.).