Influence Of Various PH And The Treating Time On Mechanical Property And Surface Structure Of Ni-ti Orthodontic Wire

Objective: To observe the performace changing of nickel-titanium (Ni-Ti) orthodontic wires induced by pH, and evaluate the influence of pH on mechanical behavior and surface structure of Ni-Ti orthodontic wires.Methods: About 3cm segments were obtained from two terminals of LH, OT, CuNiTi and IMD orthodontic arch wires respectively, maintained in EP resin tube containing artificial saliva(pH=4.0,pH=6.75,1 mL),and cultured at 37℃. Tensile strength was measured and microstructure was observed under scanning electron microscopy after 1, 7 and 28 days.Results: 1.The initial tensile strength of CuNiTi and IMD were higher than the others. At pH=6.75,compared with the control group,the tensile stress of OT dropped by 105.79N, Cuniti by 98.7N, IMD by 110.5N after 28 days, decline were higher; The initial tensile strength of LH was lowest. Compared with the control group, the tensile stress of LH dropped by 70.62N after 28 days, decline was lower than the others. At pH=4.0, compared with the control group,the tensile stress of IMD dropped by 168.68N after 28 days, decreased more significantly than the other three orthodontic arch wires. LH dropped by 71.23N, has the lowest the tensile stress. The tensile strength of 4 orthodontic arch wires in pH4.0 group were lower than pH6.75 group. Among which, compared with pH6.75 group,the tensile strength of IMD dropped by 110.17N in pH4.0 group after 1 day, decline was highest.In the same conditions, LH dropped by 53.65N, decline was lowest.2. With the time prolongation and the pH values decrease, extents of changes came to be more serious. There were more changes on the surface of IMD group than LH group and OT group. LH showed a regular surface of micro-nano-white grooved structure。Conclusion: 1.The tensile strength of orthodontic arch wires is due to the decrease of pH value, and the degree of tensile strength decreased with the elongation of culture time. Among which,with the decline of pH value,the decline of the mechanical properties of IMD was most significant.while the influence of the mechanical properties of LH was minimum.The further study is needed to show the relationship between this change and the breaks of Ni-Ti orthodontic wires in the mouth.2. Different materials show different surface structure and the forms and degrees of corrosion, especially, be closely related with the manufacturing process; Acidic environment can accelerate the corrosion of Ni-Ti orthodontic wires, with the time prolongation and the pH values decrease, corrosion comes to be more serious, the degrees of corrosion of IMD was more serious than Cuniti,OT,LH.