Introduction

The effectiveness of orthodontic movement involves the adequate interaction of factors related to the patient, mechanics, teeth and periodontal supporting structures. The outcome of treatment is particularly dependent upon the action of the orthodontic wires, according to their structural and mechanical characteristics.1

In the traditional sequence of replacing stainless steel wires during the levelling and alignment phases, the progressive transition from thinner to thicker wires alter the amount of force released.2 When the wire receives a tensile force before reaching its limit of proportionality, it will respond by returning to its original form and will therefore be in its elastic phase. After passing the elastic limit, the wire will reach the plastic phase when it changes its form, yet without returning to its original shape. If an exaggerated force is applied, a permanent deflection occurs and the wire no longer returns to its original form. This occurs because the deflection surpasses the elastic limit of the wire.3,4

When an orthodontic wire is deformed, several internal tensions occur, which means that its atoms are spatially dislocated and the interatomic forces become unbalanced. This condition of instability is due to the fact that some atoms get too close to each other while others become too distant. The atoms tend to return to their original position by diffusion with time and a consequence of this process, called stress releasing, is wire distortion.3 In order to avoid this phenomenon, after bending, the stainless steel wire is heated to a temperature of 850°F for 3 minutes until a reddish-brown colour is obtained.5 This procedure is known as “heat treatment” and recovers the normal aspect of the metal microstructure.4-6 Another method of performing this treatment is passing the bent wire repeatedly through the flame of an alcohol lamp until a reddish-brown colour is observed in its entire extension. However, this procedure does not follow a technical standardisation.

Clinically, when the orthodontist makes loops, bends or establishes a new arch form, the wire reaches a high internal pressure and should be thermally treated in order to release these tensions.7 Only conflicting and insufficient information is available regarding the alterations induced by the heat treatment, which makes this treatment a controversial choice in orthodontics. These factors, allied with the existence of only a small number of reported studies regarding this subject, were the rationale for the present study. To the best of our knowledge, all available studies refer to laboratorial investigations and do not address this procedure under clinical conditions. It is common in orthodontic practice to use an alcohol lamp or a welding machine with a specific device for heat treatment. However, neither of these procedures follows rigorous time and temperature standards. Therefore, since these procedures are routinely performed by the majority of orthodontists, it is important to investigate their effect on the physical properties of stainless steel wires. The purpose of this study was to evaluate the effect of heat treatment on stainless steel orthodontic archwires of different thicknesses when subjected to compressive and tensile strength forces.