The Comparision Of Mechanical Property Of Three Different Nickel-titanium Orthodontic Wires

Background:Nickel-titanium alloy is one of the shape memory alloys which a new-type functional material, the alloy has the superordinary function of shape memory and hyperelastic. When the laboratory of American navy Weapon found the nickel-titanium alloy had the function of shape memory at 1963, the alloy began to be used extensively with stainless steel wires in orthodontics. Nickel-titanium alloy orthodontic wires have a very potentiality in orthodontic therapeutic tools for it can release satisfactory, unremit, light and soft strength. Nickel-titanium alloy orthodontic wires contain common Nickel-titanium orthodontic wires, hyperelastic Nickel-titanium orthodontic wires and heat activation Nickel-titanium orthodontic wires.Now the present researches are focusing on the phase transformation behavior of wires. Through micro-observation, we know that hyperelastic Nickel-titanium orthodontic wires are stabile austenite state, and their elasticity are slightly better than Nitinol, but their brittleness are lower than Nitinol, and they can be changed reciprocal transformation between martensite state and austenite state. Their rigidity is larger at min state of stress than large stress state, and they can move the teeth at large stress state while stabilize the teeth at the small state of stress. The heat activation Nickel-titanium orthodontic wires are the newly rising wires. The difference between them was the different phase transformation, The heat activation Nickel-titanium orthodontic wires has higher performance retention, low rigidity and high elasticity than the hyperelastic Nickel-titanium orthodontic wires. And, doctors pay close attention to the performance of wires, especially the mechanical property of wires under the theory of light orthodontic force.Orthodontic forces of hyperelastic Nickel-titanium orthodontic wires and heat activation Nickel-titanium orthodontic wires can be changed with the temperature. Rajapakse found that the critical stress of martensitic transformation of Nickel-titanium orthodontic wires step up along with the temperature when the temperature was above the martensitic transformation start temperature, and the critical stress of martensitic anti-transformation of Nickel-titanium orthodontic wire step up along with the temperature when the temperature was above the martensitic anti-transformation start temperature. M Iijima also found that the orthodontic forces became larger when heating the hyperelastic Nickel-titanium wires from 37℃to 60℃and then to 37℃. Oral environment is not a completely permanent internal environment, its temperature can be changed when we ingest food, and the wires release different orthodontic force during the process of temperature change. When put the wire into slot it makes orthodontic force to the teeth, and the magnitude of force directly influences the progression and result of the therapy. If orthodontic force is justo major exceed the CP, it can cause local necrosis of tissue, inability to carry out physiological bone remodeling process, tooth movement slowly, excavated absorption of alveolar bone, and the dental root absorption. Besides, patients can have uncomfortable symptom such as pain.Objectives:To analyze the relationship between phase transformation piont and orthodontic force of Nickel-titanium orthodontic wires, we study the mechanics capability of different phase transformation of nickel-titanium orthodontic wires. And to investigate the changes of strain-stress curve of different phase transformation temperature under controlled temperature.Methods:Elect three nickel-titanium orthodontic wires with different phase transformation point (Af) as the samples of empirical study: nickel-titanium orthodontic round wires (Beijing you yi yan); nickel-titanium orthodontic round wires (Materials & chemistry Science institute of Zhejiang university) nickel-titanium orthodontic round wires (3M unite).(1) Constant temperature group: Three same-sized (0.016") but different phase transformation nickel-titanium orthodontic wires were examined using three-point bend test. Samples were tested at same conditions of moment and oral temperature (37℃), in different utmost strain capacity. Then got the strain-stress chart, also, we examined the phase transformation point (Af).(2) Altering temperature group: used the same method testing the samples at same conditions of moment and different temperature(0℃,10℃,20℃,30℃,37℃,50℃,60℃), then got the strain-stress curve, statistical analysis : All data demonstrated as mean±standard deviation ( x|-±S) , using statistics software of SPSS 16.0, and adopting one-factor analysis of variance, LSD mean comparison test were applied for group comparison. The significance levels used were 0.05Results:The wires had significantly difference at 3.00mm utmost strain capacity (P <0.05):The wires of the highest phase transformation point had the least orthodontic force, Contrarily the least phase transformation point had the highest orthodontic force at 3.00mm utmost strain capacity. The orthodontic forces had no difference at the little strain capacity (<0.50mm) (P>0.05), but had significantly difference from the 0.50mm (P <0.05), and the result was the same as 3.00mm.When at the same temperature, the orthodontic forces were reverse with phase transformation temperature. The highest phase transformation temperature had the least orthodontic force (P <0.05), Contrarily the least phase transformation temperature had the highest orthodontic force (P <0.05). When the test temperature was higher than the phase transformation temperature, the flat of strain-stress curve went up along with the test temperature (P <0.05);Conclusion:When the phase transformation point at high level such as 3M unit and Materials & chemistry Science institute of Zhejiang university, the orthodontic forces were at low level and more permanence, it means mechanics capability of wires were more good.Temperature alter can change the strain-stress curve, and their relationship was direct proportion. When the temperature went down below the phase transformation temperature the wire almost had no orthodontic force or very small.