Corrosion Resistance Of New And Retrieved Nickel-titanium Orthodontic Wires In Vitro

Objective To study corrosion resistance of different nickel-titanium orthodontic wires in artificial saliva with various acidities, and evaluate the biological safety of nickel-titanium orthodontic wires.Finally, direct to choose orthodontic wires in clinical practice.Methods Collected two kinds of new Ni-Ti orthodontic wires(new 0.016in IMD and new0.016in YY),also collected the same two kinds of Ni-Ti orthodontic wires used for four weeks (old 0.016in IMD and old 0.016in YY).There were four types of Ni-Ti orthodontic wires in all.Every type had 20 wires. Selected two straight parts of four types of Ni-Ti orthodontic wires (3mm).Every type wire had 40 samples, and there were 160 samples in all. Chose 30 samples of every type and divided into three groups.Every group had 10 samples. Every wire was put into EP tube.Three groups were separately added artificial saliva with various acidities 2 ml (PH2.5,PH4.0, PH6.75).The whole immersion test lasted for 28 days in 37℃constant temperature water bath.After immersion test, we observed microstructure with scanning electron microscope and detected the amount of releasing Ni+ in artificial saliva.Results①when PH values were the same, the nickel ion concentration of four types nickel-titanium arch wires were compared with each other. The results indicated that, there were no significant differences between the nickel ion's concentrations of new IMD nickel-titanium arch wires and new YY nickel-titanium arch wires(p>0.05).There were also no significant differences between the nickel ion concentrations of old IMD nickel-titanium arch wires and old YY nickel-titanium arch wires (p>0.05).There were significant differences between the other each other (p<0.05). ②when types were the same, the nickel ion concentration of three different PH values were compared with each other. The results indicated that, there were significant differences between each other (p<0.05).When the PH value was 2.5,the concentration of nickel ion was highest.③when PH value was 2.5,nickel ion's concentrations of four types of nickel-titanium arch wires were separately 0.6687ug(new IMD),18.8894ug (old IMD),0.6746ug (new YY),19.5668ug (old YY).They were all lower than the concentration of nickel ions caused by allergies (600-2500ug) and healthy adults'daily intake of nickel ions (300-500ug).④Microstructure of four types of Ni-Ti wires showed different extents of changes after dipping in artificial saliva, their forms of corrosion were also different. The corrosion was more serious with the decrease of PH value.⑤New nickel-titanium arch wires had defects. Defects of YY Ni-Ti wires were more than IMD Ni-Ti wires'.Nickel-titanium arch wires used for four weeks had more defects than new nickel-titanium arch wires.Conclusions①With the decrease of PH value, the amount of nickel ion all increase. However, the amount of nickel ions is all within the safety range.②Two brands of nickel-titanium arch wires have different morphology, and this is related with their production process. Different brands of nickel-titanium arch wires show different forms of corrosion.Observed from the surface structures,IMD nickel-titanium arch wires show better resistance to corrosion.③Observe the amount of nickel ions and surface structures of nickel-titanium arch wires,we can find anti-corrosion of used nickel-titanium arch wires is down.④We suggest patients who are nickel-sensitive should be careful with nickel-titanium arch wires.