Nickel-titanium orthodontic archwires are used with bonded appliances for initial leveling. However, precise bending of these archwires is difficult and can lead to changes within the crystal structure of the alloy, thus changing the mechanical properties unpredictably. The aim of this study was to evaluate different bending methods in relation to the subsequent mechanical characteristics of the alloy.
Materials and Methods
The mechanical behaviors of 3 archwires (Copper NiTi 35°C [Ormco, Glendora, Calif], Neo Sentalloy F 80 [GAC International, Bohemia, NY], and Titanol Low Force [Forestadent, Pforzheim, Germany]) were investigated after heat-treatment in a dental furnace at 550-650°C, treatment with an electrical current (Memory-Maker, Forestadent), and cold forming. In addition, the change in A f temperature was registered by means of differential scanning calorimetry.
Heat-treatment in the dental furnace as well as with the Memory-Maker led to widely varying force levels for each product. Cold forming resulted in similar or slightly reduced force levels when compared to the original state of the wires. A f temperatures were in general inversely proportional to force levels.
Archwire shape can be modified by using either chair-side technique (Memory-Maker, cold forming) because the superelastic behavior of the archwires is not strongly affected. However it is important to know the specific changes in force levels induced for each individual archwire with heat-treatment. Cold forming resulted in more predictable forces for all products tested. Therefore, cold forming is recommended as a chair-side technique for the shaping of NiTi archwires.
Nickel titanium (NiTi) is widely used in current orthodontic mechanics. These include archwires, springs, self-ligating brackets, and separators. By far the most common use is NiTi archwires. In contrast to other materials, NiTi alloys display unsurpassed spring back, a low module of elasticity, and relatively constant forces over large activations (superelastic behavior). These properties are particularly beneficial for archwires used in initial alignment. However, NiTi alloys are sensitive to both the composition of the alloy and the processing involved in forming the archwire. In contrast to conventional alloys such as stainless steel and beta-titanium alloy, force levels cannot be determined purely through archwire dimensions.
To date, the best method of bending NiTi archwires has not been clarified. There are 2 ways to alter the shape of the archwire: cold forming and forming with a heat source. Both techniques have been recommended for use in orthodontic practice. It is well known that NiTi alloys are highly sensitive to temperatures. Different temperatures at the time of forming and variations in intraoral temperature can lead to considerable changes in force levels. Many studies have focused on the influence of varying oral temperatures on the force levels of NiTi archwires. Clinically, force reductions that occur with low temperatures can be used by the patient to temporarily decrease the applied force by rinsing with cold beverages. Other studies have investigated the influence of temperature to program or permanently form NiTi archwires. Wide ranges of temperatures and exposure times have been used, but the results for various archwires have been inconsistent.
As a chair-side procedure, the manipulation of NiTi archwires can be achieved mechanically through cold forming. However, an overactivation of NiTi archwires during bending is necessary because a certain amount of spring back will occur. This is due to stress-induced lattice transformation from austenite to martensite. At room temperatures, the martensitic phase is stable, but, when subjected to higher temperatures comparable with the oral environment, the stress-induced martensite (low-temperature phase) will retransform to austenite (high-temperature phase). The restructuring of the austenitic lattice will result in spring back. Another possibility is to shape the NiTi archwire by using high temperatures, which induce restructuring of the lattice. This can be achieved either in a laboratory furnace or with an electric current. The latter method is more appropriate for chair-side programming of the wires. Two pliers are connected to the Memory-Maker (Forestadent, Pforzheim, Germany), and a variable electric current is transmitted through the span of wire between the pliers. Frequency and voltage can be varied so that the electric current is appropriate for the chosen archwire dimension and the intended bending.
The aim of this investigation was to identify the influence of cold forming and heat treatment of NiTi archwires on their mechanical properties.
Material and methods
Three-point bending tests according to ISO 15841 were performed on 3 archwires with the dimension of 0.016 × 0.022 in: Titanol Low Force (Forestadent), Neo Sentalloy F 80 (GAC International, Bohemia, NY), and Copper NiTi 35°C (Ormco, Glendora, Calif). In accordance with the ISO norm, the straight ends of the archwires were cut to a length of 30 mm and placed in the 3-point bending model with a span of 10 mm. The force was applied to the broader side of the rectangular wires. The crosshead speed was set at 7.5 mm per minute, and force levels were measured every 0.05 mm. All wires were deflected to 3.1 mm and deactivated without stopping at maximum deflection. The wires were all tested in a controlled environment: a dry temperature chamber at 36°C ± 0.5°C in a closed-air system connected to a thermostat (FS 18 HP, Julabo, Seelbach, Germany). All bending tests were performed with a testing machine (model 3344, Instron, Norwood, Mass). For each product (Titanol Low Force, Neo Sentalloy F 80, and Copper NiTi 35°C), 10 specimens were analyzed in every treatment group. The treatment groups were the following:
The control group, consisting of untreated archwires.
Heat treatment in a furnace. To evaluate the reaction to heat treatment, 6 groups with different temperatures and exposure times were tested. The heat treatments were performed in a laboratory furnace (KAVO EWL 5636, Biberach, Switzerland) at 550°C, 600°C, and 650°C for 2 and 5 seconds each. Two 1-kg steel cubes were heated to the intended temperature, the oven was opened, and the archwire was placed between the 2 steel cubes, where it remained for the time indicated. One archwire was treated every 15 minutes to allow enough time to reheat the steel cubes. The whole process from the opening of the furnace to the finished heat treatment of the wire lasted 15 to 20 seconds.
Heat treatment with electrical current. Programming with the Memory-Maker was conducted with an alternating current of 7 A, 50 Hz, for 5 seconds and an interplier span of 3 cm. These settings corresponded to those used clinically. The wires were not bent but only heat treated.
Cold forming. A permanent sweep of 30° was placed in the archwire by sliding the wire at an angle of 90° repeatedly over the edge of a plastic ruler. Once the permanent sweep was achieved and controlled by dipping the wire in hot water, the process was reversed by the same procedure. Thus, a straight section of wire, which was cold formed, was obtained.
Apart from mechanical testing, differential scanning calorimetry (DSC) measurements were taken from all wire specimens to define austenit finish (A f ) temperatures (MDSC 2990, TA Instruments, New Castle, Del). The heating rate was set at 10 K per minute with a temperature range of −40°C to +100°C.
Force-deflection diagrams were recorded by measuring the force levels every 0.05 mm. Force levels at a deflection of 1.5 mm on the deactivation curve were chosen for the comparison of archwires. In addition, the mechanical properties were evaluated by the ratio of variance ( Fig 1 ):
r a t i o o f var i a n c e = m i d p o int f o r c e 0.5 m m s t a r t f o r c e − 0.5 m m e n d f o r c e .