Abstract
Nickel-titanium (NiTi) orthodontic archwires are crucial in the initial stages of orthodontic therapy when the movement of teeth and deflection of the archwire are the largest. Their great mechanical properties come with their main disadvantage—the leakage of nickel. Various in vitro studies measured nickel leakage from archwires that were only immersed in the medium with little or minimal simulation of all stress and deflection forces that affect them. This study aims to overcome that by simulating deflection forces that those archwires are exposed to inside the mouth of a patient. NiTi orthodontic archwires were immersed in CACO2-2 cell culture medium and then immediately loaded while using a simulator of multiaxial stress for 24 h. After the experiment, the surface of the NiTi orthodontic archwires were analysed while using scanning electron microscopy (SEM) and auger electron spectroscopy (AES). The observations showed significant microstructural and compositional changes within the first 51 nm thickness of the archwire surface. Furthermore, the released nickel and titanium concentrations in the CACO2-2 cell culture medium were measured while using Inductively Coupled Plasma Mass Spectroscopy (ICP-MS). It was found out that the level of released nickel ions was 1.310 µg/L, which can be assigned as statistically significant results. These data represent the first mention of the already detectable release of Ni ions after 24 h during the simulation of mechanical loading in the CACO2-2 cell culture medium, which is important for clinical orthodontic praxis.
Highlights
Today, shape memory alloys (SMA) have a wide variety of medical applications, including dentistry, especially for orthodontic treatment
The chamber construction was construction was created, so that the CACO2‐2 cell culture medium was in continuous contact with created, so that the CACO2-2 cell culture medium was in continuous contact with the archwires during the archwires during testing in the simulation of multiaxial stress equipment (SMAS): The archwires that were inside the SMAS went through testing in the SMAS: The archwires that were inside the SMAS went through the chamber in one the chamber in one part of their length
The initial archwire surface is rough, with visible holes and other defects, while the deformed archwire surface is smooth and without any other visible defects. This can be attributed to the influence of mechanical loading, which led to the elongation and permanent deformation of the NiTi archwire [36] and the faster release of elements from the surface into the medium
Summary
Shape memory alloys (SMA) have a wide variety of medical applications, including dentistry, especially for orthodontic treatment. They are attractive due to their superelasticity behaviour or functional property above the temperature austenite finish (Af ), and they are characterised by martensitic phase transformation, which is caused by the initiation of stress with enough value, leading to the changes of phase or microstructure in the SMA (austenite to martensite). Clinicians remain cautious of NiTi orthodontic archwire usage, because of their large nickel content, a long period of time being required for orthodontic treatment, and the high incidence of allergic reactions to nickel [1]. Nickel may cause: lymphotoxicity [7], immunotoxicity [8], haemotoxicity [9], genotoxicity [10], and carcinogenicity [11]
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