Corrosion between orthodontic archwires and bracket couples

Abstract

Nickel-containing orthodontic wires have been reported to cause allergic reactions in sensitive individuals. However, stainless steel brackets also contain nickel and could potentially elicit a reaction. The ability of these metals to cause an allergic reaction is related to corrosion of the alloys and subsequent leaching of nickel ions into the oral cavity. The purpose of this study was to determine if there is a significant difference in the corrosive potential of stainless steel, NiTi and TMA wires either alone or when coupled with a stainless steel bracket. At least two wires and bracket/wire combinations of each type were tested using potentiostatic anodic polarization. The samples were tested in 0.9% NaCl solution at room temperature with neutral pH. Using a Wenking MP 95 potentiostat, the breakdown potential for each sample was determined from constructed polarization curves. The potentiostat was connected to an electrochemical corrosion cell and data was collected using a computer and data acquisition program. The samples were visually analyzed for surface changes and photographs were taken. The breakdown potentials of stainless steel ("A" Co.), NiTi (Ormco), TMA (Ormco), and the stainless steel bracket (Ormco) were 600 mv, 1600 mv, >2000 mv, and 200 mv respectively. When coupled with a stainless steel bracket, the breakdown potential for all three of the wire types was 200 mv. The breakdown potential of the stainless steel bracket overrode the potential for the wires themselves and the samples all broke down at the point where the bracket would have corroded by itself. The stainless steel brackets proved to be the weak ling in the galvanic couple with the three wire types and the brackets have a significantly higher corrosive potential than any of the wires themselves. If a patient has a nickel allergy, the orthodontist would be wise to avoid the use of stainless steel brackets in addition to nickel containing archwires. TMA wires do not contain nickel and they have a very high resistance to corrosion. TMA wires would be an acceptable substitute for NiTi while ceramic brackets should be considered instead of stainless steel. iii To my parents, John and Mary Ellen, whose love, support and example have allowed me to pursue all of my goals in life. iv ACKNOWLEDGEMENTS My sincere appreciation to the following individuals: Dr. Michael Bagby for serving as my chairman and helping me complete this project. Dr. Marcia Gladwin for her encouragement and serving on my committee. Dr. Peter Ngan for his commitment to excellence and his dedication to education for his residents. Dr. Hera Kim for her help and support throughout the course of this study.