Corrosion and biocompatibility of orthodontic wires

With the increasing number of orthodontic treatments using devices containing nickel and the growing prevalence of nickel allergy in the average population, biocompatibility studies of these devices have become a topic of major interest. The corrosion behavior of orthodontic wires is a decisive factor determining their biocompatibility. Therefore four nickel-titanium guiding arches, a titanium-molybdenum and a stainless steel wire were analyzed for corrosion behavior under realistic conditions. Pure potentiostatic, pure mechanical and combined potentiostatic and mechanical stresses were applied to the specimens. Subsequently, the surfaces of the wires were investigated employing atomic force microscopy (AFM) and nickel loss was measured with an atomic absorption spectrophotometer. The results yield information about the relative corrosion tendency of the wires under in vitro conditions. The wires examined can be classified into two groups, one with a high and a second group with a low tendency towards corrosion, that is American Orthodontics Memory wire as well as GAC Neo Sentalloy and Ormco Ni-Ti as well as Unitek Nitinol respectively. Although corrosion behavior under clinical conditions can not be directly derived from these results, analyses of wires after clinical usage indicate that changes of wire surfaces might show the same characteristics under in vitro conditions.