志联

　　Nitinol is a shape memory alloy. Shape memory alloys are special alloys that can automatically recover their original shape from plastic deformation at a specific temperature. Its elongation rate is over 20%, fatigue life reaches 1*10^7, and damping characteristics are 10 times higher than ordinary springs. Its corrosion resistance is superior to the best currently available medical stainless steel, therefore it can meet the application needs of various engineering and medical fields, and is a very excellent functional material.

　　In addition to its unique shape memory function, memory alloys also have excellent characteristics such as wear resistance, corrosion resistance, high damping, and superelasticity.

　　Special Properties of Nitinol

　　1. Shape Memory Characteristics (shape memory): Shape memory is when a certain shape of the parent phase is cooled from above the Af temperature to below the Mf temperature to form martensite. The martensite is deformed at a temperature below Mf, and when heated above Af temperature, accompanied by reverse phase transformation, the material will automatically recover its shape in the parent phase. In fact, the shape memory effect is a thermally induced phase transformation process of nitinol.

　　2. Superelasticity (superelastic): Superelasticity refers to the phenomenon where a specimen, under external force, produces a strain far greater than its elastic limit strain, and the strain automatically recovers upon unloading. That is, in the parent phase state, due to the action of external stress, stress-induced martensitic phase transformation occurs, resulting in the alloy exhibiting mechanical behavior different from ordinary materials. Its elastic limit is far greater than that of ordinary materials, and it no longer obeys Hooke's Law. Compared with shape memory characteristics, superelasticity does not involve heat. In short, superelasticity refers to the fact that within a certain deformation range, stress does not increase with increasing strain. Superelasticity can be divided into linear superelasticity and nonlinear superelasticity. In the former, the stress-strain curve shows a nearly linear relationship between stress and strain. Nonlinear superelasticity refers to the result of stress-induced martensitic phase transformation and its reverse phase transformation during loading and unloading in a certain temperature range above Af. Therefore, nonlinear superelasticity is also called phase transformation pseudoelasticity. The phase transformation pseudoelasticity of nitinol can reach about 8%. The superelasticity of nitinol can be changed with the change of heat treatment conditions. When the archwire is heated above 400°C, the superelasticity begins to decrease.

　　3. Sensitivity to Temperature Changes in the Oral Cavity: The orthodontic force of stainless steel wires and CoCr alloy orthodontic wires is basically unaffected by the temperature in the oral cavity. The orthodontic force of superelastic nitinol orthodontic wires changes with the change of oral temperature. When the deformation is constant, the orthodontic force increases with increasing temperature. On the one hand, it can accelerate tooth movement, because changes in oral temperature will stimulate blood flow in areas of capillary stagnation caused by orthodontic appliances, thus allowing repair cells to receive sufficient nutrition during tooth movement, maintaining their vitality and normal function. On the other hand, orthodontists cannot accurately control or measure the orthodontic force under oral cavity conditions.

　　4. Corrosion Resistance: Studies have shown that the corrosion resistance of nitinol wire is similar to that of stainless steel wire.

　　5. Toxicity: The special chemical composition of nitinol shape memory alloy, that is, it is a NiTinol-titanium equiatomic alloy containing about 50% NiTinol, and it is known that NiTinol has carcinogenic and cocarcinogenic effects. Under normal circumstances, the surface layer of titanium oxide acts as a barrier, giving the Ni-Ti alloy good biocompatibility. The surface layer of TiXOy and TixNiOy can inhibit the release of Ni.

　　6. Gentle Orthodontic Force: Commercially available orthodontic wires include austenitic stainless steel wires, cobalt-chromium-NiTinol alloy wires, NiTinol-chromium alloy wires, Australian alloy wires, gold alloy wires, and titanium alloy wires. Regarding the load-displacement curves of these orthodontic wires under tensile and three-point bending test conditions, the unloading curve platform of nitinol is the lowest and flattest, indicating that it can provide the most persistent and gentle orthodontic force.

　　7. Good Damping Characteristics: The greater the vibration caused by chewing and bruxism on the archwire, the greater the damage to the tooth roots and periodontal tissues. Studies on the results of damping experiments with different archwires have found that the amplitude of vibration of stainless steel wires is greater than that of superelastic nitinol wires. The initial vibration amplitude of superelastic nitinol archwires is only half that of stainless steel wires. The good vibration and damping characteristics of archwires are important for tooth health, while traditional archwires, such as stainless steel wires, tend to increase root resorption.

　　Clinical Applications of Nitinol Wires:

　　1. Used for early alignment and leveling of the patient's dentition: Due to the superelasticity and shape memory properties of nitinol archwires and their low stress-strain curve, nitinol archwires are routinely used clinically as the initial archwires incorporated into the orthodontic system. In this way, patient discomfort is greatly reduced. Since several different straight-wire orthodontic techniques exist, the MBT technique recommends using 0.016-inch heat-activated nitinol archwires (HANT wires), the DEMON self-ligating bracket technique recommends using copper-containing heat-activated nitinol archwires (phase transformation temperature is approximately 40 degrees) produced by Omcro, and the O-PAK orthodontic technique recommends using 0.016-inch superelastic nitinol archwires for early alignment and leveling.

　　2. Nitinol Springs: Nitinol push springs and pull springs are springs used in orthodontics. They have the special properties of nitinol superelasticity and are suitable for orthodontics to open gaps between teeth and pull teeth in different directions. A nitinol helical spring can generate approximately 50g of force when elongated by 1mm. Nitinol helical springs have high elastic properties and can generate relatively gentle and stable continuous force under tension. The force attenuation is very small, and it can generate an ideal orthodontic force that meets the clinical requirements for tooth movement. It meets physiological requirements. The high elasticity and extremely low permanent deformation rate of nitinol wire springs are 3.5-4 times different from stainless steel wires of the same diameter in terms of the orthodontic force released. Therefore, in orthodontic applications, patients not only experience less pain and feel a gentle and lasting force, but also have reduced follow-up appointments, shortening the treatment time and improving the efficacy. It is a new and excellent mechanical device in orthodontic treatment.

　　3. L-H archwires were researched and developed by Dr. Soma and others in Japan and are manufactured by Tomy. ""LH"" stands for ""Low Hysteresis,"" meaning that when this archwire is ligated to the bracket (i.e., when the archwire is activated), the difference between the stress generated and the stress generated when the archwire slowly returns to its original state while moving the teeth is very small. That is, the hysteresis is very small. Soma et al. compared the stress-strain curves of LH archwires and other NiTinol-titanium alloy wires, and the hysteresis range of L-H archwires was the smallest. This characteristic gives the archwire the advantages of low load and continuous light force. At the same time, the low initial slope of the curve indicates that the archwire has low stiffness. The hysteresis curves of other types of NiTinol-titanium alloy archwires show that they have greater rigidity. Obviously, L-H archwires have significant mechanical advantages. Because the titanium content in the NiTinol-titanium composition of LH wires is higher than that of general NiTinol-titanium archwires, it is called titanium-NiTinol wire, and experiments have shown that it has a stronger shock absorption effect. Another characteristic of LH NiTinol-titanium wire is that it can be bent and shaped using a heat treatment instrument. Therefore, LH NiTinol-titanium wire can be used from alignment and leveling, opening the bite to closing gaps, and the final finishing stage. One archwire on the upper and lower arches can complete the treatment. As long as the archwire is removed at each stage, bent into the required shape, and then shaped using a heat treatment instrument to increase the hardness. Currently, L-H archwires are clinically used for arch expansion treatment, correcting open bite, crossbite, and underbite. Due to their continuous, stable, and gentle force, the effect is better. At the same time, J-hooks are often used to improve the insufficient stiffness of the archwire. Although MEAW technology also has ideal effects on correcting the above malocclusions, the complex archwire bending often discourages many doctors. Therefore, some doctors have switched to using rocking-chair-type NiTinol-titanium archwires similar to the mechanical system plus vertical traction of the anterior teeth. Although this also has similar effects, it always feels that compared with MEAW, the independent tooth movement is not as good as MEAW technology. The reason is that the rocking-chair-type NiTinol-titanium wire is a continuous archwire and cannot be bent. Therefore, the angle of bracket bonding and the rocking-chair curvature of the archwire determine the angle of each tooth, unlike the MEAW technology, where each tooth has more individual adjustment space. Using LH NiTinol-titanium to bend the rocking chair, and then using an archwire former to bend the buccal or lingual inclination in the mouth, the effect is quite ideal.