Bond strength of orthodontic brackets to polymethylmethacrylate: effect of the surface treatment and adhesive system

Abstract

Objectives: To evaluate the influence of the surface treatment and adhesive system on the shear bond strength and the failure mode of orthodontic brackets bonded to polymethylmethacrylate surfaces. Methods: Ninety metal brackets (n=15) were bonded to aged discs of polymethylmethacrylate SR Ivocron subjected to three surface conditions (no treatment; sandblasting with 50-μm aluminum oxide; roughening with a tungsten bur), using two combinations of adhesives (methylmethacrylate monomer + Transbond XT Primer; Scotchbond Universal Adhesive) followed by the composite Transbond XT. In the control group, metal brackets were bonded with Transbond XT to 15 human mandibular incisors. The specimens were thermocycled, stored in distilled water (37ºC, 7 days), and tested in shear, using an Instron universal machine. Failure mode was classified according to the adhesive remnant index using a stereomicroscope. The data were analyzed with an analysis of variance and posthoc tests (p<0.05). Results: Significant differences were found between the bracket bond strength to polymethylmethacrylate with different surface treatments (p<0.001). Sandblasting with aluminum oxide was superior to tungsten bur roughening. The adhesive system did not significantly influence the shear bond strength nor the failure mode (p=0.8415). All experimental groups showed lower bond strengths than the control group (p=0.1566). Conclusions: Mechanical surface treatment significantly influenced the bracket’s bond strength to polymethylmethacrylate. Sandblasting with aluminum oxide was the most effective mechanical treatment. The weakest adhesive link was found at the acrylic-adhesive interface. Orthodontic bonding to polymethylmethacrylate was weaker than bonding to enamel, regardless of the surface treatment and the adhesive used.