Nanoindentation and surface characterization of clinically retrieved multi-force NiTi orthodontic archwires

Nickel-titanium (NiTi) alloys, due to their properties of superelasticity and shape memory effect, have been extensively used in orthodontic treatment. Their superelasticity allows the orthodontist to apply an almost continuously low force with larger activation that reduces tissue trauma and patient discomfort, and thus facilitates tooth movement. The producers usually do not give information about the mechanical properties of the archwires they produce on their packagings or Web pages, which makes it difficult for the dentists to choose the most suitable material and with better cost-benefit for use. In the present work we investigated and compared the mechanical and physicochemical properties of as received and used for 6 and for 9 weeks multi-force NiTi archwires, produced by American Orthodontics under the trademark TriTanium TM, to evaluate if there are changes in their properties with clinical usage. The mechanical properties were investigated using nanoindentation measurements. The surface morphology and the elemental composition of the archwires were studied by scanning electron microscopy (SEM) and energy-dispersive analysis (EDS), respectively. The comparison of mechanical properties of as received and used for 6 and 9 weeks TriTanium TM archwires showed that their indentation hardness and modulus decrease with increasing of the time for use. No significant change in the elemental composition of the archwires was observed after their clinical application.