Study of Nickel Ion Release From NiTi Orthodontic Wires Under Bending Stress

Objectives: Traditional stainless steel orthodontic wires and more recent ones such as NiTi wires are composed of metallic elements. The electrochemical degradation of these alloys may cause the release of metal ions into the oral environment, particularly Nickel ions which are known as allergens.
The aim of this study was to quantify the release of Nickel ions from NiTi orthodontic wires under mechanical stress.
Methods: The NiTi orthodontic wire Speed ​​Supercable (Universal) Speed ​​System ™ was selected. The archwires were submitted to bending stress using a blocking system without chemical interference. The samples were immersed and placed in an oven for 16 days at a temperature of 37°C in a storage medium: Ringer solution with lactate (pH = 2). Control samples were immersed without stress.
The eluates analysis was performed by inductively coupled plasma mass spectrometry to quantify the release of Nickel ions (High Resolution ICP-MS Agilent 7200; detection limit: 0.5 ppb (µg/l)).
Results: The results showed a detectable ICP-MS Nickel ion release for all the wires immersed in the storage medium. The wires tested exhibited significant levels of Nickel ion release.
The release rate varies from 599.31 ppb or 1.87 g/cm² (for control samples not submitted to stress) to 824.12 ppb or 2.57 µg/cm² (for the wires exposed to bending stress).
Conclusions: This study shows that Nickel ions are released in significant proportions during a 16 days exposition period.
Bending stress leads to a Nickel ion release rate 37% superior compared to the control samples.
This study should be completed by additional analyzes of other types of wires (composition, number of strands) and the use of different storage solutions.