# Analysis of Coating Loss from Coated Stainless Steel Orthodontic Wire
## 4 Discussion  
The rigidity of orthodontic wires is important in orthodontic treatment [3]. Although SS wire has high rigidity, it cannot be used in patients with metal allergies because it contains Ni, and only the less rigid \(\beta\)-Ti wire is available in such patients [3]. The surfaces of orthodontic wires have been modified in various ways to prevent the release of metal ions into the oral environment in efforts to overcome this issue. Corrosion resistance can be improved by coating the wire with various substances, which is expected to provide a wider selection of wires for use in patients with metal allergies [8]. We reported previously that plating of orthodontic wire with TiN improved corrosion resistance and reduced friction without compromising its original mechanical properties [9; 10]. The present study was performed to examine the resilience of such coatings to conditions of low pH and both bending and brushing of the wire. ICP-MS showed that all forms of wire coating significantly suppressed metal ion elution on immersion in HCl for 30 min in comparison to uncoated controls, suggesting that all of the coatings used in this study inhibit acid corrosion. However, the degree of inhibition differed for each type of coating, with Au plating and PEN coating showing the greatest inhibitory effects on metal ion elution. On the other hand, the amount of metal ions eluted from the Rh-coated wire was the greatest, and the amount of metal ions was significantly larger than that of HCl solution without wire immersion. Therefore, the use of Rh-coated wire should be avoided in patients with metal allergies. Plating with Au markedly suppressed acid corrosion, as its standard electrode potential is higher than that of hydrogen [21]. Conversely, as the standard electrode potential of Ti is lower than that of hydrogen, TiN-coated wire showed more corrosion than Au-coated wire. PEN has been shown to have marked resistance to chemical corrosion [22; 23], and therefore PEN-coated wire was resistant to immersion in HCl. On the other hand, Rh-coated wire showed more corrosion than Au-coated wire, even though Rh has a higher standard electrode potential than hydrogen. This increased corrosion may have been due to the greater Ra of Rh-coated wire compared to the Au-coated wire, as shown in Table 1[24]. The null hypothesis (H01) was rejected, as exposure of TiN-coated and Rh-coated wires to low pH conditions caused coating loss. It is often necessary to bend orthodontic SS arch wires up to an angle of about 120\({}^{\circ}\) to make loops for use in orthodontic treatment. This study analyzed the effects of bending of wires at angles in the range from 30\({}^{\circ}\) to 120\({}^{\circ}\), as commonly used in clinical practice, on the coating material and consequently on the release of metal ions from the substrate. Cracks in the bend area were seen on stereomicroscopy only in the TiN coating, and no such cracks or detachment of the coating were seen for the other types of coated wire. SEM analysis showed no coating loss in the PEN-coated wires after bending, while cracks and detachment were observed in the Au-coated, TiN-coated, and Rh-coated wires. At a bend angle of 30\({}^{\circ}\), the Au-coated wire showed only minor cracking, while the TiN-coated and Rh-coated wires showed clear cracks in multiple locations. This lack of crack formation may have been due to the high ductility of Au [25]. The degree of cracking and detachment became more pronounced with increasing angle in the Au-coated, TiN-coated, and Rh-coated wires, with the TiN coating showing particularly notable disruption. The application of external stress was reported to result in multiple crack formation and localized delamination of TiN coating on SS [26], and therefore TiN-coated wire may also have poor resilience to external stresses, such as bending of the wire. In addition, the decrease in fracture strength with increasing thickness of TiN coating was shown to result in a reduction in fatigue