Nickel-titanium (NiTi) alloy is a kind of shape memory alloy, which is a special alloy that can automatically restore its original shape after its plastic deformation at a certain temperature. Its ratio of expansion and contraction can reach above 20%, while the fatigue life can be up to 1*10 seven times. In addition, its damping characteristics are 10 times higher than ordinary springs, and its corrosion resistance is better than the best medical stainless steel. Therefore, nickel-titanium alloy is a very excellent functional material and can meet the needs of various engineering and medical applications.
Special properties of NiTi alloys
In the ordinary shape memory effect, a specimen which has been transformed to martensite on cooling, but not deformed, is deformed in the martensitic phase and is then reheated into the austenitic phase, whereupon it reverts to its original shape and orientation. The two-way (or “reversible”) shape memory effect, in contrast, takes place without any external stress. The specimen deforms spontaneously during the cooling half-cycle and recovers spontaneously during the heating half-cycle. In fact, the shape memory effect is a thermally induced phase transition process in Nitinol alloys.
The so-called superelasticity refers to the phenomenon that the strain produced by the specimen under the action of external force is far greater than the elastic limit strain variable, and the strain can be automatically recovered during unloading. The superelasticity of nickel-titanium alloy can be changed with the change of the conditions of heat treatment. When the bow wire is heated to more than 400oC, the superelasticity begins to decline.
Sensitivity to changes in oral temperature
The correction force of stainless steel wire and CoCr alloy dental wire was not affected by the oral temperature, while that of the superelastic Nitinol alloy dental wire changed with the oral temperature. When the deformation is constant, the temperature increases and the correction force increases. On the one hand, it speeds up the movement of the teeth, because changes in temperature in the mouth stimulate blood flow to places where blood flow is stagnant due to capillary stagnation caused by orthodontic devices. In this way, repair cells are fully nourished during tooth movement to maintain their vitality and normal function. On the other hand, orthodontists cannot precisely control or measure the orthodontic force in the oral environment.
The corrosion resistance of NiTi wire is similar to that of stainless steel wire.
Nickel-titanium shape memory alloys have a special chemical composition, that is, they are an atomic alloy such as nickel-titanium, containing about 50 percent nickel, which is known to cause and promote cancer. In general, titanium oxidation on the surface layer acts as a barrier to make Ni-Ti alloy have good biocompatibility. The TiXOy and TixNiOy in the surface layer can inhibit the release of nickel.
Gentle corrective force
The dental orthodontic wires currently in commercial use include austenitic stainless steel wire, cobalt-chrome-nickel alloy wire, nickel-chromium alloy wire, gold alloy wire, and nickel-titanium alloy wire. The load-displacement curves of some orthodontic wires under the conditions of tensile test and three-point bending test and the unloading curve of nickel-titanium alloy are the lowest and the flattest, indicating that it can provide the most durable and gentle correction force.
Good shock absorption
The greater the vibration of the archwire caused by chewing and night molars, the greater the damage to the root and periodontal tissues. It is found that the vibration amplitude of stainless steel wire is larger than that of superelastic nickel-titanium wire, and the initial vibration amplitude of superelastic nickel-titanium wire is only half of that of stainless steel wire. Good vibration and shock absorption characteristics of the archwire are important for the health of teeth, while the traditional archwire, such as stainless steel wire, has a tendency to increase root absorption.
Clinical application of Nitinol alloy wire
Nitinol alloy wire is used for the early alignment of patients’ teeth. Nickel-titanium alloy bow wire is commonly used as the most initial archwire in the orthodontic system because of its superelasticity, shape memory and low stress-strain curve. In this way, the patient’s discomfort will be greatly reduced. Due to the existence of several different straight wire orthodontic techniques, the MBT technique recommends the use of 0.016-inch thermally activated Nitinol wire (HANT wire); DEMON self-locking bracket technology recommends to use the copper-containing thermo-activated nickel-titanium alloy arc wire produced by Omicron (phase transition temperature is about 40 degrees); The O-PAK correction technique recommends the 0.016-inch superelastic Nitinol wire for early alignment.
Nickel-titanium push spring and tension spring is a kind of spring used for orthodontic teeth. It has nickel-titanium superelasticity, which is suitable for opening up the space between teeth and pulling teeth in different directions in orthodontic treatment. The nickel-titanium coil spring extends by 1mm to produce a force of about 50g. Nickel-titanium spiral spring has very high elastic properties and can produce relatively soft and stable continuous force in the state of tension. The force attenuation is very small, which can produce ideal orthodontic force for clinical tooth movement and meet the physiological requirements. Nickel-titanium wire tension spring has high elasticity and very low permanent deformation rate. Compared with the stainless steel wire of the same diameter, the correction force released by it differs by 3.5-4 times. In the application of the orthodontic treatment, patients not only have less pain but also reduce the time of return visit, as well as shorten the course of treatment and improve the curative effect. Therefore, nickel-titanium spring is a new excellent mechanical device in orthodontic treatment.
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