Bending affects surface and mechanical properties of Stainless-Steel wire

Objectives: The aim of this study was to investigate the corrosive behaviour of stainless-steel (SS) orthodontic wires in a more clinical relevant way by bending and exposing to various pH.
Methods: Rectangular SS wires (n=120, 0.43×0.64mm) were randomly assigned into 4 groups, each containing 15 bent and 16 straight wires. Wires in group 1-3 were exposed to artificial saliva at pH 5.6, 6.6 and 7.6 respectively and wires in group 4 were kept in air. 4 weeks later, surface roughness (Ra-value) was measured by a profilometer. Young’s modulus and maximum force were determined by a four-point flexural test. Scanning electron microscopy (SEM) was used to observe surface morphology. Differences were examined using ANOVA, p<0.05.
Results: Ra-value of bent wires was significantly higher than that of straight wires in all groups (p<0.000), where straight wires in group 4 showed (8.2±0.9)×10^-2μm (the lowest) and bent wires in group 1 (pH 5.6) showed (24.1±4.5)×10^-2μm (the highest). Young’s modulus was distinctively reduced after bending (p<0.000). In group 4 Young’s modulus of straight wires was 496±18.62GPa and that of bent wires was 345±85.90GPa. This discrepancy between bent and straight wires remained and enlarged in groups of various pH. Maximum force of bent wires was significantly higher than that of straight wires throughout 4 groups. Yet pH didn’t make significant influence on maximum force in either bent or straight wires. Bigger surface irregularities were observed on SEM pictures of straight wires immersed in pH5.6 compared to other pH values (p<0.05). The influence of pH on surface roughness quantified by 3D construction of SEM images was consistent with the surface roughness measured by profilometer.
Conclusions: Bending has a significant influence on surface roughness and mechanical properties of rectangular SS wires. pH plays a synergistic effect on the change of mechanical properties of SS wires along with wire bending.