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Special Properties of Nitinol _ Orthodontics

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Original Title: Special Properties of Nitinol Special Properties of Nickel-Titanium Alloy 1. Shape memory characteristics Shape memory is that when the parent phase of a certain shape is cooled from above Af temperature to below Mf temperature to form martensite, the martensite is deformed below Mf temperature, and then heated to below Af temperature, accompanied by reverse phase transformation, the material will automatically restore its shape in the parent phase. In fact, the shape memory effect is a thermally induced phase transformation process in NiTi alloys. 2. Superelastic The so-called superelastic refers to the phenomenon that the sample produces a strain far greater than the elastic limit strain under the action of external force, and the strain can be automatically restored when unloaded. That is to say, in the parent phase state, due to the external stress, the stress-induced martensite transformation occurs, so the alloy shows different mechanical behavior from ordinary materials, its elastic limit is far greater than that of ordinary materials, and it no longer obeys Hooke's law. In contrast to shape memory properties, superelasticity is not thermally involved. In a word, hyperelasticity means that the stress does not increase with the increase of strain within a certain range of deformation, and the clinical manifestation is that the orthodontic force generated by the archwire in the process of deformation remains constant and no longer gradually loses with the movement of teeth towards the orthodontic direction. According to the characteristics of the stress-strain curve corresponding to hyperelasticity, hyperelasticity can be divided into two types: linear and nonlinear hyperelastic. In the stress-strain curve of the former, the relationship between stress and strain is nearly linear. Nonlinear hyperelasticity is the result of stress-induced martensitic transformation and its reverse transformation during loading and unloading in a certain temperature range above Af, so nonlinear hyperelasticity is also called phase transformation pseudoelasticity. The phase transformation pseudoelasticity of NiTi alloy can reach about 8%. The superelasticity of nitinol varies with the conditions of heat treatment, nickel titanium wire , and begins to decrease when the archwire is heated above 400 oC. When the heat treatment temperature exceeds 600oC, the superelasticity is substantially small. According to this characteristic, the non-orthodontic area of the archwire can be thermally treated to lose its superelasticity in clinic, so as to avoid the influence of the orthodontic process on the teeth in the non-orthodontic area, and the archwire in the orthodontical area still has good elasticity. 3. Sensitivity of temperature change in oral cavity: The orthodontic force of stainless steel wire and CoCr alloy orthodontic wire is basically not affected by the temperature in oral cavity. The orthodontic force of the superelastic NiTi alloy orthodontic wire varies with the oral temperature. When the deformation is constant. The orthodontic force increased with the increase of temperature. On the one hand, it can accelerate the movement of teeth, because the temperature change in the oral cavity will stimulate the blood flow in the blood stasis area caused by the capillary stagnation caused by the orthodontic device, so that the repair cells can be fully nourished during the movement of teeth to maintain their vitality and normal function. On the other hand, orthodontists cannot precisely control or measure the orthodontic force in the oral environment. 4. Corrosion resistance: Studies have shown that the corrosion resistance of nickel-titanium wire is similar to that of stainless steel wire. 5. Toxicity resistance: Nickel-titanium shape memory alloy has a special chemical composition, that is, it is a nickel-titanium alloy containing about 50% nickel, which is known to have carcinogenic and cancer-promoting effects. In general, the surface layer of titanium oxide acts as a barrier, which makes the Ni-Ti alloy have good biocompatibility. TixOy and TixNiOy in the surface layer can inhibit the release of Ni. 6. Gentle orthodontic force: The currently used orthodontic wires include austenitic stainless steel wire, cobalt-chromium-nickel alloy wire, nickel-chromium alloy wire, Australian alloy wire, gold alloy wire and nickel-titanium alloy wire. Load-displacement curves for these orthodontic wires under tensile and three-point bending test conditions. Nitinol has the lowest and most flat plateau of the unloading curve, indicating that it provides the most durable and gentle orthodontic force. 7. Good shock absorption characteristics: The greater the vibration of the arch wire caused by chewing and bruxism, the greater the damage to the root and periodontal tissue. Through the study on the attenuation experiment of different archwires, 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 archwire is only half of that of stainless steel wire. The good vibration and shock absorption characteristics of archwires are very important for the health of teeth,Titanium 6Al4V wire, while traditional archwires, such as stainless steel wires, tend to aggravate root resorption. Contact us Back to Sohu to see more Responsible Editor:.

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