Bonding orthodontic acrylic resin to enamel

Bonded acrylic orthodontic appliances are a recent alternative to banded methods. However, the physical properties demonstrated by bonding adhesives at the acrylic-enamel interphase have not been well documented. The purpose of this study was to use a laboratory testing model to evaluate the ultimate shear and tensile strengths and fracture sites of five bonding adhesives: unfilled (Bracketbond and Genie); filled (Unite, Excel, and Concise). One hundred bovine mandibular incisors were embedded in dental stone, labial surfaces ground flat for uniform acrylic-enamel adaptation, and stored in modified Fusyama's artificial saliva. Twenty 7-mm diameter acrylic cylinders were bonded in each group. Ten samples from each group were tested for shear strength and ten for tensile strength with an Instron testing machine. Bonding adhesive retained on the cylinder was measured by means of a Bioquant digitizer. Scanning electron microscopy was used to analyze fracture sites. The results showed that the percentage of bonding adhesive that remains on the acrylic surface after fracture is decreased by the addition of inorganic fillers to the adhesive and the use of separate liquid resin sealants, and is increased by the use of plastic bracket primers and bonding adhesives that are chemically similar to methylmethacrylate. Few significant differences in shear and tensile strengths were found among the bonding adhesives. Unfilled bonding adhesives cause less enamel damage and are indicated for clinical bonding of acrylic orthodontic appliances to enamel.