Biodegradation of orthodontic appliances. Part I. Biodegradation of nickel and chromium in vitro.

The purpose of this study is to compare in vitro the corrosion rate of a standard orthodontic appliance consisting of bands, brackets and either stainless steel or nickel-titanium arch wires. The corrosion products analyzed were nickel and chromium. Evaluation was conducted with the appliances immersed for 4 weeks in a prepared artificial saliva medium at 37 degrees C. Ten identical sets were used, each simulating a complete orthodontic appliance used on a maxillary arch with a full complement of teeth. Five sets were ligated to stainless steel arch wires, and the other five sets were ligated to nickel-titanium arch wires. Nickel and chromium release was quantified with the use of a flameless atomic absorption spectrophotometry. The analysis of variance was used to determine if differences existed between the nickel and chromium release according to arch wire type, as well as with time (days 1, 7, 14, 21, and 28). The results indicate that (1) orthodontic appliances release measurable amounts of nickel and chromium when placed in an artificial saliva medium. (2) The nickel release reaches a maximum after approximately 1 week, then the rate of release diminishes with time. On the other hand, chromium release increases during the first 2 weeks and levels off during the subsequent 2 weeks. (3) The release rates of nickel or chromium from stainless steel and nickel-titanium arch wires are not significantly different. (4) For both arch wire types, the release for nickel averaged 37 times greater than that for chromium. How much of these corrosive products are actually absorbed by patients still needs to be determined.