Nickel ion release from orthodontic NiTi wires under simulation of realistic in-situ conditions

Abstract

The increasing use of nickel containing devices in orthodontics and the growing prevalence of nickel allergy in the population significantly increases the interest in biocompatibility studies of these devices. The decisive factor determining the biocompatibility of orthodontic wires is their corrosion behaviour. Therefore seven nickel titanium levelling arches, one titanium molybdenum, a cobalt chromium and three stainless steel wires were analysed with respect to their corrosion behaviour under realistic conditions. Potentiostatic tests to determine rupture potentials in artificial saliva and static immersion tests in artificial saliva (AS) or lactic acid (LA), as well as immersion tests with mechanical, thermal and combined mechanical and thermal stresses were performed. Subsequently, the surfaces of the wires were investigated employing scanning electron microscopy (SEM) with energy dispersive X-ray analysis (EDX) and the nickel release into the corrosion media of the specimens was measured with inductively coupled plasma mass spectrometry (ICP-MS). The results yield information not only about the relative corrosion tendency of the wires under in vitro conditions but also give a quantitative estimation about the nickel ion release of the orthodontic wires during in vivo treatment. Generally, the maximum release of nickel ions was two orders of magnitude below the daily dietary intake level. Mechanical and thermal loading increases nickel release in the immersion tests by a factor of 10 to 30. Two NiTi wires (Dentaurum Tensic, Forestadent Titanol Low Force) examined showed lower rupture potentials and a higher tendency towards corrosion in the immersion tests than the others due to their surface composition. However these differences are levelled off by long-term mechanical and thermal loading.