Introduction
Our purpose was to study the mechanical properties and phase transformations of orthodontic wires submitted to in-vivo exposure in the mouth for different periods of time.
Methods
Stainless steel wires were tied to fixed orthodontic appliances of 30 patients from the orthodontics clinic of Universidade Federal do Rio de Janeiro School of Dentistry in Brazil. According to the duration of the clinical treatment, the patients were divided into 3 groups. After in-vivo exposure, the samples were studied by mechanical testing (torsion) and ferromagnetic resonance. Statistical analyses were carried out to evaluate the correlation between time of exposure, mechanical properties, and austenite-to-martensite transformation among the groups.
Results
The results were compared with as-received control samples. The torque values increased as time in the mouth increased. The increase in torque resistance showed high correlations with time of exposure ( P = 0.005) and austenite-martensite phase transformation.
Conclusions
The resistance of stainless steel orthodontic wires increases as the time in the mouth increases; this effect is attributed to the austenite-to-martensite transformation.
Editor’s comment
In-vivo aging of orthodontic materials has been a field of intense activity in the past decade. This derives from the supposedly critical importance of intraoral conditions on the surface of, and the structural and mechanical alterations on, the material that change the overall performance relative to the in-vitro testing. This topic has shown important implications in clinical orthodontics for a wide array of materials, including nickel-titanium alloys, bracket materials, and adhesives. These authors assessed for the first time the mechanical properties of stainless steel archwires after exposure in the mouth for different periods of time. The study of used archwires in this article showed significant structural alterations caused by intraoral service as well as a small increase in resistance to torsional loads.
Q & A
Eliades: What are the clinical implications of your findings?
Elias: These results show that torque resistance increases with time of exposure in the mouth. The clinical implication is that, when the archwire is subjected to the same torsion angle, the wire transfers a different force to the teeth depending on the time of exposure in the mouth.
Eliades: Can you suggest any potential precautions that will address the changes that accompany the long-term intraoral placement of stainless steel wires, such as variation in the form of variation of third order activation?
Elias: To reduce the effects of variation of the force applied to the teeth after a long time of exposure in the mouth, we suggest using low-twist angles or changing the material to a nickel-titanium alloy during the first phase of the orthodontic treatment.
Eliades: Do you plan to extend the scope of this investigation to bracket steel alloys?
Elias: Although the mechanical properties of brackets made of stainless steel can change due to phase transformation, there are no clinical implications regarding orthodontic treatment because, in this case, the most important property is the friction coefficient. We are currently analyzing bacterial colonization and the changes of surface morphology of wires exposed to the oral environment and the frictional force between wires and brackets.
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