Effects of recycling on metallic direct-bond orthodontic brackets.

Recycling of metallic direct-bond orthodontic brackets is of current interest to the profession. The methods of three recycling companies (Esmadent, Ortho-Cycle, and Ortho-Bonding) as well as the author's flame method were examined for their effects on bracket base torque, slot width, and mechanical properties. There was no statistically significant difference among the four methods of recycling in terms of change in base torque angle and slot width. A slightly statistically significant number of brackets in the control group had a change in slot width of not more than 0.0015 inch (0.038 mm.), and the number of brackets with any change in slot width was less than 20 percent. The recycling methods of the author and Ortho-Bonding caused a loss of ferromagnetism in the recycled brackets, suggesting annealing of the metal. The effect of thermal treatment on brackets was evaluated by measuring hardness and theoretical tensile strength and by evaluating microstructure. The Ortho-Cycle method did not alter the mechanical properties of the bracket metal while the Esmadent method caused subtle changes. The author's process and the Ortho-Bonding method caused carbode separation. While it appears that the amount of dimensional changes in the brackets is of little clinical significance, the changes in the metallurgic microstructure suggest susceptibility to metallic intergranular corrosion.